Ultrafast hot-carrier-dominated photocurrent in graphene
Ultrafast hot-carrier-dominated photocurrent in graphene
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DOI：
10.1038/nnano.2011.243
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发表时间：
2012-02-01
影响因子：
38.3
通讯作者：
Xu, Xiaodong
中科院分区：
文献类型：
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作者：
Sun, Dong;Aivazian, Grant;Xu, Xiaodong
其高电子迁移率（1-5）、宽带吸收（6）和超快发光（7-10）的组合使得石墨烯对于光电和光子应用（11-13）具有吸引力，包括透明电极（14）、锁模激光器（15）和高速光调制器（16）。尚未冷却到石墨烯晶格温度的光激发载流子被称为热载流子，并且可能限制器件速度和能量效率。然而，它们在电荷和能量传输中的作用尚未完全理解（17-20）。在这里，我们使用时间分辨扫描光电流显微镜来证明，热载流子，而不是声子，占主导地位的能量传输通过超快激光脉冲激发的可调石墨烯p-n结。光电流响应时间从室温下的1.5 ps变化到20 K下的4 ps，这意味着基频带宽接近500 GHz（参考文献12、13、21）。依赖于门的泵浦-探测测量表明，热电和内置电场效应都有助于光电流，从每个依赖于结配置的贡献。由单个脉冲激光产生的光电流也显示出作为载流子密度的函数的多个极性反转，这是碰撞电离的可能特征（19，22，23）。
The combination of its high electron mobility(1-5), broadband absorption(6) and ultrafast luminescence(7-10) make graphene attractive for optoelectronic and photonic applications(11-13), including transparent electrodes(14), mode-locked lasers(15) and high-speed optical modulators(16). Photo-excited carriers that have not cooled to the temperature of the graphene lattice are known as hot carriers, and may limit device speed and energy efficiency. However, their roles in charge and energy transport are not fully understood(17-20). Here, we use time-resolved scanning photocurrent microscopy to demonstrate that hot carriers, rather than phonons, dominate energy transport across a tunable graphene p-n junction excited by ultrafast laser pulses. The photocurrent response time varies from 1.5 ps at room temperature to 4 ps at 20 K, implying a fundamental bandwidth of similar to 500 GHz (refs 12,13,21). Gate-dependent pump-probe measurements demonstrate that both thermoelectric and built-in electric field effects contribute to the photocurrent, with the contribution from each depending on the junction configuration. The photocurrent produced by a single pulsed laser also displays multiple polarity reversals as a function of carrier density, which is a possible signature of impact ionization(19,22,23).