Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, USA.
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Nat Commun. 2017 Jun 1;8:15734. doi: 10.1038/ncomms15734.
Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical pump-probe spectroscopy often render assignments of separate electron and hole carrier dynamics ambiguous. Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin films are directly and simultaneously observed by ultrafast transient absorption spectroscopy in the extreme ultraviolet at the germanium M edge. We decompose the spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8 × 10 cm. Separate electron and hole relaxation times are observed as a function of hot carrier energies. A first-order electron and hole decay of ∼1 ps suggests a Shockley-Read-Hall recombination mechanism. The simultaneous observation of electrons and holes with extreme ultraviolet transient absorption spectroscopy paves the way for investigating few- to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions.
理解半导体中的激发载流子动力学对于光伏和高效光子器件的发展至关重要。然而，在光泵浦探测光谱学中，光谱特征的重叠常常使得单独的电子和空穴载流子动力学的分配变得模糊。在这里，通过超快瞬态吸收光谱学在极端紫外线的锗 M 边缘直接且同时观察到锗纳米晶薄膜中的超快电子和空穴动力学。我们在载流子密度为 8×10^18 cm^-3 的情况下，将光谱分解为电子态阻塞和光致能带位移的贡献。随着热载流子能量的变化，我们观察到了单独的电子和空穴弛豫时间。~1 ps 的一级电子和空穴衰减表明存在肖克利-里德-霍尔复合机制。利用极端紫外线瞬态吸收光谱学同时观察电子和空穴，为研究复杂半导体材料和界面中电子和空穴的几到亚飞秒动力学铺平了道路。
