D.J. Williams

Growth of CdxHg1-xTe by a pressurised cast-recrystallise-anneal technique

Growth of large-grained CdxHg1-xTe ingots with 0.15<x < 0.30 has been achieved using a casting and recrystallisation technique in which the sealed quartz growth ampoule is subjected to a high external pressure of helium gas. This opposes the internal vapour pressure, permitting the use of thin-walled silica tube and the exploration of a wide range of casting rates. This investigation indicates that the optimum casting rate is a balance between the formation of porosity at high rates and segregation of the constituents at low rates. The cast structure and recrystallisation temperature gradient play a major part in determining the rate and efficacy of the recrystallisation process and details are given of changes in the structure of CdxHg1-xTe which result from changes in recrystallisation procedures. Finally, we present data on the correlation between annealing conditions and the observed electrical carrier concentration, which indicate that, at low carrier concentrations, the electrical activity is dominated by the presence of a donor impurity.

The influence of substrate surface preparation on the microstructure of CdTe grown on (001) GaAs by metalorganic chemical vapour deposition

Abstract A cross-sectional transmission electron microscopy study has been performed on the microstructure of CdTe epitaxial buffer layers, which were grown upon 2°-off (001) GaAs substrates by metalorganic chemical vapour deposition (MOCVD). The substrates were prebaked at various temperatures instead of the usual acid etching treatment. Ut was shown that single crystal films were only obtained for baking temperatures of 550 and 590°C. These single crystal deposits exhibited a distinctive anisotropic defect structure with subgrain boundaries and 90°-type misfit dislocations perpendicular to the off-cut axis and stacking faults and a mixture of 60°-type and 90°-type misfit dislocations parallel to the off-cut axis. All of the stacking faults and 60°-type dislocations lie on the plane (111) producing a 6° rotation between the substrate and deposit lattices about the offcut axis. The development of this microstructure has been ascribed to the formation of high aspect ratio island nuclei and differences in Schmid factors due to the offcut. This microstructure was compared to that produced on an etched substrate and it appears that fine scale substrate surface roughness eliminates both of these influences.

Sub-grain boundaries in CdxHg1−xTe and CdTe

Abstract Sub-grain boundary structure in Cd x Hg 1− x Te and CdTe has been investigated and is found to result primarily from slip and polygonisation. Sub-grain coalescence occurs during Cd x Hg 1− x Te high-temperature annealing, and using this process boundary-free material can be obtained.

Growth of CdTe by solvent evaporation

Recent improvements to the Solvent Evaporation (SE) process are described. It is shown that good crystallinity is achievable provided that the solid-liquid interface is convex with respect to the solid and that the pyrolysed carbon coating of the ampoule adheres firmly to the wall. Breakdown of the coating leads to carbon-rich inclusions in the material and to dissolved carbon in the crystal lattice. C-V measurements indicate that carbon-rich areas are higher n-type than carbon-free regions, suggesting that carbon is a residual donor impurity in CdTe.

Transmission electron microscopy of LPE grown CdHgTe

The defect structure in CdxHg1-xTe layers grown by LPE on {111} B oriented CdTe and CdZnTe substrates has been studied by transmission electron microscopy. The interface between the epilayer and the substrate is characterized by a dislocation network and a variation in Hg/Cd concentration both of which have a finite thickness. The geometry and nature of the interfacial dislocations have been determined by examination of a combination of transverse and planar bevelled TEM specimens. One source for the interface dislocations has been shown to be the dislocations in the substrate. Layers grown on CdZnTe show a much smaller density of interfacial dislocations. These are in the epilayer rather than at the chemical interface.

Surface damage of CdTe by mechanical polishing investigated by cross-sectional TEM

Abstract The damage caused to CdTe substrates by five mechanical polishing treatments has been assessed and compared by cross-sectional transmission electron microscopy. The best polishes were 0.05 μm Al 2 o 3 in ethanediol and Syton/H 2 O 2 slurry.

The control of growth dynamics by the use of designed precursors for the MOCVD of electronic ceramics: Part II1

Abstract MOCVD is increasingly being used for the deposition of ceramics such as Pb(Zr,Ti)O3, TiO2 and SrBi2Ta2O9 which are important in the fabrication of devices. In order to exploit the full potential of MOCVD the properties of the precursors need to be tailored in order that parameters in the MOCVD process are optimised. In this paper we describe our approach to ‘molecular design’ and discuss how the substitution of simple alkoxide groups by β-diketonates or donor-functionalised ligands can result in precursors with improved physical properties and optimum MOCVD characteristics.

The role of the initial nucleation stage in microstructural development for CdTe grown on heat-cleaned 2°-off (001)GaAs by metalorganic chemical vapour deposition

Abstract A transmission electron microscopy (TEM) study of the initial nucleation layers for the two-step growth of CdTe on 2°-off (001)GaAs substrates by metalorganic chemical vapour deposition (MOCVD) is presented. For substrates which are degreased and etched in the usual manner, the initial deposits are oriented with their axes parallel to those of the substrates and the defects are distributed isotropically. Initial deposits grown on substrates which are prepared by thermal etching at 545–585°C, however, contain both (001)- and (111)-oriented grains. When these polycrystalline initial deposits are heated to the “bulk” growth temperature, epitaxial regrowth occurs giving a single crystal film with a very distinctive anisotropic defect microstructure with stacking faults and a mixture of 60°-type and 90°-type misfit dislocations parallel to the off-cut axis and subgrain boundaries and 90°-type misfit dislocations in the orthogonal direction. All of the 60°-type misfit dislocations and most of the stacking faults lie on the plane (111) producing a rotation of 5–8° between the substrate and deposit lattices about the off-cut axis. These effects are explained in terms of differences in resolved shear stresses as secondary defects are introduced during the regrowth process.

A cross-sectional tem study of MOVPE grown CdHgTe

Abstract MOVPE CdHgTe (CMT) grown on (001) CdTe by the interdiffusion multilayer process (IMP) has been examined by cross- sectional transmission electron microscopy. The dislocation network between epilayer and substrate is narrower than for LPE and the epilayer retains no traces of the IMP structure. Low, crystallographic facets on the top surface of the CMT do not have below them any obvious defects in the epilayer. The taller, more irregular features hide a conical twinned volume originating from a point on the substrate surface. A specially prepared MOVPE double layer of HgTe and CdTe shows a puzzling difference in dislocation density between CdTe/HgTe and HgTe/CdTe interfaces.

Eutectic microstructures in the Cd-Hg-Te system

Abstract The microstructures of specimens from the Te-rich region of the Cd-Hg-Te system have been examined. Observations have revealed the presence of a true two-phase CdTe-Te eutectic and have confirmed the existence of a pseudobinary eutectic valley joining the Hg-Te and Cd-Te binary systems.