Study On Low Dimensional Wide Bandgap Nitrides Based Heterojunctions And Devices

Faced with the complex characteristics of semiconductor devices approaching and entering the sub nanometer scale,semiconductor integrated circuit integration density entering the billions level,and increasingly presenting three-dimensional heterogeneous multifunctional integration in the post Moore’s era,various new elements and new dimensional materials are explosively entering the research and application fields of post Moore’s integrated circuits.Wide bandgap semiconductors,represented by nitride semiconductors,have shown excellent properties in the fields of optoelectronic devices,radio frequency devices,power devices,and related integrated circuits due to their unique physical properties.They have demonstrated performance surpassing traditional semiconductor materials such as silicon and gallium arsenide.For information regulation devices such as logic and storage,low dimensional wide bandgap semiconductors are also expected to play a huge role in nano and sub nano scale chips in the post Moore’s era.The low dimensionality of wide bandgap semiconductors is expected to become a new approach to enhance the gate control capability of semiconductor devices and improve their transport characteristics.Therefore,exploring the electronic structure and quantum effects of low dimensional wide bandgap semiconductors at the nanoscale is of great significance for breaking through the size and functional bottleneck of devices in the post Moore’s era.This thesis focuses on two representative wide bandgap III-nitrides—GaN and h-BN,and systematically studies the influence of quantum effects on their physical properties at the two-dimensional limit.In addition,it reveals the coupling relationship between electrons and phonons,as well as charge and spin.Further,it explores their potential applications in electronics,optoelectronics,spintronics,and heterostructures.The research contents of this thesis are as follows:1.Revealed the influence of precursor partial pressure on GaN growth mode during the SCNR process,and thus achieved the preparation of large-area two-dimensional GaN single crystal films.In this study,the stronger nitriding ability of W compared to Gawas utilized to suppress the vertical growth of GaN.In addition,the kinetic dominance of the epitaxial process was achieved by reducing the precursor partial pressure,thereby promoting the layered growth of GaN.Based on this,two-dimensional GaN single crystal thin films with lateral dimension of 20μm and thickness of 3.7 nm were prepared.Under the influence of quantum confinement effect,the prepared two-dimensional GaN films exhibited larger optical bandgaps and higher internal quantum efficiencies compared to GaN bulks.This study gave a preliminary understanding of the physical properties of two-dimensional GaN and provided an experimental basis for the subsequent implementation of two-dimensional GaN based devices.2.The steady-state structures of two-dimensional wide bandgap nitrides were studied,and the influences of quantum effects on their band structures and carrier characteristics were explored.For WZ GaN with non-layered structure,the competition between the surface energy and the bulk energy was studied to reveal its spontaneous relaxation mode at the nanoscale.Furthermore,the layer dependence of its steady-state structure was utilized to achieve the large-scale regulation of its mobility,band gap,and band edge position.In addition,the effect of surface passivation on the physical properties of two-dimensional GaN was studied.The results indicated that the buckled phase of two-dimensional GaN under H passivation retained the sp³hybridization and the direct bandgap of GaN bulks,and the band gap was jointly controlled by quantum confinement effect and quantum confinement Stark effect.For h-BN with vd W-layered structure,the effects of interlayer coupling on its stacking mode,band gap and band edge position were studied,and the sources of band splitting between the bottom of conduction band and the top of valence band was revealed.This study systematically evaluated the carrier characteristics of two-dimensional wide bandgap nitrides,and provided theoretical guidance for the subsequent design of related heterojunctions and devices.3.The interface and band alignment characteristics of low-dimensional heterojunctions based on two-dimensional buckled GaN and two-dimensional h-BN were studied.For two-dimensional buckled GaN,the electronic structures of GaN/Mo S₂heterojunctions under different spontaneous polarization directions were studied.The results showed that the polarization inversion would lead to the band alignment switching of the heterojunction.Besides,with the assistance of external electric field,the band gap could be tuned in a large range.For two-dimensional h-BN,a scheme using it as as an electron acceptor for H-diamond was proposed,in which the introduction of the twist angle between components significantly affected the carrier behaviors at the interface.Based on this,a H-diamond/h-BN heterojunction with an interlayer angle of 90^°was designed.The strain-induced sp³rehybridization of h-BN was conducive to obtain H-diamond surface with high 2DHG density.In addition,the designed heterojunction could effectively shield the transmission of carriers,thereby suppressing the gate leakage.This study combined the rich tunning means of two-dimensional materials and the unique physical properties of wide bandgap materials,providing ideas for the design of novel low-dimensional heterojunctions.4.A Ti-GaN-Ti contact scheme with low tunneling barrier,strong orbital coupling and zero Schottky barrier was designed,and the ballistic transport characteristics of sub-10 nm monolayer planar GaN n-FET were studied.The results showed that it exhibited excellent subthreshold characteristics,high switching speed and low switching power consumption when the gate length was not less than 5 nm.The subthreshold swing,on-state current,delay time and power delay product of 5 nm monolayer planar GaN n-FET could meet the corresponding requirements of IRDS for both HP and LP devices.Lastly,the on-state current and carrier effective mass of 15 kinds of 5 nm ballistic transistors were summarized,and the general law that"0.6 m₀is the critical point in the competition between the carrier drift velocity and the band-edge density of states"was obtained.This study showed that monolayer planar GaN was expected to extend Moore’s Law to the 5 nm node.It also provided a reference standard for the evaluation of channel materials in ballistic transistor design.5.The introduction mechanism and regulation means of d⁰magnetism in two-dimensional GaN and two-dimensional h-BN were studied.For two-dimensional planar and buckled GaN,the unpaired electrons brought by the Gavacancies introduced non-zero local magnetic moments.Among them,the spin charges of monolayer planar GaN were highly dispersed in the lattice,and long-range ferromagnetic couplings mediated by p-p hybridization were formed between local magnetic moments,which made it achieve a Curie temperature as high as 568 K.In addition,when the Gavacancy rate was higher than 1/18,it showed semi-metallic characteristics.For monolayer h-BN,a Si-O_xadjacent doping strategy was proposed to simultaneously solve the problems of n-type doping and d⁰magnetic introduction.On the one hand,the coordination between the central and the adjacent impurities provided a new degree of freedom for the tuning of charge and spin characteristics;On the other hand,the significant difference in electronegativity between impurities and host atoms contributed to the long-range dispersion of spin charges.On this basis,a magnetic tunnel junction with Si-O₃doped monolayer h-BN as the ferromagnetic layers was designed,which achieved robust spin filtering function at 376 K.This study not only showed the possibility of charge-spin coupling in two-dimensional wide bandgap semiconductors,but also provided a novel idea for the design and regulation of d⁰DMS.