低溫強磁場橢偏儀之設計和量測及低能隙量子井厚度相關之電子傳輸有效質量和遷移率

Abstract

論文的第一部分涵蓋了光譜橢偏儀在近紅外波段（700-1000nm）可使樣品置於低溫（〜4.2 K）和強磁場（磁場最高達14T）下量測的設計和實現。論文中將詳細探討在低溫環境下系統的光學和機械的各個組成部分。我們把主要的光學原件都結合在一個可插入到常規的長頸液態氦杜爾瓶的探測器上，這樣的設計使得系統擁有很長的空宇光路徑（〜 1.8 m × 2）。在偏振光的解析方面，我們使用偏振片-樣品-（四分之一波片）-旋轉偏振片的橢偏儀配置。在光路方面，我們用兩個介電反射鏡，一個在樣品前，另一在樣品之後；而在樣品座下的兩軸壓電驅動傾角器則可用來調控反射光的方向，使光能順利反射回旋轉偏振片而被量測。系統的功能性量測結果和其隨機誤差的分析都將在文中展示。我們用此自行設計的橢偏儀系統探索砷化鎵極化子在磁場下傳播的特性。藉此，我們可同時量測到砷化鎵激子橢偏光譜的振幅和相位頻譜以及其相位譜在能量接近砷化鎵激子躍遷時，光左、右旋的轉變。更重要的，我們藉此量測方法觀察到有趣且未曾被發表過的砷化鎵激子光譜的精細結構，且這些精細結構的磁光行為無法由已知特性的激子態做解釋。鑑於此，我們把樣品的表面和磊晶界面都當作砷化鎵極化子的邊界，如此可解釋這些精細結構的由來並歸咎其為多重極化子模態間的干涉結果。而對於此干涉精細結構的磁響應，我們提出了一個同時考慮極化子的空間色散和激子的中心運動與相對運動耦合所導致的有效質量提升的模型對其做定性的解釋。 在論文的第二部分，我們提出了一個設計特殊半導體量子井的想法，這種量子井相對於一般的量子井而言；即使厚度減小，仍能提供較小的電子傳輸有效質量和較高的遷移率。在理論計算中，我們同時考慮了能帶的非拋效應和量子井能障所帶來的影響。在低溫情況下，主要的散射機制包括界面粗糙度，合金無序和雜質的散射均被用來評估電子在量子井中的遷移率。四種不同組合的低能隙量子井的結果和比較都將在文中展現。通過適當選取合適的能帶組合的量子井和其能障的材料，此新穎的傳輸特性可被實現。

The first part of this thesis covers the design and implementation of a spectral ellipsometer at near-infrared wavelength (700-1000nm) for samples placed in high magnetic fields (up to 14 Tesla) at low temperatures (~4.2 K). It details both the optical and mechanical aspects of the system in the low temperature environment. The main optical components are integrated in a probe, which can be inserted into a conventional long-neck He dewar and has a very long free-space optical path (~1.8 m x 2). A polarizer-sample-(quarter-wave plate)-rotating analyzer configuration was employed. Two dielectric mirrors, one before and one after the sample in the optical path, helped to reflect the light back to the analyzer and a two-axis piezo-driven goniometer under the sample holder was used to control the direction of the reflected light. Functional test results and analysis on the random error of the system are shown. The properties of GaAs polariton propagating in magnetic field have been explored using this self-designed ellipsometry system. We obtained both the amplitude and phase ellipsometric spectra simultaneously and observed helicity transformation at energies near the GaAs exciton transitions in the phase spectra. Interesting fine structures, which have not been reported before, have been observed and their magneto-optical behavior cannot be accounted by the known properties of excitonic states. Treating the surface and the growth interface as boundaries, we attribute the fine structures to the interference among various polariton modes. A model considering both the polariton spatial dispersion and the exciton effective mass enhancement induced by the coupling of the exciton center of mass and relative motions is proposed to explain the magnetic response of the interference ellipsometry spectra. In the second part of this thesis, we propose an ideal to design a special kind of semiconductor quantum wells, which, in contrary to conventional quantum wells, are able to provide smaller electron transport effective mass and higher mobility when the quantum well thickness is decreased. The theory used accounts for both the nonparabolicity effect and the influence of the barrier. Major scattering mechanisms at low temperatures, including the scatterings by the interface roughness, the alloy disorder, and impurities have been considered in mobility calculations. The results of four different combinations of quantum wells are shown and compared. By properly choosing the well/barrier materials with proper band lineups, the novel transport property is achievable.