利用穿透式泵-探技術量測氧化鋅薄膜之超快載子動力學研究

本實驗主要是利用飛秒短脈衝雷射為激發光源，利用穿透式泵-探技術來量測五十奈米厚的氧化鋅薄膜的超快載子動態響應;當激發光子的能量高於氧化鋅能隙之上，被激發的載子會產生能隙填滿的效應使得穿透率變化為正，接著由於載子的冷卻使得穿透率變化降低，利用分量分解法得到載子熱化的時間在一個皮秒以內，接著我們可以觀察到能隙重整的效應使得穿透率變化變為負值，由於能隙填滿與能隙重整的競爭，在不同的激發光子能量下，發生負的穿透率變化會有時間上的延遲；當激發光子的能量接近能隙的時候，由於有限的費米分佈，使得所量到的穿透率變化都是正值，此外當光子的能量接近氧化鋅激子的能階時，由於能階密度的增加，能隙填滿的效應會變得更加明顯使得正穿透率變化的值最大，此外我們也觀察到較長的鬆弛時間約47~53皮秒，推論為自發性輻射的時間;當我們增加激發光源的功率時，由於熱聲子的效應會延長載子冷卻的時間；在淺層的能隙尾部，我們也觀察到能隙重整效應發生的時間延遲，當激發光子的能量在更低的時候，由於缺陷的吸收飽和使得穿透率變化為正，藉由聲子的幫助所得到穿隧載子的鬆弛時間會延長。

關鍵字

熱載子冷卻時間；能隙重整回復時間；瞬時放射時間

並列摘要

In this work, we use femtosecond Ti:sapphire laser as the photo-excited source to study the ultrafast carrier dynamics of a 50 nm-thick ZnO thin film by using transmission pump-probe measurements. As the photo-excitation energy is above band-gap states, the positive transmission change can be seen initially due to the band filling (BF) effect. Due to the carrier cooling, the decrease of the transmission change can be seen following. Then, we observe the negative transmission change due to the band-gap renormalization (BGR) effect. For the different photo-excitation energy, the negative transmission changes occur with different time delay due to the competition of BGR with BF. As the photo-excitation energy is near band-edge states, all measured transmission changes are positive due to the finite Fermi-Dirac distribution of photo-excited carriers. As the photo-excitation energy is near exciton resonance of ZnO epitaxial film, the influence of the BF effect becomes more obvious to result in the maximum positive transmission change due to the increase of density of states. We also observe the longer relaxation time about 47~53 ps that might be resulted from spontaneous emission (SPE). As we increase the power of the photo-excitation, the carrier cooling process becomes longer due to hot phonon effects. At higher energy side of shallow band-tail states, we also observe that the occurring of BGR effect depend on photon-excited energy. At lower energy side of shallow band-tail states, we observe the positive transmission change due to saturation of the defect absorption. Besides, the carrier relaxation time is prolonged by phonon-assisted-tunneling.

並列關鍵字

hot carrier cooling time ； band-gap renormalization recovery time ； spontaneous emission time

參考文獻

chapter 1

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利用穿透式泵-探技術量測氧化鋅薄膜之超快載子動力學研究

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