砷化銦量子點及量子環在磁場下之光激發螢光光譜研究

Abstract

本論文主要是利用在磁場下光激發螢光光譜研究自聚式砷化銦量子點以及量子環的特性。從量子點演變到量子環的過程中，發光能量會因為高度變小而往高能量移動。對量子點做快速熱退火實驗，由於銦原子和鎵原子在量子點中混合嚴重，而導致發光能量藍移且均勻度增加。 在外加垂直於樣品表面方向的均勻磁場下，光激發螢光光譜會受到反磁性效應以及塞曼分裂(Zeeman splitting)的影響而產生光譜的遷移。反磁性效應會導致量子結構的發光能量隨著磁場的增加而藍移，且能量與磁場的平方成正比。基態的反磁性位移會隨著激發功率密度越大而變大，這是由於在高激發功率下，受到遮蔽效應的影響而使得激子的半徑變大，導致反磁性位移變大。 此外，在高激發功率密度時，我們研究量子點及量子環在激發態的螢光光譜。在外加磁場下，由於塞曼效應的影響使得激發態的能階會分裂。其分裂的狀況在量子點中與量子環中又有所不同。第一激發態在量子點與量子環中均為一分為二。但在第二激發態，量子點的能階會一分為三，而量子環則維持一分為二。利用塞曼分裂的能量差與磁場的線性關係，我們求得了激子在量子點與量子環中的有效質量。我們所獲得的值與他人實驗觀測的值相吻合。我們並發現量子環的激子有效質量較量子點的大，這可歸究為(1)因量子環成長所需的熱處理，使得環中有較多的鎵原子。(2)量子環的發光能量較高，因能帶的non-parabolicity而造成的。

Magneto-Photoluminescence spectroscopy was used to the optical transition properties of self-assembled InAs quantum dots and quantum rings. With the evolution from quantum dots to quantum rings, the emission peaks of the corresponding optical spectra shift to higher energies because of the enhancement of energy quantization from the reduced structure height. Rapid thermal annealing (RTA) performed on the quantum dots reveals an emission energy blueshift as well as a reduction of inhomogeneous broadening due to the atomic intermixing of In and Ga inside the dots. When a uniform magnetic field is applied normal to the sample surface, the emission peak shifts due to the diamagnetic effect and the levels with non-zero angular momentum split due to Zeeman effect. The diamagnetic effect leads to an energy blueshift in proportion to the square of the magnetic field. The diamagnetic shift amount of the ground state emission was found to increase with the excitation power. This increase is attributed to the more extended exciton radius due to screening effect at higher excitation power. Using higher excitation power, we have also studied the magneto-optical response of the excited states of quantum dots and quantum rings. In a magnetic field, the excited state emission splits into several peaks due to orbital Zeeman effect. Different splitting behavior was observed for the dots and the rings. The first excited state splits into two for both the dots and the rings but the second excited state splits into three for the dots and two for the rings. From the amount of splitting, we were able to deduce the effective reduced mass of the excitations. The obtained values agree with what people generally agreed upon. A larger effective mass was observed for rings as compared with that of the dots, and it can be attributed to a higher degree of Ga-In intermixing because of the thermal annealing cycle during the growth of the rings and the higher emission energy, which gives rise to the band non-parabolicity effect.