鍺錫/鍺異質結構奈米元件之電導量子化效應

Abstract

本論文研究鍺錫/鍺異質結構之奈米尺度元件電導量子化效應，其可應用於量子與自旋電子元件，例如量子點接觸與量子點。在量子計算中，量子點是承載電子自旋的重要平台，而量子點接觸常用作自旋讀取。矽基材料為過去數十年超大規模積體電路的基幹材料。在四族材料中，鍺錫因為具強自旋軌域耦合作用而被視為極具發展潛力的材料，因為它可以實現對自旋的電控制以及高速操作的量子計算。 本論文首次展示鍺錫材料平台之量子點接觸元件，並以矽基量子點接觸元件作為基線。n型矽基量子點接觸元件被觀測到4e^2/h的電導量子化，這個量子化值對應到波谷簡併性數值2 (二維電子系統)與自旋簡併性數值2 (零磁場條件)。在矽基元件中亦觀察到量子點的特性，包含庫侖阻絕振盪以及電荷穩定圖菱形訊號。在元件中存在量子點結構的證據為週期性出現的電荷穩定圖菱形訊號以及庫侖阻絕振盪的峰值訊號間距符合電容分析。在量子點接觸通道中出現的雜質會形成位能壁壘，將通道與源極、汲極阻隔，雜質很可能會與量子點接觸的閘極共同形成量子點。然而，因為雜質是隨機分布且不可被操控，使得雜質輔助形成量子點不具有實用性。 對於鍺錫材料之量子點接觸元件，我們首先使用霍爾量測分析鍺錫/鍺異質材料之電性，並於大磁場下觀測到清晰的量子霍爾平台與Shubnikov-de Haas振盪。鍺錫/鍺異質磊晶材料量子點接觸元件在零磁場條件顯示e^2/h電導量子化訊號，且e^2/h電導量子化平台隨著磁場增大而變得明顯，這可能歸因於強自旋軌域耦合作用以及電子間交互作用（擴大自旋極化效果），但尚須更進一步的查證來支持這一論點。

This thesis aims for quantized conductance nanoscale GeSn/Ge heterostructure nanoscale devices for quantum and spintronic applications, such as quantum point contacts (QPCs) quantum dots. Quantum dots are the important platform for quantum computing to host electron spins while quantum point contacts are commonly used for spin readout. Si-based materials are the backbone materials for VLSI technology in the past decades. Among group-IV materials, GeSn is promising due to its strong spin-orbit interaction (SOI), which enables electrical control of spins and quantum computing with fast operations. In this thesis, GeSn QPCs are demonstrated for the first time and Si QPCs are served as a baseline. Clear quantized conductance of 4e^2/h are observed in n-type Si-based QPCs. The 4e^2/h quantization is corresponding to 2-fold valley degeneracy in two-dimensional electron system and 2-fold spin degeneracy under zero magnetic field Coulomb blockade and diamond-like charge stability diagrams, the characteristics of quantum dot (QD), are also observed. The diamond-like patterns appear periodically and the spacing of Coulomb blockade peaks is consistent with capacitance analysis. The impurities in QPCs channel act as energy barrier, which blockade the channel and source/drain, constitute with gates defined QPCs and form impurity assisted QD. However, impurity assisted QD is not good for utility because impurities are randomly distributed and uncontrollable. For GeSn-based QPCs, we first characterize the GeSn/Ge heterostructure by performing Hall measurements with clear quantum Hall plateaus and Shubnikov-de Haas oscillations at large magnet fields. GeSn-based QPCs show quantized conductance of e^2/h under zero magnetic field. The e^2/h quantized plateaus become more apparent with a larger magnetic field, which could be attributed to strong spin-orbit interaction and electron-electron interactions (enlarge the spin polarization). Further investigation is required to support this argument.