| The high energy efficiency,low-power consumption and high extreme performance of the third-generation semiconductor devices have exhibited wide application prospects in electronic power,microwave radio frequency and optoelectronics.Based on the piezoelectricity property of third-generation semiconductors,piezotronics as a new emergent field has attracted widespread attention in the design of high-performance devices.However,with further reduction of semiconductor device size,Moore’s Law is going to its limitation,and the traditional semiconductor device design will face many limiting factors such as quantum size,quantum tunneling and surface effect.In this case,the development of piezotronics devices will also face severe challenges.Under such a background,quantum piezotronics has been proposed to bring the development of third-generation semiconductor devices to a new level.However,up to now,there are still many problems to be explored and solved for the basic theory and device design of quantum piezotronics,especially for those spin-related new materials and new physics.This doctoral thesis focuses on the design of high-performance quantum piezotronics devices,and explores fundamental physics and device models based on spin properties of low-dimensional piezoelectric semiconductors.The content of this thesis involves the basic theory of spin-orbit coupling（SOC）,topological phase transition of quantum materials and spintronics device design,aiming at giving a deep insight into the unique spin properties of piezotronics effect from the quantum perspective,revealing the basic physical laws of spin modulation in third-generation semiconductors,and providing possible ways for designing high-performance,low-power and high-sensitivity spin devices.These studies will be of great significance for the development of quantum piezotronics in the future.The main contents of this thesis are as follows:1.Modulation of two-dimensional electron gas by polarization field in AlGaN/GaN semiconductor heterojunction.By using the classical triangular well model,strain-induced polarization field could effectively control two-dimensional electron gas concentration,and the mechanism of high-performance piezotronic transistors was also clarified.Additionally,the band bending caused by polarization field could not only confine quantum states of the electron,but also modify the asymmetry of interface potential and further modulate the SOC.These results reveal that polarization fiel in low-dimensional piezoelectric semiconductors have great potential to control spin properties.2.Modulation of the SOC by polarization field in normal and topological insulating quantum wells.By using L（?）wdin perturbation method,both of Rashba and Dresselhaus SOC were investigated in III-V nitrides and II-VI oxide semiconductor quantum wells with normal insulator.The results showed that polarization field could simultaneously enhance both SOC but Dresselhaus SOC was predominant.In the case of Zn O/Cd O and GaN/In N quantum wells topological insulator,strong polarization field could drive band inversion,leading to a large SOC with nonlinear behavior.The SOC coefficient was up to 80 me V·nm.The analyses showed that the coupling between electrons and light holes was responsible for these interesting SOC.This work not only provides a new method to induce strong SOC,but also has great significance to study nonlinear SOC physics.3.Based on the theory of SOC modulation by polarization field,phase transition in topological insulators and transition metal dichalcogenides（TMDs）was investigated.For Zn O/Cd O and GaN/In N quantum wells,strong polarization field（about 10 MV/cm）could trigger topological phase transition.By using L（?）wdin perturbation method,an effective six-band Hamiltonian was proposed to confirm the existence of gapless edge states,where the inverted gap could reach 7.2 me V.In addition,for two-dimensional piezoelectric semiconductor materials,the in-plane piezoelectric polarization field described by electromagnetic theory exhibited high localization,which could accurately control edge states and topological phase transition in TMDs.In comparison with deformation potential and uniform electric field,polarization field was more robust to control topological phase transition.This work offers the new means to control the electronic properties of topological quantum materials,and broadens the basic content of piezotronics,which makes it possible to design quantum piezotronics devices.4.Based on the modulation of SOC and topological phase transition,high-performance device models were proposed based on low-dimensional piezoelectric semiconductors.By constructing quantum transport device model and performing transport software package Kwant,the horizontally and vertically controllable structures were proposed in device designs by studying charge transport,spin-polarized transport and spin current transport.The horizontal controllable structure was based on Hg Te/Cd Te,GaN/In N and Ga As/Ge quantum well topological insulators.By building quantum point contact（QPC）device structure,three kinds of high-performance devices were proposed including piezotronic transistors,logic nanodevices and quantum information memory.Piezotronic transistors were characterized by ultrahigh conductance switching ratio(higher than 1010),low-power and high sensitivity（about 104）.5.The vertically controllable structure involved spin-polarized transport and spin current.The former was based on Zn O/Cd O quantum well topological insulator with strong SOC.By building a long channel QPC device structure,spin flip and spin polarization are explored.The results showed that spin polarization exhibited highly oscillatory,demonstrating an excellent control of spin polarization by polarization field.The increase of QPC width was not beneficial to controlling spin polarization.In addition,polarization field would also be able to control the‘ON’and‘OFF’state of spin transport,which could be used to design spin-polarized switching devices.Spin current transport was another study of the vertically controllable structure based on Zn O/Cd O quantum well normal insulator with large SOC.A four terminal device model was constructed to study the impact of polarization field on spin-charge current conversion.The results showed that polarization field could significantly enhance spin current and spin Hall conductance,and the conversion ratio from charge current to spin current could be improved by a factor of four,greatly reducing energy consumption of spin devices.These horizontal and vertical controllable structures in device design not only broaden basic content of piezotronics,but also provide new avenue for achieving novel quantum piezotronics devices. |
