The segregation of impurities and the self-compensation problem in II-VI compounds

Abstract

The electrical and optical properties of ZnTe or CdTe crystals are generally inhomogeneous after annealing. This is shown to be due to the redistribution of preexisting impurities. Indeed, after growth, these crystals contain some excess tellurium which must precipitate since the solidus line has a retrograde shape. The liquid Te droplets are then able to purify the surrounding materials by a solid-liquid segregation mechanism. On annealing, the tellurium excess can disappear more or less rapidly: the Te precipitates and the small inclusions shrink progressively while the impurities are released into the crystal. This mechanism explains the bright dots observed in the scanning electron microscope (cathodoluminescence mode) and due to the high lithium concentration. Under other conditions, dark dots are obtained and the role of copper is suspected. This model is discussed on the basis of the available data (segregation and diffusion coefficients). Further confirmation is obtained from direct chemical analysis of the crystal and of the inclusions itself. The consequences of the segregation of impurities on the interpretation of the self-compensation problem are analyzed. A fully satisfactory interpretation of conductivity variation with zinc pressure previously attributed to the zinc vacancy, can be derived with the single hypothesis that acceptor and donor impurities in sufficient amount are stored in the crystal. The dominant role of residual impurities in the close control of the II-VI materials is particularly emphasized.