化學氣相沉積法備製之石墨烯其超快載子動力學研究

我們利用光激發-兆赫波探測頻譜來了解化學氣象沉積法所備製之石墨烯其超快載子動力學，並同時藉由外加閘極偏壓於固態高分子聚合物上來改變石墨烯的費米能階，以研究石墨烯在不同參雜狀況下之載子弛豫行為。我們發現化學氣象沉積法所備製的石墨烯頻譜，相較於早期磊晶成長在碳化矽的石墨烯研究，當探測光能量落在費米面附近，其光激發載子的行為有幾個迥異之處。首先，我們觀察到光激發後穿透度變化為正值，也就是說，得到載子激發後呈現負值的光電導率。另外，光激發後頻譜回復的曲線滿足指數遞減，且其衰減時間與光強度和費米能階位置都有很大的關連。為了更加了解激發載子弛豫行為的機制，我們以理論模擬考慮載子與光學聲子之間的散射，並包含載子溫度與光聲子數目隨時變的關係以擬和弛豫過程中激發載子的行為。從模擬中我們發現以往單純考慮石墨烯為一半導體的模型架構是不足的，石墨烯的行為更近似於一個沒有禁止能帶的材料，導致激發的電子與電洞彼此可以更快速地達到熱平衡，且此熱平衡可以用高溫且電子電洞對應到共同費米能階的近費米分布來描述。另外，值得注意的是不同於半導體，載子的平均散射率會隨著溫度而增加，類似於金屬的行為，而且在載子弛豫的過程裡面扮演極重要的角色，導致我們所觀察到負的電導率，以及隨光強度或者是費米能階不同而改變的衰減時間。同時，載子的平均散射率也會隨著費米能階增加，呈現兩個截然不同的行為區間。在費米能階相對於電荷中性點(charge neutral point)大於91 meV之後，越遠離電荷中性點，載子平均散射率越高；反之，在91 meV之間，越靠近電荷中性點，載子平均率散射率越高。這是因為當費米能階在靠近電荷中性點附近，電荷雜質會主導載子在低能量的散射，這時增加載子濃度會使雜質受到屏蔽效應的影響，導致載子平均率下降。改變參雜量而得到截然不同的平均散射率行為，除了提供載子在低能量的區間有不同的弛豫行為外，研究中所觀察'到不同的參雜量下影響載子降溫速度，也為石墨烯增加一個新的調變機制，在未來高溫載子傳導的石墨烯元件上開啟新的應用性。

關鍵字

石墨烯；化學氣象沉積法；超快載子動力學；兆赫波；閘極偏壓；固態高分子電解質薄膜

並列摘要

Abstract We present optical-pump THz-probe spectroscopy to measure the ultrafast relaxation dynamics of quasi-particle on monolayer chemical vapor deposited (CVD) graphene, and the Fermi levels can be tuned in a wide range by applying gate voltage on the casted polymer electrolyte. As probing the photoexcited carriers around the Fermi level, it has shown several controversial results from the ultrafast studies on graphene grown on SiC. One is a positive pump induced differential transmission (PIDT), that is, a transient negative conductivity after photoexcitation, and the others is the extracted exponential decay times of PIDT depend on the pump fluence and the doping levels. To better understand the relaxation dynamics, we model the relaxation process involving electron-phonon coupling together with a set of rate equations to describe the transient responses of quasi-particles and optical phonons. The simulation results indicate that graphene cannot be treated as semiconductor as usual; it is more like a zero bandgap material that gives rise to a fast thermalization among carriers after pumping and reaches a quasi-Fermi distribution described by a common Fermi level with a relatively high carrier temperature. It is also noted that the metallic temperature-dependent carrier scattering rate plays a significant role in the cooling process, leading to different observations of the relaxation dynamics. The carrier scattering rate shows two distinct behaviors with the Fermi level separated by a turning point at 91 meV. As approaching charge neutral point, the carrier scattering rate is dominated by charge impurities and decreases with increasing the doping levels due to the screening effect. Different scattering mechanism in two distinct regions leads to the change of relaxation dynamics and gives a clear new cooling control variable. The cooling rate can be changed by varying the doping levels providing means for a variety of new applications that rely on hot-carrier transport.

並列關鍵字

graphene ； chemical vapor deposition ； ultrafast carrier dynamics ； THz ； gating ； polymer electrolyte

參考文獻

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化學氣相沉積法備製之石墨烯其超快載子動力學研究

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