Design of a vapor transport deposition process for thin film materials

Abstract

A vapor transport process for continuous deposition of elemental and compound thin film materials is presented. The process saturates a carrier gas with a vapor from a subliming source. The saturated mixture is directed over a substrate at lower temperature, resulting in a supersaturation condition and subsequent film growth. The process geometry, comprising the dimensions of the saturation and deposition zones, carrier gas pressure and flow rate, and saturation zone temperature are determined by calculating worst-case characteristic times and simply insuring that the residence time of the carrier gas sufficiently exceeds these times. A model was used to design a system, which is currently being used to deposit 1–10μm thick CdTe films on a 10×10cm2 translating substrate. The process produces film thickness uniformity to within ±5% in the translation direction and across the deposition zone, with a material utilization of 50%. Linear translation speed of 12.5cm∕min has been demonstrated in depositing a 4.5μm CdTe film. The vapor transport process has also been used to deposit CdxZn1−xTe alloy films over a wide range of compositions by addition of ZnTe to the source. Photovoltaic conversion efficiencies of >13% for CdTe and >12% for CdxZn1−xTe have been achieved by devices fabricated from vapor transport deposited films deposited on to moving CdS coated substrates. Refinements are suggested for commercial-scale deposition.