  We present ab initio density-functional calculations for acceptors, donors,
and native defects in aluminum nitride, showing that acceptors are deeper (Be ~
0.25 eV, Mg_ 0.45 eV) and less soluble than in GaN; at further variance with
GaN, both the extrinsic donors Si_Al and C_Al, and the native donor V_N (the
anion vacancy) are found to be deep (about 1 to 3 eV below the conduction). We
thus predict that doped AlN will generally turn out to be semi-insulating in
the normally achieved Al-rich conditions, in agreement with the known doping
difficulties of high-x AlGaN alloys.
