Growth Preparation And Physical Property Modulation Of Two-dimensional Layered GaSe

Ferroelectricity is a collective polarization effect arising from the spontaneous alignment of electric dipoles,in which the polarization states can be switched by an external electric field.The ferroelectric field-effect transistor（Fe FET）is considered as a good candidate for next-generation non-volatile memory because of its non-destructive read and fast repeatable write characteristics,and its non-volatile functionality is realized by the bistable ferroelectric polarization in the ferroelectric layer.Conventional ferroelectric materials are generally oxides with three-dimensional（3D）perovskite structures,such as Ba Ti O₃,Pb Ti O₃ and Bi Fe O₃,mostly insulators with large band gaps and low mobility,which are difficult to integrate into existing Si-based semiconductor devices.In recent years,two-dimensional（2D）van der Waals（vd W）ferroelectric materials have attracted much attention because of their atomically flat interfaces,weak interlayer interactions,and mostly high mobility semiconductors with moderate band gaps.2D ferroelectrics can be classified into two major categories in terms of generation mechanism,ion displacement-induced intrinsic ferroelectrics such as Sn Te,Cu In P₂S₆,In₂Se₃,T_d-Mo Te₂,and sliding ferroelectrics with out-of-plane ferroelectricity induced by interlayer-sliding,such as bilayer h-BN,1T’-Re S₂,TMDs（WTe₂,WSe₂,Mo Se₂,WS₂,Mo S₂,Mo S₂/WS₂）.However,intrinsic 2D ferroelectrics are relatively rare due to the strict limitation of lattice symmetry.Sliding ferroelectricity can only occur in bilayer or multilayer systems,which generally exhibit out-of-plane ferroelectricity,lack of in-plane ferroelectricity,and much lower polarization strength than intrinsic ferroelectrics.Ga Se is theoretically predicted to have two-dimensional sliding ferroelectricity in the above bilayer system,but no relevant experiments have been reported.Therefore,exploring 2D ferroelectricity in more different crystal structures can not only expand the family of ferroelectric materials,but also further refine the generation mechanism of 2D ferroelectricity.The ultra-high vacuum molecular beam epitaxy（MBE）system enables precise epitaxial growth of 2D materials with controlled elemental ratios and single atomic layers.In combination with scanning tunneling microscope（STM）,the surface morphology and crystal structure characteristics of the synthesized samples can be observed in situ.Moreover,the local density of states（LDOS）of the samples can be further detected by scanning tunneling spectroscopy（STS）.The detection of LDOS allows direct observation of the electronic states in real space.Conventional two-dimensional electron gas（2DEG）and quantum well states（QWS）usually occur at semiconductor heterojunction interfaces and noble metal surfaces,while are rarely observed in single 2D semiconductors.The combination of MBE and STM systems allows not only the synthesis of high-quality monolayer 2D semiconductor materials,but also the further study of their electronic structure properties without contamination.In this thesis,few-layer gallium selenide（GaSe）have been synthesized by the MBE method,and then the 2DEG and QWS in few-layer Ga Se have been observed by STM/STS in real space.Then,the role of monolayer Ga Se/HOPG moiré-pattern on the modulation of its electronic state was investigated.The 2D ferroelectricity in Ga Se was explored using atomic force microscopy（AFM）.The main contents are as follows:1.Using the MBE technique,we have realized the molecular beam epitaxy growth of In Se and Ga Se thin films on mica,Sr Ti O₃,HOPG,and 6H-Si C substrates with large area and atomic level thickness.The surface morphology of the samples on different substrates was characterized by atomic force microscopy（AFM）,with the flattest and most regular surface morphology on mica and HOPG substrates.The crystal structures were characterized by X-ray photoelectron spectroscopy（XPS）,Raman spectroscopy,and X-ray diffraction（XRD）,which showed that our samples were well crystallized and free of heterogeneous phases.Then,atomic-level In Se/Ga Se vertical and horizontal heterojunctions were successfully epitaxially grown on HOPG substrates.The thin film samples prepared by MBE on HOPG substrates can be further used for STM test.2.The epitaxially grown few-layer GaSe on highly ordered pyrolytic graphite（HOPG）substrate was probed by STM/STS.We observe that the QWS and 2DEG coexist in few-layer Ga Se.The QWS host in the valence band exhibit peak feature,and the number of quantum wells is exactly equal to the number of atomic layers.Meanwhile,the 2DEG hosts in the conduction band and exhibits standing-wave feature.Moreover,monolayer Ga Se/HOPG heterostructure with different stacking angles（0°,33°,8°）form different moiré-patterns,different moiré-patterns arising from lattice mismatch and angular rotation between adjacent atomic layers in 2D materials,thus effectively modulating the electron effective mass,charge redistribution and band gap of Ga Se.Our results demonstrate that moiréengineering is a promising approach for modulating electronic structures and promoting the emergence of new physical phenomena.3.Ferroelectricity in ultrathin 2D materials has attracted broad interest due to potential applications in nonvolatile memory,nanoelectronics and optoelectronics.However,ferroelectricity is barely explored in materials with native centro or mirror symmetry,especially in the 2D limit.Here,we report the first experimental realization of room-temperature ferroelectricity in van der Waals layered Ga Se down to monolayer with mirror symmetric structures,which exhibits strongly intercorrelated out-of-plane and in-plane electric polarization.The origin of ferroelectricity in Ga Se comes from spontaneous intralayer sliding of the Se atomic sublayers,which breaks the local structural mirror symmetry and forms dipole moment alignment.Ferroelectric switching is demonstrated in nano devices fabricated with Ga Se nanoflakes,which exhibit exotic nonvolatile memory behavior with a high channel current on/off ratio.Our work reveals that intralayer sliding is a new approach to generating ferroelectricity within mirror symmetric monolayer,and offers great opportunity for novel nonvolatile memory devices and optoelectronics applications.