自組式第一型量子環與第二型量子點之磁場光學研究

Abstract

在這篇博士論文，我們研究了自組式的第一型量子環、第二型量子點的磁場光學特性，這些量子結構使用分子束磊晶成長，並且被預期具有環狀載子波函數，能展現新穎的磁場響應，例如Aharnov-Bohm效應，也就是載子磁場下波函數的量子干涉現象。利用磁場光譜技術量測磁場能量偏移，我們探知侷限在這些量子結構的載子波函數空間分布，他們展現與傳統第一型量子點截然不同的物理現象，我們建立數個計算模型，模擬結果跟我們的實驗發現相符合。 在單一銦化鎵/砷化鎵量子環，雙激子呈現了一個相較於中性激子更大許多的反磁位移，這暗示了量子環有較擴張的雙激子波函數，我們發現由於量子環結構的不完美造成了兩側的位能井，激子波函數被侷限於量子環一側的位能井。然而雙激子會由於電洞間的庫倫排斥而能夠散佈整個量子環。我們的實驗結果可以推斷，預期的量子環中性激子的量子干涉現象將被如此的波函數侷限現象所破壞。 第二型銻化鎵/砷化鎵量子點的磁場光譜量測包含法拉第組態跟沃伊特組態，當磁場施加在法拉第組態，我們觀察到標準的反磁藍移。然而，當磁場施加在沃伊特組態，量子點展現了一個反常的磁場紅移伴隨著急遽增加的螢光強度。我們發現這是因為沃伊特組態的磁場提供一個額外的垂直侷限，改善了發光複合率，因此量子點內的穩態電洞濃度下降，導致了我們觀察到的反常磁場紅移。此外，既然電子波函數分布在量子點的上下，電子波函數並非環狀，這抹煞了觀察到Aharnov-Bohm震盪的機會。 第二型砷化鎵/銻化鎵量子點的製作是利用晶格不匹配產生的天然張應力，利用反磁位移的實驗結果，我們分析不同尺寸量子點的載子波函數空間分布。我們發現，隨著發光能量增加(量子點尺寸減小)，反磁係數會快速上升到一個飽和值。這個不尋常的趨勢是歸因於量子點侷限的電子波函數，會隨著量子點減小而逐漸外擴到沾濕層。這個去侷限化效應會在這個材料系統內被強化，因為量子點的張應力釋放會抬升傳導帶而超過沾濕層。 因此，要觀察到光性的Aharnov-Bohm震盪，可行的方法是在銻化鎵/砷化鎵量子點的上下各放置一層砷化鋁鎵侷限層，可以推擠電子到量子點側邊，因此形成環狀波函數。模擬結果顯示了Aharnov-Bohm震盪的發生。

In this dissertation, the magneto-optical properties of self-assembled type-I quantum rings and type-II quantum dots are studied. These quantum structures grown using molecular beam epitaxy are expected to possess ring-like carrier wave functions and exhibit novel magnetic responses, such as Aharnov-Bohm effect, i.e. the quantum interference of the carrier wave function in magnetic fields. The magnetic energy shifts measured by magneto-photoluminescence technique were used to probe the spatial wave function extent of the carriers confined in these quantum structures. They revealed anomalous physical phenomena compared with the conventional type-I quantum dots. Several calculation models were carried out and agree well with our experimental finding. In single InAs/GaAs quantum rings, the biexciton showed a considerably larger diamagnetic shift than the neutral exciton, implying the more extended biexciton wave functions in the ring. We found that the structural imperfections of the quantum ring induces two potential valleys inside the ring, and the exciton wave function tends to be localized in one of the valleys. However, the biexciton wave function is able to spread over the ring due to the hole-hole Coulomb repulsion. Our results suggest that the expected novel quantum interference of neutral excitons in quantum rings will be destroyed by such wave function localizations. The magneto-photoluminescence measurements were performed on type-II GaSb/GaAs quantum dots in both Faraday and Voigt configurations. When the magnetic field was applied in a Faraday configuration, a typical diamagnetic blue shift was observed. However, when the field was in a Voigt configuration, the QDs exhibited an anomalous magnetic red shift together with a rapid increase of the PL intensity. We found that the magnetic field in the Voigt configuration provides an additional vertical confinement and hence increases the radiative electron-hole recombination rate. The resulting decrease of the steady-state hole concentration in the QDs gives rise to the observed anomalous magnetic red shift. Furthermore, since the electron wave function was found to be distributed above and below the QD, the electron wave function is not ring-like. This eliminates the probability of the observation of the Aharnov-Bohm oscillation. Type-II GaAs/GaSb quantum dots were fabricated by the natural tensile strain from lattice mismatch. We analyzed the carrier wave function extent of the dots with different sizes using the diamagnetic shift results. With the increase of the energy (the reduction of the dot size), the diamagnetic coefficient was found to rises quickly to a saturated value. This unusual tendency is attributed to the gradual spreading of the electron wave function from the quantum dots to the wetting layer as the dots get smaller. This delocalization effect is enhanced in this material system due to the tensile strain relaxation within the dots, which raises the conduction band edge over that in the wetting layer. As a result, in order to observe optical Aharnov-Bohm oscillation, two AlGaAs confinement layers are placed above and below the GaSb/GaAs quantum dots. The electrons are therefore pushed to surround the dots with ring-like wave functions. The simulation results reveal the occurrence of the Aharnov-Bohm oscillation.