利用瞬態泵探量測技術研究氧化鋅的超快載子動態及同調聲子震盪

Investigation of ultrafast carrier dynamics and coherent acoustic phonons oscillation in ZnO by the transient pump-probe technique

沈昱凱(Yu-Kai Shen)

指導教授 ： 林家弘

國立臺北科技大學/電資學院/光電工程系研究所/碩士(2013年)

https://doi.org/10.6841/NTUT.2013.00746

摘要

在此研究中，我利用簡併反射式泵-探技術，量測成長在矽基板上氧化鋅薄膜的超快載子動態以及同調聲學聲子共振行為，採用的量測光源為二倍頻飛秒鈦藍寶石雷射。為了成功在矽基板上成長氧化鋅，此樣品使用釔二氧化鉿作為緩衝層。由樣品的反射與螢光光譜發現有兩個峰值存在，其波長分別落在378與385奈米（3.28、3.22電子伏特），對應到氧化鋅的激子複合輻射、激子-縱向光學聲子耦合輻射。在泵-探量測中，所使用光脈衝的中心波長可由355奈米調整到385奈米，此量測波段對應量測樣品的導電帶至能帶尾端（3.49-3.22電子伏特）。當激發光子能量落在導電帶時（355-369奈米，相當於3.49-3.36電子伏特），所量測到的瞬態反射率為正值，它是由於能隙填滿的效應所造成，而自由載子的快速衰減時間是由於非輻射復合，慢速衰減時間是由於輻射復合所造成。此外，在接近激子共振時（375-379奈米，相當於3.31-3.27電子伏特），可以得到極快的鬆弛時間，它是由於激子-激子散射所造成。當激發光子的能量位於樣品的淺層能階時（380-385奈米，相當於3.26-3.22電子伏特），瞬態反射率的變化由正值變為負值，它是由於激子共振效應所造成。此外，同調聲學聲子震盪訊號疊加在此瞬態反射率的圖形上長達200皮秒，此同調聲子震盪的振幅量級約為10-4、震盪頻率約為800億赫茲。此同調聲學聲子的產生是由於電位能形變產生的電應力所造成。利用實驗量測所得到振盪頻率可以反推氧化鋅的折射率，發現所得到的數值與利用Sellmier程式所得到的折射率近似。此外，我將所得到的震盪頻率對波向量作圖，由它的斜率得到聲波在氧化鋅的傳播速度為6089m/s，此數值與理論值接近。

關鍵字

氧化鋅 ； 超快載子動態 ； 同調聲學聲子 ； 泵探量測技術

並列摘要

In this thesis, we investigated the ultrafast carrier dynamics and the coherent acoustic phonons (CAPs) in high quality epitaxial ZnO thin films on c-Si substrate by the degenerate transient reflection pump-probe technique, using a frequency doubling femtosecond Ti:sapphire laser as the light source. In order to success grow the ZnO on the silicon substrate, yttrium-doped hafnium oxide (YDH) is used as a buffer layer. The reflection and photoluminescence spectrum of the sample reveal two emission peaks at 378 and 385 nm (3.28 and 3.22 eV) that corresponds to the free exciton emission and LO-phonon replica. In pump-probe measurement, the central wavelength of the UV pulse is tuned from 355 to 385 nm corresponding to the conduction band to shallow band-tail (3.49-3.22eV) of this sample. As excited photon energy above conduction band (355-369 nm, 3.49-3.36 eV), the measured positive transient differential reflectance change (TDRC) is due to the band filling effect. The free carrier relaxations include two exponential decays, the fast decay is non-radiation recombination time, and the slow decay is due to radiation recombination time. In addition, a relative fast decay time below 1-ps around exciton resonance is due to exciton-exciton scattering (375-379 nm, 3.31-3.27 eV). When the photo-excited energy is located at shallow band-tail (380-385 nm, 3.26-3.22 eV), the TDRC transfer from positive to negative is due to the resonance. The subsequent coherent acoustic phonons (CAPs) oscillation is superimposed on the TDRC within 200 ps, with the order of amplitude around 10−4, and the oscillation frequency around 80 GHz. It is recognized that the CAPs is mainly due to electric stress of deformation potential coupling. From the oscillation frequency of CAPs, the refractive index of ZnO is obtained, which is similar with the estimated value from Sellmeier equation. The velocity of CAPs around 6089 m/s propagation in ZnO can be obtained, from the plot of the angular frequency versus wave vector. The obtained velocity is close to the theory calculation of sound wave propagation velocity in ZnO.

並列關鍵字

zinc oxide ； ultrafast carrier dynamics ； coherent acoustic phonon ； pump probe technique

參考文獻

[1] H. T. Grahn, D. A. Young, H. J. Maris, J. Tauc, J. M. Hong and T. P. Smith, "Sound velocity and index of refraction of AlAs measured by picosecond ultrasonics", Appl. Phys. Lett., vol. 53, no. 21, 1988, pp. 20232024.

[2] Akira Harata, Hirouki Nishimura and Tsuguo Sawada, "Lases-induced surface acoustic waves and phtothermal surface gratings generated by crossing two pulsed laser beams", Appl. Phys. Lett., vol. 57, no. 2, 2001, pp. 132-134.

[3] C.-K. Sun, J.-C. Liang and X-Y. Yu, "Coherent Acoustic Phonon Oscillations in Semiconductor Multiple Quantum Wells with Piezoelectric Fields", Phys. Rev. Lett. vol. 84, no. 1, 2000, pp. 179-182.

[4] O. Matsuda, O. B. Wright, D. H. Hurley, V. E. Gusev and K. Shimizu, "Coherent Shear Phonon Generation and Detection with Ultrashort Optical Pulses", Phys. Rev. Lett., vol. 93, no. 9, 2004, pp. 095501.

[5] I. Bozovic, M. Schneider, Y. Xu, R. Sobolewski, Y. H. Ren, G. Lupke, J. Demsar, A. J. Taylor and M. Onellin, "Long-lived coherent acoustic waves generated by femtosecond light pulses", Phys. Rev. B, vol. 69, no. 23, 2004, pp. 132503.