新穎奈米材料與半導體複合之高效能薄膜電晶體之研究

Studies of high-performance thin-film transistors based on novel nanomaterials and semiconductor composites

戴銘昆(Min-Kun Dai)

指導教授 ： 陳永芳

國立臺灣大學/理學院/物理研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.10235

摘要

在本論文中，我們主要研究的方向為半導體與各式各樣奈米材料所組合而成的複合物電晶體之特性研究及應用。實驗上，我們發現許多複合奈米材料的新奇、有趣特性，使其應用在元件上發揮更優異的特性、應用價值和發展的空間。我們相信結合跨領域的知識及材料，能夠開發出許多高效能固態電子元件，對未來的發展及應用有極大的幫助。本論文共包含四大主題，其摘要如下:

1.藉由向列型液晶介面修飾之高效能聚噻吩薄膜電晶體
本文以向列相液晶分子對高分子聚合物半導體的聚(3-己基噻吩）(P3HT）的微觀結構及電性影響進行了研究。結果發現在有機電晶體中，向列型液晶分子的引入可以顯著改善P3HT有機薄膜電晶體的性能，提供更好的電性和增強漂移率。經由液晶分子修飾後的P3HT電晶體之載子遷移率相對於純P3HT 電晶體可以提高一個數量級。我們從紫外-可見光吸收頻譜、X-射線繞射和原子力顯微鏡量測來說明電荷的載子遷移率提高的原因是由於P3HT晶體結構在液晶分子修飾之下，P3HT 薄膜可以擁有更高度的分子組織排列和降低缺陷密度。因此，從得到的結果來看，我們對於製造高效能有機薄膜電晶體提供一個相當可行的方法，這對有機電子元件的實際應用鋪陳了相當關鍵的一步。

2.石墨烯量子點、高介電材料與有機薄膜電晶體整合之高效能浮閘非揮發性記憶體
在本研究中，我們藉由具有均勻和良好分散性的石墨烯量子點與高介電材料五氧化二鉭(Ta2O5)的一個整合，應用於浮閘非揮發性記憶體。該石墨烯量子點是經由蠟燭燃燒的煙灰來進行合成。合成後，再將高密度石墨烯量子點嵌入在Ta2O5/PVP 閘極介電層的堆疊結構作為電荷捕捉層。我們對元件施加電壓來進行寫入和擦除功能。製作的有機薄膜記憶體在-10伏特的閘極電壓操作下呈現相當的截止電壓漂移7.3伏特，這是由於電荷通過PVP穿隧層注入至石墨烯量子點中。值得注意的是，此元件在寫入/擦除狀態下可維持超過105秒和可重複超過1000次寫入/擦除操作之良好耐久性。因此，本研究以石墨烯量子點為基底的高效能浮閘非揮發性記憶體之創造為下一代非揮發性記憶體的發展開闢了一條新的路線。

3.石墨烯奈米片和非晶態氧化銦鎵鋅組成的奈米複合材料之透明可撓性高效能無機薄膜電晶體
在本章節中,以石墨烯薄片(GNSs)與非晶態氧化銦鎵鋅氧化物（a-IGZO）的複合材料所製成之透明可撓性高性能薄膜電晶體已被開發。高達 23.8 cm2V-1s-1的電子遷移率已被實現，相較於為純 a-IGZO TFT 的（0.82 cm2V-1s-1）和氫化非晶矽（< 1 cm2V-1s-1）提高約30倍。開/關電流比例保持於106，證明了薄膜電晶體良好的持續性。此外，將透明GNSs/a-IGZO與Ta2O5介電層結合之薄膜電晶體，在反複100次的力學彎曲測試後，GNSs/a-IGZO元件顯示出優異的抗力學彎曲，載子漂移率只有8％的下降。我們的研究結果證實了，石墨烯奈米片不僅在薄膜電晶體的主動層材料中對於傳導途徑的影響發揮了重要作用之外，更增強了此複合材料的力學彎曲穩定性。因此，它為下一代高效能透明可撓性的電子元件之大型應用開發開闢了相當關鍵的一步。

4.由石墨烯奈米片與非晶態銦鎵鋅所集成之高光響應度和持久記憶之光電晶體
在本研究中，發現到二維石墨烯（GNSs）和非晶銦鎵鋅氧化物（a-IGZO）半導體為基底所集成光電晶體，成功地顯示出巨大的光敏感特性和長時間的持久記憶效應。在紫外光的照射下，可得到高達 2.0×107光敏感度，這比純 a-IGZO 元件大了約三個數量級。此外，當關掉紫外光後，GNSs/a-IGZO光電晶體的電流需要超長的時間恢復。其基本機制是由於光產生的電子和電洞在通過GNSs和a-IGZO 接面時因適當的能帶關係產生空間分離和石墨烯的優良的導電性所導致。基於長時間的持久性光電流觀察，此光學儲存元件的操作可作為下一代的全息儲存上之新穎應用。我們的結果對於具有優異性能的氧化物半導體光電元件之發展，提供了一個非常優異的選擇。

關鍵字

向列相液晶 ； 共軛高分子聚合物 ； 石墨烯量子點 ； 高介電係數材料 ； 石墨烯奈米片 ； 非晶態銦鎵鋅氧化物 ； 高感光特性 ； 浮閘式非揮發性記憶體 ； 薄膜電晶體

並列摘要

In this thesis, we have designed, fabricated, and characterized several novel nanocomposites based on semiconductors and nanomaterials. Many intriguing properties have been discovered, which not only open new routes for academic interest, but also should be very useful for the next generation of high-performance solid state electronics. It is believed our studies shown here can serve as a key step for the further development of novel functional optoelectronics. This thesis contains four main topics, and the highlight of our scientific achievement is briefly described as follows.

1. High-performance polythiophene thin-film transistors with nematic liquid crystal modification
The influence of nematic liquid crystal molecules on the electrical properties and microstructure of polymer semiconductors poly(3-hexylthiophene) (P3HT) was investigated. It was found that the introduction of nematic liquid crystal molecules can significantly improve the performance of P3HT thin-film transistors, providing better electrical characteristics and enhanced mobility. The field-effect mobility of the device with liquid crystal modification can be enhanced by up to a factor of ten with respect to that of the pure P3HT device. UV–visible absorption spectroscopy, X-ray diffraction, and atomic force microscopy measurements show that the enhancement of charge-carrier mobility is achieved through a more highly organized morphology and a reduction in the density of traps presents in the P3HT/liquid crystal structure. The results shown here therefore illustrate a high-performance solution-processable thin-film transistors, which is quite feasible and can pave a key step for the practical applications of organic electronic devices.

2. High-performance floating gate non-volatile memory based on a novel integration of graphene quantum dots, high-k Ta2O5 dielectric, and organic thin film transistors
The integration of organic thin-film transistors with uniform and well-dispersed graphene quantum dots (GQDs) in high-k dielectrics for fabrication of floating gate non-volatile memories (FGNMs) was demonstrated. The GQDs were facilely synthesized from the soot of a burning candle. The high-density GQDs embedded in Ta2O5/PVP gate dielectrics with a stacked structure serve as the charge trapping layer. Program and erase functions were performed by applying a voltage across a poly(3-hexylthiophene) (P3HT) element. The fabricated MOS memories exhibit a wide memory threshold voltage shift of 7.3 V under -10 V programming operation because of charge injection through the PVP tunneling layer into the embedded GQDs. Notably, the devices show good charge retention characteristics over 105 s in programmed/erased states and show excellent endurance properties with 1000 cycles of repeated programming/erasing. This study, therefore, puts forth an outstanding alternative method for the creation of high-performance GQDs-based organic FGNMs, which opens up a new route for the development of next-generation non-volatile memories.

3. High-performance transparent and flexible inorganic thin film transistors: a facile integration of graphene nanosheets and amorphous InGaZnO
Transparent and flexible thin film transistors (TFTs) with high performance based on solution processed graphene nanosheets (GNSs)–amorphous indium–gallium–zinc-oxide (a-IGZO) composites have been developed. A high electron mobility of 23.8 cm2V-1 s-1 has been achieved, which is about thirty times higher than those of the pristine a-IGZO TFTs (0.82 cm2V-1 s-1) and hydrogenated amorphous silicon (<1 cm2V-1s-1). The on/off current ratio remains in a high order of 106 demonstrating the sustainability of the TFT devices. In addition, transparent GNSs–a-IGZO TFTs with a Ta2O5 dielectric layer show superior resistance to mechanical bending with the variation of only 8% in mobility after 100 times of repeated cyclic bending compared with the degradation of more than 70% for the pristine a-IGZO device. Our results demonstrate that GNSs not only play an important role in forming a conducting network in the active matrix, but also enhance the mechanical bending stability of GNSs–a-IGZO composites. It therefore paves a key step to develop large-scale applications for next-generation transparent and flexible electronics.

4.Colossal photosensitivity and persistent photoconductivity memory in hybrid graphene nanosheets/amorphous indium–gallium–zinc oxide phototransistors
It is discovered that phototransistors based on the integration of two-dimensional graphene nanosheets (GNSs) and amorphous indium-gallium-zinc-oxide (a-IGZO) semiconductors exhibit a colossal photosensitivity and long life-time persistent memory effects. Under the illumination of UV light (350 nm) with 50 mW/cm2, photosensitivity up to 2.0 × 107 was obtained, which is about three orders of magnitude higher than its pure a-IGZO devices counterpart. Moreover, the GNSs/a-IGZO phototransistor possesses an extremely long life-time for the recovery of the transfer characteristics after switching off the UV light. The photoconductivity can persist for several days with negligible degradation. Additionally, the electron mobility is enhanced by more than ten times after GNSs insertion. The underlying mechanism is attributed to the spatial separation of the photo-generated electrons and holes and charge trapping due to the appropriate band alignment across the interface between the GNSs and a-IGZO and excellent conductivity of GNSs. Based on the discovered long-time persistent photocurrent, we demonstrated the operation of optical memory device, which may lead to the novel application of holographic storage in addition to next-generation flat, flexible, and transparent display devices. Our result therefore provides an outstanding alternative for the future development of solution-processable metal oxide semiconducting optoelectronic devices with unprecedent performances.

並列關鍵字

Nematic liquid crystal ； conjugated polymer ； graphene quantum dots (GQDs) ； high-k dielectric ； graphene nanosheets (GNSs) ； amorphous InGaZnO (a-IGZO) ； colossal photosensitivity ； floating gate non-volatile memories (FGNMs) ； transparent thin-film transistors (TFTs)

參考文獻

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