Elementary Excitations at Semiconductor Surfaces with Controlled Adsorption

随着吸附覆盖率的增加，掺杂半导体表面通常会形成载流子耗尽层或载流子累积层。在本工作的第一阶段，我们使用局域密度近似（LDA），分析了在耗尽层（D-L）形成过程中的基态和热平衡态的变化。我们发现了载流子密度分布的变化和温度效应。其次，在热平衡态计算结果的基础上，利用含时LDA方法，对D-L形成过程中表面元激发的演化进行了系统的分析。这里研究的基元激发是两个耦合的载流子等离子体激元（PL）-极性声子（PH）模式A和B和表面极性PH模式C涉及屏蔽电荷由于载流子的存在。主要研究结果如下：（1）研究了各模式的能量色散和能量损失强度的演化 关于我们 在D-L形成过程中。在模式A或B中，初始向上色散分支向下移动，并演化为弯曲色散。(2)通过研究诱导电荷密度分布中载流子和PH分量的相位关系和振幅比，阐明了D-L形成过程中PL和PH耦合特性的变化。(3)感应电荷密度分布的等值线图分析表明，在D-L形成过程中，模式A或B的空间结构发生了显著变化。在模式C下，研究了表面极性声子如何沿着表面拖曳屏蔽电荷传播，以及屏蔽电荷分布在此过程中的变化。(4)对高分辨电子能量损失谱（HREELS）的实验结果进行了解释，并对镜面反射结构下的HREELS进行了运动学分析，结果表明，在掠入射角固定、入射能量变化范围较大的情况下，我们可以扫描一个狭窄的探测色散区，以便仔细观察这种表面的色散关系，分析堆积层形成过程中表面元激发的演化已成为未来的研究课题。少

With increase in adsorption coverage, a carrier-depletion or carrier-accmulation layer is often formed at a doped semiconductor surface. At the first stage of the present work, using the local-density approximation (LDA), we have analyzed the variation in ground and thermal-equilibrium states in the depletion-layer (D-L) formation process. We have found the variation and the temperature effect in the carrier-density profile. Next, on the basis of the calculated result of thermal-equilibrium states and by means of the time-dependent LDA, we have made a systematic analysis of evolution of surface elementary excitations in the D-L formation process. The elementary excitations studied here are two coupled carrier plasmon (PL)-polar phonon (PH) modes A and B and the surface polar-PH mode C involving screening charges due to presence of carriers. The major results are summarized as follows :(1) We have examined the evolution of the energy dispersion and the energy-loss intensity of each mode … More in the D-L formation process. In the mode A or B, an initial upward dispersion branch shifts downward, and evolves into a warped one.(2) The variation in coupling character of PL and PH in the D-L formation process has been clarified by investigating the phase relation and the amplitude ratio of the carrier and the PH components in the induced charge-density distribution.(3) The contour-map analysis of the induced charge-density distribution has exhibited a significant variation in spatial structure of the mode A or B in the D-L formation process. In the mode C, it has shown how the surface polar phonon propagates along the surface dragging screening charges and how the screening-charge distribution varies in the process.(4) Our analysis has given a clear explanation of the experimental result of high-resolution electron energy-loss spectroscopy (HREELS).In addition, our kinematic analysis of HREELS with the specular-reflection geometry has indicated that, with a grazing incident angle fixed and with the incident energy varied in a broad range, we can scan a narrowed probing dispersion region to make a close observation of the dispersion relation of such surface-excitation modes as studied here.It has become a future problem to analyze the evolution of surface elementary excitations in an accumulation-layer formation process. Less