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

Using iodine as transport agent siliconarsenide migrates in a temperat ure gradient. The direction of the migration depends on the chosen tem perature and the concentration of the transport agent. The transport r ates 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 t o ($) over bar T less than or equal to 1000 degrees C). For low temper atures (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 i s 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 tempe ratures (e.g. T-2 = 1050 degrees C-->T-1 = 950 degrees C) and higher i odine 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 endothe rmic reaction is responsible for the migration of SiAs to the region o f the lower temperature: SiAss + SiI4,g = 2SiI(2,g) + 1/4As(4,g) The h eterogeneous and homogeneous equilibria will be discussed and an expla nation of the non equilibrium transport behaviour of SiAs is given. Th ermochemical data of SiAs are characterized by the quartzmembrane zero manometer technique and further verified by model calculations.