COPYROLYSIS OF ASH3 AND PH3 IN THE EPITAXIAL-GROWTH OF TERNARY AND QUATERNARY III-V ALLOYS

In the epitaxial growth of InGaAsP or GaAsP by metalorganic molecular beam epitaxy (MOMBE), the group V hydrides may be cracked separately or combined in a single stream before entering the cracker. Combining AsH3 and PH3 yields the mixed gaseous species AsP, As2P2, As3P and AsP3 which are stable arriving in the ultra-high vacuum (UHV) chamber in addition to the species As, As2, As4, AsH, AsH3, H, H-2 and their P analogs that would emanate from separate crackers. From statistical thermodynamics, we have evaluated all the thermodynamic functions (free-energy function, entropy, etc.) of these mixed species. Subsequently, employing a robust free-energy minimization technique, the equilibrium product distribution for the 18 gaseous molecules at various input flowrates of AsH3 + PH3 were determined. In MOMBE growth the concentration of AsP between P2 and AS2 is intermediate between P2 and As2. The calculations have also been extended to metalorganic vapor phase epitaxy (MOVPE) conditions where the dominant species are P4 and AsP3. Here the findings are more significant for hot-walled than for cold-walled reactors. The results of the thermodynamic analysis are in good accord with mass spectrometric measurements.