Structural disorder and in-gap states of Mg-implanted GaN films evaluated by photothermal deflection spectroscopy

Abstract

Both structure of the valence band maximum (VBM) and deep-level defects for box-profile Mg-ion implanted (4 × 1019 cm−3) GaN samples are characterized by photothermal deflection spectroscopy (PDS). Compared with the results evaluated by positron annihilation spectroscopy, the variations caused by the thermal annealing is discussed with respect to Urbach energy, defect levels in the band gap, and photoluminescence. Forming Urbach tail gradually at the VBM at the 1000 °C annealing, vacancy-type defects clusters without drastic decrease of deep-level defects. Green and yellow luminescence emits slightly as the PDS signal at the deep-level is decreased by the annealing at 1100 °C due to the decrease of non-radiative recombination centers. The Urbach energy is improved by the further annealing so that the luminescence becomes intense due to the energy transfer through the phonon from Urbach region. The shift of Fermi level towards the valence band at the 1300 °C annealing, the sign of p-conduction, is confirmed from the valence band spectra. Compared to the Mg-doped GaN with p-conduction, it is considered that the improvement of Urbach energy is crucial for p-type activation.