J. Kanasaki

Excitation-induced structural instability of semiconductor surfaces

The present work reviews laser-induced electronic processes of structural instability on covalent semiconductor surfaces. In particular, we concentrate on the mechanism of the instability that takes place at the intrinsic sites of reconstructed surface structures. In order to elucidate the primary processes involved, we focus our attention on experimental results obtained by scanning tunnelling spectroscopy studies, to reveal surface structural changes at the atomic level, and by post-ionization spectroscopy studies, to probe desorption processes with high sensitivity. First, the results obtained for reconstructed Si surfaces and {110} surfaces of III?V compound semiconductors are systematically surveyed. The instability is characterized by local bond rupture at intrinsic surface sites that show important common features. Also, we find significantly different aspects of the instability, which depend on the basic properties of surfaces. Based on the characteristics revealed by the experimental results, we then propose a mechanistic model based on the generalized two-hole localization mechanism, and demonstrate, through the quantitative analysis of typical experimental results, that the model describes all the important features quantitatively and consistently.

State-resolved ultrafast dynamics of impact ionization in InSb

Impact ionization (IMP) is a fundamental process in semiconductors, which results in carrier multiplication through the decay of a hot electron into a low-energy state while generating an electron-hole pair. IMP is essentially a state selective process, which is triggered by electron-electron interaction involving four electronic states specified precisely by energy and momentum conservations. However, important state-selective features remain undetermined due to methodological limitations in identifying the energy and momentum of the states involved, at sufficient temporal resolution, to reveal the fundamental dynamics. Here we report state-resolved ultrafast hot electron dynamics of IMP in InSb, a semiconductor with the lowest band-gap energy. The ultrafast decay of state-resolved hot-electron populations and the corresponding population increase at the conduction band minimum are directly captured, and the rate of IMP is unambiguously determined. Our analysis, based on the direct knowledge of state-resolved hot electrons, provides far deeper insight into the physics of ultrafast electron correlation in semiconductors.