利用瞬態泵-探量測技術研究非極化氧化鋅薄膜的超快載子動態

在此研究中，我們使用二倍頻飛秒鈦藍寶石雷射作為量測光源，利用簡併反射式泵-探量測技術，研究在r平面鈦藍寶石基板上所成長薄約97奈米和厚約291奈米a平面氧化鋅薄膜的超快載子動態行為，經由極化反射光譜的量測，發現入射電場極化方向與樣品c軸垂直和平行時所測得的能帶位置不同，此兩能帶分別對應到氧化鋅樣品的E1與E2能帶其能量差約為23 meV的，由於受到應力影響，使得此樣品兩個能帶的光子能量比氧化鋅塊材的A和C能帶大。在瞬態反射率量測中，當所採用的光子能量落在能帶共振位置附近，並且激發光的電場垂直樣品c軸時，觀察到雙光子吸收、能帶填滿與能帶縮減效應；此外，相對較薄的樣品，較厚樣品所量測到最大能帶縮減的位置不會隨著量測波長而改變，這是由於載子擴散效應所造成的。在激發光的電場平行c軸時，瞬態反射率變化的量測僅觀察到能帶填滿效應。接著，當我們將激發光子能量落在激子共振位置附近時，我們比較入射電場極化方向與樣品c軸垂直與平行時所量測到激發的瞬態反射率變化圖形，發現在入射電場垂直與平行c軸所得到的行為類似，但是約有19 meV的能量差，這個差值與氧化鋅樣品的E1和E2能帶差類似，同樣在較厚的樣品也觀察到相同的現象。

關鍵字

非極化氧化鋅；超快載子動態；泵探量測技術

並列摘要

In this study, we investigated the ultrafast carrier dynamics by different pump polarization and sample thickness at states above the bandgap and near exciton resonance. 97 nm and 291 nm a-plane ZnO epifilm grown on r-plane sapphire substrate were subjected to the degenerate reflection-type pump-probe technique with a frequency doubling femtosecond Ti:sapphire laser as the light source. From the polarized reflection spectrum using the white light source with the electric field perpendicular and parallel to c-axis of the sample, respectively, the two valence bandedge energy levels of ZnO film were measured that corresponds to the E1 and E2 band with the energy difference about 23 meV. Owing to the anisotropic strain, the energy levels of these two bands are larger than the A and C bands in the bulk ZnO. As the excitation energy above the bandgap, the two photon absorption (TPA), band filling (BF) and bandgap renormalization (BGR) effects were observed while polarization of the pump beam was perpendicular to c-axis. Compared to the thin ZnO film, the transition point from BGR to recovery regime in various excitation energies above the bandgap would not become faster as excitation energy decrease with the pump polarization was perpendicular to c-axis in thick ZnO sample. Besides, only the BF effect was observed in the transient differential reflectance (TDR) trace while the polarization of the pump beam was parallel to c-axis of ZnO film. Using two different pump polarization (perpendicular and parallel to the c-axis), the similar TDR trace trends were observed as the excitation energy at near the exciton resonance. The energies difference about 19 mV between similar TDR traces reconfirmed the energy difference between E1 and E2 band. The similar experiment results were observed in the thick ZnO epifilm as well.

並列關鍵字

non-polar zinc oxide ； ultrafast carrier dynamics ； pump probe technique

參考文獻

[1] C. Klingshirn, "ZnO: From basics towards applications", Physica Status Solidi B, vol. 244, no. 9, 2007, pp. 3027-3073.

[2] Yefan Chen, D. M. Bagnall, Hang-jun Koh, Ki-tae Park, Kenji Hiraga, Ziqiang Zhu, and Takafumi, "Plasma assisted molecular beam epitaxy of ZnO on c -plane sapphire: Growth and characterization", J. Appl. Phys., vol. 84, no. 7, 1998, pp. 3912-3918.

[3] D. M. Bagnall, Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen and T. Goto, "Optically pumped lasing of ZnO at room temperature", Appl. Phys. Lett., vol. 70, no. 17, 1997, pp. 2230-2232.

[5]C.Agashe, O.Kluth, G.Schope, H.Siekmann, J.Hupkes and B.Rech, "Optimization of the electrical properties of magnetron sputtered aluminum-doped zinc oxide films for opto-electronic applications", Thin Solid Films, vol. 442, 2003, pp. 167-172.

[6] L. Cao, B.Zou, C. Li, Z. Zhang and S.Xie,G. Yang, "Laser emission of low-threshold excitationfrom ZnO nanowires",Euro. Phys.Lett, vol. 68, no. 1, 2004, pp. 740-745.

國際替代計量

利用瞬態泵-探量測技術研究非極化氧化鋅薄膜的超快載子動態

未授權

主題瀏覽