Electronic surface and dielectric interface states on GaN and AlGaN
GaN and AlGaN have shown great potential in next-generation high-power electronic devices; however, they are plagued by a high density of interface states that affect device reliability and performance, resulting in large leakage current and current collapse. In this review, the authors summarize the current understanding of the gate leakage current and current collapse mechanisms, where awareness of the surface defects is the key to controlling and improving device performance. With this in mind, they present the current research on surface states on GaN and AlGaN and interface states on GaN and AlGaN-based heterostructures. Since GaN and AlGaN are polar materials, both are characterized by a large bound polarization charge on the order of 1013 charges/cm2 that requires compensation. The key is therefore to control the compensation charge such that the electronic states do not serve as electron traps or affect device performance and reliability. Band alignment modeling and measurement can help to determine the electronic state configuration. In particular, band bending can determine how the polarization bound charge is compensated; however the band bending is extremely sensitive to the specific processing steps such as cleaning, dielectric or metal deposition, post-deposition or post-metallization treatments, which affect oxygen coverage, carbon contamination, structural defects, bonding configurations, defect states, absorbates, and Fermi pinning states. In many cases, the specific effects of these treatments on the surface and interface states are not entirely clear as the nature of the electronic states has been obscured in complexity and subtlety. Consequently, a more systematic and methodical approach may be required.