Spin-dependent direct gap emission in tensile-strained Ge films on Si substrates

TL;DR

This study investigates the spin-dependent optical properties of tensile-strained germanium films on silicon, revealing high circular polarization in direct gap emission due to strain and spin dynamics, with implications for spintronic applications.

Contribution

It provides the first detailed analysis of spin-dependent emission in tensile-strained Ge on Si, demonstrating high polarization degrees and clarifying the role of light hole contributions at room temperature.

Findings

High circular polarization (~85%) at low temperatures for cG-LH transition.

Light hole contribution dominates room temperature PL at low tensile strain.

Strain and spin effects significantly influence Ge's optical emission properties.

Abstract

The circular polarization of direct gap emission of Ge is studied in optically-excited tensile-strained Ge-on-Si heterostructures as a function of doping and temperature. Owing to the spin-dependent optical selection rules, the radiative recombinations involving strain-split light (cG-LH) and heavy hole (cG-HH) bands are unambiguously resolved. The fundamental cG-LH transition is found to have a low temperature circular polarization degree of about 85% despite an off-resonance excitation of more than 300 meV. By photoluminescence (PL) measurements and tight binding calculations we show that this exceptionally high value is due to the peculiar energy dependence of the optically-induced electron spin population. Finally, our observation of the direct gap doublet clarifies that the light hole contribution, previously considered to be negligible, can dominate the room temperature PL even at low tensile strain values of about 0.2%.