Vincent M. Cowan

Low-temperature noise measurements of an InAs/GaSb-based nBn MWIR detector

Recent experiments on conventional p-on-n and n-on-p Type II superlattices (SLS) infrared detectors still indicate larger than theoretically predicted dark current densities, despite the well known suppression of the Auger recombination mechanism. Rather, dark current in SLS is thought to still be limited by trap-assisted tunneling in the depletion region and surface leakage currents resulting from lack of fully passivated mesa sidewalls. An emerging infrared detector technology utilizing a unipolar, single-band barrier design, the so-called nBn architecture, potentially suppresses these remaining noise current mechanisms. In this report, measurements of the noise current spectral density of a mid-wave infrared nBn detector, composed of a type-II InAs/GaSb strain layer superlattice (SLS) absorber (n) and contact (n) layers with an AlGaSb barrier (B), under low-temperature, low-background conditions are presented. Here, noise was measured using a transimpedance amplifier incorporating a dewar-mounted feedback resistor RF and source-follower MOSFET, both held at 77 K. This configuration confines high detector impedance issues to the dewar, minimizes Johnson noise due to the electronics, and enhances bandwidth by reducing stray capacitance. Features of the detectors noise spectrums at different bias are examined.

Diffusion current characteristics of defect-limited nBn mid-wave infrared detectors

Mid-wave infrared, nBn detectors remain limited by diffusion current generated in the absorber region even when defect concentrations are elevated. In contrast, defect-limited conventional pn-junction based photodiodes are subject to Shockley-Read-Hall generation in the depletion region and subsequent carrier drift. Ideal nBn-architecture devices would be limited by Auger 1 generation; however, typical nBn detectors exhibit defect-dominated performance associated with Shockley-Read-Hall generation in the quasi-neutral absorbing region. Reverse saturation current density characteristics for defect-limited devices depend on the minority carrier diffusion length, absorbing layer thickness, and the dominant minority carrier generation mechanism. Unlike pn-based photodiodes, changes in nBn dark current due to elevated defect concentrations do not manifest at small biases, thus, the zero bias resistance area product, RoA, is not a useful parameter for characterizing nBn-architecture photodetector performance.

Comparison of superlattice based dual color nBn and pBp infrared detectors

Long-wave infrared (LWIR) detector technologies with the ability to operate at or near room temperature are very important for many civil and military applications including chemical identification, surveillance, defense and medical diagnostics. Eliminating the need for cryogenics in a detector system can reduce cost, weight and power consumption; simplify the detection system design and allow for widespread usage. In recent years, infrared (IR) detectors based on uni-polar barrier designs have gained interest for their ability to lower dark current and increase a detectors operating temperature. Our group is currently investigating nBn and pBp detectors with InAs/GaSb strain layer superlattice (SLS) absorbers (n) and contacts (n), and AlGaSb and InAs/AlSb superlattice electron and hole barriers (B) respectively. For the case of the nBn structure, the wide-band-gap barrier material (AlGaSb) exhibits a large conduction band offset and a small valence band offset with the narrow-band-gap absorber material. For the pBp structure (InAs/AlSb superlattice barrier), the converse is true with a large valence band offset between the barrier and absorber and a small or zero conduction band offset. Like the built-in barrier in a p-n junction, the heterojunction barrier blocks the majority carriers allowing free movement of photogenerated minority carriers. However, the barrier in an nBn or pBp detector, in contrast with a p-n junction depletion layer, does not contribute to generation-recombination (G-R) current. In this report we aim to investigate and contrast the performance characteristics of an SLS nBn detector with that of and SLS pBp detector.

A recent review of mid-wavelength infrared type-II superlattices: carrier localization, device performance, and radiation tolerance

The last two decades have seen tremendous progress in the design and performance of mid-wavelength infrared (MWIR) type-II superlattices (T2SL) for detectors. The materials of focus have evolved from the InAs/(In)GaSb T2SL to include InAs/InAsSb T2SLs and most recently InGaAs/InAsSb SLs, with each materials system offering particular advantages and challenges. InAs/InAsSb SLs have the longest minority carrier lifetimes, and their best nBn dark current densities are <5X Rule ’07 at high temperatures, while those of InAs/GaSb SLs and InGaAs/InAsSb SLs are <10X Rule ’07. The quantum efficiency of all three SL detectors can still be improved, especially by increasing the diffusion length beyond the absorber length at low temperatures. Evidence of low temperature carrier localization is greatest for the two SLs containing ternary layers; however, the interface intermixing causing the localization is present in all three SLs. Localization likely does not affect the high temperature detector performance (>120 K) where these SL unipolar barrier detectors are diffusion-limited and Auger-limited. The SL barrier detectors remain diffusion-limited post proton irradiation, but the dark current density increases due to the minority carrier lifetime decreasing with increased displacement damage causing an increase in the trap density. For these SL detectors to operate in space, the continued understanding and mitigation of point defects is necessary.

Gamma-ray irradiation effects on InAs/GaSb-based nBn IR detector

IR detectors operated in a space environment are subjected to a variety of radiation effects while required to have very low noise performance. When properly passivated, conventional mercury cadmium telluride (MCT)-based infrared detectors have been shown to perform well in space environments. However, the inherent manufacturing difficulties associated with the growth of MCT has resulted in a research thrust into alternative detector technologies, specifically type-II Strained Layer Superlattice (SLS) infrared detectors. Theory predicts that SLS-based detector technologies have the potential of offering several advantages over MCT detectors including lower dark currents and higher operating temperatures. Experimentally, however, it has been found that both p-on-n and n-on-p SLS detectors have larger dark current densities than MCT-based detectors. An emerging detector architecture, complementary to SLS-technology and hence forth referred to here as nBn, mitigates this issue via a uni-polar barrier design which effectively blocks majority carrier conduction thereby reducing dark current to more acceptable levels. Little work has been done to characterize nBn IR detectors tolerance to radiation effects. Here, the effects of gamma-ray radiation on an nBn SLS detector are considered. The nBn IR detector under test was grown by solid source molecular beam epitaxy and is composed of an InAs/GaSb SLS absorber (n) and contact (n) and an AlxGa1-xSb barrier (B). The radiation effects on the detector are characterized by dark current density measurements as a function of bias, device perimeter-to-area ratio and total ionizing dose (TID).

Empirical trends of minority carrier recombination lifetime vs proton radiation for rad-hard IR detector materials

The continuous effort to improve space-based infrared (IR) detectors has led to a search for greater fundamental understanding of radiation damage phenomena effects on key material properties. The material parameter of interest in this paper is the minority carrier recombination lifetime (MCRL), which is directly related to detector performance and can be empirically determined. As radiation damage is incurred upon a detector structure, the MCRL can be significantly affected, and tracking this in a step-wise, in-situ fashion at a radiation source can reveal rates of defect introduction. This has been accomplished by the development of a portable MCRL measurement system employing time resolved photoluminescence (TRPL) while maintaining operational temperatures. Using this methodology is more insightful than the so-called ‘bag tests’ (i.e. characterization before and after a single 100krad dosage) due to complex parameter changes witnessed with annealing as temperatures change. In addition to the system description, MCRL data on IR detectors from its inaugural deployments at a proton radiation source are analyzed and reveal a linear relationship between inverse MCRL and proton fluence.

High operating temperature midwave infrared (MWIR) photodetectors based on type II InAs/GaSb strained layer superlattice

Midwave infrared (MWIR) photodetectors that do not require cryogenic cooling would significantly reduce the complexity of the cooling system, which would lead to a reduction in the size, weight, and cost of the detection system. The key aspect to realize high operating temperature (HOT) photodetectors is to design device structures that exhibit significantly lower levels of dark current compared to the existing technologies. One of the most attractive material systems to develop HOT photodetectors is InAs/GaSb Type II Strained layer Superlattice (SLS). This is due the ability of Type II SLS materials to engineer the band structure of the device, which can be exploited to make devices with unipolar barriers. It has been shown that, compared to the traditional homojunction SLS devices, band-gap engineered unipolar barrier SLS devices can obtain significantly lower levels of dark current. In this work, we report on the design, growth, and fabrication of mid wave infrared detectors based on type-II InAs/GaSb strained layer superlattice for high operating temperatures. The device architecture is the double-barrier heterostructure, pBiBn design. Under an applied bias of -10 mV and an operating temperature of 200 K, the tested devices show a dark current density of 4 x 10-3 A/cm2 and a quantum efficiency of 27%. At 4.5 μm and 200 K, the devices show a zero-bias specific detectivity of 4.4 x 1010 Jones.

Noise spectrum measurements of a midwave, interband cascade infrared photodetector with 33 nm wide electron barrier

Interband cascade infrared photodetectors (ICIPs) potentially offer mid-wave infrared detection at very high operating temperatures due to their nearly ideal photovoltaic operation. An ICIP typically makes use of several cascade stages grown in series, each of which consists of an active absorption region with a mid-wave cutoff wavelength, an intra-band relaxation region for electron transport and an inter-band tunneling region to enable electron transport to the next stage. The latter two also effectively act as a hole-barrier (hB) and an electron-barrier (eB), respectively, forming a preferential path for each carrier. Here, an ICIP with a relatively large eB was investigated. One of the key parameters to measure for detector performance is the noise spectrum, particularly to observe the behavior at low frequencies where the noise is often much larger than estimates based on the ideal shot noise expression would predict. This paper presents the results of noise spectrum measurements of differently sized ICIP devices, taken using an external trans-impedance amplifier with a cooled, internal impedance converter and a cooled feedback resistor. Measurements were taken at different operating temperatures and voltage biases in order to determine the noise-dependence on each.

Proton radiation effects on the photoluminescence of infrared InAs/InAsSb superlattices

Infrared detector arrays operating in space must be able to withstand defect-inducing proton radiation without performance degradation. Therefore, it is imperative that the proton-radiation hardness of infrared detector materials be investigated. Photoluminescence (PL) is sensitive to defects in materials, and thus can be used to quantify the effects of proton-radiation-induced defects. The excitation intensity-dependent PL was used to examine of a set of InAs/InAsSb superlattices before and after 63-MeV-proton irradiation. A proton dose of 100 kRad(Si) was applied to a different piece of each superlattice sample. The low-temperature excitation intensity dependent PL results reveal minimal increases in the carrier concentration, non-radiative recombination, and the PL full-width half-maximum. These results suggest that InAs/InAsSb superlattices are quite tolerant of proton irradiation and may be suitable for space infrared detector arrays.

Radiation tolerance of a dual-band IR detector based on a pBp architecture

Infrared (IR) detectors operated in the space environment are required to have high performance while being subjected to a variety of radiation effects. Sources of radiation in space include the trapped particles in the Van Allen belts and transient events such as solar events and galactic cosmic rays. Mercury cadmium telluride (MCT)-based IR detectors are often used in space applications because they have high performance and are generally relatively tolerant of the space environment when passivated with CdTe; often, the readout-integrated circuit is far more susceptible to radiation effects than the detector materials themselves. However, inherent manufacturing issues with the growth of MCT have led to interest in alternative detector technologies including type-II strained-layer superlattice (T2SLS) infrared detectors with unipolar barriers. Much less is known about the radiation tolerance properties of these SLS-based detectors compared to MCT. Here, the effects of 63 MeV protons on variable area, single element, dual-band InAs/GaSb SLS detectors in the pBp architecture are considered. When semiconductors devices are irradiated with protons with energies of 63 MeV the protons are capable of displacing atoms within their crystalline lattice. The SLS detectors tested here utilize a pBp architecture, which takes advantage of the higher mobility electrons as the minority photocarrier. These detectors are also dual-band, implying two absorbing regions are present and separated by the unipolar barrier. The absorbers have cutoff wavelengths of roughly 5 and 9 μm allowing for mid-wave (MWIR) and long-wave (LWIR) infrared detection, respectively. The radiation effects on these detectors are characterized by dark current and quantum efficiency as a function of total ionizing dose (TID) or, equivalently, the incident proton fluence.