用扫描光电流显微镜测定强关联材料的光响应
Photoresponse of a strongly correlated material determined by scanning photocurrent microscopy
摘要：
通过光产生电流是光电子和光伏器件中的关键过程。在能带半导体中，与界面相关的耗尽场分离长寿命的光生载流子。然而，在具有强电子-电子和电子-声子相关性的系统中，尚不清楚什么物理学将主导光响应。在这里，我们研究光电流在二氧化钒，一个示例性的强相关材料已知其戏剧性的金属-绝缘体转变（MIT）在Tc = 68 C，这可能是有用的光电检测和切换到紫外波长。使用扫描光电流显微镜（SPCM）对个别悬浮VO 2纳米梁，我们观察到的光响应峰值在金属-绝缘体边界，但在整个绝缘和金属相延伸。我们确定的响应是光热，这意味着有效的载流子弛豫到局部平衡的方式与强相关性一致。温度相关的测量揭示了绝缘状态内的细微相变。我们进一步展示了通过光学控制的金属-绝缘体边界排列的光电流的开关。我们的工作显示了SPCM应用于纳米晶体研究强关联材料的价值，其结果对于设计和控制使用这种材料的光电子器件具有重要意义。
英文摘要：
The generation of a current by light is a key process in optoelectronic and photovoltaic devices. In band semiconductors, depletion fields associated with interfaces separate long-lived photo-induced carriers. However, in systems with strong electron-electron and electron-phonon correlations it is unclear what physics will dominate the photoresponse. Here we investigate photocurrent in a vanadium dioxide, an exemplary strongly correlated material known for its dramatic metal-insulator transition (MIT) at Tc = 68 C which could be useful for optoelectronic detection and switching up to ultraviolet wavelengths. Using scanning photocurrent microscopy (SPCM) on individual suspended VO2 nanobeams we observe photoresponse peaked at the metal-insulator boundary but extended throughout both insulating and metallic phases. We determine that the response is photo-thermal, implying efficient carrier relaxation to a local equilibrium in a manner consistent with strong correlations. Temperature dependent measurements reveal subtle phase changes within the insulating state. We further demonstrate switching of the photocurrent by optical control of the metal-insulator boundary arrangement. Our work shows the value of SPCM applied to nanoscale crystals for investigating strongly correlated materials, and the results are relevant for designing and controlling optoelectronic devices employing such materials.
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