Solid State Communications, Vol. 78, No. 5, pp. 449-453, 1991. Printed in Great Britain. 0038-1098/91 $3.00 + .00 Pergamon Press plc ON THE NATURE OF ALLOYING IN HgTe-CdTe SOLID SOLUTIONS AT LOW TEMPERATURES FROM ELECTRODE POTENTIAL MEASUREMENTS R.D.S. Yadava and A.V.R, Warrier Solid State Physics Laboratory, Lucknow Road, Timarpur, Delhi 110054, India (Received 10 November 1990 by D. Van Dyek) Electrode potentials at HgTe, CdTe and Hg08Cd02Te in 0.1 M KOH solution are measured at 20°C with reference to a saturated calomel electrode. A model for the interfacial electron transfer reaction in equilibrium is proposed. An analysis is presented to show that the alloying of HgTe and CdTe at low temperatures is non-ideal. The enthalpy of formation of the x = 0.2 alloy from the pure component phases in solid state at 20°C is found to be + 2.1 kcal mol I. INTRODUCTION THE PHYSICS and chemistry of Hg~ ,CdxTe sur- faces have been subjected to extensive investigation over the past nearly one decade. It is quite evident from the proceedings of annual U.S. workshops from 1981 through 1989 [1] and the EMIS data reviews on the properties of Mercury Cadmium Telluride [2]. A variety of questions related to the interfaces of the semiconductor with vacuum, metal, oxide or electro- lyte have been addressed. Major efforts however, have gone into resolving the complex chemistry of these interfaces. In a recent publication [3] we presented a com- parative study of the anodic polarization behaviour of HgTe, CdTe and Hg0.~Cd02Te in an electrolyte of 0.1 M KOH in 90% ethylene glycol and 10% deion- ized water. The findings of other authors on this subject is also summarized there. It was concluded that the polarization behaviour of these electrodes could be consistently explained if one assumes that the equilibrium charge transfer reaction at the semiconductor-electrolyte interface is the metal cation dissolution and deposition as per the redox process M ,~-M 2+ + 2e. Although we provided a qualitative support to this reaction model on the basis of Cd and Hg being electropositive and Te being electronegative, still it requires considerable qualifi- cation and discrete evidence, especially in view of the fact that the electrolyte does not contain these cations. In an earlier report [4] also, based upon the a.c. impedance study of the anodization kinetics of x = 0.225 alloy, the same reaction was proposed to be the first step in oxidation. In this paper we argue further in favour of this equilibrium redox process, and present an analy- sis of its energetics with reference to the reversible reaction at a saturated calomel electrode (SCE). It emerges from this analysis that a knowledge of a set of equilibrium cell e.m.f, containing HgTe, CdTe and Hg~ xCd,Te as working electrodes in KOH solution and any non-polarizable electrode (e.g. the SCE) as the reference would yield the free energy of the alloy formation from its component phases. In the follow- ing we report on the measurement of these cell e.m.f. at 20°C and present an analysis of the x = 0.2 alloy. It is concluded that this alloy behaves non-ideally at low temperatures. Earlier experimental and theoreti- cal studies on this subject are discussed and compared with the present finding in Section 4. 2. EXPERIMENTAL Measurements were carried out with single crys- tals of HgTe, CdTe and Hg0.~Cd02Te grown by cast- recrystallise-anneal method in our laboratory [5-7]. The HgTe and CdTe crystals were as-grown without any special annealing treatment. The Hg~Cd~_Te crystals were n-type prepared by annealing in a mercury atmosphere; these crystals had electron densities ,-,2 x 10~Scm ~ at 77K. The samples of sizes about 5 x 1 x 0.5ram ~ were prepared by lapping with I/xm alumina and chemomechanical polishing with 2% bromine meth- anol solution. They were then etched in the same solution for 30s, rinsed thoroughly with methanol and blown dry with nitrogen. Copper leads were attached to them with indium solder. The electrolyte was 0.1 M KOH in 90% ethylene glycol and 10% deionized water. The electrodes were dipped into the electrolyte to about 2/3 of their length keeping the metal contacts completely out of the solution. 449