Band gap and partial density of states for ZnO: Under high pressure

Abstract

In this work, the effect of pressure on the electronic band structure, partial density of states (PDOS), and the band gap of the four phases of ZnO, namely B4 (wurtzite), B3 (zinc-blende), B1 (rocksalt) and B2 (CsCl-type), has been investigated using the plane-wave pseudo-potential code CASTEP with three different schemes: the generalized gradient approximation (GGA) in the latter approach Perdew–Burk–Ernzerhof (PBE) with and without spin–orbit (SO) coupling, the new hybrid exchange–correlation functional named B3LYP functional and Hartree–Fock+Local Density Approximation (HF+LDA). These schemes are employed in order to treat the exchange–correlation effects. In addition, we will illustrate how the orbital motion of crystal electrons is affected by spin–orbit (SO) coupling. To our knowledge, this is the first theoretical study reported on ZnO using the B3LYP method. Our investigation shows that the increase of the pressure causes the nature of the band gap to change from direct to indirect. The mechanism responsible for this change of band structure is analyzed. The wide band gap of the B4 (wurtzite) phase at p=0 as determined by the precedent methods is ∼3.221 and 3.222eV (PBE) with and without spin–orbit (SO) coupling, respectively, 9.186eV (HF+LDA) and 2.451eV (B3LYP). The first two approaches provide the best agreement with the experiments. The band gap of B3 (zinc-blende), B1 (rocksalt) and B2 (CsCl-type) and the strong contribution of d orbitals of Zn atoms on the structure of the bands will be discussed. The SO coupling effect on the band structure for all phases is presented. This effect on the electronic properties of the various phases of ZnO in particularly for the B3 (zinc-blende), B1 (rocksalt) and B2 (CsCl-type) phases at high pressures are not well-known. The aim of this work is to improve the DFT band gap error by introducing different schemes and therefore this study is important for future experimental work on this potential semiconductor material. In this study we have shown the effect of SO coupling on the bands structure of ZnO as a function of pressure, and analyzed the mechanism of this effect. The understanding of spin–orbit coupling related phenomena is very important in both fundamental research and in applications of semiconductors systems.