Effect of well-width on the electro-optical properties of a quantum well

TL;DR

This study investigates how the width of Ge/GeSi quantum wells influences their electro-optical behavior under high electric fields, revealing a transition from quantum confinement to bulk-like effects.

Contribution

It demonstrates the impact of well-width on the Franz-Keldysh effect transition using experimental photoreflectance and theoretical wavefunction calculations.

Findings

Narrower wells show quantum confinement effects.

Wider wells exhibit tunneling and bulk-like Franz-Keldysh behavior.

Simulated spectra match experimental results without adjustable parameters.

Abstract

We record photoreflectance from Ge/GeSi modulation doped quantum wells possessing $10^4$ V/cm perpendicular electric fields. Qualitatively very different spectra are obtained from samples of well-width 100 \AA and 250 \AA. Comparing the wavefunctions calculated from an $8 \times 8$ \textbf{k.p} theory, we find that while they remain confined in the narrower 100 \AA QW, the electric field causes them to tunnel into the forbidden gap in the 250 \AA\ well. This implies that the samples should show a transition from the quantum confined Franz-Keldysh effect to the bulk-like Franz-Keldysh effect. Close to the band-edge where Franz-Keldysh effects are important, simulated photoreflectance spectra reproduce the essential features of the experiment, without any adjustable parameters.