Mid-gap electronic states inZn1−xMnxO

Abstract

Electronic absorption, magnetic circular dichroism (MCD), photoconductivity, and valence-band x-ray photoelectron (XPS) spectroscopic measurements were performed on epitaxial ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ films to investigate the origin of the mid-gap band that appears upon introduction of ${\text{Mn}}^{2+}$ into the ZnO lattice. Absorption and MCD spectroscopies reveal ${\text{Mn}}^{2+}$-related intensity at energies below the first excitonic transition of ZnO, tailing well into the visible energy region, with an onset at $\ensuremath{\sim}2.2\text{ }\text{eV}$. Photoconductivity measurements show that excitation into this visible band generates mobile charge carriers, consistent with assignment as a ${\text{Mn}}^{2+/3+}$ photoionization transition. XPS measurements reveal the presence of occupied ${\text{Mn}}^{2+}$ levels just above the valence-band edge, supporting this assignment. MCD measurements additionally show a change in sign and large increase in magnitude of the excitonic Zeeman splitting in ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ relative to ZnO, suggesting that $sp\text{\ensuremath{-}}d$ exchange in ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ is not as qualitatively different from those in other II-VI diluted magnetic semiconductors as has been suggested. The singular electronic structure feature of ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ is the presence of this ${\text{Mn}}^{2+/3+}$ ionization level within the gap, and the influence of this level on other physical properties of ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ is discussed.