The Study of Regular Quantum Dots Growth and One-Dimensional Quantum Transport
Edward Yi Chang
|關鍵字:||量子點;電子束微影術;奈米T型閘極;熱回流光阻;自組鍺點;一維制限;量子點接觸;量子線;Quantum dots;Electron beam lithography;Nanometer T-gate;Thermally reflowed resist;Self-organized Ge dots;One-dimensional constriction;Quantum point contacts;Quantum wires|
This dissertation is devoted to the study of low-dimensional semiconductor structures and mainly consists of two parts: regular quantum dots growth and one-dimensional (1D) quantum transport. Prior to getting into the two parts, the electron beam lithography incorporating thermally reflowed resist technique for fabricating nanometer T-shaped gate is introduced and demonstrated. In the first part, the controlled placement of self-organized Ge dots on patterned Si (001) substrate is presented. The sizes of the Ge dots are characterized and estimated to be around 10 nm. The Ge dots tend to form along the Si mesa edge, and their distribution could be controlled by the pattern arrangement. In addition, the formation of Ge dots on patterned Si mesas was further calculated by a local strain-mediated surface chemical potential. The simulation results are quite consistent with the experimental data. It may be possible using substrate morphology to improve the ordering and uniformity of the Ge dots formed on Si substrate. In the second part, the transport characteristics on 1D narrow constrictions defined by a triple-gate structure are investigated. The device structures include single quantum well (SQW) and double quantum well (DQW) GaAs/AlxGa1-xAs heterostructures. On one hand, we focus on SQW device. The pinch-off voltages at zero center gate voltage (VCG) for various channel widths W (= 0.4-0.8 μm) and lengths L (= 0.2-2 μm) are well described by pinned-surface model. Comparison between samples with and without a center gate reveals that the center gate, even when zero-biased, significantly affects the surface potential and thereby facilitates the 1D confinement in a deep 2DEG. Nonlinear transport spectroscopy shows that subband energy separation (ΔE) changes linearly with VCG and can be enhanced by 70% for VCG = 0.8 V. A simple model is used to calculate the lowest subband energy separation (ΔE1,2), which well reproduces the overall behavior of the measured ΔE1,2. In addition, effects of impurities, occasionally found for long-channel devices (L > 1 μm), are shown to be greatly suppressed by applying a positive VCG and thereby enhancing ΔE1,2. We also present data for the transport anomaly below the first conductance plateau, the so-called ‘0.7 anomaly’, to demonstrate that the triple-gate structure is useful for the study of density-dependent phenomena in a 1D system. On the other hand, we put emphasis on DQW device. The upper electron layer is supplied via modulation doping, while the lower one is induced through back gate. Vertically aligned constrictions in DQW with separate Ohmic contacts have been fabricated. Clear conductance plateaus for both layers were observed showing that ΔE of the upper quantum well is larger than that of the lower quantum well. Finally, the frictional drag signal caused by narrow constriction was observed.
|Appears in Collections:||Thesis|