Native point defects in CuIn1−xGaxSe2: hybrid density functional calculations predict the origin of p- and n-type conductivity

Abstract

We have performed a first-principles study of the p- and n-type conductivity in CuIn_1−xGa_xSe₂ due to native point defects, based on the HSE06 hybrid functional. Band alignment shows that the band gap becomes larger with x due to the increasing conduction band minimum, rendering it hard to establish n-type conductivity in CuGaSe₂. From the defect formation energies, we find that In/Ga_Cu is a shallow donor, while V_Cu, V_In/Ga and Cu_In/Ga act as shallow acceptors. Using the total charge neutrality of ionized defects and intrinsic charge carriers to determine the Fermi level, we show that under In-rich growth conditions In_Cu causes strongly n-type conductivity in CuInSe₂. Under increasingly In-poor growth conditions, the conductivity type in CuInSe₂ alters to p-type and compensation of the acceptors by In_Cu reduces, as also observed in photoluminescence experiments. In CuGaSe₂, the native acceptors pin the Fermi level far away from the conduction band minimum, thus inhibiting n-type conductivity. On the other hand, CuGaSe₂ shows strong p-type conductivity under a wide range of Ga-poor growth conditions. Maximal p-type conductivity in CuIn_1−xGa_xSe₂ is reached under In/Ga-poor growth conditions, in agreement with charge concentration measurements on samples with In/Ga-poor stoichiometry, and is primarily due to the dominant acceptor Cu_In/Ga.