Effect of low-temperature annealing on the electronic- and band-structure of (Ga,Mn)As epitaxial layers

TL;DR

This study investigates how low-temperature annealing affects the electronic and band-structure of (Ga,Mn)As layers, showing outdiffusion of Mn interstitials, increased hole concentration, and changes in band-gap energies, supporting the valence-band model of ferromagnetism.

Contribution

It provides experimental evidence linking annealing-induced Mn outdiffusion to band-structure modifications and ferromagnetic properties in (Ga,Mn)As, confirming the valence-band origin of holes.

Findings

Mn outdiffusion confirmed by X-ray and SQUID measurements

Band-gap transition energy decreases in as-grown layers

Annealing increases band-gap energy due to Moss-Burstein shift

Abstract

The effect of outdiffusion of Mn interstitials from (Ga,Mn)As epitaxial layers, caused by post-growth low-temperature annealing, on their electronic- and band-structure properties has been investigated by modulation photoreflectance (PR) spectroscopy. The annealing-induced changes in structural and magnetic properties of the layers were examined with high-resolution X-ray diffractometry and SQUID magnetometery, respectively. They confirmed an outdiffusion of Mn interstitials from the layers and an enhancement in their hole concentration, which were more efficient for the layer covered with a Sb cap acting as a sink for diffusing Mn interstitials. The PR results revealing a decrease in the band-gap-transition energy in the as-grown (Ga,Mn)As layers, with respect to that in the reference GaAs one, are interpreted by assuming a merging of the Mn-related impurity band with the host GaAs valence band. On the other hand, an increase in the band-gap-transition energy in the annealed (Ga,Mn)As layers is interpreted as a result of the Moss-Burstein shift of the absorption edge due to the Fermi level location within the valence band, determined by the enhanced free-hole concentration. The experimental results are consistent with the valence-band origin of mobile holes mediating ferromagnetic ordering in (Ga,Mn)As, in agreement with the Zener model for ferromagnetic semiconductors.