Here the authors report systematic studies on the epitaxial growth and properties of Zn1−xCuxO (x=0.02–0.1) thin films deposited onto sapphire c-plane single crystals using pulsed-laser deposition. X-ray diffraction and high resolution transmission electron microscopy (HRTEM) were employed to study the epitaxial relations of Zn1−xCuxO with the substrate, and x-ray photoelectron spectroscopy was used to establish the bonding characteristics and oxidation states of copper inside the ZnO host. Room temperature ferromagnetism was observed in the Zn1−xCuxO films with magnetic moment per Cu atom decreasing with an increasing Cu content. The presence of any magnetic phase was ruled out using HRTEM. Thus, the ferromagnetism was attributed to Cu ions substituted into the ZnO lattice.

We report microstructural characteristics and properties of gallium-doped ZnO films deposited on glass by pulsed laser deposition. The Zn0.95Ga0.05O film deposited at 200 °C and 1×10−3 Torr showed predominant ⟨0001⟩ orientation with a metallic behavior and a resistivity of 2×10−4 Ω cm at room temperature. Low resistivity of the ZnGaO films has been explained in terms of optimal combination of carrier concentration and minimized scattering, and is correlated with the microstructure and the deposition parameters. Power conversion efficiency comparable to indium tin oxide-based devices (1.25±0.05%) is achieved on a Zn0.95Ga0.05O∕Cu-phthalocyanine∕C60 double-heterojunction solar cell.

We have investigated the properties of Zn1−xMnxO (x=0.01–0.1) thin films grown on c-plane sapphire single crystals by pulsed laser deposition. The electrical, magnetic, optical, and microstructural properties of these thin films have been characterized systematically, with a primary focus on establishing a correlation between magnetic properties and electrical conductivity. We have shown that this system exhibits ferromagnetism at room temperature when in the conducting as-deposited state. However, upon high temperature annealing in excess oxygen, the samples become insulating and exhibit nonferromagnetic behavior at room temperature. Thus, it is possible to tune ferromagnetism in Zn1−xMnxO diluted magnetic semiconductors by controlling the concentrations of oxygen vacancies and substitutional Mn dopants.

Unstrained and defect-free VO single crystals undergo structural (from high-temperature tetragonal to low-temperature monoclinic phase) and electronic phase transitions simultaneously. In thin films, however, in the presence of unrelaxed strains and defects, structural (Peierls) and electronic (Mott) transitions are affected differently, and are separated. In this paper, we have studied the temperature dependence of structural and electrical transitions in epitaxially grown vanadium dioxide films on (0001) sapphire substrates. These results are discussed using a combined kinetics and thermodynamics approach, where the velocity of phase transformation is controlled largely by kinetics, and the formation of intermediate phases is governed by thermodynamic considerations. We have grown (020) VO on (0001) sapphire with two (001) and (100) in-plane orientations rotated by 122°. The (100)-oriented crystallites are fully relaxed by the paradigm of domain-matching epitaxy, whereas (001) crystallites are not relaxed and exhibit the formation of a few atomic layers of thin interfacial VO. We have studied the structural (Peierls) transition by temperature-dependent in situ X-ray diffraction measurements, and electronic (Mott) transition by electrical resistance measurements. A delay of 3 °C is found between the onset of structural (76 °C) and electrical (73 °C) transitions in the heating cycle. This temporal lag in the transition is attributed to the residual strain existing in the VO crystallites. With this study, we suggest that the control of structural and electrical transitions is possible by varying the transition activation barrier for atomic jumps through the strain engineering.

Here we report on systematic studies of the epitaxial growth and properties of Co and Cu codoped ZnO thin films deposited onto sapphire c-plane single crystals using pulsed-laser deposition. The films display ferromagnetic behavior at room temperature. Detailed atomic scale characterization rules out the presence of clusters and secondary phases as the source of ferromagnetism. Optical measurements and x-ray photoelectron spectroscopy confirm the direct substitution of dopant atoms into Zn lattice sites. At low concentrations of Cu (∼5%) the magnetic moment of Zn1−(0.05+x)Co0.05CuxO materials appears to be additive. At higher concentrations of Cu the net magnetic moment per atom drops off sharply and seems to be relatively insensitive to the Co content. There is a dramatic increase in resistivity of the Co-doped films that accompanies Cu doping. Yet, this change of resistivity does not affect the magnetic moment, suggesting that free carrier mediated mechanism is not a feasible explanation for ferromagnetism in these films. The known presence of high oxygen vacancies in these films does allow for possible defect mediated mechanisms (e.g., bound magnetic polarons) for mediating exchange coupling of the dopant (Co,Cu) ions resulting in room temperature ferromagnetism.