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276
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 5, MARCH 1, 2007
Cladding Layer Impedance Reduction to Improve
Microwave Propagation Properties in p-i-n
Waveguides
Fang-Zheng Lin, Yi-Jen Chiu, and Tsu-Hsiu Wu
Abstract—The cladding layer effect on microwave propagation
properties of semiconductor p-i-n waveguides is investigated in this
letter. Through the optical excitation in quantum wells of p-i-n
waveguides, high-speed photocurrent is used to examine the microwave propagation. Two devices of p-i-n waveguides with different cladding layers are fabricated and measured, showing that
a higher speed is found in the waveguide of wider cladding width.
Verified by the microwave propagation properties, the higher speed
is mainly attributed to lower microwave propagation loss due to the
lower impedance in the wider cladding layer, suggesting this kind
of structure can be applied to high-speed waveguide-based devices.
Index Terms—Cladding layer, high speed, microwave propagation loss, optical–electrical (OE) response, p-i-n waveguide,
semiconductor, undercut etching.
I. INTRODUCTION
H
IGH-SPEED high-efficiency semiconductor devices
based on p-i-n waveguides have been widely used in
the optoelectronics application. Due to the highly confined
electric field in i-layers of p-i-n waveguides, the devices can
be operated in a high-electric-field-driven regime with low
driving voltage, allowing wide applications in optoelectronic
fields, such as high-speed electroabsorption modulators, electrooptical phase modulators, and waveguide photodetectors
[1]–[6]. Among these devices, the traveling-wave structure
has been widely used to overcome the resistance capacitance
element in order to obtain high-speed and also high-efficiency in long waveguides [2]–[9]. However, due to the finite
resistance of cladding layers (p- and n-layers), the highly
loaded capacitances resulted from the highly confined electric
fields in the small volume of i-layers will cause the so-called
“slow-wave” problems, i.e., the electrical wave propagation
properties of 1) high propagation loss, 2) slower velocity than
optical wave, and 3) low impedance. These factors mainly
determine the overall performance of the devices and limit
the whole design tolerance. Therefore, it is quite important to
investigate the cladding-layer effect in p-i-n structures on the
microwave propagation properties of waveguides. Based on
the p-i-n heterostructure, p-i-n waveguides can be served as
optical and also microwave waveguides. In this letter, based on
Manuscript received September 1, 2006; revised November 28, 2006. This
work was supported by grants from NCF (NSC 93-2215-E-110-018 and NSC
93-2215-E-110-013), by the project “Aim for the Top University Plan,” Ministry of Education of Taiwan, and by the Technology Development Program for
Academia (92-EC-17-A -07-S1-025) in Taiwan.
The authors are with the Institute of Electro-Optical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, R.O.C. (e-mail:
[email protected]).
Digital Object Identifier 10.1109/LPT.2006.890044
Fig. 1. Schematic plots for the two cross sections of two p-i-n waveguides,
UEARW and RW, and their corresponding equivalent circuits.
the device structures of [2], the high-speed photocurrent from
different cladding layers of waveguides is generated to test the
device performance and microwave propagation through electrical-field-driven optical absorption in multiple quantum-wells
(MQWs), namely quantum-confine-stark effect [10]. It is found
that lowering the impedance of cladding layers can improve the
device bandwidth by lowering propagation loss, further verified
by measuring the basic electrical wave properties.
Fig. 1 shows two schematic cross sections of p-i-n waveguides with the equivalent transmission-line-circuit model,
namely the conventional ridge waveguide (RW) and the
undercut-etching-active-region waveguide (UEARW). Two
governing parameters of microwave properties dominating the
high-speed performance are 1) characteristic impedance
(related to the electrical reflection) and 2) complex propagation
(the real and imaginary parts stand for microwave
constant
loss and wave number). These two parameters are extracted by
the methods cited in [7], [8], [11], and [12], where the waveguide is primarily formed by several elements, namely cladding
layer resistance
, the intrinsic capacitance
of the
and metal impedance
waveguide, and the inductance
of conducting metal. In this experiment,
is set as
the controlled factor, while the remaining parameters of both
waveguides (RW and UEARW) are intentionally kept similar.
For example, with the same metalization on the non-Ferroand
can
magnetic semiconductor material, the same
be obtained in two structures. By undercut-etching the active
region, UEARW active region can be engineered to get the
same width as RW’s. Filled with the low-dielectric constant
in UEARW is mainly
material into the undercut portion, the
); therefore,
attributed to the active region (i.e.,
the similar can be achieved in both structures. According the
circuit model [7], [8], [12], the characteristic impedances and
microwave velocities of the waveguide can be approximately
and , suggesting that the high-speed
determined just by
responses for both structures due to such effects can thus be
neglected. However, as for electrical propagation loss, it might
1041-1135/$25.00 © 2007 IEEE
LIN et al.: CLADDING LAYER IMPEDANCE REDUCTION TO IMPROVE MICROWAVE PROPAGATION
be different. Based on the general conditions of
,
and midfrequency range (10–100 GHz) in
p-i-n diode, the complex electrical propagation constants can
in the
be expressed by
first-order approximation [7], [8], [12], where is the radius
frequency. On the other hand, in the generation of distributed
photocurrent, the high-speed optical–electrical (OE) response
can be obtained from the photon-generated current propagating
in the p-i-n waveguide as [7], [14]
277
Fig. 2. Schematic diagram of photocurrent experiments. The lines of from A to
B and from C to D are CPWs situated on semi-insulating (S.I.) InP layers. The
line from B to C is the p-i-n waveguide served as optical, and also electrical,
waveguides, where the cross sections of p-i-n waveguides are shown in Fig. 1.
The distributive photocurrent (OE conversion) is generated through the input
optical power and collected through the load CPW line (C to D , forward term).
(1)
where
is a constant depending on the material,
is the microwave reflection coefficient
) and waveguide,
at the junction of load (with impedance
is the microwave transmission coefficient, is the
optical wave propagation constant inside the waveguide, and
is the waveguide length. The overall OE response includs the
microwave multireflection effects on junction [denoted in term
(a)], electrical forward wave [term (b)] with respect to optical
wave and electrical backward wave [term (c)]. It could be
can lead to the
expected that the effect of cladding layers
quite different electrical loss and also the device performance.
II. FABRICATION, MEASUREMENT, AND RESULTS
The p-i-n epi-layer is grown in a metal–organic chemical
vapor deposition system. The active region (nondoping) sandwiched by top p-InP and bottom n-InP layers is deposited with
InGaAsP–InGaAsP MQWs, serving as optical and electrical
waveguides. The i-layer thickness is 185 nm. UEARW and RW
of 400- m length are first processed on the same wafer with
corresponding cladding layer widths of 2 and 6 m. The active
region of UEARW is then further selectively undercut-etched by
a H PO H O H O solution from InP until the same width
with RWs is obtained. The p- and n-layers contact metalizations
are Ti–Pt–Au and Ni–AuGe–Ni–Au. A PMGI (Polymethylglutarimide, MicroChem Inc.) layer was then spun and patterned
for passivating the etching surface, planarization, and bridging
the high-speed interconnection. Finally, the same metalizations
of 2- m-thick coplanar waveguide (CPW), shown in Fig. 2,
are deposited as the connecting feed and load lines, forming
a traveling-wave structure [2], [4]. Fig. 2 plots the schematic
setup for measuring high-speed photocurrent and also electrical
scattering matrix ( -parameters) of the devices. As a high-speed
modulated optical signal travels through the waveguide (from
point “ ” to point “ ”), the forward distributive photocurrent
can thus be created and then propagate along the waveguide,
forming the “edge-coupled photodetector” structure [5], [6].
Through the collected high-speed photocurrent, the electrical
propagation on waveguides can thus be examined. A CW laser
diode with power level of 1 mW at a wavelength of 1550 nm
Fig. 3. Normalized OE-response with frequency. The experiment data are
shown in close-circle points for UEARW and open-circle points for RW. The
lines are the calculated results for both structures.
modulated by a high-speed LiNbO optical modulator is used
as a high-speed modulated optical source. A high-speed microwave vector network analyzer (40 GHz, VNA, Anritsu) is
then employed to characterize the high-speed OE response
(photocurrent) by feeding and collecting the microwave power
through two high-speed microwave probes. The forward
photocurrent (point “ ” is collecting point) is the signal to
be characterized. Both feed-line and loaded-line CPWs are
terminated by a 50- resistor or 50- instrument. In all the
measurements, the optical power level is kept low enough in
order to avoid the nonlinear optical-power dependent behavior.
Fig. 3 shows the normalized OE responses for UEARW and
RW structures from dc to 30 GHz. The bias is set as reverse
bias of 1 V. It can be clearly seen that the UEARW speed is
faster than RWs, where the corresponding 3-dB bandwidth is
12 GHz for UEARW and 8 GHz for RW. The difference in response is found to be about 2 dB at a frequency of 30 GHz. It also
should be noted that the difference of responses between both
structures increases monotonically as the modulated frequency
increases, indicating that the better high-speed performance can
be achieved through the wider cladding layers structure.
In order to further analyze the high-speed performance of
the device, the electrical propagation properties of waveguides
should be considered. Through direct measurement on the
straight waveguides (without CPW feed and load lines), we obtain the two-port microwave scattering matrix ( -parameters)
from VNA, by which the microwave propagation properties,
and , can be extracted by the method described by
i.e.,
are about (21 ) for UEARW and
[12]. The extracted
(22 ) for RW. Fig. 4(a) plots the obtained microwave propagation loss coefficient in term of dB/100 m (real part of ) and
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 5, MARCH 1, 2007
(squares), and electrical loss with impedance mismatch (lines).
All the factors illustrate similar responses in UEARW and RW
except the electrical loss mechanism. And also, as seen in the
simulated results [line curves of Figs. 3 and 4(a)], the calculated
OE response and microwave propagation properties are quite
in agreement with the experimental results, suggesting that the
lower impedance in the cladding layer is the main factor enhancing the performance of UEARW.
Fig. 4. (a) Extracted microwave propagation constants (including microwave
loss and velocity) of UEARW and RW structures. (b) Calculated OE response of
UEARW and RW in the conditions of velocity mismatch (triangles), impedance
mismatch (circles), and electrical loss (squares) and loss with impedance mismatch (line). The main difference of both structures is mainly attributed by the
microwave propagation loss.
TABLE I
PARAMETERS OF UEARW AND RW FOR THE OE RESPONSE CALCULATIONS
III. CONCLUSION
Two structures of p-i-n waveguides with different cladding
layers are tested for investigating the microwave propagation
properties of semiconductor p-i-n waveguides. Through the optical-to-electrical conversion in quantum wells of p-i-n diodes,
the higher speed is found in the waveguide of wider cladding
width. By extracting the microwave propagation properties from
electrical transmission on waveguides, we demonstrate that the
higher speed is mainly attributed to the lower microwave propagation loss from the lower impedance in wider cladding layer.
REFERENCES
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