稀磁性碲化鋅/硒化錳鋅量子點的成長與光學特性研究

Abstract

利用分子束磊晶法在硒化錳鋅磊晶層中成長第二型能帶排列的碲化鋅/硒化錳鋅量子點，量子點的厚度分別是1.8、2.2、2.4、2.7與3.0個原子層。並利用光激發螢光譜、變溫光激螢光譜、變功率光激螢光譜、變磁場光激螢光譜與時間解析光譜等實驗技術來探討其物理特性。我們發現峰值能量隨量子點厚度增加呈現兩個不同斜率的紅位移。當碲化鋅的覆蓋厚度小於臨界厚度時，低溫的光激螢光譜峰值能量隨覆蓋厚度增加而快速的減小。當覆蓋厚度大於臨界厚度時，光激螢光譜峰值能量趨於緩慢減小。在變溫的光激螢光實驗中，電洞在升溫的過程獲得熱能由小量子點脫逃並被其他鄰近的大量子點捕捉，這使得光激螢光譜峰的半高寬會先變窄而後變寬的趨勢。從不同激發強度的光激螢光譜中我們可見其譜峰值能量隨激發強度增加而呈現藍位移，此關係為第二型半導體能帶結構的特性。而且我們可由能帶彎曲及電子與電洞波函數的重疊情形說明時間解析光譜中的非單一指數衰減曲線。另外也觀察到，樣品之圓偏振度會隨磁場增加而增加最後達到飽和，此為布里淵形式的順磁性的效應。由研究量子點發光峰值隨時間演化的光譜中，發現因有激子磁極化子的形成使得峰值隨時間而紅移。

Type-II ZnTe/ZnMnSe quantum dots (QDs) were grown in the ZnMnSe matrix by molecular beam epitaxy (MBE). The QDs coverage varies from 1.8 to 2.2, 2.4, 2.7, and 3.0 monolayers (MLs). The photoluminescence (PL), temperature dependent PL, power dependent PL, magneto-PL and time-resolved PL spectra were used to investigate the interesting physical properties. Two different red-shift slopes are observed from the plot of the PL peak energy versus the thickness of ZnTe coverage. The peak energy of PL decreases sharply when the ZnTe coverage is less than the critical thickness and gently when the ZnTe coverage exceeds the critical thickness. The initial decrease which then increases with temperature for the full width at half maximum (FWHM) of PL is attributed to the hole which thermal escapes from the smaller QDs then transfers and is re-captured to the neighboring-larger QDs. The peak energy linearly depends on the cube root of the excitation power, which is a characteristic of all quantum structures with type-II band alignment. In addition, the non-mono-exponential decay profiles could be explained by the band-bending effect and electron-hole wave function overlap. Furthermore, the dependence of circular polarization degree on the magnetic field shows the Brillouin-type para-magnetism. The formation of exciton magnetic polaron (EMP) was illustrated by the time-evolution of QDs emission peak energy.