低溫合成氧化鋅基奈米結構之元件特性與應用之研究

Abstract

本論文旨在研究氧化鋅(ZnO)、摻雜鋁之氧化鋅(Al-doped ZnO)與氧化鎳(NiO)不同奈米結構之成長製備暨其電子、光電元件之應用及相關特性之研究。 首先，利用低溫85 °C水熱合成法成長摻雜鋁之氧化鋅奈米結構於玻璃基板上，並且探討不同摻雜鋁之濃度對於奈米結構表面型態、結晶性、場發射特性與化學鍵結型態之影響，當摻雜鋁之濃度由0 at.%變化至3 at.%時，摻雜鋁之氧化鋅奈米結構為準直之奈米線，而當濃度由變化3 at.%變化至5 at.%時，開始有奈米片結構產生，並且與奈米線共存，當濃度大於7 at.%時，奈米結構將完全以奈米片的形式存在，另外，發現當摻雜鋁之濃度≦3 at.%時，摻雜鋁之氧化鋅奈米線皆為單晶之纖鋅礦結構，而摻雜鋁之氧化鋅奈米片為多晶態之結構。進一步發現當摻雜鋁之濃度為3 at.%時，擁有較好之結晶性、較少的結構缺陷密度與較好的導電性，因而可展現出較高之電流密度、較高之場增強因子(3131)、較低之開啟電場(2.17 V/µm)與臨界值電場(3.43 V/µm)。 我們所合成摻雜鋁之氧化鋅奈米線皆比低溫(＜600 °C)成長之奈米碳管特性佳，但摻雜鋁之氧化鋅奈米線仍會有不小的場發射電流變動(Fluctuation)，這對應用於場發射顯示器上將有相當程度的影響，像是驅動電路的複雜度提高。為了增加場發射電流之穩定性，我們結合了複晶矽薄膜電晶體(Poly-Si TFT)與摻雜鋁之氧化鋅奈米線，來製作一個主動控制的場發射元件。而摻雜鋁之氧化鋅奈米線可直接整合在電晶體的汲極端(Drain)，透過電晶體中受閘極電壓所控制的穩定飽和(Saturated)汲極電流，來提供摻雜鋁之氧化鋅奈米線一個穩定的電子流來源。為了要能夠承受較高汲極電壓，一種具有偏移式閘極之薄膜電晶體被設計與製作，當閘極電壓由0 V增加到30 V時可達到開關電流比值為3.05×105，不同大小且非常穩定的場發射電流都可藉由閘極電壓來控制，此種新穎主動控制的元件架構不但結構簡單，且可低溫製作，並可提供一個可低電壓操作且非常穩定的場發射電流，其場發射電流變動程度小於2 %，比未受控制的15.6 %優異許多，相信對於場發射顯示器將有非常大的助益。 針對生醫感測器開發，我們利用低溫85 °C水熱合成法成長摻雜不同鋁濃度之氧化鋅奈米結構於玻璃基板上，並利用上述之摻雜鋁之氧化鋅奈米結構作為延伸式閘極感測場效電晶體(Extended-Gate Field-Effect Transistor, EGFET)之酸鹼離子感測膜。同時與商品化金屬-氧化物-半導體場效電晶體(Metal-Oxide-Semiconductor Field-Effect Transistor, MOSFET)元件結合後，將EGFET元件置於不同pH緩衝液，探討此感測膜材料之元件特性與酸鹼離子感測度的變化情形。由實驗結果顯示當摻雜鋁之濃度為3 at.%時，擁有較好之結晶性、較少的結構缺陷密度與較好的導電性，因而可展現出較高之電壓感測度(57.95 mV/pH)、較高之電流感測度(0.96 μA1/2/pH)、較佳之線性度(0.9999)、較小之遲滯寬度(4.83 mV)、較低之時漂速率(1.27 mV/hour)、較低之臨界值電場(1.32 V)、較小之參考電極電壓漂移量與較廣之感測範圍(pH=1~pH=13)。除此之外，我們進一步將其應用於葡萄糖感測(Glucose Biosensor)，發現摻雜鋁濃度為3 at.%之元件於0 mM~14 mM感測範圍具有極高之感測度(60.5 μA．cm-2．mM-1)與線性度(0.9996)，由於此感測器結構易於製造與封裝，且量測方法簡單，故可應用於軟性與拋棄式生物感測器。 針對光感測器的改善，我們提出一種新穎全透明紫外線感測器，藉由連續性低溫水熱合成法製程可製作出具p型氧化鎳奈米花與n型氧化鋅奈米線之奈米異質接面紫外線感測器。除此之外，我們也探討此奈米異質接面紫外線感測器於氧氣氛之下活化所造成之影響，經過氧氣氛下活化之奈米異質接面紫外線感測器，皆比未經過活化處理之元件擁有較好之整流特性(IForward/IReverse = 427)、較低之臨界值電壓(VTH = 0.98 V)、較低之漏電流(1.68×10-5 A/cm2)且對於紫外線(325 nm)有較高之感測度，此經過氧氣氛下活化處理之元件光電特性大幅提升的原因，主要歸因於較少的結構缺陷密度、較少的介面缺陷密度與較好之結晶性。另外，我們進一步對經過氧氣氛下活化處理之元件進行紫外線動態反應量測，發現此元件具有快速之反應時間與光反應之再現性，因此非常適用於未來奈米光電元件之應用。 針對薄膜電晶體開發，我們提出新穎且高效能之薄膜電晶體，經由低溫水熱合成法製程可製作出具晶界控制之氧化鋅底閘極薄膜電晶體。首先，我們藉由設計汲極與源極(Source)電極位置來控制晶粒之成長方向與晶粒邊界位置，且將製作出單一垂直於通道電流的晶粒邊界，以此方法所製作出之具晶界控制氧化鋅底閘極薄膜電晶體皆比傳統濺鍍機(Sputter)所成長之多晶氧化鋅底閘極薄膜電晶體，擁有較高之載子遷移率(9.07 cm2/V•s)、較低之臨界值電壓(2.25 V)、較高之開關電流比值(~106)與較優異的電流趨動特性，歸因於具晶界控制氧化鋅底閘極薄膜電晶體具有較好之結晶性與較少垂直於通道電流之晶粒邊界。 最後，亦提出論文結論與針對未來研究可著重的工作方向。

In this thesis, the growth and physical/chemical properties of nanostructures, including zinc oxide (ZnO), Al-doped ZnO (AZO), and nickel oxide (NiO) were investigated. This study also demonstrates the potential of these nanostructures in the applications of the nano electronic, optoelectronic devices, and biosensors. At first, the AZO nanostructures with different Al concentrations were synthesized on AZO/glass substrate via a simple hydrothermal growth method at a temperature as low as 85 °C. The morphologies, crystallinity, optical emission properties, and chemical bonding states of AZO nanostructures show evident dependence on the aluminum dosage. The morphologies of AZO nanostructures were changed from vertically aligned nanowires (NWs), and NWs coexisted with nanosheets (NSs), to complete NSs in respect of the Al-dosages of 0~3 at.%, 5 at.%, and 7 at.%, correspondingly. The undoped ZnO and lightly Al-doped AZO (≦3 at.%) NWs are single-crystalline wurtzite structure. In contrast, heavily Al-doped AZO sample is polycrystalline. The AZO nanostructure with 3 at.% Al-dosages reveals the optimal crystallinity and less structural defects, reflecting the longest carrier lifetime and highest conductivity. Consequently, the field-emission characteristics of such an AZO emitter can exhibit the higher current density, larger field-enhancement factor (β) of 3131, lower turn-on field of 2.17 V/µm, and lower threshold field of 3.43 V/µm. Secondly, the AZO NWs arrays incorporating an offset-thin film transistor (offset-TFT) have been proposed to achieve high field emission (FE) stability. The AZO NWs field emission arrays (FEAs) were hydrothermally grown in the drain region of offset-TFT at a low temperature of 85 °C. A large on/off current ratio of 3.05×105 was achieved for the gate voltage (VGS) switching from 0 V to 30 V, indicating that the driving voltage of AZO NWs FEAs can be significantly suppressed. Furthermore, the uncontrolled AZO NWs FEAs demonstrated the superior FE characteristics (i.e., turn-on field of ~ 2.17 V/µm and threshold field of ~ 3.43 V/µm) compared with those of the conventional CNT FEAs grown at a temperature below 600 °C. However, uncontrolled AZO NWs FEAs show a larger current fluctuation of 15.6 %. Therefore, the offset-TFTs were used to control the AZO NWs FEAs. Consequently, the fluctuation of AZO NWs FEAs could be significantly reduced to be less than 2 %. This novel field emission device exhibits good emission stability, low-voltage controllability, low-temperature processing, and structural simplicity, making it promising for applications in flat panel displays. For the biosensor development, the pH sensing properties of an extended-gate field-effect transistor (EGFET) based on the aluminum-doped ZnO (AZO) nanostructures are first time investigated. The AZO nanostructures with different Al concentrations were synthesized on AZO/glass substrate via a simple hydrothermal growth method at 85 °C. The AZO sensing nanostructures were connected with the metal-oxide-semiconductor field-effect transistors (MOSFET). Afterwards, the current-voltage (I-V) characteristics and the sensing properties of the pH-EGFET sensors were obtained in different buffer solutions, respectively. Consequently, the pH sensing characteristics of AZO pH-EGFET sensors with Al-dosage of 3 at.% can exhibit the higher voltage sensitivity of 57.95 mV/pH, higher current sensitivity of 0.96 μA1/2/pH, larger linearity of 0.9999, smaller hysteresis width of 4.83 mV, lower drift rate of 1.27 mV/hour, lower threshold voltage of 1.32 V, smaller variation of reference electrode voltage, and has a wider sensing range (pH1 ~ pH13). In addition, the sensing characteristics of AZO NWs (3 at.%) glucose biosensors exhibit excellent sensitivity (60.5 μA．cm-2．mM-1) and linearity (0.9996) from 0 mM to 14 mM. It is easier to fabricate and package the sensitive membrane structure and the measurement is simple for the application of flexible and disposable biosensor. For the photodiode development, a transparent ultraviolet (UV) sensor using nanoheterojunctions (NHJs) composed of p-type NiO nanoflowers (NFs) and n-type ZnO nanowires (NWs) was prepared through a sequential low-temperature hydrothermal-growth process. The devices that were annealed in an oxygen (O2) ambient exhibited better rectification behavior (IForward/IReverse=427), a lower forward threshold voltage (VTH=0.98 V), a lower leakage current (1.68×10-5 A/cm2), and superior sensitivity (IUV/IDark=57.8; IVisible/IDark=1.25) to UV light (λ=325 nm) than the unannealed devices. The remarkably improved device performances and optoelectronic characteristics of the annealed p-NiO-NF/n-ZnO-NW NHJs can be associated with their fewer structural defects, fewer interfacial defects, and better crystallinity. A stable and repeatable operation of dynamic photoresponse was also observed in the annealed devices. The excellent sensitivity and repeatable photoresponse to UV light of the hydrothermally grown p-NiO-NF/n-ZnO-NW NHJs annealed in a suitable O2 ambient indicate that they can be applied to nano-integrated optoelectronic devices. For the thin film transistors (TFTs) development, high-performance bottom-gate (BG) TFTs with ZnO artificially location-controlled lateral grain growth have been prepared via low-temperature hydrothermal method. For the proper design of source/drain structure of ZnO/Ti/Pt thin films, the grains can be laterally grown from the under-cut ZnO beneath the Ti/Pt layer. Consequently, the single one vertical grain boundary perpendicular to the current flow will be produced in the channel region as the grown grains from the source/drain both sides are impinged. As compared with the conventional sputtered ZnO BG-TFTs, the proposed location-controlled hydrothermal ZnO BG-TFTs demonstrated the higher field-effect mobility of 9.07 cm2/V•s, lower threshold voltage of 2.25 V, higher on/off current ratio above 106, and superior current drivability, reflecting the high-quality ZnO thin films with less grain boundary effect in the channel region. Finally, conclusions as well as prospects for the further research are also proposed.