Characterization of optical induced defect-band-transitions in MBE grown galliumnitride by optical admittance spectroscopy

N-type GaN layers grown by molecular beam epitaxy on sapphire substrates were characterized using optical admittance spectroscopy (OAS). The found transitions are classified into four different spectral regions. (1) Analyzing the temperature dependence of the near bandgap (NBG) region we separated three different transitions between the valence band and donorlike states having ionization energies of 23, 44, and 57 meV, respectively, below the gap. These donors were interpreted as silicon, oxygen, and nitrogen vacancy, respectively. (2) Within the blue band (BB) deeper states with ionization energies of 350, 500, and 750 meV below the gap are visible only at low temperatures. These states are assigned to deep electron traps well known from the literature. (3) The energy range between 2.0 and 2.5 eV, the so-called yellow band (YB), is dominated by two deep level-to-band-transitions at 2.2 and 2.4 eV which are discussed as possible participants to the yellow luminescence. (4) Further deep level-to-band-transitions appear around 1.85 and 0.82 eV. We demonstrate the separation of transitions with nearly the same ionization energy by using their different frequency dependence for the example of the 0.82 eV group, which is probably induced by the used electron cyclotron resonance (ECR) nitrogen source.