Today¡¦s electronic components draw high levels of power and run at high temperature, which can present overheating problems for engineers and designers. They must find ways to keep the equipment cool or watch them fail prematurely. The conventional thermoelectric devices are high power consµming and slow response. We need more integrated and high performance thermoelectric device. Because of the limit of material characteristics, the figure of art rising with the quality of epitaxial layer. We gave a cheaper and easier fabrication to realized this demand. We present a micro thermoelectric device fabricated by Bi2Te3 electrochemical process. By using rotary cathode electrode, the current density can be well-proportioned. The thermal conduction and resistivity can be optimizing by this design. Also using the MEMS technology with repeated exposure and development of multiple photoresist layers, several different metals (Au, Cr) and thermoelectric materials (Bi2Te3, Sb2Te3) are fabricated. The SiO2 of 0.5µm was grown. Then the 0.3£gm-thick Au on oxidized Si sputtered with a 1£gm thick layer of Cr. And the bottom electrode was patterned by lift-off. Thick positive photoresist with one set of holes developed. Bi2Te3 was deposited by electrochemical deposition. And the Sb2Te3 is growing with the same method. The upper electrode sputtered with thin Au film and pattern by lift-off. Finally, Cr was etched to electrically isolate the bottom interconnects. The area of Bi2Te3 is about 50x50£gm2. And it¡¦s high about 5£gm. The ZT value of Bi2Te3, which is measured and verified to be around 0.0088.

The work presented in this thesis investigates the behaviour of the Rashba semi-conductor BiTeI and of the topological insulator $\text{Bi}_2\text{Te}_3\,$ under pressure. Using Shubnikov-de Haas quantum oscillation measurements, the evolution of the Fermi surface of both materials was tracked as a function of pressure. At ambient pressure, two distinct quantum oscillation frequencies in BiTeI, corresponding to inner and outer Fermi surface orbits as a result of spin-splitting caused by the Rashba effect, were observed. Using a model Hamiltonian with a Rashba interaction term to model this system, experimental results were fitted to determine model parameters. Based on this model, carrier densities for the samples were calculated and there was good agreement with Hall effect measurements. The phase of the oscillations showed that both Fermi surfaces have a Berry phase of $\pi$ associated with them, consistent with theoretical predictions for a Rashba system. As pressure is applied, it was observed that the inner Fermi surface expands while the outer Fermi surface shrinks. Phase analysis of the oscillations showed deviations from the ambient pressure value, hinting at a topological transition. For $\text{Bi}_2\text{Te}_3\,$, we report the observation of two oscillation frequencies ($\sim 40$ T and $\sim 340$ T) at ambient pressures. Based on the angular dependence of the oscillation frequencies, phase analysis, and comparison against band structure from published ARPES results, it is deduced that the higher frequency oscillation corresponds to the surface state of $\text{Bi}_2\text{Te}_3$. Non-linear behaviour in the Hall measurement also suggests the presence of multiple bands, and a two-band model with parameters derived from quantum oscillation measurements is used to fit the experimental data. Under pressure, a slight decrease in the low field Hall coefficient and a new frequency appearing at $\sim 20$ kbar was observed. These may be signatures of a change in the Fermi surface of $\text{Bi}_2\text{Te}_3\,$ caused by an electronic topological transition.

碲化鉍((Bi2Te3)是熱電材料轉換效率較高的元件，其優質係數ZT值約為1。希望藉由奈米的量子效應提升它的熱電性質，我們製作一系列低維度的奈米線和薄膜來進行研究。本實驗使用的碲化鉍奈米線乃利用薄膜樣品與基板的熱膨脹係數不同，經由熱處理在碲化鉍的薄膜上長出奈米線。由掃描式電子式顯微鏡和穿隧式電子顯微鏡可以觀察到菱形晶胞(Rhombohedral unit cell)結構的碲化鉍奈米線沿著(110)方向生長，直徑約150-330 nm長度約20-30 μm。將碲化鉍奈米線轉移到矽晶片上，運用半導體製程中的熱蒸鍍(Evaporator)以及電子束曝光系統(E-Beam writer)製作電極、熱電偶和加熱器來量測席貝克(Seebeck) 係數、電傳導率和熱傳導率。最後成功的製作與量測出p型(107 μV/k) 和n型(-52.8 μV/k) 的奈米線，雖然其席貝克係數小於塊材，但奈米線的熱傳導率低於塊材兩倍以上，研究發現最好的碲化鉍奈米線的熱電優值(ZT value) 可達1.18略大於塊材。 碲化鉍薄膜是以分子束磊晶 (Molecular Beam epitaxy)成長，分子束磊晶是在高真空下以物理的方式將高純度的材料4N (99.99%)將原子傳遞至基板上進行沉積反應形成，鍍率可低於0.1 nm/秒以下，因此可以製備出高品質的薄膜樣品，製造出各種不同比例的Bi-Te的薄膜。藉由X光繞射儀可以得知薄膜是菱形晶胞結構並且延著(0,0,l)的平面所成長。並用熱電偶成功的量測出薄膜的席貝克係數在室溫下座落於80-80 μV/k，電阻率5-30 μΩ-m，計算出功率因子(power factor)最高可達2000 μW/mK^2，與塊材相比低於一半，但是薄膜的熱傳導率同樣也低於塊材兩倍以上。最後得到最佳的碲化鉍薄膜的熱電優值(ZT value) 可達到1.01等同於塊材。 / Bismuth telluride (Bi2Te3) is the thermoelectric material used for high-efficiency energy conversion. The figure of merit ZT of bulk is around 1. To study the promising positive effects on the thermoelectric properties, low dimensional nanowires and thin films of Bi2Te3 were prepared and measurements were performed. Here the method applied to nanowires growth on Bi2Te3 thin films is the mismatch of thermal expansion between substrate and thin films. By annealing at 300-350℃ for a week, the nanowires were grown on the thin films. Rhombohedral structure of Bi2Te3 nanowires with diameter ~150-330 nm and length ~20-30 μm grew along (110) direction was confirmed by Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction Pattern (SAED). To measure the Seebeck coefficient, electrical conductivity and thermal conductivity, Bi2Te3 nanowires were moved to silicon chips. Electrodes, thermometers and heaters were fabricated through thermal evaporation and E-Beam lithography processes. We successfully grew p-type(107 μV/k) and n-type(-52.8 μV/k) nanowires. Although Seebeck coefficient of nanowires is smaller than that of bulks, its thermal conductivity is less than half of that of bulks. The best ZT value of nanowires we obtained was 1.18, which was slightly larger than that of the bulks. Molecular beam epitaxy (MBE) is a technique to grow Bi2Te3 thin films under extremely high vacuum, which is undergoing a physical vapor deposition to atomically grow thin films layer by layer. Due to the deposition rate is lower than 0.1 nm/s, we can deposit the high-quality thin films and adjust the ratio between bismuth and telluride. Rhombohedral structure of thin films grew along (110) plane was confirmed by X-Ray Diffraction (XRD). The Seebeck coefficient (80-80 μV/k) and electrical resistivity (5-30 μΩ-m) in room temperature are obtained by the thermocouples. The highest power factor can reach to 2000 μW/mK^2. While the power factor of thin films is about half of bulk ‘s value, the thermal conductivity of thin films is also half of that of bulks. The best ZT value of thin films obtained was nearly as same as that of bulks, 1.01.

In this thesis, we investigate the quantum transport properties of disordered three dimensional topological insulator (3DTI) nanostructures of BiSe and BiTe in detail. Despite their intrinsic bulk conductivity, we show the possibility to study the specific transport properties of the topological surface states (TSS), either with or without quantum confinement. Importantly, we demonstrate that unusual transport properties not only come from the Dirac nature of the quasi-particles, but also from their spin texture. Without quantum confinement (wide ribbons), the transport properties of diffusive 2D spin-helical Dirac fermions are investigated. Using high magnetic fields allows us to measure and separate all contributions to charge transport. Band bending is investigated in BiSe nanostructures, revealing an inversion from upward to downward bending when decreasing the bulk doping. This result points out the need to control simultaneously both the bulk and surface residual doping in order to produce bulk-depleted nanostructures and to study TSS only. Moreover, Shubnikov-de-Haas oscillations and transconductance measurements are used to measure the ratio of the transport length to the electronic mean free path ltr/le. This ratio is measured to be close to one for bulk states, whereas it is close to 8 for TSS, which is a hallmark of the anisotropic scattering of spin-helical Dirac fermions. With transverse quantum confinement (narrow wires or ribbons), the ballistic transport of quasi-1D surface modes is evidenced by mesoscopic transport measurements, and specific properties due to their topological nature are revealed at very low temperatures. The metallic surface states are directly evidenced by the measure of periodic Aharonov-Bohm oscillations (ABO) in 3DTI nanowires. Their exponential temperature dependence gives an unusual power-law temperature dependence of the phase coherence length, which is interpreted in terms of quasi-ballistic transport and decoherence in the weak-coupling regime. This remarkable finding is a consequence of the enhanced transport length, which is comparable to the perimeter. Besides, the ballistic transport of quasi-1D surface modes is further evidenced by the observation of non-universal conductance fluctuations in a BiSe nanowire, despite the long-length limit (L > ltr) and a high metallicity (many modes). We show that such an unusual property for a mesoscopic conductor is related to the limited mixing of the transverse modes by disorder, as confirmed by numerical calculations. Importantly, a model based on the modes' transmissions allows us to describe our experimental results, including the full temperature dependence of the ABO amplitude.

In this thesis, we investigate the quantum transport properties of disordered three dimensional topological insulator (3DTI) nanostructures of BiSe and BiTe in detail. Despite their intrinsic bulk conductivity, we show the possibility to study the specific transport properties of the topological surface states (TSS), either with or without quantum confinement. Importantly, we demonstrate that unusual transport properties not only come from the Dirac nature of the quasi-particles, but also from their spin texture. Without quantum confinement (wide ribbons), the transport properties of diffusive 2D spin-helical Dirac fermions are investigated. Using high magnetic fields allows us to measure and separate all contributions to charge transport. Band bending is investigated in BiSe nanostructures, revealing an inversion from upward to downward bending when decreasing the bulk doping. This result points out the need to control simultaneously both the bulk and surface residual doping in order to produce bulk-depleted nanostructures and to study TSS only. Moreover, Shubnikov-de-Haas oscillations and transconductance measurements are used to measure the ratio of the transport length to the electronic mean free path ltr/le. This ratio is measured to be close to one for bulk states, whereas it is close to 8 for TSS, which is a hallmark of the anisotropic scattering of spin-helical Dirac fermions. With transverse quantum confinement (narrow wires or ribbons), the ballistic transport of quasi-1D surface modes is evidenced by mesoscopic transport measurements, and specific properties due to their topological nature are revealed at very low temperatures. The metallic surface states are directly evidenced by the measure of periodic Aharonov-Bohm oscillations (ABO) in 3DTI nanowires. Their exponential temperature dependence gives an unusual power-law temperature dependence of the phase coherence length, which is interpreted in terms of quasi-ballistic transport and decoherence in the weak-coupling regime. This remarkable finding is a consequence of the enhanced transport length, which is comparable to the perimeter. Besides, the ballistic transport of quasi-1D surface modes is further evidenced by the observation of non-universal conductance fluctuations in a BiSe nanowire, despite the long-length limit (L > ltr) and a high metallicity (many modes). We show that such an unusual property for a mesoscopic conductor is related to the limited mixing of the transverse modes by disorder, as confirmed by numerical calculations. Importantly, a model based on the modes' transmissions allows us to describe our experimental results, including the full temperature dependence of the ABO amplitude.

No description available.

我們主要研究拓樸絕緣體與鐵磁物質之間的自旋幫浦效應（spin pumping effect），我們選用的鐵磁材料是具有鐵磁性的鎳鐵合金（Py），厚度固定為40nm，而拓樸絕緣體則是選用碲化鉍（Bi2Te3），厚度範圍是0～100nm，碲化鉍已被確定為一個三維拓撲絕緣體，拓撲絕緣體其表面電子態呈線性色散關係，本身中心是絕緣體，但其表面容許有導電態。此導電態一個最有用的特性是其電子的動量與自旋維持一定方向關係（spin-momentum locking），這使得以自旋來傳遞訊息成為可能。但是實驗上要達到中心是絕緣體相當困難。 過去的實驗已驗證鐵磁共振（Ferromagnetic resonance，FMR）現象在鐵磁/一般金屬雙層膜以及鐵磁/半導體雙層膜，可以使其鐵磁層產生一純自旋流流向非磁性層，這被稱為自旋幫浦效應（spin pumping effect）。當此自旋流跨越膜面介面時，不同自旋的電子由於自旋軌道耦合作用（Spin–orbit interaction），將發生逆自旋霍爾效應（ISHE）並產生一橫向電荷流。在我們的研究中，鐵磁共振（FMR）現象透過網路分析儀在設定的外加磁場下掃描頻率。測得的共振頻率與磁場作圖並以Kittel equation擬合（fitting）出有效場（effective field）。我們發現於絕對溫度5K，隨著碲化鉍（Bi2Te3）膜厚從0nm到15nm增加時，其有效場也增加，但當薄膜厚度大於15nm時，有效磁場將下降。我們分析碲化鉍（Bi2Te3）的表面態（surface state）與塊材（bulk）對有效場變化之貢獻。