First-principles investigation of isolated band formation in half-metallic Tix Ga1-x P (x=0.3125-0.25)

Abstract

Several alloy semiconductors containing a transition metal atom of the type M Ga4 X3 with X=As or P and M=Sc or Ti have been found previously to present an isolated partially filled intermediate band within the usual band gap of the host semiconductors and have been proposed as highly efficient photovoltaic materials. In this paper, we carry out an ab initio investigation of band structures and electronic properties for the more chemically stable Tix Ga1-x P compound as a candidate for isolated intermediate band formation. We have calculated the electronic structures using self-consistent density functional theory method in both local density approximation and generalized gradient approximation (GGA) approaches and compared the GGA results with those obtained with the exact exchange method that we have implemented in the code SIESTA. Using spin-polarized localized wave functions to represent the valence electrons states and nonlocal pseudopotentials for the core electrons, we have also studied in detail the Tix Ga1-x P compounds at different dilution levels of the Ti transition metal atom (x=3.125%,6.25%,12.5%,25.0%) and for two (cubic and tetragonal) different crystal cells. Results at the different dilutions show in all cases a fully spin-polarized structure and, except for the case of immediate Ti neighbors, indicate a rather small spin coupling between Ti atoms and confirm the presence of the isolated narrow partially filled intermediate band for this compound. They also show the higher suitability of isotropic crystal structures for obtaining in these materials the intermediate band with the desired small band width. © 2006 The American Physical Society.