Direct vs. indirect optical recombination in Ge films grown on Si substrates

TL;DR

This study investigates why Ge films on Si exhibit different optical emission spectra than bulk Ge, revealing that thin film effects and non-equilibrium conditions favor direct band gap emission over indirect gap emission.

Contribution

The paper introduces a new understanding of optical recombination in Ge-on-Si films, highlighting the roles of self-absorption and non-equilibrium conduction band conditions.

Findings

Ge films show dominant direct band gap emission unlike bulk Ge.

Lack of self-absorption in thin films influences emission spectra.

Shorter recombination lifetime causes deviation from quasi-equilibrium.

Abstract

The optical emission spectra from Ge films on Si are markedly different from their bulk Ge counterparts. Whereas bulk Ge emission is dominated by the material's indirect gap, the photoluminescence signal from Ge films is mainly associated with its direct band gap. Using a new class of Ge-on-Si films grown by a recently introduced CVD approach, we study the direct and indirect photoluminescence from intrinsic and doped samples and we conclude that the origin of the discrepancy is the lack of self-absorption in thin Ge films combined with a deviation from quasi-equilibrium conditions in the conduction band. The latter is confirmed by a simple model suggesting that the deviation from quasi-equilibrium is caused by the much shorter recombination lifetime in the films relative to bulk Ge.