Epitaxial growth of (0001) oriented ${[\mathrm{Zn}\mathrm{O}\left(20\mathrm{\AA }\right)∕{\mathrm{Co}}_{0.7}{\mathrm{Fe}}_{0.3}\left(x\mathrm{\AA }\right)]}_{25}$ multilayers (MLs) with nominal thickness $x=1$ , 2 and 5 has been prepared on $\alpha \text{-}{\mathrm{Al}}_2{\mathrm{O}}_3$ (0001) substrate by ion-beam sputtering. The magnetic properties over a temperature range of $6–350\mathrm{K}$ and structures probing by x-ray absorption spectroscopy (XAS) are reported. Above room-temperature ferromagnetism has been observed for $x=1$ and $x=2$ MLs, while superparamagnetic behavior dominates for $x=5$ ML. The field-cooled magnetization-temperature $M\left(T\right)$ curves of $x=1$ and $x=5$ MLs can be fitted by a standard three-dimensional (3D) spin-wave and a Curie–Weiss model, respectively. For $x=2$ ML, however, neither a 3D spin-wave nor a Curie–Weiss model, but a combination of the two fits the $M\text{-}T$ curve. The XAS studies together with the magnetic measurements further reveal that $x=1$ sample behaves as a diluted magnetic semiconductor (DMS) ML, while $x=2$ ML shows a mixed structure consisting of a minor component of DMS and a major component of CoFe clusters. A predominant clustering phase appears for $x=5$ ML.

The crystallinity and intrinsic defects of transparent conducting oxide (TCO) films have a high impact on their optical and electrical properties and therefore on the performance of devices incorporating such films, including flat panel displays, electro-optical devices, and solar cells. The optical and electrical properties of TCO films can be modified by tailoring their deposition parameters, which makes proper understanding of these parameters crucial. Magnetron sputtering is the most adaptable method for preparing TCO films used in industrial applications. In this study, we investigate the direct and inter-property correlation effects of sputtering power (PW) on the crystallinity, intrinsic defects, and optical and electrical properties of Al-doped ZnO (AZO) TCO films. All of the films were preferentially c-axis-oriented with a wurtzite structure and had an average transmittance of over 80% in the visible wavelength region. Scanning electron microscopy images revealed significantly increased AZO film grain sizes for PW ≥ 150 W, which may lead to increased conductivity, carrier concentration, and optical band gaps but decreased carrier mobility and in-plane compressive stress in AZO films. Photoluminescence results showed that, with increasing PW, the near band edge emission gradually dominates the defect-related emissions in which zinc interstitial (Zn_i), oxygen vacancy (V_O), and oxygen interstitial (O_i) are possibly responsible for emissions at 3.08, 2.8, and 2.0 eV, respectively. The presence of Zn_i- and O_i-related emissions at PW ≥ 150 W indicates a slight increase in the presence of Al atoms substituted at Zn sites (Al_Zn). The presence of O_i at PW ≥ 150 W was also confirmed by X-ray photoelectron spectroscopy results. These results clearly show that the crystallinity and intrinsic-defect type of AZO films, which dominate their optical and electrical properties, may be controlled by PW. This understanding may facilitate the development of TCO-based optoelectronic devices for industrial production.

The effect of chromium doping on the defect-induced ferromagnetism of Zn1−xCrxO films was investigated in this study. X-ray diffraction and x-ray photoelectron results confirm Cr substitution at Zn sites in the ZnO host lattice. A parabolic dependence of the saturation magnetization on the stoichiometric variable x was observed. Photoluminescence and magnetic results indicate that magnetic moments in Zn1−xCrxO films stem from Zn vacancies on the film surface or at grain boundaries. As the variation in saturation magnetization is consistent with that in surface morphology, the ferromagnetism in Zn1−xCrxO films is sensitive to the morphological modification caused by Cr doping.