This paper examines a selected group of basic properties of the 4-6 family of compound semiconductors and related materials. The goal is to show how knowledge accumulated over a thirty-year period has led to changes in the way these properties are described and interpreted, has focused increasing attention on the unifying characteristics of the 4-6 family, and has clarified the relationships of these compounds to one another, and to other, better-known groups of insulators, semiconductors, and semimetals.

The ultimate ability of an infrared detector to sense radiation and subsequently yield a measurable output signal is intimately related to the basic physical properties and transport mechanisms of the detector material. While spectral response measurements provide a monitor on the ultimate device performance and can yield information on the detector interface physics, still further insight into the devices is provided by an investigation of their electrical properties. An epitaxial thin film IV-VI detector technology providing both self-filtering and multicolor capability has been developed at the Naval Surface Weapons. Center (NSWC). Measurements have been made of a number of basic electrical characteristics of a series of these photodiodes, including their I-V, C-V and R0A-T dependencies. This has yielded information on the barrier profiles and current conduction mechanisms of these devices which is directly related to their optical sensing ability. Device performance has been found to be distinctly improved or degraded with thermal cycling, depending on the detector material used. These results can be correlated with lattice-matching investigations made on these detectors.

Photovoltaic diode infrared detectors are fabricated by depositing metallic lead on p-type lead-salt epitaxial films. The films are grown from the vapor phase by a modified hot wall technique, under pressures on the order of 5 x 10-6 Torr. The carrier concentration of these films is typically in the low 1017 cm-3, and the mobility is a few thousands cm /V sec at 77 K. The lead salts for the growth process are prepared by heating the elemental materials in sealed quartz ampoules. The substrates used for these films are usually BaF2 with a {111} cleavage plane. The formation of Schottky barrier junctions on lead salt films has been reported to be dependent on the chemistry of the interface, such as the presence of oxygen. Furthermore it was found recently that the presence of chlorine vastly improved the I-V characteristics of diode junctions. A simple process for introducing chlorine to the semiconductor surface prior to lead deposition was developed. This process has proved to yield good quality diodes with consistency.

Surface analysis has indicated that improved IV-VI metal-semiconductor photodiode performance results from the incorporation of chlorine at the Pb/semiconductor interface. Lower reverse bias leakage current and steeper forward bias slope are observed in devices fabricated from material treated with chlorine than devices which are processed normally. RoA values of 250 Ω cm2 and Dλ* values of 1 x 10^11 cm Hz1/2 W-1 at 80K for 10-and 80-mil diameter active area devices have been obtained.

Progress in determining the parameters which characterize the PbTe space charge layers is surveyed. Work for both accumulation and inversion is discussed and areas requiring more effort are pointed out.

A brief outline is given of some of the more important magnetoresistance effects seen in the IV-VI semiconductors. In addition, an interesting strong magnetic field magnetoresistance phenomenon is exhibited and the use of weak-field magnetoresistance to examine strain in thin film samples is presented in more detail. Data showing the effects discussed are given using PbTe thin films as examples.

We describe the Liquid Phase Epitaxial (LPE) growth of p-Hgl-xCdxTe on CdTe substrates up to 2cm x 3cm in area, using an atomospheric pressure slider with a tellurium-rich growth liquid. Controlled growth of Hgl-x CdxTe with x values of 0.2, 0.3, and 0.4 will be described, and measurements presented on the morphology and semiconductor characteristics of the grown layers.

(Hg,Cd)Te photoconductive detectors in the 3-5m wavelength range were irradiated with an ultraviolet source of wavelength λ = 0.36611m and power density P0 = 50μW/cm2. The performance such as detectivity (D*λ), responsivity (Rλ), the noise current (iN) and the detector resistance (RD) were measured as a function of irradiation time for a fixed temperature T = 110°K. The measurement indicated that the responsivity changed from Rλ = 0.7 x 103 (A/W) to Rλ = 1.3 x 103 (A/W) for 22 minutes of ultraviolet irradiation. During the same ultraviolet exposure, the noise current increased from iN = 1 x 10-11 (A/Hz1/2) to iN = 1.4 x 10-11 (A/Hz1/2), the detectivity increased from D*λ = 2.7 x 1011 (Cm-Hz1/2/W) to D*λ = 3.8 x 1011 (Cm-Hz1/2/W) and the resistance changed from 680Ω to 950Ω. The corresponding change in the surface potential was calculated and was shown to be from ψs = 0.089 (volt) to ψs = 0.0946 (volt). The ultraviolet irradiation is believed to fill the traps within the insulator with electrons which in turn increases the surface potential. The data indicates that ultraviolet wavelength used for the investigation is only partially effective in filling the insulator traps.

The advantages of r.f. triode sputtering in a Hg atmosphere for deposition of (Hgl-xCdx)Te thin films for low-cost photoconductor and ohotovoltatic infrared detector arrays on electronically-active Si substrates are discussed. It is shown by optical absorp-tion data that the composition of films can be changed from x = 0.1 to 0.27 by changing the relative percentages of HgTe and CdTe particles in sputtering targets, made by cold-pressing a homogenized mixture of these particles. Changes in the physical topography and composition of the surface of pressed-powder targets when bombarded with Hg are described. The mobility and implied carrier concentrations at 24 and -188°C of n and p-type films with different compositions in the 0.18 < x <0.27 range were measured by the Van der Pauw technique. The films were deposited under different sputtering conditions onto high resistivity CdTe and Si substrates with (111) surface orientation. The film properties were improved by post-deposition annealing in a Hg atmosphere using a two-zone furnace. An increase in electron mobility of n-type films to values up to 17 and 50 percent of those for bulk material at -188 and 24°C respectively were obtained using unoptimized annealing parameters.

Two commonly used passivants of Hg0.8Cd0.2Te, the anodic oxide and ZnS, have been studied by x-ray photoelectron spectroscopy combined with ion sputtering. Chemical depth profiles of anodic oxide films of 360 to 1600 A showed that the oxide composition is constant with depth and independent of oxide thickness. Chemical shifts and line shape analysis of the Cd M45N45N45 Auger transition in the oxide, CdO, Cd(OH)2, and CdTeO3 demonstrate that CdTeO3 is the major constituent of the anodic oxide. The oxide composition is interpreted as 44% CdTeO3 , 29% CdTe205, 17% HgTe03, and 10% HgTe205. Anodization of HgCdTe depletes the semiconductor of 30% - 40% of its Mg near the interface. The spatial extent of this Hg depletion is a function of oxide thickness for thin oxides (<1000 Å) but is a constant (150-200 Å) for thick films. No significant change in the Cd concentration is seen. A ZnS film deposited on a chemically etched sample forms a graded interface of a (ZnHgCd)Te alloy. In this case, no Hg depletion is seen. Deposition of ZnS on an anodized substrate in high vacuum leads to a reaction of the Zn with the residual 02 in the chamber to form ZnO on the anodic oxide before the ZnS. The ZnO then diffuses throughout the anodic oxide.

For extrinsic silicon detector arrays, performance is optimized by a careful compromise among several material characteristics affected by growth and processing parameters. In this paper we discuss the role of the major and minor dopant concentrations on spectral response, spatial uniformity of responsivity, impact ionization and impurity band conduction. The impact of these on detector operational constraints is a key element in focal plane detector array design.

The performance of an extrinsic silicon detector array depends on achieving uniform distribution of both the major dopant and compensating impurities in the array. While in some cases uniformity can be achieved during device processing (for example, by neutron transmutation doping to control compensation), usually the uniformity must be grown in. We report the influence of changing the pull rate and rotation rate in the float zone growth of gallium-doped silicon on the resulting distribution of gallium in the crystal. The gallium distribution was monitored by etching studies and by spreading resistance and four-point probe measurements. High rotation rates and low pull rates favor higher uniformity. These results can be understood in terms of steady-state segregation theory. We also examined the effects of prolonged diffusion on originally nonuniform Si:Ga by monitoring spreading resistance and measuring Hall effect vs. temperature. Observed improvements in uniformity were consistent with diffusion theory; 16 days of diffusion at 1300°C produced marked improvement in Ga uniformity.

Electrical and optical measurements were used to characterize silicon-rich Si-Ge alloys up to 11.3% Ge content. Both high-resistivity p-type material, for use as p-i-n detectors for 1.06-μm radiation, and In-doped material, for use as an extrinsic IR detector beyond 8 μm, were studied. The measurement techniques include Hall-effect measurements versus temperature and photoresponse, photoconductivity, and absorption spectra. A major result is that multiple energy levels were found for acceptors. These energy levels arise from the interaction of the acceptor with its nearest-neighbor shell, and each level corresponds to a different number of Ge atoms in this shell. The concentration of acceptors having a given level corresponds to the probability of the nearest-neighbor shell having the corresponding number of Ge atoms; the probability of a Ge atom occupying a lattice site in the nearest-neighbor shell is from 0.5 to 1.0 times the probability of occupying a random lattice site. These results strongly affect the use of this material for extrinsic IR detectors because an extended response to long wavelengths results. We will also discuss the effects of the disorder in these materials on mobility and on spectral response and diffusion length measurements made on p-i-n detectors fabricated from these materials.

Recently, a class of shallow acceptor levels in silicon, called X-levels, have been discovered which are associated with all the Group III A acceptors. The indium related X-level at 0.113eV appears to be ideally suited to match the 8-12μm atmospheric window. This defect has been shown to arise from a substitutional In and C nearest neighbor pair. We have performed a variety of measurements to determine the feasibility of utilizing the In-X defect in a photoconductor. In particular, silicon crystals heavily doped with both indium and carbon have been grown by thermal gradient transport from indium solutions. Annealing studies have been done to determine whether In-X concentrations in excess of 1-1017cm-3 can be produced in reasonable annealing times. Such a concentration would be necessary to produce detectors with adequate quantum efficiency.

A method for growth of single-crystal silicon doped heavily, with thallium has been developed to make use of the good match of the thallium acceptor cutoff wavelength (5.0μ) to the 3-5μ atmospheric window. The method involves recrystallization of silicon from a tin-thallium solution kept saturated by a silicon source wafer. Growth conditions used success fully to date range from 1100°C to 1370°C in growth temperature, and from 7% to 50% tin fraction in the tin-thallium solvent. The thallium doping increases monotonically with increas, ing growth temperature or thallium fraction in the solvent. A model of the dependence of the thallium doping on temperature and solvent composition is presented, as well as the estimated solubility limit of thallium in silicon. The most heavily doped crystal was grown at 1370°C from a Sn.14T1.86 solvent, and produced a maximum photoionization absorption of 2cm-1 at 3μ. This corresponds to a predicted quantum efficiency of 18% in a 1-mm thick detector. The Hall mobility of the grown material near liquid nitrogen temperature is found to approach 2000cm2/V-sec.