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ACTIVITIES
OPTIMIZATION OF ABSORPTION IN MID-WAVE INFRARED TYPE-II INAS/GASB STRAINED
LAYER SUPERLATTICE (T2SL)
REU Student: Eric Frantz
Graduate Student Mentor: Lilian Acosta, Brianna Klein
Faculty Mentor: Sanjay Krishna, Elena Plis
A. Introduction
Infrared (IR) radiation is the portion of the electromagnetic spectrum between 1 to 300µm
and is just outside the visible light spectrum. Interest in IR detectors has focused primarily on
two regions of the IR spectrum, the mid-wave IR (MWIR, 3-5µm) and the long-wave IR (LWIR,
8-14µm) [1]. These regions are particularly useful for three reasons: most objects emit IR
radiation at room temperature in this spectrum, most chemical species have spectral signatures in
the IR regime, and that the atmosphere has a clear transmission in these spectral windows [2].
MWIR IR detectors are found in a wide range of applications, such as: night vision, thermal
imaging, chemical spectroscopy, gas sensing, pollution monitoring, optical remote sensing, and
medical diagnostics. Important properties of high-performance MWIR detector are high signalto-noise ratio (SNR), i.e. low noise and high signal levels; high operation temperature; large
format and affordable cost.
Currently HgCdTe (MCT) photodetectors are the state-of-the-art devices in MWIR spectral
range [1]. Although MCT devices have large quantum efficiency (QE), they are sensitive to
small changes in the alloy composition ratio during the epitaxial process, making the spatial nonuniformity a challenging problem for large format focal plane arrays (FPAs). Moreover, MCT
detectors are characterized by a low electron effective mass (~0.009 mo) [3] resulting in large
dark current due to tunneling especially at longer wavelengths.
Bulk InSb photodetectors are also used for MWIR applications due to their high QE,
however they are limited to cryogenic operation temperatures. The necessity of cryogenic
cooling increases both size and cost of IR detection systems. Therefore, there is a need for highperformance MWIR detectors operating near or at ambient temperature.
Of the alternatives we focus on photodetection using InAs/GaSb T2SL photodetectors. T2SL
offer us high operating temperature, high QE, and are relatively cheap to produce [2]. In
addition, the larger effective mass (~ 0.04m0) in T2SL leads to a reduction of tunneling currents
compared with MCT detectors of the same bandgap. Moreover, commercial availability of low
defect density substrates as well as a high degree of uniformity for III-V growth and processing
over a large area also offers technological advantages for the InAs/GaSb T2SL technology
For our study we focus on InAs-GaSb T2SL for application in MWIR detection.
B. Background
B.1. Principle of Semiconductor Photodetection
IR detectors may be divided into two broad classes, thermal and photon detectors.
Thermal detectors respond to the heat generated by the absorbed energy of the optical radiation
whereas the principal of operation of photon detectors is based on the direct interaction of the
optical radiation with the atomic lattice of material.
In thermal detectors (Si-based microbolometers, pyrometers, thermocouples, Golay cells,
and superconductors) incident optical radiation increases the temperature of the detector which
leads to a change in physical parameters, such as resistance or voltage. Some of the characteristic
features of thermal detectors are broad spectrum, room temperature operation, low cost, and slow
response. Photon detectors generate free electrical carriers through interaction of photons and
bound electrons. The interaction of light and matter produces a parameter change (resistance,
conductance, voltage or current) that is detected by an associated circuitry. Photon detectors for
IR are typically realized using III-V and II-VI semiconductors, and can further be divided into
intersubband and interband detectors.
Examples of intersubband detectors are quantum well infrared photodetectors (QWIPs)
and quantum dot infrared photodetectors (QDIPs), where intersubband transition energies in the
conduction band can be tailored to detect infrared radiation. Examples of interband detectors are
HgCdTe (MCT), InSb, and InAs/GaSb type-II strained layer superlattice (T2SL) systems.
The principle of operation of photon detector may be further described as follows: when a
photon with energy higher than the semiconductor’s bandgap is incident upon an electron in the
valence band the photon will be absorbed as energy. After photon absorption the electron is
excited from the semiconductor’s valence band to its conduction band and an electron-hole pair
is created, with electron in conduction band and the corresponding hole in the valence band from
where the electron left. Figure 1 illustrates the absorption of a photon and consequential creation
of an electron-hole pair in a pin photodiode.
The energy of a photon is dependent upon its wavelength and the wavelength past which
the photon no longer has enough energy to excite an electron is called the cutoff wavelength. The
wavelength to energy relation is shown in Equation 1, were h is planks constant, c is the speed
of light, and is the wavelength. The condition for photon absorption is shown in Equation
2.The equation for cut-off wavelength can be derived from these two formulas and is shown as
Equation 3.
(1)
(2)
(3)
While the cutoff wavelength is the limit of photons being able to excite electrons
response is also shown to decrease as the photons have lower and lower wavelengths past the
cutoff wavelength [4]. Due to this phenomenon optimal light absorption occurs for light of
wavelength near, but not past, the cutoff wavelength.
B.2 pin Photodiode Design
The pin photodiode design consists of three regions, a positively doped p region, a
negatively doped n region, and a non-intentionally doped (n.i.d.) intrinsic i region. In this design
of photodiode the p and n regions act as contacts and the i region acts as the absorption layer. A
band diagram for a pin photodiode is shown in Figure 1.
Figure 1 - Heterostructure schematic of pin photodiode and pin photodiode band diagram
For application as a photodetector we operate the diode in reverse bias, with negative
voltage applied to the p region and positive voltage applied to the n region. In this configuration
carriers generated in the absorption layer are swept away by the electric field towards either the n
or p contact and contribute to the photocurrent. Despite the simplicity of pin detector design it
has several significant drawbacks. In particular, pin detectors demonstrate high noise due to the
large Shockley-Read-Hall (SRH) associated with the depletion layer.
B.3 pBiBn Photodiode Design
The pBiBn photodiode, like the pin photodiode, has n and p contact layers and an n.i.d.
absorption layer. Unlike the pin design the pBiBn has an additional two barrier layers. These
barrier layer are located on either side of the i layer blocking minority carriers from moving from
the contact layers to the absorber region. The barrier located between the p and i region blocks
electrons from moving from the p to the i region and is called the electron barrier (EB), and the
barrier between the n and i region blocks holes from moving from the n to the i region and is
called the hole barrier (HB). The design also reduces the electric field drop across the absorber
region, resulting in most of the electric field drop across the barrier layers. The reduction in
electric field across the absorber layer leads to a small depletion region, this in turn reduces SRH
current. Band-to-band (BTB) and trap-assisted-tunneling (TAT) are also reduced due to the
significant reduction in electric field drop across the absorber region [5]. The reduction of
minority carrier diffusion, SRH, BTB, and TAT currents reduces the dark current of the devices,
reducing the overall noise and increasing its sensitivity [5]. A diagram of the pBiBn photodiode
is shown below in Figure 2. For use as a photodetector the pBiBn is used in the same bias
configuration as the pin photodiode.
Figure 2 - pBiBn Energy Band Diagram. Reprinted from [5]
B.4 Semiconducting Superlattices
Research into semiconductor superlattices began in 1969 with a proposal by Esaki and
Tsu for a one-dimensional potential structure composed of alternating layers thin enough to
allow for electron tunneling [6]. The proposed superlattice materials were calculated to have a
negative differential resistance. However, superlattice structures remained theoretical as no
fabrication techniques where able to demonstrate this quantity. It wasn’t until 1972 when Esaki
et al. grew a GaAs-GaAlAs superlattice through molecular beam epitaxy (MBE) that negative
resistance was observed [6]. Five years later the first InAs-GaSb SLS was proposed for IR
detection by Tsu et al. (1977) [2].
Superlattices work on the principle of quantum confinement. The principle of “quantum
confinement describes the discretization of the energy spectrum which occurs when a particle is
restricted in one or more dimensions by a potential well whose dimensions are comparable to the
Bohr radius or de Broglie wavelength in the material system” [2]. In the superlattice structure the
quantum wells are created by the energy bands of the alternating semiconductor. These quantum
wells cause the quantum confinement of electrons. As mentioned earlier the layers of
semiconductor must be thin enough to allow for electron tunneling between quantum wells. The
tunneling of electrons between quantum wells causes the formation of electron and hole
minibands [2]. It’s these minibands that act as the conduction and valence bands of the
superlattice structure as a whole. Figure 3 shows minibands formed in a GaSb-InAs T2SL band
diagram generated by electron tunneling.
There are four different classifications of superlattices based on the layers’ band energies
[6]. Type I superlattices have what is called a nested band gap, where the bandgap of one
semiconductor is completely enclosed by that of the other semiconductor. Type II-staggered
superlattices have the valence band of one semiconductor between the valence and conduction
bands of the other, while its valence band is above that of the other semiconductor. Type IImisaligned superlattices have both the conduction and valence bands of one semiconductor
above the conduction band of the other semiconductor. Type III superlattices are classified as
such when one of the layers of the superlattice is metallic. In our testing we will be focusing on
the type II superlattice composed of InAs-GaSb.
It has been shown that the introduction of strain into a superlattice, by having
mismatched lattice constants of the two materials, causes the splitting of valence bands into light
and heavy hole components. This leads to larger electron effective mass which results in reduced
diode tunneling currents, reducing the dark current [7].
By using SLS material for photodetection we are able to optimize photodiode design by
engineering the band structure. As mentioned previously optimal absorption occurs near the
cutoff wavelength. Through band engineering SLS materials offer bandgaps that can be very
close to the cutoff wavelength. Band engineering also allows us to create barriers better suited
for our needs for a more optimal device.
Figure 3 - GaSb / InAs T2SL diagram
C. Research Objective
The objective of this study is to optimize the absorption in MWIR InAs/GaSb T2SL
photodetectors by varying (1) thickness of detector absorber region, (2) T2SL period thickness,
and (3) strain between T2SL and GaSb substrate.
By researching what effect these parameters have on absorption we hope to further optimize
T2SL materials for use in MWIR photodetectors.
The drive behind this study is the potential in the T2SL material for IR MWIR detection
against existing technologies. T2SL detectors offer us high operating temperatures, high
quantum efficiency, spectral tuneability, low cost, high responsivity, normal incidence
absorption, and multicolor operation. Comparison of T2Sl photodetectors against existing
technologies is shown below in Table 1.
Characteristic
Normal incidence
absorption
Operation
Temperature
Spectral
Tunability
High Quantum
Efficiency
High
Responsivity and
Detectivity
Multicolor
Operation
Homogeneity
Material Yield
HgCdTe
Photodiodes
InSb
Photodiodes
Type II
StrainedLayer
Superlattice
Quantum
Well Infrared
Photodetector
Quantum Dot
Infrared
Photodetector
yes
yes
yes
no
yes
high
low
high
low
high
no
no
yes
yes
yes
yes
yes
yes
no
no
yes
yes
yes
no
no
yes
no
yes
yes
yes
no
low
yes
low
yes
high
yes
high
no
high
Table 1- Comparison of T2SL against other photodetector technologies
D. Methodology
For our study we use InAs-GaSb T2SL to study the effects of absorption layer thickness,
T2SL composition, and interfaces between layers on T2SL detectors performance for use in
the MWIR spectrum. The study is divided into three parts, one for each parameter:
1. Part 1: Effects of absorption layer thickness on detector performance.
2. Part 2: Effects of T2SL composition on detector performance.
3. Part 3: Effects of strain between T2SL detector structure and GaSb substrate on
detector performance.
For the first part of the study, MWIR homojunction pin and pBiBn architectures based on
8 monolayers (MLs) InAs/8 MLs GaSb T2SL were utilized. Thickness of non-intentionally
doped absorber layer was 1.5 um, 2.5 um, and 3.5 um.
For the second part of the study, MWIR detectors with pBiBn designs were grown. The
thickness of absorber region and the T2SL constituent InAs layer thickness were kept the
same (1.5 um and 8 MLs, respectively) whereas the T2SL constituent GaSb thickness was
varied as 6 MLs, 8 MLs, and 10 MLs.
In the third part of the study, pBiBn T2SL detectors with 8 MLs InAs/8MLs GaSb T2SL
composition and different thickness of InSb strain compensation layer were intended to be
grown. However, due to the issues with MBE reactor this part of study was not realized.
Table 2 shows a summary of devices for the first two parts of the study, and their
variables. Figures 4 and 5 illustrate the heterostructre schematics of pin and pBiBn InAsGaSb T2SL detectors, respectively.
Completion of the study requires the following steps:
1. Growth of InAs – GaSb T2SL detector samples.
2. Photoluminescence (PL) and high-resolution x-ray diffraction (HRXRD)
measurements of as-grown T2SL samples to evaluate relative material quality.
3. Fabrication
a. Mesa definition (etching)
b. Contact Metal Deposition
c. Sidewalls Passivation
4. Spectral Response testing
5. Dark current measurements
6. Responsivity Testing
7. Dark Current Testing
8. Spectral Response Testing
After completion of device testing we are able to use the responsivity, spectral response, and
dark current measurements to calculate the quantum efficiency, a standard assessment metric for
photodetectors, for each device.
8/8 InAs/GaSb SLS P+
contact Layer (10 P)
50.6 nm
8ML InAs/8ML GaSb
SLS
Varied thickness
(n.i.d)
N+ Contact Layer 8/8
InAs/GaSb SLS
(145 Periods) (730 nm)
GaSb:Te
2” wafer
5/7.5 InAs/GaSb SLS P+
contact Layer (30 P) 100nm
EB 7ML GaSb/5ML AlSb SLS
(40Periods) (156nm)
8ML InAs/8ML GaSb SLS
Varied thickness
(n.i.d)
40P (11ML InAs/5ML AlSb)
(n.i.d) (200nm)
N+ Contact Layer 6/7
InAs/GaSb SLS
(150 Periods) (585nm)
GaSb:Te
2” wafer
Figure 4 - pin structure used for the
study
Figure 5 - pBiBn structure used for the study
Sample ID
Part of Study
Design
L13-42
L13-44
L13-45
G13-169
G13-172
G13-175
L13-46
L13-47
L13-49
1. Absorber Thickness
pBiBn
pin
2. Composition of T2SL
pBiBn
Absorber
thickness (µm)
1.5
2.5
3.5
1.5
2.5
3.5
1.5
1.5
1.5
T2SL
composition
8/8
8/8
8/8
8/8
8/8
8/8
8/8
8/6
8/10
Table 2 - Summary of device parameters for study.
E. T2SL Material Growth
The detector material used in this study was grown on n-type (Te-doped) GaSb (100) epiready substrates using a solid source molecular beam epitaxy (MBE) system equipped with
valved cracker sources for the group V Sb2 and As2 fluxes, and Ga/In SUMO® cells.
MBE is a modern technique for growing III-V compound semiconductors as well as several
other materials. In MBE, thin epitaxial films crystallize via reactions between thermal-energy
atomic beams of the constituent elements and a substrate surface which is maintained at an
elevated temperature in ultrahigh vacuum. Since growth rates are typically less than an atomic
monolayer per second and the beams can be shuttered in a fraction of a second, nearly atomically
abrupt transitions from one material to another as well as good control of thickness, doping and
composition can be realized.
Diagram of MBE growth chamber is presented in Figure 6 (a). Figure 6 (b) shows the growth
chamber of VG-80 MBE system utilized for the material growth during this study.
Figure 6. (a) Diagram and (b) photo of VG-80 solid-source MBE growth chamber
F. Material Characterization
F.1. HRXRD Measurements
The structural quality of as-grown T2SL detector material was assessed by HRXRD
performed with a Philips double-crystal X-ray diffractometer using the Cu-Kα1 line and all
measurements were carried out at 300K.
XRD is a non-destructive technique for the structural characterization of thin crystalline
films. X-rays interact with electrons in matter and are scattered in various directions by the
atomic electrons. If distances comparable to the wavelength of the X-rays separate the scattering
centres then interference between the X-rays scattered from particular electron centres can occur.
For an ordered array of scattering centres this can give rise to interference maxima and minima.
In T2SL samples there are two ordered arrays, the atomic arrangement and the alternating
layers constituting the T2SL. The atomic arrangement gives rise to the characteristic 0th order
peak, while the T2SL layers are basically a diffraction grating, causing multiple diffraction
orders to occur. If there is strain in the system, the 0th order T2SL peak will be at a different
angle from the substrate peak. The main T2SL peak positioned to the left of the substrate
indicates compressive strain, to the right indicates tensile strain. Higher diffraction orders due to
the T2SL are satellite peaks and are symmetric about the 0th order peak. Depending on the
thicknesses of the T2SL layers, the spacing of the satellite peaks from the zeroth order will be
different. Thus, information about the layer thicknesses can be found. If multiple types of T2SL
are grown, such that one type has one set of layer thicknesses and another set has different layer
thicknesses, two sets of satellite peaks will be present. If not enough periods of T2SL are grown,
the x-ray diffraction pattern will be very weak, or not detectible. Therefore, to view the
diffraction pattern, there are a minimum number of SLS periods needed.
To observe the x-ray diffraction from a crystalline lattice, the Bragg condition must be
satisfied (eq. 4). The Bragg condition depends on the angle of the incident x-ray beam as it enters
the crystal lattice and the direction at which the diffracted beam exits the lattice. It is met only
when the scattered waves from all the atoms in the lattice are in phase, and interfere
constructively.
(4)
Here dhkl is the distance between the atom planes (h,k,l are the Miller indices), ʘ is the incident
angle, n is the order of diffraction and  is X-ray wavelength.
Parameters typically deduced from such a HRXRD T2SL spectrum are strain relation
between substrate and epilayer, thickness of T2SL period, full width of half maximum (FWHM)
of the T2SL satellite peaks and composition of epilayers. The number of observed T2SL satellite
peaks is related to the number of grown T2SL periods, their reproducibility and the intensity of
the X-ray source. The relative intensity of the satellite SLS peaks is influenced by the interfacial
roughness and composition.
F.2 PL measurements
Photoluminescence (PL) testing is the measurement of light emitted by a material after it
has been placed in an excited state by another light source. The principle behind this test is that
one of the forms of recombination in a semiconductor is radiative recombination. When an
electron hole pair recombine radiatively they emit a photon corresponding to their bandgap
energy. PL testing measures this type of recombination in a material sample.
In our testing we excited InAs/GaSb T2SL materials using a laser focused on the sample.
The light generated by the sample is then focused into an InSb monochromator through CaF2
lenses. The intensity of collected light is recorded as a function of wavelength. A diagram of our
test setup is shown in Figure 7.
Figure 7 - Diagram of Photoluminescence Test Setup, courtesy of Sadhvikas Addamane
We use the results from PL testing to compare material quality across samples. The
principle for this comparison is that higher PL intensities in a sample the higher radiative
recombination. We attribute the other recombination primarily to unwanted SRH and Auger
recombination. So by comparing sample PL tests against each other we can gain a relative idea
of the level of unwanted recombination between samples, with a higher PL intensity relating to
lower SRH and Auger recombination, and better material quality.
G. Fabrication
Basic fabrication process for the single-pixel InAs/GaSb T2SL detectors includes following
steps: (1) mesa isolation etch, (2) top and bottom contact metallization, and (3) passivation. Each
of processing steps is described below in details.
G.1. Mesa isolation etch
Etching is the process of taking the solid material sample and etching the device patterns
into it. The process of etching begins with photolithography to pattern the sample, for our study
we used a single pixel device etch pattern.
The wafer must typically be pretreated before resist application in order to obtain a
smooth, uniform coverage of the photoresist to the wafer. The first step in the pretreatment is the
cleaning and a dehydration bake. Wafer is cleaned consecutively in acetone, isopropanol (IPA)
and de-ionized (DI) water (5 minutes each) and then blow-dried with nitrogen (N2). The
dehydration bake is performed at 150°C for 6 minutes. This step is intended to drive off most of
the adsorbed water on the surface of the wafer. A dehydration bake is also effective in
volatilizing organic contaminants, further cleaning the substrate. At 150°C, it is estimated that
about one monolayer of water remains on the surface.
Immediately following the bake the wafer is primed with hexamethyldisilazane (HMDS),
which acted as an adhesion promoter. Liquid HMDS is applied directly to the wafer by
dispensing a fixed volume and spinning the wafer to spread out the liquid to a very thin uniform
coating. For spinning HMDS wafer is mounted on a vacuum chuck. Torque is applied to the
chuck to rapidly accelerate in with rotational speed of 5000 rpm during 30 seconds. One
monolayer of HMDS bonded with the surface of the wafer, even it is partially hydroxylated. The
other side of the molecule bonded readily with the resist. After HMDS application the wafer
undergo a softbake at 150°C for 30 seconds. Softbake is required to promote adhesion. Then
positive photoresist AZP4330 is spun on the wafer with rotational speed of 4000 rpm for 30
seconds. Photoresist was dried out at 90°C for 90 seconds (hard bake). Then wafer is exposed for
11 seconds at 200 mJ/cm2 on MJB- 3 mask aligner. Finally, wafer is developed in AZ400K
developer diluted in
four parts of water for 30 sec. Then the wafer is rinsed in DI water bath for two minutes and
blow-dried with N2. Finally, patterns are examined under Nomarski microscope and measured
by an alfa-step profilometer. The resulting PR thickness is typically equal to 3.2 µm.
After photolithography the sample is then etched in an Inductive Coupled Plasma (ICP)
etcher down to the n contact layer of the device. Plasma assisted etch with combination of BCl3,
CH4, and Ar gases provides nearly vertical profiles and clean sidewalls of InAs/GaSb SLS
detector free from etch residues.
G.2. Contacts Metallization
After etching the device in making is then cleaned with HCl to remove any oxides that
have built up on the device. After the acid clean the device is again patterned with photoresist to
the metal deposition mask. First the wafer is cleaned in acetone, IPA and DI water (5 minuted
each) and then blow-dried with N2. Then dehydration bake is performed at 150°C for 6 minutes.
Next, wafer is primed with HMDS with rotational speed of 5000 rpm for 30 seconds. Conditions
for the following HMDS soft bake are 150◦C for 30 seconds. The wafer is coated with
AZ5214E-IR photoresist after it is primed.
Photoresist is spun on the wafer with a rotational speed of 5000 rpm for 30 seconds.
These conditions verified negative wall profile needed for the lift-off of evaporated metal. After
resist application, the wafer is baked at 90°C for 90 seconds. The function of this step is to drive
off most of the solvent in the resist and to establish the exposure characteristics. After bake, the
wafer is exposed for 3.5 seconds at 200 mJ/cm2 on MJB-3 mask aligner. For the image reversal
of the photoresist, the most critical parameter is reversal-bake temperature (it should not exceed
125◦C). The exposed wafer is baked during one minute at 112°C. Then flood exposure (without
mask) during one minute at the same light intensity is applied to the wafer. Finally, wafer is
developed in AZ400K developer diluted in five parts of water for 30 sec. Then the wafer is
rinsed in DI water bath for two minutes and blow-dried with N2. Patterns are examined under a
Nomarski microscope and measured by an alfa-step profilometer. The resulting thickness of the
resist is typically equal to 1.3 µm.
After finishing the photolithography process, metals are deposited. For both p and n-type
contacts, 50 nm Ti/ 50 nm Pt/ 350 nm Au is used. This composite on provided good ohmic
behavior of contacts for the n-type GaSb and p-type SLS contacts without annealing. Metals are
thermally evaporated in e-beam thermal evaporator. Finally, lift-off process is performed by
soaking of the samples in acetone for 2 hours with subsequent acetone-brush treatment.
Resulting thickness of metal patterns is verified by an alfa-step profilometer.
G.3. Passivation
The final step in device fabrication is passivation. Passivation is the placement of substance
of the device in order to prevent the growth of unwanted oxides on the device, which can
contribute to dark current. For our devices we used the photoresist SU-8 applied through
photolithography for passivation in according to the procedure listed in [8].
An illustration of a fully fabricated device structure with pBiBn design is shown in Figure 8,
where the gold is the metal contacts and the purple is the SU-8 used for passivation. It should be
noted that fabrication procedure for pin and pBiBn device is inherently the same.
Figure 8- Fabricated Device Illustration
H. Characterization
H.1 Spectral Response Measurements
The spectral response test is used to determine the cutoff wavelength of a photodetector.
The cutoff wavelength is the wavelength of light past which the energy of the photons is too low
to generate current in the detector. For our application in the MWIR we are expecting to see
cutoff wavelengths of around 5µm. The cutoff wavelength is one of the properties that is used in
calculating the QE of a device. Figure shows a spectral response test setup.
Figure 9 - Spectral Response Test Setup
H.2 Dark Current Measurements
Dark current testing is used to measure the amount of dark current generated by a
photodetector, that is to say how much current the device generates without any light applied to
it. The dark current is typically a result of Auger recombination, SRH process, tunneling leakage,
and surface leakage (currents along the edge of the device) and is the primary source of noise in
a photodetector [9]. High dark current is typically a sign of poor material quality in a device.
Photodiode designs, such as the pBiBn design, and passivation techniques can help reduce the
dark current in a device.
H.3 Responsivity measurements
Responsivity testing is a measure of the amount of current generated by a device for a
given intensity of light. This test is an application of the photoelectric effect, discussed in section
B.1, in order to measure device performance. By measuring the current generated at a given
intensity of light we are able to get the responsivity, in units of A/W, of the device under a given
temperature and voltage bias. Figure 10 illustrates a responsivity test setup.
Figure 10 - Responsivity Test Setup
I. Refrences
[1] A. Rogalski. Infrared detectors: status and trends. Progress in quantum electronics 27.2
(2003): 59-210.
[2] E. Plis, J-B Rodriguez, S. Krishna. InAs/(In)GaSb Type II Strain Layer Superlattice
Detectors. Comprehensive Semiconductor Science and Technology Vol. 6. (2011): 229-264
[3] A. Rogalski. HgCdTe infrared detector material: history, status
and outlook. Reports on Progress in Physics. 68, 2267 (2005)
[4] A. Rogalski, K. Adamiec, and J. Rutkowski. Narrow-gap semiconductor photodiodes. Vol.
77. Bellingham: SPIE Press (2000).
[5] N. Gautam, S. Myersa, A.V. Barve, B. Klein, E.P. Smith, D. Rhiger, E. Plis, M.N. Kutty , N.
Henry, T. Schuler-Sandy, S. Krishna. Band engineered HOT midwave infrared detectors based
on type-II InAs/GaSb strained layer superlattices. Infrared Physics & Technology (2013).
[6] L. Esaki. A bird's-eye view on the evolution of semiconductor superlattices and quantum
wells. Quantum Electronics, IEEE Journal of Quantum Electronics Vol. QE-22 No. 9 (1986):
1611-1624
[7] D. L Smith, C. Mailhiot. Proposal for strained type II superlattice infrared detectors. Journal
of Applied Physics 62.6 (1987): 2545-2548.
[8] H. S. Kim, E. Plis, A. Khoshakhlagh, S. Myers, N. Gautam, Y. D. Sharma,L.R. Dawson, S.
Krishna, S. J. Lee, S. K. Noh. Performance improvement of InAs/GaSb strained layer
superlattice detectors by reducing surface leakage currents with SU-8 passivation. Applied
Physics Letters 96, 033502 (2010).
[9] D. Z-Y. Ting, A. Soibel, L. Höglund, J Nguyen, C.J. Hill, A. Khoshakhlagh, S.D. Gunapala.
Type-II superlattice infrared detectors. Advances in Infrared Photodetectors 84 (2011): 1-57