Spectroscopic Studies of Surface Layers of Semiconductor Microcrystals

一般来说，固体表面的光学研究是非常困难的，因为与入射光相互作用的原子数量非常少，包含关于表面信息的光学信号非常弱。然而，对于微晶体表面层的研究，本质上是具有大表面积与体积比的系统，光学光谱有望是有效的。在本研究项目中，我们主要使用红外和拉曼光谱来证明微晶表面光学研究的有效性，结果表明，气相蒸发的硅和锗微晶的表面氧化层可以很容易地观察到红外吸收。此外，还可以详细研究天然氧化物和热氧化物的形成过程。在表面等离子体激发条件下，可以清楚地观察到银小颗粒表面层的表面增强拉曼散射。还观察到了极小的Ge微晶(直径小于50A)表层的拉曼信号。这些实验结果证明了光谱学用于微晶表面研究的有效性。我们还利用拉曼光谱研究了气相蒸发W03微晶的相变行为。我们发现了一个新的相，这是到目前为止还没有报道的大块晶体。新相的出现似乎是由表面的存在引起的。对于各种微晶体，我们通过共振拉曼散射、光吸收和光致发光的测量，证明了量子尺寸效应。在某些情况下，我们提出了表面效应的重要性，表面效应叠加在量子尺寸效应上。

In general, optical studies of solid surfaces are extremely difficult, because the number of atoms interacting with incident light is extremely small and the optical signals containing the information about the surfaces are extremely weak. However, for the study of surface layers of microcrystals, which are essentially the systems having large surface-tovolume ratios, the optical spectroscopies are expected to be effective. In this research project, we used mainly the infrared and Raman spectroscopy in an attempt to prove the effectiveness of the optical studies of the micocrystalline surfaces.It turned out that infrared absorption by surface oxide layers of gas-evaporated Si and Ge microcrystals can easily be observed. Furthermore, processes of the formation of the natural and thermal oxides could be investigated in detail. Under the conditions of surface plasmon excitation, the surface-enhanced Raman scattering from the surface layers of Ag small particles could be clearly observed. The Raman signals from surface layers of extremely small Ge microcrystals (less than 50A in diameter) could also be observed. All these experimental results demonstrate the effectiveness of the optical spectroscopy for the studies of the microcrystalline surfaces.we also studied the phase transition behavior of gas-evaporated W03 microcrystals using the Raman spectroscopy. We found a new phase which is not reported so far for the bulk crystals. The appearance of the new phase seems to be caused by the existence of the surfaces. For various kinds of microcrystals, we have demonstrated the quantumsize effects from the measurements of resonance-Raman scattering, optical absorption and photoluminescence. In some cases, we suggested the importance of the surface effects, which are superposed on the quantum size effects.