The origin of persistent photoconductivity and its relationship with yellow luminescence in molecular beam epitaxy grown undoped GaN

The results of persistent photoconductivity (PPC) and photoluminescence measurements made on radio-frequency plasma assisted molecular beam epitaxy grown, undoped, GaN are reported in this work. Hexagonal GaN (h-GaN) epilayers grown on sapphire and cubic GaN (c-GaN) epilayers grown on GaAs and cubic SiC substrates, are employed in this study. Three clear experimental evidences are reported to claim that the commonly seen persistent photoconductivity and yellow luminescence (YL) are related to each other through the same defect. First, PPC is observed only in those samples which show YL. Second, the threshold (the minimum photon energy required) to observe PPC is determined as 1.6±0.2 eV, which is almost at the same energy at which the YL band starts raising. Third, the photocurrent increases monotonically from 1.8 to 2.2 eV, which is consistent with the broad nature of YL band.