Impact of charged basal stacking faults on the mobility of two-dimensional electron gas in nonpolar a-plane AlGaN/GaN heterostructures

Abstract

The development of nonpolar nitride enhancement-mode high electron mobility transistors (E-HEMTs) has been severely hindered by the low and strongly anisotropic mobility of two-dimensional electron gas (2DEG) in nonpolar nitride heterostructures. The poor 2DEG transport properties could be correlated with the presence of basal stacking faults (BSFs) in nonpolar nitride heteroepitaxial films. A theoretical model of 2DEG transport in a-plane AlGaN/GaN heterostructures containing a large number of charged BSFs is developed. The electric current along an applied field is taken to be formed by a process in which two-dimensional electrons tunnel through a BSF and diffusively transport between consecutive BSFs. The room temperature 2DEG mobility of an a-plane Al0.3Ga0.7N/GaN heterostructure influenced by charged BSFs is calculated, and the result is consistent in magnitude and anisotropy with the experimental data in the literature. Analysis shows that charged BSFs lower the 2DEG mobility to about one tenth or less in the 2DEG density range from 1.0 × 1012 cm−2 to 1.5 × 1013 cm−2, and the effect is dependent on the direction of the external field. The anisotropy of the 2DEG mobility is dominated by the anisotropic electron tunneling process, but the 2DEG mobility nearly parallel to BSFs can be decreased obviously by the enhanced interface roughness scattering at relatively higher 2DEG density.