John H. Tregilgas

The effect of low temperature annealing on defects, impurities, and electrical properties of (Hg,Cd)Te

Many methods for the preparation of (Hg,Cd)Te alloys rely on a low temperature processing step to convert the as‐grown p‐type material to n‐type, or to otherwise adjust the concentration of native acceptors. During this anneal, tellurium precipitates in the material are annihilated by in‐diffusing mercury, resulting in a substantial multiplication of dislocations. For substantially long anneals (>1 day at 270 °C) the depth of the p–n junction is found to vary as the square root of the anneal time and inversely as the square root of the excess tellurium concentration. Rapidly diffusing impurities such as silver are gettered out of the skin and into the remaining vacancy‐rich core. The kinetics of these processes are analyzed for self‐diffusion on the metal sublattice involving only vacancies, only interstitials, and for a mixed vacancy–interstitial model. Comparison with experimental data shows best agreement with the mixed interstitial–vacancy model.

The minority carrier lifetime in doped and undoped p-type Hg 0.78 Cd 0.22 Te liquid phase epitaxy films

This paper will describe: (1) the first comparative study of recombination mechanisms between doped and undoped p-type Hg1-xCdxTe liquid phase epitaxy films with an x value of about 0.22, and (2) the first determination of τA7i/τA1i ratio by lifetime’s dependence on both carrier concentration and temperature. The doped films were either copper- or gold-doped with the carrier concentration ranging from 2 x 1015 to 1.5 x 1017 cm-3, and the lifetime varied from 2 μs to 8 ns. The undoped (Hg-vacancy) films had a carrier concentration range between 3 x 1015 and 8 x 1016 cm-3, and the lifetime changed from 150 to 3 ns. It was found that for the same carrier concentration, the doped films had lifetimes several times longer than those of the undoped films, limited mostly by Auger 7 and radiative recombination processes. The ineffectiveness of Shockley-Read-Hall (SRH) recombination process in the doped films was also demonstrated in lifetime vs temperature curves. The important ratio of intrinsic Auger 7 lifetime to intrinsic Auger 1 lifetime, τA7i/τA1i, was determined to be about 20 from fitting both concentration and temperature curves. The reduction of minority carrier lifetime in undoped films can be explained by an effective SRH recombination center associated with the Hg vacancy. Indeed, a donor-like SRH recombination center located at midgap (Ev+60 meV) with a capture cross section for minority carriers much larger than that for majority carriers was deduced from fitting lifetime vs temperature curves of undoped films.

The activation energy of copper shallow acceptors in mercury cadmium telluride

The thermal activation energy (E0) of isolated copper shallow acceptors in bulk Hg0.8Cd0.2Te has been determined using variable‐temperature Hall measurements on samples with 77‐K carrier concentrations ranging between 1015 and 1017 cm−3. It was found that the decrease of the activation energy with the increasing hole concentration can be adequately described by the screening of the acceptor potential by free carriers. The measured E0 (11.5 meV) also agrees well with that predicted by the effective‐mass theory.

Precipitation of tellurium in (Hg, Cd)Te alloys

The precipitation of tellurium in alloys of Hg1−xCdxTe with x = 0.2 to 0.3 prepared under conditions of excess tellurium by the solid state recrystallization method have been studied using chemical defect etching and transmission eloctron microscopy. Precipitation of tellurium occurred during the quench from the recrystallization anneal. During the initial part of the quench, precipitation occurred as a result of nucleation of precipitates on pre-existing dislocations. This precipitation resulted in the formation of dislocation helices with precipitates in the interior of the helix loops. Both monoclinic and trigonal phases of tellurium were found in these precipitate defects, as well as the twinned matrix phase. Later in the quench, an additional nucleation mechanism, either homogeneous or heterogeneous on an impurity occurs resulting in the formation of complex defects in the bulk. During a subsequent post-anneal at temperatures of approximately 300°C, the tellurium precipitates dissolve, and interstitial perfect and faulted dislocation loops are formed. Both the precipitation process and the subsequent post-annealing result in considerable multiplication of dislocations.

Dislocations and electrical characteristics of HgCdTe

Abstract The electrical characteristics of HgCdTe samples have been experimentally examined as a function of dislocation density. MIS measurements on samples with a graded dislocation density ranging from about 2×10 5 to 2×10 6 cm 2 showed the net doping concentration is independent of local dislocation density, but that MIS dark current varies linearly with dislocation density. No evidence for impurity segregation to dislocations was observed between the low and high dislocation density region. Hall measurements on n-type HgCdTe samples both before and after 2% plastic deformation showed that the net donor density increased slightly by (1−2)×10 14 cm -3 possibly from residual point defects generated by deformation. Conversely, the electron mobility at low temperatures decreased by about an order of magnitude due to increased scattering from charged dislocations whose density increased by a factor of more than 100. Deformed HgCdTe shows a larger decrease in the electron mobility after low temperature plasma hydrogenation than do undeformed samples. Dislocation density may influence the hydrogen solubility in these samples. Collectively, both the MIS and Hall results suggest dislocations themselves do not contribute to the carrier concentration in HgCdTe.

Surface segregation of impurities induced by photon absorption in CdTe and (Hg,Cd)Te

Noble metal impurities have been observed to segregate toward the surface of p‐type CdTe and heavily p‐type (Hg,Cd)Te samples during room temperature aging in ambient room light. This behavior has not been observed in n‐type CdTe or lightly p‐type (Hg,Cd)Te. The photon‐induced segregation is believed to involve the redistribution of positively charged impurity interstitials in a field generated by absorbed photons with energies greater than the band gap. A qualitative electromigration model based on the Dember effect and simple ionic conduction, has been used to explain the experimental results.

Trace copper measurements and electrical effects in LPE HgCdTe

Recent improvements in sputter initiated resonance ionization spectroscopy (SIRIS) have now made it possible to measure copper in HgCdTe films into the low 1013 cm−3 range. We have used this technique to show that copper is responsible for type conversion in n-type HgCdTe films. Good n-type LPE films were found to have less than 1 x 1014 cm−3 copper, while converted p-type samples were found to have copper concentrations approximately equal to the hole concentrations. Some compensated n-type samples with low mobilities have copper concentrations too low to account for the amount of compensation and the presence of a deep acceptor level is suggested. In order to study diffusion of copper from substrates into LPE layers, a CdTe boule was grown intentionally spiked with copper at approximately 3 x 1016 cm−3. Annealing HgCdTe films at 360°C was found to greatly increase the amount of copper that diffuses out of the substrates and a substrate screening technique was developed based on this phenomenon. SIRIS depth profiles showed much greater copper in HgCdTe films than in the substrates, indicating that copper is preferentially attracted to HgCdTe over Cd(Zn)Te. SIRIS spatial mapping showed that copper is concentrated in substrate tellurium inclusions 5–25 times greater than in the surrounding CdZnTe matrix.

Type conversion of (Hg,Cd)Te induced by the redistribution of residual acceptor impurities

Bulk (Hg,Cd)Te grown by the quench‐anneal process has been found to contain residual acceptor impurities which are normally depleted from the n‐type surface by an internal gettering mechanism during low temperature postannealing in saturated Hg vapor. When the n‐type slices are postannealed for much longer periods, the Te saturated core is annihilated, and the gettered residual acceptor impurities can redistribute into the surface region. If upon homogenization the residual acceptor concentration exceeds the residual donor concentration, the material will convert to p type. A new method for applying neutron activation analysis (NAA) to (Hg,Cd)Te has been used to identify this acceptor impurity as Cu at levels below 1 ppm. An extension of the NAA technique has employed quartz encapsulated silicon samples to demonstrate Cu may come from either the quartz or annealing environment.

Observation of a new gettering mechanism in (Hg, Cd)Te

Abstract A new gettering mechanism is proposed for substitutional impurities which diffuse by an interstitial process. In this mechanism, an externally imposed gradient of self interstitials generates a gradient in impurity interstitials leading to the segregation of fast diffusing impurities to low interstitials, high vacancy regions. Data are presented to support this model in (Hg, Cd)Te alloys, as well as explicitly rule out gettering by dislocations, by segregation to precipitates, or by enhanced solubility arising from the interaction of the impurity with a varying Fermi level.

Surface segregation of Ag in CdTe induced by photon absorption

The first observation of a new impurity segregation or gettering phenomenon has been demonstrated in CdTe using radioactive Ag. Noble metal dopant impurities have been found to segregate to the surface region of CdTe during exposure to ambient light. This phenomenon occurs while the sample is held at room temperature and is not induced by localized heating or surface chemical reactions. Experimental results suggest that a redistribution of excess carriers produced by photon absorption near the CdTe surface may be responsible for the behavior.