Vacancies in Hg1−xCdxTe

Measurements have been performed of the carrier concentrations in vacancy-doped Hg_1−xCd_xTe with x=0.22, 0.29, 0.45, and 0.5. Anneals to establish the carrier concentrations were performed on both the mercury- and tellurium-rich sides of the phase field. When these results were added to earlier data for x=0.2 and 0.4, and assuming that all vacancies are doubly ionized, then vacancy concentrations for all values of x and anneal temperature can be represented by simple equations. On the mercury side of the phase field, the vacancy concentrations varied as 2.50×10²³(1−x) exp[−1.00/kT] for low concentrations, and as 3.97×10⁷(1−x)^1/3n _i ^2/3 exp[−0.33/kT] for high concentrations, where n_i is the intrinsic carrier concentration. On the tellurium rich side, the vacancy concentrations varied as 2.81 × 10²²(1−x) exp[−0.65/kT] for low concentrations and as 1.92×10⁷(1−x)^1/3n _i ^2/3 exp[−0.22/kT] for high concentrations.     

Large VLWIR Hg1−xCdxTe photovoltaic detectors

Very long wavelength infrared (VLWIR; 15 to 17 μm) detectors are required for remote sensing sounding applications. Infrared sounders provide temperature, pressure and moisture profiles of the atmosphere used in weather prediction models that track storms, predict levels of precipitation etc. Traditionally, photoconductive VLWIR (λ_c >15 μm) detectors have been used for sounding applications. However, photoconductive detectors suffer from performance issues, such as non-linearity that is 10X – 100X that of photovoltaic detectors. Radiometric calibration for remote sensing interferometry requires detectors with low non-linearity. Photoconductive detectors also suffer from non-uniform spatial optical response. Advances in molecular beam epitaxy (MBE) growth of mercury cadmium telluride (HgCdTe) and detector architectures have resulted in high performance detectors fabricated in the 15 μm to 17 μmm spectral range. Recently, VLWIR (λ_c ∼ 17 μm at 78 K) photovoltaic large (1000 μm diameter) detectors have been fabricated and measured at flux values targeting remote sensing interferometry applications. The operating temperature is near 78 K, permitting the use of passive radiators in spacecraft to cool the detectors. Detector non-AR coated quantum efficiency >60% was measured in these large detectors. A linear response was measured, while varying the spot size incident on the 1000 μm detectors. This excellent response uniformity, measured as a function of spot size, implies that low frequency spatial response variations are absent. The 1000 μm diameter, λ_c ∼ 17 μm at 78 K detectors have dark currents ∼160 μA at a −100 mV bias and at 78 K. Interfacing with the low (comparable to the contact and series resistance) junction impedance detectors is not feasible. Therefore a custom pre-amplifier was designed to interface with the large VLWIR detectors operating in reverse bias. A breadboard was fabricated incorporating the custom designed preamplifier interfacing with the 1000 μm diameter VLWIR detectors. Response versus flux measurements were made on the large VLWIR detectors and non-linearity <0.15% was measured at high flux values in the 2.5×10¹⁷ to 3.5×10¹⁷ ph-cm⁻²sec⁻¹ range. This non-linearity is an order of magnitude better than for photoconductive detectors.     

Electrical properties of Hg1−x MnxTe-based photodiodes

Anstract Electrical properties (at 80K) of p-n junctions fabricated by ion milling of p-type Hg_0.91Mn_0.09Te are analyzed. The forward current-voltage characteristics at low biases is shown to be governed by carrier recombination in the space charge region and at higher biases its voltage dependence is deformed due to the voltage drop across the high-resistance layer in the diode structure. Under reverse bias, carrier tunneling suppresses other transport mechanisms. At higher reverse biases, impact ionization by high-energy carriers is responsible for the additional increase in the diode current. Fiz. Tekh. Poluprovodn. 33, 1438–1442 (December 1999)     

Hole scattering mechanisms in Hg1−xCdxTe

In this paper, we analyze and discuss the roles of nine different scattering mechanisms—ionized impurity, polar and nonpolar optical, acoustic, dislocation, strain field, alloy disorder, neutral impurity, and piezoelectric—in limiting the hole mobilities in p-type Hg_1−xCd_xTe crystals. The analysis is based on obtaining a good fit between theory and experiment for the light and heavy hole drift mobilities by optimizing certain unknown (or at the most vaguely known) material parameters such as the heavy hole mobility effective mass, degree of compensation, and the dislocation and strain field scattering strengths. For theoretical calculations, we have adopted the relaxation time approach, keeping in view its inadequacy for the polar scattering. The energy dispersive hole relaxation times have been drawn from the published literature that take into account the p-symmetry of valence band wave functions. The temperature dependencies of multiple charge states of impurities and of Debye screening length have been taken into account through a numerical calculation for the Fermi energy. Mobility data for the present analysis have been selected from the HgCdTe literature to represent a wide range of material characteristics (x=0.2–0.4, p=3×10¹⁵–1×10¹⁷ cm⁻³ at 77K, μ_peak≅200-1000cm²V⁻¹s⁻¹). While analyzing the light hole mobility, the acoustic deformation and neutral impurity potentials were also treated as adjustable. We conclude that

Arsenic incorporation in MBE grown Hg1−xCdxTe

The p-type doping of Hg_1−xCd_xTe (MCT) has proven to be a significant challenge in present day MCT-based detector technology. One of the most promising acceptor candidates, arsenic, behaves as an amphoteric dopant which can be activated as an acceptor during Hg-rich, low temperature annealing of as-grown molecular beam epitaxy (MBE) samples. This study focuses on developing an understanding of the microscopic behavior of arsenic incorporation during MBE growth. In particular, the question of whether arsenic incorporates as individual As atoms, as As₂ dimers, or as As₄ tetramers is addressed for MBE growth with an As₄ source. A quasithermodynamical model is employed to describe the MCT growth and As incorporation, with parameters fitted to an extensive database of samples grown at the Microphysics Laboratory. The best fits for growth temperatures between 175 and 185°C are obtained for arsenic incorporation as As₄ or possibly as As₄ clusters, with lower probabilities for As₂ and individual As atoms. Based on these results, we investigate the relaxed atomic configurations of As₄ and As₂ in bulk HgTe by ab initio total energy calculations. The calculations are performed in the pseudopotential density-functional framework within the local density approximation, employing supercells with periodic boundary conditions. The lattice distortions due to As₄ and As₂ in bulk HgTe are predicted to be modest due to the small size of these arsenic clusters.     

Improved operability in Hg1−xCdxTe detector arrays

As liquid phase epitaxial (LPE) growth and array fabrication processes have matured to give excellent wafer average performance, the yield limiter for infrared focal plane arrays (IRFPAs), especially large ones, have become outages. In this work, significant progress has been made in identifying the source and eliminating outages from LPE grown Hg_1−xCd_xTe P-on-n structures. Historically, studies of the sources of outages have employed defect etches to look for dislocations and other crystalline defects, and secondary ion mass spectroscopy (SIMS), imaging SIMS, and sputter initiated resonance ion spectrometry (SIRIS) to look for impurities at critical interfaces. Using these techniques, trends were established, but direct correlation with outages have been observed. In LPE grown materials, where the dislocation densities are always below 5×10⁵ cm⁻², and often below 1×10⁵ cm⁻² on CdZnTe substrates, dislocations only account for a few outages. In order to understand the source(s) of outages, a failure analysis was performed on several long wavelength IRFPAs. Using a dilute etchant, the metals and then cap layers of some 64×64 pixel IRFPAs which had excellent average performance, but suffered from a high density of pixels with excessive leakage current, were removed. Using a scanning electron microscope with energy dispersive spectroscopy capability, the presence of carbon particles was correlated with excessive leakage current on a 1:1 pixel basis. A series of experiments was then conducted which isolated the source of the particles to the cap layer growth process, which was consequently changed to eliminate them. The process improvements have reduced the particle density to below the measurement limit of the optical measurement technique implemented to monitor the density of particles on witness wafers. These improvements are resulting in IRFPAs with significantly improved operability.     

Structure and properties of Cd x Hg1−x Te-metal contacts

A metal-semiconductor contact is a composite structure consisting of several nanodimensional layers. The contact properties depend strongly on the technique of metal deposition. A metal forms chemical compounds with the components of Cd _x Hg_1?x Te (CMT), thus changing the properties of the surface layer. Mercury is accumulated at the interface with the metal, while tellurium is accumulated on the metal surface. The CMT compounds with metals, heats of their formation, and the Fermi level shifts are reported. The structure and properties of the interfaces between CMT and gold, silver, indium, aluminum, copper, and other metals, as well as the effect of sublayers of other metals and insulators, are described.     

Valence band offset in HgTe/Hg1−xCdxTe superlattices

The valence band offset (Λ) between HgTe and CdTe has been determined by means of an optical investigation of (112)B oriented HgTe/Hg_1−xCd_xTe superlattices. Based on the fact that the difference in energy between the first heavy hole and the first light hole subband is to a good approximation due primarily to Λ, it has been shown that Λ=580±40 meV at 5K. In addition Λ has a significant temperature dependence with a linear coefficient of −0.34±0.02 meV/K, i.e., Λ is 480±40 meV at room temperature.     

Microscopic defects on MBE grown LWIR Hg1−xCdxTe material and their impact on device performance

Long wavelength infrared molecular beam epitaxy (MBE) grown p-on-n Hg_1−xCd_xTe double layer planar heterostructure (DLPH) detectors have been characterized to determine the dominant mechanisms limiting their performance. Material defects have been identified as critical factors that limit 40K performance operability. This effort has concentrated on identifying microscopic defects, etch pit density (EPD) and relating these defects to the device performance. Visual inspection indicates defect densities as high as 10⁵ per cm² with a spatial extent as observed by atomic force microscope in the range of micrometers extending several micrometers beneath the surface. At high EPD values (greater than low 10⁶ cm⁻²) zero bias resistance (R₀) at 40K decreases as roughly as the square of the EPD. At 78K, however, measured R₀ is not affected by the EPD up to densities as high as mid-10⁶ cm⁻². Visual defects greater than 2–3 μm than ∼2 μm in size (micro-void defects) result in either a single etch pit or a cluster of etch pits. Large variations in a cross-wafer etch pit distribution are most likely a major contributor to the observed large spreads in 40K R₀. This study gives some insight to the present limitation to achieve higher performance and high operability for low temperature infrared applications on MBE grown HgCdTe material.     

Transient behavior of Hg1−xCdxTe film growth on (111)B CdTe substrates by chemical vapor transport

As part of a systematic investigation of the effects of substrate surfaces on epitaxial growth, the transient behavior of Hg_1−xCd_xTe film growth on (111)B CdTe by chemical vapor transport (CVT) has been studied as a function of growth time under vertical stabilizing (hot end on top) and vertical destabilizing (hot end at bottom) ampoule orientations. The experim ental results show the morphological transition of the Hg_1−xCd_xTe deposition on (111)B CdTe at 545°C from three-dimensional islands to layers within about 0.5 and 0.75 h for the growth under vertical stabilizing and destabilizing conditions, respectively. The combined effects of small convective flow disturbances on the growth morphology and defect formation are measurable. The overall trends of the time dependent growth rates and compositions of the Hg_1−xCd_xTe epitaxial layers under stabilizing and destabilizing conditions are similar. The system atically higher growth rates of the Hg_1−xCd_xTe films by about 10% under vertical destabilizing conditions could be influenced by a small convective contribution to the mass transport. The combined results show that improved Hg_1−xCd_xTe epitaxial layers of low twin density on (111)B CdTe substrates can be obtained by CVT under vertical stabilizing conditions.     

Hg1—xCdxTe重力分离研究

通过实验证实了Hg_(1-x)Cd_xTe熔体中存在HgTe与CdTe之间的重力分离。理论分析表明,Hg_(1-x)Cd_xTe熔体中的HgTe粒子服从玻尔兹曼分布律,HgTe粒子的质量为4.07×10~(-18)g(在835℃)。     

Hg1—xCdxTe光学常数测量

根据半导体光学常数间的关系,通过测量Hg_(1-x)Cd_xTe晶片不同厚度时的透射比,求得了Hg_(1-x)Cd_xTe的光学常数。本文采用迭代法精确求解有关方程组,避免了计算过程引入的误差,提高了测量结果的精度。这种方法也适用于其他半导体光学常数的测量。     

In-situ evaluation of the anodic oxide growth on Hg1−xCdxTe (MCT) using ellipsometry and second harmonic generation

In-situ measurements of ellipsometry and second harmonic generation (SHG) were carried out to monitor the electrochemical growth of native anodic oxide films on Hg_1?xCd_xTe (MCT). Growth of the anodic oxide was performed using two different methods viz., by linear sweep voltammetry and by applying a constant current density. The influence of scan rate and the magnitude of the applied current density on the properties of the growing films were examined. From the ellipsometry data, we have shown that the measured refractive index value of 2.19 for the oxide film remains unchanged for moderate and high oxide growth rates. Only at very slow growth rates were significant increases in the refractive index observed (n=2.4), indicating an increase in the compactness of the layer. For film thicknesses in excess of ～1200 Å, a non-zero value for the extinction coefficient was found, indicating the incorporation of HgTe particles within the anodic oxide film. SHG rotational anisotropy measurements, performed on the MCT with and without an anodic oxide film showed only the four-fold symmetry associated with the MCT and so confirmed that the oxide was centrosymmetric. However, an increase in the SH intensity was observed in the presence of the oxide and this has been attributed to multiple reflections in the thin oxide film and also to the increase in the χ⁽²⁾ non-linear susceptibility tensor as a result of charge accumulation at the MCT/anodic oxide interface.     

Surface second harmonic generation in the characterization of anodic sulphide and oxide films on Hg1−xCdxTe (MCT)

Rotation anisotropy by second harmonic generation (SHG) is carried out on epitaxial Hg_1−xCd_xTe (MCT) and oxide- and sulphide-covered MCT surfaces and shows the fourfold symmetry pattern expected from the {100} surface (C_4v symmetry). The uneven nature of the four peaks confirm the vicinal surface obtained from the growth of the MCT on GaAs {100} substrate orientated 4° toward the 〈110〉 direction. The increase in the SH intensity observed for the oxide-covered MCT surface is associated with charge accumulation at the MCT/oxide interface since the oxide is centrosymmetric and cannot generate SH. The CdS layer on the other hand is strongly nonlinear active and generation here comes from a composite of one noncentrosymmetric layer on top of another. This leads to interactions in the observed SH arising from the coupling depths (∼40 nm) at the two interfaces and from the coherence length (∼1200 nm) in the CdS layer. The in-situ SHG measurements during the growth of the anodic oxide and sulphide layers would suggest that a species, most likely HgTe is embedded in the anodic layer during the initial stages and absorbs the SH radiation at 532 nm. The rotational anisotropy of the sulphide-covered MCT surface confirms that the CdS layer formed maintains the cubic closed pack symmetry of the underlying MCT.