Electrical characterisation of Mg-related energy levels and compensation mechanism in Mg-doped GaN

Using admittance spectroscopy and optical deep-level transient spectroscopy (ODLTS) we investigate activation of acceptors in GaN:Mg samples induced by annealing. Conductance measurements reveal two peaks, H1 and H2, with activation energies 130 and 170 meV, respectively, from the valence band. The concentration of H1 is proportional to the acceptor concentration in the samples. Capacitance measurements show that H1, which we attribute to a Mg-related acceptor level, is the shallowest level in our samples. ODLTS spectra exhibit two electron traps at 0.28 and 0.58 eV from the conduction band. Their concentrations are too weak to influence the free carrier concentration. We conclude that compensation occurs through passivation of Mg acceptors by hydrogen, rather than self-compensation by new donor levels.