溶液法半導性氧化鋅薄膜製備及其薄膜電晶體特性研究

Abstract

氧化鋅屬於n型透明之寬能隙(wide energy band gap, 3.2 eV)半導體，近來廣泛應用於光電元件及平面顯示器領域。本論文利用溶液法(solution method)成長氧化鋅(zinc oxide；ZnO)半導性薄膜，並對其薄膜電晶體(thin film transistor；TFT)之物理及電性做一系列研究；採用的溶液法包含化學浴鍍著法(chemical bath deposition；CBD)與溶膠-凝膠(sol-gel)法。此外利用陽離子調和(cat ion- mediation)之化學溶液鍍浴方法探討氧化鋅薄膜結晶取向與透光性質，更運用摻雜的原理探討溶液中鎂離子(magnesium ion；Mg2+)對氧化鋅結晶成長的影響，藉由鎂離子在溶液中之干擾作用改變氧化鋅結晶之優選方向(prefer orientation)，並獲得90 %良好透光性氧化鋅薄膜，將能隙(energy band gap)由3.38 eV 降低至3.23 eV。最後利用不同溫度退火的溶膠-凝膠氧化鋅薄膜作為溶液浴鍍著氧化鋅之種子層，研究溶膠-凝膠結晶種子對溶液浴鍍著氧化鋅之結晶取向的影響，並且以其作為主動層的全透明薄膜電晶體(transperent thin film transistors；TTFT)元件之電性研究。 在氧化鋅薄膜特性上，主要探討合成之純氧化鋅的結晶性(crystallization)、光學性質 (optical property)、晶粒大小以及載子濃度(carrier concentration)等物理性質，以及研究氧化鋅薄膜之形貌，結構，晶相與組成的關係。 在薄膜電晶體應用研究中，探討低溫成長之氧化鋅薄膜在沒有退火(annealing)的情況下製作之元件基本電性，其次以溶膠-凝膠法製備不同鎂離子摻雜之氧化鋅薄膜電晶體，分別探討多元氧化物薄膜與其電晶體之物性與電性，研究發現氧化鋅藉由鎂的摻雜可以提高膜厚均勻性與透光度，並且使其薄膜電晶體有較高之電流開關比。 最後利用溶膠-凝膠法與化學溶液浴鍍著法混合成長氧化鋅薄膜，以自體結晶成核的方式，解決離子摻雜效應對電晶體的影響，並提高載子遷移率，增加元件開關速度，同時使用铟錫氧化物(ITO)為閘極、源極、和汲極材料，製作全透明薄膜電晶體元件。針對電性上解決常開效應，並且提高元件開啟時的電流，此元件為常關型(normally off)，或稱增強型(enhancement mode)薄膜電晶體，經元件結構與薄膜成長方法改善後，可獲得較佳之薄膜電晶體元件(電子遷移率為0.647 cm2 V-1 s-1；元件之開關比大於10E7)。

Zinc oxide is an n-type transparent semiconductor with wide band gap of 3.2 eV, which has been extensively applied to photoelectric components and flat panel displays. This thesis was aimed at ZnO thin film preparing by solution method, the properties of the grown ZnO films, and the physical and electrical characteristics of the thin film transistors. The solution methods were including chemical bath deposition (CBD) and sol-gel deposition. In addition, this investigation explored the influence of cation-mediation on the orientation and transmittance of ZnO crystals in chemical bath deposition. Moreover, the doping effect of ZnO crystalline which grown in Mg ion (Mg2+) contained solution was investigated. In particular, the prefer orientation of the ZnO crystalline was changed by the Mg ion mediating in chemical solution. The ZnO crystalline film exhibited good optical transmittance of over 90%. Additionally, the direct band gap value was 3.22eV, which is less than 3.38eV obtained using a non-mediation solution bath. Finally, the ZnO sol-gel films were annealed at different temperatures, which were used as seed layers and nuclei sites for further ZnO films fabrication by chemical bath deposition. Base on the investigation, the influence of sol-gel seed layers on crystallinity and orientation of ZnO by chemical bath deposition were discussed. Also, the electrical characteristics of transparent thin film transistors obtained using this active layer were demonstrated. In the ZnO films study, the physical properties of the ZnO film by solution methods, including the growth mechanism on crystallization, the optical property, the crystal size, and carrier concentration of the ZnO thin film were investigated. In the ZnO thin film transistors (TFTs) study, the electrical characteristics of ZnO thin film transistors by using low temperature solution process without annealing were demonstrated. In addition, the electrical characteristics of ZnO TFTs with Mg doped and without Mg doped by sol-gel method were demonstrated respectively. The A significant improvement of surface roughness with Mg doping could be noticed. In addition, this result was in good agreement with the discussions of higher current on and current off ratio. Finally, the ZnO film were grown by sol-gel and chemical solution bath combination method, to resolve the doping effect on thin film transistors by using the self-crystal as the growing sites for further deposition. Since indium tin oxide was used as the conducting electrodes (source, drain, and gate) for transparent thin film transistors. Current-voltage properties measured through the gate infer that the ZnO channel was n-type enhancement mode device due to a positive gate voltage were required to induce a conducting channel, and the current increases with increasing positive gate bias. The optimum device had field-effect mobility of 0.67 cm2 /V s and an on-off ratio more than 10E7.