Surface/Interface Effects Of GaN/InN Core/Shell Nanowires And Cu Interconnection Under External Field

Recently, as important one-dimension heterostructure nanostructures, core/shell nanowires (CSNWs) have attracted broad interest. Semiconductor heterostructure NWs exhibit unique electronic, optical, and mechanical properties. Thus, there are of importance in fundamental science and nanotechnology application. GaN and InN with wide and narrow band gaps are the most important semiconductor materials. Different growth directions ([0001], [10(1|-)0], and [11(2|-)0]) and different across shapes (hexagonal and triangular shapes) of NWs can be synthesized by controlling different growth conditions. It has been reported that GaN and InN NWs have great application prospects and can be widely used in photovoltaic devices, photocatalyst, spintronic devices, light emitting diode, etc.In ultra-large scale integration (ULSI), Cu interconnection replaces Al and Al(Cu), because of its high conductivity, and good electromigration (EM) resistance. Cu interconnection is one of the most essential concerns and determines the further development of ULSI. Upon miniaturization of ULSI with the feature sizes reduced to 22 or 14 nm, further thinning of Cu interconnection leads to more serious EM. The Cu/low-k dielectrics or Cu/barrier layers interfaces are main EM paths, due to the weak bonding strength at the interfaces. Thus, a vital issue addressed has arisen to seek a material that can effectively improve the EM reliability of Cu interconnection.In nanodevices with the miniaturization, the size becomes smaller and smaller. The performances of many nanoscale devices are sensitive to the variation of local atomic configurations, leading to that electronic state analysis at the scale of individual atoms becomes more and more important. In particular, when the diameter of NWs is less than 5 nm, it becomes a great challenge for studying in experimental studies. Thanks to the development of computer techniques, the first?principles calculations become much more powerful, and are beneficial to study the detail effects of the atomic and electronic structures on the properties of NWs. The surface modifications, interfaces, doping, electric field, strain field, ect, are essential to the properties of nanodevices. Especially, the coexistence of the external circumstance has significant effect on the performance of nanodevices. It is promising for GaN/InN CSNWs in the application as photovoltaic devices because of the unique electronic and optical properties of GaN/InN CSNWs induced by surface modifications and strain field. In addition, the good doping at the interfaces can enhance the interfacial stability and bonding strength of Cu interconnection, which effectively improves the EM reliability under electric field. The results provide a new way of designing the miniaturization of ULSI. In this thesis, using the first-principles calculations based on density-functional theory, we investigate the effects of the surface modifications and strain fields on the electronic and optical properties of GaN/InN CSNWs, and the effects of the doping at the interfaces on the EM realiablity of Cu interconnection.The main results obtained in the thesis are divided into three parts as following:Firstly, we investigate the effects of surface modifications on atomic structures, electronic and optical properties of triangular and hexagonal GaN/InN CSNWs along [0001] derection. The results show that the band gaps Eg decrease, as the thicknesses of InN shell ( radii of GaN core) increase in hydrogenated GaN/InN CSNWs with fixed radii of GaN core (thicknesses of InN shell), due to the effects of quantum confinement and intrinsic strain. Furthermore, our calculations demonstrate that surface modifications with H and F atoms substantially modulate the band gaps dependent on the adsoprtion sites and the F/H ratio. It is worthy noting that surface modifications induce the separation of electrons and holes. The results elucidate that surface modifications change electronic structures of CSNWs with a transition from type-â… band alignment to quasi-type-â…¡, which open a new way in the field of renewable energy applications.Secondly, we investigate the influence of the intrinsic and external uniaxial strains on the atomic structures, electronic and optical properties of GaN/InN CSNWs with hexagonal and triangular shapes along [0001] direction. It is found that the band gap of the GaN core in hydrogenated CSNW differs from that of the pure GaN NW due to the intrinsic strainεi between GaN core and InN shell. Under the external uniaxial strainεe on GaN/InN CSNWs, the direct band gap decreases (increases) with increasing the external tensile (compressive) strain. The efficient dissociation of electrons and holes excitons is enhanced by the external tensile strain. The results demonstrate thatεi andεe substantially influence electronic and optical properties of CSNWs, which probably reach the requirement of GaN/InN CSNWs applied in practic devices.Thirdly, the effects ofα-Al₂O₃ films and the Al atoms doped at theα-Al₂O₃/Cu interfaces on atomic structures of Cu(Al) films are investigated under external electric fields F. The Cu films with large deformation indicate that EM of Cu slabs occurs as F reaches 0.040 au (1 au = 51.4 V/(?)), in contrast, the Cu(Al) films coated byα-Al₂O₃ films slightly change and are more stable under the same and larger F. The main reason is that Cu?O covalent-ionic and Al?O ionic bonds exist at the interfaces, indicating that the stronger bonding strength effectively improves the reliability of Cu atoms. These results provide a solution of enhancing the EM reliability of Cu interconnection in ULSI.