Deep levels in MOCVD n-type hexagonal gallium nitride studied by high resolution deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) is performed in MOCVD α-GaN either doped with two different concentrations of Si or unintentionally doped. Capacitance transients are measured in Schottky diodes made of an Au or Ni rectifying contact and an Al/Ti ohmic contact, both on the top of the samples. Only two peaks are detected in each sample in the energy range from the conduction band edge down to 1.1 eV below it, respectively close to 0.50 and 0.92–1.05 eV, by Fourier Transform DLTS (FTDLTS) with concentrations not exceeding 3×1015 cm−3. These two results testify the high crystalline quality of the samples. The deeper level characteristics depend on the shallow impurity, either Si or the unintentional shallow donor, in deep states which comprise in fact a fine structure not evidenced by FTDLTS. A high resolution DLTS (HRDLTS) method is implemented to resolve this fine structure into several sub-levels which cannot be related to distinct chemical environments. The study of emission and capture kinetics confirms that at least three charge states (+,0,-) are involved. It is concluded that MOCVD α-GaN comprises deep centers which are stabilized in such a form with a concentration in the range of a few 1014–1015 cm−3.