本實驗以溶液法製備鋅錫氧化物薄膜電晶體(Zinc Tin Oxide Thin Film Transistor, ZTO TFT)，並以波長為405nm、532nm及635nm雷射光作為光源(相對應的光子能量為3.06eV、2.33eV及1.95eV)，針對其作為光電晶體(photo-TFT)，在不同波長的照射光下得到的性質作分析。此外，實驗也製備了紅汞敏化鋅錫氧化物薄膜電晶體(Mercurochrome-sensitized ZTO TFT)，並以532nm雷射光作為光源，針對其作為可見光光電晶體的性質做分析。電性量測部分包含照光與未照光下的汲極電流-閘極電壓曲線(ID-VG curve)以及汲極電流-汲極電壓曲線(ID-VD curve)的電性分析，計算出照光前後的基本電性變化：臨界電壓偏移量(Threshold voltage shift, △VTH)、次臨界擺幅偏移量(Subthreshold swing shift, △S.S.)，用以討論ZTO TFT的光反應機制；以及photo-TFT性質參數：光敏性(Sensitivity)、光響應(Responsivity)以及外部量子效率(External Quantum Efficiency, EQE)，用以判斷photo-TFT的優劣。
本研究第一部分主要針對ZTO TFT之光感測性質作討論，紫外光-可見光吸收光譜(UV-vis absorption spectra)計算出的 ZTO薄膜能隙為3.85eV，對於入射光小於能隙之光反應，與ZTO內部的中性氧空缺(Vo)受光激發為帶電氧空缺(Vo+/ Vo2+)而產生多餘載子有關；波長越短(即入射光能量越大)、功率密度越大，受光激發產生的載子數目就越多；而閘極偏壓越大，電晶體通道層的累積電子會增多，能移動的載子數也越多，造成光電流越大，光響應也越高。然而閘極偏壓越大，原有之暗電流也較高，則會使得光敏性較差。因此ZTO TFT適合作為紫光-紫外光感測器，在適當的閘極偏壓下，可以達到高光敏性，並能藉由控制閘極偏壓的大小，而達到訊號放大的效果。
本研究第二部分則針對紅汞敏化(Mercurochrome-sensitized) ZTO TFT之可見光感測性質作討論。根據UV-vis absorption spectra結果，紅汞在500nm左右會有一個吸收峰，且其吸收度隨紅汞濃度增加(0.16mM~1.32mM)而增加。實驗中在ZTO上吸附不同濃度的紅汞溶液，形成Mercurochrome-sensitized ZTO TFT，其對532nm的光感測性質隨紅汞濃度不同而有不同程度的提升。Mercurochrome-sensitized ZTO TFT的光反應機制，除了原本ZTO內部的中性氧空缺受激發提供額外載子，mercurochrome吸收可見光而激發的載子也能有效傳遞至ZTO的導帶(conduction band, Ec)上，促進Mercurochrome-sensitized ZTO TFT對可見光的光反應。從實驗結果可以獲得紅汞敏化的最佳濃度值為0.66mM，而此最佳參數之Mercurochrome-sensitized ZTO TFT對405nm和635nm的光反應也有所提升，可以作為寬能譜光感測器。

In this study, zinc tin oxide thin film transistors (ZTO TFTs) are fabricated via solution process. As a photo-TFT, the photosensing properties under illumination of 405nm, 532nm and 635nm lasers are investigated. In addition, mercurochrome, which absorbed visible light, is incorporated to ZTO and the photo-TFT properties of mercurochrome-sensitized ZTO TFTs under illumination of 532nm laser are investigated.
Transfer characteristics (ID-VG curves) and output characteristics (ID-VD curves) in the dark and under illumination are measured on these TFTs. The VTH shift (△VTH) and Subthreshold swing shift (△S.S.) before and after light illumination are obtained to demonstrate the photoresponse mechanism of ZTO TFTs. Furthermore, photo-TFT parameters such as sensitivity, responsivity and external quantum efficiency(EQE) are also extracted.
In the first part, the photosensing properties of ZTO TFT are analyzed. UV-vis spectra indicate that the bandgap of our ZTO thin film is 3.85eV. The photoresponse of below bandgap illumination is related to the ionization of neutral oxygen vacancies (Vo) to positively-charged oxygen vacancies (Vo+/ Vo2+). The larger the energy and the power density of the incident light, the more the photogenerated carriers. On the other hand, as the gate voltage gets more positive, the number of free carriers increase so that the photocurrents as well as responsivities are enhanced. However, the larger the gate biases, the larger the original dark current, leading to worse sensitivity. As a result, ZTO TFTs can be used as Purple - Ultraviolet light photosensors, the high sensitivity can be achieved at an appropriate gate bias and the signals can be magnified by applying larger gate biases.
In the second part, the visible light photosensing properties of mercurochrome-sensitized ZTO TFT are investigated. According to UV-vis absorption spectra of mercurochrome, there is an absorption peak at approximately 500nm and the absorbance is increased with increasing concentrations (0.16mM~1.32mM) of the mercurochrome. The degree of promotion of the photoresponse under illumination of 532nm laser of mercurochrome-sensitized ZTO TFT depended on the concentration of the mercurochrome. In addition to the photogenerated electrons from ionization of neutral oxygen vacancies in ZTO, the photogenerated electrons in the mercurochrome successfully transferred to the conduction band of ZTO enhanced the photoresponse in the visible light region. The photoresponse of the optimal mercurochrome(0.66mM)-sensitized ZTO TFT are also improved under illuminations of 405nm and 635nm lasers and thus it can be applied as a wide spectral photosensor.

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