At 150 K, the short-wave channel exhibited a quantum efficiency of 55%, a dark current density of 1.0 109 A/cm2at50 mV bias voltage, providing an associated shot noise detectivity of 3.0
Trang 1Demonstration of high performance bias-selectable dual-band
short-/mid-wavelength infrared photodetectors based on type-II
InAs/GaSb/AlSb superlattices
A M Hoang, G Chen, A Haddadi, and M Razeghia)
Center for Quantum Devices, Department of Electrical Engineering and Computer Science, Northwestern
University, Evanston, Illinois 60208, USA
(Received 12 October 2012; accepted 14 December 2012; published online 4 January 2013)
High performance bias-selectable dual-band short-/mid-wavelength infrared photodetector based on
InAs/GaSb/AlSb type-II superlattice with designed cut-off wavelengths of 2 lm and 4 lm was
demonstrated At 150 K, the short-wave channel exhibited a quantum efficiency of 55%, a dark
current density of 1.0 109 A/cm2at50 mV bias voltage, providing an associated shot noise
detectivity of 3.0 1013Jones The mid-wavelength channel exhibited a quantum efficiency of 33%
and a dark current density of 2.6 105A/cm2at 300 mV bias voltage, resulting in a detectivity of
4.0 1011Jones The spectral cross-talk between the two channels was also discussed for further
optimization.V C 2013 American Institute of Physics [http://dx.doi.org/10.1063/1.4773593]
High performance infrared detectors in the short-wave
infrared (SWIR) and mid-wave infrared (MWIR) spectral
bands are highly needed in a number of tracking and
recon-naissance missions A multi-color imager sensing the SWIR
and MWIR in a spatially co-incident fashion and all-in-one
package allows improved target identification, and detection
of chemical signatures specific to a wave-band (e.g., CO2in
the MWIR) that are otherwise not possible with single color
imagers In addition, the combination of the SWIR and
MWIR provides the flexibility to perform active and passive
imaging in a single camera The reflective nature of SWIR
light allows detailed and high contrast objects similar to the
visible spectrum that can be obtained by illuminating a light
source or under night sky radiance known as “night-glow.”
Under low light conditions or situations where use of a light
source is prohibitive, the MWIR may prove to be more
use-ful without requiring active illumination
Bulk semiconductor materials are often limited to a
par-ticular detection window near the semiconductors’ bandgap
and not suitable for multi-color detection For example,
InGaAs compound is the mature technology for the SWIR
regime, but InGaAs is incapable to extend its detection limit
beyond 2.6 lm.1 InSb only covers the MWIR regimes
HgCdTe is a special family which, by changing the molar
fraction of Cd, can tailor the cutoff from SWIR to very long
wavelength infrared (VLWIR) The current state-of-the-art
MWIR/SWIR dual-band detectors are HgCdTe technology
However, HgCdTe which is a II-VI based technology is
reported to suffer from toxicity and high cost.2
Type-II InAs/GaSb/AlSb superlattice3 (T2SL) has
emerged as a candidate highly suitable for multi-spectral
detection due to its versatility in band-gap engineering, while
retaining lattice-matched conditions Based on the stability
and robustness of the mature III–V compound technology,
T2SL has demonstrated the feasibility of covering a large
infrared detection range, from SWIR to VLWIR,47and the
capability of growing complex devices with different
super-lattice structures such as W-structure8 and M-structure.9 T2SL material system has demonstrated high performance SWIR4and MWIR5photodetectors, as well as the dual-band LWIR-LWIR, MWIR-LWIR, MWIR-MWIR photodetec-tors10–12 and imaging.13–15 However, up to date, no dual-band SWIR-MWIR photodetector performance has been reported yet We present, in this letter, the demonstration of high performance dual-band SWIR-MWIR photodetectors, introducing T2SL InAs/GaSb/AlSb a competitor in this active/passive imaging area
Several device architectures of stacked-detectors have been implemented for the dual-band detection, including si-multaneous and nearly sisi-multaneous bias-selectable detec-tors.2Simultaneous detectors require additional contact and therefore are more complicated in device fabrication In this work, we use the structure of two back-to-back p-i-n-n-i-p photodiodes in which the channels can be addressed alterna-tively by changing the polarity of applied bias voltage The schematic diagram of device structure and the band align-ment of superlattices for absorption regions are shown in Fig 1 The SWIR channel is grown on top of the MWIR channel in order to have the response from both channels while the SWIR channel will also play a role as a low-pass filter for the underneath MWIR channel in front-side illumi-nated measurement This structure takes advantage of the simplicity of bipolar stacked photodiodes without additional middle contact and keeps each channel quite independent of each other The first challenge in the realization of two-color SWIR/MWIR type-II photodetectors requires developing separate materials sensitive to the SWIR and MWIR with high performance After that, the integration of co-located photodetectors requires taking different priority order of optimizing optical or electrical performance in different channels Indeed, it is essential to enhance the quantum effi-ciency (QE) of the SWIR channel and at the same time, improve the electrical performance of the MWIR channel
On one hand, the higher the quantum efficiency of SWIR is, the less light can go into the MWIR channel and thus less cross-talk On the other hand, given the difference in band
a) Email: razeghi@eecs.northwestern.edu.
0003-6951/2013/102(1)/011108/4/$30.00 102, 011108-1 V C 2013 American Institute of Physics
Trang 2gap, the electrical performance of SWIR is several orders of
magnitude better than the MWIR channel; a higher operating
temperature is dependent on the electrical performance of
the MWIR channel The strategies of improving individual
channel performance are thus different for each channel;
however, they affect profitably to each other and improve the
overall dual-band photodiode performance
The structure of SWIR channel was previously
described in detail in Ref.4 It is a homojunction p-i-n
pho-todiode with superlattice design of 6/1/5/1 monolayers
(MLs) of InAs/GaSb/AlSb/GaSb This design gives a 50%
cut-off wavelength of 2 lm at 150 K Single color SWIR
diodes were grown to verify the performance as well as
theirs dynamics in response towards the applied bias Under
illumination, the samples showed photoresponse at reverse
and zero bias When we applied slight forward bias, the
pho-tocurrent was still present and it was only completely off
when the forward bias, called open-circuit voltage, reached a
certain value This will later explain the spectrum of the
dual-band SWIR-MWIR detectors For the MWIR channel,
we use the same superlattice structure as in Ref 5 The
absorption region of the MWIR channel is designed with
6.5/12 MLs of InAs/GaSb to have a 50% cut-off wavelength
of 4 lm at 150 K Unlike standard p-i-n homojunction, in
p-p-M-N device architecture, the M barrier was inserted to
block the tunneling current After that, the active region p
was p-doped heavily to suppress the dark current The M
bar-rier was also carefully engineered to eliminate the
misalign-ment in the conduction band which can result in bias
dependence of quantum efficiency.5
designed to consist of 0.5 lm thick bottom p-contact, 2 lm
thick p-doped active region in MWIR, 0.5 lm thick
M-barrier, 1 lm thick common n-contact (0.5 lm MWIR
super-lattice and 0.5 lm SWIR supersuper-lattice), 1 lm undoped SWIR
active region, and 0.5 lm p-contact at the end Prior to the
detector, a 0.5 lm thick p-doped InAsSb lattice-matched to
the GaSb substrate was grown as an etch-stop layer Our
photodetectors were grown on n-type (001) GaSb substrate
using the state-of-the-art molecular beam epitaxy (MBE) equipped with group III SUMOVR
cells and group V valved crackers We introduce silicon (Si) in InAs and beryllium (Be) in GaSb as n-type and p-type dopants, respectively Af-ter the growth, the sample was structurally characAf-terized using the atomic force microscopy (AFM) and high resolu-tion X-ray diffracresolu-tion (HR XRD) The AFM showed a stand-ard morphology with rms roughness of 1.3 A˚ over a
10 10 lm2
area The satellite peaks in the high resolution x-ray diffraction rocking curves show the thicknesses of
62 A˚ and 45 A˚ for each period of MWIR and SWIR active regions, respectively
The photodiodes were then fabricated with linear sizes ranging from 100 to 400 lm The processing technique was described thoroughly elsewhere.16The photodiodes were left unpassivated for optical characterization but care was paid in order to minimize the surface leakage After the processing, the photodiodes were wire-bonded onto a leadless ceramic chip carrier (LCCC) and loaded into a cryostat for optical and electrical characterizations
Shown in Figures2and 3are the electrical and optical performances of the device at 150 K The current-voltage characteristic exhibits the behavior of a SWIR photodiode in the bias range from negative to positive 200 mV Above
250 mV, the I-V curve starts to behave like a MWIR photo-diode when the differential resistance of MWIR becomes greater than SWIR The sample exhibits an absolute dark current density of 1.0 109A/cm2at50 mV The value
of R0A is around 5 107X cm2
At zero or negative bias, the diode shows photo-response at the SWIR channel and the MWIR operation is completely off The SWIR response is still present even when we apply a positive 200 mV bias while the MWIR channel starts showing significant response from 270 mV The quantum efficiency of MWIR channel quickly saturates with 33% at peak responsivity for a 2 lm active region at around 300 mV bias This bias for saturated quantum efficiency is small enough to be suitable for focal plane array application At this bias, the measured dark cur-rent density is 2.6 105A/cm2
Between the bias values of
100 mV and 300 mV, we switch from saturated SWIR optical signal to saturated MWIR optical signal In the middle of this range, the polarity of the photocurrent depends on the
FIG 1 Schematic diagram of a dual-band SWIR-MWIR back-to-back
p-i-n-n-i-p photodiode structure and schematic band alignment of superlattices
in two absorption layers The colored rectangles in the insets represent the
forbidden gap of component materials Dotted lines represent the effective
band gaps of superlattices.
FIG 2 Dark current and differential resistance-area product vs applied bias
of the diode at 150 K SWIR and MWIR arrows represent the operation bias which shows the dominant behavior of each channel.
Trang 3wavelength The photocurrent may be positive for certain
wavelengths and negative for others, depending on which
diode is generating the higher current at a given wavelength
This QE bias dependence of the dual-band
photodetec-tors can be explained by the difference of built-in voltages
and the dynamics of generated photocurrent in the two p-n
junctions The schematic band alignment of a dual-band
SWIR-MWIR p-i-n-n-i-p structure is shown in Fig 4 It is
essential to remind that each optimized individual
photo-diode did not exhibit QE bias dependence when operated
separately Therefore, the origin of QE bias dependence in
this dual-band detector is not from the band misalignment
between different regions that could block the photocurrent
The only band misalignment in this structure is from the
junction in SWIR and MWIR n-contacts However, these
common contacts were heavily n-doped to form an Ohmic
contact, which permits electrons to easily tunnel through a
thin barrier In this device architecture, the QE bias
depend-ence of the channels comes from the competition of built-in
voltages which are the result of the formation of p–n
junc-tions Since the diodes are positioned back to back, the
built-in voltage built-in one channel creates a potential barrier to block
the photo-current of the other channel
The built-in voltage in dark condition is given by
Vbuiltin¼kT
q ln
NaNd
n2 i
where T is the temperature, q is the electron charge, Naand
Ndare the concentrations of acceptors and donors, niis the intrinsic carrier concentration (ni2 / exp(Eg/kT) and is decided by the operating temperature and the band gap of the material, which is fixed for specific requirement of cut-off wavelength A rough estimation shows that if the doping
of Naand Ndis almost the same in the two channels, the dis-crepancy in built-in voltages of the two channels is the dif-ference in band gap Under illumination or applied bias, the generation and redistribution of mobile charges will either reduce or increase the potential barrier At negative or small positive bias (<200 mV), the photoresponse is expected to
be only present for SWIR channel since its built-in potential
is much higher than that of MWIR The photocurrent of MWIR channel is completely blocked by the SWIR built-in voltage In the bias range (200–270 mV), where the disparity
in built-in voltages is reduced, the sensitivity of the diodes at
a specific wavelength will decide the polarity and the magni-tude of photocurrent The MWIR channel absorbs light at the MWIR spectrum and shows a low photoresponse, whereas the SWIR photodiode governs at the wavelength in SWIR re-gime since the applied bias is not enough to reach the open-circuit voltage of SWIR diode In order to fully extract the photocurrent from the MWIR channel and eliminate the SWIR signal, a high positive bias must be applied, this is mainly to make the SWIR channel return to forward opera-tion mode and thus, no more potential barrier is present for the photocurrent generated from MWIR channel The fact that we have to apply large bias to fully extract the photocur-rent from the MWIR channel will also generate more dark current It is, therefore, necessary to reduce this QE high bias dependence As indicated in Eq.(1), higher Naand Ndwill result in a higher built-in voltage As a result, in order to solve the large bias dependence issue, the strategy will be to lower the doping level of the SWIR channel and increase the doping of MWIR channel
Shown in Figure5is the detectivity of the sample in its two operation modes The SWIR channel exhibits high per-formance with the detectivity of 3.0 1013
Jones At
300 mV, the saturated QE in MWIR channel provides a detectivity of 4.0 1011 Jones The cross-talk is not com-pletely eliminated at the SWIR regimes where the MWIR diode still shows response A thicker SWIR absorption region might be a solution to that The high performance of the SWIR channel grown on top of MWIR channel confirms the high quality of T2SL material for complex thick structure
In summary, we reported the demonstration of the bias-selectable dual-band SWIR-MWIR photodetectors based on the type-II InAs/GaSb/AlSb superlattices The mechanism of the QE bias dependence of the dual-band p-i-n-n-i-p photodi-odes was also discussed and the method to reduce this issue was suggested The SWIR channel achieved a quantum effi-ciency of 55% at peak responsivity, a dark current density of 1.0 109 A/cm2 at 50 mV, providing a detectivity of
FIG 3 Quantum efficiency spectrum of the photodiode at 150 K as function
of applied bias The SWIR signal starts to attenuate at 200 mV and the
MWIR signal saturated at 300 mV.
FIG 4 Schematic band diagram at zero bias and under dark condition V S
and V M are the built-in voltages of SWIR and MWIR channels, respectively.
E c , E v , and E F are the conduction band, valence band, and Fermi level at
zero bias.
Trang 43.0 1013 Jones The MWIR channel has its saturated QE
of 33%, a dark current density of 2.6 105 A/cm2
atþ 300 mV, exhibiting a detectivity of 4.0 1011 Jones
This high performance allows us to operate this dual-band
device at higher temperature This work has made it possible
for the InAs/GaSb/AlSb type-II superlattice to become an
active candidate in the area of active/passive imaging
The authors would like to acknowledge Dr Nibir Dhar
from Defense Advanced Research Projects Agency and
Dr Priyalal Wijewarnasuriya from U.S Army Research Lab for their support, interest, and encouragement The authors also would like to thank Simeon Bogdanov for fruitful discussions
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FIG 5 Calculated detectivities of the two channels at 150 K The MWIR
detection operates at 300 mV positive bias The detectivity calculation uses
the equation in the inset, where k is wavelength, g is QE, J is dark current
density, R A is differential resistance-area product, h, c, and K b are basic
constants.