THE TOTAL PRESSURE OF ARSENIC OVER MOLTEN GALLIUM-ARSENIDE AT 1260-DEGREES-C

In the design of Bridgman type growth systems for the growth of GaAs, the use of a hermetically sealed fused quartz ampoule is often required. In addition, if the sample is to be directionally solidified in some manner, the melt is usually held at a temperature above the melting point of 1238-degrees-C, such as 1260-degrees-C. The literature generally supports the value of 0.97 atm for the total pressure of As over molten GaAs at the melting point, although there is not agreement on the ratio of the As2 and As4 species. However, only Richman [J. Phys. Chem. Solids 24 (1963) 11311 lists any data for the total pressure of arsenic over GaAs at temperatures above the melting point. His data indicate a value of 1.2 atm at 1260-degrees-C. In this paper several experiments are reported, which were conducted using a pressurized furnace and any subsequent deformation of the fused quartz ampoule, which at 1260-degrees-C is in viscous flow, was observed. Thus, for no deformation of the ampoule to occur, the internal ampoule pressure due to the total pressure of arsenic over the molten GaAs must be balanced by the externally applied pressure. Based on the results of these experiments, a value of 2.2 atm (18 psig) was measured for the total pressure of arsenic over molten GaAs at 1260-degrees-C. This value was used to construct ampoules for several different growth systems, all of which yielded successful ampoules with no deformation.