石墨烯超快載子釋能機制及次兆赫同調聲波激發研究

Abstract

石墨烯(Graphene)是一種由碳原子組成呈六角形蜂巢結構的二維薄膜，其擁有許多 極佳的特性如透明、特高的熱傳導率及載子遷移率、極其強韌的機械特性等，因 此石墨烯在實際應用上有極高的價值，例如可成為下一世代超高頻場效電晶體的 導電通道和取代銦錫氧化物 (ITO)成為觸控面板、LCD 和太陽能電池中的透明導 電薄膜。然而在此篇論文中，我將探討石墨烯的另一項尚未被發掘潛力：基於石 墨烯的極薄特性，其厚度比任何實際用來當聲能轉換器(acoustic transducer)的金屬 薄膜都薄了十倍，而由金屬薄膜產生的同調聲子(coherent acoustic phonons)頻寬大 略與其厚度成反比，因此石墨烯有極大潛力擔任聲能轉換器來產生極寬頻的同調 聲子。使用石墨烯作為聲能轉換器將有一項無可取代的優勢，其易於製備及轉印 於任何基板的特性，使其有機會取代其他寬頻聲能轉換器，將奈米超音波顯微技 術應用到任何的材料上。 在上述的應用中，石墨烯載子受光、電激發後的動力及釋能機制顯得格外重 要，因此在本論文中，我們首先利用飛秒雷射激發探測(femtosecond pump probe) 技術來研究石墨烯載子的釋能動力學。透過此項技術及理論分析，受激載子透過 載子-載子、載子-聲子、聲子-聲子交互作用來釋能的特徵時間、發生順序等特性 都可被了解。此外，透過量測在本論文中使用的石墨烯樣本的載子釋能動力學， 並與其他研究團隊對原始石墨烯(無基板、無參雜等)的理論及實驗研究做比較，我 們使用的石墨烯樣本因基板的使用、製程等引起的差異可進一步被了解。 接著，我們同樣利用激發探測技術來研究透過飛秒雷射激發有基板支撐的單 層石墨烯是否可產生垂直表面傳遞的寬頻同調聲子。首先，藉著觀測反向布呂淵 振盪 (backward Brillouin oscillation)，我們印證了透過雷射激發由玻璃、藍寶石、 銦化鎵基板支撐之單層石墨烯，可產生具有垂直表面動量的同調聲子。因此我們 證實了將石墨烯轉印至基板上便作為聲能轉換器。 為了能了解透過飛秒雷射激發產生的同調聲子的完整頻譜，我們利用具有壓 電性質的氮化銦鎵量子井(在氮化鎵基底中)作為基板。透過使用3nm 的量子井， 我們可獲得高達2THz 的偵測頻寬。實驗結果顯示透過飛秒雷射激發有基板支撐之 單層石墨烯產生的同調聲子脈衝擁有兩極的波形，且其頻率高達一兆赫(最高值在 約0.2 兆赫)。這一部分的實驗結果證明了，儘管石墨烯擁有二維性質，透過飛秒 雷射激發由基板支撐的單層石墨烯，兆赫頻寬的同調聲子可被產生且傳遞進入基 板內，因此利用石墨烯作為超寬頻聲能轉換器的可行性已被證實。進一步，為了 了解同調聲子的產生機制，基於我們對於石墨烯載子釋能動力學的研究，我們提 出了一個假說，此假說可成功的解釋實驗中量測到的同調聲子頻譜及脈衝波形。

Graphene is a two-dimensional honeycomb lattice of carbon atoms. Due to its superior qualities, for example, it’s transparent, its thermal conductivity outperforms any other materials, its charge-carrier mobility is extremely high, and it is mechanically extremely strong, it has tremendous potential for future applications. For example, it’s a promising candidate for the conducting channel of future ultrafast field effect transistors and the replacement for the transparent electrode in touching screens, liquid crystal display (LCD), and solar cells. In this thesis, one of graphene’s undiscovered potential will be studied: graphene’s atomic thickness is one order thinner than any realistic metal films, which are usually used as the acoustic transducers in ultrafast acoustic devices and acoustic microscopy. Hence, graphene is a promising material to generate extremely broad band-width coherent acoustic phonons, because metal film transducers typically generate longitudinal coherent acoustic phonons with band-width inversely proportional to the film thickness. Moreover, unlike other broad band-width acoustic transducers, graphene is easily fabricated can be transferred onto all kinds of substrates. Thus, the usage of graphene can help applying acoustic microscopy to all kinds of materials. In the above applications, clear understanding about the carrier dynamics of supported monolayer graphene is critical. In this thesis, femtosecond infrared pump-probe spectroscopy together with theoretical analyses were used to study the ultrafast carrier dynamics of graphene. The characteristic times, sequences, and dominance of several hot carriers’ efficient relaxation channels, including carrier-carrier, carrier-phonon, and phonon-phonon interaction were studied. Moreover, by comparing the ultrafast carrier dynamics of our own graphene samples and pristine graphene, which was theoretically predicted or experimentally observed in previous literatures, we can have a clue of the differences between our own sample and pristine graphene due to fabrication methods and the usage of substrates. Then, pump-probe spectroscopy was also used to study whether coherent acoustic phonons that propagate into the substrate can be generated by photo-excitation of the supported monolayer graphene. First, through the observation of backward Brillouin oscillations with graphene deposited on glass, sapphire and GaN, it was verified that via photo-excitation of the monolayer graphene, coherent acoustic phonons with momentum in the out-of-plane direction were generated and had propagated into the substrate. This result suggested that by depositing monolayer graphene on top of a substrate, it can serve as an acoustic transducer. Afterwards, in order to reveal the full spectrum of the generated coherent acoustic phonons, a GaN crystal with a piezoelectric InGaN quantum well buried inside was used as the substrate. With our 3nm InGaN quantum well, detection band-width up to 2THz was achieved. The experimental results showed that the generated coherent acoustic phonon pulse had a bipolar shape and frequency components extending up to 1 THz (with the peak at 200GHz). In summary, by photo-exciting supported monolayer graphene, in spite of the two-dimensional nature of graphene, THz band-width coherent acoustic phonons that propagate perpendicularly to the surface and into the substrate can be generated. Hence, our study has confirmed the feasibility of using supported monolayer graphene as a THz acoustic transducer. A hypothesis based on the ultrafast carrier dynamics study of our supported graphene sample was brought up to elucidate the generation mechanism. A good correspondence was reached between the characteristics of the experimentally observed coherent acoustic phonon pulse and the prediction of the hypothesis.