| Summary: | 碩士 === 國立中山大學 === 物理學系研究所 === 101 === Semiconductors of （1-100） face ZnO with Wurtzite structure aren’t symmetric in two axes which are perpendicular or parallel to c-axis. This provides an optical technique to measure properties of ZnO sample. Reflectance anisotropy spectroscopy (RAS) is an important optical technique to determine surface properties of semiconductors. Although optical penetration depths are often in the range of several hundred nanometers, we can still observe difference signals. That’s because our ZnO sample is a bulk. A photo-elastic modulator (PEM) was often used to change polarization-state of incident light to measure reflectance difference of polarization between (E⊥c) and (E∥c) these two directions. There are some features around critical points and their amplitudes or/and shapes are dependent on carrier concentration of the sample or/and structure of the surface. If photon-energy of the incident light is fixed at critical point energy, it can be used to in-situ monitor the growth of GaAs by molecular beam-epitaxy.
However, the PEM can be operated only at a fixed frequency (50 kHz or 100 kHz) and if the state of polarization is set to change from x to y linear polarization, there are elliptical polarizations lying between them. In this work, a method using TN liquid crystals (LCs) to change the direction of polarization of the incident light was used. A linear-polarized light will be rotated by 90 degrees by passing through TN LCs. Nevertheless, the polarized direction of the incident light will not be changed if there are enough voltages applied on LCs because they are aligned perpendicular to front and back electrodes. Using a synthesized signal generator, the direction of the incident light can be changed between x and y directions and there are almost no elliptical polarizations lying between them.
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