自聚性二六族半導體量子點之成熟機制與特性研究

Abstract

本論文旨於探討自聚性二六族半導體量子點之成熟機制與特性分析。利用分子束磊晶法在砷化鎵基板上成長自聚性硒化鎘/硒化鋅與碲化鋅/硒化鋅兩種量子點結構，藉此研究各種效應（包括部份覆蓋層、活性氧原子以及快速熱退火處理）對於成熟機制的影響。我們發現對於硒化鎘/硒化鋅異質結構而言，260 oC是成長量子點最理想的基板溫度。藉由原子力顯微鏡的觀察，證實硒化鎘量子點結構的形成是從二維轉變為三維的成長模式，此轉變的臨界厚度（沾溼層；wetting layer）約2.5個原子層。隨著硒化鎘沈積量增加，我們觀察到量子點的成長模式發生一連串的轉變，從起先的二維磊晶模式（Frank van der Merve mode）轉變為穩定的三維成長模式（Stranski–Krastanow growth mode），最後進入成熟模式（ripened mode）。利用一個圖示的模型，我們描述了整個量子點成長模式的轉變。 針對成熟機制，我們進一步做了一系列的研究：利用硒化鎘/硒化鋅量子點探討硒化鋅部份覆蓋層以及活性氧原子對於成熟機制的影響。從表面形貌的觀察，發現部份覆蓋層會加快成熟機制的速度，這是由於部份覆蓋層增加了應變能量所造成的結果；當硒化鋅部份覆蓋層厚度低於3個原子層，隨著部份覆蓋層厚度增加，其光激螢光的能量呈現明顯的紅位移，這個結果與原子力顯微鏡觀察到的結果是一致的。另一方面，在硒化鋅緩衝層表面鋪上活性氧原子也可以加快成熟機制的速度，活性氧原子在硒化鋅緩衝層表面可能會改變其局部的化學性質，並且在表面上扮演凝核中心的角色，因此，在硒化鎘沈積厚度相同的條件下，成長在有加活性氧原子處理的硒化鋅緩衝層表面上的量子點密度會較高，且尺寸較小。此外，我們也利用碲化鋅/硒化鋅第二型能帶結構的量子點探討快速熱退火對於成熟機制與能帶彎曲效應的影響。自聚性碲化鋅/硒化鋅量子點也是以從二維轉變為三維的成長模式形成量子點結構。當量子點經過400 oC以上的熱退火處理，其光激螢光的能量會發生很大的紅位移，這是由於當熱退火溫度超過400 oC，熱能啟動了成熟機制，使得某些量子點因為聚集了鄰近小量子點的材料而尺寸變大。在時間解析的光激螢光實驗中，較快的再結合通道被抑制了，暗示快速熱退火處理減小了碲化鋅/硒化鋅量子點的能帶彎曲效應。最後，我們利用改變激發光強度的光激螢光實驗證實了碲化鋅/硒化鋅量子點的能帶彎曲效應被減小是由於熱退火啟動成熟機制造成量子點尺寸變大的關係。

This dissertation is devoted to study the ripening dynamics of the self-assembled II-VI semiconductor quantum dots (QDs). The self-assembled CdSe/ZnSe and ZnTe/ZnSe QDs, which were grown on GaAs (001) substrate by molecular beam epitaxy (MBE), were employed to study various effects on the ripening dynamics. In the CdSe/ZnSe QDs system, the optimum growth temperature was found to be 260 oC. The Stranski–Krastanow (SK) growth mode was confirmed clearly by atomic force microscopy (AFM) images. The thickness of the wetting layer of the CdSe QDs is about 2.5 mono-layers (MLs). As the coverage increases, a complete transfer of the QD growth mode from the Frank van der Merve (FM) mode to the SK mode, followed by the ripened mode, was observed. A schematic diagram of the growth mechanism of self-assembled CdSe QDs was presented. The effects of ZnSe partial capping and atomic oxygen on the ripening dynamics were investigated in the CdSe QDs system. AFM images show that the ripening of QDs is dramatically accelerated by the deposit of a ZnSe partial capping layer. The driving force of ripening enhancement is attributed to the increasing strain energy with capping thickness. For a ZnSe partial capping layer of below 3 MLs, photoluminescence (PL) exhibits a clear red-shift with increasing ZnSe MLs. It is attributed to the increasing size of the CdSe QDs with the ZnSe partial capping, in a manner that is consistent with the results of the AFM study. On the other hand, the ripening of CdSe QDs can be significantly enhanced by introducing atomic oxygen and was confirmed by a PL study. The atomic oxygens on the surface of ZnSe buffer layer probably change the local chemistry of the ZnSe surface and play the role of the nucleation sites. Therefore, the incorporations of atomic oxygen cause the increasing density of QDs and the decreasing size of CdSe QDs when the CdSe coverage thickness is kept the same. Moreover, we investigated the effect of rapid thermal annealing (RTA) on ripening and band-bending effect for the type-II ZnTe/ZnSe QDs with 3.0 MLs. The self-assembled ZnTe/ZnSe type-II QDs were grown with the SK mode. The PL spectra of samples that were annealed at temperatures of over 400 oC reveal a strong red-shift in the peak energy. This significant red-shift is understood by the QD ripening, the increasing of the dot size is caused by the migration of atoms from the neighboring smaller QDs, activated by RTA process. In a time-resolved PL study, the fast recombination channel in annealed QDs is suppressed, implying that RTA reduces band-bending effect of ZnTe/ZnSe QDs. Finally, studies of the dependences of excitation power demonstrate that the observed reduction in the band-bending effect is attributed to the increase in the dot size upon the RTA process.