Isothermal Liquid Phase Epitaxial Growth of Mercury Cadmium Telluride

Abstract

A method is presented for the isothermal growth of mercury cadmium telluride from a mercury or tellurium-rich melt. In accord with a proposed model, growth proceeds by HgTe deposition from the melt onto the substrate and interdiffusion between the Hg and Cd within the growing layer. Mercury-rich interdiffusion coefficients are determined from an analysis of experimental composition profiles.Mercury cadmium telluride consists of a continuous series of solid solutions between mercury telluride and cadmium telluride. Currently it is of great interest for use in infrared detectors in the long wavelength regime, for use in optical communications systems, and in scientific spectroscopic applications. This interest is based on the continuous change of the direct bandgap as the composition varies from HgTe (-0.32 eV) to CdTe (1.6 eV) (1). However, there are substantial problems with growth and processing of this material, see for example Ref.(2). A large liquidus-solidus lens in the pseudobinary temperature-composition diagram makes it difficult to grow homogeneous bulk material. In addition, the material is mechanically weak, the mercury vapor pressure is high during processing and growth, and it is difficult to experimentally define the material thermodynamically since it is a ternary system. A great many growth techniques have been developed, both bulk and epitaxial. In this paper we present a new technique for growth from an isothermal melt and provide an analysis of the growth mechanism. In addition, mercury-rich interdiffusion coefficients are obtained from an analysis of the composition profiles. ExperimentalThe experimental geometry for growth from a mercury-rich melt is shown in Fig. 1. A CdTe {111} substrate is initially placed above a reservoir of mercury large enough to cover the substrate when the ampul is tilted 180 ~ Sufficient Te is added to the mercury to establish the equilibrium tellurium atomic fraction in the melt at the growth temperature for the composition of interest. We have been using a growth temperature of 500~ and a tellurium fraction of 8-10%; this produces material with X values (X in Hgl_xCdxTe) in the range of 0.3-0.2. The ampul is then evacuated and sealed. Growth takes place within a tipping furnace equipped with a sodium heat pipe to ensure that the system is isothermal. The ampul is first annealed at the growth temperature to dissolve the Te into the liquid Hg. The furnace is then tipped 180 ~ thereby immersing the substrate in the melt which then saturates by dissolving some of the substrate. Growth is allowed to proceed for 60-70h. At the end of the growth cycle, the melt is tipped off the substrate. The furnace is then turned off and allowed to cool to below 360~ before removal of the ampul in order to avoid boiling the Hg melt which can spoil the layer surface quality. A schematic of this process is given in Fig. 2. Growth from a tellurium melt is similar except that the melt consists of the tellurium-rich liquid in equilibrium with the composition of interest. After growth is co...