ON THE MIGRATION OF SIAS WITHOUT USING A TRANSPORT AGENT - EXPERIMENTS AND THERMOCHEMICAL CALCULATIONS

SiAs migrates in a temperature gradient ((T) over bar = 0.5 . (T-1 + T -2) = 850 to 1000 degrees C) without adding a transport agent, into th e cooler part of the silica ampoule. The migration rate depends on the temperature and the partial pressure of elemental arsenic in the sili ca tube. The migration rates were measured for various arsenic concent rations (0 less than or equal to n(As) less than or equal to 4 mmol/20 cm(3)) and for various mean transport temperatures (850 less than or equal to (T) over bar less than or equal to 1000 degrees C). In case o f increasing the temperature the migration rate rises (e.g. (T) over b ar = 850 degrees C, m(exp.) = 0.006mg/h; (T) over bar = 1000 degrees C , m(exp.) = 0.044mg/h). Adding arsenic (e.g. n(As) = 0.11 mmol, m(exp. ) = 0.067 mg/h; n(As) = 4.0 mmol, m(exp.) = 0.82 mg/h), gives also the result of an increasing migration rate. Augmenting the pressure by ad ding argon as inert gas has only a small effect to the migration rate of SiAs. To explain the mechanism of the migration by using model calc ulations, the thermochemical data of the gaseous species SiAsg and SiA s3,g have to be estimated. According to model calculations an endother mic reaction like the following one is responsible for the migration o f SiAs to the region of the lower temperature: SiAss + 2/n As-n,As-g = SiAs3,g (1 less than or equal to n less than or equal to 4).