ON THE CHEMICAL-TRANSPORT OF SIAS USING IODINE - EXPERIMENTS AND THERMOCHEMICAL CALCULATIONS

Using iodine as transport agent siliconarsenide migrates in a temperature gradient. The direction of the migration depends on the chosen temperature and the concentration of the transport agent. The transport rates were measured for various transport agent concentrations (0.0002 less than or equal to C(I-2) greater than or equal to 0,02 mmol/cm(3)) and for various mean transport temperatures (650 less than or equal to ($) over bar T less than or equal to 1000 degrees C). For low temperatures (e.g. T-1 = 750 degrees C --> T-2 = 850 degrees C), low iodine concentrations (e.g. C(I-2) = 0.001 mmnol/cm(3) and in the presence of H2O (from wall of silica ampoule) the following exothermic reaction is responsible for the deposition of SiAs-crystals in the sink region: SiAss + 4HI(g) = SiI4,g + 2H(2,g) + 1/4As(4,g) In case of higher temperatures (e.g. T-2 = 1050 degrees C-->T-1 = 950 degrees C) and higher iodine concentrations (e.g. C(I-2) = 0.02 mmol/cm(3)) Si-4,Si-g is the transport agent. According to model calculations the following endothermic reaction is responsible for the migration of SiAs to the region of the lower temperature: SiAss + SiI4,g = 2SiI(2,g) + 1/4As(4,g) The heterogeneous and homogeneous equilibria will be discussed and an explanation of the non equilibrium transport behaviour of SiAs is given. Thermochemical data of SiAs are characterized by the quartzmembrane zero manometer technique and further verified by model calculations.