J.B. Mullin

A new MOVPE technique for the growth of highly uniform CMT

Abstract A new method for growing cadmium mercury telluride (CMT) is described where alternate thin layers of CdTe and HgTe are grown by metalorganic vapour phase epitaxy (MOVPE) and allowed to interdiffuse at the growth temperature. Using this technique epitaxial layers have been grown with both good depth and lateral uniformity and a reflective surface finish. Preliminary electrical measurements indicate that for the best layers the electrical properties are dominated by mercury vacancies rather than impurities.

Organometallic growth of II–VI compounds

Abstract Developments in the organometallic growth of II–VI compounds are reviewed with an emphasis on epitaxial growth related problems. The advantages and disadvantages of organometallic growth are compared with analogous growth using conventional VPE techniques. Consideration is given to the growth of specific compounds using alkyls of Zn, Cd or Hg with the elements O, S, Se and Te as hydrides or alkyls. Special reference is given to the decomposition characteristics of typical alkyls such as (C2H5)2Te, (CH3)2Te and (CH3)2Cd by pyrolysis in H2. Reaction mechanisms involving homogeneous and heterogeneous pyrolysis, adduct formation and the potential role of adsorption on parameters such as growth rate are discussed.

The growth by MOVPE and characterisation of CdxHg1−xTe

Abstract Epitaxial growth of Cd x Hg 1− x Te has been achieved using sources of diethyltelluride, dimethylcadmium and liquid Hg in an H 2 stream. The major problem encountered in the growth of Hg rich Cd x Hg 1− x Te is the relatively high equilibrium Hg pressure that is needed to avoid decomposition. A low temperature limit to growth is set by the stability of diethyltelluride, this problem and the effect of addition compound formation in the vapour is discussed. Depth profiles for Cd, Hg and Te using EDX and SIMS have indicated good depth uniformity and an interdiffusion region less than 1 μm. A comparison is made for the Cd/Hg interdiffusion between MOVPE and some other epitaxial techniques. Optical microscopy and SEM studies of epitaxial layers grown on CdTe oriented 2° off (100) revealed a rough surface finish for HgTe but smooth surfaces for compositions above x = 0.2. These results are discussed in terms of a step growth model. A preliminary study of the Hall coefficient as a function of temperature has indicated a high free electron concentration at the lowest temperatures (∼4.2 K). SIMS analysis of one of these epitaxial layers has indicated a high level of In, but this has not been the case for other epitaxial layers investigated in this preliminary survey.

Photosensitisation: A stimulant for the low temperature growth of epitaxial HgTe

Abstract Epitaxial growth of HgTe layers on CdTe and InSb substrates is reported where ultraviolet (UV) radiation is used to dissociate diethyltelluride (Et2Te). Good quality single crystal growth has been achieved over a range of temperatures from 200 to 300°C, well below temperatures expected to pyrolytically dissociate Et2Te. It was dound that high Hg pressures (>1×10-2 atm) were needed to successfully dissociate Et2Te and grow HgTe. Mechanisms for the dissociation of Et2Te are discussed and a model is proposed which relies on a photosensitisation mechanism. This model is based on a surface selective reaction which is necessary for good epitaxial growth and high surface quality. The crystalline quality of these epitaxial layers has been established using X-ray diffraction and electron channeling. Layer thickness nd growth rates were determined using a Scanning Electron Microscope (SEM). The results of growth rate measurements on different substrate orientations are discussed together with growth rate dependence on temperature and Et2Te concentration. It is found that surface kinetics as well as transport are important in this growth process.

Liquid encapsulation crystal pulling at high pressures

Abstract The technique of Liquid Encapsulation that is the use of an inert liquid as a seal in preventing the loss of volatile components from dissociable compounds or their melts is discussed with particular reference to the growth of compounds having dissociation pressures in excess of one atmosphere. A pressure chamber is described in which it is possible to pull crystals in a gas pressure of 200 atm. The practical application of the technique to the growth of good quality InP and GaP is also described; and a comparison of the impurity content of these crystals grown either from silica or vitreous carbon crucibles is made. The relationship between the impurity content of the crystals and their electrical properties is also briefly considered.

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.

The growth of highly uniform cadmium mercury telluride by a new MOVPE technique

Abstract A novel modification for the MOVPE of CMT is described and the quality of these epitaxial layers is discussed. This new method, interdiffused multilayer process (IMP), has enabled the growth of CMT with very good compositional uniformity comparable with LPE.

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.

A study of transport and pyrolysis in the growth of CdxHg1−xTe by MOVPE

Abstract A study has been made of some of the factors affecting transport in the metalorganic vapour phase epitaxy (MOVPE) growth of cadmium mercury telluride (CMT), and their influence on the layer composition. Layers have been grown at different reactor wall temperatures to investigate the problems of (a) maintaining a sufficient mercury pressure and (b) premature pyrolysis of metalorganics on the reactor wall. Experiments have also been carried out on the role of tilting the susceptor with respect to the reactor axis and on adjustments in the ratio of the inlet dimethylcadmium (Me2Cd) to diethyltelluride (Et2Te) concentrations. The results of these growth experiments have been interpreted as changes in the ratio of effective Cd and Te concentrations on the substrate. These changes have been attributed to a number of processes such as changes in inlet concentrations, premature pyrolysis on the walls and incomplete pyrolysis on the substrate. At low mercury pressures, where it can be less thab the equilibrium value at the tellurium rich solidus, the composition moves towards the CdTe rich part of the pseudo-binary in order to accommodate this drop in mercury pressure. These changes in composition will occur whether the growth rates are controlled by transport or surface kinetics.