THERMODYNAMIC PROPERTIES OF ASH3 AND ITS SUBHYDRIDES

The thermodynamic properties of AsH3 and its subhydrides AsH and AsH2 have been evaluated from critically assessed or estimated spectroscopi c data. The calculation of thermodynamic functions (free-energy functi on, entropy, enthalpy, and heat capacity) is based on statistical ther modynamics. For the first time, for all three species a complete set o f these functions has been generated between 0 and 1600 K in tabular f orm. A combination of the free-energy functions with the standard enth alpies of formation of hydrides (derived from the photoionization mass -spectrometric bond energy values of Berkowitz) permits the determinat ion of the gas phase composition in the pyrolysis of AsH3 during the M OM BE (CBE), HS-MBE, or MOCVD growth of III-V epitaxial layers that in clude As. Using a free-energy minimization technique, the equilibrium concentrations of AsH, AsH2, AsH3, As, As2, As4, H and H-2 have been o btained at 1.013, 3.039 x 10(3) and 1.013 x 10(5) Pa (1 atm) in the te mperature range between 800 and 1500 K. In the case of MOMBE, under eq uilibrium conditions in the hydrate cracker, the removal of carbon-con taining radicals or oxygen is facilitated by atomic H and AsH with par tial pressures of approximately 3.33 X 10(-4) and 1.87 x 10(-5) Pa, re spectively, at 1300 K. In contrast, in low pressure MOCVD the species AsH and AsH2 are equally prominent, while in atmospheric pressure MOCV D the dominant subhydride is AsH2.