Properties of Tellurium based II-VI semiconducting materials

Abstract

Opto-electronic devices operating as radiation detectors in the infra-red region of the electromagnetic spectrum are currently of interest. By operating in the infra-red, particularly in the 8-12 µm wavelength range, it is possible to detect the infra-red radiation emitted by objects at ordinary temperatures and so to image in darkness. Furthermore, at such wavelengths, vision is also possible in mist, fog or smoke. Semiconducting materials which have an energy gap corresponding to the photon energy of the radiation of interest are suitable for fabricating such devices. The growth and characterisation of two such materials both formed from elements in groups IIB and VIA of the periodic table and generally refered to as II-VI compounds, forms the subject matter of this thesis. The first of these materials is the ternary compound mercury cadmium telluride ((Hg,Cd)Te). This is a well established infra-red material and was grown for this work in thin film form by Metal Organic Vapour Phase Epitaxy (MOVPE) using the Inter- diffused Multilayer Process (IMP). The resulting layers were characterised optically and electrically and were shown to be of excellent compositional uniformity, an important consideration for infra-red devices, but to contain extremely high acceptor concentrations in the as grown state. These high acceptor concentrations were attributed to mercury vacancies present due to the inherent weakness of the material. Fitting of the electrical data obtained from p-type samples using a multicarrier/multilayer transport model suggested that the mercury vacancy concentrations were also highly non-uniform. A more novel alternative to (Hg(_1)Cd)Te is the HgTe:ZnTe superlattice system. By forming a superlattice from the two constituent binary compounds, rather than alloy, quantum confinement and strain effects may, in principle, be used to tailor the optical and electronic properties to some extent independently of the composition. The resulting material may also be structurally more stable than an equivalent alloy. Here the development of a thermal MOVPE growth process for this superlattice system is described and it is shown that such superlattices may be preferable to the equivalent alloys as they are easier to grow by MOVPE. Initial structural and optical studies and theoretical calculations have confirmed the suitability of MOVPE for the growth of this superlattice system and its applicability for infra-red applications.