銻化鎘/硒化鎘/硫化鋅多殼層奈米粒子之磁光性質研究

Abstract

量子力學系統中，儘管同調帶電粒子波函數的絕對相位無法由儀器量測，但卻可獲得其相對相位。著名的阿哈羅夫-波姆效應 (Aharonov-Bohm effect) 中討論電子在一環狀路徑運動，產生的干涉圖案可得知電子波函數相位改變與磁通量變化有關。近來，一連串有關於光學阿哈羅夫-波姆效應(optical Aharonov-Bohm effect) 開始引起物理界的討論與關注。光學阿哈羅夫-波姆效應理論預測半導體的激子性質 (excitonic properties)，如：電子和電洞在磁場下的相對相位；激子能量隨著磁場改變而震盪；光激發冷光 (photoluminescence) 積分強度隨著磁場改變逐漸消逝 (或震盪)。實驗上，在砷化鎵銦/砷化鎵量子環 (quantum ring) 系統中的帶負電激子 (negatively charged exciton) 以及第二類半導體 (type-II semiconductor) 砷化鎵/銻化鎵系統中的極化激子 (polarized exciton) 亦證實具有光學阿哈羅夫-波姆效應。 本文中我們將討論光學阿哈羅夫-波姆效應在銻化鎘/硒化鎘/硫化鋅多殼層奈米粒子系統。奈米粒子利用化學方法合成，其大小約六奈米。銻化鎘(核)/硒化鎘(殼)形成一第二類半導體能帶結構。奈米粒子外圍長成一硫化鋅層，其目的為了減少硒化鎘表層能帶 (surface state) 吸收致使增強發光效率。磁光激發冷光(Magneto-photoluminescence) 實驗部分，我們利用最大磁場為14特斯拉的超導磁鐵搭配一綠光波段的二極體雷射 (diode laser) 與單光儀 (monochromator)，在1.4度絕對溫度的環境下進行。 光激發冷光頻譜中，我們發現發光位置波峰能量以及積分發光強度對應磁場強度改變而震盪。此兩項發現我們將其解讀成類光學阿哈羅夫-波姆效應 (optical Aharonov-Bohm-like effect) 。

Although the absolute phase of a quantum state is not measurable, the relative phase of a coherent charged particle wave could be manipulated. In the famous Aharonov-Bohm effect for an electron traveling in a ring, interference pattern was observed with changing magnetic flux. Recently, optical Aharonov-Bohm has received much attention. The effect of the relative phase of the electron and hole in a magnetic field on the excitonic properties in semiconductors was studied theoretically and oscillation of the excionic energy and quenching (or oscillation) of the integrated photoluminescence intensity with magnetic field were predicted. Experimentally, evidence of the optical Aharonov-Bohm effect was observed with negatively charged exciton in InGaAs/GaAs quantum ring and polarized exciton in type-II GaAs-GaSb system. In this thesis we’ll present our studies on the optical Aharonov-Bohm in CdTe/CdSe/ZnS system. The nanoparticles were grown by chemical method and have size of about 6 nm and the band alignment between the core (CdTe) and the shell (CdSe) is a type–II band alignment. The addition of ZnS layer is to passivate the surface of CdSe and to enhance the light emitting efficiency. Magneto-photoluminescence experiment was performed at T=1.4 K with a 14 T superconducting magnet in conjunction with a green diode laser and a monochromator. Oscillation on the peak energy of the photoluminescence spectra as well as oscillation in the integrated intensity as a function of magnetic field were observed and are attributed to the optical Aharonov-Bohm-like effect.