C.L. Jones

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.

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.

Acceptor doping of bridgman-grown CdxHg1−xTe

Abstract The behaviour of various dopants known to be acceptors in Cd x Hg 1− x Te has been studied in crystals grown by the Bridgman process. Dopants from Groups I (Li, Cu, Ag) and V (P, As, Sb) were added, in elemental form, to the initial melts. Chemical analysis was used to determine the segregation behaviour along the grown crystals and, linked to Hall effect measurements, to establish the electrical activity of each dopant. More extensive studies were then carried out on examples (Ag and Sb) from each group. These two dopants are believed to be the slowest diffusing species within their group, making electrical characterisation and doping control easier. Linear relationships were found between carrier concentration and amount of dopant added for both elements. Hall effect measurements down to 20 K have been performed and acceptor ionisation energies determined. The values for doped material are compared to those from samples containing native acceptor defects and are found to be shallower. Isothermal annealing in mercury vapour has been used to study the stability of the dopants and to assess the suitability of the material for use as substrates for diode fabrication by ion implantation.

Factors affecting isotherm shape during Bridgman crystal growth

Abstract Isotherm shape is an important factor controlling the quality of Bridgman grown crystals. A convex (relative to the solid) isotherm will often lead to the best crystallinity but if segregation is important a flat isotherm is desirable. A thermal model is used to establish the effects of temperature dependent conductivity, ampoule transparency and furnace temperature profile on heat flows and temperature distributions in the growing crystal. The shape of the first-to-freeze isotherm is found to depend on the furnace profile and the way heat is lost from the base of the melt. A convex isotherm requires a shallow profile and high end losses. As growth progresses end losses are less important and the growth isotherm shape is determined by the crystal-melt conductivity and the furnace profile.

Thermal modelling of bridgman crystal growth

Abstract Electrical analogues have been used to model the thermal behaviour of a Bridgman crystal growing system. Computer analysis of the analogues yields detailed information on temperatures and heat flows in a complete system, i.e. furnace, water-jacket, ampoule and crystal. For the particular system modelled it is found that isotherm shapes within the crystal during the critical early stages of growth are strongly affected by the way heat is lost from the bottom of the ampoule. By changing the shape of the ampoule bottom or by changing the conductivity of the stem supporting the ampoule the isotherm shapes can be readily altered. The model also shows that the crystal growth rate is 15% slower than the ampoule lowering speed early in the growth cycle.

Effect of process changes on the properties of CdxHg1-xTe crystals grown by Bridgman using ACRT

Abstract Changes to the basic accelerated crucible rotation technique (ACRT) as applied to the growth of CdxHg1−xTe (CMT) have been effected and the resultant crystal properties determined. Faster stable growth rates are found to be possible with ACRT. The use of flat-based growth ampoules leads to improved macrocrystallinity and radial compositional uniformity in the early stages of growth. Predictions made from quenching experiments on the effect of maximum rotation rate have been confirmed. Addition of convective mixing to that produced by ACRT gives rise to a deterioration in crystal properties. Changes in starting composition together with changes to the maximum rotation rate enables a degree of “tuning” of the process, in terms of x, to be achieved. It appears that an optimum amount of melt mixing can be set up which results in large amounts of material with the desired composition and relatively free from grain boundaries, which are known to have adverse effects on devices. Larger diameter crystals have also been produced with good radial compositional uniformity.

Comments on segregation during bridgman growth of CdxHg1-xTe

Abstract During Cd x Hg 1- x Te growth CdTe and HgTe segregate between solid and melt, leading to composition gradients in the crystal. A method is described for calculating an effective segregation coefficient, k , from axial composition profiles. The variation of k with composition is plotted for a number of growth conditions.