Observation of a phonon bottleneck effect on the thermal depopulation from a photoexcited shallow defect in silicon

TL;DR

This study provides experimental evidence of a phonon bottleneck effect that delays the thermal depopulation of shallow defects in photoexcited silicon, revealing important insights into carrier relaxation dynamics and electron-phonon interactions.

Contribution

First observation of a phonon bottleneck effect affecting shallow defect depopulation in silicon using time-resolved THz spectroscopy.

Findings

Delay in photoconductivity due to shallow trap states

Temperature dependence confirms phonon bottleneck

Delay invariant with photon flux

Abstract

We report the observation of a phonon bottleneck effect impacting the thermal depopulation of photoexcited shallow defects in high-resistivity silicon. Using time-resolved terahertz (THz) spectroscopy, near-band-gap excitation produces a pronounced temporal delay in photoconductivity, indicating that a fraction of photogenerated charge carriers is temporarily trapped immediately after excitation. By analyzing the frequency-resolved complex photoconductivity as a function of pump-probe delay and photon energy, we attribute this delay to the presence of a localized shallow state situated approximately 40 meV from the band edge, which competes with silicon's indirect band-to-band absorption. The zero-order kinetic profile of the temporal delay, its invariance with respect to photon flux, and its temperature dependence collectively support the existence of a phonon bottleneck that hinders the thermal release of electrons from this shallow trap. this represents experimental evidence of a phonon bottleneck effect associated with the thermal activation of shallow traps in photoexcited silicon. These findings provide microscopic insight into carrier relaxation dynamics in silicon and highlight the significance of electron-phonon interactions in the ultrafast processes governing materials used in optoelectronic applications.