Study of the electron dynamics on semiconductor surfaces by the use of the time-resolved two-photoelectron spectroscopy with the femto-second pulse-laser.

在本研究中，我构建了由fsec脉冲激光时间分辨双光子光电子能谱仪（TR2PPE）、扫描隧道显微镜（STM）和样品处理系统组成的实验系统。利用该系统对Si（001）表面的电子性质和动力学进行了研究，得到了以下结果：1)研究了在光激发作用下，电子暂时占据未占据的表面悬空键态（D_<down>）和体导带的时间演化。干净Si（001）表面光激发后，π^* （D_< - >处的电子居群）和C^*（表面附近的体导带处的电子居群）的sub-ns衰减。π^*和C^*在亚ns时域内均存在明显的激发波长依赖性衰减，且在激发后超过40 ps的时间范围内两者成正比。结果揭示了该表面上的载流子动力学中体-表面跃迁的关键作用，以及电子在D_<down>状态下的短寿命（<10ps）。通过比较扩散方程的解，确定了表面复合速度S。2)利用2PPE光谱研究了用STM检测c缺陷密度的Si(001)（2x1）表面的电子特性和电子动力学，发现当p型Si(001)（2x1）表面c缺陷密度大于0.05单层时，相对于CBM在~ 0.8eV发生费米能级钉钉。Si（001）表面的c缺陷影响其电子动力学。研究了体积到表面电子跃迁速率S与表面激发电子消光寿命τ的乘积的反比随c缺陷密度的变化，发现当表面c缺陷密度达到0.1 ML时，它变成了两倍。

In this study, I constructed the experimental system consisting of a time-resolved two-photon photoelectron spectroscopy (TR2PPE) with a fsec-pulse laser, scanning tunneling microscopy (STM), and the sample treatment system. Using this system, the electron property and dynamics of the Si(001) surface is investigated, and the following results are obtained.1)The temporal evolution of electrons which temporarily occupy the unoccupied surface dangling bond state (D_<down>) and the bulk conduction band as a result of the photoexcitation is studied. The sub-ns decay of π^* (electron population at D_<down>) and C^* (electron population at the bulk conduction band near the surface) after the photo-excitation of the clean Si(001) surface. A prominent excitation-wavelength dependent decay in sub-ns temporal domain was observed for both π^* and C^*, and they are proportional with each other at temporal regime longer than 40 ps after the excitation. The results reveal a crucial role of the bulk-to-surface transition in the carrier dynamics on this surface, together with a short lifetime (<10ps) of electrons at the D_<down> state. The surface recombination velocity S is determined by comparing the solution of the diffusion equation.2)2PPE spectroscopy was utilized for investigating the electronic properties and electron dynamics on the Si(001)(2x1) surface on which the density of C-defect was examined with STM It was found that the Fermi-level pinning occurs at 〜0.8eV with respect to CBM when C-defect density is more than 0.05 monolayer on the p-type Si(001)(2x1) surface. The electron dynamics on Si(001) is affected by the C-defect on the surface. A change in the inverse of the product of the bulk-to-surface electron transition rate S and the extinction lifetime τ of the surface excited electron is examined as a function of the C-defect density, and found that it becomes twice when the C-defect density reaches to 0.1 ML on the surface.