Growth Of Bi₂Te₃low-dimentional Structures By Hot Wall Epitaxy And Its Laser Heating Effect

As a kind of room temperature thermoelectric material, Bi2Te3has beenintensively investigated. Recently, much more attention has been paid to this materialagain since it is found to be one kind of three-dimensional topological insulator. Thetopological insulator is a conventional insulator in the bulk with a gapless metallicstate on the surface, which is significant for application in the fields of quantumcomputing, spintronics and so on. Bi2Te3low-dimensional structures such as films,nanoplates and nanoribbons have more advantages in the researches of topologicalsurface states and fabrication of devices since compared with bulk materials they havelarge surface-to-volume ratios, facilitate external manipulation of the surface states bydoping and varying gate voltages, and are compatible with modernmicro/nanofabrication technologies. In this dissertation, we have prepared Bi2Te3films and ultrathin nanoplates by self-designed hot wall epitaxy (HWE) system andinvestigated the laser heating effects of Bi2Te3films and nanoplates in Raman spectra.The major contents and results are summarized as follows:1. We have designed and built by ourselves the hot wall epitaxy system for thegrowth of Bi2Te3films and nanoplates. High quality Bi2Te3films with flat and smoothsurface along c-axis have been prepared by this HWE system in the Volmer-Webergrowth mode. We have investigated the influence of the substrate temperature andcrystal orientation on microstructures of Bi2Te3films.2. We have prepared ultrathin Bi2Te3nanoplates with triangular or hexagonalmorphologies by a modified HWE system, in which a quartz plate with holes wasplaced between the source and substrate in the growth tube in the layer-by-layergrowth mode. In Raman spectra the peak from the nanoplates exhibits anobvious red shift with decreasing the thickness. The Raman map derived from thefrequency of the peak central position was used to characterize the thicknessdifference of ultrathin Bi2Te3nanoplates.3. XPS measurement shows that the surface of Bi2Te3films exposed to air isoxidized and further study by Raman spectra and XRD reveals that the oxides wereα-Bi2O3and TeO2. Similarly, the peaks from the above oxides appeare in the Ramanspectra of Bi2Te3nanoplates and the degree of oxidation was related to the crystal quality of the Bi2Te3nanoplates. This is profitable to the research of oxidationmechanism and the surface states of Bi2Te3, Bi2Se3and so on.4. By Raman spectra, Raman mapping, SEM and in situ AFM, We haveinvestigated the evolution of the chemical property and morphology of Bi2Te3filmswhich were irradiated by the laser with different powers in air. Under the laserirradiation with a low power, a swelling in which Bi2Te3is oxidized is formed in thesurface of the film. Under the laser irradiation with a high power, the material in thecenter of the swelling is ejected in the form of liquid or gas, which leads to a deephole with the nail shape and the Bi2TeO5particles formed during the ejection weredispersed over the swelling and its nearby area. For thick Bi2Te3nanoplates, a similareffect of laser heating is observed. When the thickness of Bi2Te3nanoplates is thinnerthan tens of nanometers, however, a crater is formed without oxidation. Since thediameter of the crater is comparable with the size of the laser spot and the morphologyof the area around the crater is little affected by the laser irradiation, ultrathinnanostructures can be micro/nanofabricated by laser micro/nanoprocessing.