We present the pressure dependence of the defect emissions in the chalcopyrite alloy semiconductor Ag x Cu 1--x GaS 2 for values of the alloy concentration x varying between 0 and 1. A large variation in the pressure coefficients of the different defect emissions with x was found. In one alloy concentration x ¼ 0.25 deep levels were found to appear under pressure. Plausible explanations of our results have been proposed.
Recently alloys of chalcopyrite semiconductors such as CuGa x In 1--x Se 2 have received attention for improving the performance of solar cells [1]. As a result, the effect of alloying on the electronic band structure and vibrational properties of several chalcopyrite alloy systems have been investigated [2][3][4][5]. However, the effect of alloying on the defect properties in chalcopyrite semiconductors is still largely unexplored [3]. It is now well-established that donors in alloys of zincblende-type semiconductors, like AlGaAs, exhibit a shallow-to-deep donor transformation as a function of both pressure and alloy concentration, see e.g. [6]. In view of the close relation between the chalcopyrite and the zincblende structures, it is interesting to ask whether this phenomenon can also occur in the chalcopyrite semiconductors. In this paper we have studied the pressure dependence of defect photoluminescence (PL) in Ag x Cu 1--x GaS 2 for several alloy compositions [7]. While we did not see shallow-to-deep transformation as a function of alloy concentration, we observed signatures of such transformation as a function of pressure in the alloy Ag 0.25 Cu 0.75 GaS 2 . A lack of reliable band structure calculation in the Ag x Cu 1--x GaS 2 alloys precludes at present a definitive explanation of this result. However, qualitative explanations are proposed.
We have grown alloys of Ag x Cu 1--x GaS 2 with x ¼ 0.25, 0.5, and 0.75 with the horizontal Bridgman method. The samples were not intentionally doped. They have been characterized by X-ray diffraction and found to be single-phased and crystalline. Details of the properties of these samples have been presented elsewhere [5] and will not be reproduced here. The high pressure optical experiments were performed with the sample placed inside a standard gasketed diamond-anvil cell using a mixture of methanol and ethanol as the pressure medium. The pressure inside the cell was determined by