氮化鋁於五苯環有機薄膜電晶體上應用之研究

Abstract

在有機半導體材料和元件的領域中，以有機薄膜電晶體的研究為主要的範疇。有機薄膜電晶體有許多優點，例如:低溫製程、低成本與製程簡單、結構的可撓性…等，因此在可撓式顯示器、感應器、RFID上都是很好的應用。為了預防電晶體間的crosstalk現象及降低漏電，必須將有機主動區圖形化，此篇論文之研究，是與周政偉學長所共同進行開發的；而在本論文中，將結合傳統的黃光微影製程開發出新式樣有機元件的圖形化流程，並研究分析當中的反應機制；此外，為開發有機半導體元件製程的穩定度，本論文亦針對在氮化鋁表面累積環境中碳污染所造成元件特性的影響作深入探討。 在此論文當中，我們利用低表面能高介電係數的氮化鋁作為介電層，並搭配氧電漿與傳統的黃光微影調變其表面能。隨即五苯環薄膜將沉積在此氮化鋁表面，經過去離子水浸泡後，在氧電漿處理區域的五苯環薄膜便會剝落，而達到我們圖形化五苯環的目的。由表面化學材料分析中，可以觀察到氮化鋁的表面將被氧化而組成氧化鋁鍵結，這是其表面能上升的主因，而氮化鋁表面與五苯環之間的吸附與水的侵入反應，則因表面極性的差異而改變，水將與極性較高的表面有較大的反應，致使上方的五苯環被剝離，達到了圖形化的目的。 另外，我們觀察到氮化鋁的表面能因環境的碳吸附將會隨著置放時間不同而有所改變，愈長的置放時間其表面能愈低。並且在我們的研究中亦發現，五苯環成長在碳吸附量較大而表面能較低的表面上有較好的元件特性，其元件的載子遷移率隨著置放時間增加而增加；在此研究當中發現，元件置放14天其場效載子遷移率將由0.05 cm2/Vs 增加至 0.67 cm2/Vs。究其原因，是由於表面的碳吸附降低了氮化鋁的表面能，導致pentacene在初期成長的覆蓋率增加，因此改善了載子遷移率。

The study of pentacene based organic thin film transistors (OTFTs) has been a major area of research on organic semiconducting materials and devices. OTFTs have many advantages such as low process temperature, low cost and simple process fabrication, structure flexibility. However, the high operating-voltage remains a limitation on organic transistors. To lowering the operating-voltage, a high-k materials, aluminum nitride (AlN), has been proposed as a low operating-voltage gate-dielectrics in pentacene based OTFTs. This work was studied with my senior classmate, Cheng-Wei Chou. In this thesis, the AlN dielectric application and aging effect in pentacene based OTFTs are reported. To prevent cross-talk between transistors in close proximity and to achieve a low off-current in OTFTs array, it is necessary to pattern the organic semiconductor layer. A pentacene patterning method on the AlN dielectric surface that can be combined with conventional lithography to pattern pentacene film was reported. The AlN surface was patterned using a conventional photo lithography process and then treated with oxygen (O2) plasma on uncovered AlN to modify surface polarity. The surface energy of O2 plasma treated region was increased drastically. Following pentacene deposition, the sample was dipped in water to remove pentacene from the O2 plasma treated area. The polar surface energy was attributed to the increase of Al-O bonds on the surface based on XPS measurements. The enhancement of the polar surface energy explains the water-removable pentacene patterning mechanism. Likewise, the lowered AlN surface energy over time when the AlN film stored aged in a non-vacuum environment was observed. To confirm the AlN aging effect, the relationship between the aged AlN film and the pentacene based OTFTs were demonstrated in this thesis. The lowered surface energy over the aging time was due to the carbon absorption on the AlN surface. Pentacene based OTFTs fabricated on the aged AlN surface showed a higher performance in electrical characteristics. The mobility was enhanced from 0.05 cm2/Vs to 0.67 cm2/Vs with the limited aging time (14 days). This improved mobility was due to the carbon accumulation on the AlN surface, lowers the surface energy and increased the pentacene coverage in the first few layers.