On the thermodynamic behavior of cadmium in Te-saturated HgTe-CdTe and CdSe-CdTe solid alloys
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I.
INTRODUCTION
IN recent years, chalcogenides of group IIB elements have gained prominence for their applications in various electronic devices, such as infrared, X-ray and y-ray detectors, light-emitting diodes, photoresistors, thin-film transistors, and high-efficient solar cells. Although extensive research has been directed toward the development of device applications by studying the electron transport properties of CdxHgl-~ Te and CdTe~Se~_x alloys, their thermodynamic properties have not been systematically investigated until recently. As the accurate knowledge of phase diagram and thermodynamic properties of the system is essential for adjusting the semiconducting characteristics by varying the alloy composition, systematic and precise thermodynamic measurements on CdTe, I~j CdSe, r2} HgTe, [3j CdTexSet_x, 141 and Cd~Hgl_xTe I5~ have been recently reported by our laboratory. In this article, we present the measurement of activity of cadmium in Te-saturated CdxHgl_xTe and CdTe~Se2_x alloys by electrochemical technique using molten salt electrolyte. Various partial and excess thermodynamic quantities have been calculated from activity data. The results have been discussed in the light of the existing phase diagram of the HgTe-CdTe and CdSe-CdTe systems, Darken's stability, and excess stability parameters, and physical properties, i . e . , energy gap and magnetic susceptibility.
B.
A.
Materials
EXPERIMENTAL
and Preparation
of Alloys
A series of alloys at 10 mol pct interval of CdTe in both the systems HgTe-CdTe and CdSe-CdTe were prepared by pyrosynthesis of high purity (99.999 pct pure) constituent elements. The details of experimental procedure have been discussed elsewhere.L4'5] M. SHAMSUDDIN, Professor, and A. NASAR, Research Associate, are with the Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi-221005, India. Manuscript submitted March 16, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
Measurements
Activity of cadmium in Te-saturated HgTe-CdTe and CdSe-CdTe alloys was determined by measuring the open circuit electromotive force (emf) of the following electrochemical cells, respectively: Cd (1)/LiC1 - KC1 + 5 wt pct CdCI2/Cd~Hgl_~Te (s) + Te (1)
[I]
Cd (1)/LiC1 - KC1 + 5 wt pct CdC12/CdTexSe~_~ (s) + Te (s, 1)
[II]
A eutectic mixture of LiC1-KC1 containing 5 wt pet CdCI2 was used as the electrolyte. Cell assemblies made of BOROSIL glass containing five lower limbs (8-mm internal diameter) below a tubular electrolyte compartment (50-mm internal diameter) were used in this investigation. At the bottom of each limb, an electrode contact wire of tungsten (0.4-ram diameter and 20-cm length) was sealed. The design of the cell assembly, experimental technique, and procedure for attainment of correct reversible cell emf have been described by Shamsuddin and coworkers, t4,5,6] Ill. A.
II.
Electrochemical
RESULTS AND DISCUSSION
Activity of Cadmium
The virtual cell reaction in the galvanic cells [I] and [II] may be represented as follows: C
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