第三章、 實驗方法
4.7 總結
綜合本章節所觀察到 ZnO 於 R 面藍寶石基板之成長行為，沉積 溫度在 600C 以上與以下，其兩者間存在著顯著的差異。
較低的沉積溫度，其具有較高的沉積速率，以基板表面平台的島 狀成核為主，由於結構異向性，隨著沉積量的增加，其晶粒很快的會 沿著 c 軸方向拉長，較低的沉積溫度、較高的沉積速率以及較粗糙的 表面，將導致低溫製程之 ZnO 磊晶沿著
[ 1 1 00 ]
ZnO方向存在著相當程度 的殘留張應變，而在[0001]ZnO方向，受到晶格錯配應力的影響，其呈 現壓應力，不過由於 LT-ZnO/sapphire 界面相對的不完美，其壓應力 仍有些微的釋放。在高溫沉積條件下，ZnO 的成核將傾向於沿著基板的階梯成核，
然而，當基板表面完全被 ZnO 覆蓋後，又會轉成於平坦表面島狀成 核/成長的方式；其成長異向性大約需超過 4000pulses 才會顯現，在 這之前，表面相對平坦，由於較低的沉積速率與較高的沉積溫度，將 導致沿著
[ 1 1 00 ]
ZnO方向存在著殘留壓應變；然而，當其表面的結構異向 性也顯露後，可發現此試片沿著[0001]ZnO軸存在相當程度的彎曲，並 且沿著[ 1 1 00 ]
ZnO方向之殘留應變也轉成張應變的形式。在[0001]ZnO方向122
上，相對於 LT-ZnO，HT-ZnO/sapphire 界面存在著較少的缺陷以及較 完美的接合行為，故在此方向上，ZnO 的晶格趨近於被基板完全應變 的狀態。
值得注意的是，在降溫過程中，不論沉積溫度高低，RHEED 繞 射圖形都會發生明顯的變化，而經由改變降溫速率，也可以發現到較 高的降溫速率下，ZnO 表面會出現沿著
[ 1 1 00 ]
ZnO方向之凹痕，光激發 光譜亦可發現到高速降溫試片存在著較高的缺陷發光行為，以及較低 的載子復合發光效率；由於 ZnO 與藍寶石在[0001]ZnO方向上存在著 顯著的熱膨脹錯配度，推測此與其熱膨脹係數的差異有關。123
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