Heterostructure Band Engineering of Type-II InAs/GaSb Superlattice ...

Invited Paper
Heterostructure Band Engineering of Type-II InAs/GaSb Superlattice
based Longwave Infrared Photodiodes using Unipolar Current
Blocking Barriers
N. Gautam a , E. Plis a , H.S. Kim a , M. N. Kutty a , S. Myers, A. Khoshakhlagh a , L. R. Dawson a and
S. Krishna a
a Center for High Technology Materials, Department of Electrical and Computer Engineering,
University of New Mexico, Albuquerque, NM 87106
ABSTRACT
We report heterojunction bandgap engineered long wave infrared (LWIR) photodetectors based on type-II InAs/GaSb
strained layer superlattices (SLS) which show significant improvement in performance over conventional PIN devices.
For this study, a device with unipolar barriers but same absorber region as PIN has been studied and compared. Unipolar
barriers reduce the tunneling currents and SRH recombination current in the active region due to reduced electric field
drop across the active region, while maintaining the photocurrent level. Moreover, they also reduce the diffusion current
by blocking the minority carriers from the two sides of the junction. We report three orders of magnitude reduction in the
dark current with the use of unipolar barriers. The reduction in the dark current results in significant improvement in
signal to noise ratio, resulting in measured specific detectivity of 2×10 10 (cm-√Hz)/W and dark current density of 8.7
mA/cm 2 at -0.5 V applied bias, for the 50% cutoff wavelength of 10.8μm.
Keywords: InAs, GaSb, superlattice, LWIR, infrared, detectors
1. INTRODUCTION
Type-II InAs/GaSb based strained layer superlattice (SLS) system has shown promising progress in the field of infrared
detection over the last few years. The main advantage of this system is the flexibility of changing the bandgap by
changing the thicknesses of InAs and GaSb layers. Due to bandgap tunability, infrared detectors based on type-II SLS in
midwave infrared (MWIR) 1 , longwave infrared (LWIR) 2 and very-longwave infrared (VLWIR) 3 regimes have been
realized. Other advantages of type-II SLS system are reduced Auger recombination rate 4 due to strain induced splitting
of heavy hole and light hole bands, and reduced tunneling currents due to higher effective masses of electrons and holes.
Moreover type-II SLS based detectors have demonstrated high quantum efficiency. Advances in molecular beam epitaxy
based SLS growth promises to deliver uniform and reproducible material for device fabrication. Further performance
enhancement can be done by heterojunction bandstructure engineering of type-II SLS based detectors. A number of
architectures such as nBn 5 , CBIRD 6 and M-structure 7 have already been proposed.
We report here heterojunction bandgap engineered type-II SLS based photodetector in LWIR regime which show
superior performance over conventional PIN detector. This device uses PbIbN architecture which consists of N contact
layer followed by a hole blocking layer “b” and then the absorber region “I”, which is followed by an electron blocking
layer “b” and P contact layer, respectively, in the growth direction. This device architecture reduces diffusion current,
Shockley-Reed Hall (SRH) current and tunneling currents due to the placement of unipolar blocking layers. Figure 1
shows the band diagram for conventional PIN and PbIbN under reverse bias operation. It is to be noted that the
complimentary barrier engineered infrared detector (CBIRD) design reported by Jet Propulsion Lab (JPL) has a very
Infrared Technology and Applications XXXVI, edited by Bjørn F. Andresen, Gabor F. Fulop, Paul R. Norton,
Proc. of SPIE Vol. 7660, 76601T · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.849889
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similar heterostructure design 6 , with n-type top and bottom contacts and bottom contact made of bulk material unlike the
PbIbN structure reported here.
(a)
(b)
Figure 1. (a) Schematics of PIN photodetector under reverse bias (b) schematics of PbIbN detector under reverse
bias operation which shows negligible field drop across active region.
2. MATERIAL GROWTH AND CHARACTERIZATION
Material was grown on Te doped epi-ready (100) GaSb substrate using a VG80 solid source molecular beam epitaxy
system. The system was equipped with SUMO® cells for gallium and indium, a standard effusion cell for aluminum and
cracker cells for antimony and arsenic. Growth rates were calibrated by monitoring the intensity oscillations in the
reflected high-energy electron diffraction (RHEED) patterns and confirmed with X-Ray diffraction (XRD) by growth of
calibration superlattice (SL) samples with different thicknesses of SL periods. The reported PbIbN design has 637 nm
thick N contact layer made of 16ML InAs/ 7ML GaSb doped with Te (n=3×10 18 cm -3 ) followed by non-intentionally
doped (n.i.d) 450 nm thick hole blocking (hB) layer made of 13ML InAs/ 4ML GaSb SLS. This is followed by a 2.2 µm
thick n.i.d absorber region of 14ML InAs/ 7ML GaSb SLS and an eB layer of n.i.d 325 nm thick 8ML InAs/ 8ML GaSb
SLS. A 138 nm thick 13ML InAs/ 8ML GaSb SLS P contact layer doped with Be (p=2.8×10 18 cm -3 ) completes the
structure. For the purpose of illustration, heterostructure schematic and XRD data of PbIbN has been shown in figure 2.
The performance of PbIbN structure has been compared with conventional PIN design consisting of same absorber
region. The PIN device under consideration has 609 nm thick Te doped N contact layer (n = 3×10 18 cm -3 ) of 14ML InAs/
7ML GaSb SLS followed by 159 nm thick graded n-doping region. This is followed by 2 µm thick n.i.d absorber region
of 14ML InAs/ 7ML GaSb SLS, followed by p-type graded doping region, 159 nm thick, of 14/7 SLS. The top most
layer is 100nm thick Be doped GaSb P contact layer (p = 2.8×10 18 cm -3 ).
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138nm 13ML/8ML
(p = 2.8×10 18 cm -3 )
325nm eB 8ML/8ML (n.i.d.)
GaSb substrate
2.2μm 14ML/7ML SLS (n.i.d.)
450nm hB 13ML/4ML SLS
637nm 16ML/7ML SLS
(n=3×10 18 cm -3 )
GaSb:Te
Counts/s
10 6
10 5
10 4
10 3
28 29 30 31 32 33
10 7 Ω/2θ ( ο )
(a)
(b)
Figure 2. (a) Heterostructure schematic PbIbN device, (b) XRD plot of PbIbN
3. DEVICE FABRICATION AND CHARATERIZATION
Material was processed into 410×410 µm 2 mesas single pixel devices with varying circular aperture from 25 to 300 µm
in diameter using conventional deep mesa etch. Standard photolithography and dry inductively coupled plasma (ICP)
etching was used. SU8 passivation was performed to reduce surface leakage currents and finally top and bottom contacts
were made by metal deposition,
Spectral response measurements for these devices were performed at 77K using Nicolet 670 Fourier transform infrared
(FTIR) spectrometer and a Keithley 428 preamplifier. Figure 3 shows the spectral response measurements of PIN and
PbIbN devices, with 50% cutoff wavelength specified on the plot. Radiometric measurements were carried out using a
band pass filter (8.4 µm-11.5µm), in order to obtain the responsivity and detectivity in LWIR range.
.
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Spectral Response (a.u.)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
λc=10.8 μm
PbIbN
PIN
λc=11.0 μm
0.0
8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5
Wavelength (μm)
Figure 3. Spectral response of PIN and PbIbN devices at 77K with cutoff wavelength of 11.0 µm and 10.8 µm
respectively.
Current-voltage (IV) characteristics were measured for all the devices from 30K to 250K using HP4145 semiconductor
parameter analyzer. Figure 4(a) shows temperature dependent dark current characteristics for PbIbN detector and figure
4(b) compares it with PIN detector at 77K. At lower temperatures, these devices show asymmetric diode characteristic
while at higher temperatures the I-V curves become symmetric owing to the dark current due to thermal generation of
carriers.
J (A/cm 2 )
100
10
1
0.1
0.01
1E-3
1E-4
1E-5
-3 -2 -1 0 1 2 3
Bias (V)
(a)
77K
150K
200K
250K
J (A/cm 2 )
100
10
1
0.1
0.01
1E-3
1E-4
1E-5
1E-6
77K
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Bias (V)
(b)
PIN
PbIbN
Figure 4. (a) Variable temperature dark current characteristics of PbIbN detector, (b) dark current density comparison
of PbIbN design with PIN design at 77K.
As shown in figure 4(b), PbIbN design shows significantly improved performance over PIN detector. The major sources
of dark current in a PIN detector are thermal generation of carriers, SRH current, tunneling currents and minority carrier
diffusion current. In PbIbN design, an electron barrier of wider bandgap material is placed between P contact layer and
absorber region I. When the detector is operated under reverse bias, photogenerated holes move towards P contact while
electrons move towards N contact. The eB layer allows unimpeded flow of holes while blocking electrons and also leads
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to majority of electric field drop across the eB layer and reduced field drop across the active region as compared to PIN
diode. Due to significant reduction in field drop in active region, smaller bandgap material, tunneling currents are
reduced and also activity of SRH centers in the active region is reduced leading to reduction in dark current. Further, eB
layer blocks the minority carrier diffusion from P contact layer into the active region and hence reduces dark current.
Similarly, hB layer offers barrier for holes while it allows unimpeded flow of electrons when the device is operated in
reverse bias, similar to the role played by eB for holes. Field drop across the active region is further reduced as there are
barriers on either side, made of wider bandgap material, which reduce SRH and tunneling dark currents as mentioned
above. Also, hB layer reduces dark current further by blocking minority carrier diffusion from N contact layer into the
active region. The PbIbN design has all the layers made of SLS, which is a clear demonstration of bandgap as well as
band offset tunability in this system by merely changing the thicknesses of InAs and GaSb layers. PbIbN design shows
dark current reduction, over PIN design, by more than two orders of magnitude. At -0.5V, PbIbN reduces it by factor of
36 over PIN design. However, this reduction is much more significant at lower bias values, for example, at -50 mV
PbIbN shows more than three orders of magnitude reduction in dark current.
Radiometric measurements were carried out at 77K and as mentioned earlier, longwave bandpass filter was used for all
the measurements. It is to be noted that photocurrent due to 300K background under 2π field of view (FOV) was at least
a factor of four higher than the dark current implying background limited (BLIP) operation 9 at 77K for all the three
devices. The specific detectivity (D*) was measured for 2π FOV under BLIP condition, and was scaled to f/2 optics.
Figure 5(a) shows measured D* (f/2 FOV), not calculated from shot noise, and 5(b) shows 77K responsivity and external
quantum efficiency (QE) for PbIbN device at 10 µm wavelength.
77K
4
77K
40
10 9
10 8
D*(cm-Hz 1/2 /W) 1010
Measured D* 2π FOV
Measured D* F/2 FOV
Responsivity (A/W)
3
2
1
30
20
10
QE (%) at 10 μm
.
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
Bias (V)
(a)
0
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
Bias (V)
(b)
Figure 5. (a) Plot of measured specific detectivity, (b) responsivity and quantum efficiency of PbIbN detector at 77K.
As shown in figure 5(a), peak D* for PbIbN design is 2×10 10 (cm-√Hz)/W at -0.5 V. The measured value of responsivity
at this bias is 3.4 A/W and QE of 35% is attained at 10 µm wavelength. It is to be noted that the calculated value of shotnoise
limited D* is 5.2×10 10 (cm-√Hz)/W for 2π FOV.
4. CONCLUSIONS
It has been shown that heterojunction type-II SLS based LWIR photodetectors with unipolar current blocking layers
show improved performance over conventional PIN devices. The reported barrier engineered device with two unipolar
barrier layers, contact layers made of SLS showed much lower dark current and better signal to noise ratio due to optimal
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and lineup. Nature of unipolar current blocking layer was investigated and dark current reduction mechanisms were
discussed. The peak D* for the PbIbN design is 2×10 10 (cm-√Hz)/W at -0.5 V. The measured value of responsivity at this
bias is 3.4 A/W and QE of 35% for 10 µm wavelength.
Work supported by AFRL, AFOSR and MDA.
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