Neutron transmutation doping effects in GaN

The effects of neutron transmutation doping were studied for undoped (residual donor concentrations <1015 cm−3) GaN films grown by metalorganic chemical vapor deposition. After irradiation with reactor neutrons (equal fluences of 1.5×1017 n/cm2 of thermal and fast neutrons) the sample became semi-insulating, with the Fermi level pinned near Ec−0.8 eV. Isochronal annealing from 100 to 1000 °C showed three stages—slight recovery of conductivity at 200–300 °C, reverse annealing at 300–500 °C, and a broad recovery stage from 600 to 1000 °C. After annealing at 1000 °C, the donor concentration in the sample was close to the expected concentration of Ge donors transformed from Ga atoms upon interaction with thermal neutrons (2×1016 cm−3). Admittance spectroscopy showed that the donors had ionization energies ∼Ea=0.2 eV, much deeper than substitutional Ge donors. For intermediate annealing temperatures of 800 °C the donors were deeper (Ea=0.47 eV), but the proximity of concentrations of all these different centers suggests that they are due to transformation of complexes of Ge donors with radiation defects.