A. Royle

Crystal growth and properties of group IV doped indium phosphide

Abstract InP crystals have been grown from the melt by the high pressure liquid encapsulation technique. A study of the segregation of group IV elements based on measurements of free electron concentrations in doped crystals indicates that the mean effective distribution coefficients for Ge and Sn are 2.4 × 10 −2 and 2.1 × 0 −2 respectively. Whereas doping with Ge and Sn gave high quality and reproducible n-type crystals, controlled doping experiments using Si and Pb were not successful. The role of these latter elements together with that of C is discussed. The characteristic photoluminescence spectra of these doped crystals were measured at 77°K, and in conjunction with the electrical and chemical evidence thepeaks were assigned to specific impurity/vacancy interactions. The highest energy peak at about 1.41 eV is associated with donors and is sensitive to Ge and Sn doping, while the acceptor level appears to lie at 1.37 eV. The peaks at 1.32 eV and 1.35 eV are attributed to the effects of growth from B 2 O 3 and copper doping respectively, and the broad peak between 1.12 and 1.20 eV may be associated with phosphorus vacancies.

Recent developments in the MOVPE of II–VI compounds

This review is concerned with developments that have taken place over the last year or so in the field of metalorganic vapour phase epitaxy of II–VI compounds. In this rapidly developing field the compounds attracting most study are CdTe, CdxHg1−xTe, HgT e, ZnSe, ZnSxSe1−x, ZnS, CdS and ZnO. These developments, which are considered within the framework of existing knowledge, cover organic reagents or precursors, alternative substrates, reaction mechanisms and developments in growth technique. Prominence is given to Photo-MOVPE and low temperature growth. Improvements in material quality and recent device advances using MOVPE are also reported.

The growth of high quality CdxHg1−xTe by MOVPE onto GaAs substrates

Abstract The growth of Cd x Hg 1− x Te layers by metalorganic vapour phase epitaxy (MOVPE) onto (100) 2°→(110) GaAs substrates is reported. The mirror smooth epitaxial layers have been grown with a reproducible structural quality that is comparable to layers grown onto the best CdTe substrates. By growing a sufficiently thick buffer layer to isolate the active layer from the substrate, the potential problem of Ga diffusion out of the substrate into the layer has been successfully controlled. As a consequence the Ga concentration in the active layer has been reduced to a level well below that for normal background contamination.

Recent developments in the pyrolytic and photolytic deposition of (Cd,Hg)Te and related II–VI materials

Abstract Recent results for the pyrolytic MOVPE of cadmium mercury telluride (CMT) and related compounds are reported. In pyrolytic MOVPE the role of “new” tellurium precursors and their potential for developing low temperature epitaxy is reviewed. Very high purity CMT layers which show classic p-type behaviour are discussed. The limitations of the cool down process in giving rise to complex (“anomalous”) Hall measurements are analysed. The interdiffused multilayer process (IMP) has been used to grow very uniform CMT ( Δx = 0.004 over 2.5 cm) using iPr 2 Te at 350°C. Growth on GaAs substrates has been shown to give some of the best crystallographic quality CMT layers yet reported. MOVPE CMT has been fabricated into infrared detectors showing state-of-the-art performance in the 8–14 μm band. Developments in photolytic growth of CMT and particularly CdTe are reported, in particular the use of laser photo-induced epitaxy, to achieve local area patterning is demonstrated.

The potential for abrupt interfaces in CdxHg1−xTe using thermal and photo-MOVPE

This paper reviews the current progress in the growth of abrupt structures in the infrared detector alloy CdxHg1−xTe, with special reference to metalorganic vapour phase epitaxy (MOVPE). Recent results on the growth of heterostructures using the interdiffused multilayer process (IMP) are described for epitaxy onto GaAs as well as CdTe substrates. It is envisaged that useful heterostructure devices can be grown where the interface widths are of the order of 0.3 μm. For more abrupt structures, lower growth temperatures are needed and this has been demonstrated using the new photolytic MOVPE process. Epitaxial growth at temperatures as low as 200°C has enabled measured interface widths of approximately 100 A to be realised for a HgTe/CdTe structure. Lower growth temperatures also reduce the rates of diffusion of dopants such as Ga from a GaAs substrate. Ga concentrations of just 0.05 ppma have been measured 500 A from a CdTe/GaAs interface. Detailed Hall measurements on photo-MOVPE HgTe and HgTe/CdTe structures have shown that high quality epitaxial layers can be grown. A study of the limitations on the electrical quality has shown some influence of impurity diffusion in very thin layers but for thicker layers ( > 1 μm) this has little effect. The possibility of using photo-MOVPE for the growth of CdTe/HgTe superlattices has been explored by predicting the extent of interdiffusion at 200°C and 150°C. Even at 150°C, the predicted interdiffusion in just 10 min is significant. However, interdiffusion may depend critically on the dislocation structure and strain at the interface. Structural studies on thin epitaxial layers shows the critical dependence of structure, and strain, on the substrate orientation and layer thickness.

Epitaxial growth of HgTe by a MOVPE process

Abstract Epitaxial layers of HgTe have been grown onto insulating CdTe substrates by the pyrolysis of (C 2 H 5 ) 2 Te in the presence of Hg vapour using a H 2 flow system. Temperature-dependent Hall effect and conductivity measurements have shown that the electrical properties of the layers are comparable with good quality bulk HgTe.

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.

Crystal growth and characterisation of cadmium telluride: A modified solvent evaporation technique

Abstract The growth of CdTe crystals completely free from twins and visible grain boundaries is described using a modified solvent evaporation technique. The method was designed to avoid the temperature fluctuations that can be an inherent feature of the original technique of Lunn and Bettridge (Rev. Physique Appl. 12 (1977) 151). The growth and characterisation of undoped and doped crystals using In, Ga, Cl and Cr as dopants are described. The principal growth factors controlling the resistivity of the undoped crystals are discussed together with a review of the deep levels that control the resistivity.

Boron-segregation in Czochralski-grown CdTe

Abstract Chemical analysis of CdTe ingots grown by the liquid encapsulated Czochralski technique using B 2 O 3 encapsulant show that the material contains up to 90 ppma boron and that the boron distribution is non-uniform. High concentrations are found at twin and grain boundaries. Boron-rich precipitation is also observed. The observation of low carrier concentrations ( n 77 K = 8×10 14 cm -3 ) and high electrical mobility (μ 77 K = 4800 cm 2 V -1 s -1 ) in this material also supports the thesis that most of the boron is not electrically active.

On the electrical properties and Hall effect behaviour of MOVPE CdxHg1−xTe

Abstract The electrical properties of as-grown MOVPE layers of Cd x Hg 1− x Te are discussed in the context of the fundamental and the preparation-dependent parameters which determine their characteristics. A simple model is used to predict the equilibrium vacancy concentration as a function of temperature and Hg pressure in the range 250–430°C. The preparation-dependent parameters associated with substrates, growth and post growth processes are considered with references to experimental examples. The results of both two layer and single layer Hall models are used to illustrate the role surfaces and interfaces and their significance on the measured Hall results.