- 學位論文
運用第一原理計算探討矽摻雜對晶相與非晶相三氧化二銦的結構與電子性質之影響
First-principles Study of the Si-doping Effect on the Structural and Electronic Properties of Crystalline and Amorphous Indium Oxides
摘要
近年，隨顯示器科技朝向高畫素發展，薄膜電晶體(TFTs)中半導體主動層所需要的電子遷移率也提高，傳統使用的非晶相矽(a-Si)因而面臨被新的材料取代。目前，以三氧化二銦為母相的非晶相氧化物，被視為是下個世代顯示器的重要材料，因為三氧化二銦的導帶由5s軌域組成，使其在非晶相中依然保持相當高的電子遷移率，不過三氧化二銦容易形成氧空缺，這些缺陷結構使得薄膜電晶體在正偏壓、負偏壓後，其臨界電壓會產生偏移，使三氧化二銦無法直接應用於薄膜電晶體中，因此科學家透過摻雜鎵(Ga)、鋅(Zn)、矽(Si)…等元素來減少氧空缺濃度，不過目前對於矽摻雜會如何影響三氧化二銦的性質，並沒有深入的了解，因此，本研究透過第一原理計算來探討矽摻雜對三氧化二銦之結構、電子性質、氧空缺生成之影響。 矽摻雜非晶相三氧化二銦之結構，我們透過第一原理計算與分子動態模擬來建立，根據我們所建立的結構，矽摻雜並不影響In-O鍵長及銦原子-銦原子多面體間的連接關係，然而，氧氣氛圍會影響銦原子之配位數，在符合化學劑量平衡下，銦原子以五配位、六配位為主，在氧缺乏氛圍下，則會出現二配位、三配位…等低配位數的銦原子。對其電子性質，我們觀察到在氧缺乏氛圍下，低配位數的銦原子間會補陷電子(trap electron)，形成局域態，我們認為將造成導電度的下降；而在符合化學劑量平衡下，能隙值將隨矽摻雜濃度上升而上升，不過導帶底部依舊保持以銦原子s軌域為主的電子性質。 針對三氧化二銦的氧空缺計算，我們觀察到氧空缺的生成能與其氧原子周圍鍵結的元素有關，只與銦原子鍵結的氧原子之氧空缺生成能並不隨矽摻雜濃度而有所改變，但氧原子若有與矽原子鍵結，其氧空缺生成能將會大幅提升，因此，我們的結果顯示矽摻雜能有效增加非晶相三氧化二銦之氧空缺生成能，然而，對於晶相三氧化二銦，我們的結果顯示矽摻雜並無法增加晶相三氧化二銦之氧空缺生成能。此外，矽摻雜所導致能隙值的上升，使自發性游離氧空缺的比例將從33 %下降至10 %左右，表示矽摻雜能減緩正偏壓所導致之電性不穩定；矽摻雜也穩定氧空缺缺陷能階，使氧空缺缺陷能階下降，更為接近價帶，因此我們認為在負偏壓操作下所產生之游離氧空缺，可更容易回填電子，恢復成中性氧空缺，改善負偏壓操作所導致之電性不穩定。最後，我們計算在不同電子化學位勢能下，氧空缺帶不同電荷量之生成能，矽摻雜使從帶+2價的氧空缺轉換為中性氧空缺的電荷轉換能階(charge transition level)移至導帶下，顯示矽摻雜會使只與銦原子鍵結之氧原子所形成的氧空缺，更傾向維持中性氧空缺，改善氧空缺對臨界電壓偏移之影響。
關鍵字
三氧化二銦；氧空缺；薄膜電晶體並列摘要
Amorphous indium-oxide-based materials have much higher electron mobility than amorphous silicon, making it a strong candidate to replace Si-based materials as the channel material in thin film transistors (TFTs) for future generation high-definition flat-panel displays. However, the instability of the threshold voltage of the indium-oxide channels due to the formation of oxygen vacancies is still a bottleneck that hampers its further applications. One way to solve this problem is to inhibit the formation of the oxygen vacancies by doping with other elements such as Ga, Zn, or Si, but up to now the atomistic mechanisms for how those dopants affect the vacancy formation remain unclear. In this thesis, we have used first-principles calculations in conjunction with molecular dynamics simulations to inverstigate the structure, electronic properties, as well as the oxygen vacancy formation and relevant induced defect states of Si-doped In2O3. For Si-doped amorphous indium oxide, our results show that the inclusion of Si dopants has no effect on both the In-O bond length and the ratio of corner- and edge-sharing In-In pairs. However, in oxygen-deficient (non-stoichiometric) environment, under-coordinated indium atoms appear in heavily doped a-In2O3, which tend to trap electrons and generate localized states. In oxygen-rich (stoichiometric) environment, under-coordinated indium atoms are not observed and the addition of silicon leads to the band gap opening for the indium oxides. Furthermore, We observed that conduction band is still characterized by In-5s orbital in Si-doped a-In2O3. For the analysis of the oxygen vacancy, our results showed that the inclusion of Si dopants can effectively increase the vacancy formation energy in amorphous indium oxide but have no effect on crystalline indium oxide. Furthermore, based on the all oxygen vacancies we analyzed, we noticed that the ratio of spontaneously ionized oxygen vacancies decrease as a-In2O3 is doped with silicon, leading to the improment of positive threshold voltage shift observed under positive gate bias stress. Our calculations also showed that the level of the neutral defect states become deeper with the increase of Si content, thus ionized oxygen vacancy may be prone to recover to its neutral defect level, indicating the instability caused by negative bias illumination stress can be improved. Finally, we calculated formation of oxygen vacancy in different charged states. We observed that ionized oxygen may be prone to recover to its neutral state in Si-doped a-In2O3, indicating the instability caused by negative bias illumination stress can be improved.