用飛秒時間解析激發-探測系統研究氧化鋅薄膜之超快光譜現象

Abstract

以氧化鋅為基底的半導體材料，因其具有潛力做為藍光到紫外光波段的光電元件而備受重視。由於室溫下氧化鋅的激子束縛能約為60毫電子伏特，在氧化鋅的相關結構物中已經被觀察到光激發之受激輻射，包含激子與激子的散射與電子電洞的電漿型態。過去的文獻已經報導過氧化鋅薄膜中電漿形成及其受激放光的時間演化過程。此外，在時間解析螢光的研究中，大部分著重在載子的雷射動力學與激子的復合。對於了解氧化鋅中之超快載子動力學是必要的，因為這將有助於促進光增益與高速光電元件的發展。本論文中，我們利用了光學式激發-探測系統，研究室溫下氧化鋅薄膜的超快載子動力學，以及氧化鋅/氧化鎂鋅多層量子阱中的非線性光學吸收。 我們首先進行了室溫下吸收與螢光光譜的量測，藉此了解所使用樣品之線性光學特性。接下來，我們利用了標準激發-探測技術去量測1微米與70奈米氧化鋅薄膜的自由載子動力學。用瞬時反射量測的實驗結果表示，在1微米樣品中的自由載子熱化時間約為1.0至1.5皮秒，且隨著激發光能量的增加而增加。至於用瞬時穿透量測70奈米樣品的結果表示，觀測到較長的自由載子熱化時間約為10至15皮秒以及較緩慢的恢復時間。藉由比較這兩種不同薄膜厚度的結果，我們分析研究出厚度對於自由載子熱化時間的影響。受激自由載子將優先與薄膜表面發生碰撞，讓載子與聲子之間的碰撞機率降低，造成自由載子的熱化時間延長。由於70奈米樣品受到表面捕捉載子之效應，將造成受激載子的數量減少，導致出現較緩慢的恢復時間。 在共振能量激發且強度為10 μJ /cm2的條件下進行70奈米氧化鋅薄膜的瞬態穿透光譜量測，我們得到自由激子的超快鬆弛時間約在1皮秒之內，以及最大之吸收係數變化量為1.8×104 cm−1。我們進一步發現，自由激子的鬆弛時間將隨著光激發功率的增加而變短，這顯示著當激發功率增加時，激子的鬆弛行為將從激子與聲子的交互作用轉變為激子與激子的交互作用。由瞬態穿透光譜量測結果可以發現最大的瞬態穿透變化率恰好發生於自由激子的共振能階上。此外，在考量能帶填滿與能隙重整的物理機制下，我們使用模型理論去計算氧化鋅中折射率的變化量，進而成功地解釋了瞬態反射光譜之實驗結果。 除了研究氧化鋅薄膜之超快光譜現象，我們利用了開口式Z軸-掃描技術來量測氧化鋅/氧化鎂鋅多層量子阱中的非線性光學吸收。量測結果顯示，單光子飽和吸收效應發生於能量在靠近能隙與自由激子之間的區段，並隨著能量的降低而變大。受飽和吸收之影響，最大的非線性吸收係數發生於激子的共振能階上。當操作於波尾區段時，非線性吸收會開始發生雙光子吸收飽和特性。當我們的光激發波長操作在375奈米時，非線性吸收量測同時表現出單光子與雙光子飽和吸收之特性，而當波長操作於378奈米時，此特性逐漸轉為由雙子吸收飽和主導。

ZnO-based wide bandgap semiconductors have attracted much attention as a promising candidate for photonic devices in the blue to ultraviolet spectral region. Due to its large exciton binding energy of 60 meV at room temperature (RT), optically pumped stimulated emission from the exciton-exciton scattering and electron-hole plasma (EHP) have been demonstrated in ZnO related structures. There are some reports about temporal evolution of EHP formation and stimulated emission in ZnO thin films. Besides, most picoseconds time-resolved photoluminescence (PL) studies focused on the lasing dynamics and excitonic recombination, it is essential to understand the carrier dynamics and optical nonlinearities since they play important roles in realizing the optical gain process and developing ZnO-based photonic devices. Therefore, we investigated the ultrafast free carrier/exciton dynamics and nonlinear absorption in ZnO and ZnO/ZnMgO MQWs at RT in this thesis. First, we measured the absorption spectrum and PL spectra at RT in the used samples to realize the linear optical properties. Then, we performed the free carrier dynamics in a 1-μm and a 70-nm ZnO epitaxial film by utilizing the standard pump-probe technique. The results in a 1-μm sample show that ultrafast thermalization time around 1.0-1.5 ps increases slightly with increasing the excited photon energy by measuring the transient differential reflectance. As for the carrier dynamics in a 70-nm sample shows that prolonged thermalization time of 10-15 ps and slow recovery time for well-above band-gap states by measuring the transient differential transmission. By comparison of these two extremely thickness samples in order to investigate the thickness effect on ultrafast thermalization of free carriers for above band-gap states, the interaction between free carriers and sample surface induces the inefficient carrier–phonon scattering to result in the prolonged thermalization. And the loss of excited carrier density via surface trapping effect causes the slow recovery times in a 70-nm epifilm. We also obtained the free exciton relaxation time to be less than 1 ps and the maximal differential absorption to be 1.8×104 cm−1 with pumping fluence of 10 μJ /cm2 by demonstrating the free exciton dynamics in a 70-nm epifilm under resonant excitation. The relaxation time of free exciton decreases with increasing the pumping fluence that indicates the transition of relaxation from the exciton-phonon scattering into the exciton-exciton scattering. From the measurement of spectral dependent transient differential transmission, the largest induced transparency occurs at near exciton-resonance associated with absorption saturation. Moreover, based on a theoretical model to calculate the change of refractive index in ZnO, the measurement of spectral dependent transient differential reflectance can be successfully analyzed as a result of combination of band-filling (BF) and band-gap renormalization (BGR) effects. In addition to the investigation of ultrafast spectral phenomena in ZnO thin films, we also utilized the standard open-aperture Z-scan method to measure the optical nonlinear absorption in ZnO/ZnMgO multiple quantum well structures (MQWs). The saturation of one-photon absorption (OPA) increases as decreasing the excited photon energy from the conduction band-edge approaching the free exciton transition of ZnO QWs. The maximal nonlinear absorption coefficient can be observed near exciton resonance associated with absorption saturation. The contribution of the two-photon absorption (TPA) increases as tuning excitation in the shallow band-tail region. The characteristics of nonlinear absorption reveal the concurrence of OPA and TPA saturation for excitation wavelength at 375 nm, and gradually transit to the dominance of TPA as further increasing the photon wavelength at 378 nm.