P. Capper

Effects of annealing on the electrical properties of CdxHg1−xTe

CdxHg1−xTe with 0.17<x<0.31 has been annealed using closed‐tube and open‐tube methods. In both methods the mercury vapor pressure during an anneal was controlled by a mercury reservoir held at a temperature either the same as the CdxHg1−xTe (isothermal anneal), or lower than the CdxHg1−xTe (two‐temperature anneal). Isothermal anneals carried out in closed‐tube and open‐tube systems convert material which is initially p type by native defects to n type. Two‐temperature, closed‐tube anneals can be used to convert n type to p type, the acceptor concentration being controlled by the CdxHg1−xTe temperature and the mercury vapor pressure. Two‐temperature, open‐tube anneals also result in conversion from n to p, however, the mercury vapor pressure (over the range studied) does not influence the final acceptor concentration but does affect the time required to reach equilibrium. The results are discussed in terms of the pressure‐temperature diagram and the defect/impurity balance in CdxHg1−xTe .

Application of the accelerated crucible rotation technique to the Bridgman growth of CdxHg1− xTe: Simulations and crystal growth

Abstract Simulation studies have been carried out to determine the fluid flows induced in the tall narrow containers used in the Bridgman growth of Cd x Hg 1 - x Te by the influence of ACRT. Spiral shearing, Ekman and transient Couette flows have all been observed. Ekman flow and Couette flows are the dominant stirring mechanisms at the crystal/melt interface and crucible wall respectively. A comparison is given between experimentally determined critical rotation rates for the onset of Couette flow and those predicted using Rayleighs criterion. Crystals of Cd x Hg 1 - x Te grown using ACRT are found to have a much greater degree of axial and radial composition uniformity than equivalent non-ACRT crystals. In addition an improvement in crystallinity has been obtained which is thought to be due to Couette flows at the crucible walls.

Substrate orientation effects in CdxHg1−xTe grown by MOVPE

Abstract Layers of Cd x Hg 1− x Te (CMT) have been grown by thermal metalorganic vapour phase epitaxy (MOVPE) at temperatures between 420 and 350°C on CdTe substrates of various orientation. The differences in surface morphology, crystalline quality and electrical behaviour between these layers are described. Surface hillocks on layers grown close to (100) are the main features of interest and their behaviour at the different growth temperatures and on the various orientations will be described and compared. In (111) layers twinned regions are observed and evidence from X-ray, electrical and chemical assessment is presented which suggests that these twins may act as donor sites. For layers grown on substrates oriented 18° off (111) no twins are seen and the surface features are less prominent.

Minority carrier lifetime in n‐type Bridgman grown Hg1−xCdxTe

Minority carrier lifetimes have been mapped on slices of n‐type Hg1−xCdxTe (x∼0.3) grown by the Bridgman technique. The mean lifetime on a slice has been measured as a function of extrinsic carrier density and composition. Lifetimes at selected positions on slices have been measured as a function of temperature down to 30 K. Aging effects have also been investigated. We show that the lifetime variation with temperature cannot be explained by direct band‐to‐band recombination alone. The assumption that recombination also occurs via Shockley–Read type recombination centers, situated 10–30 meV below the conduction band edge, enables us to calculate theoretical values of the lifetime in good agreement with the experimental results. Such a model can also be used to explain the variation of lifetime with composition.

Growth of CdxHg1−xTe: Comparison of some properties with the predictions of two melt growth models

Abstract The theoretical predictions of the two normal freeze growth models involving no convective mixing and complete mixing are described. Crystals have been grown using a range of growth speeds and initial compositions in two temperature profiles designed to affect these conditions in the Cd x Hg 1− x Te (CMT) system. The axial and radial compositional uniformity found in these crystals is compared with the predictions of the two models. The relative effects on radial compositional uniformity of isotherm shape, radial segregation, density-driven convective flow and convection imposed by the temperature profiles are also discussed. It is found that radial variations in composition decrease as the growth rate decreases and that these variations must be taken into account when interpreting axial composition profiles. It is considered that density-driven convective flow is the most significant force in determining radial variations in composition. Convective flow induced by the temperature profile is also seen to influence these variations while isotherm effects and radial segregation are thought to be much less significant.

Effect of ACRT rotation parameters on Bridgman grown CdxHg1−xTe crystals

A microprocessor-controlled Bridgman grower is described which has been specifically designed for ACRT growth of CdxHg1−xTe crystals. The improved control and monitoring of both growth and rotation parameters has enabled us to increase the understanding of the effects of ACRT on crystals properties. Crystals have been grown using various ACRT sequences and characterised in terms of axial and radial x uniformity and macrocrystallinity. It is shown that rapid acceleration/deceleration is the most critical part of the ACRT cycle while stop time, run time at constant rate and rotation reversal are of less importance. One of the ACRT sequences has produced a crystal with radial x variations which are inverted to the normal direction. This, and the improved macrocrystallinity in this crystal, suggest that a convex growth interface may have been achieved.

Quenching studies in bridgman-grown CdxHg1−xTe

Rapid quenching of crystals of CdxHg1−xTe (CMT) while growing at the slow rates of the Bridgman process reveals the solid/liquid interface at the point of quenching. The shape and extent of these interfaces, revealed by etching longitudinally-cut sections, have been related to the resulting radial composition variations as determined by infra-red transmission measurements. In the singly-quenched crystals features are seen at the interface which are believed to be diffusional boundary layers. Optical microscopy and electron microprobe analysis have been used to determine the thickness of these layers. The evidence suggests that melt stirring reduces the thickness as expected for diffusional boundary layers.

Quenching studies in CdxHg1−xTe crystals grown using ACRT

Abstract Quenching of Cd x Hg 1− x Te (CMT) crystals while growing under conditions of accelerated crucible rotation (ACRT) has revealed solid-liquid interfaces. Optical microscopy and electron microprobe analysis have been used to study these interfaces in crystals grown over a range of ACRT parameters as well as at different growth rates and in both conical and flat-based ampoules. Trends are seen in the shape and depth of interface and therefore crystal compositional uniformity with increasing maximum rotation rate. A “pasty” region is seen in front of some interfaces consisting of alternating cadmium enriched and depleted zones. Faster stable growth rates appear possible using ACRT and flat-based ampoules lead to a further improvement in crystallinity.

Interfaces and flow regimes in ACRT grown CdxHg1−xTe crystals

An attempt has been made to explain quantitatively the interface depths seen in crystals of CdxHg1−xTe quenched while growing under ACRT conditions. Three regimes of acceleration are identified and linked to the interface depths at various rotation rates. Stability in the Ekman flow is found to be critical in maintaining a flat growth interface. Attempts to freeze-in Couette flows in ACRT melts were not successful, although they are believed to exist. Axial composition variations have revealed a stirred Ekman region ahead of a growth interface in one crystal. As the starting melt composition increases a slush region forms ahead of the interface which could be associated with constitutional supercooling.

Incorporation and activation of group V elements in MOVPE-grown CdxHg1-xTe

Abstract Reproducible acceptor doping has been achieved in MOVPE-grown layers of Cd x Hg 1- x Te using Group V elements. Dopant element concentrations have been controlled over the range 5×10 15 -8×10 17 cm -13 for As. Agreement has been obtained between these levels and Hall measurements made on annealed samples, at least for concentrations above 5×10 16 cm -3 . At lower levels residual impurities may exert a significant influence on the electrical behaviour of layers. In order to achieve dopant activation it has been found necessary to establish “metal-rich” conditions during the CdTe growth cycle. High temperature anneals to activate the dopant are thus avoided which permits the growth of heterolayers with sharpe interfaces, both in terms of x and dopant concentration.