# Electronic Melting of Silicon in Nanostructures using X-ray Forbidden Bragg Reflections

## TL;DR

This paper investigates the electronic melting of silicon nanostructures using x-ray diffraction to observe changes in covalent bonds after laser excitation.

## Contribution

The study introduces a novel method using x-ray forbidden Bragg reflections to observe ultrafast electronic processes in silicon nanostructures.

## Key findings

- The Si(222) reflection is extinguished on femtosecond time scales after laser excitation.
- The acoustic response of the lattice is determined by the speed of sound over the device thickness, occurring in the mid-picosecond range.

## Abstract

We carried out a short beamtime at the Pohang Accelerator Laboratory x-ray Free Electron Laser to perform a pump-probe (PP) laser excitation diffraction experiment on the silicon (222) forbidden Bragg peak. To limit the x-ray penetration, we used a “device layer” silicon film wafer bonded to a silicon substrate. The sample, specially fabricated by MEMC Electronic Materials, had a Si(100) substrate bonded to a 170 nm Si(100) film rotated at 45° for crystallographic isolation. A second sample was reactive-ion-etched down to 52 nm thickness. In the silicon lattice, the covalent bonds are seen exclusively at the 222 reflection. Upon laser excitation, these electrons are expected to be excited to the valence band on femtosecond electronic time scales. The Si(222) reflection is therefore expected to be extinguished on this fast time scale, while the electron–phonon coupled acoustic response is determined by the lattice dynamics. The latter is determined by the speed of sound over the device thickness, which is in the mid-picosecond range.