  We present a comprehensive study on the structure and optical properties of
Mn-and Co-doped ZnO samples prepared via solid-state reaction method with
different dopant concentrations and atmospheres. The samples were structural
and chemically characterized via X-ray diffraction, scanning electron
microscopy, energy-dispersive X-ray spectroscopy, and X-ray excited
photoelectron spectroscopy. The optical characterization was performed via
Raman, photoluminescence, and diffuse photoreflectance spectroscopies. Emphasis
was done on the studies of their vibrational properties. The structural data
confirm the incorporation of Mn and Co ions into the wurtzite ZnO lattice. It
is demonstrated that the usual observed additional bands in the Raman spectrum
of transitional metal (TM) doped ZnO are related to structural damage, deriving
from the doping process, and surface effects. The promoted surface optical
phonons (SOP) are of Fr\"ohlich character and, together with the longitudinal
optical (LO) polar phonons, are directly dependent on the ZnO electronic
structure. The enhancement of SOP and LO modes with TM-doping is explained in
terms of nonhomogeneous doping, with the dopants concentrating mainly on the
surface of grains, and a resonance effect due to the decrease of the ZnO
bandgap promoted by the introduction of the 3d TM levels within the ZnO
bandgap. We also discuss the origin of the controversial vibrational mode
commonly observed in the Mn-doped ZnO system. It is stated that the observation
of the analyzed vibrational properties is a signature of substitutional doping
of the ZnO structure with tuning of ZnO optical absorption into the visible
range of the electromagnetic spectrum.

Materials Science