Controllable Growth And Characterization Of Ge Nanowires On Patterned Si(001) Substrate

Low dimensional nanomaterials have attracted attentions due to their novel physical properities and potential applications in quantum devices.1D semiconductor Ge nanowires are ideal material system for spin or even topological qubits because of the high carrier mobility,low hyperfine interaction,high Landau g factor and the Si compatibility.Conventionally,the Ge nanowires were obtained by vapor-liquid-solid(VLS)method.In this method,metallic catalysts are needed and the wires are grown out-of-plane.The metallic catalysts are deleterious to the Ge wires.In addition,the transfer and arrangement of such out-of-plane wires are big challenges.Site-cotronlled growth of in-plane Ge nanowires without the use of metallic catalyst is a solution to such challenges.In this thesis,we firstly introduce the growth of in-plane Ge nanowire on flat Si substrate and its research progress in quantum devices.Then,by combining bottom-up self-assembly and top-down nanofabrication technologies,we demonstrate for the first time the wafer-scale growth of site-controlled Ge nanowires on Si which lays a good material foundation for the scalability and addressability of the Si based quantum devices.The details are as follows:Firstly,I introduce the growth of catalyst free and in-plane Ge nanowires on Si(001)through the growth of pure Ge with in-situ annealing.The Ge wire is 2 nm in height with two(105)side facets.Stability diagram and g factor are obtained from Ge nanowire single quantum dot device fabricated by our collaborators.Furthermore,a superconductor microwave resonator is coupled to a Ge single quantum dot device and the resonator can accurately detect the charge state in the quantum dot.The first Ge hole spin qubit is also formed in a Ge double quantum dot device.Secondly,in-plane ordered Ge nanowires are grown on Si substrate by combing the top-down micro-fabrication technology and bottom-up self-assembly growth method.They were achieved by a novel growth process which uses a SiGe strain-relaxation template and can be generalized to other material combinations.Periodically arranged deep trenches along [100] or [010] crystallographic direction form on Si(001)substrate by electron beam lithography and reactive ion etching technologies.Then site-controlled Ge nanowires form at the two edges of the trenches after the deposition of SiGe layer which will form SiGe mound and subsequently Ge layer with in-situ annealing by MBE.The so called(105)faceted Ge hut wires have a height of 3.8 nm with a standard deviation of merely 0.11 nm and their position,period,length and structrure can all be precisely controlled.L-shaped and square-shaped Ge nanowires are also realized by tuning the structure of the trenches.STEM image indicates the high crystal quality of the nanowires.Thermodynamic model shows that Ge nanowires prefer to form on the SiGe mound rather on the flat area because of the enhanced strain relaxation.Stability diagram,the anisotropy of g factor and spin-orbit coupling strength are extracted from the single quantum dot device on the site-controlled Ge nanowires.The spin-orbit coupling strength is comparable to the In As and In Sb wires and can be tuned by electric field.Moreover,the coupling between two quantum dots can be realized on two closely spaced parallel Ge nanowires and it is promising for the scalability of quantum devices.Furthermore,we systematically study the influence of thickness of SiGe layer,Ge concentration,growth temperature and annealing on the SiGe mound and the morphology of Ge nanowires.Strain induced multilayer growth of ordered Ge nanowires is also realized on this kind of patterned substrate.Finally,in order to get more differernt structures of nanowires like nanowire networks,we also try to get Ge nanowires inside the trenches.Narrow and shallow trenches along [100] or [010] crystallographic direction are fabricated on the Si(001)substrate by electron beam lithography and reactive ion etching technologies.Ordered GeSi nanowires can grow inside the trenches after the deposition of SiGe layer with low Ge concentration with subsequently annealing at higher temperature.The(105)faceted GeSi nanowires have a height of about 7 nm and the position,structure and period can be precisely controlled by these of the trenches.Growth mechanism is qualitatively explained through the research on the influence of annealing temperature,Ge concentration and pattern period on the formation of GeSi nanowires.Furthermore,the growth of GeSi nanowires with higher Ge concentration is realized by the growth of strain induced Ge nanowires on Si spacer layer.