【作者】 马红；

【导师】 马国宏；

【作者基本信息】 上海大学 ， 无线电物理， 2011， 博士

【摘要】 目前,以荷电性为基础的传统半导体电子器件的运行速度和集成度仍然以Moore律发展。但是,随着器件尺寸的缩小,量子效应越来越明显,并严重地制约着传统电子学的发展,因此必须寻找传统电子学器件的替代品,自旋电子学便应运而生。半导体自旋电子学(spintronics=spin+electronics)是一个由多学科交叉形成的新兴领域,主要研究如何有效地操作/控制电子自旋自由度,以期在器件设计中用自旋自由度结合或代替电荷自由度。与传统的半导体器件相比,自旋电子器件具有稳定性更好、数据处理速度更快、功率损耗更低以及集成密度更高等优点。自旋弛豫时间常数是研制自旋电子器件必须考虑的一个参数。利用自旋弛豫过程的超快特性,可以实现超快的自旋开关;另一方面,在量子信息存储以及自旋输运中要实现较长的输运距离则又需要保持较长的自旋弛豫时间。影响自旋弛豫时间的因素很多,例如,温度,载流子浓度,半导体带结构和外场等。因此,自旋弛豫动力学的实验现象相当丰富,很多实验结果不能被合理解释,理论工作也需要进一步的深入探究。本文集中研究了半导体及纳米结构中的自旋弛豫时间与激发光子能量、载流子浓度之间的关系,取得了一些有意义的结果。主要的创新点如下:第一、利用抽运探测技术研究了常温下CdTe单晶的载流子超快动力学。载流子的弛豫过程存在2个成分,随着激发强度的不断增加,与载流子冷却有关的快过程逐渐变慢,而慢的复合时间基本上与激发光强度无关。第二、系统研究了CdTe单晶和InP单晶的电子自旋弛豫时间τs和激发光子能量E的依赖关系。结果显示随着抽运光光子能量的增加,自旋弛豫时间单调减小,这和传统的D’yakonov- Perel’(DP)机制是一致的。第三、CdTe单晶和InP单晶的电子自旋弛豫时间τs随着载流子浓度n的增加并非单调变化,而是出现一个峰值τsmax,而且常温的τsmax对应的电子浓度要明显高于低温(70K)时的电子浓度,这与基于全微观动力学自旋Bloch方程方法预言结果相同。第四、InP单晶的电子自旋弛豫过程表现出了明显的激发光子能量依赖关系。从近带隙开始增加光子能量,反射率的变化率ΔR /R出现了变号行为,由漂白变为吸收。考虑带填充效应和带隙重效应计算了ΔR /R随着激发光子能量的变化曲线,与实验结果基本吻合。第五、利用抽运探测技术研究了CdSe核结构量子点的激子自旋弛豫动力学,自旋弛豫只有一个几皮秒的快过程。量子点的尺寸变小,比表面积增加,表面缺陷迅速捕获电子和空穴,导致自旋极化消失。而对于表面修饰了ZnS之后的核/壳结构量子点中缺陷态的捕获被减弱,自旋弛豫过程与核结构明显不同,除了快过程之外,还有一个慢过程,常温下共振激发的自旋弛豫时间达到纳秒量级,比体材料的自旋弛豫时间长1-2个量级,这是因为量子点中与自旋-轨道相互作用有关的自旋弛豫机制被抑制。第六、利用法拉第旋光技术研究了CdTe量子点的磁光效应。结果发现,随着抽运光激发功率的增加,法拉第旋转角和椭圆率线性增加;随着抽运光波长的变化,在最低激子吸收峰附近法拉第旋转角的符号发生改变,而椭圆率达到极值,二者满足K-K关系,这是由于量子限制效应引起的能级分裂所致。更多还原

【Abstract】 Processing speed and integration of traditional electronic devices based on the degree of freedom of a charge still follow the prediction of Moore’s law. However, quantum effect of the device becomes more and more predominant with the decrease of the size, which restricts the development of the traditional electronic devices. Semiconductor spintronics aims at utilizing or incorporating the spin degree of freedom in electronics for a new generation of spintronic devices. Spintronic devices are more stable, smaller size, faster speed, and lower power consumption than traditional electronic devices.Spin relaxation/dephasing time is an important parameter in the fabrication of spintronic devices. Experimental results about the spin relaxation/dephasing time are extreme complex because it is affected by many factors, such as temperature, density, bandgap structure, and external field etc. In this dissertation, carrier density and photon energy dependence of spin dynamics in bulk semiconductor and semiconductor nanostructrues are systematically investigated by time resolved pump-probe technique. The main results in this dissertation are concluded as follows:1. The transient carrier dynamics of intrinsic cadmium telluride (CdTe) was investigated by femtosecond time-resolved pump-probe reflectivity (fs-TRPPR) method at different photon energy and carrier density. Two relaxation processes are observed. The fast one related to carrier cooling process with typical decay time of several picoseconds (ps) increases with the carrier density, while the slow one owing to the electron hole recombination almost keeps a constant.2. Electron spin dynamics in intrinsic bulk CdTe and Indium Phosphide (InP) semiconductor crystal was studied by fs-TRPPR technique using the co-circularly and counter-circularly polarized femtosecond pulses at room temperature and 70 K. The results show that spin relaxation time decreases monotonously with increasing photon energy.3. With increasing carrier density, the electron spin relaxation time in bulk semiconductor increases initially and then decreases after reaching a maximum value. Our experimental results agree well with the recent theoretical prediction based on a fully microscopic kinetic spin Bloch equation (KSBE) approach and D’yakonov-Perel’mechanism is considered as a dominate contribution to the electron spin relaxation in intrinsic bulk semiconductor.4. The reflectivity change from bleaching to absorption in InP crystal is observed with increasing pump photon energy, which has been explained successfully in terms of the spin sensitive band filling and band gap renormalization effects.5. Exciton spin dynamics of CdSe as well as CdSe/ZnS quantum dots (QDs) was investigated with circularly polarized pump-probe transmission spectroscopy at room temperature. The excitation photon energy is tuned to be on-resonant with the 1S(h)-1S(e) exicton of the CdSe QDs. The spin dynamics of core CdSe QD shows single exponential decay with typical time constant of about several ps, while the spin dynamics in the CdSe/ZnS core/shell structure shows biexponential relaxation: a several-ps-fast-component and a slow-component with time constant of hundreds of ps. The time constant of slow process decreases with increasing the excited power. The fast spin relaxation component in both CdSe and CdSe/ZnS arises from the surface-state-trapping effect. ZnS-capped-CdSe QD can greatly reduce the surface states at the CdSe surface, and the slow spin decay comes from the long exicton lifetime in core/shell structure. Spin relaxation mechanisms based on the spin-orbit interaction are strongly inhibited in spatial confined systems, and the absence of translational motion in QDs prolongs the carriers spin lifetimes as compared to the bulk counterpart.6. Magneto-optical dynamics of water dispersed CdTe quantum dots is investigated by time-resolved Faraday rotation technique at room temperature. Wavelength dependence of Faraday rotation and ellipticity are studied across the lowest exciton absorption in CdTe QDs. It is found that a sign reversal of Faraday rotation occurs around the exciton abosprtion peak, where the Faraday ellipticity reaches the maximum. This phenomenon is caused by the level splitting owing to the quantum confinement effect.更多还原