高效能有機薄膜電晶體與其在感測元件上之運用研究

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國

立

交

通

大

學

光電工程研究所

博

士

論

文

高效能有機薄膜電晶體與其在

感測元件上之運用研究

Study on High Performance Organic Thin Film Transistor

and its Application on Sensor Devices

研究生：顏國錫

指導教授：冉曉雯博士

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高效能有機薄膜電晶體與其在

感測元件上之運用研究

Study on High Performance Organic Thin Film Transistor and

its Application on Sensor Devices

研究生：顏國錫 Student：Kuo-Hsi Yen

指導教授：冉曉雯 Advisor：Hsiao-Wen Zan

國立交通大學

光電工程研究所

博士論文

A Thesis Submitted to Institute of Electro-Optical Engineering

National Chiao Tung University in partial Fulfillment of the Requirements for the Degree of Ph.D. in Electro-Optical Engineering

December 2008

Hsinchu, Taiwan, Republic of China

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高效能有機薄膜電晶體與其在感測元件上之運用研究

學生：顏國錫指導教授：冉曉雯博士國立交通大學光電工程研究所博士班摘要本論文提出一種可製作出高效能有機薄膜晶體的介電材料。我們利用射頻濺鍍法沉積低溫氮化鋁薄膜，該低溫沉積的氮化鋁薄膜具有相當高的輸水特性，與並五苯有機薄膜有相近的表面能特性。在研究中，我們首先調變沉積溫度來降低氮化鋁的表面粗操度與結晶率，當氮化鋁的表面粗操度與結晶率隨沉積溫度而降低時，氮化鋁的介電層漏電流、在元件的操作區間內可降低到 10-9 A/cm2 的水準，且介電層厚度也可以近一步降低到 100 奈米以下。我們亦嚐試調控濺鍍時的氬氣與氮氣混合比率來近一步降低氮化鋁介電層漏電並提升可靠度。研究中發現，較高氮氣的比率可以降低漏電流，且我們進一步發現一個可能與氮空缺相關的缺陷分佈將往深層能階移動，這個近似 Poole-Frenkel 的缺陷態一但位於較深的能階，則氮化鋁的漏電則可以進一步獲得控制。在掌握了氮化鋁的介電特性之後，我們在該低溫介電層上進行有機薄膜電晶體的製作，我們所製作的氮化鋁有機薄膜電晶體可以操作在相當低的電壓（小於 5V），但具有相當高的場效載子漂移率（大於 1.6 cm2 /V-sec）與相當優良的次臨界擺福（小於 0.2 V/decade），與國際上有機薄膜電晶體的領先研究團隊的成果相當。另一方面，我們也利用有機薄膜電晶體作為光與氨氣體的感測器。在有機薄膜光偵測器的研究中，我們嘗試用紫外光來改變介面態、來影響元件對光的響應。我們發現存在於有機薄膜與介電層間的帶電缺陷態可能有助於提升對光的響應，在光激發下有助於提升光電流生成而在光激發除後將會延長元件回覆時間。在實驗中所獲得的有機薄膜光感測器

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IV 的響應可高達 10 安培每瓦（A/W），與目前所知的高光響應有機電晶體相當。在有機薄膜氣體感測中，我們初步地研究了氨氣與有機薄電晶體的反應。我們發現提高環境氨氣濃度將會降低電晶體輸出電流並提高元件臨界電壓，並討論金屬接面端與有機薄膜本身在氨氣環境下的電阻變化。我們亦發現元件的尺度與通道比例可能是影響氣體感測靈敏度的一個因素。最後我們提出一種新穎垂直通道的電晶體結構，並研究改善該新穎元件的關閉區域漏電流並提升元件開關比例的方式。

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Study on High Performance Organic Thin Film Transistor

and its Application on Sensor Devices

Student: Kuo-Hsi Yen Advisor: Hsiao-Wen Zan Degree of Ph.D. in Electro-Optical Engineering

Abstract

In this thesis, we proposed a dielectric layer for the application of high performance organic thin film transistors (OTFTs). By using the radio frequency (RF) sputtering system, we deposited the alumni nitride (AlN) film as the dielectric layer under a very low temperature. The low-temperature deposited AlN film is highly hydrophobic and its surface energy is similar to that in pentacene film. In our study, we varied the AlN film deposition temperature to lower the AlN film surface roughness and suppress its crystallization. When the surface roughness and the crystallization decreased with the lowering of deposition temperature, the dielectric leakage current of AlN film can be as low as 10-9A/cm2 when the devices were operated and biased. The AlN dielectric thickness can also be reduced to less than 100nm. Furthermore, we also adjusted the argon (Ar) and nitrogen (N2) ratio during the AlN film sputtering to lower the

dielectric leakage and to increase the AlN film reliability. It was also found that higher N2 ratio in sputtering process may lower the AlN dielectric leakage. A nitrogen related

vacancy defect may also distribute toward a deeper energy level under higher nitrogen ratio. When the Poole-Frenkel liked defect distribution is situated on deep energy level, which helped to further decrease the AlN dielectric leakage. After we gained the experiments of AlN dielectric leakage control, we fabricated the OTFTs on the AlN dielectric layer. The fabricated OTFT with AlN dielectric layer (AlN-OTFTs) can be operated under a low voltage (less than 5V) with high field effect mobility (more than 1.6cm2/V-sec), and its subthresold swing is still good (less than 0.2V/decade). Besides

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the development of high performance AlN-OTFTs, we also applied the OTFTs to act as optical and gas sensors. In the study of optical OTFT sensors, we used the ultra violent light (UV-light) to modify the interface states, which may influence the device optical response. It was also found that the charged defect states between the organic film and dielectric layer may help to increase the photo-responsivity in optical OTFT sensors. That will enhance photo-current generation under illumination and prolong the device recovering time when the illumination was removed. The observed photo-responsivity in our organic photo detector can be as high as 10 A/W, which value was similar to that in high performance organic photo detector. In the study of organic thin film gas sensors, we studied the interaction between NH3 and OTFTs

primitively. It was found that the OTFT output current will be reduced and the threshold voltage will be increased with the increasing of NH3 concentration. The

contact resistance between metal electrode/organic interface and channel resistance were also discussed under different NH3 concentration. The device geometry and

channel length may be important factors that influenced the sensitivity of organic gas sensor. Finally, we proposed an novel vertical channel OTFTs. We studied the device leakage properties and improved device leakage current in the device off state region.

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VII 誌謝什麼是人生最大的快樂？對我而言，就是能做自己想做的事，從一無所知到小有收穫；而什麼是人生最大的幸福？對我而言，就是遇到再大的困難與挫折時，身邊永遠都有可信可愛的家人、老師、與朋友，陪著我一起走過這些難忘的歲月。在博士班的日子裡，最感謝我的父母，給我ㄧ個安穩且無後顧之憂的環境，讓我一無返顧、無止盡的追求學問與解答，並且時時提醒我要照顧好身體。感謝我的指導教授－冉曉雯老師，以最大的信任、耐心、與關懷，不斷的給我機會，讓我進步並受益滿懷，並讓我到世界各地與傑出的研究人員互動。感謝蔡娟娟老師，深刻的引導我一窺業界深厚的研究經驗，給我機會到國外進行研究交流。實驗室的學弟妹們，有你們日以繼夜的努力，才會有今天實驗室的規模！更感謝幾位實驗室剛成立時，最辛苦的幾位伙伴：傑斌、溥寬、睿志、文馨、與俊傑等，你們的付出我們才有許多振奮人心的研究成果。還要感謝其他協力實驗室的夥伴們：勳哥、小銘、宏澤、明達、坤益、貓貓、小白、以及工研院的夥伴們。給我許許多多技術上與儀器上的資助，我們的論文才得以順利進行。當然，還有中正大學的老朋友們，這幾年在新竹與我共同走過。以及，曾經與我一起熬夜患難的朋友，感謝你們！一個學位的完成，代表人生一個階段的結束與開始。此時此刻，滿足與感恩，是完成這著作帶來的最大感動。願您也能接受到，我們的感動！

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Figure Caption

Chapter 1

Fig. 1-1. Four fundamental OTFT structures with different electrode and gate

configuration………2

Fig. 1-2. Pentacene chemical and crystalline structures……….3

Fig. 1-3. Three types of growth mode in conventional molecular beam deposition………..4

Fig. 1-4. The solution-based process of fabricating OTFT dielectric and organic film………...8

Fig. 1-5. The inject-printing system and its printing head………10

Fig. 1-6. The surface treatment on dielectric for OTFT fabrication……….13

Chapter 2 Fig. 2-1. Bonding and anti-bonding molecular orbit energy……….19

Fig. 2-2. Bonding and anti-bonding molecular orbits………...20

Fig. 2-3. σ bonding, π bonding and δ bonding molecular orbits………...21

Fig. 2-4. Band diagram of extended states and mid-gap states……….22

Fig. 2-5. Tunneling model for organic transport………...25

Fig. 2-6. HMDS, OTS, ODS for surface treatment in OTFTs………..33

Fig. 2-7. Polymer dielectrics for OTFTs………...34

Fig. 2-8. Effects of polar dielectric result in local disorder and DOS extension...35

Fig. 2-9. Energy level of singlet and triplet states……….……36

Fig. 2-10. Three types of excitons inside the crystalline materials………….……37

Fig. 2-11. Defect related optical transitions……….……...43

Fig. 2-12. Influence of local energy (potential) fluctuation on DOS……….…….43

Fig. 2-13. DOS of localized states and its energy distribution………….………..44

Chapter 4 Fig. 4-3-1. The leakage current of the Au-AlN-Si structure as a function of the electrical field and gate voltage……….……….61

Fig. 4-3-2. (a) The transfer characteristics of OTFTs with 150℃, 200℃, and 250℃ AlN gate dielectric………..61

Fig. 4-3-2. (b) The output characteristic of the OTFTs with 150℃ AlN gate dielectric……….62

Fig. 4-3-3. The field effect mobility of OTFTs with 150℃, 200℃, and 250℃ AlN gate dielectric……….62 Fig. 4-3-4. The AFM images of AlN films deposited at 250℃, 200℃, and 150℃

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grown on the 250℃, 200℃, and 150℃ AlN dielectrics are shown in Fig. 4(d), 4(e), and 4(f)………...63 Fig. 4-3-5. (a) The AFM image of room temperature deposited AlN film surface, and (b) is the SEM image of AlN dielectric profile……….……..64 Fig. 4-3-6. The x-ray diffraction spectrum of AlN film deposited on the Si wafer

under different deposition temperature………..65 Fig. 4-3-7. (a) The room temperature AlN dielectric leakage is plotted as a function

of electric field and operation voltage, and (b) is the transfer characteristic of AlN-OTFT, which was fabricated on the room temperature AlN dielectric……….66 Fig. 4-4-1. (a) The x-ray photoemission spectrum (XPS) of three distinct AlN

samples, the AlN samples were sputtered under different Ar/N2 gas ratio: 2/5, 2/7 and 2/9, respectively……….71 Fig. 4-4-2. (a) The leakage current as a function of electric field in log(J)-log(E)

scale and (b) in ln(J/E)-(E)1/2……….72 Fig 4-4-3. The Poole-Frenkel plots under different temperatures and electric

field……….73 Fig. 4-4-4. The output characteristic of the OTFTs with AlN gate dielectric……...74 Fig. 4-4-5. (a) The transfer characteristics of AlN-OTFTs under different drain

voltages. (b) The squared drain current is plotted as a function of gate voltage………74 Fig. 4-5-1. (a) The AFM image shows the 150°C AlN film, and the scanning size is

5×5 µm2. (b) The SEM image is the cross-section view of the AlN film on substrate. (c) The leakage current of the Au-AlN-Si structure as a function of the electric field and gate voltage………79 Fig. 4-5-2. (a) The transfer characteristics of OTFTs with AlN gate dielectric. (b) The corresponding output characteristic………80 Fig. 4-5-3. (a) The water, (b) ethylene glycol, and (c) diiodo-methane drops on the

l50°C AlN film for the surface-energy measurement……….81

Chapter 5

Fig. 5-1. The illumination system of light-excitation on OTFTs………..89 Fig. 5-2. The transfer characteristics of standard PMMA-OTFT (opened circle)

and UV-treated PMMA-OTFT (solid square)………90 Fig. 5-3. (a) The absorption spectra of pentacene films on standard PMMA

dielectric and UV-treated PMMA dielectric. (b) The X-ray diffraction spectra of pentacene films on standard PMMA dielectric and UV-treated PMMA dielectric………...91

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Fig. 5-4. The photo responsivity of standard PMMA-OTFT (opened circle) and UV-treated PMMA-OTFT (solid square)………...92 Fig. 5-5. (a) The threshold voltage shift of standard PMMA-OTFT (opened circle)

and UV-treated PMMA-OTFT (solid square) are plotted as a function of time. (b) The photo-induced drain current of standard PMMA-OTFT (opened circle) and UV-treated PMMA-OTFT (solid square) are also plotted as a function of time………...93 Fig. 5-6. The fitted photocurrents (standard and UV-treated) are plotted as a function of time………..94 Fig. 5-7. (a) The rising time constants and the recovering time constants are

plotted as a function of gate-voltage………..95 Fig. 5-8. The optimized structures and dipole moments of standard PMMA

monomer and UV-treated PMMA monomer are plotted as ball and bond type. (Calculated by Gaussian 03, Hartree-Fork method)………….….96 Fig. 5-9. The illustration of UV-treatment induced charged sites on the interface between PMMA dielectric and pentacene film………..97

Chapter 6

Fig. 6-1. (a) The geometry of Gated-four-probed OTFTs, and (b) the photo image of gas sensing chamber………104 Fig. 6-2. Transfer characteristics of Gated-four-probed OTFTs were under varied

concentrations of NH3 gas………105

Fig. 6-3. Normalized turn-on current, threshold voltage shift, subthreshold swing, and intrinsic mobility are as a function of NH3 concentration………106 Fig. 6-4. (a) Pentacene film resistance and contact resistance are plotted as a

function of under different NH3 concentrations. (b) The measured

(L=1200µm) and estimated (L=500µm and 100µm) intrinsic mobility and extrinsic mobility of different channel lengths are plotted as a function of NH3 concentration……….107

Chapter 7

Fig. 7-1. The structure and the process flow of vertical-channel organic thin film transistors with top drain contact and bottom source contact. (TBC-OTFTs)………..………113 Fig. 7-2. Topview microscope images of the TBC-OTFTs. The shapes of

source/drain electrodes are also shown………114 Fig. 7-3. Topview microscope images of the TBC-OTFTs with meshed source

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are also shown……….……….…114 Fig. 7-4. (a) Output characteristics and (b) transfer characteristics of Group-A

TBC-OTFTs……….…….……115 Fig. 7-5. Output characteristics of Group-B devices………..…….115 Fig. 7-6. The ln(ID/VD) v.s. 1/|VD| plots for: (a) Group-A devices with ultra short channel length and (b) Group-B devices with long channel length….116 Fig. 7-7. (a) The output characteristics and (b) the transfer characteristics of

TBC-OTFTs with meshed source electrode……….117 Fig. 7-8. The simulated electric field distribution of: (a) Group-A TBC-OTFTs

and (b) the TBC-OTFTs with meshed source. The gate bias is 20V and the drain bias is -20V, the source is connected to ground………118

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Table Caption

Chapter 4

Table. 4-5-1. Contact-angle measurements and the corresponding surface free energy of the commonly used dielectrics in the OTFTs fabrication…….…….82

Chapter 5

Table 5-1. The comparison of liquid contact angle and surface energy between standard PMMA (non-treated) and UV-trated PMMA………...…89

高效能有機薄膜電晶體與其在感測元件上之運用研究

國

立

交

通

大

學

光電工程研究所

博

博

博

博

士

士

士

士

論

論

論

論

文

文

文

文

高效能有機薄膜電晶體與其在

感測元件上之運用研究

Study on High Performance Organic Thin Film Transistor

and its Application on Sensor Devices

研 究 生：顏國錫

指導教授：冉曉雯 博士

高效能有機薄膜電晶體與其在

感測元件上之運用研究

Study on High Performance Organic Thin Film Transistor and

its Application on Sensor Devices

研 究 生：顏國錫 Student：Kuo-Hsi Yen

指導教授：冉曉雯 Advisor：Hsiao-Wen Zan

國 立 交 通 大 學

光電工程研究所

博 士 論 文

高效能有機薄膜電晶體與其在感測元件上之運用研究

Study on High Performance Organic Thin Film Transistor

and its Application on Sensor Devices

Table of Contents

Figure Caption

Table Caption

研究生：顏國錫

指導教授：冉曉雯博士

研究生：顏國錫 Student：Kuo-Hsi Yen

國立交通大學

博士論文