In recent years, Type II InAs/GaSb superlattices grown on GaSb substrate have achieved significant
advances in both structural design and material growth, making Type II superlattice infrared detector
a rival competitor to the state-of-the-art MCT technology. However, the limited size and strong
infrared absorption of GaSb substrates prevent large format type-II superlattice infrared imagers from
being realized. In this work, we demonstrate type-II superlattices grown on GaAs substrates, which is
a significant step toward third generation infrared imaging at low cost. The device performances of
Type II superalttice photodetectors grown on these two substrates are compared.
The Type-II InAs/GaSb superlattice photon detector is an attractive alternative to HgCdTe photodiodes and QWIPS. The use of p+ - π - M - N+ heterodiode allows for greater flexibility in enhancing the device performance. The utilization of the Empirical Tight Binding method gives the band structure of the InAs/GaSb superlattice and the new M- structure
(InAs/GaSb/AlSb/GaSb) superlattice allowing for the band alignment between the binary superlattice and the M- superlattice to be determined and see how it affects the optical performance. Then by modifying the doping level of the M- superlattice an optimal level can be determined to achieve high detectivity, by simultaneously improving both photo-response and reducing dark current for devices with cutoffs greater than 14.5 μm.
A challenge for type-II InAs/GaSb superlattice (T2SL) photodetectors is to achieve high fill factor, high aspect ratio
etching for third generation focal plane arrays (FPAs). Initially, we compare the morphological and electrical results of
single element T2SL photodiodes after BCl3/Ar inductively coupled plasma (ICP) and electron cyclotron resonance
(ECR) dry etching. Using a Si3N4 hard mask, ICP-etched structures exemplify greater sidewall verticality and
smoothness, which are essential toward the realization of high fill factor FPAs. ICP-etched single element devices with
SiO2 passivation that are 9.3μm in cutoff wavelength achieved vertical sidewalls of 7.7μm in depth with a resistance area product at zero bias of greater than 1,000 Ωcm2 and maximum differential resistance in excess of 10,000 Ωcm2 at 77K. By only modifying the etching technique in the fabrication steps, the ICP-etched photodiodes showed an order of
magnitude decrease in their dark current densities in comparison to the ECR-etched devices. Finally, high aspect ratio
etching is demonstrated on mutli-element arrays with 3μm-wide trenches that are 11μm deep.
Recent advances in the design and fabrication of Type-II InAs/GaSb superlattices allowed the
realization of high performance long wavelength infrared focal plane arrays. The introduction of an Mstructure
barrier between the n-type contact and the π active region reduced the tunneling component of the
dark current. The M-structure design improved the noise performance and the dynamic range of FPAs at
low temperatures. At 81K, the NEDT of the focal plane array was 23 mK. The noise of the camera was
dominated by the noise component due to the read out integrated circuit. At 8 μm, the median quantum
efficiency of the detectors was 71%, mainly limited by the reflections on the backside of the array.
LWIR and VLWIR type II InAs/GaSb superlattice photodetectors have for long time suffered from a
high dark current level and a low dynamic resistance which hampers the its emergence to the infrared
detection and imaging industry. However, with the use of M-structure superlattice, a new type II
binary InAs/GaSb/AlSb superlattice design, as an effective blocking barrier, the dark current in type II
superlattice diode has been significantly reduced. We have obtained comparable differential resistance
product to the MCT technology at the cut-off wavelength of 10 and 14μm. Also, this new design is
compatible with the optical optimization scheme, leading to high quantum efficiency, high special
detectivity devices for photon detectors and focal plane arrays.
Recent progress made in the structure design, growth and processing of Type-II InAs/GaSb
superlattice photo-detectors lifted both the quantum efficiency and the R0A product of the detectors.
Type-II superlattice demonstrated its ability to perform imaging in the Mid-Wave Infrared (MWIR)
and Long-Wave Infrared (LWIR) ranges, becoming a potential competitor for technologies such as
Quantum Well Infrared Photo-detectors (QWIP) and Mercury Cadmium Telluride (MCT). Using an
empirical tight-binding model, we developed superlattices designs that were nearly lattice-matched to
the GaSb substrates and presented cutoff wavelengths of 5 and 11 μm. We demonstrated high quality
material growth with X-ray FWHM below 30 arcsec and an AFM rms roughness of 1.5 Å over an
area of 20x20 μm2. The detectors with a 5 μm cutoff, capable of operating at room temperature,
showed a R0A of 1.25 106 Ω.cm2 at 77K, and a quantum efficiency of 32%. In the long wavelength
infrared, we demonstrated high quantum efficiencies above 50% with high R0A products of 12 Ω.cm2
by increasing the thickness of the active region. Using the novel M-structure superlattice design, more
than one order of magnitude improvement has been observed for electrical performance of the
devices. Focal plane arrays in the middle and long infrared range, hybridized to an Indigo read out
integrated circuit, exhibited high quality imaging.
In recent years, Type-II InAs/GaSb superlattice photo-detectors have experienced significant improvements in material
quality, structural designs, and imaging applications. They now appear to be a possible alternative to the state-of-the-art
HgCdTe (MCT) technology in the long and very long wavelength infrared regimes. At the Center for Quantum Devices,
we have successfully realized very high quantum efficiency, very high dynamic differential resistance R0A product
LWIR Type-II InAs/GaSb superlattice photodiodes with efficient surface passivation techniques. The demonstration of
high quality LWIR Focal Plane Arrays that were 100% fabricated
in-house reaffirms the pioneer position of this university-based laboratory.
In the past few years, significant progress has been made in the structure design, growth and
processing of Type-II InAs/GaSb superlattice photodetectors. Type-II superlattice demonstrated its ability
to perform imaging in the middle and long infra-red range, becoming a potential competitor for
technologies such as QWIP and HgCdTe. Using an empirical tight-binding model, we developed a
superlattice design that matches the lattice parameter of GaSb substrates and presents a cutoff wavelength
of 12 &mgr;m. Electrical and optical measurements performed on single element detectors at 77 K showed an
R0A averaging 13 &OHgr;.cm2 and a quantum efficiency as high as 54%. We demonstrated high quality material
growth with x-ray FWHM below 30 arcsec and an AFM rms roughness of 1.5 Å over an area of
20x20 &mgr;m2. A 320x256 array of 25x25&mgr;m2 pixels, hybridized to an Indigo Read Out Integrated Circuit,
performed thermal imaging up to 185 K with an operability close to 97%. The noise equivalent temperature
difference at 81 K presented a peak at 270 mK, corresponding to a mean value of 340 mK.
Infrared sensors utilizing Type II superlattice structures have gained increased attention in the past few years.
With the stronger covalent bonds of the III-V materials, greater material uniformity over larger areas is obtained as
compared to the weaker ionic bonding of the II-VI materials. Results obtained on GaSb/InAs Type II superlattices have
shown performance comparable to HgCdTe detectors, with the promise of higher performance due to reduced Auger
recombination and dark current through improvements in device design and material quality. In this paper, we discuss
advancements in Type II IR sensors that cover the 3 to >30 μm wavelength range. Specific topics covered will be device
design and modeling using the Empirical Tight Binding Method (ETBM), material growth and characterization, device
fabrication and testing, as well as focal plane array processing and imaging. We demonstrate high quality material with
PL linewidths of ~20 meV, x-ray FWHM of 20-40 arcsec, and AFM rms roughness of 1~.2 Å over a 20 μm×20μm area.
Negative luminescence at 10 μm range is demonstrated for the first time. Device external quantum efficiency of >30%,
responsivity of ~2A/W, and detectivity of 1011 Jones at 77K in the 10 μm range are routinely obtained. Imaging has been
demonstrated at room temperature for the first time with a 5 μm cutoff wavelength 256×256 focal plane array.
Recently, several groups have investigated the aspects of positive and negative luminescence behavior in infrared materials. Under forward bias voltage, charge carriers are injected into the active region of a p-n junction, giving rise to positive luminescence. In contrast, a p-n junction under reverse bias conditions can exhibit negative luminescence caused by a reduction of the electron-hole recombination of the device, such that the photon flux is below that of the black body emission in equilibrium. In the present work, we show measurements of both positive and negative luminescence of binary Type II InAs/GaSb superlattice photodiodes in the 3 to 13 μm spectral range. Through a radiometric calibration technique, we demonstrate temperature independent negative luminescence efficiencies of 45 % in the midwavelength (MWIR) sample from 220 K to 320 K without anti-reflective coating and values reaching 35 % in the long wavelength infrared (LWIR) spectrum sample. With the radiative recombination constant obtained in the framework of k • p theory a model is obtained to describe the temperature dependent behavior of the results near thermal equilibrium in both samples. In the long wavelength regime, we demonstrate that the dominant non-radiative recombination channel in n-type material is Auger recombination with an electron-hole-electron (CHCC) Auger recombination coefficient of Cn = 1 x 1024 cm6s-1. While in the mid wavelength infrared window, the primary non-radiative recombination is Shockley-Read-Hall recombination giving rise to a p-type residual background capture cross-section of σn = 7 x 10-16 cm2.
Nanopillar devices have been fabricated from GaInAs/InP QWIP material grown by MOCVD. Using electron beam lithography and reactive ion etching techniques, large, regular arrays of nanopillars with controllable diameters ranging from 150 nm to less than 40 nm have been reproducibly formed. Photoluminescence experiments demonstrate a strong peak wavelength blue shift for nanopillar structures compared to the as-grown quantum well material. Top and bottom metal contacts have been realized using a polyimide planarization and etchback procedure. I-V and noise measurements have been performed. Optical measurements indicate photoconductive response in selected nanopillar arrays. Device peak wavelength response occurs at about 8 μm with peak device responsivity of 420 mA/W. Peak detectivity of 3×108 cmHz1/2/W has been achieved at -1V bias and 30 K.
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