透明ITO導體與ZnO半導體薄膜之熱電勢研究

Abstract

在今日透明導電薄膜已廣為被應用於生活中，其中ITO是常見的材料，因為ITO具有高導電率及高透光率的特性，從液晶螢幕面板、觸控式螢幕、太陽能電池電極、、、等，皆能看見其身影。但由於銦含量短缺、價格昂貴，導致人們開始研究其他材料。 ZnO為另一種透明薄膜，由於有許多良好的光電特性：直接能隙、高激子束縛能，且鋅在地球上含量豐富，ZnO價格上當然比ITO便宜；然而想要取代ITO之前，必然要先知道兩者傳輸特性。 本論文透過量測不同退火條件與未退火ITO樣品的熱電勢，觀察其室溫到低溫的變化行為；從實驗結果中發現，經過熱處理的樣品，載子濃度明顯地變小，造成電阻率上升，而未退火的樣品，則是受到本身無序程度，影響碰撞時間的大小。 藉由改變製程中氧氣通量，得到一系列氧缺陷不同的ZnO樣品，量測ZnO的熱電勢，依照熱電勢對溫度圖的趨勢可分為兩類：一種是氧通量較少的樣品，熱電勢在量測區間平滑地往零收斂，低溫區發覺傳輸機制慢慢從導帶電子傳輸轉變成Mott variable-range hopping；另一類則是屬於氧通量較大的樣品，從其熱電勢對溫度圖可發現有劇烈的變化，根據行為趨勢判斷傳輸機制的轉變為：nearest-neighbor hopping到Mott variable-range hopping最後為Efros- Shklovskii variable-range hopping。

The transparent conductive oxide thin films have been applied extensively in our life. ITO is the most common material especially, since it possesses high electrical conductivity and high optical transparency. LCD panels, touch panels and electrodes on solar cell are all made of ITO. But there is lack of Indium and it is very expensive, people is looking for other materials. ZnO is one of the other TCOs. It is with well optoelectronic properties: direct band gap and high exciton binding energy. Zinc is abundant, so ZnO is cheaper than ITO. We have to know the transport mechanism in both of them, before we would like to use ZnO to replace ITO. We measured the thermoelectric powers (TP) of ITO thin films which were annealed or non-annealed from room temperature to low temperature. From the results, the samples after annealing have lower carrier concentrations and higher resistivities. In the non-annealed samples, the collision times are mainly affected by the degrees of disorderness. We had a series of samples with different oxygen vacancies through changing oxygen flux during RF sputtering. Measuring the TP, we classified the ZnO samples into two groups. In one group, we found the TP were smoothly toward zero between 300 K and 10 K, and them changed to Mott VRH type in low-T regime. In the other group, the dramatic changes in TP were observed in low-T. According to the tendency of the TP, we supposed the transport mechanism were from nearest-neighbor hopping to Mott VRH and ES VRH in very low temperature.