Direct observation of $\Gamma - X$ energy spectrum transition in narrow AlAs quantum wells

TL;DR

This study observes a transition in the energy spectrum of electrons in narrow AlAs quantum wells, revealing a change in valley occupation and effective mass enhancement as the well width decreases below 5.5 nm.

Contribution

It provides direct experimental evidence of a valley transition and effective mass increase in narrow AlAs quantum wells, advancing understanding of quantum confinement effects.

Findings

Energy spectrum transition occurs below 5.5 nm well width.

Transition from in-plane to out-of-plane valley occupation.

Significant enhancement of cyclotron effective mass.

Abstract

Spectra of magnetoplasma excitations have been investigated in a two-dimensional electron systems in AlAs quantum wells (QWs) of different widths. The magnetoplasma spectrum have been found to change profoundly when the quantum well width became thinner than $5.5$~nm, indicating a drastic change in the conduction electron energy spectrum. The transformation can be interpreted in terms of transition from the in-plane strongly anisotropic $X_x - X_y$ valley occupation to the out-of-plane isotropic $X_z$ valley in the QW plane. Strong enhancement of the cyclotron effective mass over the band value in narrow AlAs QWs is reported.