第二型能帶結構量子井與量子點之成長與光學研究

Abstract

利用分子束磊晶法成長第二型能帶排列的硒化碲鋅/硒化鋅多重量子井、碲化鋅/硒化鋅量子點、碲化錳鋅/硒化鋅量子點與碲化鋅/硒化錳鋅量子點。並且利用光激螢光譜、變溫光激螢光光譜、時間解析光譜、磁性光激螢光光譜等實驗技術來探究其物理特性。 硒化碲鋅/硒化鋅多重量子井的發光波長、震盪強度以及光激螢光發光效率可依量子井的井寬以及雷射激發強度做調控。隨著量子井井寬由5奈米降至1奈米我們可以觀察其光激螢光譜譜峰值有一個260毫電子伏特的藍位移。在井寬最寬的樣品中我們可以觀察到一個特別長的結合時間，而在最窄的井寬中此間接激子束縛能為12毫電子伏特。另外在高雷射激發功率下光激螢光強度因獲得熱能而降低的效應被大大的減小。 同樣也利用變溫光激螢光光譜和時間解析光譜來研究第二型能帶結構的碲化鋅量子點其物理特性。電洞在升溫過程中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。而因為載子在量子點中脫逃與再捕捉的過程使其時間衰減曲線呈現非單一指數趨勢，而我們以Kohlrausch延伸指數可以完美的擬合碲化鋅量子點的時間衰減曲線。 碲化錳鋅/硒化鋅量子點因為摻雜錳原子進入量子點內使得其磁光效應與碲化鋅/硒化鋅量子點有極大的不同。我們利用分子束磊晶法嘗試不同成長方式成長碲化錳鋅/硒化鋅量子點，其中發現先供應錳原子數秒之後再提供碲與鋅的方式所成長的碲化錳鋅量子點擁有較佳的發光效率。利用右旋與左旋極化光的光激螢光譜和時間解析光譜來研究碲化錳鋅/硒化鋅量子點內的載子自旋動力學，發現Kohlrausch延伸指數可以精確地擬合右旋與左旋極化光的時間衰減曲線，進而可以得到自旋鬆弛時間大約為23奈秒。此外我們也在碲化錳鋅/硒化鋅量子點中觀察到磁極化子(magnetic polarons)的形成，此磁極化子主要是由被侷限在量子點內的電洞與錳離子中的3d電子之間的自旋交互耦合作用而形成。在此系統中我們觀察極化子可維持至100 K，且幾乎不隨溫度而有所改變。 最後我們在硒化錳鋅中成長碲化鋅量子點並且研究其光學特性，以瞭解其與碲化錳鋅/硒化鋅量子點的異同。在碲化鋅/硒化錳鋅量子點中我們觀察到圓極化率隨著磁場變化遵守Brillouin函數，表示此量子點擁有順磁特性。由不同磁場下的圓極化程度我們可以推得其自旋鬆弛時間約為35奈秒。而在低溫中我們也可在此樣品中觀察到磁極化子的形成。此外碲化鋅/硒化錳鋅量子點相較碲化錳鋅/硒化鋅量子點有一較佳的發光效率。

Type-II ZnSeTe/ZnSe multiple quantum wells (MQWs), ZnTe/ZnSe quantum dots (QDs), ZnMnTe/ZnSe QDs, and ZnTe/ZnMnSe QDs were grown by molecular beam epitaxy (MBE). The photoluminescence (PL), temperature dependent PL, time-resolved PL and magneto-PL were used to investigate the interesting physical properties. The tunability of the emission energy, oscillator strength and PL efficiency by varying the well thickness and excitation density was demonstrated in the ZnSe0.8Te0.2/ZnSe multiple quantum wells. A significant blueshift about 260 meV of the PL peak energy was observed as the well width decreased from 5 to 1 nm. An extraordinary long lifetime (300 ns) of the recombination for the widest sample was detected. The binding energy of the indirect excitons is determined as 12 meV for the thinnest sample. The reduction of PL efficiency by thermal energy is greatly suppressed by employing a high excitation power. The optical properties of type-II ZnTe QDs were also investigated by temperature-dependent and time-resolved PL spectroscopy. The initial decrease then increase with temperature for the full width at half maximum (FWHM) of PL is attributed to the hole thermal escape from the smaller QDs then transfer and re-capture to the neighboring-larger QDs. The non-mono-exponential decay profiles reflect the processes of carrier transfer and recapture. We show that the Kohlrausch’s stretching exponential well fits the decay profile of ZnTe/ZnSe QDs. ZnMnTe/ZnSe QD structures have distinguished difference in magneto-optical properties from ZnTe/ZnSe QD structures due to the existence of Mn in the QDs. Different growth modes of ZnMnTe/ZnSe QDs were studied by using MBE. The ZnMnTe QD system grown by opened the Mn for a few seconds and then Zn+Te growth mode exhibit better emission efficiency. The σ+ and σ– circularly polarized time-integrated and time-resolved PL were employed to investigate the carrier spin dynamics of ZnMnTe/ZnSe QDs grown on GaAs substrates by molecular beam epitaxy. The Kohlrausch’s stretching exponential function well correlates both the σ+ and σ– decay profiles. The measured spin relaxation time is about 23 ns. In addition, we present a magneto-optical study of magnetic polarons (MP) in ZnMnTe/ZnSe QDs. The polarons are found due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the QDs. In this system, the MP was detected at temperature up to 100 K and the formation energy is roughly independent of temperature. Finally, the ZnTe QDs were grown in ZnMnSe matrix. The magnetic field dependence of PL circular polarization degree follows the Brillouin function and evidences the Mn magnetism in ZnTe/ZnMnSe QDs. The magnetic field dependence of PL circular polarization degree shows the long spin relaxation time of about 35 ns. The magnetic polaron formation was observed in low temperature. The ZnTe/ZnMnSe QDs have better PL emission efficient than ZnMnTe/ZnSe QDs.