HgCdTe Alloys Research Articles

Molecular beam epitaxial growth of HgCdTe mid-wave infrared dual-band detectors

The technique of molecular beam epitaxy (MBE) has been utilized to fabricate high-performance HgCdTe infrared detectors, which exhibit sensitivity to mid-wave infrared radiation in adjacent spectral bands for two-color thermal imaging applications. The growth of a multilayer HgCdTe device structure by MBE enables the use of an n-p-n device architecture, which facilitates pixel-level registration of images in two separate spectral bands. Device structures are grown on CdZnTe (211)B substrates using CdTe, Te, and Hg sources with in situ indium (In) and arsenic (As) doping. The composition of the HgCdTe alloy layers is adjusted to achieve detection of infrared radiation in adjacent spectral bands within the wavelength range of 3.0–5.0 µm. The as-grown device structures are characterized using X-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. The 640 × 512, 20-μm-pixel-pitch dual-band MWIR1/MWIR2 focal-plane arrays (FPAs) are fabricated by mesa etching, sidewall passivation and other processes. Ohmic contacts are made to the two n-type layers for operation of the detectors in a sequential detection mode. The MWIR1/MWIR2 FPAs, which are fabricated using these HgCdTe heterostructures with exceptionally high-quality layers, exhibit a band 1 (MWIR1) operability of 98.65 % and a band 2 (MWIR2) operability of 99.93 %. The NETD values of both MWIR1 and MWIR2 modes are less than 15 mK.

Non-Radiative Transitions of Holes on Mercury Vacancies in Narrow-Gap HgCdTe

Mercury vacancies, acting as double acceptors, are the dominant point defects in ternary HgCdTe alloys. Though HgCdTe is one of the leading materials in infrared optoelectronics, the energy spectra of the vacancies are still a matter of some debate. This work investigated the rates at which holes are captured to a singly ionized mercury vacancy via acoustic phonon emission in narrow-gap Hg1−xCdxTe with technologically relevant x~0.22. Combined with the calculated rates of intracenter transitions, the data allow one to predict the most pronounced optical transitions in the emission spectrum of a double-charged acceptor. The results are sustained by the photoluminescence spectroscopy in the terahertz domain, allowing one to identify the emission band that is related to neutral vacancies.

Figure of merit for infrared detector materials

At present stage of infrared detector technology two material systems, bulk HgCdTe ternary alloys and type-II superlattices are used in fabrication of both mid wavelength infrared and long wavelength infrared photodetectors. Proposal of a new metric, so called “Law 19”, for estimation of the HgCdTe performance photodiodes forces the ultimate properties of HgCdTe photodiodes. In these circumstances, the evaluation for assessing the material merit of novel III-V material systems relative to bulk HgCdTe alloys should be reconsidered. This paper attempts to collect and compare the properties of various material systems in terms of their suitability for active detector region.

Calculation of discrete and resonant states of Coulomb acceptor in HgCdTe alloys

The energies and wavefunctions of both localized and resonant states are calculated for Coulomb acceptor in narrow-gap HgCdTe alloys with cadmium mole fraction from 0% to 30%. The simulation is performed with scattering matrix method within a spherically-symmetric three-band Kane model that takes into account the conduction band and two valence bands. The energies of both localized and resonant states are determined using the localization rate magnitude, introduced in this work.

Расчет температурной зависимости энергии состояний кулоновского акцептора в узкозонном твердом растворе HgCdTe

Localized and resonant state energies of coulomb acceptor in narrow-gap HgCdTe alloy are calculated at different temperatures. A universal technique for estimation of localized and resonant state energies is suggested. It is shown theoretically that these energies are туфкдн constant in a wide range of temperatures from 0 to 100 K. The simulation is performed with scattering matrix method within spherically-symmetric three-band Kane model that takes into account the conduction band two valence bands.

Van der Waals Epitaxial Growth of Mosaic-Like 2D Platinum Ditelluride Layers for Room-Temperature Mid-Infrared Photodetection up to 10.6 µm.

Mid-infrared (MIR) photodetection, covering diverse molecular vibrational regions and atmospheric transmission windows, is vital to civil and military purposes. Versatile use of MIR photodetectors is commonly dominated by HgCdTe alloys, InSb, and quantum superlattices, which are limited by strict operation demands, high-cost, and environmental toxicity. Despite the rapid advances of black phosphorus (BP)-based MIR photodetectors, these are subject to poor stability and large-area integration difficulty. Here, the van der Waals (vdW) epitaxial growth of a wafer-scale 2D platinum ditelluride (PtTe2 ) layer is reported via a simple tellurium-vapor transformation approach. The 2D PtTe2 layer possesses a unique mosaic-like crystal structure consisting of single-crystal domains with highly preferential [001] orientation along the normal direction, reducing the influence of interface defects and ensuring efficient out-of-plane carrier transportation. This characteristic, combined with the wide absorption of PtTe2 and well-designed vertical device architecture, makes the PtTe2 /Si Schottky junction photodetector capable of sensing ultra-broadband light of up to 10.6 µm with a high specific detectivity. Also, the photodetector exhibits an excellent room-temperature infrared-imaging capability. This approach provides a new design concept for high-performance, room-temperature MIR photodetection based on 2D layered materials.

Calculation of Wave Functions of Resonant Acceptor States in Narrow-Gap CdHgTe Compounds

Electron wave functions in the conduction band of zero-gap HgCdTe alloy are calculated in the Coulomb acceptor field. The resonant-state energies are determined.

Расчет волновых функций резонансных состояний акцепторов в узкозонных соединениях CdHgTe

Wavefunctions of electron have been calculated for conduction band of zero-gap HgCdTe alloy with Coulomb acceptor. Energies of resonant states have been estimated.

Suppressed Auger scattering and tunable light emission of Landau-quantized massless Kane electrons

The Landau level laser has been proposed a long time ago as a unique source of monochromatic radiation, widely tunable in the THz and infrared spectral ranges using an externally applied magnetic field. In spite of decades of efforts, this appealing concept never resulted in the design of a reliable device. This is due to efficient Auger scattering of Landau-quantized electrons, which is an intrinsic non-radiative recombination channel that eventually gains over cyclotron emission in all materials studied so far: in conventional semiconductors with parabolic bands, but also in graphene with massless electrons. The Auger processes are favored in these systems by Landau levels (or their subsets) equally spaced in energy. Here we show that this scheme does not apply to massless Kane electrons in gapless HgCdTe alloy, in which undesirable Auger scattering is strongly suppressed and the sizeable cyclotron emission observed, for the first time in the case of massless particles. The gapless HgCdTe thus appears as a material of choice for future technology of Landau level lasers.

Interdiffusion Effects on Bandstructure in HgTe-CdTe Superlattices for VLWIR Imaging Applications

In this paper, a systematic study of interdiffusion in (112)B oriented HgTe/CdTe superlattice (SL) structures has been undertaken in order to investigate the viability of employing SL as the absorber layer for very long wavelength infrared (VLWIR, 15 μm and longer) applications in imaging focal plane arrays. Using numerical analysis, the optimal superlattice parameters and annealing time at the growth temperature (∼ 180°C) are found, which result in better control of the cut-off wavelength in superlattice absorbers compared to the corresponding HgCdTe alloy absorber. Simulations show that, by appropriate adjustment of annealing time, it is possible to achieve the same cut-off wavelength with a larger HgTe well width while the sensitivity of the SL to well width variations remains at 15 meV/nm. Furthermore, the electron effective mass in a SL absorber is larger than a HgCdTe alloy absorber, which results in lower tunneling dark current. This work focuses on optimization of the superlattice absorber using the stationary Schrodinger equation. A complete photodetector device design based on a SL absorber structure will require a comprehensive numerical modeling using a Schrodinger–Poisson solver and drift–diffusion solver, or a combination of both approaches, which will be undertaken in the future.

Type-II InAs/GaSb (InAsSb) superlattices for interband cascade midwavelength detectors

Type-II superlattice (T2SL) interband cascade infrared detectors (IB CIDs) proved to be a promising candidate for short response time devices operating in room and higher temperatures. The current status of the higher operating temperature (HOT) T2SLs InAs/GaSb and InAs/InAsSb IB CID is presented. We compare both materials with HgCdTe alloy, which is widely described in literature. The detectivity of midwave infrared (MWIR) T2SLs InAs/GaSb and InAs/InAsSb based IB CID has been demonstrated up to 380 K.

Optimization of Superlattice Barrier HgCdTe nBn Infrared Photodetectors Based on an NEGF Approach

Unipolar nBn photodetector structures have recently emerged as a viable alternative to the traditional p-n junction infrared photodiode approach. However, realization of a unipolar nBn detector technology using the mercury–cadmium–telluride (HgCdTe) alloy system is a challenging task because of the lack of a barrier material with a favorable valence band offset. In this paper, advanced quantum mechanical calculations, based on the nonequilibrium Green’s function (NEGF) formalism, are used to demonstrate that it is possible to achieve diffusion-limited dark current performance in HgCdTe nBn detectors by incorporating a type-III HgTe/CdTe superlattice (SL) barrier layer. Optimal design parameters for CdTe layer thickness, HgTe layer thickness, and total number of periods are presented in order to achieve maximum hole current transmission through the barrier layer, and therefore diffusion-limited dark current performance. The NEGF simulation framework herein presented allows greater insight into effects associated with electron and hole wave function propagation in the SL barrier layer as well as the calculation of individual carrier current components. The presented results form a good basis for the fabrication of high-performance SL barrier HgCdTe nBn detectors.

Massless Dirac fermions in semimetal HgCdTe

Abstract Magneto-transport results obtained for the strained 100 nm thick Hg 1 − x CdxTe (x=0.135) layer grown by MBE on the CdTe/GaAs substrate are interpreted by the 8×8 kp model with the in-plane tensile strain. The dispersion relation for the investigated structure proves that the Dirac point is located in the gap caused by the strain. It is also shown that the fan of the Landau Levels (LL's) energy calculated for topological protected surface states for the studied HgCdTe alloy corresponds to the fan of the LL's calculated using the graphen-like Hamiltonian which gives excellent agreement with the experimental data for velocity on the Fermi level equal to v f ≈ 0.85×106 m/s. That characterized strained Hg 1 − x CdxTe layers (0.13 x 0.14) are a perfect Topological Insulator with good perspectives of further applications.

New concepts in infrared photodetector designs

In 1959, Lawson and co-workers published the paper which triggered development of variable band gap Hg1−xCdxTe (HgCdTe) alloys providing an unprecedented degree of freedom in infrared detector design. HgCdTe ternary alloy has been used for realization of detectors operating under various modalities including: photoconductor, photodiode, and metal-insulator-semiconductor detector designs. Over the last five decades, this material system has successfully overcome the challenges from other material systems. It is important to notice that none of these competitors can compete in terms of fundamental properties. The competition may represent more mature technology but not higher performance or, with the exception of thermal detectors, higher operating temperatures (HOTs) for ultimate performance. In the last two decades, several new concepts for improvement of the performance of photodetectors have been proposed. These new concepts are particularly addressing the drive towards the so called HOT detectors aiming to increase detector operating temperatures. In this paper, new strategies in photodetector designs are reviewed, including barrier detectors, unipolar barrier photodiodes, multistage detectors and trapping detectors. Some of these new solutions have emerged as a real competitor to HgCdTe photodetectors.

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.

HOT infrared photodetectors

AbstractAt present, uncooled thermal detector focal plane arrays are successfully used in staring thermal imagers. However, the performance of thermal detectors is modest, they suffer from slow response and they are not very useful in applications requiring multispectral detection.Infrared (IR) photon detectors are typically operated at cryogenic temperatures to decrease the noise of the detector arising from various mechanisms associated with the narrow band gap. There are considerable efforts to decrease system cost, size, weight, and power consumption to increase the operating temperature in so-called high-operating-temperature (HOT) detectors. Initial efforts were concentrated on photoconductors and photoelectromagnetic detectors. Next, several ways to achieve HOT detector operation have been elaborated including non-equilibrium detector design with Auger suppression and optical immersion. Recently, a new strategies used to achieve HOT detectors include barrier structures such as nBn, material improvement to lower generation-recombination leakage mechanisms, alternate materials such as superlattices and cascade infrared devices. Another method to reduce detector’s dark current is reducing volume of detector material via a concept of photon trapping detector.In this paper, a number of concepts to improve performance of photon detectors operating at near room temperature are presented. Mostly three types of detector materials are considered — HgCdTe and InAsSb ternary alloys, and type-II InAs/GaSb superlattice. Recently, advanced heterojunction photovoltaic detectors have been developed. Novel HOT detector designs, so called interband cascade infrared detectors, have emerged as competitors of HgCdTe photodetectors.

The bulk generation-recombination processes and the carrier lifetime in mid-wave infrared and long-wave infrared liquid nitrogen cooled HgCdTe alloys

Comprehensive study of the bulk generation-recombination mechanisms and the carrier lifetime in long wavelength and mid wavelength infrared indium-doped as well as arsenic- and mercury vacancies-doped HgCdTe ternary alloys at liquid nitrogen temperature has been done. The excess minority carrier lifetime in HgCdTe materials has been calculated by solving the set of non-linear transport equations under conditions of small deviation from equilibrium. The results of numerical calculations of the carrier lifetime determined by the Auger 1, Auger 7, and Shockley-Read-Hall mechanisms related to mercury vacancies have been compared with experimental data available in the literature. We re-examine the carrier lifetime, including the impact of the electrical screening on Coulomb interaction of carriers in the Auger process to explain certain inconsistency between theoretical prediction and experimental data especially in highly doped p-type HgCdTe material. Moreover, the marginal significance of the interband radiative recombination has been indicated.

Accurate electronic properties for (Hg,Cd)Te systems using hybrid density functional theory

Hybrid screened density functional theory better describes the electronic structure of HgTe, CdTe, and HgCdTe systems in comparison with standard density functional theory. The unique hybrid functional reproduces the band inversion in the popular HgCdTe alloy, justifying it as a better method than standard density functional theory in the search for different topological insulators. In addition, the 0.53-eV valence-band offset obtained using the hybrid functional supports the recently observed higher band offset in the HgTe/CdTe heterostructure.

HgCdTe heterostructures on Si (310) substrates for midinfrared focal plane arrays

Results of studies of the molecular beam epitaxial growth of HgCdTe alloys on Si substrates as large as 100 mm in diameter are presented. Optimum conditions for obtaining HgCdTe/Si(310) heterostructures of the device quality for the spectral range of 3–5 μm are determined. The results of measurements and discussion of photoelectric parameters of an infrared photodetector of a format of 320 × 256 elements with a step of 30 μm based on a hybrid assembly of a matrix photosensitive cell with a Si multiplexer are presented. A high stability of photodetector parameters to thermocycling from room temperature to liquid-nitrogen temperature is shown.

Arsenic Diffusion Study in HgCdTe for Low p-Type Doping in Auger-Suppressed Photodiodes

Controllable p-type doping at low concentrations is desired for multilayer HgCdTe samples in a P +/π/N + structure due to the promise of suppressing Auger processes, and ultimately reduced dark current for infrared detectors operating at a given temperature. In this study, a series of arsenic implantation and annealing experiments have been conducted to study diffusion at low Hg partial pressure with the goal of achieving effective control over dopant profiles at low concentration. Arsenic dopant profiles were measured by secondary ion mass spectroscopy (SIMS), where diffusion coefficients were extracted with values ranging between 3.35 × 10−16 cm2 s−1 and 6 × 10−14 cm2 s−1. Arsenic diffusion coefficients were found to vary strongly with Hg partial pressure and HgCdTe alloy composition, corresponding to variations in Hg vacancy concentration.