Growth Mechanism Of Group ? Low-dimensional Materials And Field Effect Transistor For Terahertz Detection

Carbon,silicon and germanium are three kinds of typical group IV elements with electrical structures and a varity of allotropes.Recently,the successful preparation of group ? low-dimensional materials has greatly expanded the research field of materials.For example,the discovery of graphene aroused great interest in two-dimensional materials.With the decreasing of the dimensions,low-dimensional materials show many promising mechanic,optoelectronic and quantum properties,which greatly expand the prospect of applications in electronics,optoelectronics,energy conversion and storage,thermal electronics and so on.In this paper,we focused on two representative types of group IV for systematic study.One is the study of the mechanism of the heteroepitaxy of in-plane germanium nanowires and the.The other is the study of non-catalytic chemical vapor deposition of graphene,and the graphene and graphene-like structure material Bi₂Te₃ in the terahertz photodetectors.The full text is divided into the following three chapters:1.In-plane nanowires were fabricated via the growth of Ge onto rib-patterned Si?1 1 10?templates separately oriented in the[551^<sub>]and[11^<sub>0]direction.The self-assembled,self-extending and self-connected structures of SiGe heterostructure were observed at the rib top and sidewalls.According to the experimental results,the models of the SiGe heteroepitaxy were simplified by the finite element method?FEM?.The corresponding models were established for all possible structures on the rib-patterned silicon substrates.We analyzed the distribution of strain in the heterostructure.And,furthermore,the total energy density in different structures was calculated by FEM.After comparation of the total energy density,the explanation was obtained from the thermodynamic equilibrium.The expected results are consistent with the theoretical results.The self-assembled,self-connected and self-extending growth processes is proved to be an effective route to reduce the total energy density.2.We proposed a new method for catatyst-free growth of graphene quantum dots and nanographene on dielectric substrates by using organic liquid as carbon source.This aromatic hydrocarbon molecule(C₁₄H₉)SiH[N?C₂H₅?₂]₂ was sepecially designed and optimized.We designed and built a special chemical vapor deposition system combined with an atomic layer control function to achieve the direct growth of nanographene on silica and silicon substrates.In the atmosphere of Ar/H₂ gas mixture at 800?,the benzene ring of organic molecules was combined with each other to form period hexagonal structure.Further,with the help of an atomic layer deposition control system,the thickness of nanographene films can be precisely controlled.The growth process is simple and controllable,and was independent of the substrates.The resistance of the nanographene films was measured to be about 2000ohms/sq,and the transmittance was present above 90%,which can be applied for transparent conductive film.3.We explored the mechanism of graphene-based terahertz photodetectors.The graphene field effect transistor was designed and fabricated by using the fabrication technology of top-gate field-effect transistor.The output characteristic curve,the transfer characteristic curve,the photoelectric response and the response speed at microwave/terahertz range of our devices were tested through the detection system in our laboratory.The photoelectric response mechanism of the top-gate graphene field effect transistor at microwave and terahertz range was then studied.We obtained the response and the noise equivalent power of the device at terahertz frequency,which were 158V/W and 67pW/Hz^1/2 respectively.According to the impulse response,the response speed of the device was about 30?s.We verified the diffusion transport model.The functional relationship between the terahertz response and the conductivity of graphene follows as?35?u?1/s×ds/dV_G under zero bias voltage.We also clarified the source of the photoelectric response of the top-gate graphene field effect transistor.The temperature gradient was formed in the graphene channel under the irradiation of microwave or terahertz light,causing the diffusion of hot carriers and following form the photocurrent.Meanwhile,we compared the graphene with the topological insulator Bi₂Te₃ by fabricating the Bi₂Te₃-based devices with a log-periodic circular-toothed antenna as source and drain contact.Using the the detection system in our laboratory,the output characteristic curve of the device and the photoelectric response at terahertz frequency were tested.The response at 0.1THz reached 15V/W and the corresponding noise equivalent power was measured as 0.6nW/Hz^1/2.The response speed of the device was about 9?s,which was one order of magnitude faster than that of the graphene-based devices.It shows that the topological surface state with ultra-high carrier mobility plays an important role in the photoelectric response.Under the built-in electric field,the photogenerated carrier quickly reached the source and leakage to form the photocurrent,which improves the response speed of the device.In this paper,we discussed the mechanism of growth methods and photodetectors of two typical IV low-dimensional materials.Our research findings revealed the guiding significance for the foundations and applications of these materials in photoelectric detection.