THERMODYNAMIC ANALYSIS OF ASH3 AND PH3 DECOMPOSITION INCLUDING SUBHYDRIDES

We have determined the gas-phase composition produced by the pyrolysis of AsH3, and PH3 in chemical equilibrium over a wide range of temperatures and pressures (P-tot). Although the main interest in the analysis is the distribution of species in a cracker employed in metalorganic molecular-beam epitaxy (MOMBE) the results given as a function of P-tot are also applicable to metalorganic vapor-phase epitaxy (MOVPE). To perform the calculations, we have updated the thermodynamic properties (free-energy function, entropy, etc.) of PH, PH2, and PH3 via statistical thermodynamics and derived the standard enthalpies of formation for these hydrides from bond energy measurements. In a previous article we provided similar data for As hydrides. Next, using a Gibbs energy minimization technique, we evaluated the equilibrium gas-phase concentrations of P, P-2, P-4, PH, PH2, PH3, H, and H-2 and the corresponding As-related species as a function of temperature and P-tot. In the case of MOMBE, P-tot is the approximate steady-state pressure. As expected, the decomposition of AsH3 is essentially complete at all temperatures, while PH3 is more stable. The major subhydride in MOMBE operations is AsH; its mole fraction reaches similar to 10(-4) at a cracker temperature of 1500 K and p(tot)=1O(-5) atm. AsH2 becomes more prominent in MOVPE applications. We have found that the atomic H concentration rises with temperature; it reaches a mole fraction of similar to-3x10(-4) at 1300 K and 10(-5) atm which translates to an estimated beam flux of 0.011 monolayers (ML)/s and equals the atomic As flux. However, for PH3 decomposition the atomic H is about an order of magnitude more abundant than P. With rising temperature there is a rapid increase in the dimer/tetramer ratio. In MOMBE P-2 and As-2 are the dominant species and their concentrations are relatively insensitive to temperature. The calculations suggest that a cracker pressure exceeding similar to 10(-5) atm is advisable to attain an As-2 or P-2 beam flux of 1 ML/s (i.e., growth rate of 1 mu m/h) in growing GaAs or InP. As low pressure MOVPE conditions are approached the tetramers become more prominent than the dimers and reach a dominant position at atmospheric pressure.