本論文旨在透過Sentaurus TCAD模擬軟體分析具蕭基金屬覆蓋層(capping layer，CL)氧化銦鎵鋅(Indium Gallium Zinc Oxide, IGZO)薄膜電晶體之紫外光(UV)響應特性，探討蕭基金屬覆蓋層對紫外光響應性能之影響。模擬結果顯示於IGZO背通道加入蕭基金屬覆蓋層可有效改善於紫外光感測器(UV photodetectors，UVPD)應用之光響應特性與減輕Idark與Iph間的消長(trade-off)問題。
本研究針對IGZO TFT下閘極結構，利用Sentaurus TCAD進行光電模擬。於材料部分，閘極金屬採用功函數為5.0 V的金屬，介電層high-k材料選用模擬軟體內建的氧化鉿(HfO2)，其相對介電常數與厚度分別為22與60 nm，SiO2等效厚度(equivalent oxide thickness, EOT)約為10 nm。本研究使用蕭基金屬覆蓋層於背通道形成一金半(MS)蕭基接面，目的在給予背通道一額外的空乏區進而使增厚的通道由原本(即無金屬覆蓋層)的部份空乏狀態轉為全空乏狀態，有效降低暗電流(Idark)。由於增加通道厚度可增加照光下產生光生載子的空間，使光電流(Iph)及元件光響應特性提升，然若僅單純增加通道厚度將使通道由增厚前的全空乏狀態變成部分空乏狀態，將導致暗電流上升。採用蕭基金屬覆蓋層可有效地改善此一問題，它可以給予通道額外的空乏區並增強其空乏能力，於不增加暗電流下，提升光電流，使Idark與Iph之間的消長問題得以解決。
本論文內容主要分為「元件結構與材料參數對IGZO TFTs光電特性之影響」、「具蕭基金屬CL IGZO TFTs之元件光電特性之模擬分析」與「具通道蝕刻與蕭基金屬CL IGZO TFTs元件光電特性之模擬分析」等三個部分，茲依序簡述如下：
於第一部份「元件結構與材料參數對IGZO TFTs光電特性之影響」方面，本模擬所採用傳統下閘極結構IGZO TFT通道厚度(Tch)分為四個，依序為30、40、50及60 nm，所得元件依序命名為Type A-30、Type A-40依此類推。元件靜電參數以Von 、Ion、Ioff、Ion⁄Ioff 、SS與Vth等為評估項目，光響應則以光響應度(Rph)、光靈敏度(Sph)及光檢測度(D*)做為評估依據，Rph可以評估一光檢測器光電轉換的能力，而Sph為判斷一檢測器其訊號與背景訊號之間的關係，D*則為檢測較弱能量入射光的能力。模擬結果顯示，Type A-30之元件表現出較良好的靜電特性，其Von 、Ion、Ioff、Ion⁄Ioff 、SS與Vth分別為-0.01 V、2.62×10^(-5) A、4.10×10^(-12) A、6.39×10^6、66 mV/dec以及0.35 V，隨Tch的提升，Vth逐由正轉負，至Type A-60的-0.06 V，顯示通道已從全空乏狀態轉移至部分空乏狀態。通道增厚使閘極控制力衰減，次臨界擺幅SS隨Tch增加而增大，開關性能減弱。本研究亦針對Type A-30元件進行材料參數(如通道濃度、通道缺陷密度以及閘極功函數)調變。隨通道濃度從1×10^17 cm^(-3)下降至1×10^16 cm^(-3)，開啟電壓Von從原本的-0.01 V變為0.24 V，此一結果指出隨通道濃度的下降，通道之空乏區逐漸變大，導致開啟電壓上升。當Type A-30之閘極功函數從5.0 V調整為5.2 V時，開啟電壓與臨界電壓均增加0.2 V，Ioff與SS則無變化。於通道缺陷密度從1×10^17 cm^(-3)變為1×10^19 cm^(-3)，臨界電壓出現右移的趨勢(0.35 V→0.39 V)，間接導致元件的導通電流下降。於UV光照射下的光響應上，模擬結果顯示Type A-60因其光電流最大，擁有最高的Rph。然由於Type A-60之Ioff(1.29×10^(-10) A)最高，其Sph值僅為1.36×10^3 A/A，低於Type A-30的9.35×10^3 A/A，此一結果顯示Ioff與Iph間的消長問題。
第二部份模擬工作聚焦於具蕭基金屬CL IGZO TFTs之元件(稱Type B TFT)光電特性之模擬分析，探討蕭基金屬CL功函數與長度(LCL)對結構參數的元件光電特性之影響並找出最適化的參數。模擬分析顯示，當CL功函數值越大時，其對於元件之空乏能力越強，於Tch=60 nm情況下，當CL之功函數為5.0 V時，其元件的光電特性最佳，基此於後續模擬中乃選用5.0 V為蕭基金屬之功函數。由於蕭基金屬CL遮蔽UV光進入通道，我們亦探討調變LCL對所增加空乏區區域大小與臨界電壓所帶來的影響，茲就下列LCL之長度(25、50、100、200、300、400與500 nm)依序將元件命名為Type B1至Type B7。模擬結果顯示，於不照光下，增大LCL可提升通道的空乏能力降低截止電流(如Type B1的1.40×10^(-12) A減小至Type B7之8.60×10^(-13) A)。然過於增加空乏區範圍，將使Vth增大與電流路徑之面積減少，降低導通電流，進而使開關電流比Ion⁄Ioff 亦隨之下降(2.58×10^7 A⁄A→2.50×10^7 A⁄A)，若以開關電流比為最佳LCL之評估依據，顯然Type B5(其開關電流比達2.58×10^7 A⁄A之最高值)所使用LCL=300 nm即屬一最適化參數。然於UV光檢測器應用上，過大的LCL將影響元件的照光面積，降低光電流導致光響應特性下降。茲比較未照光下具最佳靜電特性之Type B5元件與最大化照光面積的Type B1元件兩者之光電特性，模擬結果顯示，Type B5其在250 nm之紫外光照射下之光響應度Rph僅有43.19 A⁄W，顯示其所採用300 nm之CL長度對於照光性的影響較為嚴重。相較之下，Type B1展現了較高的光響應度1325.14 A⁄W，其Sph亦高達9.21×10^4 A⁄A，顯示Type B1為一較合適的UV光檢測器。
於檢視蕭基金屬CL之效益上，茲分別就將Type A-30與Type B1的光電特性進行比較。模擬結果顯示，Type B1之截止電流1.40×10^12 A小於Type A-30的4.10×10^(-12) A，顯示蕭基金屬CL對於抑制通道的效果表現優異。於紫外光照射下，厚度增為60 nm的Type B1 TFT因採用蕭基金屬CL解決了Type A-30礙於通道厚度的光電流限制所表現出的較低光響應(Rph=383.24 A⁄W)，增加通道厚度的Type B1仍可處在全空乏狀態下，展現極佳的光響應(Rph=1325.14 A⁄W)。
本研究亦利用通道蝕刻方式進一步提升通道厚度以提升元件光響應，第三部分之研究係以Type B TFT結構參數為基礎，以蝕刻的方式使增厚通道的中央區域減為60 nm，以利通道維持全空乏狀態。具通道蝕刻與蕭基金屬CL之元件命名為Type C TFT。所擬探討之結構參數主要為兩種，其一為未蝕刻區通道厚度，分別為150、175、200 nm，其二為兩側的未蝕刻長度，分別為150、200、250 nm，我們將其搭配成9種結構並且依序命名為Type C1至Type C9，探討其靜電特性與照光特性。Type C元件之靜電特性模擬結果顯示，隨通道厚度的提升，元件轉移特性曲線出現左移的趨勢，表示臨界電壓降低。依據Type C1、C2與C3之模擬結果，顯示隨未蝕刻長度Lside的增加，元件的導通電流有提升的趨勢，此應源自通道電流路徑的截面積之提升以及Vth降低所致。於UV光照射下，模擬結果顯示，通道厚度的增加使Type C TFT光響應度較Type B TFT呈現進一步的提升，從Type B1之1325.14 A⁄W進一步提升至Type C9之1578.77 A⁄W；另從Lside的增加可以發現，在能量較弱的紫外光波段350 nm，Type C3的光響應度比Type C1來的高，此應為整體元件通道體積增大所帶來的效益。
本研究於具通道蝕刻與蕭基金屬CL IGZO TFTs元件光電特性之模擬分析結果證實，蕭基金屬CL於Type B TFT可提供增厚通道額外的空乏區使之處於全空乏狀態有效抑制暗電流，同時藉增厚之通道提升光電流，改善元件的光電響應特性。於採用通道蝕刻則更可以進一步提升通道厚度以增加光電流。模擬結果顯示結合通道蝕刻與蕭基金屬覆蓋層之Type C9 TFT擁有最大的開關電流比2.39×10^8 A⁄A、最高的光響應度(1578.77 A⁄W)與光靈敏度(1.37×10^6 A⁄A)。
本研究所提出具通道蝕刻與蕭基金屬CL IGZO TFTs元件，模擬分析結果顯示蕭基金屬CL可解除傳統TFT於暗電流Ioff與光電流Iph間之消長關係，而通道蝕刻則可於維持不增加暗電流Ioff下進一步提升光電流，此在TFT於紫外光感測器的應用上具有學術性與應用性價值，對紫外光感測器產業於先進紫外光感測器之開發可提供助益。

This study explores the use of a Schottky metal capping layer (CL) to mitigate the trade-off between dark current (Ioff) and photocurrent (Iph) in ultraviolet photodetectors (UVPDs) utilizing IGZO thin-film transistors (TFTs) through Sentaurus TCAD simulation. The IGZO channel thickness is found to be a crucial factor in influencing the photoelectric characteristics of devices. To enhance the photodetection performance of TFTs, a Schottky metal CL is applied on the back-channel surface to form a metal-semiconductor contact. Additionally, back-channel etching is also investigated to optimize channel thickness and reduce the trade-off between Ioff and Iph.
Simulation results reveal that the Type C9 TFT, having a thickness of 60 nm for the central channel and 200 nm for unetched channel on both sides, achieves the highest on/off current ratio of 2.39×10^8 A⁄A and an excellent subthreshold swing of 111 ( mV)⁄dec in darkness. Under UV illumination, Type C9 TFT exhibits exceptional photodetection capabilities, with a photoresponsivity of 1578.77 A⁄W, photosensitivity of 1.37×10^6 A⁄A and specific detectivity of 8.22×10^14 Jones.
The improvement is attributed to the combination of the Schottky metal CL and the back-channel etching, which increase the space available for producing photo-induced electron-hole pairs during UV illumination. These results findings indicate that IGZO TFTs with Schottky metal CL and back-channel etching structure are highly promising for advanced UVPD applications.

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