Orbital Hybridization and Defective States of Vacancy Defects in AlN

Intrinsic atomic defects pose a great effect in wide-bandgap semiconductors owing to the high tendency of forming in-gap defective states. Utilizing density-functional theory, the atomic vacancy defects of Al and N vacancies in hexagonal wurtzite aluminum nitride (AlN) are investigated. We find that the most energetically favorable form of vacancies is predicted to be negatively trivalent charged Al vacancy (VAl−3) in n-type AlN and positively monovalent charged N vacancy (VN+1) in p-type AlN, respectively. N vacancy generally has a low formation energy and could be either an acceptor or a donor: Under p-type conduction, it is a deep donor with a positively monovalent charge of +1, acting as a compensating center for holes; Under n-type conduction, it is a deep acceptor carrying charge of -3, acting as a compensating center for electrons. For the Al vacancy, it tends to be a shallow acceptor in p-type AlN but a high formation energy or a deep acceptor in n-type AlN with low formation energy, most possibly at a negatively trivalent charged state. Our orbital hybridization analysis shows that these vacancies can be facilely generated which are rooted in the antibonding coupling Al3s-N2p states in valence band. The above finding is independent of the chemical potential of nitrogen and aluminum and holds true under both nitrogen rich and poor conditions. Our work provides a guide to experimental works by providing the energetics and atomic-scale mechanism of doping nature of atomic vacancies in AlN.