有機磁性半導體—富勒烯與鈷的交互作用探討
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2022
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在自旋電子學中，磁性半導體是其中一個重要研究領域，其中有機材料與磁性材料的電子交互作用，是如何影響有機-磁性複合材料的磁性與電子傳輸行為，更是一個需要深入探討的領域。本研究使用物理氣相沉積法 ( Physical Vapor Deposition, PVD )，於超高真空系統 ( Ultra-High vacuum system, UHV ) 中，選擇在Al2O3 與Si兩種基板上，成長了C60/Co/C60與C60/Co的三層膜與雙層膜結構。透過探討薄膜磁性、表面形貌、光致螢光光譜( Photoluminescence, PL ) 與拉曼光譜 ( Raman Spectrum ) 、電壓-電流性質、磁阻響應與霍爾效應 ( Hall effect ) 在不同溫度的真空熱退火前後的變化，並以共鍍方式成長了不同比例的Co-C60 複合材料，並與上述退火實驗結果進行比較。本實驗分為兩大主軸，第一部分為C60薄膜與C60/Co 層膜在500 ℃ 下的真空退火，由表面形貌量測中，發現成長於Al2O3基板的C60/Co 雙層膜於退火後，形成了以Co原子為主的奈米島分區結構，以及C60 薄膜經過退火後，形成了近十奈米的原子團簇；在使用拉曼光譜分析碳基材料振動模式後，發現C60裂解為無定型碳的程度，因Co原子的參與下變得更高，說明了Co與C原子之間的交互作用，不僅增強了C60的裂解行為，同時限制了無定型碳的脫附行為；在磁滯曲線量測中，經過500 ℃ 退火後薄膜鐵磁行為明顯增強，包含了矯頑場 ( Coercivity, Hc ) 增大了至少5倍以上，以及薄膜由無磁性/順磁性轉變為鐵磁性；在光致螢光光譜量測中，可觀察到C60 與無定型碳之PL峰值強度皆受到Co原子的含量影響；在電壓-電流特性的量測中，注意到C60/Co 雙層膜無論退火前後皆屬於導體；在磁阻量測中，注意到退火後C60/Co 雙層膜磁阻率增大了將近50 %；在霍爾效應量測中，C60/Co 雙層膜經過500 ℃ 退火後，薄膜主要載子由電洞變為電子，並量測到載子濃度為2.32 × 1021 cm-3，載子遷移率為10.9 cm2V-1s-1。第二部分則是製作不同比例的Co-C60 複合材料，並注意到Co原子比例越低，薄膜內材料就以蕭特基接觸為主，以及C60分子的發光特性受到Co原子的熱蒸鍍過程破壞，最後則是C60在共鍍過程中受到Co-C60電子交互作用影響，導致C原子間的鍵能改變，進而改變C60的分子振動模式。上述實驗結果說明了Co與C60的交互作用增強了C60的裂解行為，且C60裂解後所形成的無定型碳，與Co原子混合後誘發了更明顯的磁性行為，同時在光學量測發現退火後的C60/Co仍保有半導體性質，暗示了只要適當調整Co原子與C60含量，就可利用真空退火製作出以Co-C為主成分的磁性半導體，對改善有機自旋閥中的電導率不匹配，具有相當大的潛力。
In spintronics, magneto-semiconductor is an important part of the connection with spin and devices. To improve the conductivity mismatch between metal and organic semiconductor (OSC), the electrion interaction organic-magnetic interface is a potential solution. In this thesis, we choose Al2O3 and Si substrate to prepare C60/Co/C60 tri-layer and C60/Co bi-layer structure with physical vapor deposition in an ultra-high vacuum system. Through the analysis of morphology image, magnetism, Photoluminescence (PL), Raman spectrum, I-V curve, MR and Hall effect, we compare these results before and after annealing in vacuum.There are two main parts in the experiment setup. One is the annealing experiment of C60 thin film, C60/Co/C60 tri-layer and C60/Co bi-layer. During 1 hour annealing at 250 ℃, 500 ℃ and 750 ℃, morphology image show the nanostructural transition of C60/Co/Al2O3 and the columnar-like structure of C60 thin film after annealing at 500 ℃；MOKE reveal that the enhancement of magnetism in C60/Co and C60/Co/C60, such as the amplification of coercivity；PL exhibited cobalt atom reduced the PL intensity of C60；In the Raman spectrum analysis, we observe the characteristic peak shift of C60 which illustrate the formation of amorphous carbon；I-V curve show that C60/Co is a conductor weather it is annealed or not；MR show the MR ratio of C60/Co bi-layer enhanced about 50 % after annealing at 500 ℃；Hall effect measurement show that after annealing at 500 ℃, the majority carrier of C60/Co changed from hole to electron.The other part is the preparation of Co-C60 composites. First we notice that the lower content of cobalt would cause more schottky contact in the composites. Then we observet the chacteristics peak of C60 was reduced by the cobalt atom deposition by the PL measurement. Finally, the raman vibration mode of C60 is shift at least 20 cm-1 by the electrion interaction of cobalt and C60.The morphology show the interaction between cobalt and C60 enhance the formation of amorphous carbon. Annealing in vacuum contribute to the stronger magnetism due to the mixture of cobalt and amorphous carbon. In additional, PL and Raman show that semiconductor properties remain after annealing. By controlling the ratio of cobalt and C60, we can make Co-C magnetic-semiconductor by annealing in vacuum.
In spintronics, magneto-semiconductor is an important part of the connection with spin and devices. To improve the conductivity mismatch between metal and organic semiconductor (OSC), the electrion interaction organic-magnetic interface is a potential solution. In this thesis, we choose Al2O3 and Si substrate to prepare C60/Co/C60 tri-layer and C60/Co bi-layer structure with physical vapor deposition in an ultra-high vacuum system. Through the analysis of morphology image, magnetism, Photoluminescence (PL), Raman spectrum, I-V curve, MR and Hall effect, we compare these results before and after annealing in vacuum.There are two main parts in the experiment setup. One is the annealing experiment of C60 thin film, C60/Co/C60 tri-layer and C60/Co bi-layer. During 1 hour annealing at 250 ℃, 500 ℃ and 750 ℃, morphology image show the nanostructural transition of C60/Co/Al2O3 and the columnar-like structure of C60 thin film after annealing at 500 ℃；MOKE reveal that the enhancement of magnetism in C60/Co and C60/Co/C60, such as the amplification of coercivity；PL exhibited cobalt atom reduced the PL intensity of C60；In the Raman spectrum analysis, we observe the characteristic peak shift of C60 which illustrate the formation of amorphous carbon；I-V curve show that C60/Co is a conductor weather it is annealed or not；MR show the MR ratio of C60/Co bi-layer enhanced about 50 % after annealing at 500 ℃；Hall effect measurement show that after annealing at 500 ℃, the majority carrier of C60/Co changed from hole to electron.The other part is the preparation of Co-C60 composites. First we notice that the lower content of cobalt would cause more schottky contact in the composites. Then we observet the chacteristics peak of C60 was reduced by the cobalt atom deposition by the PL measurement. Finally, the raman vibration mode of C60 is shift at least 20 cm-1 by the electrion interaction of cobalt and C60.The morphology show the interaction between cobalt and C60 enhance the formation of amorphous carbon. Annealing in vacuum contribute to the stronger magnetism due to the mixture of cobalt and amorphous carbon. In additional, PL and Raman show that semiconductor properties remain after annealing. By controlling the ratio of cobalt and C60, we can make Co-C magnetic-semiconductor by annealing in vacuum.
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有機-磁性介面, 磁性半導體, 原子力顯微儀, 柯爾磁光效應, 拉曼光譜儀, 光致螢光光譜, Co-C60 複合材料, 磁阻量測, 霍爾效應, Organic-magnetic interface, Magnetic-semiconductor, MOKE, AFM, Co-C60 composite, Magnetoresistance, Raman spectrum, Photoluminescence, Hall effect