Mechanism of high conduction on the N polar face of GaN

Abstract

High conductivity on the N-face of GaN has long been observed. For many years, sapphire was the basic substrate used for growing GaN layers. Hence, such conductivity was associated with a high concentration of defects at the GaN/sapphire interface. Due to recent advances in GaN growth techniques, bulk GaN in polar and non-polar orientations has become available. In this study, we compare GaN grown on sapphire with bulk GaN. It is found by numerical fitting that the sheet resistance of GaN layers on sapphire substrates is determined by both bulk GaN conduction and polar-induced screening-charge conduction. The same high conductivity on the N-faces of GaN layers is observed even without the interface with sapphire substrates, i.e., when the latter are peeled off and the GaN layers are chemically mechanically polished. A further comparison of three samples with different orientations (c-, a-, and m-planes) suggests that the observed high conductivity is caused by a mobile screening charge attracted to the N-face by the polar charge present on it. This conclusion follows from the fact that no high conductivity is observed in the samples with non-polar orientations.