J.T. Mullins

A novel “multi-tube” vapour growth system and its application to the growth of bulk crystals of cadmium telluride

Abstract A novel crystal growth apparatus which separates the source and growth regions of a vapour transport system into separate vertical furnaces connected by a horizontal transport passage has been developed. The transport passage incorporates a flow restrictor which regulates the mass flow and decouples it from the source–sink temperature difference. Growth takes place on a seed crystal past which the growth envelope is dynamically pumped to ensure that the transport is diffusionless. This “multi-tube” arrangement offers several advantages over conventional linear vapour transport systems. In addition to control of the mass transport, source and growth regions are thermally decoupled and there is noninvasive optical access to both source and crystal growth regions and for in situ vapour pressure monitoring. In this paper, the results of in situ vapour pressure measurements made during the growth of a boule of cadmium telluride are presented and a flow model, which accounts for the significant molecular component of the transport, is developed and used to calculate the corresponding mass flows. This system offers the potential to give improved control of the nucleation and subsequent growth of large single crystals of high-vapour pressure opto-electronic materials such as the II–VI compounds.

Partial pressure monitoring in cadmium telluride vapour growth

Abstract A simple system, based on optical absorption, which allows the partial pressures of cadmium and tellurium vapour over CdTe to be monitored under typical vapour-growth conditions is described. Use of the 326, 479 and 508 nm lines of a cadmium spectral lamp allows the narrow 326nm elemental absorption of cadmium together with the broad (350–600 nm) absorption of the tellurium dimer to be easily monitored with silicon photodiodes and optical bandpass filters. As the two absorptions do not overlap significantly their optical densities may be determined and related to the concentration and hence partial pressures of cadmium and tellurium. The present apparatus has been calibrated for cadmium pressures over the range 1.4–95.2 mbar and for tellurium pressures from 0.2 to 80.3 mbar.

Thick epitaxial CdTe films grown by close space sublimation on Ge substrates

This paper reports, for the first time, the successful growth of 200 µm thick CdTe films on mis-oriented Ge(1 0 0) substrates by a cost-effective optimized close space sublimation method. It is found that, as the thickness increases to a few hundred micrometres, subgrains are formed probably as a result of the large density of dislocations and strain within the initial interfacial layers. The films are of high quality (x-ray rocking curve width ~100 arcsec) and high resistance (~109 Ω cm), and are thus candidates for x-ray and γ-ray detectors.

Optical absorption tails in Cd0.97Zn0.03Te and CdS at elevated temperatures

Abstract Low energy optical absorption in Cd 0.97 Zn 0.03 Te and CdS has been studied at temperatures up to 1014 and 1161 K, respectively, by measurements of the incandescence spectra of wafers of both materials. Exponential absorption (Urbach) tails were observed, characterised by a spreading energy which increased exponentially with increasing temperature. This contrasts with the inverse temperature dependence widely assumed to be the high temperature limit for this parameter. It may be that defects are dominating the absorption tail at high temperatures in these samples, rather than phonon induced electric fields.

Heteroepitaxial growth and properties of crystals of CdTe on GaAs substrates

This paper reports on the use of a seeded vapour phase technique to grow bulk crystals of CdTe onto commercially available 50 mm diameter (211)B GaAs substrates. High quality crystals, several mm in thickness were grown on the GaAs at linear growth rates of ~ 120 μm/h. Characterisation by double and triple axis XRD showed the best crystals to have θ-2θ FWHMθ values of ~ 24 arc sec corresponding to low strain dispersion (< 2×10-4). Rocking curve scans included two to three sharp peaks, indicative of some small mosaicity. When mapped across a the surface of the crystal, the FWHM was uniform and < 93 arc sec. Contactless resistivity showed a similar degree of uniformity with a mean value of 4.4 × 109 ± 1.6 × 109 Ω cm. Infrared microscopy showed that within the resolution of the microscope (~ 5 μm) there were very few Te inclusions.

Vapour phase growth of 100 mm diameter cadmium telluride boules on GaAs seeds

The Multi-Tube physical Vapour Transport (MTPVT) crystal growth technique has been scaled to grow single crystals of cadmium telluride 100 mm in diameter.

Growth of high crystalline quality CdTe layers on 100 mm diameter Ge substrates for room temperature radiation detection

Uniform layers of cadmium telluride have been successfully grown hetero-epitaxially on germanium seeds up to 100 mm in diameter. X-ray diffraction studies show high structural quality for layers up to 1.4 mm thick. By doping with chlorine, resistivities up to 1 × 109 Ωcm have been achieved.

In-situ monitoring during vapour phase growth of bulk crystals of cadmium zinc telluride

In-situ laser reflectance measurements and photography have been made during the hetero-epitaxial nucleation and growth of cadmium zinc telluride on GaAs seeds. Fabry-Pèrot oscillations are observed in the reflectance signal which persist up to several tens of micrometres layer thickness and the average reflected intensity provides information on the surface morphology of the growing crystal.