| 研究生: |
李國任 Lee, Guo-Ren |
|---|---|
| 論文名稱: |
不同金屬電極及傳輸通道設計之氧化鋅薄膜電晶體電性研究 Electrical Properties of ZnO Thin Film Transistors with Various Metal Electrodes and Channel Designs |
| 指導教授: |
陳貞夙
Chen, Jen-Sue |
| 學位類別: |
碩士 Master |
| 系所名稱: |
工學院 - 材料科學及工程學系 Department of Materials Science and Engineering |
| 論文出版年: | 2009 |
| 畢業學年度: | 97 |
| 語文別: | 中文 |
| 論文頁數: | 110 |
| 中文關鍵詞: | 通道長度 、薄膜電晶體 、金屬電極 |
| 外文關鍵詞: | TFT, channel, electrodes |
| 相關次數: | 點閱:76 下載:0 |
| 分享至: |
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本研究針對薄膜電晶體之金屬電極與半導體層之接觸進行探討,以不同金屬作為電極,量測其接觸電阻,並探討接觸電阻與薄膜電晶體電性之間的關係。此外,亦透過不同通道設計變化來探討不同金屬於不同通道設計下對薄膜電晶體元件表現的差異。實驗上以p-型矽(100)作為薄膜電晶體的閘極,而以500 nm二氧化矽層作為薄膜電晶體之絕緣層,並利用氧化鋅靶材以反應性磁控濺鍍法將厚度為50 nm之氧化鋅濺鍍於二氧化矽上。之後,透過電子束蒸鍍系統將四種金屬鋁(Al)、鈦(Ti)、鉭(Ta)、鎳(Ni)分別鍍於氧化鋅薄膜上,金屬層厚度控制在大約120 nm左右。完成金屬電極製作後,於同一試片上進行薄膜電晶體電性量測以及接觸電阻之量測,藉以比較由不同金屬與氧化鋅接合所產生之接觸電阻對整個薄膜電晶體元件電性表現影響並且從材料分析角度探討其原因。
本實驗利用X光繞射儀鑑定氧化鋅薄膜的晶體結構與結晶特性;使用X光光電子能量分析儀觀察由薄膜表面至內部的化學組成及鍵結的變化,再輔以拉賽福背向散射分析儀對氧化鋅薄膜進行組成成份鑑定;以穿透式電子顯微鏡觀察電晶體結構之截面影像以及氧化鋅薄膜之微結構影像;最後,以紫外光-可見光光學儀測得薄膜穿透率並計算得到光能隙(Eg)。
薄膜電晶體電性表現以Agilent 4156C量測之,並萃取出TFT元件在不同金屬作為電極材料時之電性如飽和移動率( )、飽和電流值(ISat.)、起始電壓值(VTH)及開關比(On/Off ratio);利用Transmission Line Model(TLM)並以”金屬(Al、Ti、Ta、Ni)/氧化鋅(50nm)/二氧化矽(500nm)/矽”之試片進行接觸電阻值的量測與計算。
X光繞射儀之結果確定本實驗所製備之氧化鋅為多晶結構; 使用X光光電子能量分析儀發現,本實驗所製備之三種不同金屬鋁、鈦、鉭皆會與鋅競爭氧原子,根據熱力學原理,鋁、鈦、鉭三種金屬形成氧化物所需之生成自由能皆比鋅小很多,而鎳金屬則反;從拉賽福背向散射分析儀之氧化鋅薄膜成分分析顯示,當氧化鋅薄膜厚度為50 nm時,O:Zn=0.95,確定本實驗之氧化鋅薄膜之傳導機制為氧空缺(Oxygen Vacancies);另外,透過穿透式電子顯微鏡之電晶體結構之截面影像以及氧化鋅薄膜之微結構影像,進一步確定了金屬層與氧化鋅層界面氧化情形以及氧化鋅薄膜之多晶結構;紫外光-可見光光學儀測得薄膜穿透率約為85%~90%,並計算得到光能隙(Eg),並了解由厚度不同所造成的Eg差異可能來自於不同厚度時,薄膜具有數量不同之氧空缺。
接觸電阻的量測結果顯示,其鋁、鈦、鉭金屬與氧化鋅接面之接觸電阻值分別為0.0029( )、1.252( )、0.315( ),而鎳金屬與氧化鋅接觸部分由本實驗結果顯示無法量得接觸電阻值,亦無法萃取其TFT電性,主要原因是鎳金屬的功函數為5.15eV,比本實驗所製備之n-型氧化鋅之功函數(4.5eV)高0.65eV,構成形成蕭特基接觸之條件,為造成非歐姆接觸之主因。
本實驗藉由不同電極通道之設計,獲得不同的TFT電性結果並且可歸納出一系列的電性趨勢。在相同金屬電極的TFT中,電極間傳輸通道越短,所量測出的飽和移動率( )、飽和電流值(ISat.)及開關比(On/Off ratio)皆隨電極傳輸通道長度下降而漸序上升,而起始電壓值(VTH)則反。相同電極寬度/傳輸通道長度(W/L)比值時,除了鎳金屬因為與氧化鋅形成蕭特基接觸與而鉭金屬數據部分可能因為其電極氧化形成Ta2O5,此氧化物的絕緣特性可能影響TFT電極處之current injection程度,導致其開關比、飽和電流值及漏電流之數據不甚理想,造成不可比較,故本實驗可歸納出在飽和電流值(ISat.)和開關比(On/Off ratio)方面,以鋁作為金屬電極之TFT大於以鈦作為金屬電極之TFT,而飽和移動率( )和起始電壓值(VTH)之趨勢則相反。
As the design of the electrodes on Thin Film Transistor (TFT) plays a curial role in the determination of TFT electrical properties, we have investigated the differences in electrical properties of TFTs fabricated with different types of metals, such as Al, Ti, Ta, and Ni, used as source and drain electrodes along with various designs of channel width to length ratios (W/L). The structure of the TFTs was designed as bottom-type, in which the gate was made with p-type Si (100). In addition, 500 nm thick SiO2 serving as the insulator was thermally grown on to the gate. 50 nm thick active layer material, Zinc Oxide (ZnO) was prepared by magnetron sputtering prior to the growth of the metals on the ZnO surface by E-beam Evaporation.
The crystal structure of fabricated ZnO thin film was identified by grazing incident angle x-ray diffractometer (GIAXRD). X-ray photoelectron spectroscopy (XPS) was applied for the composition and chemical bonding analysis of the interface between metals and ZnO layer. Rutherford backscattering spectroscopy (RBS) was utilized to examine the composition of ZnO thin film. The cross-sectional images and microstructure of the ZnO thin film were observed through high-resolution transmission electron microscopy (HRTEM). The optical transmittance of the ZnO thin films with various thicknesses were measured by UV-vis. spectrophotometer and their energy band gaps were calculated.
The experimental results of GIXRD and HRTEM reveal that the ZnO thin film is polycrystalline structure. With RBS, we confirmed that the O/Zn ratio for 50 nm ZnO is around 0.95, so the conducting mechanism through oxygen vacancies of the ZnO thin film was implicated. XPS analysis showed the oxidation behaviors at the interface of different metals and ZnO layer with the support of HRTEM cross-sectional images. From the results of UV-vis., the transmittance of the ZnO thin film was measured around 85%~90% and energy band gap was calculated.
The electrical properties of the TFT, such as Saturation Mobility( ), Saturation Current(ISat), Threshold Voltage(VTH), and On current and Off current ratio (On/Off ratio) were measured by Agilent 4156C; The contact resistance on samples of ”metals(Al, Ti,Ta,Ni)/ZnO(50nm)/SiO2(500nm)/Si” were measured and calculated through Transmission Line Model (TLM).
The results of the contact resistance measurement and calculation of the Al/ZnO、Ti/ZnO、Ta/ZnO are 0.0029( )、1.252( )、0.315( ) respectively , the contact resistance of Ni/ZnO was not able to be measured due to its metal work function(5.15eV) is 0.65eV higher than the fabricated ZnO(4.5eV), making the contact a non-ohmic contact(shocktty contact).
With various designs of channel width and length ratios (W/L), a series of electrical properties were summarized. If the metal material is the same, the , ISat , On/Off ratio all increased with only the VTH went down when the distance of the electrodes was shortened. If the electrode width and conducting channel length ratio (W/L) is fixed, the ISat and On/Off ratio of Al electrodes TFTs are higher than that of Ti electrodes TFTs’, while the and VTH of Al electrodes TFTs are lower than that of Ti electrodes TFTs’. The failure of the Ta electrodes is possibly due to the degradation of the properties of metal electrodes during its fabrication process, and the non-ohmic Ni/ZnO also leads to the failure of its TFT electrical properties.
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