The tri-service Vital Infrared Sensor Technology Acceleration (VISTA) program rapidly matured III-V semiconductor epitaxy to produce tactically viable detectors using Type II Superlattice (T2SL) structures. The T2SL material system allows tunable band gaps for creating lattice-matched heterojunction devices. Heterojunction devices are integral to suppressing sources of dark currents, such as internal Shockley Reed Hall (SRH) and device surface currents. Once the VISTA program demonstrated that T2SL detectors offered competitive performance to traditional indium antimonide (InSb) detectors at an operating temperature 40K to 50 K higher, many opportunities emerged. This elevation in operating temperature provides two benefits to infrared (IR) sensors. The first is to miniaturize the integrated Dewar-electronicscooler assembly (IDECA) such that it can support small aerial vehicle and soldier mounted sensors. The second is to increase the mean time to failure (MTTF) of an existing InSb IDECA. To benefit from T2SL higher operating temperature (HOT) detectors, the overall cost of the IDECA must be competitive with InSb. This drives a manufacturing capability that is equivalent to InSb. At the L3 Space and Sensors Technology Center (L3 SSTC), the III-V detector foundry processes 125 mm diameter InSb wafers. The development of 125 mm diameter T2SL detector wafers started with the gallium antimonide substrates. The greater size and weight of these substrates required extra care to avoid breakage. Leveraging the learning reported from the silicon industry, we developed a specification for the substrate thickness and edge bevel to provide a robust platform for wafer processing. Next, we worked with commercial III-V epitaxy suppliers to develop multi-wafer growth capability for 125 mm diameter substrates. The results of this effort, funded by the Office of the Secretary of Defense (OSD) Defense-wide Manufacturing Science and Technology (DMST) program through the Army Night Vision and Electronic Sensor Directorate (NVESD), we were able to improve focal plane array (FPA) yield from virtually zero to InSb manufacturing levels.
We are developing resonator-QWIPs for long wavelength applications. Detector pixels with 25 μm pitch were
hybridized to fanout circuits for radiometric measurements. With a moderate doping of 0.5 x 1018 cm<sup>-3</sup>, we achieved a
quantum efficiency of 37% and conversion efficiency of 15% in a 1.3 μm-thick active material and 35% QE and 21%
CE in a 0.6 μm-thick active material. Both detectors are cutoff at 10.5 μm with a 2 μm bandwidth. The temperature at
which photocurrent equals dark current is about 65 K under F/2 optics. The thicker detector shows a large QE polarity
asymmetry due to nonlinear potential drop in the QWIP material layers.
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 10<sup>11</sup> 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.
We present our most recent results and review our progress over the past few years regarding InAs/GaSb Type II superlattices for photovoltaic detectors and focal plane arrays. Empirical tight binding methods have been proven to be very effective and accurate in designing superlattices for various cutoff wavelengths from 3.7 μm up to 32 μm. Excellent agreement between theoretical calculations and experimental results has been obtained. High quality material growths were performed using an Intevac modular Gen II molecular beam epitaxy system. The material quality was characterized using x-ray, atomic force microscopy, transmission electron microscope and photoluminescence, etc. Detector performance confirmed high material electrical quality. Details of the demonstration of 256×256 long wavelength infrared focal plane arrays will be presented.
Leakage currents limit the operation of high performance type II InAs/GaSb superlattice photodiode technology. Surface leakage current becomes a dominant limiting factor, especially at the scale of a focal plane array pixel (< 25 μm) and must be addressed. A reduction of the surface state density, unpinning the Fermi level at the surface, and appropriate termination of the semiconductor crystal are all aims of effective passivation. Recent work in the passivation of type II InAs\GaSb superlattice photodetectors with aqueous sulfur-based solutions has resulted in increased R<sub>0</sub>A products and reduced dark current densities by reducing the surface trap density. Additionally, photoluminescence of similarly passivated type II InAs/GaSb superlattice and InAs GaSb bulk material will be discussed.
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 3×10<sup>8</sup> cmHz<sup>1/2</sup>/W has been achieved at -1V bias and 30 K.
Dark current has become a significant limiting factor for the development of the Type II InAs/GaSb superlattices technology. Experimental results showed that at liquid nitrogen temperature the dominating dark current under reverse bias is the generation-recombination current before the tunneling current turns on. Recent research on the source of the dark current indicated that the Auger recombinations might play a very important role in the superlattice diode dark current. With proper design of the superlattice structure, we have been able to reduce the dark current several orders of magnitude in the LWIR range. The superlattice diode performance was also improved dramatically. Infrared focal plane arrays based on these superlattices will also be discussed.
The recent advances in the experimental work on the Type II InAs/GaSb superlattices necessitate a modeling that can handle arbitrary layer thickness as well as different types of interfaces in order to guide the superlattice design. The empirical tight-binding method (ETBM) is a very good candidate since it builds up the Hamiltonian atom by atom. There has been a lot of research work on the modeling of Type II InAs/GaSb superlattices using the ETBM. However, different groups generate very different accuracy comparing with experimental results. We have recently identified two major aspects in the modeling: the antimony segregation and the interface effects. These two aspects turned out to be of crucial importance governing the superlattice properties, especially the bandgap. We build the superlattice Hamiltonian using antimony segregated atomic profile taking into account the interface. Our calculations agree with our experimental results within growth uncertainties. In addition we introduced the concept of Ga<sub>x</sub>In<sub>1-x</sub> type interface engineering, which will add another design freedom especially in the mid-wavelength infrared range (3~7 μm) in order
to reduce the lattice mismatch.
The absorption or emission wavelength in optoelectronic devices such as quantum well infrared photodetectors, quantum cascade lasers, and type II superlattice photodiodes can be controlled by the thickness and composition of the quantum wells that constitute their active layers. By further confining the charge carriers, for instance in a quantum dot, even more control can be gained over energy transitions within the semiconductor crystal. We propose a method for manipulating the semiconductor band structure by confining carriers within nanopillar structures. Using electron beam lithography and dry plasma etching, we can precisely control the pillar placement, density and dimensions, and thus the performance characteristics, of the optoelectronic device. Furthermore, by patterning different size structures, it is possible to create arrays of multi-color devices on the same substrate, a technique that lends itself to large-scale monolithic integration. We demonstrate the fabrication of nanopillar arrays in the GaSb, GaInP, GaInAs, and type II InAs/GaSb superlattice material systems and show initial photoluminescence data, which seems to indicate quantum confinement within these structures.
In the very long wavelength infrared (VLWIR) band, λ>14 microns, the detector materials are currently limited to extrinsic semiconductors. These extrinsic materials can be either heavily doped bulk semiconductor, like silicon or germanium, or a doped quantum well heterostructure. An alternative choice that provides the opportunity for higher temperature operation for VLWIR sensing is an intrinsic material based on a type-II InAs/Ga(In)Sb superlattice. There are many possible designs for these superlattices which will produce the same narrow band gap by adjusting individual layer thicknesses, indium content or substrate orientation. The infrared properties of various compositions and designs of these type-II superlattices have been studied. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection beyond 15 microns. An overview of the status of this material system will be presented. In addition, the latest experimental results for superlattice photodiodes with cut-off wavelengths as long as 30 microns will be covered.
The authors report the most recent progress in Type II InAs/GaSb superlattice materials and photovoltaic detectors developed for focal plane array applications with a cutoff wavelength of ~8 μm. No turn-on of tunneling current was observed even at a reverse bias of -3 V for a 3 μm thick p-i-n photodiodes. The thermally-limited zero bias detectivity under 300 K 2 π FOV was 2~3×10<sup>11</sup> cm•Hz<sup>1/2</sup>/W at liquid nitrogen temperature, with a current responsivity of 2~3 A/W and a mean quantum efficiency of ~50%. Initial passivation using SiO<sub>2</sub> has shown to decrease the dark current by ~30% at a reverse bias of -1 V. The same detector structure was used for focal plane arrays with silicon readout integrated circuit. Concept proof of imaging was demonstrated with a format of 256×256 at liquid nitrogen temperature.
New infrared (IR) detector materials with high sensitivity, multi-spectral capability, improved uniformity and lower manufacturing costs are required for numerous long and very long wavelength infrared imaging applications. One materials system has shown great theoretical and, more recently, experimental promise for these applications: InAs/In<sub>x</sub>Ga<sub>1-x</sub>Sb type-II superlattices. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection beyond 15 microns. The infrared properties of various compositions and designs of these type-II superlattices have been studied. The infrared photoresponse spectra are combined with quantum mechanical modeling of predicted absorption spectra to provide insight into the underlying physics behind the quantum sensing in these materials. Results for superlattice photodiodes with cut-off wavelengths as long as 25 microns will be presented.
The authors report the most recent advances in type II InAs/GaSb superlattice materials and photovoltaic detectors. Lattice mismatch between the substrate and the superlattice has been routinely achieved below 0.1%, and less than 0.0043% as the record. The FWHM of the zeroth order peak from x-ray diffraction has been decreased below 50 arcsec and a record of less than 44arcsec has been achieved. High performance detectors with 50% cutoff beyond 18 micrometers up to 26 micrometers have been successfully demonstrated. The detectors with a 50% cut-off wavelength of 18.8 micrometers showed a peak current responsivity of 4 A/W at 80K, and a peak detectivity of 4.5<SUP>10</SUP> cm x Hz<SUP>1/2</SUP>/W was achieved at 80K at a reverse bias of 110mV under 300K 2(pi) FOV background. Some detectors showed a projected 0% cutoff wavelength up to 28~30 micrometers . The peak responsivity of 3Amp/Watt and detectivity of 4.25<SUP>10</SUP> cm x Hz<SUP>1/2</SUP>/W was achieved under -40mV reverse bias at 34K for these detectors.
We report on the demonstration of high performance p-i-n photodiodes based on type-II InAs/GaSb superlattices operating in the very long wavelength infrared (VLWIR) range at 80 K. Material is grown by molecular beam epitaxy on GaSb substrates with excellent crystal quality as evidenced by x- ray diffraction and atomic force microscopy. The processed devices with a 50% cutoff wavelength of (lambda) <SUB>c</SUB> equals 22 micrometers show a peak current responsivity about 5.5 A/W at 80 K. The use of binary layers in the superlattice has significantly enhanced the uniformity and reproducibility of the energy gap. The 90% to 10% cut-off energy width of these devices is on the order of 2 kT which is about four times smaller compared to the devices based on InAs/Ga<SUB>1-x</SUB>In<SUB>x</SUB>Sb superlattices. Similar photovoltaic devices with cut-off wavelengths up to 25 micrometers have been measured at 80 K. Our experimental results shows excellent uniformity over a three inch wafer area, indicating the possibility of VLWIR focal plane arrays based on type-II superlattices.