Native point defects in Culn_(1-x)Ga_xSe2: hybrid density functional calculations predict the origin of p- and n-type conductivity

摘要

We have performed a first-principles study of the p- and n-type conductivity in Culn_(1-x)Ga_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 ln_(Cu) causes strongly n-type conductivity in CulnSe2. Under increasingly In-poor growth conditions, the conductivity type in CulnSe2 alters to p-type and compensation of the acceptors by ln_(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 Culn_(1-x)Ga_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).