Thermodynamical properties of thallium-based III-V materials

Abstract

Thermodynamical factors that affect growth of the thallium-bearing zinc-blende alloys InTIP and InTlAs are examined within the local density approximation (LDA), using the linear muffin-tin orbital method. The LDA predicts TIP and TIAs to be unstable with respect to decomposition into the elemental constituent solids, or marginally stable if conservative estimates of the LDA errors are made. Several thallium-rich and anion-rich compounds have also been examined; some are found to have excess energies per atom comparable to the zinc-blende phase. The equilibrium partial pressures over InTIP and InTlAs have also been calculated as a function of composition. Even with conservative error estimates, we predict that only low concentrations of thallium can be achieved in InTIP (<5% at 350°C) using gas source molecular beam epitaxy (GSMBE), far less than the 67% needed for long-wave infrared (LWIR) applications. Although much less than 1% thallium is predicted to be soluble in InTlAs for GSMBE growth at 350 °C, the addition of error estimates into the calculation indicates that obtaining the 15% thallium needed for LWIR applications may be possible. Native defect populations have been calculated for alloy compositions corresponding to band gaps in the LWIR, and the anion antisite densities are predicted to be quite high, especially in InTIP, reflecting the comparable stability of the TIP and T1P3 phases.