Orbital Engineering For Two-dimensional Quantum Structures Of Nitride Semiconductors And Their Deep Ultraviolet Optoelectronic Devices

During the fast scale-down and dimension reduction of materials and functional devices,conventional manipulating technologies and theories on material properties have met their limit for those two-dimensional(2D)and quasi-2D structures such as multiple quantum wells(MQWs)and 2D monolayer materials.As the fundamental entity of matter at the quantum scale,atomic orbitals play a crucial role in determining various properties,just like gene segments in DNA.By modulating orbitals and their interactions in between,one is able to precisely manipulate the microscopic electronic states of low dimensional as well as macroscopic scaled structures.Consequently,the properties of materials and functional structures could be modified intentionally.Based on the above idea,in this thesis,we have proposed and established the theorem and systematic techniques on "orbital engineering" aiming at modifying properties in 2D nitrides and their optoelectronic devices.By manipulating the electron orbitals in the multi-dimensions and multi-scales,the electronic states,band structures,photon propagation modes,and impurity behaviors can be modulated.We have applied this orbital engineering to the two-dimensional quantum structures of group-Ⅲ nitride semiconductors and solved the key problems of deep ultraviolet(DUV)light-emitting devices,including carrier non-confinement,strong optical anisotropy,low light extraction efficiency,and the difficulty of high conductivity.Important results are summarized below:1.Modulating the optical anisotropy of AlGaN MQWs by elevating the orbital potentials.Combining the orbital coupling,Coulomb interaction,and built-in polarization field,the precise energy band profiles of AlN/AlGaN MQWs are revealed.The px/py orbitals enhanced and pz orbital compensated quantum confinement mechanism is discovered.By applying the orbital potential elevating technology,the px/py potentials are shifted up by the orbital coupling and Coulomb additional potentials induced by Mg dopant,which leads to the inversion of the valence band arrangement and the enhancement of TE-polarized emission in the AlN/AlGaN MQWs.2.Optical field modulation by the strong quantum confinement of orbitals.The optical performances of AlGaN and h-BN are enhanced greatly through the strong quantum confinement of orbitals via compressing the scale and constructing 2D,onedimensional(1D),and zero-dimensional(OD)artificial quantum structures.By applying the(AlN)8/(GaN)2 ultra-short period superlattice(SPSLs),the orbitals and electronic states can be strongly confined,which leads to the dominant emission of TE polarization at 234 nm DUV wavelength.By applying nanoimprint lithography,a hexagonal truncated pyramid nanohole structure are introduced to the lateral dimension of(AlN)8/(GaN)2 SPSLs.This 0D nanohole can effectively modulate the light propagation and extraction patterns to overcome the light extraction limit via multiple reflections and enhanced scattering.Finally,the total luminosity of this unique nanostructure is greatly increased by 191%compared to that of a conventional planar structure.3.Orbital-modulated p,n-type conduction in 2D h-BN monolayer.The critical roles of the orbital and its coupling from impurity doping for the conduction of h-BN are revealed.We find that the orbital interaction of low energy s orbitals prefers the BeB,MgB,and ZnB as effective acceptors in h-BN.In contrast,a high conduction band contributed by p-orbitals of B and strong electron-attraction of N result in the extreme difficulty in obtaining n-type conduction in h-BN.Experimentally,by using the lowpressure chemical vapor deposition method,the p-type h-BN monolayer has been successfully achieved by Mg doping and the hole current can reach 14 μA.4.Towards n-type conductivity in h-BN by sacrificial impurity coupling.The introduction of side-by-side O to Ge donor can effectively push up the donor level by the formation of another sacrificial deep level due to the strong coupling between the O 2 pz and Ge 4 pz orbitals.Consensually,a Ge-O2 trimer brings an extremely shallow donor level and very low ionization energy.Experimentally,we obtain the in-situ GeO doping in the h-BN monolayer by using GeO2 as an impurity precursor and successfully achieve both through-plane(-100 nA)and in-plane(～20 nA)n-type conduction.The electron concentration of n-hBN can reach 1016 cm-3.This effective ntype conductivity in monolayer h-BN is obtained for the first time in the world.A vertically-stacked n-hBN/p-GaN heterojunction was fabricated and show good diode behavior.5.Orbital decoupling and unidirectional elimination of hydrogen from semiconductors through a strong local electric field.By applying the interactive behavior of chloridions and the local electric field in a solution-mediated electrochemical system,the orbital coupling of the hydrogen complex can be broken.Efficient hydrogen elimination(>52%)from various semiconductors wafers including p-GaN,p-AlGaN,SiC,AlInP,and also completed light-emitting diodes(LEDs)has been achieved.The p-type conductivity and light output efficiency of H-eliminated UVC LEDs have been significantly enhanced,and the lifetime is almost doubled.Moreover,we confirm that under one-second irradiation of UVC LEDs,bacteria and COVID-19 coronavirus can be completely killed(>99.9%).The RNA of coronavirus also can be completely eliminated by UVC LEDs.These in-depth studies of orbital interactions and related electronic effects in the 2D quantum structures of group-Ⅲ nitrides will help us to better understand the influences of orbitals on the macroscopic properties of materials.By flexibly adopting various orbital engineering techniques in multi-dimensions and multi-scales,controlling of orbitals and electronic states in the Ⅲ-nitride quantum systems has been achieved.This further overcomes the critical problems of AlGaN-based and hBN-based materials and devices,paving a leap toward highly efficient DUV light-emitting devices,and provides new strategies for the fundamental studies and practical applications of group-Ⅲ nitride semiconductors.In addition,the electronic state manipulating techniques by orbital engineering are believed to extend to other material systems.This will further overcome the limitations of conventional methods and promote the development of new materials and functional devices.