Vapor Deposition of CdS Films

A similiar behaviour can be observed for ZriS which is a material of interest for the formation of Cd Zn- S compounds in order to get better matching of the electron affinity and lattice constants to the Cu S. The vapor pressure of ZnS is about one order of magnitude lower than the vapor pressure of CdS. The vapor pressure of both materials as a function of temperature is plotted in figure 3. 

It can be deduced from these curves that the composition of the vapor from mixed compound sources depends strongly on the temperature. Since CdS evaporates at higher rates films from mixed compound sources exhibit gradients of composition. Better reproducibility can be obtained by two source evaporation, yet a lateral gradient of the film composition makes this method inapplicable for large area devices. 

The evaporation of both compounds from one coaxial source with nozzles having a hole size adjusted to the evaporation rates of both compounds proved to be the best deposition method. A review on the deposition of mixed compound films is given by Burton /6/. 

The evaporation of CdS from open boats requires low source temperatures and a wide distance between source and substrate in order to prevent solid particles reaching the siibstrate ( spattering ).  

Coaxial indirect heating provides a homogeneous temperature distribution in the zone where the CdS load and the quartz frit are located. Uniform layers of an area of 7 x 7 cm and a thickness of more than lOO urn at a rate up to 2 ym/min can be deposited with this source /7/. 

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