Correlation between deep levels and the persistent photoconductivity in Mg-doped GaN grown MOCVD

Electrical properties of Mg-doped GaN epilayers grown by metal organic chemical vapour deposition (MOCVD) were investigated using photocapacitance measurements and deep level transient spectroscopy (DLTS). Annealing at different temperatures and different time durations gave gradual activation of Mg acceptors in samples taken from the same as-grown wafer. The samples exhibit a clear persistent photocapacitance (PPC) at low temperature. The photocapacitance versus illumination energy measurements shows the presence of energy levels labelled O1 and O2 at 1.1 and 1.9 eV, respectively, from the valence band. Their concentrations increase with the annealing duration and the temperature and thus they can be inferred to originate from Mg. They are found to be metastable. Our results support the hypothesis that the PPC observed in GaN: Mg originates from these metastable deep centres related to Mg. DLTS measurements show the presence of a deep level T1 at 0.35 eV from the valence band. When the sample has been illuminated at 80 K with photon energy of I eV or higher, the DLTS signal from this Centre decreases with the illumination time. The energy 0.35 eV is therefore likely to be the thermal activation energy of the O1 Centre.