低溫氧化物半導體及有機半導體之電晶體及感測器研究

Abstract

軟性電子與顯示器的開發在近年來是新世代在電子領域之研究的重要方針， 目前具有潛力的電晶體如有機半導體電晶體與無機氧化物半導體電晶體，兩者相 較於傳統非晶矽(a-Si)電晶體元件有以下幾項優勢，如較低的製程成本及較低製 程溫度(<200℃)，且易於大面積液態製程，所以近年來被許多專家學者廣泛的研 究及探討。然而，後者的載子遷移率相較於前者具有更好的表現，所以在應用層 面上較為廣泛如在高頻電路上等。 首先，在論文第二章節裡，提出一個簡易的製程技術，在元件的通道做處理， 製作出許多的短通道，且利用電漿處理大幅提通道與通道間的載子濃度，進而改 善元件的載子遷移率從4 到約80 cm2 /V s。相較於傳統的銦化鎵氧化物半導體， 大多數的研究都是利用電漿處理元件的源極及汲極介面，以去降低元件的接觸電 阻，進而提升元件的特性，且一般傳統的載子遷移率往往都是小於40 cm2 /V s， 除了一些研究團隊利用一些特殊的技術，如雙層的氧化物半導體層(IGZO/ITO)， 或是利用高介電常數(Ta2O5)的絕緣層，去提升元件的載子遷移率為60~100 cm2 /V s。在此，我們的研究中也討論到奈米點摻雜的數量以及電漿處理的時間都會 影響到元件的特性。由於元件目前製程尚未把元件封裝，且通道孔洞化的結構 受到外界水、氧分子的影響使得元件的穩定性較差，未來我們會覆蓋保護層 (passivation layer)於元件之通到處，且期望搭配一些規則陣列的圖案取代現有的 製程技術，以達到元件的最佳特性。在本論文中，我們也討論到了有機光(章節 三、四)和氣體感測器。此兩種光的偵測器，如光偵測器二極體及光偵測器電晶 體，在章節三提到的光偵測器二極體中，我們首先發現，垂直式元件於垂直方向 的載子遷移率會影響元件對光偵測的頻寬的大小，在此我們利用富勒烯(C60)當 作光偵測器的受體(acceptor layer)，並改變元件在垂直方向的電洞之載子遷移率 從2.3×10−5 到 2.8×10−4 cm2 /Vs，可以提升元件偵測的頻寬從10 到80 MHz(元 件的操作偏壓為4 伏特)。本論文在第三章節有詳細的討論。另外，在章節四提 到的垂直式光電晶體上，為了方便光載子的拆解，我們將我們現有的(p-type)垂 直式電晶體中摻雜少許的(n-type)材料，且在照光下的反應，我們發現外部量子 效率(EQE)可以提升至360%在620 奈米下。在此，我們推斷為元件在照光下屏 蔽了基極的控制所以大幅提升EQE，且本論文在第四章節有進一步的討論 。最 後，在氣體感測器上，在本論文第五章節裡有討論到利用水平式並五苯環 (pentacene)有機薄膜電晶體當作氨氣感測器，我們在此利用紫外光(UV)處理元件的絕緣層(PMMA)，為了去修飾絕緣層表面的形成氫氧官能基進而提升元件的反 應靈敏度，且在有處裡的絕緣層最低可以偵測到的氨氣濃度為0.5ppm。此偵測 的範圍剛好可以判斷一般病人是否患有肝病(健康病人呼出的氨氣含量為 0.745ppm；肝硬化病人呼出的氨氣含量為0.278ppm)。因此非侵入式有機電晶體 當氣體感測器在未來電子產品的發展是相當具有潛力的。本論文最後章節為垂直 式有機電晶體的開發，此項工作為本人與趙宇強博士、陳俊宇博士、姜鈞銘、姜 淑玲、古明哲共同完成，垂直式有機電晶體相較於傳統的水平結構電晶體有幾項 優勢：(1)可以不利用到黃光微影技術就能使通道長度縮至小於1(微米)等級，(2)低的操作電壓，(3)高輸出電流。在此章節裡，我們有改變不同的半導體材料製作不同的空間電荷限制垂直式電晶體，並探討其工作原理與元件特性。在我們的研究之中，高分子垂直式電晶體具有較好的操作特性，如其操作電壓在為1 伏特下最大輸出電流為0.14 mA/cm2，元件的開關比為24310，電流增益為104 個數量級，另外我們的半導體層也有嘗試利用蒸鍍的方式成膜，但得到的成果不盡理想，再本論文中會再加以討論其原因。

The development of flexible electronics and displays are crucial for nextgeneration electronics. Metal-oxide thin-film transistors (TFTs) and organic TFTshave higher potential than a-Si TFTs because of the following advantages：1) low-cost;2) solution fabrication process on a large area array system; 3) low fabrication temperature (< 200 ℃); and 4) the applications are on a flexible substrate.

In the Chapter 2 of this dissertation, we proposed a high performance amorphous In-Ga-Zn-O (a-IGZO) thin film transistor with nano-dot doping (NDD).The nano dot structure creates many short channel and high concentration region in the channel region. Also, the performance of a-IGZO TFT can be controlled by the dots concentration and the Ar plasma treatment time. The proposed nano-dot doping reduces the effective channel length, lowers down the energy barrier, and enhances the effective field-effect mobility 19 times larger than that of the control sample without NDD. The proposed device is promising for the development of low-cost, lithography-free, and high-performance flexible electronics. In the Chapter 3 and Chapter 4 of the dissertation, we introduce the two organic photodetectors, including photodiode and phototransistor. In the Chapter 3, bilayered organic photo diodes with controllable carrier mobility in the p-type donor are demonstrated. Vertical carrier mobility of pentacene is demonstrated to be increased while the deposition rate is high. With C60 as the acceptor material and pentacene deposited at high deposition rate as donor material, the detection of 80-MHz signals is demonstrated. Using high mobility pentacene in stead of poly(3-hexylthiophene) (P3HT) to serve as the hole transport layer effectively improves the hole mobility over 1 orders and hence enlarges the operation bandwidth. The results enable the development of low-cost large-area organic image sheets for the detection of high-frequency signals. In the Chapter 4, we introduce a vertical polymer phototransistor with low operational voltage (-1.5 V). A blended polymer layer with both acceptor and donor materials was used as a IV channel material in the vertical space-charge-limited transistor (SCLT). Under illumination, we obtained external quantum efficiency (EQE) as high as 360% at 620nm. We propose the effects of base-field shielding as a means to explain high EQE. This proposition has been supported by two-dimensional simulation of the device. Moreover, we also study the pentacene-based organic thin film transistor (OTFT) as ammonia sensor in the Chapter 5 of this dissertation. Non-invasive ammonia sensors are attractive alternatives for the diagnoses of a variety of chronic diseases such as liver cirrhosis and renal failure. A low cost pentacene-based organic thin film transistor (OTFT) fabricated by a novel and simple process was demonstrated to be highly sensitive and specific for ammonia gas. Various measurement parameters that reflected OTFT device characteristics for ammonia detection were investigated. Significant variations of the turn-on current, intrinsic mobility, and threshold voltage (Vth) were observed while subthreshold swing (S.S.) was almost unchanged to the alteration of ammonia concentration. The OTFT device detected 0.5~5 ppm concentration ammonia gas at room temperature, which is in the critical range that can distinguish between healthy person and patients with liver cirrhosis and renal failure. The sensitivity of the device was further enhanced following a simple UV irradiation treatment to modify the functional groups on poly(methyl methacrylate) (PMMA) dielectric layer. Possible interference for ammonia detection such as humidity effect and selectivity among nitrogen, alcohol, carbon dioxide, acetone, methane and ammonia were also examined. We concluded that the proposed pentacene-based OTFT is a promising device for the future application in non-invasive medical diagnoses. In the Chapter 6 of this dissertation, an organic vertical transistor is investigated. In this work, I was cooperating with Dr. Yu-Chiang Chao, Dr. Chun-Yu Chen, Shu-Ling Jiang, Chun-Ming Chiang, and Ming-Che Ku. In the Chapter 6, the operation mechanism of vertical transistor is introduced. The turn-on current, on-off current ratio, current gain, and operation voltage of P3HT-based vertical transistor is 0.14 mA/cm2, 24310, 104, and 1 V. In order to improve the controllability and solve the accumulation of polystyrene spheres problems, the nano-imprint technology is used to replace the polystyrene spheres fabrication process and to from the grid structure.