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Carrier-driven disordering in semiconductors: Time-resolved x-ray diffraction and density functional perturbation theory investigations.

机译：半导体中的载流子驱动无序：时间分辨X射线衍射和密度泛函微扰理论研究。

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Time-resolved x-ray science has opened the door to a previously inaccessible experimental world. Now the possibility of imaging ultrafast events with atomic spatial resolution is a reality. This dissertation highlights these new experimental techniques and uses them to study the effects of carrier photo-excitation in semiconductors using both time-resolved x-ray diffraction and time-resolved x-ray absorption spectroscopy.;I have probed the ultrafast atomic disordering in InSb after intense photoexcitation with ultrafast x-ray diffraction measurements at the Sub-Picosecond Pulse Source (SPPS), The results indicate that three disordering regimes exist, depending on the photoinduced carrier density. At lower carrier densities, disordering occurs via a thermal mechanism, occurring on a picosecond time scale with the dominant relaxation mechanism being the transfer of energy from hot carriers to the lattice. At intermediate carrier density values, the potential energy surface flattens, allowing the atoms to move with the inertial room temperature velocities for approximately ~500 fs at which point other processes take over including thermal energy transfer, atomic collision, and diffusion. At higher carrier densities, it is observed that accelerated atomic disordering occurs, indicating the formation of a repulsive potential energy surface.;These experimental observations are in contrast with previous theoretical work and therefore, I have performed calculations using Density Functional Perturbation Theory (DFPT) to more clearly outline the role of excited carriers in lattice destabilization. The calculations show that with increasing carrier density the transverse acoustic modes soften and the lattice destabilizes first in the (100) direction (X point) with 3.7% of the valence band electrons excited into the conduction band. Increasing the carrier density leads to the entire transverse acoustic mode becoming unstable, indicating a repulsive interatomic potential. A model has been developed for converting the lattice dynamics calculations into predicted diffraction intensities, leading to theoretical verification of the three disordering regimes that were observed experimentally.;I have also studied carrier excitation in Cu2O using time-resolved x-ray absorption spectroscopy (XAS) at the Cu L3 edge and the O K edge using ~70 ps x-ray pulses at the Advanced Light Source. After photoexcitation of carriers above the band gap, changes in the XAS spectrum are monitored at 70 ps time delay. By probing the two different atomic sites, changes in the XAS spectrum can be extracted that are both atomic and angular momentum specific. It is observed that the energetic shift in the XAS spectra after photoexcitation of Cu2O is much less than the band gap. The relative change in absorption is observed to be 50% larger at the Cu site than at the O site and there is no integral change in absorption in the transient spectra.;The desire to understand the mechanism that underlies chemical, physical and biological transformations motivates the majority of time-resolved studies. While ultrafast laser spectroscopy has greatly enhanced our understanding of dynamical phenomena, many important and interesting processes remain unexplained. Ultrashort pulses of x-rays provide the ability to access atomic and electronic structure with a detail and clarity absent from most optical spectroscopy measurements. It was time-resolved x-ray diffraction using the high per pulse intensity of SPPS that allowed the disordering mechanism in highly excited InSb to be clearly defined in contrast with previous work using optical probes and laser plasma-generated x-ray probes. Furthermore, time-resolved x-ray absorption spectroscopy provided a new experimental tool with which the excited state properties of Cu2O were able to be further elucidated. This dissertation directly shows the significant influence that the introduction of new experimental tools can have on scientific advancement.
机译：时间分辨的X射线科学为以前无法​​进入的实验世界打开了一扇门。现在，以原子空间分辨率成像超快事件的可能性已成为现实。本文重点介绍了这些新的实验技术，并利用时间分辨X射线衍射和时间分辨X射线吸收光谱技术研究了载流子在半导体中的光激发效应。在亚皮秒脉冲源（SPPS）上用超快速x射线衍射测量进行强烈的光激发后，结果表明存在三种无序状态，具体取决于光诱导的载流子密度。在较低的载流子密度下，无序现象是通过热机制发生的，发生在皮秒级的时间尺度上，主要的弛豫机制是能量从热载流子传递到晶格。在中间载流子密度值处，势能表面变平，允许原子以惯性室温速度移动约500 fs，在这一点上其他过程将接管其他过程，包括热能转移，原子碰撞和扩散。在较高的载流子密度下，观察到发生了加速原子无序现象，表明形成了排斥势能表面。这些实验观察与以前的理论工作相反，因此，我使用密度泛函扰动理论（DFPT）进行了计算更清楚地概述了激发的载流子在晶格失稳中的作用。计算结果表明，随着载流子密度的增加，横向声模变软，晶格首先在（100）方向（X点）失稳，其中有3.7％的价带电子被激发进入导带。载流子密度的增加导致整个横向声模变得不稳定，表明排斥的原子间电势。已经开发了一个模型，用于将晶格动力学计算转换为预测的衍射强度，从而对实验观察到的三种无序态进行了理论验证。;我还使用时间分辨X射线吸收光谱法（XAS）研究了Cu2O中的载流子激发）在Cu L3边缘和OK边缘使用高级光源的〜70 ps X射线脉冲。在带隙以上的载流子进行光激发之后，以7​​0 ps的时间延迟监测XAS频谱的变化。通过探测两个不同的原子位点，可以提取XAS光谱中既有原子动量又有角动量的变化。可以观察到，Cu2O的光激发后XAS光谱中的能量位移远小于带隙。观察到Cu位置的吸收相对变化比O位置大50％，并且在瞬态光谱中吸收没有整体变化。;了解化学，物理和生物转化基础的机制的动机激发了大多数时间分辨的研究。尽管超快激光光谱极大地增强了我们对动力学现象的理解，但许多重要而有趣的过程仍然无法解释。 X射线的超短脉冲提供了访问原子和电子结构的能力，而大多数光学光谱学测量却没有这种细节和清晰度。使用SPPS的高每脉冲强度进行时间分辨X射线衍射，与以前使用光学探针和激光等离子体生成X射线探针的工作相比，可以清楚地定义高激发InSb中的无序机制。此外，时间分辨X射线吸收光谱学提供了一种新的实验工具，借助该工具，可以进一步阐明Cu2O的激发态性质。本文直接表明了引入新的实验工具对科学发展的重大影响。

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