利用分子束磊晶系統成長碲化鋅量子點耦合 硒化鎘鋅量子井以延長載子生命期之研究

Abstract

本論文以分子束磊晶技術將自聚性成長之碲化鋅/硒化鋅第二型量子點耦合第一型硒化鎘鋅/硒化鋅量子井結構，當量子點和耦合量子井之間的間隔層厚度降低至2奈米以下時，我們成功地將載子的生命期由約20奈秒大幅延長至130奈秒。我們利用光激螢光光譜及時間解析光譜探討量子點和耦合量子井之間不同間隔層的厚度對發光與載子復合機制的影響。隨著間隔層厚度的減少，耦合碲化鋅/硒化鋅量子點發光的譜峰能量往低能量移動。透過不同波長的雷射激發與變激發功率的螢光光譜，我們發現耦合量子點的光激螢光是由兩種第二型躍遷組成，其一為在硒化鋅能障層內的電子與侷限在碲化鋅量子點內的電洞復合所產生的較高能量發光，另一來源為侷限於硒化鎘鋅/硒化鋅量子井的電子與侷限在碲化鋅量子點內的電洞復合所產生的較低能量發光。由時間解析光譜譜線的擬合，發現載子經由第一種躍遷的生命期不隨間隔層厚度降低而變化且維持在10奈秒左右，但對於第二種躍遷，當間隔層厚度低於2奈米時不但生命期延長至130奈秒，且主導了大部份的載子復合過程。因此，我們認為可以透過耦合量子井增加電子與電洞的空間分離，而達到延長載子生命期的目標。

In this work, we demonstrated the growth of a self-assembled type-II ZnTe/ZnSe quantum dot (QD) structure coupled with a type-I Zn0.88Cd0.12Se/ZnSe quantum well (QW) on the (001) GaAs substrate by molecular beam epitaxy (MBE). As the spacer thickness is less than 2nm, the carrier lifetime increasing from 20ns to 130ns was successfully achieved. By utilizing the time-resolved photoluminescence (TRPL) and PL with difference excitation wavelength and power, we identify the PL emission from the coupled QDs consists of two recombination mechanism. One is the recombination between electrons in ZnSe barrier and holes confined within ZnTe QDs, and the other is between electrons confined in Zn0.88Cd0.12Se QW and holes confined within ZnTe QDs. According to the band diagram and power-dependent PL, both of the two recombinations reveal the type-II transition. Based on the fitting of time-resolved PL decay curve, the lifetime of the first recombination remains about 10 ns, while that of the second recombination increases with decreasing spacer thickness. In addition, the second recombination mechanism dominates the whole carrier recombination as the spacer thickness is less than 2nm. A significant extension of carrier lifetime by increasing the electron – hole separation is illustrated in a type-II ZnTe/ZnSe QD structure coupling with a type-I ZnCdSe/ZnSe QW.