Thesis Examination Committee

Prof Zhigang LI, MAE/HKUST (Chairperson)
Prof Kevin Jing CHEN, ECE/HKUST (Thesis Supervisor)
Prof Yang CHAI, Department of Applied Physics, The Hong Kong Polytechnic University (External Examiner)
Prof Jianan QU, ECE/HKUST
Prof Zhiyong FAN, ECE/HKUST
Prof Baoling HUANG, MAE/HKUST

Abstract

Benefiting from the superior material properties, wide bandgap semiconductor GaN has emerged as one of the most promising candidates for momentous device applications such as power electronics, RF/microwave/millimeter-wave electronics and optoelectronics. The GaN surface/interface properties, especially the atomic configurations and electronic structures, are of particular significance to the performance of lateral heteroterojunction devices in which the critical conducting channel is in close proximity of the surface/interface. Surface nitridation has been experimentally proved to be an effective surface/interface treatment technique to mitigate the GaN surface/interface trap density (Dit) and enhance the device performance, stability and reliability. In this work, a comprehensive investigation is carried out to obtain an atomistic understanding of the nitridation effects on GaN surface and interface, utilizing first-principles calculation and material/device characterizations. The study aims at revealing the intrinsic nature of the atomic configurations, modified surface/interface state distribution and the underlying mechanisms for the enhanced device performances.

Firstly, we investigated the effect of nitridation on GaN surface and SiNx/GaN interface which are vital for GaN-based lateral MIS-gate power devices. It is revealed that GaN surface features two surface state energy bands, both of which could be modified towards the valence band by surface nitridation. The modified surface band distribution of GaN surface clearly validates a surface-state ionization model for GaN band-edge emission in metal-AlGaN/GaN Schottky-on-heterojunction diode. As for the SiNx/GaN interface, the underlying mechanism for the greatly suppressed Dit was identified to be the formation of replacement of the Si-Ga bonds by interfacial Si-N bonds. The low Dit in the MIS-gate region clearly explains the enhanced threshold voltage stability in nitridized GaN MIS-gate devices. 

Secondly, a MoS2/GaN 2D/3D hybrid heterostructure is investigated. Both theoretical and experimental results explicitly depict that the band edges of monolayer MoS2 side increase after nitridation treatment, leading to better electron accumulation capability at GaN side. The nitridized 2D/3D heterostructure exhibits a clean bandgap and substantial optical absorption ability, and could be potentially used as efficient photocatalyst for hydrogen generation by water splitting.