David Horwat

Exciton and core-level electron confinement effects in transparent ZnO thin films

The excitonic light emission of ZnO films have been investigated by means of photoluminescence measurements in ultraviolet-visible region. Exciton confinement effects have been observed in thin ZnO coatings with thickness below 20 nm. This is enhanced by a rise of the intensity and a blue shift of the photoluminescence peak after extraction of the adsorbed species upon annealing in air. It is found experimentally that the free exciton energy (determined by the photoluminescence peak) is inversely proportional to the square of the thickness while core-level binding energy is inversely proportional to the thickness. These findings correlate very well with the theory of kinetic and potential confinements.

Compression and strong rarefaction in high power impulse magnetron sputtering discharges

Gas compression and strong rarefaction have been observed for high power impulse magnetron sputtering (HIPIMS) discharges using a copper target in argon. Time-resolved ion saturation currents of 35 probes were simultaneously recorded for HIPIMS discharges operating far above the self-sputtering runaway threshold. The argon background pressure was a parameter for the evaluation of the spatial and temporal development of the plasma density distribution. The data can be interpreted by a massive onset of the sputtering flux (sputter wind) that causes a transient densification of the gas, followed by rarefaction and the replacement of gas plasma by the metal plasma of sustained self-sputtering. The plasma density pulse follows closely the power pulse at low pressure. At high pressure, the relatively remote probes recorded a density peak only after the discharge pulse, indicative for slow, diffusive ion transport.

Transmittance enhancement and optical band gap widening of Cu2O thin films after air annealing

Cu2O thin films have been grown on glass substrates at room temperature by reactive magnetron sputtering. As-deposited films exhibit high electrical resistivity and low optical transmittance. To improve the film properties, post annealing treatments in air at various temperatures have been performed. Low temperature annealing (<300 °C) avoids the film oxidation into CuO and the films remain single-phased. In this temperature range, the annealing in air enhances the transmittance in the visible region due to the decrease of the defect scattering. Moreover, the optical band gap of Cu2O thin films is enlarged from 2.38 to 2.51 eV with increasing annealing temperature. The increase of optical band gap accompanying the reduction of Urbach energy indicates that the widening of optical band gap may result from the partial elimination of defect band tail after thermal annealing in air. Combining experimental results with recent reported calculations, the peak at about 1.7 eV in photoluminescence spectra is assign...

On the deactivation of the dopant and electronic structure in reactively sputtered transparent Al-doped ZnO thin films

We report on the possible origin of electrical heterogeneities in 4 at% Al-doped ZnO (AZO) reactively sputtered films. It is found through the Zn L3 and Al K edge x-ray absorption near-edge structure that a fraction of the Al dopant is deactivated by its positioning in octahedral conformation with oxygen. This fraction as well as the conductivity, optical bandgap and c-axis parameter of ZnO wurtzite are all found to depend on the sample position during deposition. The present results suggest the formation of a metastable Al2O3(ZnO)m homologous phase that degrades the electrical conductivity.

Spatial distribution of average charge state and deposition rate in high power impulse magnetron sputtering of copper

The spatial distribution of copper ions and atoms in high power impulse magnetron sputtering discharges was determined by (i) measuring the ion current to electrostatic probes and (ii) measuring the film thickness by profilometry. A set of electrostatic and collection probes were placed at different angular positions and distances from the target surface. The angular distribution of the deposition rate and the average charge state of the copper species (including ions and neutrals) were deduced.The discharge showed a distinct transition to a high current mode dominated by copper self-sputtering when the applied voltage exceeded the threshold of 535 V. For a lower voltage, the deposition rate was very low and the average charge state was found to be less than 0.4. For higher voltage (and average power), the absolute deposition rates were much higher, but they were smaller than the corresponding direct current (dc) rates if normalized to the same average power. At the high voltage level, the spatial distribution of the average charge state showed some similarities to the distribution of the magnetic field, suggesting that the generation and motion of copper ions is affected by magnetized electrons. At higher voltage, the average charge state increases with the distance from the target and locally may exceed unity, indicating the presence of significant amounts of doubly charged copper ions.

Ion acceleration and cooling in gasless self-sputtering

Copper plasma with hyperthermal directed velocity (8.8 eV) but very low temperature (0.6 eV) has been obtained using self-sputtering far above the runaway threshold. Ion energy distribution functions (IEDFs) were simultaneously measured at 34 locations. The IEDFs show the tail of the Thompson distribution near the magnetron target. They transform to shifted Maxwellians with the ions being accelerated and cooled. We deduce the existence of a highly asymmetric, pressure-driven potential hump which acts as a controlling “watershed” between the ion return flux and the expanding plasma.

Near-room temperature single-domain epitaxy of reactively sputtered ZnO films

Single-domain epitaxial ZnO films are grown near-room temperature (below 40 °C) on (0 0 0 1)-sapphire using reactive magnetron sputtering of a zinc target in the transition between the metallic and compound sputtering regimes. As the oxygen content in the reactive gas mixture is increased, the in-plane six-fold symmetry of hexagonal wurtzite ZnO, probed by -scan measurements, develops. Transmission electron microscopy analyses confirm that single-domain epitaxial layers are formed. This is accompanied by the incorporation of oxygen interstitial defects associated with oxygen over-stoichiometry and by compressive stresses. A model is proposed to explain the observed behaviour based on the transformation of the kinetic energy of fast oxygen particles into the mobility of the adatoms.

Structure-properties relationship in reactively sputtered Ag-Cu-O films

Ag–Cu–O films were deposited on glass substrates by pulsed dc sputtering of a silver–copper target (Ag50Cu50) in reactive Ar–O2 mixtures. The film chemical composition was estimated by x-ray energy dispersive spectrometry and the structure was studied by x-ray diffraction (XRD). Optical properties (reflectance and transmittance) and room temperature electrical resistivity were evaluated using spectrophotometry and the four point probe method, respectively. Since silver atoms are less reactive versus oxygen than copper ones, the increase in the oxygen flow rate introduced into the deposition chamber induced the preferential oxidation of sputtered copper atoms. XRD analysis showed that the structure of the deposited films can be divided into three domains. At low oxygen flow rate, the films were biphased (metallic silver-based solid solution and crystalline copper-based oxide). At intermediate oxygen flow rate, the films were x-ray amorphous (grain size lower than 2 nm). At high oxygen flow rate, the films contained a crystalline silver–copper oxide phase and a crystalline unknown phase. Thanks to the absorption band of silver in the UV range, reflectance measurements were used to show the occurrence of metallic silver phase in the films. It was shown that the chemical environment of silver atoms in the x-ray amorphous region evolved from metallic to oxide when the oxygen flow rate increased. Transmittance evolution versus the oxygen flow rate were well correlated with that of the electrical resistivity. The evolution of Ag–Cu–O film properties was discussed in connection with the structure and chemical composition.

Bacterial adhesion on biomedical surfaces covered by micrometric silver Islands

A set of Cu-Mn-O and Ag-Cu-Mn-O films were sputter-deposited onto polished Ti-6Al-4V coupons and the microbiological adherence of Staphylococcus sp. was studied in these biomedical surfaces modified by using advanced ternary and quaternary oxides, these latter incorporated micrometric silver islands. Silver is known to have a natural biocidal character and its presence in the surface of Ti-6Al-4V forming large micrometric islands. In principle, predicted to enhance the antimicrobial properties of biomedical surfaces. Microbial adhesion tests were performed using collection strains and six clinical Staphylococcus aureus and Staphylococcus epidermidis strains. The adherence study was performed using a previously published protocol by Kinnari et al. Collection strains and clinical strains showed decreased adherence to modified materials; however, only on the clinical strains were there statistically significant differences between Cu-Mn-O and Ag-Cu-Mn-O containing silver islands. Nanocrystalline silver dissolves and releases both Ag(+) and Ag(0) whereas other silver sources release only Ag+. We can conclude that nanocrystalline silver coating, confirmed by XRD, appears to alter the biological properties of the solution, particularly antimicrobial activity.

Thermochromic effect in NdNiO3−δ thin films annealed in ambient air

The synthesis of NdNiO3 perovskite structure was achieved by soft post deposition annealing of initially amorphous thin films reactively sputter deposited on silicon substrates. The physical measurements were fully consistent with the properties expected for the thermochromic NdNiO3 phase. Upon heating, the optical transmission that was correlated with the electrical properties decreased in the infrared domain showing a thermochromic effect in this optical region. The metal–insulator transition temperature was found to be −68 °C for the specimen tested and the jump in resistance at the transition was 1.4 orders of magnitude. The state-of-the-art methods that normally involve an annealing at a high oxygen pressure (200 × 105 Pa) or epitaxial stabilization were considerably improved in this work by the use of dc sputtering and the optimization of the deposition conditions. Therefore, the novel soft process proposed here opens up numerous research possibilities.