Electronic properties of the residual donor in unintentionally doped beta-Ga2O3

Electron paramagnetic resonance was used to study the donor that is responsible for the n-type conductivity in unintentionally doped (UID) beta-Ga2O3 substrates. We show that in as-grown materials, the donor requires high tempeature annealing to be activated. In partly activated materials with the donor concentration in the 10(16) cm(-3) range or lower, the donor is found to behave as a negative-U center (often called a DX center) with the negative charge state DX- lying similar to 16-20 meV below the neutral charge state d(0) (or E-d), which is estimated to be similar to 28-29 meV below the conduction band minimum. This corresponds to a donor activation energy of E-a similar to 44-49 meV. In fully activated materials with the donor spin density close to similar to 1 x 10(18) cm(-3), donor electrons become delocalized, leading to the formation of impurity bands, which reduces the donor activation energy to E-a similar to 15-17 meV. The results clarify the electronic structure of the dominant donor in UID beta-Ga2O3 and explain the large variation in the previously reported donor activation energy. Published by AIP Publishing.Funding Agencies|Linkoping Linnaeus Initiative for Novel Functional Materials (LiLi-NFM); Council for Science, Technology and Innovation (CSTI); Cross-ministerial Strategic Innovation Promotion Program (SIP); NEDO, Japan