The results of electrical characterisation of a series of MOVPE layers of CdxHg1−xTe (CMT) grown by the interdiffused multilayer process (IMP) are reported. It is shown that the properties of the CMT layers when grown as a “sandwich” between CdTe buffer and cap layers display classical p-type Hall effect curves. These have been modelled and the sensitivity of the fitting parameters are reported. It is shown that there is evidence of complex (“anomalous”) two-layer like behaviour in low x material which is not attributable to inversion behaviour.

Mercury cadmium telluride layers were grown at 350°C by metal organic vapour phase epitaxy (MOVPE) using the interdiffused multilayer process (IMP) and doped with indium or aluminium. Aluminium doping yielded a low electrical activity and displayed large concentration oscillations with the IMP period. This has been attributed to the formation of oxide and carbide inclusions. Indium doping does not display this oscillatory behaviour and the electrical activity was close to 100% after low temperature annealing at concentrations up to 3 × 1017 cm−3.

Epitaxial layers of CdxHg1−xTe (CMT) have been grown by the interdiffused multilayer process (IMP) at 350 °C. The electrical properties are fixed by the equilibrium conditions of the interdiffused structure and subsequent cooling of the layer in a Hg ambient. Indiffusion of Hg during cooldown can be prevented by the growth of a 1000-Å-thick cap layer of CdTe. X-ray double-crystal rocking curves for the 400 reflections have been measured to find whether IMP introduces any residual structure or strain into the CMT. For CMT grown onto CdTe/GaAs, a postgrowth anneal of 30 min or longer reduces rocking curve widths to below 100 arc s, with a best value of 55 arc s. The best value on CdTe substrates was 72 arc s. Depth profiling of x-ray peak widths for CMT/CdTe/GaAs structures has shown that the CMT/CdTe interface improves the crystalline quality of the epilayer. One contribution to peak width broadening has been identified as pyramidlike morphological defects. Epitaxial layers have been grown onto GaAs substrates with regions >1 cm2 free of pyramids.

Recent developments in Metalorganic Vapour Phase Epitaxy (MOVPE) of CMT are reviewed and the potential of the technology for producing advanced IR cadmium mercury telluride (CMT) detectors, particularly planar diode arrays, are assessed. The assessment covers the current status of layer production, including compositional uniformity, surface quality and crystalline perfection as well as the electrical properties and doping. In addition, the role of the newer, low temperature precursors as a means of producing advanced detector structures are considered. Recent diode results are reported and it is shown that MOVPE CMT, either on CdTe or GaAs substrates, is capable of producing state-of-the-art performance for diode arrays. The future scope of photolysis for in situ device fabrication is also considered, together with some recent results on photo-patterning of epitaxial CdTe.

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 iPr2Te 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.

Chemical analysis of CdTe ingots grown by the liquid encapsulated Czochralski technique using B2O3 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 (n77 K = 8×1014 cm-3) and high electrical mobility (μ77 K = 4800 cm2 V-1 s-1) in this material also supports the thesis that most of the boron is not electrically active.

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 Å 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 Å 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.

The electrical properties of as-grown MOVPE layers of CdxHg1−xTe 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.

The Interdiffused Multilayer Processing (IMP) technique has been developed as a way of growing uniform layers of cadmium mercury telluride (CMT). The principle of the technique is discussed in the context of an interdiffusion coefficient so as to assess the potential applicability of the technique to the growth of other II–VI or III–V alloys. The development of IMP for the growth of CMT layers, together with the properties of the layers and their suitability for use in the fabrication of IR detectors, are reviewed.

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 CdxHg1−xTe 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.

The Hall coefficient RH of specimens of cadmium mercury telluride, suitably doped with acceptors, appear anomalously n-type at low temperatures (circa 50K and below). This has been shown to correlate with the extent of surface damage and oxidation. A two-layer model has been used to predict the double sign change and anomalous low temperature RH and is in good agreement with the experimental results.

A preliminary study has been carried out on the electrical properties and chemical purity of epitaxial layers of cadmium mercury telluride (CMT) grown by metal-organic vapor phase epitaxy (MOVPE). It has been shown that the mobility of such layers grown at 410 °C are comparable with bulk CMT of equivalent carrier concentration. SIMS profile studies of trace impurities in the epitaxial layers and CdTe substrates used for epitaxial deposition revealed significant impurity concentration enhancement (ICE effect) for a number of elements (Li, Na, K, Al, Ga, In and Mn) at the CdTe/CMT interface: some of these elements (Li, Na, K, Al and In) also showed this ice effect at the subsequent CMT/CdTe interface. The magnitude of the ICE effect could be quite large (a factor of ∼ 100 for Li) but its origin was not identified although potential mechanisms for its formation are discussed.

Epitaxial layers of HgTe have been grown onto insulating CdTe substrates by the pyrolysis of (C2H5)2Te in the presence of Hg vapour using a H2 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.

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.

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 results of a study on the LEC growth of GaAs doped with Si, Ge, Sn, Se and Te are compared where appropriate with similar results on Bridgman, non-LEC, and epitaxial methods. The study involves distribution coefficient measurements and the effect of doping on mobility carrier concentration and compensation. The study on compensation has shown that Se and Tedoped LEC crystals have compensation ratios (NA/ND) typically ∼0.3 over a wide doping range in agreement with various studies on epitaxial GaAs. These results and those for Si and Ge doped LEC crystals which show values of compensation ratio of 0.4 and 0.8, respectively, are analysed using current physico-chemical and electrical models of compensation.

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 B2O3 and copper doping respectively, and the broad peak between 1.12 and 1.20 eV may be associated with phosphorus vacancies.