本實驗以氧化鋅(ZnOx)與氧化錫(SnOx)二種主動層進行單層主動層薄膜電晶體與雙主動層薄膜電晶體的製作與電性量測，在製程方面，將單層主動層薄膜電晶體進行三種厚度的變化(10, 20, 40nm)；而雙主動層的部分則是在固定下層主動層的厚度為20nm，變化上層主動層的厚度(10, 20, 40nm)。電性量測部分包含汲極電流-閘極電壓曲線(ID-VG curve)、正閘極偏壓應力測試(Positive bias stress test, PBS)以及負閘極偏壓應力測試(Negative bias stress test, NBS)等的電性分析。將分析的結果計算出基本電性：臨界電壓(Threshold voltage, VTH)、次臨界擺幅(Subthreshold swing, S.S.)、載子遷移率(Mobility, µ)、開關電流比(on-off current ratio, ION/IOFF)以及臨界電壓偏移量(Threshold voltage shift, △VTH)。除此之外我們也將得到的基本電性數值進行換算，以獲得載子捕捉能態密度(由S.S.值換算而得)與載子濃度(由ID,max、VD及mobility換算而得)，用於討論薄膜電晶體VTH值與厚度的關係，以及施加閘極偏壓後其VTH值穩定性。
從X光光電子能譜儀(X-ray Photoelectron Spectroscopy, XPS)材料分析中可看出ZnOx所含氧空缺比例較低，SnOx所含氧空缺比例較高；同時，電性量測所推得之載子濃度與主動層中的氧空缺比例呈現正相關，說明氧空缺在本實驗中為提供載子的來源之一。由於ZnOx載子較少，SnOx載子較多，因此本實驗中量測兩種薄膜電晶體的性質的電性參數設定也有所不同。對於ZnOx薄膜電晶體: 設定參數為VD=40V，VG=-20~40V；對於SnOx薄膜電晶體: 設定參數為VD=1V，VG=-70~10V。經由量測基本元件：Al/ZnOx(20nm)/SiO2/p+ Si、Al/SnOx(20nm)/SiO2/p+ Si、Al/ZnOx(20nm)/SnOx(20nm)/SiO2/p+ Si以及Al/SnOx(20nm)/ZnOx(20nm)/SiO2/p+Si，可以得知雙主動層薄膜電晶體適合用下層主動層的量測參數設定進行電性分析，且下層主動層為薄膜電晶體性質主導層，主導著元件的mobility及VTH值。
在單層薄膜電晶體中，ZnOx主動層載子濃度較少，約略1016 cm-3且隨ZnOx厚度增加(10~40nm)載子濃度下降，導致VTH值往正的方向偏移；SnOx主動層載子濃度較多，約略1019 cm-3且隨SnOx厚度增加載子濃度上升，導致VTH值往負的方向偏移。除此之外，從XPS分析中得知ZnOx主動層中氧空缺比例隨厚度增加而減少；而SnOx主動層中氧空缺比例隨厚度增加而增加，故氧空缺訊號的比例與載子濃度有正相關。而在閘極偏壓穩定性方面，ZnOx主動層其載子捕捉能態密度隨厚度增加而增加，再加上載子濃度較少的，正閘極偏壓穩定性隨厚度增加而變差(△VTH變大)；SnOx主動層中材料的載子濃度較多，儘管元件內部的載子捕捉能態密度也隨厚度增加而增加，元件的穩定性因高載子濃度的影響，減緩了正閘極偏壓下電荷捕捉行為，使得偏壓穩定性隨著SnOx厚度增加而有所改善(△VTH變小)。
而在雙主動層薄膜電晶體中，Al/SnOx(h nm)/ZnOx(20 nm)/SiO2/p+Si中載子濃度隨著上層SnOx厚度增加而增加，元件的VTH值也隨之往負的方向偏移(h=10 nm, VTH=18.6 V; h=40 nm, VTH= -1.02 V)，改善原本ZnOx單層電晶體過於偏正的VTH值。除此之外，元件整體的載子捕捉能態密度隨著SnOx厚度增加而增加，其閘極偏壓穩定性應隨SnOx厚度增加而變差，然而在SnOx高載子濃度的作用下，可以減緩元件因正閘極偏壓所造成的載子捕捉效應，所以此處元件的穩定性隨厚度增加而改善(△VTH變小)。在Al/ZnOx(h nm)/SnOx(20 nm) /SiO2/p+Si載子濃度隨著上層ZnOx厚度增加而減少，可說明為什麼VTH隨ZnOx厚度增加而往正的方向偏移(h=10 nm, VTH= -68.5 V; h=40 nm, VTH= -27.3 V)；在閘極偏壓穩定性討論方面，載子捕捉能態密度隨ZnOx厚度增加而減少，儘管在此試片組中載子濃度隨著厚度增加有減少的趨勢，但從數值的大小仍可說明載子濃度是較多的(~1019 cm-3)，故在高載子濃度且載子捕捉能態密度隨厚度增加而減少這二種效應的作用下可解釋為什麼閘極偏壓穩定性(△VTH)隨著ZnOx厚度增加而改善。在XPS分析中，Al/SnOx(h nm)/ZnOx(20 nm)/SiO2/p+Si試片組之氧空缺訊號隨SnOx厚度增加而增加；而在Al/ZnOx(h nm)/SnOx(20 nm)/SiO2/p+Si試片組中氧空缺訊號隨ZnOx厚度增加而減少，再次說明氧空缺的訊號比例與載子濃度呈現正相關的趨勢。
由以上說明可知道儘管下層主動層為薄膜電晶體的主導層，但上方主動層對元件整體載子濃度的貢獻及載子捕捉能態密度有所貢獻，進而改變薄膜電晶的基本電性。而在雙主動層薄膜電晶體中，以Al/SnOx(20 nm)/ZnOx(20 nm)/SiO2/p+Si這個試片有最好的薄膜電晶體性質，不僅提升了薄膜電晶體的mobility，同時也有較適中VTH值與S.S.值，因此在進行薄膜電晶體疊層分析時，可針對上下主動層的特性分別加以調整，可預期提升薄膜電晶體的電性表現與穩定性。

關鍵字: 雙主動層, 載子濃度, 載子捕捉能態密度, 基本電性, 薄膜電晶體

In this study, sputtered zinc oxide (ZnOx) and tin oxide (SnOx) thin film transistors (TFTs) are fabricated into single-active-layer TFTs and dual-active-layer TFTs. The single-active-layer TFTs are made with different active-layer thickness (10, 20, 40 nm) for each metal oxide semiconductor. Also, we fabricated the dual-active-layer TFT with different top-layer thickness (10, 20, 40 nm) and constant bottom-layer thickness (20 nm) to investigate the influence on the electrical properties of the top layer and bottom layer in dual-active-layer TFT.
Transfer characteristics (ID-VG curves), positive bias stress test and negative bias stress test, were measured on these TFTs. Based on the ID-VG curves, the threshold voltage(VTH), subthreshold swing(S.S.), saturation mobility(µ), and the on-off current ratio (ION/IOFF) are extracted. The VTH shift (△VTH) under bias stress test is also obtained to perceive the device stability. Furthermore, density of trap states (Ntotal) and carrier concentration (ne) are calculated, to explain the dependence of VTH and △VTH (under bias stress) on active-layer thickness. According to the XPS analysis, there is a lower ratio of oxygen deficiency in ZnOx and a higher ratio in SnOx. Meanwhile, the carrier concentratin and the ratio of oxygen deficiency are in positive correlation, which means that the oxygen deficiency is a kind of carrier supply source.
The electrical test settings for single-active-layer SnOx TFTs and single-active-layer ZnOx TFTs are quite different. For single-active-layer ZnOx TFT, the setting is VD=40V, VG=-20~40V; and for single-active-layer ZnOx TFT, setting is VD=1V, VG=-70~10V. After measuring the following devices: Al/ZnOx(20nm)/SiO2/p+Si, Al/SnOx(20nm)/SiO2/p+Si, Al/SnOx(20nm)/ZnOx(20nm)/SiO2/p+Si and Al/SnOx(20nm)/ZnOx(20nm)/SiO2/p+Si, it is observed that the electrical-test setting must match the setting of bottom active layer and the bottom active layer dominates the mobility and VTH value of dual-active-layer TFTs.
For single-active-layer TFTs, the carrier concentration in ZnOx active layer (~1016 cm-3) is less than in SnOx active layer (~1019 cm-3). When the ZnOx thickness increases (10~40 nm), the carrier concentration decreases and the VTH values shift positively. As the SnOx thickness increases (10~40 nm), the carrier concentration increases and there is a negative shift in VTH value. Besides, XPS analysis reveals that the ratio of oxygen deficiency subpeak decreases as the ZnOx thickness increases, while it increases as the SnOx thickness increases. Therefore, the ratio of oxygen deficiency subpeak has a positive correlation with carrier concentration. The gate-bias stability of single-active-layer ZnOx TFTs gets worse (△VTH increasing) because the density of trap states in ZnOx active layer increases as the ZnOx thickness increases in addition to the decreasing of carrier concentration. In contrast, for single-active-layer SnOx TFTs, the density of trap states increase as the SnOx thickness increases. Because the high carrier concentration of SnOx may compensate the effect of charge trapping caused by positive gate-bias, the gate-bias stability of single-active-layer SnOx TFTs are improved (△VTH decreasing).
In the discussion of dual-active-layer TFTs with different top-active-layer thickness, as the SnOx thickness increases in Al/SnOx(h)/ZnOx(20)/SiO2/p+Si TFTs, the carrier concentration increases, which leads to the decreasing of VTH values (h=10 nm, VTH=18.6 V; h=40 nm, VTH= -1.02 V). Besides, the density of trap states increases as the SnOx thickness increases. However, the high carrier concentration can reduce the trapping effect caused by positive gate-bias. So, the gate-bias stability of Al/SnOx(h)/ZnOx(20)/SiO2/p+Si TFTs are improved (△VTH decreasing). In Al/ZnOx(h)/SnOx(20)/SiO2/p+Si TFTs, the carrier concentration decreases when the ZnOx thickness increases; therefore, VTH values increase with the increasing ZnOx thickness (h=10 nm, VTH= -68.5 V; h=40 nm, VTH= -27.3 V). As the ZnOx thickness increases, the density of trap states decreases. Though the carrier concentration reduces when adding ZnOx, it is still a high value for carrier concentration (~1019 cm-3). Due to both high carrier concentration and decreasing in density trap of states, the positive gate-bias stability is improved. According to the XPS analysis, the ratio of oxygen deficiency increases as the SnOx thickness increases in Al/SnOx(h)/ZnOx(20)/SiO2/p+Si TFTs, while in Al/ZnOx(h)/SnOx(20)/SiO2/p+Si TFTs, the oxygen deficiency decreases when the ZnOx thickness increases. That means the oxygen deficiency in both single-active-layer TFTs and dual-active-layer TFTs is a kind of carrier supply source.
Finally, although the bottom active-layer dominates the mobility of dual-active-layer TFT, the top active-layer will affect overall carrier concentration and density of trap states. Consequently, electrical performance of the dual-active-layer TFT can be modified. In our study, the Al/SnOx(20nm)/ZnOx(20nm)/SiO2/p+Si TFT has the best electrical performance. This device not only improves the mobility, but also do have the moderate S.S. and VTH value. Therefore, the electrical performance and bias stability of TFTs, can be improved by tuning the top-and-bottom-active layers characteristics individually.

Key words: dual-active-layer, carrier concentration, density of trap state, electrical properties, thin film transistor

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