Optical absorption and electronic structure of Zn1−xMnxO alloys studied by spectroscopic ellipsometry

Abstract

Dielectric functions of ${\mathrm{Zn}}_{\text{1−x}}{\mathrm{Mn}}_x\mathrm{O}$ $\text{(x⩽0.19)}$ thin films were obtained by spectroscopic ellipsometry at room temperature in the 1.5–5 eV photon energy region. A strong absorption structure is observed near 3.1 eV, below the optical band-gap absorption ${\mathrm{(E}}_0)$ edge, and it does not shift with x. It is interpreted as due to transitions between the crystal-field-split ${\text{3d}}^5$ multiplet levels of the tetrahedral ${\mathrm{Mn}}^{\text{2+}}$ ion that substitute for ${\mathrm{Zn}}^{\text{2+}}.$ A redshift is observed for the $E_0$ edge near $\text{x=0.06}$ from that of pure ZnO, interpreted as due to the spin–exchange interaction between d electrons of the ${\mathrm{Mn}}^{\text{2+}}$ ion and $\mathrm{sp}$ electrons near the band gap. The $E_0$ edge shows a blueshift for larger x. The excitonic character of the $E_0$ edge is gradually reduced and replaced by a three-dimensional shape as x increases, and this is attributed to increased disorder.