第二型能帶結構碲化鋅/硒化錳鋅量子點之光學特性研究

Abstract

本文係利用光激螢光光譜和時間解析光譜量測技術來分析碲化鋅/硒化錳鋅量子點的光學特性。碲化鋅/硒化錳鋅量子點是以分子束磊晶系統成長，碲化鋅覆蓋厚度分別為1.8、2.2、2.4、2.7與3.0個原子層。根據反射式高能電子繞射圖形以及光激螢光譜峰值隨覆蓋層厚度增加而紅移的趨勢，可以判斷量子點是以Stranski-Kronstonov (S-K) 的模式成長的。隨著雷射激發的強度增加，我們可以觀測到光激螢光譜峰值能量會有明顯的藍移，但是，此藍移現象對量子點樣品以及二維結構樣品而言，卻是個別受到量子點大小分佈以及能帶彎曲效應，不同的兩個物理機制所導致。由時間解析光譜來分析載子生命期，我們發現載子生命期會隨著碲化鋅覆蓋厚度的不同而改變，其現象可由能帶彎曲的情形，以及電子電洞波函數的重疊情形加以說明。進一步量測不同溫度下的時間解析光譜，發現在低溫下載子會以輻射方式結合，然而當溫度高於50度絕對溫度，載子復合會轉為非輻射形式。最後，我們發現無外加磁場時，此第二型能帶結構樣品會有非零圓形極化率產生，主要是因為載子受激發後，受到碲化鋅/硒化錳鋅介面不均勻電場的影響，進而產生等效內建磁場，造成自旋能帶分裂的現象。

The optical properties of ZnTe/ZnMnSe quantum dots were investigated by photoluminescence (PL) and time-resolved photoluminescence (TRPL) in this thesis. The ZnTe/ZnMnSe quantum dots (QDs) were grown by the molecular beam epitaxy (MBE) and the coverages of ZnTe are 1.8, 2.2, 2.4, 2.7, and 3.0 monolayers (MLs). Stranski-Kronstonov (S-K) growth mode was identified by the reflection high energy electron diffraction patterns and different red-shift slopes with ZnTe coverage for the PL peak energy. In order to verify the type-II band alignment, power-dependent PL was investigated. There is a significant blue-shift of PL peak energy as the excitation power increases. However, the blue-shift of 0D QDs and 2D layer are caused by two different mechanisms. The QD size distribution and band-bending effect result in the blue-shift for 0D QDs and 2D layer, respectively. In order to understand the mechanism of carrier recombination process, the TRPL was also investigated. The coverage dependence of lifetime can be well-explained by band-bending effect and electron-hole wave function overlap. In addition, the temperature-dependent TRPL shows that the carrier recombination process dominates by radiative channel in low temperature and nonradiative recombination takes over above 50K. Finally, we observed a nonzero circular polarization rate for the type-II structures without external magnetic field. It is attributed to the existence of a built-in magnetic field induced by the motion of carriers under the presence of nonuniform electric field across the interface of ZnTe QDs and ZnMnSe layer. This results in the spin splitting responsible for the nonzero circular polarization rate.