A variety of optical and electrical properties of ZnSiP2 have been studied. Single crystals of this material were grown from solution in zinc or tin, using an accelerated crucible rotation technique. Its lattice parameters have been measured between room temperature and 1100 C and the anisotropy of the thermal expansion coefficients interpreted in terms of the bonding of the crystal. The absorption edges of the best crystals showed well defined structure due to pseudodirect transitions between the three crystal field and spin-orbit split valence bands and the lowest conduction band minimum. The lowest band gap, after taking account of the exciton binding energy, was found to be 2.082 eV at room temperature. The valence band splittings and the relative strengths of the transitions were interpreted in terms of the quasicubic model. The absorption due to the two strongest excitons was fitted to a simple model, and the exciton binding energy estimated to be 22 meV. From the magnitude of the absorption coefficient, the matrix element for pseudodirect transitions was estimated to be about a thousand times weaker than that for direct transitions in the III V compounds. An absorption peak found only in low resistivity n-type material in the near infra-red was attributed to transitions between the Gamma3 and Gamma2 conduction bands (x 1 - x3 in zinc blende) at about 0.7 eV. The electrical properties of the crystals were measured between liquid nitrogen temperature and 200 C. Crystals grown from Sn, and some of those from Zn were fairly low resistivity n-type ( .5 - 20 ohm cm) while others grown from Zn were high resistivity (~108 ohm cm) p-type. Electron mobilities up to 160 cm 2 V-1 S-1, and hole mobilities up to 22 cm2 V-1 S-1 were measured. The dominant scattering mechanism was probably due to ionised impurities. Measurements of photoconductivity and photovoltaic effect at a rectifying metal contact had spectral responses substantially in agreement with the absorption measurements, although the structure due to pseudo-direct transitions was not resolved. Strong trapping of excess carriers was observed even in the best optical material.
|Date of Award||1975|