Studies of Ultrafast Carrier Dynamics in ZnO thin film and Nanostructures

氧化鋅相關材料由於在元件應用上具極佳的材料特性而愈來愈受到重視。近年來，愈來愈多奈米結構的氧化鋅被合成出來。然而，奈米結構氧化鋅的超快載子動態甚至塊材的氧化鋅自由載子動態仍然很少被研究過。發展以氧化鋅為基礎的元件，了解其物理機制是非常根本的。 因此，在本篇論文中，我們利用飛秒暫態穿透式量測研究氧化鋅奈米柱和氧化鋅薄膜在室溫下激子和「超越能隙自由載子」的動態。從光激發「超越能隙自由載子」後，可以觀察到非常快速的外部熱化時間在200飛秒的數量級。在高功率的激發下，發現熱聲子效應會延緩載子冷卻的過程。當激發光子能量調到和氧化鋅奈米柱的束縛激子躍遷吻合時，發現激子穩定形成而沒有明顯的激子解離過程除非當光激發激子密度超過莫特密度(Mott Density)；反之，當激發光子能量調到和氧化鋅薄膜的自由激子躍遷吻合時，即使光激發激子密度未超過莫特密度，仍然可以觀察到時間常數數量級在800飛秒的微弱的激子解離。 除了氧化鋅載子動力學的研究，我們也利用反射式激發探測技術測量氧化鋅薄膜的聲速。然而，量得的聲速數量級為4000m/s，較過去文獻發表的數值數量級為6000m/s慢得多。根據實驗結果分析，我們探討幾個可能的原因。另一方面，觀察到的一些有趣現象引起我們更廣泛地討論。ZnO-related material has got more and more attentions because of its excellent material properties in device application. In recent years, more and more nanostructure ZnO have been synthesized. However, ultrafast carrier dynamics in nanostructure ZnO even free carrier dynamics in bulk ZnO have been less investigated. To develop ZnO-based devices, it is essential to know the physical mechanisms For this reason, in the thesis, exciton and above-band-gap free carrier dynamics in ZnO nanorods and ZnO thin films have been investigated at room temperature with a femtosecond transient transmission measurement. Following the photoexcitation of above-band-gap free carriers, an extremely fast external thermalization time on the order of 200 fs can be observed. Under high excitation, hot phonon effects were found to delay the carrier cooling process. While the photoexcitation energy was tuned to match the bound exciton transition in ZnO nanorods, stable exciton formation can be uncovered while no evident exciton ionization process can be found unless the photoexcited exciton density exceeded the Mott density. Whereas, while the photoexcitation energy was tuned to match the free exciton transition in ZnO thin films, a weak exciton ionization process with an exciton ionization time of 800 fs can still be found even photoexcited exciton density below the Mott density. In addition to the investigation carrier dynamics in ZnO, we have also utilized femtosecond reflectivity pump-probe technique to measure sound velocity in ZnO thin films. However, the measured sound speed on the order of 4000 m/s appear much slower than the previous reported value on the order of 6000 m/s. Some possible reasons were explore according to the analysis of experimental results. On the other hand, some interesting phenomena observed prompted us to discuss extensively.Acknowledgement i Abstract iii Contents v Chapter 1 Introduction 1 1.1 Properties of Zinc Oxide and Its Nanostructures 1 1.1.1 Properties of ZnO 1 1.1.2 ZnO Nanostructures 2 1.2 Femtosecond Pump-Probe Spectroscopy 2 1.3 Motivation and Thesis Structure 5 References Chapter 2 Carrier Dynamics in Semiconductors 9 2.1 Ultrafast Dynamic Processes 11 2.1.1 Carrier-carrier Scattering 11 2.1.2 Carrier-phonon Scattering 11 2.2 Rate Equation Model 14 2.3 Effects of Carrier Generation 15 2.3.1 Band-filling Effect 15 2.3.2 Band-gap Renormalization 16 2.3.3 Hot Phonon Effect 17 2.4 Basic Concepts of Exciton 17 2.4.1 Free Exciton 18 2.4.2 Bound Exciton 18 References Chapter 3 Ultrafast Carrier Dynamics in ZnO 21 3.1 Sample Preparation 22 3.1.1 ZnO Nanorods 22 3.1.2 ZnO Thin Films 26 3.2 Optical Transmission Pump-Probe Setup 28 3.3 Experimental Result 30 3.3.1 Above-band-gap Free Carrier Dynamics 30 3.3.2 Exciton Dynamics 37 3.4 Summary 39 References Chapter 4 ZnO Sound Velocity Measurement 43 4.1 Principle of Acoustic Wave Detection 43 4.2 Sample Preparation and Experimental Setup 45 4.2.1 Au-coated ZnO Thin Films 45 4.2.2 Optical Reflectivity Pump-Probe System Setup 46 4.3 Experimental Result 48 4.4 Discussion 53 References Chapter 5 Conclusion 6