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.
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