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標題: | 無氫臨界溫度電漿增強氣相沉積石墨烯於光調變及鎖模之應用 Hydrogen-free PECVD growth of few-layer graphene at threshold temperature for optical modulation and mode-locking applications |
作者: | Kaung-Jay Peng 彭冠傑 |
指導教授: | 林恭如(Gong-Ru Lin) |
關鍵字: | 多層石墨烯,低溫,薄鎳,穿透率,拉曼光譜,石墨烯光調製元件,石墨烯超快雷射, Few-layer graphene,low temperature,thin nickel film, transmission, Raman scattering,graphene base modulators,graphene ultra-fast laser, |
出版年 : | 2011 |
學位: | 碩士 |
摘要: | 在本篇論文中,我們以電漿輔助化學氣象沉積在低溫無氫的環境中以及極薄的鎳上合成多層石墨烯,沉積的碳原子一開始被鎳吸收然後在鎳的表面析出形成多層石磨烯。我們製程的溫度大約可以降到475oC,鎳厚度大約可以縮小到30 nm。因為鎳在低溫(475 oC )時的溶解度非常低,所以我們析出之石墨烯的層數不會太多,可以控制在很小的數值。我們用拉曼分析發現,在不同的鎳厚度去合成石墨烯,其中D與2D的強度幾乎保持不變,但是G的強度會隨著鎳厚度增加而增加,換而言之石墨烯的層數也隨之增加,直到鎳的厚度大於50 nm之後,G的強度才停止增加,會造成這個原因主要是因為由PECVD所提供的碳原子有限,所以鎳厚度超過50 nm之後所吸收的碳原子並沒有再增加,所以析出的石墨烯的層數相仿,拉曼圖形也相似。當我們將沉積時間從600縮減到100 s時,多層石墨烯在波長550 nm的光源下之線性穿透率由83%上升至93%,代表層數由八層縮小至三層,而拉曼光譜中的ID/IG值由1.8縮小到0.2,此外G峰之半高寬由67縮小至37.2 cm-1 ,這些證據都一再顯示當我們將石墨烯的沉積時間縮短,其品質會變好。
三種不同的石磨烯光調製元件在本論文中製作,並討論。 其中兩個光調製器是藉由電壓來控制光強度,另外一個是藉由光來控制光強度。第一個是穿透式石墨烯光調製器。在調製深度為20%的情況下,當介電質厚度由200 nm降到10 nm,則驅動電壓可由30 V降到5 V。反射式的石磨烯光調製器的效果沒有穿透式的效果來的好。在介電質的厚度為50 nm時,需要8 V才有5%的調製深度 。這不好的表現來於較厚的介電質以及元件在大電壓較易短路。最後一個光調製器,是石磨烯放在側磨光纖所做成。當輸入功率為500 mW 時, 在雷射功率為85 mW照射下,輸出光功率可提升至525, 519以及516 mW,其中輸入光波長依序為1520, 1550以及1580 nm,對應的調製深度為5, 3.8 以及 3.2 %。調製深度隨著輸入光波長縮短而增加。這是因為在較短的波長下,光有較強的能量可以將石墨烯之載子激發到較高的能階,而石磨烯在較高的能階有較多的空位階所以1520 nm 的雷射可以比1550及1580 nm激發較多的載子,故調製深度較大。 當輸入功率增加到1000 mW時,照射功率一樣維持在85 mW,最大的調製深度可以被達到。調製深度為5.8, 5.1 and 4.6 % 對應之光波長為1520, 1550以及1580 nm。在1550 nm輸入功率為1000 mW下的光調製深度從1.2 提升到 5.1 %,當照射光功率從24 提升到 85 mW。調製深度上升主要是因為當照射之雷射強度上升時,有較多的石墨烯載子可以被激發。當輸入光功率提升到2000 mW時,輸入波長1550 nm 在 85 mW 之照射下,調製深度由5.1下降至3.3 %。 這是因為大部分的能階在輸入功率為1000 mW就已經被填滿,所以當光功率提升至2000 mW 時,功率並不會比1000 mW 有顯著的提升,故調製深度下降。 石墨烯被當作飽和吸收體在穿透式以及反射式的被動鎖模摻鉺光纖雷射系統中。在穿透式的系統中,脈寬由441 提升至 483 fs光譜的半高寬由6 縮小至 4.2 nm當系統的電流由 900 縮小至 200 mA。系統的復現率為 28.57 MHz。在反射式的系統中,脈寬由 796 拓寬為874 fs光譜的半高寬由 3.25縮小至2.2 nm。系統的復現率為16.66 MHz。較小的復現率是因為反射式系統需要額外長度的光纖。 較小的脈寬可由穿透式的系統達成因為在反射式的系統中,大約1.8 dB的損耗是不可避免的。 另外穿透式系統的TBP值在輸入高電流時(800~900 mA)為0.31。這與理想值很靠近。而對於兩個系統的臨界電流為400 mA。當電流小於400 mA時其值縮小的很快,代表系統不穩定。 The synthesis of few-layer graphene sheet on ultra-thin nickel film coated SiO2/Si substrate by using hydrogen-free plasma-enhanced chemical vapor deposition with in-situ low-temperature carbon dissolution is preliminarily demonstrated. The deposited carbon atoms are initially dissolved into the nickel matrix and subsequently participated out on nickel film surface. The threshold carbon dissolution temperature for synthesizing few-layer graphene is observed as low as 475oC, and the critical thickness of host nickel film is at least 30 nm. Due to the ultra-low solubility of carbon atoms into nickel film at threshold temperature of 475oC, the layer number of few-layer graphene can be precisely controlled. Raman scattering analysis indicates almost identical D and 2D peak intensities for nickel films with different thickness, whereas the G peak enhances with increasing layer number of graphene precipitated from thicker nickel films. The saturation of G peak at 50-nm thick nickel film due to the finite carbon dissolution within a limited deposition time is observed to preserve a stabilized quality of precipitated few-layer graphene. The linear transmittance of few-layer graphene at 550 nm is increased from 83 to 93% when shortenening the deposition time from 600 to 100s, corresponding to a decrease of graphene layer number from 8 to 3 layers. The Raman scattering peak ratio of ID/IG decreases from 1.8 to 0.2 and the G-band linewidth shrinks from 67 to 37.2 cm-1 accordingly, providing strong evidence for the improved quality of few-layer graphene synthesized with the hydrogen-free and threshold temperature on ultra-thin nickel host. There are three type graphene based optical switches demonstrated by us. Two of the modulators controlled the signals by the voltage and the other controlled the signals by the continuous wave laser power. The first one is transmission type graphene modulator. For 20 % of modulation depth, the drive voltage can be decreased from 30 V to 5 V when the insulator thickness decreases from 200nm to 10 nm. The reflection type modulator doesn’t performed well than that of transmission type modulator. The modulation depth is only 5% at 8V with the insulator thickness of 50 nm. That is caused by the thicker insulator and the device short easier than that of transmission type modulator. The last one is graphene on side polished fiber optical switch. The input power enhanced from 500 mW to 525, 519 and 516 mW for 1520, 1550 and 1580 nm of laser under 85 mW of exposition and the modulation depths are 5, 3.8 and 3.2 % respectively. The modulation depth increases when the wavelength decreases. There are more states available at higher energy stage for graphene so 1520 nm of laser can excite more carriers than that of 1550 and 1580 nm. When the input power is 1000 mW and the power of continuous wave laser is 85 mW, the maximum modulation depth can be obtained. The modulation depth for 1520, 1550 and 1580 nm are 5.8, 5.1 and 4.6 % respectively. The modulation depth for 1550 nm with the intensity of 1000 mW increases from 1.2 to 5.1 % when the continuous wave laser power increases from 24 to 85 mW. More carriers can be excited when the power of continuous waved laser increases. The modulation depth of 1550 nm under 85 mW of exposition decreases from 5.1 to 3.3 % when the input power increase from 1000 mW to 2000 mW. The states are almost filled at 1000 mW, so 2000 mW can not further excited much more carriers. Graphene is served as mode locker for both transmission type and reflection type passively mode-locked fiber laser of the erbium-doped fiber lasers (EDFLs). In transmission type system, the pulsewidth increases from 441 to 483 fs and the spectral FWHM decreases from 6 to 4.2 nm when the pumping current decreases from 900 to 200 mA. The repetition rate is 28.57 MHz. In reflection type system the pulsewidth increases from 796 to 874 fs and the spectral FWHM decrease from 3.25 to 2.2 nm. The repetition rate is 16.66 MHz. The smaller repetition rate of reflection type passively mode-locked EDFL is cause by the extra cavity length contributing by circulator. The shorter pulsewidth could be obtained from transmission type passively mode-locked EDFL because the inevitable loss 1.8 dB caused by circulator in reflection type passively mode-locked EDFL. The time bandwidth product for both system is 0.31 under high pumping current (800~900 mA). It is really close to the transform limited. The pumping threshold for both system is 400 mA. The time bandwidth product decrease very fast when the pumping current is smaller than 400 mA. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/63411 |
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