THERMODYNAMICAL PROPERTIES OF THALLIUM-BASED III-V MATERIALS

Thermodynamical factors that affect growth of the thallium-bearing zin c-blende alloys InTlP and InTlAs are examined within the local density approximation (LDA), using the linear muffin-tin orbital method. The LDA predicts TlP and TlAs to be unstable with respect to decomposition into the elemental constituent solids, or marginally stable if conser vative 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 equi librium partial pressures over InTlP and InTlAs have also been calcula ted as a function of composition. Even with conservative error estimat es, we predict that only low concentrations of thallium can be achieve d in InTlP (<5% at 350 degrees C) using gas source molecular beam epit axy (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 degrees C, the addition of e rror estimates into the calculation indicates that obtaining the 15% t hallium needed for LWIR applications may be possible. Native defect po pulations 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 InTlP, reflecting the comparable stab ility of the TIP and TlP3 phases.