圓盤狀氮化鎵自組性量子點之結構與光譜研究 ; Structure and optical spectroscopy studies of self-organized disk-like GaN dots

本論文中我們利用原子力顯微術和光激螢光光譜來研究氮化鎵量子點結構和光學特性。從原子力顯微鏡的觀察，我們發現量子點的形狀是圓盤狀。這些量子點的尺寸隨著TMGa的流率增加而變大。除此之外，我們還發現在成長量子點的時候，氮化鎵磊晶層必須累積到一定的厚度之後才會形成量子點。因此我們認為利用流率調制磊晶技術所成長的量子點是透過Stranski-Krastanow (SK)成長模式。從量子點的密度對磊晶層厚度之關係可以推估出濕潤層的厚度大約7.2個原子層。光激螢光光譜研究顯示量子點的躍遷能量隨著尺寸增加而變小。相對於室溫氮化鎵塊材譜峰位置有36到62 毫電子伏特的藍移量。這個藍移現象是來自量子尺寸效應所致。由變溫螢光光譜，我們發現在低溫時譜峰會隨著溫度的增加而藍移。隨著溫度繼續增加，才會跟隨著瓦希尼(Varshni) 公式所預測的紅移。而且在低溫時的藍移量會隨著量子點尺寸的增加而變大。這樣的藍移現象是以SK模式成長的量子點特徵，在其他的量子點系統都被觀察到。但是目前為止，在氮化鎵量子點系統中並沒有文獻上報導過此現象。我們認為在低溫時激子被侷限在量子點內的缺陷態，隨著溫度增加而游離到量子點的自由激子態，再進一步提高溫度，則自由激子克服量子點能障而熱活化到氮化鋁鎵緩衝層的缺陷態上。 ; In this study, structural and optical properties of self-organized GaN dots grown on Al0.11Ga0.89N/sapphire by metal organic chemical vapor deposition (MOCVD) were investigated by means of atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy. AFM studies showed the disk-like shapes of GaN dots. The disk-like dot size varies with the flow rate of TMGa. We found that the formation of disk-like GaN dots starts when the average GaN coverage exceeds 7.2 MLs. It implies that the growth of disk-like GaN dots is mainly through the Stranski-Krastanow (SK) growth mode. And the wetting layer (WL) thickness is estimated to be about 7.2 MLs from the dependence of the dot density on the GaN coverage. The PL peak energy was found to decrease as the dot size increases. The blue shift is about 36 to 62 meV at room temperature in comparison with GaN epilayers. This observation is ascribed to the quantum size effect. The temperature dependent PL peak energy exhibits an initial blue-shift with increasing temperature and then followed by a red-shift which obeys the Vashini’s relation. The strength of blue-shifts increases with dot size. Such an energy shift with increasing temperature is a signature of QDs grown by SK growth mode and has also been observed in other QDs systems except for GaN QDs. This is a typical characteristic of the exciton localization. Exciton bound to the defect state in the dots and carrier thermalization from confined well states into the defect state of buffer layer were used to explain the temperature dependent PL results.