Impact of Annealing on the Band Structure of (Ga,Mn) (P,As)

The effect of outdiffusion of Mn interstitials from (Ga,Mn)(P,As) epitaxial layers caused by post-growth low-temperature annealing is an important issue and is known to impact transport and optical properties of ferromagnetic semiconductors. Here we study the impact of post-growth annealing on the electronic and band-structure properties of $(\mathbf{G a}, \mathrm{Mn})(\mathrm{P}, \mathrm{As})$ by spectroscopic ellipsometry, high resolution X-ray diffraction, micro-Raman spectroscopy and superconducting quantum interference device magnetometry. The spectroscopic ellipsometry results reveal a decrease in the band-gap-transition energy in as-grown (Ga,Mn)(P,As) layers, with respect to that of the reference $\mathrm{Ga}(\mathrm{P}, \mathrm{As})$, interpreted as a result of a Mn impurity band pinning of the Fermi energy. On the other hand, the increase of the band-gap-transition energy observed in the annealed (Ga,Mn)(P,As) layers is interpreted as a result of the Moss-Burstein shift of the absorption edge due to the Fermi level moving deeper. This is consistent with the enhanced free-hole concentration in annealed samples. Our findings shed important light on the role of Mn acting not only as a magnetic dopant, but also altering the band structure of the host material.