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 GaN metal-insulator-semiconductor (MIS) gate power field-effect-transistor (FET) in which the critical conducting channel is in close proximity of the surface and interface. Recently, nitridation on GaN surface has already been experimentally proved to be an effective surface and interface treatment technique to mitigate the surface/interface trap density (D
it) and enhance the device performance, stability and reliability. Despite the technique process details and the enhanced device performances by nitridation on GaN surfaces have been reported, a microscopic understanding of the nitridation effects on GaN surface and dielectric/GaN interface at atomic level is still lacking. 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. To realize this objective, the work was devoted into three parts: (1) Revealing the nitridation effects on GaN surface. In this work, the surface state distribution profile as a result of nitrogen adsorption on GaN surface has been reported by means of first-principles calculation and photoelectron spectroscopy. The results prove that the nitridized GaN surface features two surface bands, and both bands are modified towards the valence band (i.e., the shallow traps become deeper, and the lower band directly overlaps with the valence band by the deployment of surface nitridation). The theoretical and experimental results insightfully proclaim the nitridation effects on GaN surface at atomic level and support a surface-state ionization model for the GaN band-edge (3.4 eV) emission in metal-AlGaN/GaN Schottky-on-heterojunction diode under forward bias. (2) Investigating the nitridation effects on amorphous-SiN
x/GaN interface. The study reveals that for the Si-rich SiN
x/GaN interface without nitridation treatment, both shallow and deep traps exist in a wide energy range within the GaN bandgap. However, with proper surface nitridation prior to SiN
x deposition, the interface exhibits much cleaner bandgap structure with significantly suppressed D
it, indicating a high-quality interface with fewer trap states after sufficient nitridation. The nitridation effects on modified D
it is further verified by C-V measurement in GaN MIS diode with interface nitridation. The low D
it in the MIS-gate region well explains the enhanced V
th stability in nitridized GaN MIS-gate devices. (3) Interface engineering on monolayer MoS
2/GaN 2D/3D hybrid heterostructure to seek potential opportunities for designing novel devices. The calculation results reveal that both interfaces demonstrate indirect bandgap, which is a benefit for a longer lifetime of the photoexcited carriers. Meanwhile, the conduction band edge and valence band edge of MoS
2 side move upwards after nitridation treatment. The modification to band alignment is validated by XPS measurement on MoS
2/GaN heterostructures constructed by a modified wet-transfer technique. The significantly increased band offset could lead to better electron accumulation capability at GaN side. The nitridized 2D/3D heterostructure with effective interface nitridation exhibits a clean bandgap and substantial optical absorption ability and could be potentially used as practical photocatalyst for hydrogen generation by water splitting using solar energy.
| Date of Award | 2018 |
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| Original language | English |
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| Awarding Institution | - The Hong Kong University of Science and Technology
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