Electronic properties of GaSe, InSe, GaS and GaTe layered semiconductors: charge neutrality level and interface barrier heights

Density functional theory calculations have been applied to study the structural and electronic properties of layered epsilon-GaSe gamma-GaS and beta-GaTe compounds. The optimized lattice parameters have been obtained using vdW-DF2-009 exchange-correlation functional, which is able to describe dispersion forces and produces interlayer distances in close agreement with experiments. Based on the calculated electronic band structures, the energy position of the charge neutrality level (CNL) in the III VI semiconductors has been estimated for the first time. The room-temperature values of CNL are found to be 0.80 eV, 1.02 eV, 0.72 eV and 0.77 eV for epsilon-GaSe, beta-GaS, GaTe and gamma-InSe, respectively. The persistent p-type conductivity of the intentionally undoped epsilon-GaSe beta-GaS and GaTe and n-type conductivity of 7-InSe crystals are discussed and explained using the concept of CNL. We also estimated the barrier heights for a number of metal/semiconductor and semiconductor/semiconductor interfaces assuming partial Fermi level pinning at the CNL. A reasonable agreement between our calculations and the available experimental data has been obtained.