Preparation Of Bismuth Telluride Nano-materials By MBE And The Thermo-electrical Measurement Of Thin Films

Thermoelectric material, a functional material converting thermal energy toelectric energy directly, is very promising in thermoelectric power generation andthermoelectric refrigeration. They have a good application prospect in thermoelectricpower generation and thermoelectric refrigeration. The efficiency is considerablyincreased in a sense that waste heat is transformed to electric power by solid statetransformers made of thermoelectric materials. Researches in thermoelectric areaincludes preparation of materials and measurement of thermoelectric properties. Inthe aspect of materials preparation, bismuth telluride semiconductors exhibitexcellent thermoelectric properties near room temperature, with its ZT value close to1. Low dimensional thermoelectric materials have improved sufficiently in ZT value,putting them in the spot light of recent research. In the field of thermoelectricmeasurement, it’s very difficult to measure properties of thermoelectric material dueto thermal radiation, which results in thermal loss of the test samples andtest composition. Moreover, because of the low dimension of samples, contact of testcomposition and sample will disturb the temperature condition of the sample,especially in the micro-system and lead to the test error.In this work, the deposition parameters of Bismuth, Telluride and BismuthTelluride by Molecular Beam Epitaxy system were investigated. The effect of celltemperature and substrate temperature on the film’s surface and thermoelectricperformances have been studied. A new test method was established to measure thethermal diffusivity and conductivity of micrometer thick thin-films more accurately.The main results are summarized as follows:1. By combining the apparatus of Steady-state Comparative-longitudinal HeatFlow (SCHF) method and the one-dimensional thermal wave in Angstr mmethod, the phase of thermal wave was analyzed and the influence of interfacebetween test probe and sample was eliminated. As a result, cross-plane thermal diffusivity of thin films were deduced.2. The chemical cleaning played an important role in pretreatment process, whichcan remove most of the impurity on the wafer. The RHEED patterns showedthat the chemical cleaning and annealing in vacuum was also an inevitable stepin the pretreatment of substrate. They all made wafers more smooth and clean.3. In the experiment of Bismuth deposition, with the increasing of substratetemperature, the sizes of Bi grains increased to about1micrometer. When thesubstrate temperature reached300℃, the film was discrete, while indicatingthe best crystallinity and sticking to (00l) as the preferred orientation.4. The substrate temperature and Telluride cell temperature were set as to280℃and270℃, while the Bismuth cell temperature was set from480℃to500℃. The result showed that with the increasing of Bismuth cell temperature,the epitaxy rate increased. The ratio of Telluride to Bismuth decreased from1.5to0.8, with decreased the power factor from13.5μW/cmK~2to7μW/cmK~2.5. Substrate temperature was set from270℃to500℃, while the Bismuth celltemperature and Telluride cell temperature were set as to490℃and270℃.The result showed that with the increasing of substrate temperature, the ratio ofTelluride to Bismuth first increased and then decreased with the conductivityfirst increasing and then decreasing. The power factor reached17μW/cmK~2when the substrate temperature was290℃.