稀磁性碲化鋅/硒化錳鋅量子點的成長與磁極化子之研究

Abstract

利用分子束磊晶系統在低濃度硒化錳鋅磊晶層上成長第二型能帶結構的碲化鋅/硒化錳鋅量子點，量子點的厚度分別是1.6、1.9、2.1、2.4、2.6、2.9與3.2個原子層。利用穿透式電子顯微鏡觀察多層碲化鋅/硒化錳鋅量子點的截面型貌，多層量子點中，每一層量子點厚度皆為2.6 個原子層。每一層量子點間皆以厚度10奈米的硒化錳鋅磊晶層隔開。再使用低溫光激螢光光譜、變溫光激螢光光譜、時間解析光譜探討樣品的物理特性。我們發現第二型能帶結構的碲化鋅/硒化錳鋅量子點，其峰值能量隨著量子點覆蓋厚度增加而呈現兩段不同斜率的紅移。當碲化鋅覆蓋厚度小於臨界厚度時其峰值能量紅位移較快，但當碲化鋅覆蓋厚度大於臨界厚度時其峰值能量紅位移則較為平緩。此外，被束縛在小量子點的電洞隨著溫度上升獲得能量而逃離，其後被大量子點捕捉。此現象造成量子點光譜訊號的半高寬隨著溫度上升而下降。但在溫度越來越高時，半高寬會因為激子-縱向聲子交互作用而隨著溫度增加而變大。而當我們研究量子點發光峰值之強度隨著時間的變化，其發光訊號的生命週期約100-200奈秒。這是由於其第二型能帶結構造成電子、電洞在空間上分離而導致生命週期比第一型能帶結構長。我們由量子點能量位置隨時間變化的光譜發現: 不論是否有錳離子的存在，量子點訊號隨著時間都會有紅移的現象。進而，得到摻有錳離子的樣品，其紅移量不只由磁極化子的形成所貢獻，也由其他非磁性機制所造成。最後，比較不同錳離子濃度的樣品。磁極化子的形成時間隨濃度增加而大幅減短，而形成磁極化子造成的能量位移量隨濃度增加而變大。

ZnTe/ZnMnSe quantum dots (QDs) of type II band alignment with different ZnTe coverages from 1.6 to 3.2 MLs were grown by molecular beam epitaxy system. The cross-section image of ZnTe/ZnMnSe multiple QDs was investigated by Transmission Electron Microscopy (TEM). The photoluminescence (PL), temperature-dependent PL and time-resolved PL were used to study the physical properties. The slopes of peak energy versus ZnTe coverages show two different red-shifts. The peak energy of PL decreases rapidly when the ZnTe coverage is less than the critical thickness and slowly when the ZnTe coverage is above the critical thickness. The full width of half maximum (FWHM) of QDs emissions initially decreases and then increases with increasing temperature. It was attributed to thermal escape of holes from the smaller QDs and re-capture by the nearby-larger QDs. When the temperature was increased further, the exciton-longitudinal (LO) phonon interaction dominated and led to the further broadening of FWHM. From the time-resolved PL measurement, the long lifetime of QDs emissions is about 100 to 200 ns, which is a signature of type II QD, and is much longer than that in type-I systems. The type-II band-alignment spatially separates the electrons and holes and results in long electron and hole recombination time. The PL peak position exhibits red-shift as the time evolves. It is attributed to two mechanisms: (1) thermal escape from the smaller QDs and re-capture by the nearby-larger QDs and (2) formation of magnetic polarons. With increasing Mn concentration, the formation time of magnetic polarons decrease and the magnetic poalron energy increases.