本研究論文主要利用溶液法製備氧化鋅錫薄膜電晶體(Zinc Tin Oxide thin film transistors, ZTO TFTs)。首先，二元金屬氧化物(M1M2O)半導體，M1，M2金屬元素例如:銦、鎵、錫、鋅、鍺等，電子組態滿足(n−1)d^10ns^0(n≥4)由具對稱性S球型軌域組成，即使在非晶無序的排列下，彼此鄰近S軌域仍重疊，提供有效電子傳輸路徑。而這些氧化物選擇上，Zinc Tin Oxide可規避選用鎵和銦等貴金屬，具價格競爭且無毒性等，具相當潛力於未來應用。接續，本研究利用旋轉塗佈法/溶液法，為可連續鍍製薄膜製程，提供簡單、低成本、高生產無需使用真空設備等優勢。基此，使本研究之目的更具實質學術價值。與此同時，本文將分為三個主要研究方向。
本研究第一部分(第3章節)，以醋酸鋅(Zinc acetate)和氯化亞錫(Tin chloride)為溶質，乙二醇甲醚(Ethylene glycol monomethyl ether)為溶劑，配製半導體ZTO薄膜之前驅溶液。藉由調變ZTO前驅溶液原子組成比例(即，錫/(鋅+錫))，作為一開始的研究基礎，調查研究ZTO於不同原子比例組成之材料特性。接續，以旋轉塗佈方式鍍製不同鋅錫比例之超薄ZTO主動層(厚度約7nm)，最後製備完成下閘極-上接觸結構之薄膜電晶體元件。根據材料分析結果，隨著添加錫含量提高，於ZTO薄膜中之氧缺陷大幅下降。研究中於未摻雜錫之氧化鋅電晶體元件無法有薄膜電晶體運轉特性，隨錫含量逐漸由小量至大量摻雜並伴隨降低氧空缺後，亦造成ZTO為主動層之電晶體特性順利運轉。此研究機制主要利用錫具高離子電位特性，藉由錫摻雜量來控制ZTO薄膜中之氧空缺含量。於ZTO薄膜電晶體在鋅錫原子比例Sn/(Zn+Sn)=0.5具有最佳場效載子遷移率2.0cm^2/Vs和極大電流開關比(on/off ratio)~10^8。ZTO材料特性和電晶體電性能之間關連性，在不同錫/(鋅+錫)比例，於此章節詳細探討。
本研究第二部分(第4章節)，此章節將ZTO溶液之前驅物溶質做一調整，使用揮發性金屬硝酸鹽類，將原醋酸鋅以硝酸鋅做為替換，ZTO薄膜退火溫度可於較低溫350oC完成。然而，此章節ZTO薄膜退火溫度仍維持500oC。下一章節(第5章)，將以退火溫度為350oC之ZTO薄膜電晶體元件做一系列穩定性研究探討。於此章節中，所有元件皆未鍍製鈍化層(Passivation layer)之ZTO薄膜電晶體建構於溶液法，並調查不同厚度之ZTO為主動層之薄膜電晶體於各種環境穩定性表現。首先，超薄ZTO主動層之薄膜厚度只有5奈米，此薄膜電晶體元件性能表現高場效載子遷移率約13~14cm^2/Vs，低次臨界電壓表現約~0.30V/decade，電流開關比約10^8。隨之ZTO薄膜厚度調整增加至12奈米和22奈米，其場效載子遷移率分別更可高達約22cm^2/Vs和29cm^2/Vs。與此同時，這些不同厚度且未鍍製鈍化層之ZTO主動層薄膜電晶體元件於各環境中穩定性之實驗結果；發現，超薄ZTO主動層薄膜厚度為5奈米之ZTO薄膜電晶體元件，不管操作於空氣(相對濕度約60%)、真空(3.3×10^-5 torr)或是乾氧(760 torr)之下，ZTO(5nm-ZTO)薄膜電晶體元件性能幾乎不受影響。於此不同ZTO薄膜厚度之相異電晶體運轉特性，調查其不同氧空缺含量和費米能階至導電帶之變化，來解釋超薄ZTO電晶體元件(5nm-ZTO)可達到抗潮濕且無須鈍化處理，於第4章節詳細探討。
本研究第三部分(第5章節)，有鑒於薄膜電晶體元件於實際應用上，元件反覆開關，即操作於正負偏壓反覆施加，且同時受到背光源影響。因此本章節研究調查於施加偏壓與照光對薄膜電晶體元件特性之影響。於此章節，溶液法製備之ZTO薄膜退火溫度只有350oC，其電晶體元件特性表現:場效載子遷移率約8cm^2/Vs，電流開關比~10^8。接續，利用雷射照光(波長為405和532nm)，亦或搭配施加偏壓(分別施加偏壓為0或-20或+20伏特)探討電晶體元件穩定性。研究調查發現臨界電壓偏移之窗口，ΔVth(Vthstress 3400s - stress 0s)在正偏壓(positive bias stress)施加下有一明顯之正偏移(ΔVth= 9.98V)，正偏壓加照光(positive bias illumination stress)為ΔVth= 6.96V，但在只有照光下(illumination stress, 405nm, ΔVth= -2.02V)，或者是負偏壓(negative bias stress, ΔVth= -2.27V)，顯示只有些微負偏移。然而，在負偏壓加照光(negative bias illumination stress)條件下卻有一非常明顯的負偏移，而其臨界電壓窗口可藉由施加PBS得以有效回復。此結果主要貢獻為當照光可將ZTO薄膜中氧空缺離子化為一價或二價(i.e., Vo, Vo+ and Vo++)。詳細的機制探討有助於了解溶液法ZTO薄膜電晶體之穩定性，於第5章節詳細探討。

In this dissertation, we fabricate solution-processed Zinc Tin Oxide thin film transistors (ZTO TFTs). First, the mixed metal oxides semiconductors (M1M2O) which have heavy metal cations (M1 and M2) are primarily composed of spatially spread metal (n − 1)d10ns0 (n ≥ 4) orbital with isotropic shape, the magnitude of this overlap among the adjacent metal ns orbital of spherical symmetry resulted in efficient path of electron transport even if distorted amorphous structure. In addition, among them, ZTO TFT that is indium- and gallium- free has gained an advantage because of the low-cost competition and non-toxic. Second, this work suggested spin-coating/solution-processing method allow for a continuous process, which is advantageous for achieving simple, low-cost and high-throughput production without the need for costly high-vacuum equipments. Hence, this thesis provided more substantial academic value. Furthermore, this thesis is divided into three aspects for research.
In the first of aspect, zinc acetate, tin chloride (solute) and ethylene glycol monomethyl ether (solvent) were used as precursors for preparing ZTO semiconductor films. We examine the fundamental studies of ZTO precursor composition (Sn/(Zn+Sn) ratio) effects at the beginning in order to figure out the role of each component in ZTO oxide semiconductors with their material characteristic. Next, ultra-thin zinc-tin oxide (ZTO) films (~7 nm thick) with different Sn/(Sn+Zn) molar ratios, fabricated by using a solution process in combination of spin coating method, are applied as channel layers in thin film transistors (TFTs) with a bottom-gate top-contact structure. With regard to material characteristics, oxygen deficiency in ZTO films can be substantially decreased with the addition of different Sn contents. The non-Sn added ZnO TFT cannot be turned on whereas the Sn added ZTO channel layers perform TFT characteristics adequately, indicating the necessity of reducing oxygen deficiency by introducing Sn during the solution synthesizing process. In the mechanism, we can control the oxygen deficiency of the ZTO TFTs using various molar ratios of Sn because of a high ionic potential. With a Sn/(Sn+Zn) molar ratio of 0.5, the ZTO TFT exhibits the best field-effect mobility of ~ 2.0 cm^2/Vs and a large on/off current ratio of ~10^8. The electrical characteristics of ZTO TFTs are explored and correlated to the levels of oxygen deficiency associated with various Sn contents (detailed in Chapter 3).
In second aspect, solution-processed ZTO films are fabricated using a volatile metal nitrate precursor species with a low annealing at 350 oC. However, this section of ZTO films at annealing temperature remained 500 oC. Next chapter of 5, ZTO TFTs will do a series of stability study at annealing temperature of 350 oC. In this chapter, an ultra-thin (5 nm-thick), unpassivated zinc tin oxide (ZTO) thin-film transistor TFT, fabricated with solution process, exhibits a good field-effect mobility (13~14 cm^2/Vs), small subthreshold swing (~0.30 V/dec.) and high on/off current ratio (~10^8). The field-effect mobility can be further enhanced by increasing the ZTO thickness to 12 nm (~22 cm^2/Vs) and 22 nm (~29 cm^2/Vs). Furthermore, ID-VG characteristics of the 5nm-thick ZTO TFT remain unaffected, regardless of working in air (60% relative humidity), vacuum (3.3×10^-5 torr) or dry O2 (760 torr) atmosphere. The dissimilar TFT characteristics are discussed in terms of oxygen deficiency content, as well as the Fermi level position (EF to EC) for ZTO of various thicknesses to explain the moisture immunity of the 5 nm-thick solution-processed ZTO TFT (detailed in Chapter 4).
In the third of aspect, in practical application, switching TFTs are continuously exposed to bias and/or illumination from the backlight. In this section, the interactions between these (such as simultaneous illumination/bias stressing) will also be discussed. Solution-processed ultra-thin (3 nm) zinc tin oxide (ZTO) thin film transistors (TFTs) with a mobility of 8 cm^2/Vs are obtained with post spin-coating annealing at only 350 oC. The effect of light illumination (at wavelengths of 405 nm or 532 nm) on the stability of TFT transfer characteristics under various gate bias stress conditions (zero, positive, negative) is investigated. It is found that the ΔVth (Vth= stress 3400s - stress 0s) window is significantly positive when ZTO TFTs are under positive bias stress (PBS, ΔVth= 9.98 V) and positive bias illumination stress (PBIS, λ=405 nm, ΔVth= 6.96 V), but ΔVth is slightly negative under only light illumination stress (IS, λ= 405 nm, ΔVth= -2.02 V) or negative bias stress (NBS, ΔVth= -2.27 V). However, the ΔVth of ZTO TFT under negative bias illumination stress is substantial, and it will efficiently recover the ΔVth caused by PBS. The result is attributed to the photo-ionization and subsequent transition of electronic states of oxygen vacancies (i.e., Vo, Vo+ and Vo++) in ZTO. A detailed mechanism is discussed to better understand the bias stress stability of solution processed ZTO TFTs (detailed in Chapter 5).

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