Valence band modulation and the p-type conducting mechanism of LiMCh2 (M = Al, Ga, In and Ch = S, Se, Te) semiconductors driven by low-electronegativity anions

Due to the strong electronegativity of oxygen ions, O 2p orbital derived valence bands lead to low hole mobility and poor hole doping, which makes it extremely difficult to obtain high-conductivity p-type transparent materials. Herein, we propose a design principle called electronegative effect modification of the valence band (EEMVB) by utilizing low-electronegativity chalcogen anions to promote valence band dispersion. The proposed design principle is tested on LiMCh2 (M = Al, Ga, and In and Ch = S, Se, and Te) compounds in the I4̄2d space group using density functional theory (DFT) calculations at the atomic scale. The DFT-D3(0) method with zero damping is adopted for geometry optimizations. The Heyd-Scuseria-Ernzerhof hybrid functional (HSE06) is subsequently used for electronic and optical property calculations. We demonstrate that the electronegativity difference between chalcogen anions has a great influence on the bonding characteristics of LiMCh2, which in turn affects the overlap states at the valence band maximum (VBM). Combining the band gap, effective mass and p-type propensity, LiAlTe2 is identified as a promising p-type transparent material. A hole effective mass as low as 0.37 m0 is predicted for LiAlTe2, which benefits from the mixing of Te 5p orbitals with Al 3p orbitals to produce highly favorable dispersion at the VBM. Defect calculations are discussed for LiAlTe2, showing that LiAlTe2 displays an intrinsic p-type behavior, originating from the Li-on-Al antisite defect, without effective hole killers in this material. The design principle proposed herein is expected to open up a pathway for high-performance p-type materials.