Unambiguous symmetry assignment for the top valence band of ZnO by magneto-optical studies of the free A-exciton state

TL;DR

This study uses magneto-optical measurements of the free A-exciton in ZnO to definitively determine the symmetry of the top valence band, revealing it has Gamma_7 symmetry and detailing magnetic field effects.

Contribution

The paper provides unambiguous experimental evidence for the Gamma_7 symmetry of the top valence band in ZnO using magneto-photoluminescence spectroscopy.

Findings

Top valence band in ZnO has Gamma_7 symmetry.

Magnetic field causes Zeeman splitting of exciton states.

In-plane magnetic field enhances oscillator strength of weak exciton states.

Abstract

We studied the circular polarization and angular dependences of the magneto-photoluminescence spectra of the free A-exciton 1S state in wurtzite ZnO at T = 5 K. The circular polarization properties of the spectra clearly indicate that the top valence band has Gamma_7 symmetry. The out-of-plane component of the magnetic field, which is parallel to the sample's c axis, leads to linear Zeeman splitting of both the dipole-allowed Gamma_5 exciton state and the weakly allowed Gamma_1/Gamma_2 exciton states. The in-plane field, which is perpendicular to the c axis, increases the oscillator strength of the weak Gamma_1/Gamma_2 states by forming a mixed exciton state.