臺灣博碩士論文加值系統

本研究使用Materials Studio 8.0軟體進行材料模擬，並採用密度泛函理論加Hubbard U (DFT+U) 方法研究Al/Ga元素雙摻可能出現的位置、Al/Ga不同濃度和不同氧空缺濃度於ZnO，並研究其光電性質。計算結果顯示純ZnO的晶格常數與實驗量測之差小於1 %，且所計算的能隙非常符合實驗值(3.3 eV)。
對於Al/Ga不同雙摻雜不同位置於ZnO時其研究結果顯示，形成能計算結果顯示雙摻結構為Als(Zn)Gas(Zn)是最容易形成，而雙摻結構中有置換氧的則是最不易形成。並從雙摻的晶格常數發現當Al/Ga形成間隙型時造成晶格扭曲並使體積上升，而Al/Ga摻雜位置為置換鋅與間隙型皆會在導帶底產生電子佔據並使光學能隙擴大。製程中在貧氧條件下使置換氧形成機會提高，會在價帶上方產生佔據態，使光學能隙縮小並使紫外光波長下平均穿透率下降。由能態密度圖可得知形成置換氧與間隙型會產生較高的載子濃度，但於能隙中產生深層能級影響其穿透率。相比於不同摻雜位置下AlS(Zn)GaS(Zn)擁有最小的有效質量，且光學能隙擴大且沒有深施體能階生成，紫外光和可見光平均穿透率高達72.3與90.85%，而其它的雙摻位置都造成穿透率下降。
對於Al/Ga不同摻雜濃度形成能結果發現，Al/Ga各摻雜一顆時有著最低的形成能。當Al/Ga摻雜濃度提高後形成能隨之增加，意謂Al/Ga若要以置換鋅的位置存在於ZnO會顯得更為困難。優化結構中提升Ga濃度其晶格常數a與c軸上升，造成其體積增加。若是增加Al濃度則會使a軸下降造成體積縮小。Al-O的鍵長會比Ga-O的鍵長來的小，從Atomic population中得知Al所丟出的電子比Ga來得多，且Al-O的Bond population比Ga-O還來的大，這說明了置換Al會比置換Ga來得更顯共價鍵特性。從DOS中得知Ga濃度的提升比其Al更有助於載子濃度的提升，於相同摻雜濃度下Ga元素是主要自由載子來源。隨著摻雜濃度的提升，施體能階的能量範圍擴大、貢獻增高，光學能隙值也隨之增大造成本質吸收邊產生藍位移，提高了UV波長下的穿透率。當Al/Ga濃度增加時於導帶底部電子佔據態增加，造成近紅外光波長下的吸收產生。
對於不同氧空缺濃度存在於AGZO對光電性質的影響，從形成能結果顯示若要減少AGZO中的氧空缺，在製程時於富氧的條件下較容易達成。氧空缺的存在使得晶格常數a與c軸下降並使體積縮小。氧空缺濃度升高會使鋅所丟出的電子減少，但氧所接收到的電子增加，並造成導帶上的未佔據態形狀改變。靠近費米能階部分空帶明顯增多，且對淺層施體能階的吸收有明顯幫助。氧空缺濃度提升會使載子濃度的主要貢獻由淺層轉移至深層。在光學性質上AGZO中氧空缺的出現造成紫外光與可見光下的平均穿透率下降。實驗中也發現同樣現象，當退火時通入氧壓增加，載子濃度隨之下降並造成電阻率的上升。而氧空缺的增加使空帶數目變多，造成長波長下的穿透率下降。除了對計算的光電性質分析外我們也與實驗上的文獻做比較，並提供不同氧空缺濃度存在AGZO下的性質做為一個參考的指標。

Using density functional theory and the Hubbard U (DFT+U) method, we investigated the doped sites and doped ratio of Al/Ga in ZnO. Also, considering the different concentrations of oxygen vacancy defect, we studied the electronic structure and optical properties of AGZO. The difference in lattice constants between the calculated results and experimental measurements is within 1%, and the calculated band gap of pure ZnO is in excellent agreement with experimental values (3.3 eV).
The calculated formation energy for the Al/Ga doped site shows that Als(Zn)Gas(Zn) is most likely to form, while the substitution of oxygen is most difficult. The lattice consistant became distortion and volume expansion when Al/Ga occurs on the interstitial site. The Al/Ga-substituted Zn atoms or interstitial sites in ZnO, which contain electrons occupying an energy level below the conduction band, enlarge the optical band gap. The possibility for Al/Ga substitution for oxygen will increase while processed under oxygen-poor condition. In addition, the electrons occupying an energy level above the valance band lead to narrowing the optical band gap and decreased average transmittance under UV wavelength. The substituted oxygen and interstitial sites have a higher carrier concentration than substituted znic, but the deep occupy state causes reduction in transmittance. Al/Ga-substituted zinc has the smallest effective mass than other doped site, further expanding the optical band gap and having no occupied state in the band gap, and thus having UV and visible wavelength average transmittance of 72.3% and 90.85%, respectively. However, oxygen substitution and interstitial site formation reduces the average transmittance.
The Al/Ga-substituted zinc has the lowest formation energy; the formation energy increases with increase in the doping concentration, which indicates that the formation of Al/Ga-substituted zinc becomes more difficult. The a-axis and c-axis increases with increase in Ga doping concentration, which expands the cell volume. Increase in Al doping concentration causes the c-axis to increase but the a-axis to decrease, which narrows the cell volume. The Al-O bond length is shorter than the Ga-O bond length. Furthermore, the Al atoms throw more electrons than the Ga atoms and the bond population of Al-O is larger than that of Ga-O, which indicates that Al-substituted zinc has more covalent bond behavior. The calculated carrier concentration shows that Ga contributes more free carriers than Al for the same total doped concentration. The shallow donor state energy range expands and the optical band gap enlarges with increased doping concentration. However, the UV wavelength average transmittance increased because of the absorption edge blue shift. In addition, increase in Al/Ga doping concentration expands the electron-occupied state below conduction band, leading to absorption under NIR wavelength.
The formation energy of different oxygen vacancy concentrations in AGZO shows that it easily reduces oxygen vacancy under oxygen-rich ambience. Oxygen vacancy leads to decrease in the a-axis and c-axis and thus narrows the cell volume. Increase in oxygen vacancy concentration make Znic throwed electrons decrease and enhance the receive electrons of Oxygen. Moreover, it enhances the unoccupied state near Fermi level; this phenomenon improves the absorption for shallow donor state. The carrier concentration transfer from shallow to deep donor state increases with increase in concentration of oxygen vacancy. Generation of oxygen vacancy leads to decrease in UV and visible wavelength average transmittance. We found the same effect in the experiment. While decrease the carrier concentration by increase in oxygen pressure during annealing process, the long wavelength transmittance decreases, because of the enhancement in empty bands. These results can theoretically explain the factors that influence the electrical and optical properties.

明志科技大學碩士學位論文指導教授推薦書 i
明志科技大學碩士學位論文口試委員審定書 ii
誌謝 iii
摘要 iv
英文摘要 vi
目錄 ix
表目錄 xii
圖目錄 xiii
第一章 緒論 1
1-1 研究背景 1
1-2文獻回顧 3
1-2-1 氧化鋅之結構與性質 3
1-2-2 Al/Ga氧化鋅實驗與理論計算文獻-摻雜位置 4
1-2-3 Al/Ga氧化鋅實驗與理論計算文獻-摻雜濃度 5
1-2-4 Al/Ga氧化鋅實驗文獻-本質缺陷 7
1-3研究動機 9
第二章 理論基礎 10
2-1 薛丁格方程式 10
2-2 Density Functional Theory 11
2-3 Kohn-Sham方法 12
2-4 局部密度和廣義梯度近似法 13
2-5 Self-consistent過程 14
2-6 計算參數選取 14
2-7 透明導電之原理 15
2-7-1 導電率 15
2-7-2 光學特性 15
2-8 形成能計算式 16
第三章 實驗方法與步驟 21
3-1 實驗設備 21
3-1-1磁控濺鍍原理 21
3-1-2射頻磁控濺鍍(radio frequency magnetron sputtering) 21
3-1-3熱退火爐 22
3-2 分析儀器 22
3-2-1電子微碳分析儀(EPMA) 22
3-2-2電子能譜化學分析儀(ESCA) 22
3-2-3 X光繞射儀(XRD) 23
3-2-4 霍爾效應量測儀(Hall effect measurement) 23
3-2-5 紫外光-可見光-紅外光分光光譜儀(UV/VIS/NIR spectrometers ) 24
3-2-6薄膜輪廓儀 (α-step) 25
3-3 實驗流程與步驟 26
3-3-1實驗流程 26
3-3-2基板準備 27
3-3-3靶材選取與薄膜鍍製 27
第四章 結果與討論 29
4-1 Al/Ga位置共摻雜於氧化鋅 29
4-1-1 形成能 29
4-1-2 晶體結構、鍵長、電荷密度 30
4-1-3 能帶結構與能態密度 32
4-1-4 光學性質分析 35
4-2 Al/Ga不同濃度摻雜於氧化鋅 37
4-2-1形成能、晶體結構、鍵長、電荷密度 37
4-2-2 能帶結構與能態密度 39
4-2-3 光學性質分析 40
4-3 實驗與計算不同氧空缺濃度於Al/Ga共摻雜氧化鋅 42
4-3-1 形成能 42
4-3-2 結構優化 42
4-3-3 能帶結構與能態密度 43
4-3-4 光學性質分析 45
4-3-4 AGZO薄膜實驗分析 45
第五章 結論 97
參考文獻 99

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