First-principles Study On Optoelectronic Properties And Interface Effects Of Gallium Nitride-based Van Der Waals Heterostructures

Low-dimensional gallium nitride(GaN)materials show huge potential for the fabrication of next-generation high-performance optoelectronic devices due to their high electron mobility,wide bandgap,and outstanding optoelectronic p roperties.The formation of low-dimensional GaN-based heterostructures is a premise for the realization of their applications in optoelectronic devices.However,the tradiational heteroexpitaxial method easily brings plenty of mismatch defects at the interface of heterostructures due to the lattice mismatch,resulting in the degeneration of overall performance of GaN devices.In this thesis,we take van der Waals heterostructures consisted of two-dimensional materials and GaN as the research target and investigate the various effects on structural stability,electronic and optical properties of GaN and its heterostructures based on first-principles calculations within the density functional theory(DFT),the main research content and results as follows,1.We investigate the size and surface effects on the structural stability,band structures,and carrier mobilities of GaN by using the DFT calculations.The research results indicate that the pristine GaN undergoes a continuous phase transition from wurtzite,Haeckelite,to graphene phase and the hydrogenated GaN has the surface reconstruction from the partial hydrogenated(2×2)to fully hydrogenated(2×2)one with the thickness reduction of GaN layer.Owing to the impact of surface effect,size effect and quantum confinement Stark effect,the band gap of GaN nanosheets can be tuned in the range of 1.32?4.35 e V and their carrier mobilities also cover a wide range from 11 to 6314 cm²V^-1s^-1.Therefore,two-dimensional GaN materials suitable for optoelectronic applications can be obtained by the control of thickness,structural phase transition,and surface passivation.2.We deeply investigate the structural stability,band structures,and optical properties of monolayer transition metal dichalcogenides(TMDs)and their Janus structures(JTMD)controlled by the component effect based on the first-principles calculation method.The calculation result shows that the component tunes the band gap of TMDs from 1.36 to 1.97 e V,thereby achieving good light absorption from near infrared to ultraviolet.The carrier mobility of JTMD obtained by combining this calculation method shows that it strongly depends on the structural composition and transportation direction,and the carrier mobility of some configurations are higher than that of traditional TMD.Based on these results,this two-dimensional TMD has excellent semiconductor characteristics and can be an excellent material for constructing heterojunction with low-dimensional GaN.3.On the basis of the results of research contents(1)and(2),we construct the van der Waals heterostructures consisted of monolayer MoS₂ and GaN and study the thickness effect of GaN layer on electronic and optical properties of heterostructures.The calculated results indicate that the thickness change leads to the transition of MoS₂/GaN heterostructures from the I-type to II-type band alignment,which explains successfully the experimental controversy about the band alignment of MoS₂/GaN heterostructures.The calculated results of band structures and optical properties show the band gap of heterostructures in the range of 1.35?1.70 e V,resulting in strong optical absorption(?10⁵ cm^-1)in visible region.The calculated results suggest the large potential of two-dimensional MoS₂/GaN van der Waals heterostructures for the photovoltaic and photocatalytic applications.4.We construct two-dimensional MXene/GaN metal-semiconductor junctions by using metallic MXenes as the electrode contact of heterojunctions and investigate their electronic structures and electrical contact properties.The calculated results indicate that the electronic structures and Schottky barrier heights(SBHs)of MXene/GaN heterojunctions can be tuned by the thickness and polarity of GaN sheets and surface termination of MXenes.Moreover,we reveal that the SBHs of these MSJs inherently depend on the screening effect of MXenes on the polarization of GaN and their carrier tunneling barriers are strongly related to the interfacial coupling strength.Based on the co mputed SBHs and tunneling probabilities,OH-terminated MXenes are identified as the most promising electrodes for the formation of low-resistance and high-speed contacts with GaN.