Electrical Properties of Polycrystalline MgZnO/ZnO Heterostructure

氧化鋅在光學元件上的應用極具潛力, 不過導電度在元件表現方面的仍是一個關鍵因素。先前研究指出在單晶氧化鋅鎂/氧化鋅異質結構內的極化效應使載子侷限在氧化鋅鎂/氧化鋅之介面，形成二維電子氣體，使得異質介面電性有數量級的改善。 本篇論文主要是探討多晶氧化鋅鎂/氧化鋅異質結構內的極化效應以及極化效應對電性的影響。本實驗之多晶氧化鋅薄膜是由射頻濺鍍方式所成長。成長完後利用高溫退火改善結晶程度，氧化鋅鎂則繼續成長在氧化鋅薄膜上。氧化鋅鎂/氧化鋅介面之電性量測證實了當氧化鋅鎂疊加上氧化鋅後，電性有了兩到三個數量級的改善，證明了即使在多晶系統內，極化效應所產生的載子能藉由屏蔽效應大大降低晶界能障，並大幅改善異質結構的電導率。 除了雙層膜之氧化鋅鎂/氧化鋅異質結構外，本實驗也採用鋁參雜之氧化鋅鎂作為調變摻雜薄膜，引入氧化鋅鎂/氧化鋅鎂摻雜鋁/氧化鋅鎂/氧化鋅之多層膜調變摻雜結構，希望能藉由氧化鋅鎂摻雜鋁之薄膜提供額外載子以注入二維電子侷限區域，更進一步改善電性。實驗結果發現，當作為緩衝層的氧化鋅鎂內含鎂含量較低時，相對雙層膜異質結構，調變摻雜的效果可以再改善接近一個數量級。但隨著緩衝層鎂含量增加，調變摻雜不再有明顯效果。 此性質將能進一步應用於高速薄膜電晶體的發展上。ZnO has shown great potential for application in optoelectronic devices and its conductivity is one of the key issues in device performances. Previous researches show that polarization effect existing in single-crystalline MgZnO/ZnO heterostructure results in carrier confinement at MgZnO/ZnO interface. The two-dimensional electron gas (2DEG) forming at the interface improves the electrical conductance to two to three orders of magnitude. In this thesis, the polarization effect on the electrical roperties of “polycrystalline“ MgZnO/ZnO heterostructure will be studied. ZnO film is grown by rf-sputtering and post-annealing is introduced to reinforce the grain formation. A thin layer of MgZnO is then deposited upon ZnO at room temperature. The electrical conductance of MgZnO/ZnO shows two to three orders of magnitude of improvement compared to ZnO single layer, confirming that polarization effect enhances electrical properties even in polycrystalline system. MgZnO:Al is used as modulation doping layer in the modulation doping structure, MgZnO/MgZnO:Al/MgZnO/ZnO, in the hope to introduce carrier from MgZnO:Al into 2DEG confinement region. The result indicates that modulation doping shows strongest improvement when Mg content of MgZnO buffer layer is low; while the improvement is less obvious as the Mg content increases. The heterostructure can be further applied in high-electron-mobility-transistor