This article reviews the history, the present status and possible future developments of HgCdTe ternary alloy for infrared (IR) detector applications. HgCdTe IR detectors have been intensively developed since the first synthesis of this material in 1958. This article summarizes the fundamental properties of this versatile narrow gap semiconductor, and relates the material properties to its successful applications as an IR photoconductive and photovoltaic detector material. An emphasis is put on key developments in the crystal growth and their influence on device evolution. Competitive technologies to HgCdTe ternary alloy are also presented. Recent advances of backside illuminated HgCdTe heterojunction photodiodes have enabled a third generation of multispectral instruments for remote sensing applications and have led to the practicality of multiband IR focal plane array technology. Finally, evaluation of HgCdTe for room temperature long wavelength IR applications is presented. (Some figures in this article are in colour only in the electronic version)

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HgCdTe infrared detector material: history, status and outlook

A method enabling numerical modeling of fluctuation phenomena in semiconductor devices is presented The method is based on the assumption that fluctuations of generation–recombination processes and carrier mobility result in the fluctuations of carrier and ionized impurity concentrations These, in turn, may be expressed by the fluctuations of the electrical potential and quasi-Fermi levels Fluctuations of the electrical potential and quasi-Fermi-levels were calculated by solving the set of “transport equations for fluctuations” in which the fluctuations of generation–recombination processes (both thermal and optical) and fluctuations of mobility play roles of random source terms The method enables the calculation of fluctuations of all physical quantities enclosed in a set of transport equations The spatial distribution of the fluctuations of the electrical potential, electron concentration, and noise current density is shown The noise spectrum in selected, cooled, long-wavelength HgCdTe photoresistors is calculated and the contribution of different noise sources is determined Theoretical results are compared with experimental data

Numerical modeling of fluctuation phenomena in semiconductor devices

The densities and mobilities of heavy and light holes have been simultaneously determined at various temperatures (1.2–300 K) in two molecular beam epitaxy-grown p-type Hg1−xCdxTe (x=0.224) samples from variable magnetic-field Hall measurements. The separation of the contribution from the light hole and heavy hole was achieved by a hybrid approach consisting of mobility spectrum analysis followed by a multicarrier fitting procedure. An acceptor energy level at ∼13 meV above the valence band, as well as various mass ratios of light to heavy holes for different temperature were obtained. In addition, the minority carrier (electron) and the surface two-dimensional electron concentrations and mobilities have also been derived as a function of temperature. The explicit experimental values obtained in this work should be useful to physics and modeling of HgCdTe infrared detectors.

Evaluation of densities and mobilities for heavy and light holes in p-type Hg1−xCdxTe molecular beam epitaxy films from magnetic-field-dependent Hall data

The effect of dislocation on the 1/f noise current in long-wavelength infrared (LWIR) reverse biased HgCdTe photodiodes working at liquid nitrogen (LN) temperature was analyzed theoretically by using a phenomenological model of dislocations as an additional Shockley–Read–Hall (SRH) generation–recombination (G–R) channel in heterostructure. Numerical analysis was involved to solve the set of transport equations in order to find a steady state values of physical parameters of the heterostructure. Next, the set of transport equations for fluctuations (TEFF) was formulated and solved to obtain the spectral densities (SD) of the fluctuations of electrical potential, quasi-Fermi levels, and temperature. The SD of mobility fluctuations, shot G–R noise, and thermal noise were also taken into account in TEFF. Additional expressions for SD of 1/f fluctuations of the G–R processes were derived. Numerical values of the SD of noise current were compared with the experimental results of Johnson et al. Theoretical analysis has shown that the dislocations increase the G–R processes and this way cause the growth of G–R dark current. Despite the fact that dislocations increase both shot G–R noise and 1/f G–R noise, the main cause of 1/f current noise in LN cooled LWIR photodiodes are fluctuations of the carriers mobility determined by 1/f fluctuations of relaxation times. As the noise current is proportional to the total diode current, growth of G–R dark current caused by dislocations leads to the growth of noise current.

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Dislocations as a Noise Source in LWIR HgCdTe Photodiodes

The theoretical analysis of long wavelength uncooled photovoltaic devices based on complex two-dimensional Hg/sub 1-x/Cd/sub x/Te heterostructures is presented. An enhanced computer program has been developed to solve the system of nonlinear continuity equations and the Poisson equation. All physical quantities of semiconductor structure are expressed as functions of electric potential and Fermi quasi-levels. The noise analysis is based on the set of 'transport equations for fluctuations' that enables calculations of spatial distribution of electrical potential and Fermi quasilevel fluctuations. Both generation recombination noise and If noise caused by mobility fluctuations were taken into account. The results of calculations are presented as maps illustrating spatial distributions of current densities, electrical gain, and fluctuations of selected physical quantities. Detectivity of 4/spl times/10/sup 7/ cmHz/sup 1/2/W/sup -1/ is predicted for a 10.6 /spl mu/m unbiased multiple heterojunction photovoltaic device with 20 /spl mu/m period. The theoretical predictions have been compared with performance of practical devices.

Enhanced numerical modelling of non-cooled long-wavelength multi-junction (Cd,Hg)Te photodiodes

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 Hg1−xCdxTe 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, μpeak≅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


• the heavy hole mobility is largely governed by the ionized impurity scattering, unless the strain field or dislocation scattering below 50K, or the polar scattering above 200K, become dominant;


• the light hole mobility is mainly governed by the acoustic phonon scattering, except at temperatures below 30K where the neutral impurity, strain field and dislocation scattering also become significant;


• the intervalence scattering transitions make negligible impact on the heavy hole mobility, but virtually limit the light hole mobility;


• the alloy disorder scattering does not dominate in any temperature region, although it exercises some influence at intermediate temperatures;


• the heavy hole mobility effective mass ratio mhh/mo∼-0.28–0.33 for crystals with x<0.4; and


• the light hole band deformation potential constant is ∼12 eV.

Hole scattering mechanisms in Hg 1−x Cd x Te

We report first results on the activated resistivity inn- type Hg08Cd02Te crystals heavily compensated through native acceptor defects (double acceptor mercury vacancies) Three activation energies are reported, all of which increase with increasing compensation In conjunction with the variation of Hall coefficient and Hall mobility with temperature we conclude, i) formation of (donor+-acceptor−) pairs during stoichiometric annealing, ii) distortion of second acceptor states of the paired mercury vacancies to raise them near, or in the conduction band, and iii) the pair states, when occupied with electrons in the presence of excess unpaired donors, behave like localized donors, and when unoccupied, like electron traps and/or recombination centres

A. V. R. Warrier

Papers

Hole scattering mechanisms in Hg 1−x Cd x Te

Donor-like localization and activated conduction in compensated n -Hg 0.8 Cd 0.2 Te