Characteristics of Sb6Te4/VO2 Multilayer Thin Films for Good Stability and Ultrafast Speed Applied in Phase Change Memory

The Sb6 Te4/VO2 multilayer thin films are prepared by magnetron sputtering and the potential application in phase change memory is investigated in detail. Compared with Sb6 Te4, Sb6 Te4/VO2 multilayer composite thin films have higher phase change temperature and crystallization resistance, indicating better thermal stability and less power consumption. Also, Sb6 Te4/VO2 has a broader energy band of 1.58 eV and better data retention （125℃ for 103/）. The crystallization is suppressed by the multilayer interfaces in Sbf Te4/VO2 thin film with a smaller rms surface roughness for Sbf Te4/VO2 than monolayer Sb4Te6. The picosecond laser technology is applied to study the phase change speed. A short crystallization time of 5.21 ns is realized for the Sb6Te4 （2nm）/VO2 （8nm） thin film. The Sb6 Te4/VO2 multilayer thin film is a potential and competitive phase change material for its good thermal stability and fast phase change speed.

Effects of Thickness and Temperature on Thermoelectric Properties of Bi2Te3-Based Thin Films

Bi_2Te_3 thin films and GeTe/B_2Te_3 superlattices of different thicknesses are prepared on the silicon dioxide substrates by magnetron sputtering technique and thermally annealed at 573 K for 30 min. Thermoelectric（TE）measurements indicate that optimal thickness and thickness ratio improve the TE performance of Bi_2Te_3 thin films and GeTe/B_2Te_3 superlattices, respectively. High TE performances with figure-of-merit（ZT） values as high as 1.32 and 1.56 are achieved at 443 K for 30 nm and 50 nm Bi_2Te_3 thin films, respectively. These ZT values are higher than those of p-type Bi_2Te_3 alloys as reported. Relatively high ZT of the GeTe/B_2Te_3 superlattices at 300-380 K were 0.62-0.76. The achieved high ZT value may be attributed to the unique nano-and microstructures of the films,which increase phonon scattering and reduce thermal conductivity. The results indicate that Bi_2Te_3-based thin films can serve as high-performance materials for applications in TE devices.

High-Quality Bi_2Te_3 Single Crystalline Films on Flexible Substrates and Bendable Photodetectors

Recently, great efforts have been made in the fabrication of arbitrary warped devices to satisfy the requirement of wearable and lightweight electronic products. Direct growth of high crystalline quality films on flexible substrates is the most desirable method to fabricate flexible devices owing to the advantage of simple and compatible preparation technology with current semiconductor devices, while it is a very challenging work, and usually amorphous, polycrystalline or discontinuous single crystalline films are achieved. Here we demonstrate the direct growth of high-quality Bi2 Te3 single crystalline films on flexible polyimide substrates by the modified hot wall epitaxy technique. Experimental results reveal that adjacent crystallites are coherently coalesced to form a continuous film, although amounts of disoriented crystallites are generated due to fast growth rate. By inserting a quartz filter into the growth tube, the number density of disoriented crystallites is effectively reduced owing to the improved spiral interaction. Furthermore, flexible Bi2 Te3 photoconductors are fabricated and exhibit strong near-infrared photoconductive response under different degrees of bending, which also confirms the obtained fexible films suitable for electronic applications.

Thermoelectric Transport by Surface States in Bi2Se3-Based Topological Insulator Thin Films

We develop a tractable theoretical model to investigate the thermoelectric （TE） transport properties of surface states in topological insulator thin films （TITFs） of Bi2Sea at room temperature. The hybridization between top and bottom surface states in the TITF plays a significant role. With the increasing hybridization-induced surface gap, the electrical conductivity and electron thermal conductivity decrease while the Seebeck coefficient increases. This is due to the metal-semiconductor transition induced by the surface-state hybridization. Based on these TE transport coefficients, the TE figure-of-merit ZT is evaluated. It is shown that ZT can be greatly improved by the surface-state hybridization. Our theoretical results are pertinent to the exploration of the TE transport properties of surface states in TITFs and to the potential application of Bi2Sea-based TITFs as high-performance TE materials and devices.

Analysis of Resonance Absorption in Multilayered Thin-Film Bi-Grating

The resonance absorption of a multilayered bi-grating which consists of thin-film corrugated periodically in two directions is investigated. The absorption in a multilayered thin-film bi-grating has been of considerable interest since we can expect more complex behaviors in the absorption phenomen by virtue of the presence of double periodicity and multilayer structure. In solving the problem, we employed a computational technique based on modal expansion. Taking a sandwiched structure /Ag/SiO2/Ag/ for an example, we observed: 1) excitation of a single-interface surface plasmon mode at the lit surface of the 1st Ag layer with strong field enhancement for thick enough Ag layer case;2) excitation of coupled short-range or long-range surface plasmon modes at each surface between vacuum and Ag layers with strong field enhancements for thin enough Ag layer cases no matter with the thickness of SiO2 layers;3) enhancements of field at surfaces between Ag and SiO2 layers in some cases related with the thickness of SiO2 layers. The coupled plasmon modes were resulted by the resonance waves on four surfaces in these cases.

Electrically Tunable Wafer-Sized Three-Dimensional Topological Insulator Thin Films Grown by Magnetron Sputtering

Three-dimensional(3 D)topological insulators(TIs)are candidate materials for various electronic and spintronic devices due to their strong spin-orbit coupling and unique surface electronic structure.Rapid,low-cost preparation of large-area TI thin films compatible with conventional semiconductor technology is the key to the practical applications of TIs.Here we show that wafer-sized Bi2Te3 family TI and magnetic TI films with decent quality and well-controlled composition and properties can be prepared on amorphous SiO2/Si substrates by magnetron cosputtering.The SiO2/Si substrates enable us to electrically tune(Bi1-xSbx)2Te3 and Cr-doped(Bi1-xSbx)2 Te3 TI films between p-type and n-type behavior and thus study the phenomena associated with topological surface states,such as the quantum anomalous Hall effect(QAHE).This work significantly facilitates the fabrication of TI-based devices for electronic and spintronic applications.

基于RTP热处理的Bi/Te多层膜的热电性能

为改善薄膜热电材料的热电性能,通过磁控溅射制备多层Bi/Te薄膜材料,并用常压高温环境对样品进行快速热退火(RTP),以期在较短时间内获得更高的热电参数.结果表明:Seebeck系数与功率因数最大值与退火温度呈正相关,在400℃退火2~11min,Seebeck系数极大值为-190.41μV/K,功率因数最大值为8.28μW/(K^2·m);随着退火温度的升高,Seebeck系数、载流子浓度、载流子迁移率、电导率和功率因数的振荡幅度也随之加大,并且载流子浓度与Seebeck系数呈反比关系,与电导率呈正比关系,这说明通过改变材料结构和高温快速退火可以得到较高的热电参数,同时保证薄膜的完整性.

Bi-Te基薄膜热电材料的研究进展

热电材料是一种能够实现热能与电能直接转换的功能材料,由于无法有效降低块体热电材料的热导率,其性能研究进展缓慢。自上世纪90年代初Hicks等提出了低维化能够显著提高热电材料性能的理论后,薄膜热电材料开始受到广泛关注。低维化提高材料性能的原因主要是材料在低维化后能够产生量子限制效应,使得电子在被压缩维度的运动受到限制。首先,在费米能级附近,与Seebeck系数呈正相关的电子态密度会增大,导致低维热电材料的Seebeck系数相比块体材料显著增大。其次,与块体材料相比,薄膜材料存在更多能够散射声子的晶界,能有效降低晶格热导率。在这两种效应的共同作用下,材料的热电优值(ZT值)能够显著增大。低维热电材料的研究初期主要是通过数学模型和数值计算,从理论上证明量子效应会影响材料的Seebeck系数和电导率,且能实现二者的独立控制,从而提高材料的ZT值。后期的实验数据证明,通过合适的热处理工艺能够有效降低薄膜材料的缺陷,提高其综合性能。因此,热处理工艺的改进对性能的提升也非常重要。热电材料性能的提升离不开制备工艺的进步。为了获得低维化的热电材料,多种薄膜材料制备工艺被用于样品的制备,且不同的制备工艺各有优缺点。Bi-Te基合金不仅可用于低温发电还可用于低温制冷,是目前应用最广泛的低温热电材料,虽然其块体状态下的热电性能研究已趋于完善,但其薄膜状态下热电性能的理论研究还相差甚远,因此Bi-Te基低温薄膜热电材料成为研究热点。本文介绍了国内外采用不同制备工艺生长Bi-Te基热电薄膜材料的发展状况以及热电性能测试方法,提出了在目前发展薄膜热电材料时需要重点关注的方面,并对低维热电材料的发展方向进行了阐述。

Synthesis of Flower-Like Bi2Te3 Nano Structure and Studying Its Structural and Thermo-electric Properties

Flower-like Bi2Te3 nanostructures were successfully synthesised for the first time by a simple magnetron technique or D.C. sputtering method. The phase and morphology of the products were characterized by X-ray diffraction （XRD）, manning electron microscope （SEM） and atomic force microscope （AFM）. It was found that the as-deposited Bi2Te3 has a well re-crystallized Rhombohedral phase and consisted of a wealth of flower-like structure, also the thermo-electric properties of Bi2Te3 were examined and we find that the Seebeck coefficient is 136.6μ volt/K.

Broadband photovoltaic effect of n-type topological insulator Bi2Te3 films on p-type Si substrates

We report the photovoltaic effects of n-type topological insulator （TI） Bi2Te3 films grown on p-type Si substrates by chemical vapor deposition （CVD）. The films containing large nanoplates with a smooth surface formed on p-Si exhibit good p-n diode characteristics under dark and light illumination conditions and display a good photovoltaic effect under the broadband range from ultraviolet （UV） to near infrared （N1R） wavelengths. Under the light illumination with a wavelength of 1,000 nm, a short circuit current （Isc） of 19.2 μA and an open circuit voltage （Voc） of 235 mV are achieved. The maximum fill factor （FF） increases with a decrease in the wavelength or light density, achieving a value of 35.6% under 600 nm illumination. The photoresponse of the n-Bi2TeB/p-Si device can be effectively switched between the on and off modes in millisecond time scale. These findings are important for both the fundamental understanding and solar cell device avDlications of TI materials.

Principle of X-ray Ф-scan and Application ofHigh T_c Bi-epitaxial Junction

The X-ray Ф-scan technique was applied to determine the in-plane orientation relationship of the high T_csuperconductins bi-epitaxial junction. The result shows that the in-plane orientation relation of the films in thetwo sides of the grain boundary can be easily obtained with this method. By controlling the growth condition,a 45° YBCO grain boundary can be made on the SrTiO_3 substrate.

BOPP膜表面化学键组装制备多层膜

通过紫外光照射将半频哪醇自由基偶合到双向拉伸聚丙烯(BOPP)膜表面,利用该偶合基团上的羟基与对苯二甲酰氯(TPC)和双酚A(BPA)进行交替反复的逐步缩合反应,制备了以共价键结合的多层组装膜。采用紫外(UV)、红外光谱(ATR-FTIR)跟踪和证实了组装膜的形成,并且膜的组装比较规则。

快速退火对磁控溅射制备Bi/Te多层膜热电性能的影响

采用磁控溅射法在玻璃衬底上室温沉积Bi/Te多层膜,并在常压氮气保护下进行快速退火处理。研究退火温度和时间对薄膜的显微结构、物相组成和热电性能的影响。结果表明：随退火时间的延长样品中依次出现片状晶粒的成核、定向生长和横向堆叠等结构的演变特征,使晶粒沿横向逐渐增厚,且退火温度越高,结构演变速率越快。退火样品以Bi2Te3为主相,350400℃退火使薄膜表层生成少量Bi-Te氧化物。随退火时间的延长,与晶粒堆叠厚度相关的量子化效应显现,使薄膜的热电性能出现明显的振荡现象。在350℃退火1117 min的薄膜,可获得稳定的高功率因数,最大值为21.91μW/（K2·cm）。

Linear magnetoresistance versus weak antilocalization effects in Bi2Te3

In chalcogenide topological insulator materials, two types of magnetoresistance （MR） effects are widely discussed： a sharp MR dip around zero magnetic field, associated with the weak antilocalization （WAL） effect, and a linear MR （LMR） effect that generally persists to high fields and high temperatures. We have studied the MR of thin films of the topological insulator Bi2Te3 from the metallic to semiconducting transport regime. In the metallic samples, the WAL is difficult to identify owing to the low magnitude of the WAL compared to the samples＇ conductivity. Furthermore, the sharp WAL dip in the MR is dearly present in samples with a higher resistivity. To correctly account for the low-field MR with the quantitative theory of the WAL according to the Hikami-Larkin-Nagaoka （HLN） model, we find that the classical （linear） MR effect should be taken into account in combination with the WAL quantum correction. Otherwise, the WAL fitting alone yields an unrealistically large coefficient in the HLN analysis. This work clarifies the WAL and LMR as two distinct effects and offers an explanation for the overly large a in the WAL analysis of topological insulators in some studies.

Pedestrian Analysis of Harmonic Plane Wave Propagation in 1D-Periodic Media

The propagation of TE, TM harmonic plane waves impinging on a periodic multilayer film made of a stack of slabs with the same thickness but with alternate constant permittivity is analyzed. To tackle this problem, the same analysis is first performed on only one slab for harmonic plane waves, solutions of the wave equa- tion. The results obtained in this case are generalized to the stack, taking into account the boundary condi- tions generated at both ends of each slab by the jumps of permittivity. Differential electromagnetic forms are used to get the solutions of Maxwell’s equations.