Vapor phase treatment of CdTe/CdS thin films with CdCl2:O2

A controllable technique for treating CdTe/CdS thin film structures to produce uniform film properties and device performance is presented. Heat treatments of physical vapor deposited CdTe/CdS thin films were carried out in an atmosphere containing CdCl₂ vapor and air, or mixtures of Ar and O₂. The heat treatment reactor permitted independent control over the reaction temperature and the CdCl₂ source temperature, hence CdCl₂ vapor phase concentration at the CdTe surface. Modelling of the thermal and mass transfer for the reactor geometry shows that at total pressure of 1 atmosphere of air, the CdCl₂ equilibrium concentration over the CdTe surface is established in less than 2 seconds and varies from 1.3 mtorr at 380°C to 32.7 mtorr at 450°C. The effects of treatment conditions on interdiffusion, grain coalescence, and device operation are presented. Devices with efficiency greater than 11% were obtained for treatment with a CdCl₂ partial pressure of 9 mtorr in the temperature range 420-430°C and at oxygen partial pressures from 8 mtorr to 570 torr