氧化物薄膜電晶體載子遷移率和穩定性提升之研究

Abstract

近幾年以透明非晶氧化物半導體取代矽材料作為薄膜電晶體的通道受到許多的關 注，以金屬氧化物為通道的薄膜電晶體有潛力地作為驅動主動矩陣液晶螢幕及有機發光 二極體顯示器的開關。近期非晶銦鎵鋅氧化物薄膜受到極高的關注，由於它有高載子遷 移率、對於可見光的高透明率並對現今固態半導體製程技術相容性像是高清顯示器製程， 然而非晶銦鎵鋅氧化物薄膜電晶體有高密度的缺陷導致高臨界電壓、高次臨界擺幅及薄 膜電晶體在負偏壓照光下的不穩定性，當然缺陷劣化氧化物薄膜電晶體的電性是一個關 鍵問題。本篇的論文研究三種不同的通道材料，包含氧化鋅、銦鋅氧化物及銦鎵鋅氧化 物，並著重於探討對於未來大尺寸面板的電子產品的各方面影響。在此主要是研究藉由 雙主動層及共濺鍍結構來提升氧化物薄膜的遷移率和穩定性，並探討雙主動層及共濺鍍 結構的非晶銦鎵鋅氧化物薄膜電晶體的電性相對於其餘薄膜電晶體的比較。首先在結構 上及光照影響下，氧化鋅及銦鎵鋅氧化物雙層結構相對於單一的銦鎵鋅氧化物層都突顯 出較少的缺陷，由於雙層結構薄膜電晶體有較低次臨界擺幅及臨界電壓，但是此雙層通 道結構因為含有多晶矽氧化鋅而導致低場效載子遷移率和高次臨界漏電流。接著探討以 高遷移率銦鋅氧化物來取代氧化鋅進而改善薄膜電晶體電性，並鍍一層非常薄的氧化鋅 在銦鋅氧化物和銦鎵鋅氧化物雙層結構中，降低銦鋅氧化物本質上的缺陷。另外研究氧 化鋅和銦鎵鋅氧化物共濺鍍為一種可代替和低價非晶態氧化物半導體材料進而優化電 ii 晶體，此材料會減少接面問題不像一般雙層結構。藉由在氧化鋅和銦鎵鋅氧化物共濺鍍 下維持鋅陽離子的摩爾比改善此結構的粗糙度和電性。最後，氧化鋅和銦鎵鋅氧化物共 濺鍍的薄膜電晶體相對於一般銦鎵鋅氧化物薄膜電晶體有較佳的電性和負面壓照光下 的穩定性，適用於低電壓和大面積的顯示器。

In recent years, the transparent amorphous oxide semiconductor (TAOS) has attracted a lot of attention to replace the silicon material as the channel layer of thin film transistors (TFTs). Metal oxide based TFTs are potential candidate for switching or driving the active matrix liquid crystal displays (AMLCD) and organic light emitting diode displays (AMOLED). Recently, amorphous InGaZnO (a-IGZO) thin films have received considerable attention owing to high carrier mobility, transparency for visible light, high process compatibility with present solid-state semiconductor technologies such as high definition displays. However, a-IGZO thin films contain large density of defects – leading higher threshold voltage (Vth), sub-threshold swing (SS) and instability of the TFTs under negatively biased light illumination stress (NBIS) test. The defects play a crucial role to degrade the electrical characteristics of oxide TFTs. In this dissertation work, various aspects of the a-IGZO TFTs by choosing three different materials, zinc oxide (ZnO), indium zinc oxide (IZO), and indium gallium zinc oxide (IGZO) have been detailed studied to shed light further on potential application of such technology to future large area electronics. Present study aims to enhance mobility and stability in the oxide based thin films by adopting their bilayer and co-sputtered structures. Also, the bilayer and co-sputtered thin film qualities vis-à-vis their thin film iv transistors characteristics are compared with a-IGZO TFTs. First, structural, optical, and photoluminescence study of ZnO/IGZO thin films has been investigated to show fewer structural defects in comparison to single layer pure a-IGZO. The bilayer TFTs thus showed better sub-threshold swing and threshold voltage than the a-IGZO TFTs. However, the ZnO/IGZO TFT displayed lower field effect mobility and large subthreshold leakage owing to channel formation by polycrystalline ZnO. Further, the channel is substituted by high mobility indium zinc oxide (IZO) to enhance the electrical characteristics of IZO/IGZO TFT. A very thin layer of ZnO is used as interlayer of IZO/IGZO to reduce the intrinsic defects within the IZO channel for the development of reliable IZO/IGZO bilayer thin film transistors. To further optimize the device performance of IGZO based material, ZnO co-sputtered IGZO is investigated as a possible alternative and cost effective amorphous oxide semiconductor (AOS), leading less interfacial problems unlike in the bilayer case. The performance of ZnO co-sputtered IGZO TFTs is optimized by maintaining the molar ratio of zinc cation in ZnO co-sputtered IGZO, which is able to improve structure, roughness and electrical performance. ZnO co-sputtered IGZO TFT performs a better electrical transfer characteristic and NBIS stability than a-IGZO TFTs- leading a potential candidate for low voltage and large area displays applications.