低高低能隙結構之氫化微晶矽薄膜電晶體

Abstract

本論文中，我們利用自行設計組裝之電漿輔助化學氣相沉積(PECVD)系統沉積的高品質氫化微晶矽薄膜和各種薄膜來製作底閘極的低高低能隙結構之氫化微晶矽薄膜電晶體，研究氫化非晶矽(a-Si:H)與氫化微晶矽(mc-Si:H)薄膜的光能隙大小及結晶化程度與氫氣¤矽甲烷流量比(H2/SiH4)的關係，以及其應用在薄膜電晶體的所有電性表現，並以氫氣、氮氣、氨氣等電漿鈍化處理研究鈍化的機制與穩定性，再以偏壓應變實驗證實低能隙氫化微晶矽通道層與高能隙氫化非晶矽垂直偏移層兩者組合，對於元件穩定性的貢獻，並說明此低高低能隙結構之氫化微晶矽薄膜電晶體應用於主動式陣列液晶顯示器的可行性。 根據實驗數據得知，我們的PECVD系統可以在氫氣¤矽甲烷流量比(H2/SiH4)於39~87.5的範圍內，成功地沉積出可以做為通道層的氫化微晶矽(mc-Si:H)薄膜，而在其它的氫氣¤矽甲烷流量比範圍，則可以沉積出氫化非晶矽(a-Si:H)薄膜。我們目前研製出的低高低能隙結構之氫化微晶矽薄膜電晶體的元件，已經改善了傳統非晶矽薄膜電晶體的低電流驅動能力、低載子移動率…等元件特性，卻仍然可以維持良好的元件關閉狀態，亦即維持了傳統非晶矽薄膜電晶體的低關閉電流，因此可以有效地提高開/關電流比。 由於在關閉狀態下的傳導載子受到高能隙垂直偏移層的高能隙能帶阻擋，使得元件在關閉狀態下因穿透能障變高而不易發生能帶到能帶穿透傳輸(band to band tunneling)，亦即降低閘極感應之汲極漏電流，以達到降低元件關閉電流Ioff的目的。另一方面，以高品質之低能隙氫化微晶矽薄膜作為通道層，由於低能隙氫化微晶矽薄膜具有較高的傳導率以及結晶化程度較大的薄膜結構，因此可以有效地提升元件的導通電流及載子移動率。 在低高低能隙結構中，因為氫化非晶矽/氫化微晶矽(a-Si:H/mc-Si:H)薄膜組合而成的通道層/偏移層及其介面，當元件承受較小的負偏壓應變時，高能隙高阻抗之非晶矽(a-Si:H)薄膜中脆弱的矽-氫鍵容易被打斷，而被打斷鍵結的氫離子(H+)被負電壓吸引至高品質低能隙之氫化微晶矽通道層中，補償了氫化微晶矽薄膜中的懸浮鍵以及因偏壓應變所產生的缺陷狀態，改善了元件的偏壓應變穩定性，我們稱這種效應為”氫原子補償效應” (H-atom compensation effect)，但是在更大的負偏壓應變時，由於更多缺陷狀態的產生導致這個效應較不明顯。在正偏壓應變方面，由於選用較高傳導率、較高結晶程度之低能隙氫化微晶矽做為通道層，因此元件因正偏壓應變所引起的次臨限振幅與臨限電壓變動值，均低於傳統非晶矽薄膜電晶體，而且其變動值趨於飽和。相對於傳統非晶矽薄膜電晶體而言，低高低能隙結構之氫化微晶矽薄膜電晶體具有較好的偏壓應變穩定性。

The characteristics of hydrogenated microcrystalline silicon thin-film transistors (TFTs) with low-high-low band gap structures have been investigated. The various thin films including high-quality hydrogenated microcrystalline silicon (mc-Si:H) thin film are deposited by homemade PECVD system. Hydrogenated amorphous/microcrystalline silicon thin films with various H2/SiH4 flow rate ratios are deposited to study their optical band gap and crystalline fraction, and then TFT devices are fabricated to investigated its device characteristics. It is found that the stress-bias-induced degradations are significantly improved for new devices. Bias-stress-induced phenomena in thin-film transistors with low-high-low band gap structure are also studied. The new TFTs with low-high-low band gap structure thus appear to be a promising candidate for application to high-resolution active-matrix liquid-crystal displays (AMLCDs). Within the range of H2/SiH4 flow ratio from 39 to 87.5, our PECVD system can deposit mc-Si:H films successfully. And out of that range, our PECVD system only deposit a-Si:H films. When compared to a-Si:H TFT with conventional inverted-stagger structure, the TFT device parameters such as field effect mobility, threshold voltage, subthreshold swing and ON-current can be significantly improved. The high-band-gap a-Si:H offset layer, which blocks the carrier conduction during OFF-state operation of the device, is used to prevent the band-to-band tunneling, and thus reducing the high OFF-current normally observed in conventional thin-film transistors. The proposed structure also employs a low-band-gap higher crystalline fraction mc-Si:H layer as its channel to improve the ON-state current and field-effect mobility. It is found that the stress-bias-induced degradations are significantly improved for the new devices. An interesting bias-stress-induced phenomenon has also been observed. Namely, an improvement in the subthreshold slope of the new devices is found after negative-bias stressing. A new mechanism, which we called “H-atom compensation”, is proposed. This mechanism results from the breaking of weak bonds in the a-Si:H offset layer. The released H+ atoms could be injected into the channel and serve to reduce the bias-stress-generated state located on the gate insulator/channel interface under negative bias stress, thus improving the switching operation for TFT. Abstract (Chinese) .........i Abstract (English) .........iii Acknowledgements ..........v Contents ..........vi Table Captions .........ix Figure Captions .........x Chapter 1 Introduction .........1 1.1 Thesis Outline .............1 Chapter 2 Plasma-Enhanced Chemical Vapor Deposition System......... 5 2.1 Introduction ..........5 2.2 Reactor Chamber ............6 2.3 Gas Distribution System ..........6 2.4 Pumping System ...........6 2.5 Radio-Frequency Generator .............7 Chapter 3 Characteristics of Various Thin Films Prepared by PECVD ............8 3.1 Introduction .............8 3.2 Characteristics of a-SiNx:H Films ............8 3.2.1 Deposition of a-SiNx:H Films ...........9 3.2.2 Experimental Results ...........9 3.3 Characteristics of Undoped a-Si:H Films ........10 3.3.1 Deposition of Undoped a-Si:H Films ..........10 3.3.2 Experimental Results .........10 3.4 Characteristics of n+ a-Si:H Films .........11 3.5 Characteristics of mc-Si:H Films .........11 3.5.1 Deposition of mc-Si:H Films ...........11 3.5.2 Measurements of Optical Energy Gap Eopt .......12 3.5.3 Measurements of Crystalline Fraction (CF) of mc-Si:H Films ........12 3.5.4 Dependence of Eopt and CF on Substrate Temperature .......12 3.5.5 Dependence of Eopt and CF on RF Power Density .........13 3.5.6 Dependence of Eopt and CF on Deposition Pressure ........13 3.5.7 Dependence of Eopt and CF on SiH4 Flow Rate .........13 3.5.8 Dependence of Eopt and CF on H2 Flow Rate ............13 3.5.9 Dependence of Eopt and CF on H2/SiH4 Flow Ratio ..........14 3.5.10 Effects of H2/SiH4 Flow Ratio on mc-Si:H and a-Si:H Films ............14 3.6 Summary ...........14 Chapter 4 Thin-Film Transistors Fabricated by PECVD .........15 4.1 Introduction ..........15 4.2 The Physics of a-Si:H TFT Devices ........17 4.2.1 Effects of PECVD a-Si:H on a-Si:H TFTs ........17 4.2.2 The Structure of Thin-Film Transistor.........19 4.2.3 The Operation for TFT Devices............19 4.2.4 The Instability Mechanism for Thin-Film Transistor ...........21 4.3 Experimental............23 4.4 Results and Discussions...........24 4.5 Summary .............27 Chapter 5 Plasma Passivation Effects on Thin-Film Transistors ...........29 5.1 Introduction ...........29 5.2 Experimental .........30 5.3 Results and Discussion ........31 5.4 Summary .......33 Chapter 6 Thin-Film Transistors with Low-High-Low Band Gap Structures .........35 6.1 Introduction ...........35 6.2 Experimental ..........36 6.3 Results and Discussion ..........37 6.4 Mechanism of Carriers Transport in Thin-Film Transistors with Low-High-Low Band Gap Structure .........41 6.5 Summary ...........43 Chapter 7 Anomalous Bias-Stress-Induced Phenomena in Thin-Film Transistors with Low-High-Low Band Gap Structures .......44 7.1 Introduction ........44 7.2 Experimental ..........45 7.3 Results .........45 7.4 Discussion .......49 7.5 H-atom Compensation .........55 7.6 Summary .........56 Chapter 8 Conclusions and Recommendations for Further Study ..........57 References .........60 Personal Information 學經歷 Publication List