Codoping of p-type GaN with Mg and oxygen was investigated. By codoping wit
h oxygen the hole concentrations increased to 2 x 10(18) cm(-3) at 295 K, a
n order of magnitude greater than in Mg-doped epilayers. The resistivity of
codoped layers decreased from 8 to 0.2 Ohm cm upon oxygen codoping. Variab
le temperature Hall effect measurements indicated that the acceptor activat
ion energy decreases from 170 +/-5 meV in Mg-doped films to 135 +/-5 meV up
on oxygen doping. The higher hole concentration results in part from a decr
ease in the ionization energy of the acceptor. (C) 2001 American Institute
of Physics.