type-II Superlattice Materials Research Articles

Detector size selection for long-range targeting performance in the extended shortwave infrared band

Interest in the eSWIR band is growing due to focal plane array technology advancements with mercury cadmium telluride and type-II superlattice materials. As design and fabrication processes improve, eSWIR detector size, weight, and power can now be optimized. For some applications, it is desirable to have a smaller detector size. Reduced solar illumination in the 2 to 2.5 μm spectral range creates a fundamental limit to passive imaging performance in the eSWIR band where the resolution benefit of small detectors cannot out-compete the reduced SNR in photon-starved environments. This research explores the underlying theory using signal-to-noise ratio radiometry and modeled target discrimination performance to assess the optimal detector size for eSWIR dependent upon illumination conditions. Finally, we model continuous-wave laser illumination in the eSWIR band to compare the effect of detector size on active and passive imaging for long-range object discrimination.

Carrier concentration and in-plane mobility in both non-intentionally and Si-doped InAsSb and InAs/InAsSb type-II superlattice materials for space-based infrared detectors

Carrier concentration and in-plane mobility in both non-intentionally and Si-doped InAsSb and InAs/InAsSb type-II superlattice materials for space-based infrared detectors

Gradual funnel photon trapping enhanced InAs/GaSb type-II superlattice infrared detector.

InAs/GaSb type-II superlattice materials have attracted in the field of infrared detection due to their high quality, uniformity and stability. The performance of InAs/GaSb type-II superlattice detector is limited by dark noise and light response. This work reports a gradual funnel photon trapping (GFPT) structure enabling the light trapping in the T2SL detector absorption area. The GFPT detector exhibits an efficient broadband responsivity enhancement of 30% and a darker current noise reduction of 3 times. It has excellent passivated by atomic layer deposition and achieves a high detectivity of 1.51 × 1011 cm Hz1/2 at 78 K.

Growth of high material quality InAs/GaSb type-II superlattice for long-wavelength infrared range by molecular beam epitaxy

By optimizing the V/III beam-equivalent pressure ratio, a high-quality InAs/GaSb type-II superlattice material for the long-wavelength infrared (LWIR) range is achieved by molecular beam epitaxy (MBE). High-resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM), and Fourier transform infrared (FTIR) spectrometer are used to characterize the material growth quality. The results show that the full width at half maximum (FWHM) of the superlattice zero-order diffraction peak, the mismatching of the superlattice zero-order diffraction peak between the substrate diffraction peaks, and the surface roughness get the best results when the beam-equivalent pressure (BEP) ratio reaches the optimal value, which are 28 arcsec, 13 arcsec, and 1.63 Å, respectively. The intensity of the zero-order diffraction peak is strongest at the optimal value. The relative spectral response of the LWIR detector shows that it exhibits a 100% cut-off wavelength of 12.6 μm at 77 K. High-quality epitaxial materials have laid a good foundation for preparing high-performance LWIR detector.

Fabrication of a 1024 × 1024 format long wavelength infrared focal plane array based on type-II superlattice and barrier enhanced structure

1024 × 1024 format long-wavelength infrared focal plane arrays are realized using type-II superlattice material with a double-barrier enhanced structure grown on 3″ GaSb substrate. The detector structure material exhibits excellent crystalline quality and uniformity. A 100% cutoff wavelength of ~11.8 μm was obtained at 82 K. A dark current density of 1.4 × 10−6 A/cm2 and a differential resistance-area product RA of 4.7 × 104 Ωcm2 at 49 K, 3.6 × 10−5 A/cm2 and 2.8 × 103 Ωcm2 at 70 K, and 2.5 × 10−4 A/cm2 and 5.5 × 102 Ωcm2 at 82 K, at a reverse bias of ~ 50 mV, were obtained, respectively. At zero bias, the R0A was 5.1 × 104 Ωcm2, 290.6 Ωcm2, and 35.6 Ωcm2 at 49 K, 70 K, and 82 K, respectively. Noise equivalent temperature differences of 37.4 mK at 82 K and 31.9 mK at 70 K were achieved with an integration time of 0.12 ms, a 300 K background and f/2.0 optics. The large format FPA also shows excellent imaging.

Vertical carrier transport in strain-balanced InAs/InAsSb type-II superlattice material

Anisotropic carrier transport properties of unintentionally doped InAs/InAs0.65Sb0.35 type-II strain-balanced superlattice material are evaluated using temperature- and field-dependent magnetotransport measurements performed in the vertical direction on a substrate-removed metal-semiconductor-metal device structure. To best isolate the measured transport to the superlattice, device fabrication entails flip-chip bonding and backside device processing to remove the substrate material and deposit contact metal directly to the bottom of an etched mesa. High-resolution mobility spectrum analysis is used to calculate the conductance contribution and corrected mixed vertical-lateral mobility of the two carrier species present. Combining the latter with lateral mobility results from in-plane magnetotransport measurements on identical superlattice material allows for the calculation of the true vertical majority electron and minority hole mobilities; amplitudes of 4.7 ×103 cm2/V s and 1.60 cm2/V s are determined at 77 K, respectively. The temperature-dependent results show that vertical hole mobility rapidly decreases with decreasing temperature due to trap-induced localization and then hopping transport, whereas vertical electron mobility appears phonon scattering-limited at high temperature, giving way to interface roughness scattering at low temperatures, analogous to the lateral electron mobility but with a lower overall magnitude.

Investigation of carrier localization in InAs/AlSb type-II superlattice material system

We investigate carrier localization in the InAs/AlSb type-II superlattice (T2SL) material system using temperature- and excitation power (Iex)-dependent photoluminescence (PL). Evidence of carrier localization in T2SLs was observed by an S-shaped temperature dependence of the PL peak position. Analysis of the Iex-dependent PL at various temperatures also shows the existence of carrier localization in the T2SLs. The thermal activation energies in T2SLs were extracted to identify the nonradiative recombination mechanisms and the possible origins of localized states. We found that there are two thermal activation energies, E1 = 8.2–1.2 meV and E2 = ∼60 meV at various Iex. We interpret E1 as a thermal activation energy that comes from Anderson localization, associated with roughness due to As2 diffusion into the interfaces. This is because the extracted E1 values are comparable to the exciton binding energy of localization in various quantum structures. Carrier trapping at a state in the InSb interfacial layer (Tamm state) may account for the origin of E2. Based on previous reports, we believe that the 60 meV state might be a Tamm state if we consider thickness variations in the InSb interfacial layer for the T2SLs.

Carrier reduction studies of type-II superlattice materials for very long wavelength infrared sensing

There are continuing efforts to develop type-II superlattice (SL) materials for very long wavelength infrared (VLWIR) detector applications. However, the SLs have high residual electron background doping densities that depend on SL growth conditions, which lead to shorter minority carrier lifetime and lower performance parameters than theoretically predicted. In this study, the authors compare the technical advantages of using InAs/GaInSb over InAs/GaSb SL with respect to reducing the electron doping levels. Our temperature-dependent electrical transport measurements show that the InAs/GaInSb SL design has a lower electron density than the InAs/GaSb SL with the same bandgap and have electron densities (mobilities) on the order of the mid 1011 cm−2 (25 000 cm2/V s). Since small period InAs/GaInSb SLs also produce greater Auger recombination suppression for a given VLWIR gap than the large period InAs/GaSb SL, the InAs/GaInSb SL appears to be a better candidate for long lifetime IR materials for future very long wavelength infrared devices.

Visible-extended mid-infrared wide spectrum detector based on InAs/GaSb type-Ⅱ superlattices (T2SL)

The authors report an innovative design of wide spectrum detector that can detect visible and mid-infrared spectrum, from 0.4 to 5 μm, simultaneously. The detector is designed with microstructure of various types of photon traps fabricated on mid-infrared InAs/GaSb type-II superlattice materials. Quantum efficiency has been significantly improved due to the action of photon traps which can decrease the reflectivity of the surface of InAs/GaSb type-Ⅱ superlattice down to less than 5%. Under the condition of 77 K and 200 mV bias voltage, the responsivity of the detector is about 0.025–0.45 A/W in the visible wavelength regime of 400–790 nm, more than 0.3 A/W in spectrum of 800–2000 nm, respectively. Through the optimized photon trap structure, the responsivity of detector is to 1.25 A/W and predominantly improve the quantum efficiency of the detector up to 52.5% at 3 μm.

InAs-based InAs/GaAsSb type-II superlattices: Growth and characterization

In this paper, the authors reported on the growth of InAs/GaAsSb type-II superlattice materials on InAs substrates. The lattice mismatch between the T2SL materials and the InAs substrates was tuned by varying As beam equivalent pressure and thus the As composition in GaAsSb. The lattice mismatch was as small as −6.5×10−5 when As BEP was 8.0×10−6Torr. T2SL materials with different InAs thicknesses were then grown and characterized by high resolution X-ray diffraction. The results showed that the lattice mismatch of the superlattices to the InAs substrates was insensitive to the InAs layer thickness in each period. Finally, a P–I–N InAs/GaAsSb T2SL photodiode was grown on an InAs substrate. The photodiode with 14ML InAs and 7ML GaAsSb in each period showed a 100% cutoff wavelength of 6.5μm at 77K. The dark current density at −30mV bias was 6.05×10−5A/cm2 and the resistance–area product at zero bias (R0A) was 435Ωcm2.The black body detectivity and peak detectivity were 1.34×1011cmHz1/2/W and 3.72×1011cmHz1/2/W, respectively. The quantum efficiency at 5.0μm was measured to be 28%.

Carrier transport properties of Be-doped InAs/InAsSb type-II infrared superlattices

The InAs/InAsSb type-II superlattice materials studied to date for infrared detector applications have been residually n-type, but p-type absorber regions with minority carrier electrons can result in increased photodiode quantum efficiency, RoA, and detectivity. Therefore, Be-doped InAs/InAsSb superlattices were investigated to determine the p-type InAs/InAsSb superlattice material transport properties essential to developing high quality photodiode absorber materials. Hall measurements performed at 10 K revealed that the superlattice converted to p-type with Be-doping of 3 × 1016 cm−3 and the hole mobility reached 24 400 cm2/Vs. Photoresponse measurements at 10 K confirmed the 175 meV bandgap and material optical quality.

Barrier infrared detectors

AbstractIn 1959, Lawson and co-workers publication triggered development of variable band gap Hg1−xCdxTe (HgCdTe) alloys providing an unprecedented degree of freedom in infrared detector design. Over the five decades, this material system has successfully fought off major challenges from different material systems, but despite that it has more competitors today than ever before. It is interesting however, that none of these competitors can compete in terms of fundamental properties. They may promise to be more manufacturable, but never to provide higher performance or, with the exception of thermal detectors, to operate at higher temperatures.In the last two decades a several new concepts of photodetectors to improve their performance have been proposed including trapping detectors, barrier detectors, unipolar barrier photodiodes, and multistage detectors. This paper describes the present status of infrared barrier detectors. It is especially addressed to the group of III-V compounds including type-II superlattice materials, although HgCdTe barrier detectors are also included. It seems to be clear that certain of these solutions have merged as a real competitions of HgCdTe photodetectors.

Growth and Characterization of Long-Wavelength Infrared Type-II Superlattice Photodiodes on a 3-in GaSb Wafer

We report the molecular beam epitaxial growth and characterization of high-performance Type-II superlattice photodiodes on a 3-in GaSb substrate for long-wavelength infrared detection. A 7.3-μm-thick device structure shows excellent structural homogeneity as demonstrated by atomic force microscopy and X-ray diffraction characterization. Optical and electrical measurements of the photodiodes reveal not only the uniformity of the Type-II superlattice material but also of the fabrication process. Across the wafer, at 77 K, photodiodes with a 50% cut-off wavelength of 11 μm exhibit more than 45% quantum efficiency and a dark current density of 1.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 50 mV, giving a specific detectivity of 6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> /W.

Characterization of LWIR diodes on InAs/GaSb Type-II superlattice material

Long wavelength infrared (LWIR) focal plane arrays (FPAs) built on Type-II strained layer InAs/GaSb superlattice materials are emerging as an alternative to LWIR HgCdTe. We have made progress in the development of this technology in a collaborative effort between Raytheon Vision Systems and Jet Propulsion Laboratory, resulting in successful devices with LWIR cutoff wavelengths. We report here two investigations related to wafer processing and superlattice material characteristics. The critical interface between the superlattice and the silicon dioxide passivation was examined at the atomic scale by high resolution transmission electron microscopy (HRTEM), showing a conformal coating on an InAs/GaSb mesa sidewall, which undulates with the superlattice periodicity due to differential etching. Electron energy loss spectroscopy (EELS) showed that oxides of the superlattice elements were present but minimal, and some occasional arsenic precipitates were observed at the passivation interface. Our previous analysis of the current–voltage curves was extended further to reveal the minority carrier lifetimes responsible for producing the generation–recombination (GR) and the diffusion dark currents. Lifetimes at 78 K were found to be 6 and 20 ns in the GR and diffusion processes, respectively. Lifetimes from both mechanisms track together with temperature. A HgCdTe diode was analyzed in the same manner for comparison.