Results of the thermodynamic analyses of the system Si-Ge-Cl-H-Ar useful for depositing Si1-xGex thin film alloys are presented. It is demonstrated that the use of argon as an inert carrier gas should lower the temperature of deposition of the alloy, increase the efficiency of deposition at a given temperature, and lower the temperature for onset of deposition composition plateaus. In addition, the use of an inert carrier gas such as argon should minimize back diffusion and flow rate perturbations during alternating cyclic (A.C.) selective area deposition of alloys.