飛秒激發二維陣列氮化鎵奈米柱之徑向呼吸模態

氮化鎵奈米柱是一個常被使用於光電元件和微機電元件的結構。然而當結構被縮小至奈米尺度時，一些有趣且和巨觀尺度底下不同之行為，常常引人注意和被研究。這些新的行為和現象將會造成未來元件縮小化後，在設計和實做上的挑戰。然而為了可以在製程的過程中，及時監控和量測材料的特性，一些量測的技術因此被發展出來。我們提出利用飛秒雷射所激發的二維陣列氮化鎵奈米柱的音波局限模態來監控奈米材料的彈性係數(elastic stiffness contant)的改變。在這裡的研究中，我們成功的利用電子束微影系統以及偶合電漿子乾式時刻技術，製作出直徑小於35nm的氮化鎵奈米柱，並利用奈米超音波的量測技術觀察不同直徑大小下之奈米柱的徑向呼吸模態。因為奈米柱的徑向呼吸模態，是屬於單一個奈米柱的自然共振模態，對於一個二維陣列結構來說，我們也設計不同的周期和不同的aspect ratio來驗證周期結構和呼吸振動膜態的關係，然而我們也分析了奈米柱的呼吸振動模態的品質因子，並發現品質因子和奈米柱直徑的相關性。最後，在我們的分析裡，我們觀察到當奈米柱直徑小於50nm的時候，材料的彈性係數(elastic stiffness contant)會變小，我們推論這是因為氮化鎵材料因尺度奈米化後，表面原子和原子之間的鍵結變弱造成的材料軟化效應(softening effect)。憑藉著我們的量測和分析的方法，得到直徑35nm的氮化鎵奈米柱，他的彈性係數(elastic stiffness contant)是193±24 GPa。另外，我們的量測也發現氮化鎵奈米柱在尺度縮小後不僅機械特性改變，他的光學特性也會有所不同。我們發現直徑越小的奈米柱會出現一個較短的載子存活時間，我們推論這個現象可能和氮化鎵表面的surface recombination和surface depletion region等載子的運動行為有關。

關鍵字

氮化鎵奈米柱；飛秒雷射；彈性係數

並列摘要

GaN nanorod is a popular structure for the optoelectronic and microlectromechanical devices. Under the nano-scale, the rod has some interesting phenomenon about optical and mechanical properties. In this thesis, we successfully fabricated 2-D arrayed GaN nanorods with E-Beam lithography and dry-etching. The rod diameter can be scaled down to 35nm with this fabrication method. Some modifications of optical and mechanical properties were observed in our measurement, compared with the bulk GaN. In order to monitor the changes of optical and mechanical properties, the ultrafast optical pump-probe technique was adopted to generate and detect the nano-confined acoustic modes. The size-dependence experiment also pointed that non-radiative surface recombination or surface depletion region on the side wall might be a dominator factor for the relaxation time of carrier recombination. Radial breathing oscillation of 2-D arrayed GaN nanorods was successfully excited and identified in rods with different diameters by using femtosecond transient reflectivity measurement. Through analyzing thus measured diameter dependent oscillation frequency, we discovered that modification of the mechanical property appeared in the 2-D arrayed piezoelectric GaN nanorods, fabricated on top of a bulk substrate, when the rod diameter was on the order of or less than 50 nm. Our measurement showed a much reduced elastic stiffness constant (C11) of 193±24 GPa in 35nm diameter nanorods, compared with the 365±2 GPa in bulk GaN. This size-reduction induced mechanical modification would be a critical factor to be considered for future sensing and energy applications. Our study also provides a new spectroscopic method to explore the size-reduction-induced softening effect through the measurement of the radial breathing oscillations.

並列關鍵字

GaN nanorod ； Femtosecond laser ； elastic stiffness contant

參考文獻

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飛秒激發二維陣列氮化鎵奈米柱之徑向呼吸模態

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