Homogeneity Ranges, Defect Phases and Defect Formation Energies in AIBIIICVI 2 Chalcopyrites (A = Cu; B = Ga, In; C = S, Se)

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Abstract
Employing the generalization of cavity model, formation energies of point defects were calculated and compared with the self-doping behaviour and homogeneity regions of Cu BIIIC2 VI chalcopyrites. Electrical compensation as well as large off-stoichiometries, observed in chalcopyrites, can be explained by this data introducing defect arrays of VCu, VC VI BIII Cu and CuB III, especially VCu - BIII Cu pairs. The ratio of defects in such pairs can be adjusted via partial pressures of chalcogen vapor species during crystal or layer growth causing p- or n-type doping. Since the homogeneity range of CuGaSe2 along the binary cut Cu2Se - Ga2Se3 was claimed to be of about 10 mole% Ga2Se3, which is beyond the size that can be explained by point defect formation energies, the phase diagram was carefully investigated in the range from 40 to 60 mole% using Differential Thermal Analysis (DTA). The solidified samples were structurally analysed by X-Ray Diffractometry (XRD). A Cu-rich chalcopyrite phase of composition Cu1.015Ga0.956Se2 was found showing a maximum in the liquidus at 1100°C and two solid-solid phase transitions at 1056 and 987°C, respectively. Three further Ga-rich chalcopyrite defect phases were identified by careful analysis of the liquidus and the course of lattice constants vs. composition curve. As a result, the wide homogeneity range is discussed as a sequence of ordered defect chalcopyrites.

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