An accurate formula for electron affinity is necessary for the correct modeling of Hg1-xCdxTe devices. We show that the most commonly used formula is inaccurate. We construct a generalized equation for the electron affinity based on certain properties of the HgCdTe alloy system and select the experimental values to insert, producing a recommended formula that is useful in device modeling. We also find that in a graded HgCdTe structure, very nearly 2/3 of the bandgap change will appear in the conduction band. In addition, we explain why the electron affinity rule (conduction band offset equals the electron affinity difference) remains valid for HgCdTe-HgCdTe heterojunctions, even though it does not apply to heterojunctions in general.

An understanding of radiation damage in HgCdTe is essential to predicting the continued performance of space-based HgCdTe infrared sensors. The ability of a sensor to perform a mission in space depends heavily on its ability to be robust to the harsh radiation environment experienced on orbit. Robust architecture design requires detailed knowledge of the various damage mechanisms, so that they can mitigated through device design. Previous studies have found that damage-induced performance degradation can be related to the nonionizing energy loss (NIEL) associated with nuclear displacement damage. The NIEL can be simulated with Monte Carlo codes (such as SRIM dagger), with the displacement energy threshold being an important parameter; this parameter is usually taken (somewhat dubiously) as a code-dependent default (25 eV in the case of Hg, Cd, and Te in HgCdTe). In this work, we describe a systematic study of the NIEL with varying displacement energy thresholds and discuss the relative importance of this parameter. We also investigate displacement damage in HgCdTe through molecular dynamics (MD) simulations using the LAMMPS double dagger code to better understand the microscopic damage mechanisms, such as vacancy and interstitial formation.

In mid-wavelength infrared detectors built with the InAs/InAsSb superlattice, a study of the absorption behavior near the cutoff wavelength was conducted. Two results are, first, an accurate method for determining the bandgap energy from the spectral response of a fabricated device, and second, an analysis of the Urbach tail of the absorption showing that its characteristic energy is consistent with the dominant phonon of InAs. Additionally, a temperature-dependent measurement of the transport of photo-generated holes in nBn devices can be used to identify the effects of localization due to layer nonuniformities, and to measure the energy step at hole-blocking features.

The analysis of forward-bias tunneling data to determine the energy position of a deep level in the gallium-free InAs/InAsSb superlattice is reported. The level is found in p–n junction devices built for mid-wavelength infrared detection. The traps that produce this level can facilitate a trap-assisted tunneling (TAT) current in forward bias that adds to the conventional diffusion dark current. The effect is manifested as a peak in the ideality factor, where the TAT current turns off at a voltage above which the band alignment no longer permits tunneling. We apply the tunneling model of Anderson developed for HgCdTe. Our analysis of data from InAs/InAsSb having a bandgap energy of 220 meV finds the trap energy to be 170 meV above the valence band edge.

We report an analysis of the current-voltage characteristics of a dual-band HgCdTe infrared detector built in an n-p-n configuration and designed for sequential mode operation in mid-wavelength (MW) and long-wavelength (LW) bands. The model treats the device as a pair of back-to-back p-n junctions and addresses both dark and illuminated conditions over a range of temperatures. We show that the assumption of ideal diode behavior (diffusion-only current) provides a very good first approximation, particularly at small bias voltages. We also find that a plot of the resistance-area product RA is the most sensitive indicator of the deviation from ideality, most of which is due to non-diffusion currents in the LW junction. We determine the apportionment of the applied voltage between the two junctions and show that for LW detection, less than half the total voltage appears across the reverse biased LW junction. Our approach should be useful in analyzing other dual-band test data and guiding design improvements.

We have studied infrared absorption near the bandgap energy in mid-wavelength (MWIR) III-V photon detectors built in the nBn configuration. The absorbing material is the InAs/InAsSb superlattice. We show that in a practical device near the infrared (IR) cutoff, the spectral response curve as a function of photon energy is proportional to the absorption coefficient, to a good approximation. Thus, in the near-gap range, the energy dependence of the device spectral response is a reliable proxy for the energy dependence of the absorption coefficient. We demonstrate this by means of an expansion of the Hovel equations in powers of the product of the absorption coefficient and hole diffusion length. One application of this result is that the point of maximum slope of the spectral response curve can be used to locate the true bandgap energy. This result also facilitates a study of absorption in the Urbach tail, which occurs at sub-gap energies. The temperature dependence of the Urbach steepness parameter was found to be consistent with the dominant phonon energy of InAs.

Random telegraph signal (RTS) noise is ubiquitous in electronic and electro-optical devices, having been observed in MOSFETs and photodiode arrays. For imaging arrays, in particular, RTS noise (blinking pixels or "blinkers") deteriorates system performance through poor nonuniformity correction (NUC) stability and degrades image quality with blinking pixel behavior that can distract human operators and confuse computer vision algorithms. To date, there exists no universally accepted identification method or description of RTS noise in photodetectors, nor a conventional analysis approach to determine its origin. Current approaches typically focus on spectral properties (RTS noise is characterized by a Lorentzian power spectrum), which can be expensive to compute through Fourier methods, and analysis is usually performed on only a small sample of pixels. Here, we propose a method to identify and characterize blinkers by training a hidden Markov model (HMM) to extract the principal parameters governing blinking behavior, including the underlying state space, the state transition probabilities, and the distribution of state output levels. We find evidence to support classifying blinking behavior with HMM parameters; the variation of the model parameters with extrinsic variables, such as the temperature and applied bias, give some indication of the underlying physical mechanisms. Specifically, we find the timescale of the blink current is longer than typical electron{phonon, electron{electron, and electron{photon interactions, which leads to the suggestion that the blinking mechanism may be related to trap occupation dynamics.

Mid-wavelength infrared detectors have been fabricated in the nBn configuration using the InAs/InAsSb superlattice as the absorber. Possible impediments in the valence band can interfere with the transport of holes that represent the signal. We demonstrate that the thermal activation energy of the photocurrent density, as a function of the applied bias voltage, can be a very sensitive probe of the valence band features. We identify and measure two types of impediments, the hole-block due to a band misalignment and the localization sites formed by fluctuations in the superlattice layer thicknesses. The latter are found to dominate the temperature dependence of the hole mobility. Our inferred localization characteristics are consistent with published results obtained by other techniques.

Valence band features affecting carrier transport in III-V superlattice nBn detectors. David R. Rhiger and Edward P. Smith, Raytheon Vision Systems. We have investigated non-ideal features occurring in the valence band profile of nBn detectors which affect the transport of minority-carrier holes representing the IR signal. The objectives are to reduce dark currents and improve quantum efficiency. The nBn device consists of an n-type absorber several microns thick, plus a very thin electron barrier B and a thin n-type collector (top contact region). In this investigation, the absorber and collector were built with the InAs/InAsSb superlattice. For normal operation, holes generated by photons in the absorber must flow to the collector. Current is promoted by a combination of diffusion and electric field drift. However, in some cases the transport of holes is limited by (1) absorber-barrier valence band misalignment, (2) bandgap difference between collector and absorber, or (3) possible localization sites in the absorber due to compositional fluctuations. These characteristics, when combined with the known limitations of hole diffusion length, can adversely affect the quantum efficiency, and require the application of an operating bias that is larger than otherwise necessary. We have been able to identify and measure these valence band features by analyzing device characteristics as a function of temperature, bias voltage, photon flux, and wavelength dependence of the response. Examples will be presented. This work was supported by Dr. Meimei Tidrow of NVESD, Contract Number W15P7T-06-D-E402, Task BD30, Agreement No. S08-092256, Purchase Order P000006939.

Mid-wavelength infrared nBn detectors built with III–V superlattice materials have been tested by means of both capacitance and direct-current methods. By combining the results, it is possible to achieve clear separation of the two components of dark current, namely the generation–recombination (GR) current due to the Shockley–Read–Hall mechanism in the depletion region, and the diffusion current from the neutral region. The GR current component is unambiguously identified by two characteristics: (a) it is a linear function of the depletion width, and (b) its activation energy is approximately one-half the bandgap. The remaining current is shown to be due to diffusion because of its activation energy equaling the full bandgap. In addition, the activation energy of the total measured dark current in each local region of the temperature–bias parameter space is evaluated. We show the benefits of capacitance analysis applied to the nBn device and review some of the requirements for correct measurements. The carrier concentration of the unintentionally doped absorber region is found to be 1.2 × 1014 cm−3 n-type. It is shown that the depletion region resides almost entirely within the absorber. Also, the doping in the nBn barrier is found to be 4 × 1015 cm−3 p-type. Minority-carrier lifetimes estimated from the dark current components are on the order of 10 μs.

We report on the design, growth, fabrication and characterization of dual-band (long-/long-wave infrared) type-II InAs/GaSb strained layer superlattice (T2SL) detectors with pBp architecture. Under operating the bias of −200 mV and +100 mV, quantum efficiencies of 37% (∼11 μm band) and 25% (∼9 μm band) were realized, respectively. To reduce the dark current in a dual-band T2SL detector, the effect of a “restoration” chemical etch treatment and ZnTe passivation on device performance were investigated.

Magnetic-field-dependent Hall-effect measurements and high-resolution mobility spectrum analysis have been employed to determine electronic transport parameters in InAsSb-based nBn structures. Three samples were studied, with nominally identical epitaxial layer structure but with barrier layers of different compositions. Two separate well-defined electron species, associated with the two distinct doping regions, were identified. The extracted electron concentrations were found to be in excellent agreement with the nominal doping density of the absorber and the back contact layers for all samples studied. For all samples, electron mobility appears to be limited by impurity scattering, whereas the relatively small differences in extracted mobility values between samples are likely to be due to unintentional variations in ionized impurity and/or defect concentration in the samples.

Mercury cadmium telluride (HgCdTe) grown on large-area silicon (Si) substrates allows for larger array formats and potentially reduced focal-plane array (FPA) cost compared with smaller, more expensive cadmium zinc telluride (CdZnTe) substrates. In this work, the use of HgCdTe/Si for mid-wavelength/long-wavelength infrared (M/LWIR) dual-band FPAs is evaluated for tactical applications. A number of M/LWIR dual-band HgCdTe triple-layer n-P-n heterojunction device structures were grown by molecular-beam epitaxy (MBE) on 100-mm (211)Si substrates. Wafers exhibited low macrodefect densities (< 300 cm−2). Die from these wafers were mated to dual-band readout integrated circuits to produce FPAs. The measured 81-K cutoff wavelengths were 5.1 μm for band 1 (MWIR) and 9.6 μm for band 2 (LWIR). The FPAs exhibited high pixel operability in each band with noise-equivalent differential temperature operability of 99.98% for the MWIR band and 98.7% for the LWIR band at 81 K. The results from this series are compared with M/LWIR FPAs from 2009 to address possible methods for improvement. Results obtained in this work suggest that MBE growth defects and dislocations present in devices are not the limiting factor for detector operability, with regards to infrared detection for tactical applications.

The crystalline structure and impurity profiles of HgCdTe/CdTe/alternate substrate (AS; Si and GaAs are possibilities) and CdTe/AS were analyzed by secondary-ion mass spectrometry, atomic force microscopy, etch pit density analysis, and scanning transmission electron microscopy. Impurities (Li, Na, and K) were shown to getter in as-grown CdTe/Si epilayers at in situ Te-stabilized thermal anneal (~500°C) interfaces. In HgCdTe/CdTe/Si epilayers, indium accumulation was observed at Te-stabilized thermal anneal interfaces. Impurity accumulation was measured at HgCdTe/CdTe and CdTe/ZnTe interfaces. Processing anneals were found to nearly eliminate the gettering effect at the in situ Te-stabilized thermal anneal interfaces. Impurities were found to redistribute to the front HgCdTe/CdTe/Si surface and p–n junction interfaces during annealing steps. We also investigated altering the in situ Te-stabilized thermal anneal process to enhance the gettering effect.

We report on an interband cascade mid-wave infrared (MWIR) detector based on type-II InAs/GaSb/AlSb strained layer superlattices (T2SL). The reported device has a seven-stage cascade region, each segment containing a MWIR absorber region, a graded T2SL transport region, and an interband tunneling region. Above room temperature spectral response was observed, with a cutoff wavelength of 7 μm at 420 K. Detailed radiometric measurements yielded a Johnson noise limited detectivity of 3.0 × 1011 cmHz1/2W−1 (8.9 × 108 cmHz1/2W−1) and a dark current density of 3.6 × 10−7 A/cm−2 (7.3 × 10−3 A/cm−2) near zero bias with a 100% cutoff wavelength of 5.2 μm and 6.2 μm at 77 K (295 K), respectively, with an estimated 36.2% QE.