Optical anisotropies of asymmetric double GaAs (001) Quantum Wells

TL;DR

This study investigates how asymmetry in double GaAs quantum wells causes in-plane optical anisotropies due to symmetry reduction, with detailed spectroscopic analysis and theoretical calculations illuminating the effects on optical transitions.

Contribution

It identifies and characterizes a new source of optical anisotropies in asymmetric DQWs caused by symmetry reduction from $D_{2d}$ to $C_{2v}$, supported by experimental and theoretical analysis.

Findings

Asymmetric DQWs exhibit increased RAS strength due to symmetry loss.

Heavy- and light-hole mixing is induced by symmetry breakdown.

Reflectance spectra help interpret optical transition energies.

Abstract

In the present work, we were able to identify and characterize a new source of in-plane optical anisotropies (IOAs) occurring in asymmetric DQWs; namely a reduction of the symmetry from $D_{2d}$ to $C_{2v}$ as imposed by asymmetry along the growth direction. We report on reflectance anisotropy spectroscopy (RAS) of double GaAs quantum wells (DQWs) structures coupled by a thin ($<2$ nm) tunneling barrier. Two groups of DQWs systems were studied: one where both QWs have the same thickness (symmetric DQW) and another one where they have different thicknesses (asymmetric DQW). RAS measures the IOAs arising from the intermixing of the heavy- and light- holes in the valence band when the symmetry of the DQW system is lowered from $D_{2d}$ to $C_{2v}$. If the DQW is symmetric, residual IOAs stem from the asymmetry of the QW interfaces; for instance, associated to Ga segregation into the AlGaAs layer during the epitaxial growth process. In the case of an asymmetric DQW with QWs with different thicknesses, the AlGaAs layers (that are sources of anisotropies) are not distributed symmetrically at both sides of the tunneling barrier. Thus, the system losses its inversion symmetry yielding an increase of the RAS strength. The RAS line shapes were compared with reflectance spectra in order to assess the heavy- and light- hole mixing induced by the symmetry breakdown. The energies of the optical transitions were calculated by numerically solving the one-dimensional Schr\"odinger equation using a finite-differences method. Our results are useful for interpretation of the transitions occurring in both, symmetric and asymmetric DQWs.