Electrical and optical properties of semi-insulating GaN

Molecular-beam-epitaxial (MBE) GaN layers can be made semi-insulating (SI) by using a high N flux during growth. For growth at low N flux, the samples have high concentrations (> 1018 cm−3) of shallow donors and shallow acceptors, and also contain a deep center producing a yellow band (2.2 eV) in photoluminescence (PL). For growth at high N flux, the PL lines attributed to shallow acceptors and the yellow band disappear, and the only remaining lines are due to the ground and excited states of the free-exciton A and B bands. The SI material does not produce a measurable Hall effect, and the conduction mechanism is assigned to hopping between deep defects. Using arguments from stoichiometry and theory, we tentatively assign the shallow donors, shallow acceptors, and deep center to the N vacancy, Ga-antisite/Ga-vacancy complex, and Ga antisite, respectively.