含氧化物之光聚合奈米複合材料及氧化鋅薄膜元件之製備與光電特性研究

Abstract

本論文研究第一部分為以凝膠法化學合成（Sol-gel Chemical Route）ZnO與Ta2O5氧化物奈米粒子，以改質劑與偶合劑分別修飾ZnO與Ta2O5，使ZnO與Ta2O5粒子得以均勻的分散在壓克力單體中，而能製備含氧化物微粒之UV光聚合奈米複合材料（UV-curable Nanocomposites），並分析其光、熱以及電特性；第二部分以矽烷偶合劑（Silane Coupling Agent）進行表面改質以製備高穩定度的ZnO奈米粒子溶液，並以旋塗法沉積薄膜應用在ZnO薄膜電晶體（Thin-film Transistor，TFT）與ZnO光偵測器（Photo-detector，PD），退火熱處理對ZnO TFT元件電性與PD元件光響應（Photoresponse）的影響亦一併探討之。 利用帶有壓克力單體的矽烷偶合劑（3-(trimethoxysilyl)propyl methacrylate，TPMA）做為ZnO奈米粒子的表面改質劑之目的在改善親水性的ZnO與疏水性的壓克力之間相容性（Compatibility），改質後的ZnO粒子與單體相混合且矽烷偶合劑上之丙烯酸甲酯基（Methylacrylate）可與單體內丙烯酸甲酯基/丙烯酸酯基（Acrylate）進行光聚合反應（Photo-polymerization），以提高有機/無機相間的相連性與相容性，進而使ZnO粒子可均勻分布於光聚合型高分子中。在ZnO壓克力奈米複合薄膜中，當ZnO的含量達到20 wt.%時，其可見光穿透度仍然超過95%，顯示出ZnO粒子在高分子基地中的分散良好。隨著ZnO含量的增加，奈米複合材料的介電常數也隨之上升，當ZnO的含量增加到20 wt.%時介電常數約為4.35。在含Ta2O5的奈米複合薄膜中，熱穩定性與介電特性隨著Ta2O5含量的增加而上升，當Ta2O5的含量增加到10 wt.%時介電常數約為4.23；熱裂解溫度（Thermal Decomposition Temperature, Td）約為323.4□C。而以Maxwell-Garnett Effective Medium Approximation之理論計算顯示Ta2O5或ZnO與壓克力間的界面交互作用扮演著重要的角色，使得奈米複合材料的介電常數高於理論值。 奈米粒子溶液必須具備高均勻性與高穩定性以獲得低孔洞率、高均勻性與平整性的高品質旋塗沉積薄膜，以矽烷偶合劑（(3-glycidyloxypropyl)-trimethoxysilane，GPTS）修飾ZnO粒子的表面使其在乙二醇（Ethylene Glycol，EG）溶劑中呈現高穩定的分散，其表面電位（Surface Potential）可達66 mV；經六個月的沉降測試，ZnO膠體溶液依舊透明且無團聚的現象，顯示出以化學吸附在ZnO表面的GPTS能夠有效地將ZnO粒子分散在EG中。比較退火前後ZnO薄膜的光激光譜（Photoluminescence，PL）可得知經退火的ZnO薄膜之直接能隙訊號增強，此歸因於ZnO結晶度程度之改善。當退火溫度達到500□C時能有效的去除GPTS偶合劑而得到最佳TFT的電性，其載子遷移率（Mobility，□） = 0.104 cm2/V□sec、臨界電壓（Threshold Voltage，Vth） = 17.1 V、開關比（On/off Ratio） = 105。在ZnO PD中，以波長365 nm能量密度為350 μW/cm2在偏壓3 V下量測得到開關比 = 105，電流上升的時間常數（Rise Time，τr） = 4.80秒而下降的時間常數（Decay Time，τd） = 2.10秒，τr較大是由於ZnO薄膜中之晶界缺陷使得電子傳遞時受到阻礙而延遲了光電流上升的時間。

This thesis studies the synthesis of ZnO and Ta2O5 nanoparticles by a sol-gel chemical route and their surface modifications for the fabrication of UV-curable nanocomposites and ZnO thin-film devices. The study includes two parts: first, the surface modification agent and coupling agent containing acrylic functional group was adopted to modify ZnO and Ta2O5, respectively, followed by the blending of modified Ta2O5 or ZnO nanoparticles with acrylic monomers for the preparation of UV-curable nanocomposites. Accordingly, characterizations of optical, thermal and dielectric properties were also accomplished. Secondly, silane coupling agent was adopted to stabilize the ZnO nanoparticles in EG solution for subsequent ZnO TFT and PD devices preparation via the spin coating process. For ZnO TFT, effects of annealing treatment at temperatures of 200-500□C on electrical performance were investigated. For ZnO PD, photoresponse properties were studied. In the study of UV-curable ZnO/acrylic nanocomposites, the coupling agent, 3-(trimethoxysilyl)propyl methacrylate (TPMA), containing acrylic functional group was adopted to modify ZnO nanoparticles. The acrylic functional group attached on ZnO not only promotes the compatibility but also forms the chemical bonds in between the ZnO and monomers so that the highly transparent nanocomposites could be prepared. The nanocomposite containing 20 wt.% ZnO exhibited a high optical transmittances > 95% in visible-light wavelength range, implying a uniform dispersion of ZnO nanoparticles in polymeric matrix. Moreover, dielectric constant of nanocomposite increased with the increase of ZnO content and dielectric constant of the nanocomposite was about 4.35 for the sample containing 20 wt.% ZnO. As to Ta2O5/acrylic nanocomposites, dielectric constant and thermal stability of nanocomposite increased with the increase of Ta2O5 content and dielectric constant and thermal decomposition temperature (Td) of the nanocomposite was about 4.23 and 323.4□C for the sample containing 10 wt.% Ta2O5. Calculation based on the Maxwell-Garnett effective medium approximation theory indicated the interfacial interactions in Ta2O5 or ZnO with acrylic matrix is essential to the enhancement of dielectric constant of nanocomposite thin-films. A stable ZnO colloidal solution is essential to obtain the uniform, nearly pore-free and highly flat ZnO thin film via the spin coating process. (3-glycidyloxypropyl)-trimethoxysilane (GPTS) was adopted to stabilize the ZnO nanoparticles in EG solution. The surface potential of colloidal solution was found to be 66 mV, indicating the chemisorption of GPTS on ZnO particles could effectively prevent the particle aggregation. Comparison the PL intensity of as-deposited and annealed ZnO thin films, grain growth of annealed ZnO resulted in the increase of near band edge emission. The ZnO layer was adopted for TFT preparation and the sample subjected to the 500□C annealing exhibited the satisfied electrical properties with mobility (□) = 0.104 cm2/V□sec, threshold voltage (Vth) = 17.1 V and on/off ratio = 105. Under the UV illumination with a power density about 350 μW/cm2 at wavelength = 365 nm and bias voltage = 3 V, on/off ratio = 105, rise time (τr) = 4.8 sec and decay time (τd) = 2.10 sec were observed for ZnO PD. Larger τr was attributed to the grain boundaries and point defects in ZnO layer which serve as the scattering and trapping centers to impede the electrons transport in the ZnO and thus prolong the τr of ZnO PD.