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Resonant cavity enhanced photonic
M. Selim lhiiia)
Boston Universiv, Department of Electrical, Computer and SystemsEngineering,
and Centerfor Photonics Research,Boston, Massachusetts02215
Samuel Strite
IBM ResearchDivision, Zurich ResearchLaboratory, 8803 Riischlikon, Switzerland
(Received 1 August 1994; accepted for publication 1 March 1995)
We review the family of optoelectronic devices whose performance is enhanced by placing the
active device structure inside a Fabry-Perot resonant microcavity. Such resonant cavity enhanced
(RCE) devices benefit from the wavelength selectivity and the large increase of the resonant optical
field introduced by the cavity. The increased optical field allows RCE photodetector structures to be
thinner and therefore faster, while simultaneously increasing the quantum efficiency at the resonant
wavelengths. Off-resonance wavelengths are rejected by the cavity making RCE photodetectors
promising for low crosstalk wavelength division multiplexing (WDM) applications. RCE optical
modulators require fewer quantum wells so are capable of reduced voltage operation. The
spontaneous emission spectrum of RCE light emitting diodes (LED) is drastically altered, improving
the spectral purity and directivity. RCE devices are also highly suitable for integrated detectors and
emitters with applications as in optical logic and in communication networks. This review attempts
an encyclopedic overview of RCE photonic devices and systems. Considerable attention is devoted
to the theoretical formulation and calculation of important RCE device parameters. Materials criteria
are outlined and the suitability of common heteroepitaxial systems for RCE devices is examined.
Arguments for the improved bandwidth in RCE detectors are presented intuitively, and results from
advanced numerical simulations confirming the simple model are provided. An overview of
experimental results on discrete RCE photodiodes, phototransistors, modulators, and LEDs is given.
Work aimed at integrated RCE devices, optical logic and WDM systems is also covered. We
conclude by speculating what remains to be accomplished to implement a practical RCE WDM
system. 8 1995 American Institute of Physics.
I. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. Theoretical formulation of RCE parameters. . . . . .
A. Formulation of the quantum efficiency for
RCE photodetectors. . . . . . . . . . . . . . . . . . . . . . .
B. Standing wave effect. . . . . . . . . . . . . . . . . . . . . .
C. Exact numerical calculation of RCE quantum
D. Resonant cavity enhancement of quantum
E. Near unity quantum efficiency
photodetection. . . . . . . . . . . . . . . . . . . . . . . . . . . .
F. Wavelength selectivity of RCE
photodetection. . . . . . . . . . . . . . . . . . . . . . . . . . . .
G. Formulation of crosstalk. . . . . . . . . . . . . . . . . . .
H. Angle dependence of quantum efficiency.. . . . .
I. Formulation of inhibited spontaneous
III. Design criteria for RCE photodetectors. . . . . . . . .
A. Material requirements for RCE
photodetection. . . . . . . . . . . . . . . . . . . . . . . . . . . .
“Electronic mail: [email protected]
B. Material system combinations for RCE
photodetection. ...........................
1. AlGaAs/GaAs/InGaAs. .................
2. InP/InGaAs/InAlAs. ...................
3. AlAsfGaAslGe. .......................
; .....
4. SilSiGe ........................
5. Si/AlP/GaP. ..........................
. High speed properties of RCE photodiodes. .....
A. High speed capabilities of conventional
photodetectors. ...........................
B. High speed performance of RCE
photodetectors. ...........................
C. Impulse response of RCE and conventional
photodiodes ..............................
D. Simulated high speed response of RCE
photodetectors. ...........................
V. Experimental results on discrete RCE devices .....
A. Schottky photodiodes. .....................
B. Avalanche photodiodes. ....................
C. P-I-N photodiodes. .......................
D. Metal-semiconductor-metal photodetectors .....
E. Heterojunction phototransistors. .............
F. Bipolar inversion channel field effect
phototransistors. ................
8 1995 American Institute of Physics
J. Appl. Phys. 78 (2), 15 July 1995
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G. Wavelength demultiplexing with RCE
photodiodes and phototransistors. ............
H. Optical modulators. .......................
I. Light emitting diodes. .....................
J. Optical amplifiers. ........................
VI. RCE optical logic and systems. ...............
k Integration of detectors with LEDs. ..........
B. Vertical cavity lasers integrated with
photodetectors. ...........................
C. Wavelength selective optical logic. ..........
VII. Towards a practical RCE WDM system. .......
VIII. Conclusions. .............................
Over the past five years a new family of optoelectronic
devices has emerged whose performance is enhanced by
placing the active device structure inside a Fabry-Perot resonant microcavity. In such structures, the device functions
largely as before, but is subject to the effects of the cavity,
mainly wavelength selectivity and a large enhancement of
the resonant optical field. The increased optical field allows
photodetectors to be made thinner and therefore faster, while
simultaneously increasing the quantum efficiency at the resonant wavelengths. Since off-resonance wavelengths are rejected by the cavity, the photodetectors have both wavelength selectivity and high speed response making them ideal
for wavelength division multiplexing (WDM) applications.
Optical modulators situated in a resonant cavity require
fewer quantum wells to absorb the same fraction of the incident light, and can therefore operate at lower voltages. In the
case of emitters, the cavity modifies the spontaneous emission of light emitting diodes (LED) improving their spectral
purity and directivity. For these reasons and others, devices
of this genre are referred to as Resonant Cavity Enhanced
While the fundamental physics of RCE has been known
for nearly 100 years,“’ and the first observation of RCE in
semiconductor devices occurred nearly twenty years ago,3
recent developments have stimulated research activity in this
area. Optical communication networks demand higher bandwidth photo& devices and devices capable of WDM.4 Molecular beam epitaxy (MBE) provides researchers with a
growth technique capable of producing epitaxial Fabry-Perot
microcavities within exacting specifications enabling the realization of RCE devices.5 This review attempts to provide a
broad overview of RCE devices and systems, including theoretical, simulation and experimental results. All classes of
RCE optoelectronic devices studied to date are covered.
However, resonant devices for which cavity resonance is
necessary for device operation, such as vertical cavity surface emitting lasers (VCSEL), are not included. In Section II,
we assemble a largely analytical formulation of the RCE
effect which is extended to numerical modeling when necessary. Section III lays out fundamental materials criteria for
RCE devices and examines the suitability of common heteroepitaxial systems. In Section IV, a simple model compares
the bandwidth of RCE and conventional photodetectors and
corroborating results from simulations of high speed RCE
photodiode response are overviewed. Section V is devoted to
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
the body of experimental work on discrete RCE photonic
devices, including photodetectors, phototransistors, modulators, and LEDs. Section VI covers integrated RCE devices
and their implementations for optical logic and WDM. In
Section VII, we speculate over what remains to be accomplished towards implementing a practical RCE WDM system.
Important for the design and understanding of RCE
based devices is the development of an analytical mathematical model describing the behavior of an active absorption (or
gain) region inside a Fabry-Perot resonator. This challenge
has been taken up by a number of workers3,6-‘3 whose efforts contribute to this section. Below, we present a formulation which allows the calculation of the quantum efficiency, finesse, and free spectral range of an .arbitrary RCE
detector structure. Special consideration is given to solutions
which yield near unity quantum efficiencies. We also consider the dependence of RCE detector properties on the
placement of the active layer within the cavity and the angle
of incidence of the detected radiation. The derivation for the
lossy cavity (detection) can also be applied to a gain cavity
without lasing (emitters) by replacing the absorption coefficient with the gain coefficient. Finally, a formulation for
spontaneous emission within an optical cavity suitable for
LEDs is presented.
A. Formulation
of the quantum
for RCE
One of the desired features for photodetectors is high
quantum efficiency. The quantum efficiency 7 of a photodetector is defined as the probability that a single photon incident on the device generates an electron hole pair which
contributes to the detector current.t4 When many photons are
present, which almost always is the case, 17is defined as the
current flux/photon flux ratio.
Incident Light -
DBRt--L-( k--L2-‘Ge
FIG. 1. Analysis model of a RCE photodetector. The active region of thickness d is a small bandgap semiconductor. The top and bottom distributed
Bragg reflection mirrors consist of alternating layers of non-absorbing larger
bandgap materials.
Appl. Phys. Rev.: M. Selim finlij and S. Strite
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For the derivation, a generalized RCE photodetector
structure is chosen (Fig. 1). In practice, the end mirrors are
made of quarter-wave stacks of GaAslAlAs, InGaAsfInAlAs
or any of a number of other semiconductor or insulator materials which are capable of providing the desired refractive
index contrast. In simplified designs, the top mirror can be
the native semiconductor to air interface which, due to the
large refractive index difference at the boundary, provides a
reflectivity of approximately 30%. The active layer, where
absorption occurs, is placed between the two mirror structures and is defined by its thickness d and absorption coefficient a. The separations between the active layer and the top
and the bottom mirrors respectively are indicated by L1 and
Lz. The absorption coefficient outside the active region is
denoted (Lag. The field reflection coefficients of the top and
the bottom mirrors are [email protected] and [email protected], respectively,
where fir and & denote phase shifts due to light penetration
into the mirrors.
The transmitted component of the incident light wave
electrical field component (Ei) equals tl. Ei . In the cavity,
the forward traveling field Ef is composed of this transmitted
field and the feedback as a result of internal reflections at the
mirrors. Therefore, for a propagation constant p, the forward
traveling wave Ef at z=O (Fig. 1) can be obtained through a
self-consistent consideration, i.e., Ef is the sum of the transmitted field and the feedback after a round trip in the cavity:
-ad-a,,(L1+L2)e-j([email protected],+~2)
The backward traveling wave (i.e., Eb at z = L) is found by
propagating Ef (2) through the detector region:
- ~
(L,+Lz!~ [email protected]+&i)+
The optical power inside the resonant cavity is given by:
)ES12 (~=f
or b),
Under the assumption that all the photogenerated carriers contribute to the detector current, 17 is the ratio of the
absorbed power to the incident optical power, i.e.,
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
A Old
FIG. 2. Wavelength dependence of 17for RCE detectors having various top
mirror reflectivities for fixed L=2 pm, Rz=0.9 and &=O.l (after Ref. 8).
7~ PI I Pi. Hence:
(e-‘ae.&l+e-%.&[email protected])
l-2$&[email protected]
cos(2pL-f JI1+ qb2)fR1R2e -2”% I
act= (rx&
+ a,,L2 + ad)lL.
In a practical detector design, the material around the active
layer absorbs negligibly ( a,,-5 - 10 cm-‘) compared to
the active layer (aa IO4 cm-‘), so acx in (6) can be neglected, except when active layer is extremely thin. Thus,
TJcan be written:
(1 +R2eWed)
1 -2aeWad
where r/O and n are the vacuum characteristic impedance of
electromagnetic waves and the refractive index of the detector material, respectively.
In this case, the light power absorbed in the active layer
(Pl) (neglecting the standing wave effect to be considered
below) can be obtained from the incident power Pi in the
(1 -rT)(e -ru,Ll+r2e-“exL~e-neL>(l
= 1-2~-,r~a-~~~ cos(2~L+$1+~~)+(rlrz)2e-2”cL
1 - r1r2e
where LY, is:
Solving for Ef gives:
ud = 0.1 , R2 = 0.9
Since the propagation constant p (/3=2nr/Xo,
X0 is the vacuum wavelength and y1 is the refractive index)
has a wavelength dependence, 17is a periodic function of the
inverse wavelength. This is easily seen in Fig. 2, which illustrates the calculated wavelength dependency of 0. The
three curves correspond to the cases of RI =0.9, 0.3, and
0.05 while Rz=0.9, crd=O.l, and L=2 ,um are fixed. 7 is
enhanced periodically at the resonant wavelengths which occur when 2/?L+ fir + G2=2 mrr (m= 1,2,3...). The spacing of
the cavity modes, i.e., resonant wavelengths, is defined as the
free spectral range FSR.” For the given cavity parameters,
the FSR is around 500 nm.
On the right hand side of (8), the term inside the braces
represents the cavity enhancement effect. This term becomes +
unity when R2=0 giving 77 for a conventional detector. The
flat dotted line in Fig. 2 indicates the maximum r,~attainable
for conventional photodetectors for the same active layer
thickness (ad = 0.1). The contrast between the two types of
detectors is clear. Conventional photodetectors provide
roughly constant ‘17across a broad wavelength range while .%;.?
. ;”
Appl. Phys. Rev.: M. Selim klti
and S. Strite
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RCE photodetectors can be designed to have significantly
improved 77at specific wavelengths.
The peak p at the resonant wavelengths can be derived
by imposing the resonant condition in (8)
(1 fR$+)
v nmx= (l-me-adj2
In the limit of a thin active layer ad+i 1, (9) reduces to
(1 -R&d.
Proper selection of the device parameters allow for RCE detectors having nearly unity 77to be designed.
(1 - &-&l
B. Standing
wave effect
In the above derivation of (8), the spatial distribution of
the optical field inside the cavity was neglected. A spatial
distribution arises from the standing wave formed by the two
counter propagating waves [Eqs. (2) and (3)]. It follows that
g, which was derived from the power absorbed in the active
region, is a function of the placement of the active region in
the optical field. We refer to this as the standing wave effect
(SWE). When detectors with thick active layers which span
several periods of the standing wave are considered, the
SWF can be neglected. For very thin active layers, which are
necessary for strained layer absorbers, the SWE must be considered.
The SWE is conveniently included in the formulation of
37 as an effective
aeff = S WE. a, which is either enhanced or decreased by the
placement of the active region. The effective absorption coefficient aeff is the normalized integral of the product of (Y
and the field intensity across the absorption region. Using a
perturbation analysis of Maxwell’s equations, including the
loss factors and assuming uniformity along the transverse
direction, the effective absorption coefficient can be expressed as?
[email protected]=
where X=X0/n, E(z,X) is the total electrical field in the
cavity at a given wavelength and the denominator is the average of the standing wave. Taking a to be negligjble outside
the active region and constant within
SWE= y
I i
To apply (12), the standing wave distribution must be
calculated. Several assumptions simplify the analysis. The
cavity is assumed to be lossless; reasonable for a thin active
layer which absorbs only a small fraction of the total power
density. The change in p in the active layer is neglected since
the real permittivity is much larger than the imaginary (absorptive) part for low loss dielectrics. Finally, reflections
from the active region interfaces are also neglected. This
approximation is valid when heterostructures having subtle
permittivity changes are considered. An exact numerical
-> . . . 20
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
FIG. 3. Optical field distribution in a RCE detector as a function of wavelength and position. Top and bottom mirrors are 5 and 15 periods C&As/
AlAs, respectively, with a center wavelength of 0.9 pm (after Ref. 11).
method for the more general case is described in the following section.
The forward (Ef) and backward (Eb) components of the
standing wave are given by (1) and (2). The total electric
field E and intensity 1El” are:
E= Ef(0)exp( -jPz)
t Eb(L)exp[jp!z-
Substituting (1) and (2) into (14) and assuming a=O:
In (15), the term in braces represents the enhancement effect
of the resonant cavity. Note that under the resonance condition with an ideal bottom mirror ( r2 = 1) this term reduces to
[ 1 - r$l 1 - r i / 2 > 1. It increases rapidly with r , representing
the resonant cavity field enhancement. The position dependence represents the influence of the standing wave. Figure 3
shows the wavelength dependent cavity field distribution calculated from (15) for a GaAs based RCE photodetector.
Substituting (15) into (12), we obtain the dependence of
the SWE on the cavity parameters:”
SWE-l+pdtl+r;)[[email protected]
The SWE is an explicit function of both the magnitude r2
and phase $2 of the bottom mirror reflectivity, and it is implicitly dependent on the top mirror phase +i through the
resonance condition (2pL-k qil + t,b2=2m7rj. The SWE varies with the h not only through /I, but also due to the strong
wavelength dependence of the phase of the quarter wave
stack mirrors ($i , 4V2).The wavelength dependencies of the
mirror reflectivity and phase are demonstrated in Fig. 4 for
several different mirror retlectivities. The reflectivity maxiAppl. Phys. Rev.: M. Selim cnlti and S. Strite
Downloaded 18 Jun 2002 to Redistribution subject to AIP license or copyright, see
g 0.4
region center is at the standing wave maximum and minimum, respectively. These extremes of the SWE are shown in
Fig. 5(b) which highlights the decreasing importance-of the
SWE for thicker absorption regions.
The above approximation (16) can be used to calculate
the photosensitivity of RCE detectors using (17). Excellent
agreement between experiment and theory was realized for a
structure having a small mole fraction In,Ga,-,As
(zKO.1) absorbing layer in a GaAs cavity.8Y’7 This success
was due to the fact that the imaginary part of the propagation
constant p was much larger than the real part (cz), making
the overall change in the refractive index negligibly small
despite drastic changes in the absorption spectrum. However,
for a more detailed analysis of the detector response, such as
crosstalk calculations, it is desirable to have a method which
accounts for any refractive index change in the active region
and the resulting reflections at these interfaces.
C. Exact numerical
of RCE quantum
To more accurately evaluate the spectral dependence of
the total cavity reflection, a simple transmission line model
can be applied” in which wavelength dependent impedance
is calculated at every interface, beginning at the substrate and
a, 0.00
8250 8500 8750
FIG. 4. (a) Magnitude of mirror reflectivity for a GaAs/AlAs (643 A/776
A) quarter wave stack mirror for 10 (solid), 15 (dashed), and 20 (dottedj
periods. ibj Phase of the same mirrors as a function of wavelength (after
Ref. 11).
mum increases and sharpens with additional periods, saturating near unity above fifteen AlAsJGaAs periods. The phase
shows a strong wavelength dependence which sharpens as
the number of periods increase. Knowledge and control of
the phase behavior is particularly important for proper positioning of very thin absorbing layers in high 77 photodetectors.
Figure 5(a) shows the wavelength dependence of the
SWE for various active layer thicknesses d. At d=XO/4n
(solid line), the SWE ranges between 0.35 and 1.7, resulting
in drastic variations in the device photosensitivity at different
wavelengths. For d=XO/2n (dotted line), the SWE is weak,
again due to the active region spanning an entire half period.
For thicker active regions, the maximum deviation occurs at
d=3XO/4n (dashed line) where the SWE ranges from 0.8 to
1.2, giving a photosensitivity variation at resonance below
210% for n=lO” cm-‘.
For an ideal bottom mirror (r2 = 1,q2= 0)) a real top
mirror reflectivity ( $t = O), and L r = L2 (centered active region), the SWE reduces to a simple form:t6
sin /Sd
sin ,Bd
SWE= 1 + cos(mw) pd
= 12
where + and - correspond to the cases when the active
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
10 -=
Wavelength X,, (A)
10 -’
Normalized active layer thickness
FIG. 5. (a) The SWE as a function of wavelength for three different active
layer thicknesses: d=650 %, (solid), d=1300 %, (dotted), and d=2000 8,
(dashed) for a cavity with 20 pairs GaAs/AlAs bottom mirror and native
GaAs surface as the top mirror (L,=L,=2
pm). (b) Dependence of the
SWE on the active layer thickness. The extremes of the SWET are shown
(after Ref. 11).
Appl. Phys. Rev.: M. Selim cnlti and S. Strite
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ending at the epilayer surface. Every semiconductor layer is
considered as a transmission line segment in which the characteristic impedance is given as:t9
~Et -jE”
where E’ and E” are the real and imaginary parts of the
dielectric constant. For a low loss dielectric (E’ % e”), the
propagation constant y can be expressed as:t9
i )I
1 d’ 2
pur’ 1-js+-
8 E’
Equating the imaginary parts yields
p-&zj 1+;( ;)‘I.
For a lossless dielectric (a = 0, E is real), the characteristic
impedance is real and the propagation constant is purely
imaginary. For practical detectors, a is small enough at the
relevant wavelengths to satisfy these assumptions. The
physical parameters, such as E and a, are well known for
common semiconductors and can be found for all pertinent
The substrate (approximated as infinitely thick) impedance (Z+t) is first computed from (18). The impedance is
then transformed for each semiconductor layer in accordance
to its thickness Zi and characteristic impedance Zai by’8,‘9:
to N
until the surface is reached. N is the total number of contrasting layers including those in the quarterwave mirror stacks.
Although this formulation can yield analytical results, we
preferred simple computational methods allowing for analysis for a variety of wavelengths and device parameters. Once
the input impedance (Zi,) of the entire detector structure is
known, the overall complex reflection coefficient can be
evaluated as:l’
Q~uantumefficiency, 7, is approximated as the difference
between the reelection coefficients for a lossless cavity (assuming (Y=O) and a cavity with a finite absorption coefficient in the active layer. This approximation is valid when
the bottom mirror has high reflectivity, a requisite for functional RCE designs. This method automatically accounts for
the position of the active layer with respect to the standing
Figure 6 compares the SWE approximation and the more
accurate calculations by the above described transmission
line analogy for d=h0/4n. The low mole fraction active region was assumed to have the same refractive index (n=3.5,
GaAs) as the spacer regions (~5, ,L2= 2 pm) and a constant
[email protected]
Zi; 1+ Zni tanh YZi
Zaf+ Zi- t tanh YZi ’
Equating the real parts of (19) leads to
Zi’ Zoi
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
Xo (A)
FIG. 6. Comparison of 17 calculated by different methods. Dashed line
shows the case in which the SWE is neglected. Solid lines (nearly coincident) compare the first order SWB approximation (A) with the numerical
method through transmission line analogy (B) (after Ref. 11).
(a= lo4 cm-‘, InCaAs) for the entire spectrum. A 20 period Al.As/GaAs quarter-wave stack and the GaAs/air interface comprised the bottom and top mirrors. The 7’s calculated by the two methods are nearly identical over the entire
wavelength range as a result of the relatively low contrast
InGaAs active layer. The importance of the SWE at small d
is illustrated by the deviation of the dashed line [plotted from
Eq. (S)], which does not take the SWE into account.
D. Resonant cavity enhancement
of quantum
The origin of the drastic enhancement in v is the greatly
increased amplitude of the electric field inside a high Q resonant cavity which causes more energy to be absorbed in the
active region. An equivalent interpretation is that an individual photon is multiply reflected at the mirrors and therefore makes many passes through the absorption region. The
field enhancement is apparent when one calculates the increase in the electric field intensity (i.e., the optical power) in
the cavity (]Ef]2+]Eb]2)/]Ei]2.
The result is identical to the
right hand term in the expression for ~7 (8), except for
( I- e - nd). Therefore, the enhancement of 7, with respect to
an idealized conventional detector with equal active layer
thickness, is simply the increase in the internal optical power.
Figure 7 presents the calculated internal power enhancement factor at resonance for varying mirror refiectivities as a
function of c&. For small ad, i.e., a low loss cavity, the
factor exceeds 10. At higher mirror reflectivities, the enhancement can exceed 50, yielding a correspondingly high
~7despite the thin active layer. As ad increasespthe enhancement falls off rapidly, especially in high Q cavities (e.g.,
RI = 0.9 and R2 = 0.99)) eventually becoming less than unity.
This is because most of the light is absorbed in the thicker
active layer before it reaches the bottom mirror negating the
RCE feedback mechanism.
Appl. Phys. Rev.: M. Selim finlij and S. Strite
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(Fig. 7), 17 is maximized at a certain ad before decreasing
to the conventional RI limited value
v~+,= 1 - R, . The maximum occurs at
: Ra = 0.99
FIG. 7. Internal power enhancement factor in resonant cavities with various
end minor refkctivities as a function of the normalized absorption coefficient (after Ref. 8).
Multiplying the internal optical power enhancement factor by (1 -E -“d) yields 7 (8) for an RCE photodetector.
Figure 8 plots 7 at resonance as a function of ad for
R2=0.9 as a function of RI. The conventional case
(Rz=O) is given by a dotted-dashed line and 7 for
R1=0.7, R2=0.99 is shown by a dashed line. For a native
semiconductor surface (R, =0.3), the R2=0 and 0.9 curves
contrast the conventional and RCE cases. RCE improves 77a
factor of 6.5 for a 0.1 pm thick absorption layer ( UY=lo4
cm- *) . Quantum efficiency can be further enhanced by more
reflective mirrors. The R2=0.99, R1=0.7 curve reaches a
maximum 7 in excess of 98%.
Because the absorption term ( 1 -e Iad) increases with
ad, while the internal power enhancement factor decreases
as obtained by differentiating (9) with respect to R, .
Figure 9 plots vrnax as a function of ad for various R,
values. Conventional photodetectors (R, =0) exceed 90%
only for active layers in excess of 2 pm. In contrast, RCE
photodetectors having R2=0.99 exceed 90% for very thin
active layers (0.05 pm thickness at (Y= 10” cm-‘). A 2 JUIYI
thick conventional detector will be bandwidth limited by carrier transit times. Increased recombination also erodes 17 in
such thick detectors. On the other hand, because of their thin
absorption regions, RCE photodetectors offer increased
speed along with enhanced v.
When ad is very small, o+, , neglected up to this point,
becomes significant. The dashed line in Fig. 9 shows 7 for
aex=5 cm-’ and LI = L2= 1 pm. The effect of external
absorption can only by seen for very large reflectivities
(R2=0.99) at small ad. For practical ad values, the discrepancy is negligibly small and LK~, can continue to be neglected.
E. Near unity quantum
Many applications, such as background limited astronomical observations” and high sensitivity interferometry
for squeezed-light measurements,‘l require photodetectors
with 7 approaching unity. Farhoomand and McMurray”
suggested that RCE photodetectors were capable of providing near unity 77 detection at the expense of wide spectral
photosensitivity. Kishino et al.* presented a detailed analysis
of near unity detection.
The critical requirements were
8r 0.8
'3 0.6
t R2 =\".gg ,' *"
.Ei 0.2
R1 = 0.3 , Ra = 0
Normalized Absorption
FIG. 8. Calculated 77as a function of the normalized absorption coefficient.
Solid lines show 7 at the resonance mode peaks for R2=0.9, dashed lines at
R2=0.99 and RI=0.7, and dotted-dashed fines the conventional detector
case (Rz=O, RI =0.3) (after Ref. 8).
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
Normalized absorption coefficient, ord
FIG. 9. Maximum 7 (R1=R2ezcd)as a function of normalized absorption
coefficient. Dashed line includes parasitic cavity absorption (~4~75
cm-‘) (after Ref. 8).
Appl. Phys. Rev.: M. Selim blti
and S. Strite
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3 2.0
uk3 1.5
Top Mirror Reflectivity
FIG. 10. Constant 77contours as a function of normalized absorption coefficient and top mirror reikctivity for fixed R2=0.99. 7 in excess of 0.99
(meshed region) can be obtained over a fairly large parameter space (after
Ref. 8).
shown to be very high bottom mirror reflectivity and a moderate absorption layer thickness.
To attain the highest 7, near unity Rz is desirable. Mirrors with near unity reflection coefficients require a large
number of periods and become impractical to grow. The use
of metal films as a unity reflection mirror has been
proposed.” However, at optical frequencies, even noble metals such as Au are not ideal reflectors. The reflectivity of
metals on different materials can be evaluated using their
wavelength dependent optical constants.22Y”3For example,
Au is 98% reflective in vacuum at wavelengths around 1
pm which reduces to about 94% on GaAs. Therefore, metal
films alone are not ideal mirrors. For near unity v detectors,
a hybrid approach of a quarter-wave stack and a metal film is
necessary for the bottom mirror. For example, a 6 period
AlAs/GaAs mirror with a deposited Au film will have a reflectivity in excess of 99% at ho= 1 pm.
Figure 10 shows the constant 77contours as a function of
RI and ad for R2=0.99. The meshed region defines the
parameter space of RI and ad which yields 77BO.99. 4t
RI =0.2, a range of active layer thicknesses from
d = 0.7- 0.95 pm at LY= lo4 cm’ is available. Similarly, high
77can be obtained for a range of RI values. For example, at
cud=0.7, the ~>0.99 region extends from RI =0.2 to
R 1=0.3. In contrast, a conventional photodetector with a perfect anti-reflection (AR) coating (R 1=0) requires an absorption layer in excess of 5 ,um to absorb 99% of the incoming
light. Such a detector design with very thick absorption region would suffer from secondary recombination of photogenerated carriers which, in turn, limits the maximum attainable quantum efficiency.
F. Wavelength
of RCE photodetection
At the off-resonance wavelengths, i.e., 2pL + $1 + qb2
(m=1,2,3...), the cavity field amplitude decreases due to the destructive interference of the forward and
SO2508500 8750 9000 9250 Q500 9750
FIG. 11. Effective optical length of GaAs/AlAs (643 A1776 A) quarterwave mirrors designed for a 0.9 pm center wavelength calculated from the
mirror phase [Fig. 4(b)] for 10 (solid line), 15 (dashed line), and 20 (dotted
line) mirror periods.
backward traveling waves, resulting in suppressed 117
(Fig. 2).
The wavelength spacing between neighboring resonant
peaks, called the free spectral range (FSR), can be expressed
as. 15-
2neff(L+ k?ff,l f L,ff,2)
where ncff is the effective refractive index, and L,ff,i are the
effective optical lengths of the mirrors. Generally, L,ff,l can
be given byz4:
L eff,i=y
l a+i
(i= 1,2).
For quarter-wave stack mirrors, Lcff reflects the strong
wavelength dependence of the mirror phase. Fig. 11 shows
the wavelength dependent Lcff of the mirror from Fig. 4.
Experimentally, a typical Leff value for two GaAslAlGaAs
mirrors is 0.7 pm.=
The finesse F, the ratio of the FSR to the wavelength
F;wHM A~,,T,. measures the wavelength selectivity of the
cavity. F can be derived from (8) as:
Ecluation (27) clearly indicates that F increases at higher mirror reflectivity and thin absorption regions. For example,
when cud=O.Ol and cu,,=O, an F as large as 100 can be
expected for R2=0.99 and RI =0.985.
G. Formulation
of crosstalk
The narrow spectral range of RCE devices can be
viewed as a potential disadvantage. Fortunately, it is quite
easy to vary the spectral response of RCE detectors by altering the effective cavity length. This can be accomplished
through a variety of means and permits the fabrication of
arrays from a single epitaxial layer which combine the high
sensitivity of RCE with a broad spectral range.
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
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Top Mirror Reflectivity Ri
FIG. 12. Crosstalk attenuation and v of the demultiplexer for &=O.l as a
function of top mirror retlectivity for R?=0.99 (solid) and 0.9 (dashed)
[after Ref. 8).
In RCE structures in which the top mirror is a simple
semiconductor/air interface, the effective cavity length can
be altered by recessing the surface using conventional wet
chemical etching.a6 Surface recessing can be followed by the
deposition of a dielectric or metallic top mirror if a high
finesse cavity is desired. It is also possible to grow a several
period quarter wave top mirror beneath a final surface recessing layer. After etching and the deposition of a high reflectivity metal, a very high reflectivity hybrid top mirror can be
A critical parameter for any ‘wavelength demultiplexer is
its crosstalk, the measure of the spectral sensitivity overlap
between adjacent detector channels. Using the wavelength
dependent expression for 17 (8), the interchannel crosstalk
attenuation (C) can be estimated as a function of F (26) and
the number of channels N:
From (28), it is clear that a very low crosstalk is obtained for
high F cavities, i.e., large mirror reflectivities and thin absorption regions. Figure 12 shows the dependence of C and
~7 on RI at a fixed ad=O.l. Crosstalk attenuation rises
monotonically with increasing RI , reflecting the higher F.
However, v is optimized by the condition (R 1= R, emzad)
and degrades rapidly once RI exceeds that value. An optimized WDM design must balance the requirement of high C
with high 7.
A suitable measure of WDM detector performance reelecting the above tradeoff is the product q. C. Figure 13
shows the product as a function of ad at optimized 7
(R,=R, e -2ryd) in a 4 channel demultiplexer. Peak performance is achieved near ad=0.07 (less as R2 increases). As
ad increases, performance suffers due to the lower F (28). At
smaller ad, the reduced performance level reflects the decrease of 77.
Wavelength selectivity of a RCE photodetector is similar
to a device comprised of a Fabry-Perot filter placed in front
of a conventional detector. In this case, the filter and detector
can be designed and optimized separately. Since the filter can
be fabricated out of transparent materials very high finesse
(low crosstalk) can be achieved. However, the design of the
overall structure will be more complicated and a different
filter for each wavelength will be required. Besides, for the
filter detector combination, the overall efficiency is simply
the product of the. filter transmission coefficient and the
quantum efficiency of the conventional detector. The advantages of the RCE approach are the simplified design and the
drastic quantum efficiency enhancement. While the simple
design allows for monolithic fabrication of WDM detector
arrays,” quantum efficiency enhancement enables the use of
thin absorbers, and thus high-speed devices.
H. Angle dependence
2. \ I‘ ,
Returning to normal incidence (8) and inserting the angular dependence of 7 as a function of F (27), h, L, neff,
and n gives
Earlier, we restricted our derivation of 77to normal incidence. This can easily be expanded to the general case by
replacing ,3 with /3 cos 8 in the equations. Here, 6, is related
to the angle of incidence 0, by Snell’s law, n sin B=sin ei.
The phases r++i(i = 1,2) are functions of p and are also angle
dependent. The resonant wavelength is also shifted, since the
optical path lengths increase. The new resonance wavelength
condition becomes 2pL cos 0+r,$(0)+&(s>=2m~.
small Bi, the wavelength shift AX can be approximated as:
ot quantum
A?- Q-v- 1%?ffw-&-f,l+~
FIG. 13. Dependence of 77.C product versus normalized absorption coefEcient for four-channel demultiplexing (after Ref. 8).
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
_ \nl
Equation (30) indicates that 7 is most sensitive to the
angle of incidence in a high F cavity at shorter A. This be
havior may be very useful in selected applications (such as
high response angle photodetection). In the general case, the
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length by as much as a factor of ten. As a result of RCE, in
an optimized structure as much as 90% of the spontaneous
emission can be coupled into the resonant wavelength.
For LEDs, ISE serves to significantly improve the spectral purity and directivity of the emission. While conventional LEDs have their spectral linewidths limited by thermal
effects to - 1.8kT, RCE LED linewidths are determined
solely by the Q of the optical cavity35:
Incident Light Angle 6, (deg)
FIG. 14. Degradation of 7 for off-angle incidence at X = 1.0 and 1.5 pm for
F=lO and 20 (after Ref. 8).
angular dependence of the detector performance is not so
large that it presents significant alignment difficulties (Fig.
14). Below 3”, 17degrades by less than 1% even for a very
high F cavity at 1 ,um. For lower F and longer A, the angular
constraints can be relaxed while mainta.ining the benefits of
I. Formulation
of inhibited
Above derivation of the quantum efficiency and its dependence on cavity parameters was carried out for detector
structures. A similar formulation for light emitters can also
be considered simply by replacing the absorption coefficient
with a gain coefficient. Such a formulation is valid as long as
the optical emission properties of the device/material is not
altered by the presence of the cavity. Additional considerations are required for systems where the cavity influences
the emission characteristics, such as onset of lasing in cavities. Spontaneous emission characteristics are also effected
by the cavity. Purce112* realized many years ago that the
spontaneous emission spectrum of a system could be altered
by modifying its external environment. It has since been
shown that the spontaneous emission of trapped electronsZ9
and atoms3’ can be inhibited if the system is placed inside a
microwave cavity which prohibits electromagnetic modes at
the transition frequency. Yablonovitch31 realized that this
concept could be profitably applied to semiconductor light
emitters. He showed that if an active region was placed in a
Fabry-Perot cavity, the spontaneous emission at the resonant
wavelength of the cavity could be greatly enhanced while
off-resonance emission would be inhibited. This is essentially the RCE effect discussed thus far for detectors, but
applied in reverse for emitters. Yokoyama3’ has published a
review of the potential applications. A detailed analysis of
inhibited spontaneous emission @SE) has been presented by
Bjijrk et al. 33,34They considered a QW in a thin Fabry-Perot
cavity and showed that the SWE could be used to enhance
the spontaneous emission of the QW at the resonant wave616
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
where L, is the total effective cavity length including the
field penetration of the mirrors.
The magnitude of the intensity enhancement for a single
dipole at the resonant wavelength is:35
:” E)
For their RCE LED design, Hunt et aZ.35estimated a maximum intensity enhancement factor of 73. In order to apply
this result to a practical LED, the cavity lifetimes and
substrate/air reflectivity would have to be considered.
The RCE scheme is capable of operation over a large
and continuous wavelength range, either by tuning within a
material system, or by moving to a complementary material
system. Having derived the basic equations describing RCE
in the previous section, we move onto the basic design criteria which satisfy the above equations and yield optimal
device performance in common semiconductor material systems. A number of semiconductor material systems are
evaluated below for RCE applications.
A. Material requirements
for RCE photodetection
The superior performance of the RCE photodetection
scheme depends critically on the realization of a low loss
cavity. This dictates that both the mirror and cavity material
must be non-absorbing at the detection wavelength and that
the mirrors have high reflectivity. The multiple mirror periods, each h/4 thick, have a combined thickness on the order
of microns. Therefore, the materials composing the mirror
must be well lattice matched to avoid the introduction of
defects into the active layer. To minimize the number of mirror periods, thereby simplifying growth and reducing the device series resistance, it is desirable to have as large a refractive index difference as possible between the mirror
The active layer material must have a smaller bandgap
than the mirror and cavity materials, but not so much smaller
that large heterojunction band offsets hinder the extraction of
photogenerated carriers. The active layer absorption coefticient should be moderate o! (i.e., 1 X 103Ca<5 X lo4
cm-“) within the operation spectrum. If (Y is too small, a
thick active layer is required, resulting in increased transit
time and degraded high speed performance. For large LY,
RCE remains beneficial only for very thin active layers leadAppl. Phys. Rev.: M. Selim blij
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TABLE I. Pertinent properties of various material systems for RCE devices. Refractive index and absorption
coefficient are given for energies at which the RCE application is most viable for the specific material system.
S: Substrate, Ml: mirror material 1, M,: mirror material 2, A: Active region material, negl.: negligible, ind:
indirect gap.
E,(eV)(300 K)
Absorption.coeff. (cm-‘)
at 1.35 eV
at 0.75 eV
at 0.75 eV
SW, 3)
1 opd.
0 @ji”d.
Refractive index
at 1.35 eV
a(&(300 K)
2 2ind.
2.4 1’“”
5.43 1
ing to SWE complications. Furthermore, a thin absorption
regions may limit the breakdown voltage for some photodetector structures. The desirability of a moderate active layer
thickness also constrains the material to be nearly lattice
matched to the cavity material so that the absorption region
remains below the pseudomorphic thickness limit.
The electronic properties of the material system should
also be considered. For high speed and low noise operation,
high carrier saturation velocities and low bulk and surface
generation/recombination rates are important. Large carrier
velocities directly reduce transit time delays. However, the
device must be designed to withstand the necessary biases
which produce high carrier velocities. Low recombination
rates reduce the dark current and noise of a detector. As the
device area is reduced to seek higher bandwidths, the surface
recombination component increases in relative importance
reflecting the larger surface area to device volume ratio.
Parasitic resistances can also limit device bandwidth. It
is important to choose materials on which low resistance
ohmic contacts can be easily formed. The heterojunction
band offsets and dopability of the mirror materials need to be
considered for their effect on the device series resistance. If
the mirror resistance cannot be suitably minimized, then an
etched mesa contact can be used to circumvent conduction
through the mirror stack, but only at the expense of added
fabrication complexity and the introduction of a lateral access resistance.
B. Material system
for RCE
Different material combinations satisfy all of the above
criteria and are therefore amenable to the RCE scheme. By
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
at 1.3 eV
at 0.8 eV
at 1.3 eV
2.4X lo4
at 0.8 eV
at 0.8 eV
at 2.4 eV
at 1.4 eV
at 0.8 eV
at 2.4 eV
at 1.4 eV
using different material combinations, RCE photodetection
can be extended to wavelengths ranging from the visible to
well beyond 1.55 pm. Table I lists the material properties of
each material relevant to RCE design. The material properties are tabulated for energies at which the fundamental requirement for RCE detectors, i.e., low loss cavity formation,
can be satisfied. Below, we discuss the capabilities and limitations of each combination.
1. AIGaAdGaAsAnGaAs
To date, this has been the most commonly studied material system for RCE detection because of the ease with which
GaAs based alloys and heterostructures can be grown by
MBE. GaAs has good electronic properties, reasonably low
carrier recombination rates, and excellent lattice matching to
AlAs. Because of the fine lattice match, AlGaAs alloys
across the entire range are easily incorporated as wide bandgap contact layers and graded heterojunctions. GaAs and
AlAs also have a good refractive index contrast which allows
a mirror of nearly unity reflectivity to be realized with a
twenty period quarter wave superlattice.
InGaAs as the active material allows the device spectrum to be extended to wavelengths at which GaAs does not
absorb. While GaAs/AlAs mirrors can function easily out to
1.55 pm,36 beyond about 1 pm the attainable pseudomorphic InGaAs active layer thickness begins to limit the potential detector performance. For this reason, GaAs based RCE
devices are expected to remain important for testing concepts
and prototype devices, but find limited commercial applications.
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semiconductor material near a heterojunction because of autodoping caused by interdiffusion. It is also a challenge to
grow GaAs on Ge.37
Thus, a workable detector must be designed as shown in
Fig 15. The growth of such a structure requires two separate
deposition systems for Ge and the arsenides. The heavily
doped upper GaAs layer minimizes the lateral access resistance without increasing the thickness of the heavily doped
(neutral) Ge contact region. Furthermore, the resonant modes
of the optical cavity can be tuned by varying the thickness of
the GaAs contact layer before Ge growth. Ge can be grown
at low temperature with very high doping levels, so interdiffusion effects at the heterojunction should not be important.
A top mirror could in principle be added. A disadvantage of
this design is that absorbing Ge contact layers are necessary
to avoid charge trapping.
Light in
undoped Ge
GaAs contact layer
GaAs Substrate
FIG. 15. Structure of a Ge photodetector grown onto a GaMAlAs
(after Ref. 11).
The Ino,53Gao.47As/Ino52Alo.48Asalloy system grown lattice matched to InP substrates is of great interest for practical
applications due to its outstanding electrical properties and
ability to function between 1.3 ,um and 1.55 ,um While
lattice matching to the substrate must be maintained, this is
easily accomplished.
One disadvantage of this system is the poor refractive
index contrast between InGaAs and InAlAs. Roughly 35 periods are required for a near unity reflectivity mirror. However, the continuing development of gas source MBE has
greatly improved the quality of epitaxial phosphides. Another potential difficulty is that the bandgap of lattice
matched InGaAs is very close to the desired 1.55 pm~wavelength range. The use of InGaAs/InAlAs mirrors is, therefore, limited to longer wavelengths. For the detectors operating at 1.3 to 1.55 pm range, quaternary materials
(InGaAlAs or InGaAsP) are required for mirror formation.
Researchers are developing Ina525Gau475-xA1,As alloys
which are lattice matched to InP across a large wavelength
range. The introduction of.these alloys will further increase
the flexibility of this material system Andyallow graded heterojunctions to be introduced. While more development
work is required, the InP/InGaAlAs system has the greatest
potential to make an impact in the commercial arena.
3. AIAs/GaAdGe
Ge active layers have been suggested” to extend the
sensitivity of GaAs based detectors to 1.55 pm and beyond,
making them applicable to fiber optic systems. The good
lattice matching (0.25%) enables dislocation free growth of
Ge on GaAs. Ge also has very low recombination rates and
the desired moderate absorption coefficient above 1 pm.
However, the lack of an intermediate bandgap alloy would
lead to charge trapping at the heterojunctions. Some difficulties arise when outstanding materid- quality is required. It is
very difficult to control the carrier concentration in either
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
4. WSiGe
Si-based photodetectors are widely used for operating
wavelengths below 1 pm. Recently, the ability to grow
strained SiGe alloys has extended the usefulness of Si-based
detectors to longer wavelengths.38 WSiGe quarter-wave reflectors have also been suggested as a candidate for wavelengths below 1 pm39 although they are absorbing at those
wavelengths. The basic advantage of Si based photodetection
is the potential for integration with Si electronics.
Kuchibhotla et aL4’ extended the RCE scheme to this
material system by demonstrating Si/SiasGeoZ reflectors optimized for 1.5 pm. WSiGe mirrors are limited by the allowable strain, refractive index contrast and absorption of the
SiGe alloy at the wavelength of interest. A Ge mole fraction
of 0.2, as chosen by these authors, remained pseudomorphic
and transparent at the required thicknesses of 0.1 ,um, but
only provided a refractive index contrast of 0.1, a factor of
4-5 less than provided by the III-V materials. As a result, a
reflectivity of only 50% at 1.5’pm was achieved. However,
because of its other inherent advantages, SiGe based detectors are highly desirable, even if it proves difficult to fully
exploit RCE in this material system.
5. Si/AIP/GaP
For shorter wavelengths, above the GaAs band edge, the
nearly lattice matched Si/AlP/GaP material system has excellent potential but requires considerable development. Although the bandgap of GaP is 2.2 eV, its absorption coefficient remains sufficiently low (i.e., 9Si) to permit RCE
detection of blue light at 2.4 eV. Therefore, RCE detectors
with GaP/AlP mirrors and Si active regions can be fabricated
to function across the entire visible spectrum. The refractive
index difference between GaP and AlP is less than that of
GaAslAlAs so more mirror periods are required. However,
for short wavelength detection, this is compensated for by
thinner mirror periods.
These detectors share many of the disadvantages of the
Ge based detectors necessitating a design similar to Fig. 15.
The need for two separate deposition chambers and absorbing Si contact layers, the lack of an intermediate bandgap
alloy, and potential for autodoping, all complicate the design
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of high performance devices. However, unlike the material
systems discussed above, this system spans the entire visible
range. Fairly conservative device designs could easily provide higher 77detection than the conventional Si photodetectors presently in wide use today.
The structure can be grown on Si substrates allowing
integration with electronic devices. Growth of GaP on nearly
lattice matched Si has already been demonstrated.41 Once the
structure is grown, well established Si processing technology
can be utilized for fabrication. The combination of long Si
carrier lifetimes and high 17RCE detection could yield high
performance optical devices.
Intuitively, RCE photodetectors are expected to have improved speed compared to conventional photodetectors because of the reduced active layer thicknesses which can be
used. Experimental reports confirming this are beginning to
A relatively simple analysis comparing the bandwidth of RCE and conventional heterojunction p-i-n photodiodes (PD) provides insight into high speed performance
limitations of each. More complex simulations support the
intuitive predictions.44
A. High speed capabilities
of conventional
The most important response speed limitations in heterojunction p-i-n PDs are drift time across the depleted region,
charging and discharging times of inherent and parasitic capacitances, the diffusion time for the carriers generated in the
undepIeted regions, and charge trapping at the heterojunctions. Good detector design minimizes the latter two by incorporating non-absorbing contact regions, grading the absorbing region heterojunctions, and placing the active layer
in the depleted region. Therefore, we neglect those speed
limitations and focus on intrinsic limitations imposed by the
transit and capacitance-charging times.
The transit time depends on the electron u, and hole
uh velocities, and is dominated by the slower holes. In a
conventional PD [Fig. 16(a)], photogenerated carriers have
to traverse the entire depletion region. The transit time limited 3 dB bandwidth of a thin detector is:45
width (pm)
FIG. 16. (a) Schematic respresentation of a conventional heterojunction
p-i-n photodetector illustrating the distances that the photogenerated carriers
carriers have to transit. (b) The bandwidth-depletion width dependence
(solid lines) are shown for 10X 10 @rns and 5X5 pm’ area conventional
p-i-n detectors. The transit time limited bandwidth (dashed line) is common
for different size devices and capacitance limited bandwidth (dotted lines for
a total output resistance of 50 0) varies with the device area (after Ref. 44).
(33) and (34), an optimum L exists for a given A and RT.
Figure 16(b) shows the bandwidth of 10X 10 pm2 and
5 X 5 ,um’ area conventional detectors as a function of depleted layer thickness for a total resistance of 50 s1. The
maximum bandwidth for this structure is reached at a 0.3
,um and 0.15 pm depleted layer thickness and are about 43
GHz an 86 GHz for the larger and smaller device dimensions, respectively.
For a high speed detector, in which the depletion width
is small, i.e., crLe1, with absorption occurring only in the
depleted region, 7 is:
For an ideal AR coating, (35) reduces to
where L is the depletion width, and uzaAs= 6 X lo6 cm/s. For
a 1 ps response, a depletion width of 0.06pm is needed.
However, as the intrinsic layer thickness is decreased,
the capacitance increases and becomes the fundamental
bandwidth limitation. The capacitance limited bandwidth
where E, is the relative permittivity of the semiconductor, A
is the area of the device, and RT is the total resistance (load
resistance + contact resistance). As can be seen from Eqs.
J. Appt. Phys., Vol. 78, No. 2, 15 July 1995
Note that 7 increases with L in contrast to the transit time
limited bandwidth. At the optimum bandwidth of the larger
device shown in Fig. 16(b) (L=O.3 ,um), vsO.26. The increasing dependence of the sensitivity on the depletion width
results in an optimum bandwidth efficiency product shifted
towards larger thicknesses as shown in Fig. 17. The maximum value of the bandwidth-efficiency
product for a
10X 10 pm2 GaAs detector with R,=50 n is 16.2 GHz
and occurs at L=O.7 pm. The maximum bandwidthefficiency product of the smaller device is only slightly
higher than the larger device despite the significant difference in their maximum bandwidths.
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depletion width (pm)
FIG. 17. Bandwidth-efficiency product versus the depletion (absorption)
region width for conventional detectors as depicted in Fig. 16(a). The transit
time and capacitance limitations are shown in dashed and dotted lines, respectively (after Ref. 44).
The inverse relation of transit time limited bandwidth
with L (33) suggests that infinitely large bandwidths can be
achieved by decreasing the depletion width so long as the
capacitance limitation is simultaneously relaxed by shrinking
the device size or reducing the total resistance. However, if
we consider the bandwidth. efficiency product for a thin detector with no capacitance limitation, we obtain from (33)
and (35):
SW- v=ftr-
77=0.45auJl(l -R.l
of RCE photodetectors
The p-i-n RCE PD structure can incorporate a smaller
bandgap absorption region of thickness d placed in a depletion region of width L [Fig. 18(a)]. In this case, the carriers
do not have to traverse the entire depletion region since they
are generated only at the active layer. The transit times, re
and rh, for electrons and holes, respectively, are given by
the distances they must travel:
3-<=and rh=*
[email protected]
The transit time is optimized when r,= rh :
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
depletion width (pm)
FIG. 18. (a) Schematic respresentation of a RCE p-i-n photodetector illustrating the distances that the photogenerated carriers carriers have to transit.
(b) The bandwidth-depletion width dependence (solid lines) are shown for
10X 10 p m ’ and SXS ,um2 area RCE p-i-n detectors. The transit time
limited bandwidth (dashed line) is common for different size devices and
capacitance limited bandwidth (dotted lines for a total output resistance of
50 Cl) varies with the device area (after Ref. 44).
which is independent of the design parameters and can be
perceived as the y-intercept of the dashed line in Fig. 17. For
AR coated (R=O)
GaAs detectors (~~-10~ cm-‘,
uh=6 X lo6 cm/s) this product is 27 GHz.
The 77of thin p-i-n PDs has been improved by collecting
the tight through the mesa edge of the device, perpendicular
to the electrical current.‘6 This configuration provides a long
(albeit narrow) absorption region and a short current path
simultaneously, enabling, the independent optimization of
speed and 17. In this waveguide configuration however, the
insertion loss of the incoming light limits the overall 7,
while the waveguide design creates charge trapping at the
6. High speed performance
Therefore, the optimized transit time limited bandwidth for
the RCE detector is
for L>d.
This represents a drastic improvement over conventional detectors given by (33), since in most compound semiconductors u,>uh (e.g., ucg I X lo7 cm/s in GaAs). The capacitance limited bandwidth (34) remains unchanged in the RCE
design. Figure 18(b) shows the dependence of bandwidth on
the depletion width L for IOX 10 pm” and 5X5 ,um’ area
GaAs RCE-detectors with a 0.1 pm thick absorption region.
When compared with the bandwidth of the conventional detector (Fig. 16), we see that the optimum bandwidth moves
toward thicker depletion regions and a maximum of 64 GHz
is reached at L= 0.5 ,um for the larger device, and maximum
bandwidth approaches 120 GHz (at L= 0.25 pm) for the
smaller device.
In the RCE scheme, large 77can be maintained independently of L. Applying the peak v relation (9), we obtain
?7=0.9 for a 0.1 ,um thick absorber (ad=O.l)
R2=0.99 and RI =0.7. 17 remains unchanged as long as
L>d = 0.1 F. When L<d, 17must be evaluated by replacing d with L in (9). The bandwidth efficiency product evaluated under these constraints for the RCE p-i-n is shown in
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depletion width (pm)
FIG. 19. Bandwidth-efficiency
product versus the depletion (absorption)
region width for RCE detectors as depicted in Fig. 18(a). The transit time
and capacitance liiitations are shown in dashed and dotted lines, respectively (after Ref. 44).
Fig. 19. The maximum bandwidth-efficiency
product of 58
GHz for the 10X 10 pm2 RCE p-i-n represents a more than
3.5-fold improvement over the conventional detector (16.2
GHz) For the 5X5 pm’ device this predicted improvement
is more drastic, over a 5-fold increase from 19 GHz to 106
In the above discussion, we presented the bandwidth
considerations for 10X 10 pm” and 5X5 pm” device sizes.
As the deirice size is reduced, the capacitance limitation shift
towards thinner depletion regions; At this extreme, the influence of the RCE-scheme is even more prominent since the
intrinsic limitation on the bandwidth-efficiency
product is
relaxed by removing the direct dependence of the quantum
efficiency on the depletion layer thickness.
To treat fully the high speed response of the RCE PDs,
we must also consider the photon lifetime in the optical cavity as one of the limiting factors. The photon lifetime rp can
be viewed as the time required to build or decay the optical
fields inside the cavity and is given by15:
p Loss 3
where TRT is the time required for photons to make one
round trip in the optical cavity, and Loss is the total decay
they encounter during this round trip. For a 1 pm long GaAs
cavity, ?-RT=23 fs. For a Fabry-Perot cavity of mirror reflectivities RI and R, and absorption region ad, the loss term
can be expressed as
0 W(v,+ v<)
FIG. 20. The theoretical current response of a photodetector with a depletion region width of L when excited by an optical impulse under constant
drift velocity and no diffusion assumption. The vertical scales are arbitrary,
and the horizontal scales are intended for a qualitative comparison of different cases. (a) The optical impulse ariving at t=O. (b) The cmrent for a
single electron-hole pair generated in the middle of the depletion region.
The response continues until the hole reaches the contact. (c) The current for
a conventional structure with uniform generation across the depletion region. The response lasts until the holes generated at the opposite end
traverse the entire depletion region. (d) The current for an optimized RCE
detector. The electrons and holes arrive at the contacts at the same time
(after Ref. 44).
C. Impulse
of RCE and conventional
In the previous two sections, we presented a simplistic
bandwidth picture for conventional and RCE p-i-n PDs
which neglects diffusion, field dependence of the mobilities,
carrier recombination, and the influence of heterojunctions.
Under these assumptions, we can further predict the impulse
response of each PD design.
The current i(t) due to a carrier of charge Q moving
with a velocity of u(t) in a semiconductor material of thickness L is given byI
- $ u(t).
Figure 20 shows a comparison of the theoretical current responses for conventional and RCE p-i-n PDs. For a single
electron-hole pair generated in the middle of the depletion
region, the detector current is as shown in Fig. 20(b). For a
conventional detector, under uniform illumination, the transit
time spread is more severe since the full width is determined
by the slower holes traversing the farthest distance [Fig.
20(c)]. By equalizing the electron and hole transit times in
the RCE device, a uniform response with a small time spread
tian be realized [Fig. 20(d)].
(R, = 0.7, Rz = 0.99, ad = 0.1) ,
~~-50 fs, which is much smaller than the carrier drift time.
Even for RCE PDs with thinner active regions (d-0.05 prnj
and higher reflectivity mirrors (R,>0.9), ~~ remains on the
order of 100 fs. Therefore, the photon lifetime will be an
important factor only in RCE detectors designed for THz
D. Simulated high speed response
of RCE
When photodetectors with picosecond response times
are considered, a more accurate representation of the transient response is desirable. Efforts have been underway towards obtaining a better understanding of the transient behavior of p-i-n PDs.“~-~“,~’
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
Appl. Phys. Rev.: M. Selim finlti and S. Striie
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l2ce #2
FIG. 21. The schematic flat band diagram (applied and built-in fields are not
shown) of the modeled p-i-n photodiodes and the qualitative representation
of the photogeneration terms. (a) Device #l: the conventional AlGaAslGaAs
p-i-n heterojunction photodiode. The photogeneration occurs in the depleted
GaAs region on an exponentially decaying profile. (b) Device #2: the GaAs
RCE photodiode with an InGaAs absorbing region. The generation is localized into the InGaAs region (after Ref. 44).
&~lii et aZ.51,52
have deveIoped a one dimensional simulation method for the transient analysis of PDs and used it to
compare the high speed response of RCE and conventional
heterojunction p-i-n PDs. The structures, shown in Figs.
21(a) and 21(b), were chosen to follow the design rules for
optimal bandwidth. For the conventional design, a depletion
width L = 0.72 pm was selected, corresponding to the maximum bandwidth-efficiency product for a 10X 10 pm’ detector. A 0.64 pm thick, normally depleted n--GaAs absorbing
region and transparent Ala.s~Gass4As contact layers completed the conventional PD structure. The small Al mole
fraction avoided large band discontinuities. The heterojunctions were graded and positioned inside the depletion region
to prevent charge trapping. W ithin the absorption edges of
the AlGaAs and GaAs, photogeneration occurs only in the
depleted GaAs region. As a result, the device speed is limited
solely by the transit time of photogenerated carriers across
the depletion region. Finally, a nearly ideal AR coating
(Ri = 0.05) was assumed, yielding 77=0.45 from (35), within
the detection range.
The RCE PD structure was assigned the same depletion
width L= 0.72 pm. The absorption region was a graded heterojunction 0.08 pm n--In0.07Gac,9sAslayer optimally positioned in the GaAs depletion region. The remainder of the
detector consisted of n- and p-type GaAs contact regions.
The resonant cavity was defined by an ideal bottom mirror
(R2= 1.0) and a high reflectivity (R, = 0.7) top mirror. 17for
this structure was 0.9 near 900 nm.’
The model allowed for the exact computation of the
transient photocurrent under arbitrary optical illumination.
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
FIG. 22. The transient short circuit photocurrent under a 5 ps E W H M optical pulse for AlGaAs/GaAs conventional p-i-n (Device #l, dashed line), and
GaAs/InGaAs RCE detector (Device #2, solid line) (after Ref. 44).
W ithin the convergence capabilities of the simulator, it was
possible to calculate the output current for an optical pulse of
less than 1 ps FWHM. Such a short pulse is a good approximation of an impulse excitation, and provided the best estimate of the speed of each device. Since the estimated transit
time for conventional and RCE device designs are about 5 to
10 ps, the transient response of the two devices are compared
under a 5 ps optical pulse.
Figure 22 shows the simulated short circuit currents for
the conventional (dashed line) and RCE (solid line) p-i-n
detectors under a 5 ps F W H M optical pulse. At first glance,
we observe a larger (magnitude) and sharper (less time
spread) current for the RCE-detector compared to the conventional case. A close inspection of the individual current
components reveals the expected transient features and superiority of the RCE-detector.
(i) The rise and fall times of the total current are comparable for the RCE design. For the conventional detector,
the fall time is much larger than the rise time as a result of
long transit time for holes.
(ii) The RCE-detector not only has larger current under
identical optical excitation owing to the enhanced quantum
efficiency, but also the response is faster than the conventional counterpart. Therefore, the bandwidth-efficiency
product is doubly improved by the RCE-detection scheme.
(iii) Due to the simultaneous arrival of both kinds of
carriers at the contacts, the time spread of the conduction
current for the RCE design is much smaher than the conventional p-i-n in which the carriers reach the contacts at different times determined by the position of the photogeneration.
For a comparison of the RCE and conventional detectors
in the frequency domain, we used Fast-Fourier-Transform
(FFT). The simulated temporal responses depicted in Fig. 22
were converted into frequency domain by FFT and shown in
Fig. 23(a). Fourier analysis of the optical excitation term
gives a flat spectrum (within 1 dB) up to approximately 100
GHz verifying the accuracy of the frequency domain repreAppl. Phys. Rev.: M. Selim ijnlti and S. Strite
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,..... .d...~~~_.L~..~.;.~
; : ::
: .:::: . .
.. . . . . . . . . . . ;......: i...:..A.:..:.:.:
I: . . .. . . ..: y; ._:. *..;.i ..: . .
: .:::::
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.. . . .. . . . .. . ..._.......... ;...:...i..;..;.:.; .....Y.....::. . .. . . .: :...:.I.:..>.:.:.
: Ii::‘Z”
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: ::..:.
\ .;<,I ::::.; :::: ::,
5 ,A . . . .....................
. . . ..I...... L .:...i...Liii
/. ...... ...... . ........ ;..:..l.;
.... :...; ..; .., * ., ............
I’ ....
.. ........
a.’ .““”
\ ... ... 1
. ....... :‘..I. :“.:“.).‘:“!.<‘:
..!. .. . ... :“7.:.<.:
. ...........
. .... ..~.....,......~.~..j.~.~............~
... ..:....;...!...r..i..~.~.~
............ . ....................................
.... .{......\’1 ......
.......... .:. ..... . ..... ;...;...;..;..;.z.;. ........... j.. ...........i.. ..:...;...:..i.i
: :::::
.................... L....;...;...j.;..;.:
..... .:. ........... i.. ..... . ........ :...:..:..:.^.:. ... .\I.. .....
:“‘z:::c ::::::
..m..i..I..i.i.l....... 1
. . . . . . . . . . . .. . . . . ..~....~.......~..~..~........................~.....
: ::::::
: : :;:::
. ....
. ....
. ..,
. ......
:_ .....
FIG. 23. (a) Frequency response of conventional (dashed) and RCE (solid
line) p-i-n photodetectors obtained by Fourier transform of pulse responses
shown in Fig. 22. The optical excitation term was deconvolved to extract the
impulse response. (b) Detector response (peak value of the normalized detector current) versus the inverse excitation pulse width for conventional
(dashed) RCE (solid line) p-i-n detectors (after Ref. 44).
sentation of the current pulses. Furthermore, to eliminate the
influence of the finite width of the optical excitation pulse, it
was deconvolved from the simulated current responses.
From Fig. 23(a), the transit time limited 3-dB bandwidth of
the conventional p-i-n detector can be calculated as 52 GHz
in comparison with 70 GHz for the RCE p-i-n, corresponding to a 35% improvement.
For a direct comparison of the bandwidths for RCE and
conventional p-i-n structures, we plot the peak value of the
normalized short circuit current (or detector response) as a
function of the inverse pulse FWHM [Fig. 23(b)]. When the
pulse duration is large, the output current reaches its maximum value determined by the internal quantum efficiency,
i.e., r)= 0.45 for conventional and o= 0.9 for RCE detectors.
As the optical pulse width becomes smaller, the electrical
current is unable to reach the steady state value. Although the
ratio of electron hole pairs to the number of incident photons
J. Appt. Phys., Vol. 78, No. 2, 15 July 1995
FIG. 24. The transient short circuit photocurrent under a long optical pulse
with 0.01 ps fall time for conventional (dashed) and RCE (solid) p-i-n detectors. The excitations is long enough to allow both devices to reach their
steady state current levels (after Ref. 44).
. .......
i/(2 pulse FWHM)
! ...
. i.<.;
remains the same, the peak current decreases since the current is spread over time. The peak short circuit current drops
to half of its maximum at a pulse FWHM of 4.5 ps and 3.5
ps for conventional and RCE p-i-n detectors, respectively.
Therefore, the bandwidth of the RCE p-i-n is approximately
30% larger than that of the conventional diode conlirming
our findings through Fourier analysis. Note that, both of
these comparisons are made for a single optical pulse of
varying duration. If we consider repetitive pulses, or sinusoidal excitation, the bandwidth difference will be even
larger due to the large fall time of the conventional p-i-n.
To this end, we also compare the fall times for RCE and
conventional p-i-n diodes. We calculated the response of
these devices (Devices #l and #2) under a long optical pulse
such that both devices can reach the steady state current levels. The optical pulse is terminated abruptly (fall time=O.Ol
ps) and we observe the decay of the short circuit current for
both cases as shown in Fig. 24. Despite the higher steady
state level,‘the current from the RCE detector decays faster
and drops to lower values than the current for the conventional p-i-n. We have computed the fall time rf as the time it
takes the current output to drop from 90% to 10% of the
steady state value. The fall time Q-~=7.1 ps for the RCE p-i-n
is 60% of that for the conventional structure ( rf= 11.7 ps)
corresponding to 65% faster response.
The simulation results demonstrated a 35% bandwidth
improvement along with a two-fold enhancement in quantum
efficiency over conventional p-i-n photodiodes optimized for
a 10X 10 ,um2 device area. For smaller area devices, even
more drastic improvements exceeding a three fold increase in
the bandwidth-efficiency product can be achieved. The simulation results suggest that high quantum efficiency RCEphotodetectors can be realized with bandwidth-efficiency
products approaching 100 GHz.
Appl. Phys. Rev.: M. Selim fin1C.iand S. Strite
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r? - GaAs substrate
FIG. 26. Schematic cross section of the RCE APD (after Ref. 57).
Figure 25 compares the photoresponse of the RCE device
with that of a similar diode having no bottom mirror. At the
resonance wavelength, a 50% enhancement in detected photocurrent was observed. The moderately thick absorption region in these devices prohibited the full exploitation of the
RCE concept. The design for bottom illumination causes
losses due to reflection at the substrate surface and limits the
highest attainable quantum efficiency. This problem can be
circumvented by a AR coating on the substrate surface.
B. Avalanche
FIG. 25. Photoresponse of (a) RCE Schottky photodiode and (b) a similar
detector wihout resonant cavity (after Ref. 53).
Avalanche PDs constitute another device in which the
v. bandwidth product can be optimized by RCE. Conventional APDs in the low gain regime are bandwidth limited by
the transit time of secondary electrons through the depletion
region.54-56 The ability to obtain high 17detection with a thin
absorption region allows the transit time limitation to be
greatly relaxed. Smaller depletion regions also translate into
higher electric fields in the gain region at a given external
bias permitting reduced power operation.
Kuchibhotla et aLS7 demonstrated the first RCE APD
(Fig. 26) which consisted of a thin (900 A) InaosGa,-,~sAs
absorbing layer (doubling as the multiplication region) situated in an optical cavity. The bottom mirror was a 15 period
Most of the theoretical relations derived in the previous
sections have been experimentally verified by the many
c workers in the field. The RCE scheme has been ,profitably
applied to many different types of photodetectors, modulators and emitters in order to enhance the efficiency, realize
wavelength selectivity, or both.
A. Schottky
An InGaAlAs Schottky PD was among the first reports
of a RCE device.9*53Chin and Chang realized that by placing
a conventional Schottky PD inside of a Fabry-Perot resonant
cavity, a thinner absorption region could be employed, reducing transit time bandwidth limitations while increasing 77 at
the resonant wavelength. Their structure was a relatively
conservative one in which an 8 period InAlAs/InGaAlAs
mirror was grown onto an InP substrate. This was followed
by a 475 nm In,,5sGaa.47As absorbing region capped by a 50
nm A&As Schottky contact layer. The cavity was completed
by the deposition of a high reflectivity Al Schottky contact.
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
FIG. 27. Measured photoresponse of the RCE APD depicted in Fig. 26 for mirror reflectivities (after Ref. 57).
Appl. Phys. Rev.: M. Selim blij
and S. Strite
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Bragg Reflector
MlyGa,, As)
UU,Ga., As)
(Al Ga
FIG. 28. Schematic layer structure of InP/InGaAsP/GaAs RCE photodiode
(after Ref. 62).
quarter wave AlAs/GaAs stack having an estimated reflectivity of &90% in the wavelength range of interest. In order to
investigate the RCE effect, RI was varied by depositing a
CaFs cap layer on some samples. Figure 27 shows the photoresponse of three RCE APD devices having RI = 15%,
30%, and 73%. As the Q of the cavity is increased, the response sharpens around the 840 nm resonance wavelength of
the optical cavity and the corresponding 77at that wavelength
increases. The maximum 17 of 49% achieved for RI ==73%
was limited by the low reflectivity of the bottom mirror. A
further limitation was the design of the cavity resonance at
840 nm were the AlAs/GaAs mirror is lossy. Because of the
rapid increase of the electric field in the active layer with
external bias, gain in the RCE APD is achievable at relatively low applied voltage. Near breakdown at 9 V, an internal gain of 200 was measured. This operating bias is the
lowest reported to date for APDs.
C. P-I-N photodiodes
P-i-n PDs are leading candidates for high speed
P-i-n PDs should be capable of speeds
exceeding 100 GHz.““~~ For applications requiring faster response, photoconductive detector8’ are currently favored,
but suffer from poor sensitivity. RCE p-i-n PDs are ideal for
overcoming the bandwidth/sensitivity tradeoff and have received the majority of the attention directed towards the high
speed RCE photodetectors.
The first RCE p-i-n PD was reported by Dentai et ~2.~~
who studied an InGaAsZInGaAsPZInP structure (Fig. 28) designed to operate near 1.55 pm. An 77of 82% was achieved
using a 2000 A InGaAs absorbing layer with Rt=0.7 and
Rz=O.95. Kuchibhotla et aL4’ have demonstrated an RCE
p-i-n PD in the SiGe system. Their 77was severely limited by
the reflectivity obtainable with Si/Sia,sGec,, mirror stacks
which suffer from a relatively small refractive index contrast.
Several refinements and variations of the basic structure have
since been reported. RCE p-i-n photodetectors have been integrated with HBT drivers.63 Huang et ~1.~ reported a 30%
improvement in their p-i-n PDs by optimally positioning the
absorption regions at cavity antinodes. Lai and Campbell65
FIG. 29. Conceptual diagram of a RCE MSM photodetector with a buried
DBR mirror (after Ref. 70).
predicted that an RCE p-i-n PD with a MQW absorption
region could be actively tuned over a 20 nm range due to the
quantum confined Stark effect but their approach is limited
by large crosstalk. Hunt et aZ.66introduced wavelength selectivity to a back illuminated InGaAs p-i-n PD by growing a
Si/SiOz resonant cavity onto the backside of the InP substrate while Corbett et LzZ.~~reproduced the RCE effect by
using epitaxial lift-off to physically place a conventional
p-i-n PD inside a cavity consisting of two Ag mirrors. Finally, Sverdlov et aL6* analyzed InGaAsZInAlAs p-i-n PDs
and observed that an added advantage of the RCE scheme is
reduced dark current resulting from the thinness of the active
D. Metal-semiconductor-metal
The MSM photodetector design is conceptually similar
to photodiodes in so far as incoming photons create an
electron-hole pair that is separated by applied electrical field
and collected at contacts causing an electrical current. However, the implementation of the MSM approach differs from
the others in being a planar rather than vertical device
structure.69 This is an advantage in applications where the
photodetector must be integrated with amplifiers43 or an
emitter. Nevertheless, the 7. bandwidth product remains
governed by similar fundamental tradeoffs as p-i-n PDs and
APDs, and is therefore amenable to RCE. The response
speed of MSM detectors can be increased by reducing the
finger spacing, and thus the carrier transit times. However,
for very small finger spacing, the thickness of the absorption
layer becomes comparable to the finger spacing and limits
the high speed performance. The RCE design allows for the
use of very thin absorption layers while maintaining large
quantum efficiency, thus eliminates the fundamental speed
limitation for MSM photodetectors.
Li et aL7’ theoretically investigated the properties of an
AlGaAs/GaAs MSM photodetector situated on top of a buried quarter wave mirror (Fig. 29) based on a comparison
with experimental results on conventional devices with simi-
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
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Incident Light
400 ps 500
FIG. 30. Measured temporal response of AlGaAs/GaAs RCE M S M photodetectors showing a E W H M of 19 ps (after Ref. 42).
FIG. 31. Structure of the AlGaAs/GaAs/InGaAs RCE HPT (after Ref. 17).
lar layer structures. As a result of the reduced active layer
thickness, a 1.5 pm channel RCE MSM photodetector was
capable of operation at twice the speed of a similar conventional MSM structure.
Prank et .1.a2171have reported substantially improved
AlGaAs/GaAs RCE MSM photodetectors by increasing the
cavity finesse and fabrication technique. A device incorporating 20 and 7 period A~A~IA&,&zz~~As lower and upper
mirror stacks exhibited a photoresponsivity peak at 846 nm
having a F W l!&I of only 1 nm. High frequency measurements indicated that these devices were capable of operation
up to at least 35 GHz as a result of the photoresponse rise
and fall times on the order of 14 ps (Fig. 30).
Litvin et aL43 have also reported high frequency measurements of an RCE MSM photodetector. Their device consisted of a GaAs absorption layer sandwiched between a high
reflectivity distributed Bragg reflector (DBR) and a transparent AlGaAs cap layer. After the growth of the detector, a high
electron mobility transistor structure was deposited. This allowed the MSM photodetectors to be integrated with a low
noise amplifier. A time Iimited bandwidth in excess of 40
GHz was measured for devices having a 0.5 pm finger spacing.
E. Heterojunction
Heterojunction phototransistors (HPT) are often employed to combine photodetection and amplification with
less noise than APDs.‘~-‘~ Their combination of excellent
sensitivity, large gain, low noise and high bandwidth places
HPTs among the most promising RCE devices.
AlGaAs/GaAs HPTs having InGaAs active layers (Fig.
31) are among the first RCE devices demonstrated.” The
resonant cavity was defined by a 10 pair AlAslGaAs bottom
mirror (R,=0.9) and the GaAs top surface (R1 0.3). The 0.1
pm InGaAs active layer extended the photosensitivity spectrum beyond the GaAs absorption edge (>900 nm) where
absorption losses in the heavily doped GaAs base and collector were negligible. The RCE effect was evaluated by
comparison to a conventional HPT having an identical structure except for the bottom mirror. Figure 32 compares the
photosensitivity of the conventional and RCE HPTs. The
spectral responses were normalized at 750 nm where the cavity effect was negligible. The conventional HPT response
decreased sharply beyond the GaAs band edge near 870 nm.
The small increase near 900 nm reflected absorption by the
InGaAs layer, which was too thin to have a large effect in a
conventional structure. Under pulsed laser illumination, the
optical gain of the HPT was measured to be above 500 at 850
nm (~=25%) and 150 at 900 nm (~=6.7%).
As expected, the photosensitivity of the RCE HPT was
greatly enhanced at resonance. At 900 nm, where the cavity
was nearly lossless, the measured photocurrent was 7 times
that of the conventional HPT. The enhancement at 850 nm
was only a factor of 1.7, reflecting absorption losses. Both
values agreed well with theory.
Higher F cavities are desirable for improving 77 and
wavelength selectivity. Bryan et a1.77have described an RCE
HPT capable of operation at 930 nm in which InGaAs QWs
placed optimally within a high F cavity benefit from the
SWE. As a result of the high Q cavity design (R2=99%,
R 1=70%), a very sharp photoresponse of 3 nm l?#HM with
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
Wavelength (urn)
FIG. 32. Spectral response of conventional (solid line) and RCE (dashed
line) floating base HPTS illustrating the enhanced v resulting from the resonant cavity (after Ref. 17).
Appl. Phys. Rev.: M. Selim &dii and S. Strite
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FIG. 33. Device cross section of the fabricated BICFJ5T/HFET detector
(after Ref. 87).
an optical gain of 600 was achieved. In this design, the resonant wavelength is set during the layer growth requiring precise control of growth rates.
High finesse RCE-HPTs with a thin 200 A semitransparent Au top mirror and a conventional AlAslGaAs
bottom mirror (R,=95%) have also been demonstrated.78An
advantage of a metal top mirror is that the cavity length can
be pre-tuned by surface recessing before the top mirror is
evaporatedT9 The FSR and F W H M of the devices were 78
mn and <6 nm respectively, resulting in F> 13, a 3.5 fold
improvement over the same devices without the Au film.
Dodabalapur and Chang” reported an InGaAs/InAlAs/
InP HPT capable of operation at the technologically important 1.3-1.6 p m wavelength range. Because of the relatively
poor contrast attainable with InAlAs/InGaAs mirrors, a high
reflectivity Au top mirror was used and the device was back
illuminated. Two different designs optimized for 1.3 and 1.55
pm were demonstrated having optical gains as large as 2500.
F. Bipolar inversion
field effect
Bipolar inversion channel field effect transistors
(BICFET)81 have been promoted for their suitability for integration with laser structures.82-84 Like HBTs, BICFETs
function as HPTs if a narrow bandgap absorbing region is
inserted into the structure, and they benefit in a similar manner from RCE. BICFET HPTs are of interest for their ability
to be integrated along with an edge emitting or vertical cavity (VC) lasers to form a completely optical switch.85’86
Daryanani et a1.87-gohave studied RCE BICFET HPTs
designed to operate at 940 run. In their structure (Fig. 33),
three InGaAs QWs doubled as both the inversion channel
and the absorbing region. The devices exhibited a sharp photoresponse with lYWHM=2 nm and an optical gain of 25.
The measured ~=80% represented a factor of 26 improvement over a single pass BICFET phototransistor structure.
G. Wavelength demultiplexing
and phototransistors
with RCE photodiodes
Wavelength Division Multiplexing (WDM) is a key
technology for increasing the transmission capacity of optical fiber communication systems.24 High-speed, high gain
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
FIG. 34. Conceptual diagram of a W D M application for the detector composed of four RCE HPTs having shifted resonance wavelengths (after Ref.
demultiplexing receivers are a crucial component of these
systems.‘l-” RCE detectors are ideal for this application be
cause they combine very high 77with low crosstalk and high
speed. Each RCE detector in an F&ay can function as a channel discriminator, detecting only a specific band from the
common incoming light. Sharing of the optical input signal
can be compensated by the enhanced 17 and gain of each
RCE W D M was first demonstrated in a simple array of 4
RCE HPTs having complementary spectral responses (Fig.
34).26 Recessing the epitaxial layer surface with standard wet
etching tuned the response wavelengths of each device by
changing its overall cavity length. Three RCE HPTs were
recessed to produce three equally spaced modes such that the
resonance of the most deeply etched device coincided with
that of the unetched device.
Figure 35(a) shows the spectral responses of three de
vices under equal illumination at a collector emitter bias of 3
V. The FSR and F W H M were 55 mn (at 900 mn center
wavelength) and 15 nm, yielding a finesse F=3.6. Maximum
crosstalk attenuation of 15 dB and - 12 dB were realized for
2 and 3 channel demultiplexing, respectively. The F was
greatly improved by the addition of semi-transparent Au top
mirrors78 allowing an experimental crosstalk attenuation of
16 dB for four channel demultiplexing to be realized. Spectral response of this four channel W D M detector is shown in
Figure 35(b). The measured finesse of more than 10 suggests
the crosstalk attenuations better than 28 dB and 14 dB for 4and lo-channel demultiplexing, respectively. The discrepancy between the experimental and theoretical results was
attributed to fabrication imperfections.
An elegant RCE based W D M scheme has been demonstrated by Pezeshki et a1.96*97(Fig. 36) in which a waveguide
is coupled into a Fabry-Perot cavity with a spatially varying
cavity length. The light propagates along the waveguide, until it reaches the region with which it is resonant where it is
transmitted to a dedicated photodetector. The tapered cavity
spatially demultiplexes the incoming light by selecting only
the band which is in resonance with the cavity at each position. Unlike the approach of Unlii et al., the incoming. light
does not have to be divided between detectors.
Appl. Phys. Rev.: M. Selim cnlij and S. Strite
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FIG. 36. Schematic of WDM device application for a tapered RCE structure
(after Ref. 96).
0.0 I.
4, ,,
use of fewer QWs thus lowering the operating voltage. In
addition, at the cavity resonance, nearly all of the incident
light is transmitted into the device which allows very low
insertion losses to be realized.
1988 saw the first reports”9*‘oo of RCE MQW reflection
modulators. The prototype GaAs/AlGaAs device of Simes
et ~1.~~exhibited an ON/OFF contrast ratio of 8:l at 873 nm.
Whitehead and Perry82*o* realized the potential for reduced
voltage operation. The reflectivity of their device dropped
from 43% to nearly zero when the applied bias was varied
from 0 to 9 V corresponding to a contrast ratio of 100: 1 (Fig.
37). A modulation in excess of 10 dB was maintained over a
bandwidth of 4 nm. Fritz et al.“* extended the bandwidth to
20 nm by using two cavities in the same device structure.
Yan et uZ.‘~~~‘~~reported a higher F reflection modulator incorporating both top and bottom mirrors. The improved F
allowed the number of QWs to be reduced to 24. As a result,
the device exhibited a 47% change in reflectivity for an operating voltage swing of only 2 V, Several other
groups’05-108 have reported RCE modulator structures along
the same lines.
In an effort to increase epitaxial yield, Karim et aL7’
combined surface recessing with an externally deposited top
mirror so that the cavity resonance could always be tuned
exactly to the exciton line. This approach is applicable over
350h etched
FIG. 35. (a) Experimentally observed photoresponses of three devices
whose resonances have been shifted by surface recessing plotted over one
FSR (after Ref. 26) (b) Spectral response of similar photodetectors with
semi-transparent Au top mirrors (after Ref. 78).
H. Optical modulators
As fiber communication bandwidth increases, the need
for devices capable of very high speed optical modulation
has arisen. Typical devices, such as the self electro-optic effect device (SEED)Y8 are based on the quantum confined
Stark effect in which an applied electric field red shifts the
exciton absorption band. Because the extent of optical modulation provided by SEED devices is limited by the amount of
absorption, their contrast ratio can be improved by RCE.
Increased absorption resulting from RCE also permits the
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
FIG. 37. Reflectivity spectra for 0 and 9 V applied bias showing high ON/
OFF ratio (after Ref. 101).
Appl. Phys. Rev.: M. Selim &dii and S. Strite
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0 5
FIG. 38. Schematic layer structure of AlGaAs/GaAs resonant cavity LED
(after Ref. 114).
the entire family of RCE devices and offers the advantage of
post-growth cavity tuning.
Whitehead et a1.to9 pointed out that a high F cavity has
undesirable elements including reduced optical bandwidth,
higher insertion losses, and greater dependence on layer
thickness and temperature fluctuations. They achieved the
desired low voltage modulation by increasing the thicknesses
of the 15 GaAs QWs to 150 A. At this width, the dominant
electro-optic effect is ionization of the lirst exciton level
which is more strongly dependent on the applied electric
field. A 6.7 dB reflection contrast was obtained for a voltage
swing of 3.5 V. Grindle et al.“’ and Choi et al.“’ have since
reported devices of similar design and performance. Modulators have also been integrated with HPTs to perform more
complicated functions.tt2
I. Light emitting
Light emitting diodes (LED) are used widely in short
and medium distance optical fiber communication because of
their low cost, excellent reliability
and temperature
insensitivity.“3 Since they produce spontaneous emission,
LEDs have a zero threshold current and therefore can be
more efficient than lasers in low power applications.
Through the use of an optical cavity, more of the spontaneous radiation can be coupled into a narrow wavelength band
whose width is solely a function of the cavity finesse. Also,
the cavity increases the directivity, allowing more efficient
coupling of the electroluminescence (EL) into an optical fiber.
The first RCE LED (Fig. 38) was reported by Schubert
et aL114 who designed a conventional LED placed in a high
Q (Rt=90%, R,=99%) optical cavity. The optical cavity
enhanced the spontaneous emission at its resonant wavelength by as much as a factor of 10, channeling more EL into
a narrow spectral band. In addition, instead of the usual isotropic emission, RCE LEDs emit their light largely through
the lower reflectivity mirror allowing the useful EL to be
doubled. Care had to be taken to place the active region at a
cavity antinode to benefit from the SWE,“’ and also to tune
the cavity resonance to the active region exciton wavelength
for optimal efficiency.
2 0
l?lOO 1150
2 5
FIG. 39. Emission spectra of LEDs having similar designs with the exception of RCE (Device A). The sharper spectra of the RCE LED leads to a
factor of 12 enhancement of the power density at resonance and a linewidth
of 3 nm (0.11 kT) (after Ref. 116).
After the initial demonstration, there have been several
reports of RCE LEDs having improved properties.*t6-‘I9
Blondelle et al. ‘I9 noted a peak external quantum efficiency
in excess of 6% in InGaAs/AlGaAs RCE LEDs with optimized Fabry-Perot cavity parameters. Hunt et al. ‘I6 reported
the narrowest EL linewidth 2.8 meV (3 nmj at 1.3 ,um which
was achieved by integrating InGaAsP LEDs into a cavity
having R1=92% and R,=95%. Figure 39 compares the EL
spectra of an RCE LED and a conventional LED with the
same structure. While the total emitted power of the devices
was similar, the spectral purity of the RCE device yielded a
factor of 12 enhancement of the power density at resonance.
The RCE linewidth of 0.11 kT, was much smaller than the
3.3 kT linewidth of the conventional LED. Because the emission mechanism of the RCE LED is spontaneous and determined only by the external cavity, the EL FWHM and wavelength were independent of the forward bias over a broad
range. This is an advantage over devices relying on optica
gain which have variable spectral purity and mode shifting,
_... .-
FIG. 40. Normalized LED output spectra highlighting the extremely narrow
spontaneous emission linewidths obtainable in RCE LEDs situated in high
finesse cavities (after Ref. 120).
Appl. Phys. Rev.: M. Selim cnlti and S. Strite
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0, lo
w& 8
s 6
8 4
FIG. 41. EL spectrum of an RCE LED (Solid line) having an organic dye
active region versus a similar LED without the cavity (dotted line, scaled up
by a factor of 3.5). The multi-mode resonant cavity dictates the three EL
wavelengths permitted by ISE (after Ref. 122).
Lott et al. 120~‘2*demonstrated red RCE LEDs fabricated
from AlInGaP heterostructures grown on GaAs substrates.
Devices incorporating AlAs/AlGaAs quarter wave mirrors
and an Ino,s~Gaa~P QW surrounded by InGaAlP barriers
emitted at 665 nm with linewidths as sharp as 0.9 nm,r2o a
factor of 19 better than a comparable conventional LED (Fig.
40). Comparable performance was obtained in a structure
incorporating AlInP/AlInGaP mirrors, although significantly
more mirror periods were required to realize a high Q
An organic semiconductor RCE LED has been reported
to emit simultaneously at three wavelengths through the use
of a multi-mode Fabry-Perot cavity.“’ The organic dye is a
suitable choice for the active region since it provides gain
across a wide wavelength range. The cavity enhanced the
spontaneous emission at its three resonant wavelengths overlapping the gain spectrum of the active layer, producing the
three primary colors together (Fig. 41). It is eventually desirable that some independent control mechanism over the
relative intensities of the colors be developed. Nevertheless,
this is an exciting result for color dispIay applications.
Hunt et al. 123investigated the performance of InAlGaAs
RCE LEDs for optical communications. An RCE LED emitting at 940 nm was compared to a conventional GaAs LED
(presently used in 50 Mbit/s systems) by coupling both into a
multimode, graded-index optical fiber. As a result of the narrow EL spectrum, chromatic dispersion of the RCE device
signal was reduced yielding sharper rise and fall times in the
output signal. After transmission through 3.37 km of fiber,
the RCE LED signal had a rise/fall time of -5 ns compared
to a value of = 16 ns for the conventional GaAs LED (Fig.
42). The enhanced directivity in the RCE LED resulted in an
intensity which was far greater than the conventional devices, and even exceeded the theoretical maximum of an
ideal isotropic LED by as much as a factor of two.‘a4 Since
the EL is collapsed both into a narrow wavelength band and
a narrow light cone, RCE LEDs are ideal for coupling into
and transmission through optical fibers. The authors pre630
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
Time (ns)
FIG. 42. Comparison of rising and falling edges of a standard ODL.50 LED
and an RCE LED when driven by a square wave. Dashed-dotted and solid
lines are the transition edges after transmission through 5 m and 3.37 km of
62.5 um core graded-index fiber, respectively. The sharper rise and fall times
of the RCE LED refiect decreased chromatic dispersion as a result of its
sharper spectrum (aktel Ref. 123).
dieted that the RCE LED communication system could function at a maximum bandwidth of 200 Mbit/s compared to a
limit of 66 Mbit/s for the standard device. Further bandwidth
gains are possible if multiple discrete wavelength channels
are transmitted and demultiplexed through a single fiber link.
J. Optical amplifiers
Er-doped optical amplifiers are important elements for
long-haul soliton fiber optic communication systems.‘25 Generally, several meters of E-doped fiber is spliced into the
transmission line. One or more semiconductor diode lasers
pump the Er3+ ions at their resonant absorption wavelengths
of either 0.98 or 1.48 pm. The pumping allows the signal
propagating at the dispersion free 1.55 pm wavelength to be
regenerated by inducing stimulated emission of the excited
4f Ers + electrons. 1267127
These pump sources are quite expensive and are only used in long haul systems where their expense comprises a small fraction of the whole. For short haul
LED systems, devices based on the spontaneous emission of
Er are attractive alternatives.
Schubert et al.‘28-‘30 have studied both theoretically and
experimentally the properties of Er-doped SiOZ situated in a
resonant cavity formed by Si/Si02 mirrors. The essential device structure, grown by rf-magnetron sputtering is shown in
Fig. 43 along with the distribution of the ion implanted Er.
When the resonant cavity wavelength was tuned to the 1.55
pm Er3+ emission,128 a factor of 50 enhancement of the
photoluminescence (PLj at that wavelength was observed
compared to a sample in.which the top mirror was removed
(Figure 44). Similarly, when the resonant wavelength of the
cavity matched that of the 980 nm excitation source,tz9 the
overall E?+ PL intensity increased by a factor of 28, although it was no longer confined within so narrow a spectral
range. The enhancement of the PL in both cases was in
agreement with theoretical calculations based on the cavity
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FIG. 43. (a) Schematic of the Si/SiO~ VC structure and (b) the corresponding &-implantation profile (after Ref. 128).
While the performance of discrete RCE devices is impressive compared to their conventional counterparts, it is
expected that RCE devices- will be most useful when integrated with other RCE and conventional devices for use in
photo& interconnect and Iogic systems. Practical optical
logic systems wihcapitalize on the inherent advantages of
light: i.e., rapid propagation over typical off-chip distances
(>lO cm) and the ability to couple multiple wavelengths into
a single fiber creating a whole new dimension of parallelism.
The wavelength selectivity of RCE devices make them
highly attractive as low crosstalk optical interconnects. RCE
detectors integrated with LEDs or lasers can function as
wavelength selective optoelectronic switches or logic gates.
The most elaborate schemes proposed to date describe cascadable, optical input/output, optoelectronic logic circuits in
which the RCE components permit the wavelength to function as an additional variable.
A. integration
The advantage of this optical amplifier is that the RCE
effect, by greatly increasing the absorption of the Er3’ ions,
offers the potential of a much more compact and lower cost
design since conventional pumps require several meters of
B-doped fiber. Furthermore, the pump light has to be
coupled into the fiber (typically with a 50% loss) which must
then be spliced into the transmission line. RCE Er-doped
amplifiers allow the pump light to be coupled into the gain
region perpendicularly and with nearly 100% efficiency so
that the amplification occurs over a much smaller volume.
The challenge of implementing this scheme is coupling the
signal into and out of the relatively narrow gain region which
would also involve losses.
In order to have an optical logic element, three functions
must be satisfied, namely detection, amplification and emission. One of the simplest ways to accomplish this is by integrating an HPT and LED in a vertical n-p-n-p
configuration.61*‘31-135 In such a device (Fig. 45) the upper
two heterojunctions form the HPT whose collector current
drives the LED in the bottom heterojunction. The resulting
structure is a photothyristor (or a Shockley diode) which is
also referred to as a light amplifying optical switch
(LAOS).131 The LAOS is designed so that some light from
the LED is re-absorbed by the HPT. When this positive feedback mechanism exceeds a threshold, the LAOS switches to
a low impedance state in which a large optical output is
generated. Depending on the external circuit and adjacent
device interconnections, one or more LAOS devices can
function as bistable optical switches, optical inverters, AND,
NAND, and NOR gates. The LAOS device structure is suitable for RCE,‘36 making these devices ideal for WDM optical interconnects.
-‘- -.-. WITHOUT
with LEDs
of detectors
InP Substrate, Semi-Insulating
,-“C -.“
__ . ..f 2.’
FIG. 44. Room temperature photoluminescence spectra of Er-implanted
Si/SiO, structures with and without resonant cavities (after Ref. 128).
Output Light
FIG. 45. Schematic of the LAOS structure. Input light (top) turns on the
HPT which drives the LED (below) (after Ref. 135).
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
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W gate/emitter
FIG. 46. Schematic of the DOES (after Ref. 141).
A similar bistable optical device named the double heterostructure optical switch (DOES)‘372’38 (Fig. 46) is an extension of the BICFET (Section V F). The key to the device
operation is the band bending introduced by the p-type
charge sheet and the n-type AlGaAsIGaAs heterojunction.
Under increasing optical or electrical forward bias, the
DOES behaves much like the LAOS in that it switches from
a high to a low impedance, tight emitting state.
In general, the broadband output of the LED limits the
usefulness of these devices by limiting the cascadability and
increasing the crosstalk. A monochromatic laser output is
much more desirable as an input signal for a next level of
optical logic. As a result, attention has been redirected towards incarnations of these devices incorporating laser
which can be achieved by placing the
active device structure in either a vertical or horizontal cavity.
FIG. 47. Band diagram of a vertically integrated VCSIZLJHIT. Incoming
light (bottomj is absorbed, turning on the HPT, whose output drives the
VCSEL (above) (after Ref. 146).
with an intermediate selective etch (Fig. 48). The goal was to
integrate a low threshold, high efficiency VCSEL and a high
7 HPT with a broader spectral bandwidth. The etch step
enabled the RCE HPT to be situated in a double VC which
provided a broadened response spectrum, independent from
that of the laser. Figure 49(a) shows the VCSEL output linewidth of 1.5 A with a differential efficiency of 0.25 W/A.
Due to its separate cavity design, the RCE HPT had a 50
A FWHM response [Fig. 49(b)].
Numai et ~2.‘~~ reported a more compact optical logic
element consisting of a thyristor situated in a VC which they
B. Vertical cavity lasers integrated with
named VSTEP (Fig. 50). The VSTEP functions as a wavephotodetectors
length selective photodetector and VCSEL emitter. Optical
or electrical excitation switches the thyristor into its low imVertical cavity surface emitting lasers (VCSEL) offer
pedance state which results in lasing.‘51p’52 Under external
several important advantages over edge emitters which make
them attractive for integration into photonics systems.27*144 electrical bias, the VSTEP is capable of amplification or
switching within a single wavelength band. A VSTEP elecMany of the WDM approaches applied to RCE detectors can
be extended to VCSELs. Also, VCSELs are structurally similar to RCE detectors and modulators and can therefore be
fabricated alongside one another or a single device can perform both functions.145 Such a combination provides both
wavelength selective photodetection and narrow band emission, ideal for multiple wavelength photonics.
The first all optical logic element incorporating a
VCSEL’46 was realized by growing the laser on top of an
HPT engineered to detect wavelengths at which the VCSEL
was transparent (Fig. 47). When illuminated, the HPT collecSI-GaAs Substrate
tor current served to drive the laser. This device is cascadable
and is therefore suitable for optical logic. Two other
groups 147~148
have reported similar structures. However, for
most applications, a planar integration rather than a vertical
one is desirable.
Kosaka et a1.‘49 realized a planar integration of a RCE
FIG. 48. Horizontally integrated H!?T and VCSBL. The HPT is situated in a
HPT and thyristor VCSEL using a two stage growth process
double VC in order to broaden its response spectrum (after Ref. 149).
J. Appl.
Vol. 78, No. 2, 15 July
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and S. Strite
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preach is cascadable, dynamically reconfigurable, and can in
principle perform arbitrarily complex logic operations. It is
also complicated due to the need to fabricate two separate
types of devices in a non-planar geometry. The PTs, being
outside of the VCSEL, are not highly wavelength selective
leading to crosstalk concerns.
C. Wavelength
optical logic
What is required for a practical, low crosstalk W D M
system is a scheme in which both the emitters and detectors
FIG. 49. (a) Emission spectrum of the VCSEL from Fig. 47, and (b) photocurrent response of the HPT together with the calculated absorption spectrum (after Ref. 149).
trical to optical conversion efficiency as large as 1 1.4%“53
was achieved by coating the device sidewalls with a reflective Au layer to reduce spontaneous emission losses.‘54,‘55
A similar device consisting of a photothyristor (PT) and
VCSEL has been reported by Z~OU.‘~~-*‘~ The PT is grown
on top of the VCSEL, not inside the optical cavity. Individual
devices are fabricated in a horizontal array (Fig. 51) and
interconnected to perform various logic operations. This ap-
i -Ak.2sGao.7sAs
i - Ino.zGao.sAs
i -AIor25Gaa75As
L- ~-
As mirror
FIG. 50. Schematic of the VSTEP device structure. The thyristor situated
within the VC functions both as a detector and VCSEL, leading to a compact, highly wavelength selective optical logic element (after Ref. 150).
operate in very narrow wavelength bands which will permit a
large number of channels with low crosstalk. Willner et al. Is9
have proposed such a scheme in the form of a two dimensional optical interconnects composed of arrays of VCSELs
cascaded through wavelength selective detector arrays (Fig.
52). The pixels are defined by a VCSEL array in which one
device emits at each of the different system wavelengths.
Each successive detector array detects one of the VCSEL
wavelengths while remaining transparent to the remaining
wavelengths. This scheme realizes both wavelength selective
emission and detection, but it is complex and bulky requiring
N+ 1 precisely aligned cascaded array planes for N channel
A simpler wavelength selective optical logic (WSOL)r2
can be realized from the basic structure of Zhou et al.‘58 if
the PT is also placed inside of an optical cavity. The major
advantage of this scheme is that the top mirrors are formed
after surface recessing which permits each laser and PT to be
tuned during the fabrication process to an individual narrow
wavelength band. ‘2*27,79,90~‘VCSELs
tuned by surface recessing before top mirror deposition have recently been demonstrated by Wipiejewski et al. 162
The WSOL device is well suited for optical interconnect
applications. The low crosstalk enables a closely packed
multiple wavelength array to be fabricated whose light is
coupled in and out of a single optical fiber or waveguide.
Such a [email protected] greatly reduces the total number of optical fibers in and out of the interconnect chip, thereby reducing the complexity and increasing the reliability of optoelectronic signal conversion. The enhanced 7 and gain of each
device can compensate for the light intensity lost from sharing.
Different horizontal integrations of the WSOL VCSELs
and PTs function as completely optical OR (Fig. 53) or AND
(Fig. 54) gates. An XOR gate can be implemented in a six
mesa structure coupled to a single fiber. The XOR output
wavelength can be chosen as any within the range of the
tunable VCSEL. This structure brings about a considerable
simplification to a previously described two level single
wavelength optical input/output XOR gate requiring nine
mesas at each level.*60 WSOL devices can also be designed
to perform different logic functions for different sets of input
Another important aspect of the WSOL device is its cascadability which enables sequential logic operations by placing wafers in series. In the cascaded configuration, wavelength can be used as an independent variable which greatly
increases the flexibility of WSOL circuits. Figure 55 illustrates how a FULL ADDER can be fabricated from only two
J. Appt. Phys., Vol. 78, No. 2, 15 July 1995
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FIG. 51. Horizontal integration of a PT and VCSEL (after Ref. 156).
layers of WSOL and three input wavelengths. The low
crosstalk resulting from wavelength selectivity relaxes the
requirements for accurate alignment between the wafers
forming the different levels. In wavelength selective logic
each local and global variable can be represented by a different wavelength. in a synchronous system, a global clock
wavelength can be broadcast over the entire system eliminating deleterious clock skew effects.
Instead of forming logic circuits by means of directly
the wavelength
selective optoelectronic
switches, a hybrid approach can also be employed. In this
approach, the optical input variables are coupled into an array of WDM detectors through a single fiber. Once the input
is demultiplexed, arbitrarily complex logic operations of
many variables can be performed by conventional electronic
circuits and the optical output can be provided by a multiple
wavelength VCSEL array as depicted in Fig. 56.163
RCE is a conceptually simple technology which is vastly
preferable in potential price and performance to today’s com-
2-D WDM Optical Interconnects
hDependent Detector Arrays
Plane A
FIG. 52. Optical plane interconnect scheme incorporating a twodimensional pixel array of identical three wavelength VCSEL mini-arrays
and sequential wavelength selective detector arrays (after Ref. 159).
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
mercial WDM systems.‘@ It is relatively easy to understand
how RCE affects common device structures and to predict
how more complicated, integrated systems might perform.
However, the complexity of implementing RCE has hindered
its development. The highly precise vertical structures require sophisticated and expensive crystal growth techniques
which nevertheless do not offer high yield. VCSELs continue
to suffer from high series resistance and other difficulties
which have thus far kept them from realizing their potential.
The horizontal and vertical integration schemes discussed in
the previous section require fairly elaborate fabrication techniques. To realize a practical, low cost prototype RCE photonic system technology, it will probably be necessary to
simplify the structures and performance goals to decrease the
complexity of the growth and fabrication. For this reason,
RCE will likely only find applications in high performance
systems in which improved speed and/or WDM are required.
A number of simplifications arising from the nature of
RCE can lend themselves towards realizing a practical RCE
WDM architecture. MBE growth yield may be increased by
monitoring the layer refiectivity and using high energy electron diffraction
during growth permitting
in situ
corrections.t6’ Devices can be designed with sacrificial top
layers for surface recessing. Epilayers then can be pre-tuned
before device fabrication and top mirror deposition.‘61*79
Fabrication might also be considerably simplified if a simple
VCSEL structure were used as both detector and emitter’45 at
the expense of some device performance. These technologies
together are realizable, suitable for WDM, and potentially
high yield and low cost.
WDM systems would be greatly improved by an innovation which enables dynamic tuning of the RCE component
devices. An architecture can be envisioned where an optical
data stream carries several multimedia channels which the
user can access by tuning an RCE detector. A tunable
VCSEL and RCE photodetector would allow a single PC
user to easily send and receive video, data and voice over the
wavelength channels dedicated to each.
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Lightin( 1, A,,)
FIG. 53. Structure of a WSOL OR gate with an equivalent circuit representation. The input and output wavelengths are set during the fabrication (after Ref.
With such applications in mind, efforts towards achieving dynamic tuning by modulating the optical length of the
VC through refractive index variations have been undertaken. Simes et LzZ.‘~observed that an electrical bias applied
across the QW active regions of their RCE reflection modulators was sufficient to provide a modest index shift leading
to a 10 a shift of the cavity resonance. Blum et aZ.1667’67
applied an electrical bias across QWs positioned inside of the
DBR mirrors. They succeeded in altering the mirror reflectivity but have not applied their technique towards actual
device tuning. Lai and Campbell65 have predicted that this
effect can provide as much as 20 nm of tuning in the GaAs/
AlGaAs system. The largest continuous tuning range in a VC
device attained to date is 2.2 nm at 970 nm by Wipiejewski
et ~1.‘~’This was achieved in a three terminal VCSEL structure in which a tuning current flowed over one DBR mirror
causing ohmic heating and a corresponding shift in the refractive index. None of the above approaches is really satisfactory for a practical WDM system which will require multiple channels, preferably over the entire 1.3 - 1.55 pm low
dispersion band.
A dynamic, wide range tuning mechanism for RCE remains a major goal of device researchers. Pezeshki and
Harrislc9 have proposed combining VC lasers and detectors
with micro-machined, movable Al mirrors (Fig. 57). In this
approach, conventional RCE structures with a DBR bottom
mirror are grown. A freely suspended Al top mirror is then
fabricated using standard micro-machining techniques. The
distance between the Al mirror and the episurface, and therefore the cavity length, can then be controlled by electrostatic
deflection of the mirror. One difficulty of the proposed struc-
- - - - - - - - - - - -*
In (
FIG. 54. Structure of a WSOL AND gate (after Ref. 12).
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
B (Id
[email protected]
Q 3)
FIG. 55. Conceptual representation of a FULL ADDER implemented in
WSOL by cascading two layers (after Ref. 12).
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digital optical
Multiple wavelength
digital optical
h 01P J-11- Ti;i,
km= j;;,+k,,
FIG. 56. Conceptual implementation of multipIe wavelength optical interconnects (after Ref. 163).
ture is maintaining the cavity Q which requires keeping the
mirror face flat and perpendicular to the cavity axis.
The most impressive demonstration to date of this concept was an optical reflection modulator consisting of a
SiN, membrane fabricated onto a doped Si substrate.‘70 A
fairly weak RCE effect was in evidence since no mirrors
were incorporated into the structure. Nevertheless, it was
shown that the membrane could be modulated (and therefore,
the cavity response could be tuned) on a 1 ,US time scale.
A more robust implementation of the electrostatic tuning
scheme is a Au mirror fabricated on the underside of a
SiN, membrane. Figure 58 is an illustration of an optimized
LED/PD structure in the InGaAs/AlGaAs material system.
The device is based on an extended vertical cavity (EVC)
defined below by the DBR mirror and above by a hybrid
mirror consisting of a several period DBR, a sacrificial GaAs
spacer layer, and the Au/air interface. Due to the SWE, two
or more QWs are required to maintain a flat device response
over a large wavelength range. The asymmetric positioning
of the QWs equalizes the electron and hole transit times to
improve the device bandwidth. By making the arms of the
membrane thin, mirror tilt and buckling should be minimized. Simulations show that both AR coatings are critical.
FIG. 58. Tunable RCE emitter/detector EVC device structure (after Ref.
The lower coating insures that the maximum amount of light
is coupled into or out of the cavity. The upper AR coating
suppresses multiple reflections between in the air gap. The
device structure is tunable across 100 run (Fig. 59) with a fiat
response ( 77=90+ 5%) across the entire range. The tuning
range is limited by the DBR mirror stop band width, not the
Q W or membrane tuning mechanism. Eight channel operation within this range can be realized with only a 125
The EVC scheme is promising because the resonant cavity can be tuned across its full FSR inside of a ,us. Once
tuned, the bandwidth is dictated by the RCE LED or PD and
will be in the 200 MHz on up to GHz range. A single EVC
device (Fig. 60) would replace the entire tapered waveguide
array of Pezeshki et aLg6 (Fig. 36). Since only a single wave-
rdy suspenaea
Al mirror
I i
! i
! ’
I i
! 1
f I
, (
active cavity region
II - substrate
FIG. 57. Proposed RCE laser/detector in which the cavity width is modulated by electrostatic deflection of a micro-machined AI top mirror (after
Ref. 169).
J. Appl. Phys., Vol. 78, No. 2, 15 July 1995
FIG. 59. Spectral response of an EVC device designed for 8 channel operation in the 900-1000 n m band. They axis corresponds to EL intensity (LED
case) or generated photocurrent (PD case) (titer Ref. 171).
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and S. Strite
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FIG. 60. Coupled EVC-waveguide device. Gnly a single wavelength is selected from the optical data bus. The non-resonant wavelengths are free to
propagate to other users.
length is detected, the remainder can continue to propagate.
The permits multiple users to share the same optical bus,
even at remote locations, without expensive and time consuming optical switching or signal regeneration between
The EVC scheme would also increase the flexibility of
the multi-wavelength LED of Dodabalapur et al. lz2 (Fig. 31)
which electroluminesced at three wavelengths, but simultaneously and without control of the relative intensities. By
seIecting the EL wavelength with an electrostatically tunable
membrane, an organic dye LED could be tuned continuously
across the visible band. LEDs with narrower gain bands
could be switched on and off, or between several emission
lines, by membrane modulation. In these implementations,
the wavelength and the brightness would be independent.
This review article has attempted to describe both the
physics and the applications of semiconductor devices situated within Fabry-Perot microcavities. Placing a device inside of a microcavity was shown to have two major advantages. Due to the wavelength selectivity of the microcavity,
only resonant wavelengths are admitted or emitted. Resonant
radiation undergoes multiple reflections within the cavity,
causing its intensity to by amplified. The wavelength selectivity and field amplification of the resonant cavity has some
important favorable effects on many common device structures, and we therefore refer to such devices as resonant
cavity enhanced (RCE).
RCE photodetectors adopt the highly wavelength selective response of the cavity in which they are situated. They
also benefit greatly from the increased amplitude of the resonant field (or in an equivalent picture, the multiple passes of
each resonant photon), allowing much thinner absorption re
gions to achieve nearly unity quantum efficiency. Thinner
absorption regions allow higher transit time limited bandwidths due both to shorter transit distances and sharper rise/
fall characteristics. The wavelength selectivity makes RCE
photodetectors attractive for low crosstalk wavelength demultiplexing.
LED structures can also be integrated into a VC which
radically alters their spontaneous emission spectra. The overall radiative efficiency is improved, and the linewidth of the
emission becomes limited by the cavity finesse rather than
thermal effects. RCE LEDs have radically sharper spectra,
increased directivity and spectral power density compared to
conventional designs which permit more optical power to be
coupled into optical fibers and higher bandwidth due to reduced dispersion.
RCE detectors and phototransistors have vertical structures which are quite similar to VCSELs, simplifying integration. Several optical logic schemes have been proposed
which combine the sharp response spectra of RCE detectors
with the nearly monochromatic output of VCSELs, resulting
in cascadable optical input/output circuits which are capable
of incorporating the wavelength as an additional logical variable. Present day optical logic schemes will increase dramatically in power and flexibility when dynamic tuning of
individual elements is fully incorporated.
For these reasons, RCE devices can be expected to play
a growing role in optoelectronics over the coming years. The
continuing push towards higher bandwidth plays right into
the natural capabilities of RCE devices for greater speed and
wavelength demultiplexing.
This work was supported in part by the National Science
Foundation under the grant .No. 9309607. The authors wish
to acknowledge the contributions of Professor K. Kishino
and Professor A. MorkoG to the development of this work
and all of our colleagues who have graciously provided preprints and original figures.
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