Special features of the electronic structure of Ga2AsSb crystals

Abstract

The ternary ABX 2 -type two-cation compounds with chalcopyrite structure (X-anion) are well known and well understood using various experimental and theoretical methods [1]. At present, interest has quickened in studies of the III 2 -V-V-type two-anion compounds for which phase diagrams have been constructed and the chalcopyrite structure has been established [2,3]. The structural, electronic, and optical properties of one of such crystals (Ga 2 PSb) were studied in [4] from the first principles. The following crystal structure parameters a and c were reported for the Ga 2 AsSb compound with chalcopyrite lattice [2]: a = 5.8922 Å, γ = c/a = 1.996, and u = 0.2294. The parameter u here characterizes the displacement of Ga atoms from sites of the face-centered cubic sublattice.In the present work, the electronic structure of Ga 2 AsSb crystal is studied from the first principles by the sublattice method. The electronic structure of Ga 2 AsSb crystal and its sublattices was calculated from the first principles in the local density approximation of density functional theory (DFT-LDA) implemented in the Quantum Espresso (QE) or PWSCF code [5] with the use of the atomic pseudo-potentials that retain their norm generated by this code. The kinetic energy was truncated at 40 Ry, which provided convergence in the total energy on a level of 10 -4 eV. The electronic density was calculated by the Monkhorst-Pack method of special points [6] on a 4 × 4 × 4 grid. To find the state density N(E), the tetrahedron method and a 16 × 16 × 16 grid were used.The idea of the sublattice method and principles of its implementation with application to crystals having the chalcopyrite lattice were presented in [1,7,8]. According to the sublattice method, the Ga 2 AsSb crystal can be represented as a set of individual Ga, As, and Sb sublattices or sublattice-type systems of anion tetrahedrons (AsGa 4 and SbGa 4 ). Consideration of the electronic structure of such subsystems allows thin effects of forming the structure of the valence band in the crystal caused by interaction of atoms of different species to be traced.Calculations of the electronic structure of the Ga 2 AsSb crystal helped us to establish the special features of its band structure shown in Fig. 1. The valence band consists of three resolved subbands subdivided by forbidden energy regions. The theoretical group analysis and the analysis of the partial state density N(E) demonstrated that the Ga 2 AsSb crystal belongs to direct band gap semiconductors with the gap E g = 0.67 eV. The special feature of the valence band structure of the Ga 2 AsSb crystal is the presence of two anion s-bands of As (near -12 eV) and Sb atoms (near -10 eV) subdivided by a narrow forbidden energy gap. The s-band of Ga atoms in the region from -5 to -7 eV also involves contributions of the p-states of As and Sb atoms. The valence band located from 0 to -4 eV contains nearly identical (~30%) contributions of the p-states of all three atoms. The absolute valence band maximum is at point Γ and has the Γ 4 s...