2, Universidade Federal de Vicosa, Rio Paranaiba, MG, Brazil
P-type transparent conducting oxides (TCOs) present many exciting problems for materials scientists due to low conductivity arising from large hole effective masses. The applicability of TCOs to technologies like flat panel displays and photovoltaic cells establishes the need for complementarity between the well documented and commercially available n-type TCOs and p-type TCOs and motivates the search for a means to delocalize the O-2p states that limit shallow acceptors and lead to large hole effective masses. CuAlO2 and AgAlO2 (XAO) show promise as p-type TCOs due to the presence of X-3d/4d states, which hybridize with O-2p states and delocalize the valence states. Additionally, p-doping with Mg may further enhance conductivity of pristine XAO.
XAO exists as three polymorphs: the delafossites 2H and 3R, and an orthorhombic polymorph which is the least stable polymorph energetically. This study is restricted to the 2H polymorph since it is the least studied of the two delafossites. In this work, a theoretical study based on first-principles calculations is presented on pure and Mg-doped (replacing Al) 2H-XAO using density functional theory as implemented in the Vienna Ab initio Simulation Package (VASP) code. Projector augmented wavefunction pseudopotentials with a cutoff energy of 400 eV are employed and the exchange correlation energy is treated using the generalized gradient approximation with the addition of a Coulomb interaction energy (Hubbard correction U) for the Cu-3d and Ag-4d states. Results are also obtained using the hybrid functional (HSE06) approach. Pure 2H-XAO is initially modelled using an 8-atom hexagonal primitive cell, which is used to relax the crystal structure. Band structure, density of states, and the complex dielectric function are obtained. By calculating the total energy per atom as a function of V/V0, where V0 is the relaxed volume, it was observed that the 2H(3R) polymorphs modelled using hexagonal primitive(unit) cell are equal in total energy down to 1 meV/atom at V=V0, with the rhombohedral 3R model 90(37) meV higher for CuAlO2(AgAlO2). This suggests that the 2H polymorph may have a slightly lower total energy. Hole effective masses are determined using parabolic curve fitting of the lower band gap edge around the high symmetry k points of the first Brillouin zone.
Once the electronic structure of the bulk using the primitive cell is well described, 64-atom hexagonal supercells are used to model pristine and Mg-doped 2H-XAO. The electronic structures, densities of states, optical properties and hole effective masses for these systems are presented and discussed in the context of experimental results from literature. A discussion of the effects of Mg-doping on the optical properties and its effectiveness in reducing hole effective masses and increasing conductivity is also presented.