Extraction of mobile impurities from CdZnTe

Abstract

To develop a process for removing impurities from the CdZnTe substrate, we have carried out experiments using a molten salt, KCN (potassium cyanide), for impurity extraction. This method was known to be effective for treating Ge but had never been tried on CdZnTe. The wafers are placed in a custom-fabricated quartz ampoule and covered with KCN powder. After evacuating both air and water vapor, the ampoule is sealed and heated to 650°C, which is above the KCN melting point of 634.5°C. The ampoule includes some metallic Cd-Zn alloy to provide an overpressure of those elements. Samples before and after this treatment were analyzed by glow discharge mass spectrometry (GDMS). Given starting concentrations in the range of 20 to 4000 ppba, it was found that the Cu and Li were very effectively removed, to near the measurement sensitivity limit, which is less than 10 ppba. Al was also diminished. These results were achieved in times as short as 15 min. The wafers take up some K from the melt, but this can be minimized with the shortest times. Chemically, the impurity extraction mechanism involves two steps in sequence. First is solid state diffusion of the impurity from the interior to surface of the wafer, and second is the dissolution of the impurity into the molten salt. It is likely that the former is rate-limiting, although fast. The procedure and results are presented, along with a discussion of the safety issues, and other characterizations of the treated wafers. Another more attractive method for removal of mobile impurities from CdZnTe, and related materials is the treatment in Bi solution. Bi has the advantage that it melts at a low enough temperature (271°C) such that it can be used in a temperature range where the semiconductor materials do not melt or interdiffuse among themselves, and yet high enough that the impurities are very mobile. Other advantages for impurity extraction are reasonable solubility of the treated material in Bi solution, good wipe-off characteristics and low diffusion rate in CdZnTe. Initial result and observations from preliminary experiments with Bi are also described.