以明膠蛋白為閘極介電層之高載子遷移率有機場效應電晶體

High carrier mobility organic field-effect transistors with gelatin as gate dielectric

毛隆凱(Lung-Kai Mao)

指導教授 ： 黃振昌

國立清華大學/工學院/材料科學工程學系/博士(2014年)

摘要

本論文利用明膠蛋白作為介電層材料，製備有機場效應電晶體並探討其相關應用。明膠是一種自然生物高分子具有良好的絕緣特性及很好的成膜性，可利用簡單的溶液製程塗佈於軟性基板上面。以明膠蛋白為介電層之P型五苯環(pentacene)有機場效應電晶體的元件特性受明膠蛋白分子量所影響，其中以300 bloom明膠為介電層之P型五苯環有機場效應電晶體具有最好的元件表現，其元件具有相當高的平均載子遷移率16 cm2V-1s-1以及很低的臨界電壓-1 V。除此之外元件也具有良好的可撓曲特性，在0.34% 的壓縮應力測試下，載子遷移率由原先的14 cm2V-1s-1，稍微降低至13.5 cm2V-1s-1。如此高的元件表現提升了五苯環有機場效應電晶體在低電壓、低成本與可撓曲元件應用的可能性。
在實際的邏輯電路應用中，例如互補式金屬氧化物半導體(CMOS)電路，元件必須同時具備P型及N型電晶體於同一元件，因此本論文亦探討N型有機場效應電晶體的元件特性。以明膠蛋白為介電層之N型PTCDI-C8有機場效應電晶體，當元件在真空中操作其載子遷移率為0.22 cm2V-1s-1以及臨界電壓55 V，然而當量測環境由真空轉移至大氣時，載子遷移率上升至0.74cm2V-1s-1而臨界電壓則大幅下降至2.6 V。最大遲滯電壓由真空中的10 V下降到大氣下的2 V，此遲滯電壓的降低與最大缺陷密度(Nss)由在真空中的2.3×1012 cm-2eV-1下降至在大氣中的5.8 ×1011cm-2eV-1之結果一致。元件特性在真空以及大氣中的大幅改變，可藉由明膠蛋白與水分子反應產生離子之機制來合理解釋。
雙極性(ambipolar)有機場效應電晶體可藉由選擇P型五苯環以及N型PTCDI-C8兩種有機半導體材料以雙層結構(bilayer)製作而成。而雙載子有機場效應電晶體的元件特性受明膠吸水解離、半導體層的沉積順序、以及半導體層之間的相對厚度所影響。當以PTCDI-C8/五苯環之沉積順序作為元件的結構，元件的電洞電流遠大於電子電流，即使改變雙層之間的相對厚度仍無法得到平衡的雙載子傳輸特性。然而以pentacene/五苯環作為結構，可藉由改變雙層之間的厚度來調控元件電子與電洞的相對電流，其中大氣下最佳參數為pentacene(40 nm)/五苯環(40 nm)，可得到平衡且高的電子遷移率0.85 cm2V−1s−1和電洞遷移率0.95 cm2V−1s−1。然而，當操作環境為真空時，明膠內的水分子被抽離，此時元件中電子電流反而會大於電洞電流，藉由改變雙層間之厚度來調控元件電子與電洞的相對電流，真空中的最佳參數為五苯環(65 nm)/PTCDI-C8(40 nm)，可得到平衡的電子遷移率0.008 cm2V−1s−1和電洞遷移率0.007 cm2V−1s−1。
最後本論文探討以五苯環以及PTCDI-C8當作P型及N型有機半導體材料製作有機互補式相反器(organic CMOS inverter)，相反器的特性可藉由TTC (tetratetracontane,C44H90)插入層而大幅改善。當加入TTC時，P型及N型電晶體在輸出曲線(output curve)具有較好的截止(pinch-off)特性及電流飽和(current-saturation)特性，此外在遲滯傳輸曲線(hysteresis transfer curve)中，最大遲滯電壓下降且元件表現對稱的P型及N型傳輸特性，製作而成的相反器具有對稱的電壓轉換特性(voltage transfer characteristics)，非常小的遲滯電壓，以及低的操作電壓10 V。其中最好的元件表現發生在VDD(supply voltage)為12 V時，可得到非常高的相反器增益60，實現了低操作偏壓、低遲滯、高相反器增益、大氣下操作的有機互補式 相反器。

關鍵字

明膠蛋白 ； 有機場效應電晶體 ； 五苯環 ； PTCDI-C8 ； 雙載子 ； 互補式相反器

並列摘要

Solution-based gelatin thin film was utilized as the gate dielectric material for organic field-effect transistors (OFETs) fabricated on flexible poly(ethylene terephthalate) (PET) substrate. Gelatin is a natural protein with good film forming and insulating properties, which was coated on PET by a low cost solution process. The performance of p-type pentacene OFETs were found to depend on the molecular weight of gelatin dielectric. The pentacene OFETs with 300 bloom gelatin as the gate dielectric exhibited the best performance with a very high average field-effect mobility (FE) of 16 cm2V−1s−1 and a low threshold voltage (VTH) of -1 V. The pentacene OFETs fabricated in the rearch also showed good flexibility under bending test. Under 0.34% compressive strain, the FE was slightly reduced from 14 cm2V−1s−1 (flatten) to 13.5 cm2V−1s−1 (bending in 0.34% compressive strain). The device performance was found decent enough to open the possibility of fabricating pentacene OFETs in applications of low-power consumption, low-cost manufacturing and being flexible structurally.
It is well known that electron transporting is also essential in circuits of complementary metal-oxide semiconductor (CMOS), which requires both p- and n-type transistors on a single device substrate. The present research found that gelatin also works well as the gate dielectric for n-type N,N-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) OFETs with FE and VTH values were (0.22 cm2V−1s−1, 55V) in vacuum and (0.74 cm2V−1s−1, 2.6V) in ambient air. The maximum voltage shift in hysteresis was also reduced from 10 V to 2 V when the operation environment changed from vacuum to ambient air, which was consistent with the reduction of the NSS value from 2.3×1012 cm-2eV-1 in vacuum to 5.8 ×1011cm-2eV-1 in ambient air. The improvement in the device performance is attributed to the charged ions generation owing to the water absorption in gelatin in ambient air.
The fabrication of ambipolar OFETs was also completed based on the bilayer structure of pentacene/PTCDI-C8 with gelatin as the gate dielectric. The ambipolar characteristics were found to depend on relative hole and electron currents that were affected by the moisture absorption in gelatin, layer sequence and relative thickness of pentacene and PTCDI-C8. In the PTCDI-C8/pentacene layer sequence, the hole current in pentacene was much higher than the electron current in PTCDI-C8 regardless of the change of layer thicknesses in ambient air and no ambipolar characteristics seemed to appear. However, the relative contribution from hole and electron currents depended on their relative layer thickness in the pentacene/PTCDI-C8 layer sequence in ambient air. The present work found that the optimum ambipolar performance occurs at pentacene (40 nm)/PTCDI-C8 (40 nm) with electron FE of 0.95 cm2V−1s−1 and hole FE of 0.85 cm2V−1s−1 in ambient air. In contrast, electron current became higher than hole current in pentacene/PTCDI-C8 ambipolar OFETs when moisture was extracted out of gelatin in vacuum. The optimum ambipolar performance occurred at pentacene (65 nm)/PTCDI-C8 (40 nm) with electron FE of 0.008 cm2V−1s−1 and hole FE of 0.007 cm2V−1s−1.
The present research also fabricated air ambient operated organic CMOS inverters using pentacene and PTCDI-C8 as semiconductor layers with a bilayer dielectric of tetratetracontane (TTC) and gelatin. The performances of inverters were greatly improved when gelatin was replaced by TTC/gelatin bilayer dielectric. With the TTC/gelatin bilayer, both types of OFETs show better pinch-off and current saturation in output characteristics and negligible hysteresis in transfer characteristics. The organic CMOS inverters can be operated at a voltage as low as 10 V with symmetric voltage transfer characteristics and small hysteresis. A high static gain of 60 can be obtained at a voltage of 12 V. Ambient air operated organic CMOS inverters with low operation voltage; negligible hysteresis and high static gain were realized.

並列關鍵字

Gelatin ； OFETs ； Pentacene ； HASH(0xcd71990) ； Ambipolar ； CMOS Inverter

參考文獻

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