利用大氣常壓電漿輔助化學氣相沉積製備氧化鋅系透明電極與氧化鋅/銦鎵鋅氧薄膜電晶體應用之特性研究

Abstract

隨著光電領域的蓬勃發展，透明導電膜的需求量急速上升，商業化銦錫氧化物具有良好的光電特性，然而銦是稀有金屬且具毒性，新的替代材料開發是必需的，氧化鋅具有低成長溫度、低成本、鋅的蘊含量豐富且不具毒性等優點而受到很大的關注，而氧化鋅當作薄膜電晶體的主動層亦具有很大的發展潛力由於可增加開口率、低光敏感度以及可做為透明電路上應用，在另一方面，近年來非晶銦鎵鋅氧化物被廣泛的研究，主要由於其良好的均勻性及高的遷移率具有很大的發展性於大尺寸主動矩陣平面顯示器以及主動矩陣有機發光二極體應用。 本論文旨在新製程技術開發透明氧化物半導體，藉由新穎式大氣壓電漿輔助化學氣相沉積開發氧化鋅系透明電極以及氧化鋅/銦鎵鋅氧化物薄膜電晶體，利用較無環境危害水溶液式硝酸金屬鹽類的前驅物，可直接在大氣環境下沉積，而非真空大氣壓電漿技術具有、低溫、低成本、適合大面積化等競爭優勢，可期望於未來商業化應用。 首先，我們探討不同製程參數包括載氣流量、電漿噴嘴與基板的距離、基板溫度、不同摻雜鎵的百分比對於氧化鋅摻雜鎵薄膜特性影響，若在電漿區域通入過多的前驅物將導致大量的氣態附著性差的成核粒子產生，電漿噴嘴與基板的距離若太高會增加可以氣態成核的時間導致薄膜特性劣化，當基板溫度升高對於薄膜結晶特性並沒明顯改變，在100oC時得到較好特性，反而在基板溫度升高時電阻率升高，由於周圍的空氣的在高溫時氧吸附造成，而當摻雜8原子百分比的鎵時有最低電阻率並具有(002)優先取向，在基板溫度100oC下，電阻率可達到7.8x10-4 Ω．cm，在可見光範圍穿透率大於80%。而在氧化鋅摻雜銦方面，在基板溫度200oC得到較低的電阻率，當銦的摻雜濃度增加時表面形成尖錐狀增加表面粗糙度，摻雜8 at.%的銦時有最低電阻率，在基板溫度200oC下，電阻率可達到1.8x10-3 Ω．cm，此低溫製程且具有良好的特性的氧化鋅摻雜鎵與氧化鋅摻雜銦薄膜具有潛力於商業化應用。 其次，我們探討在基板溫度100oC下成長氧化鋅並探討主動層厚度以及氧氣對於薄膜電晶體特性影響，透過使用壓縮空氣當做載氣以及探討電漿氣體裡摻入氧氣，可以有效修補缺陷並得到較佳的切換特性，降低主動層厚度亦改善了電晶體特性由於降低了源極與汲極之間的漏電路徑，但太薄可能由於薄膜島狀不連續結構導致較低的遷移率，主動層約在35~60nm時可得到較佳的特性，最後在電漿氣體裡摻入0.69%氧氣，可以得到遷移率2.38cm2/V-s、開關電流比達4.63x109，此低溫成長的氧化鋅適合於軟性電子應用。 最後，我們先探討熱退火溫度200-500oC對於銦鎵鋅氧化物薄膜電晶體影響，再將高介電係數氧化鋁(Al2O3)應用於銦鎵鋅氧化物薄膜電晶體，結果顯示經由熱退火處理銦鎵鋅氧化物薄膜電晶體特性有效的提升，在300oC有良好的切換特性，經過500oC退火亦呈現類非晶態，經過熱退火製程在500oC有最好的特性，臨限電壓6.74V、次臨界擺幅1.54V/dec、遷移率可達到10.31cm2/V-s、開關電流比達到3.28x108，使用高介電常數氧化鋁可以有效降低等效氧化層厚度(EOT)來達成提升電流密度及降低臨界電壓，結合氧化鋁製作銦鎵鋅氧化物薄膜電晶體具有低臨限電壓0.71V、低次臨界擺幅276mV/dec、優良的遷移率8.39cm2/V-s、高電流開關比1x108，此利用非真空大氣壓電漿技術沉積的銦鎵鋅氧化物具有良好的元件特性且將可應用於大尺吋的平面顯示器以及驅動有機發光二極體。

The demand for transparent conductive oxide (TCO) is rising rapidly because of the booming field of optoelectronics. The commercial indium tin oxide (ITO) has excellent optical and electrical properties. However, indium is a rare and toxic metal. As a result, development of new alternative materials is necessary. Zinc oxide (ZnO) has attracted much attention due to low growth temperature, low cost, abundance and nontoxicity. Furthermore, ZnO thin film transistors (TFTs) have a great interest due to the potential in increase of the pixel aspect ratio, insensitivity to visible light, and application for transparent circuit. On the other hand, amorphous indium gallium zinc oxide (a-IGZO) is extensively studied and has great development in large-size active matrix liquid crystal displays (AMLCD) and active matrix organic light-emitting diode (AMOLED) applications because of its good uniformity and high mobility. In this thesis, new process technology is developed to deposit transparent oxide semiconductors (TOSs). Novel atmospheric pressure plasma enhanced chemical vapor deposition oxide (AP-PECVD) is proposed to fabricate ZnO-based transparent electrodes and ZnO/IGZO thin film transistors. Also, water-based metal salt solution, which is an eco-friendly precursor, is adopted, and the thin film can be deposited in atmospheric environment. The non-vacuum AP-PECVD offers several competitive advantages, such as low temperature process, low cost and suitable for large area application. It is expected for commercial applications in the future. First, we study on the different process parameters including carrier gas flow rate, gap distance between plasma nozzle and the substrate, substrate temperature and the different gallium doping concentrations. Excessive precursor in the plasma region will lead to nucleation particles with poor adhesion. The longer distance increases the time to form gas phase nucleation particles resulting in a degradation of crystallinity. As substrate temperature increases, the cystallinity doesn’t change obviously. The 100oC samples exhibits a better performance, and the higher substrate temperature shows a higher resistivity. It is may be due to the adsorption of oxygen from the surrounding air which reduces the carrier concentration and mobility. Gallium-doped ZnO (GZO) has the lowest resistivity via 8 at.% doping possessing a (002) preferred orientation. The low resistivity of GZO thin film is 7.8×10-4 Ω．cm and the transmittance in the visual region is more than 80% at a substrate temperature of 100oC. Indium-doped ZnO (IZO) has the lowest resistivity via 8 at.% at a substrate temperature of 200oC. When the doping concentration becomes higher, the surface shows obviously needlelike geometry. As a result, the high indium content shows a rougher surface. The lowest resistivity of IZO is 1.8×10-3 Ω．cm at a substrate temperature of 200oC. These good characteristics of GZO and IZO with low temperature process have high potential for commercial applications. Next, ZnO active layer is deposited at a low substrate temperature of 100oC. The effect of channel thicknesses and oxygen species on the characteristics of ZnO TFTs is studied. Using compressed dry air (CDA) as a carrier gas as well as incorporating oxygen gas in the plasma gas can effectively repair the defects, and excellent switching properties is achieved. Reducing the thickness can increase the channel resistance and reduce the undesired current flow between source and drain resulting in improvement of TFT properties. The too thin channel layer might lead to a low mobility due to discontinuous island structure. The channel layer with a thickness of 35~60nm can obtain a better performance. By incorporating 0.69% O2 into plasma gas, a field-effect mobility of 2.38 cm2/Vs and an Ion/Ioff current ratio of 4.63×109 are obtained. This ZnO with low-temperature process is suitable for flexible applications. Finally, we investigate on the effect thermal annealing temperature 200-500oC on the IGZO TFTs, and then the high-k dielectric aluminum oxide (Al2O3) is integrated in IGZO TFTs. The results shows switching characteristics is effectively improved the by thermal annealing. After post annealing in higher than 300 oC, the devices show clear switching properties. The defects can be repaired effectively by post annealing. After poster annealing, IGZO thin film shows an amorphous-like phase, and no obvious crystallization is observed even at 500 oC. IGZO TFT annealed at 500oC in N2 shows excellent electrical characteristics including a VT of 6.74 V, a subthreshold swing of 1.54V/dec, a high mobility of 10.31cm2/V-s and a large Ion/Ioff ratio of 3.28x108. Using the high-k dielectric Al2O3 can effectively reduce the equivalent oxide thickness (EOT) to achieve a high drive current and a low threshold voltage. The PE-ALD Al2O3/IGZO TFT demonstrated excellent electrical characteristics, including a low VT of 0.71 V, small subthreshold swing of 276 mV/dec, a mobility of 8.39 cm2/V-s, and a large Ion/Ioff ratio of 1×108. The IGZO TFTs deposited by non-vacuum APPECVD are suitable for large-size flat panel displays and driving OLED.