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Optimized technology for PLD of highly transparent and uniform ZnO films simple, doped or partially covered with nano-clusters

ZnO thin films were coated on SiO2 (001) substrates by PLD using an excimer KrF* laser source (248 nm, 7 ns, 2 Hz), operated at a laser fluence of 2.6 J/cm2 for a 5-cm target-substrate distance. Prior to any deposition, the chamber was evacuated down to a residual pressure of 10^-4 Pa. Meanwhile, the chamber walls were heated up in order to facilitate the desorption of water vapors and other contaminants.

The 99.9% pure commercial ZnO raw powder was pressed at 20 MPa into pellets of 13 mm diameter and 2 mm thickness which were sintered in air at 1100º C for 8 h. Prior to each deposition, the target surface was cleaned by applying 5,000 laser pulses. In order to prevent the deposition of material ablated from the first layers of the target, a region, which can contain contaminants, a shutter was interposed between the target and the substrate. For avoiding the drilling effect and for ensuring surface cleanliness and uniform surface usage, the target was submitted to a rotational movement at a frequency of 0.04 Hz while translated on orthogonal directions.

The coatings were grown by applying 30,000 subsequent laser pulses on the undoped ZnO or Au (0.5% wt.) doped ZnO targets, respectively. The depositions were performed in a uniform dynamic oxygen flow of 10 Pa monitored by an MKS 50 controller. Au nanoclusters were deposited in a second PLD step starting from an Au target, in vacuum (10-4 Pa) at room temperature. In this case, to avoid uncontrolled contamination exposure to ambient air and to minimize processing time, a multi-target carousel holding both the ZnO and Au targets, was mounted inside the chamber. After completing the deposition of ZnO structures, the ZnO target was simply shifted and replaced by rotation by the Au target, without opening the chamber or performing other supplementary operation.

Highly c-axis oriented ZnO films were grown on (001) SiO2 substrate by pulsed laser deposition. All deposited thin films surfaces are smooth and uniform, which constitute a major advantage when using the m-line detection technique. The optical refractive index and extinction coefficient were very close to those of the reported reference data for ZnO thin films. The transmission of the films measured in the range 200-1400 nm had an average value of about 85%.

The good optical properties stand for the basic requirements, which allow the implementation of the obtained thin films in future optical gas sensors. Butane concentrations down to 100 ppm were detected. Moreover, we observed that nanostructure doping with noble metal nanoparticles resulted in a slight decrease of the detection sensitivity, which was probably due to enhanced light diffusion, scattering, and/or absorption.

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National Institute for Lasers, Plasma and Radiation Physics
National Institute for Lasers, Plasma and Radiation Physics
77125 Bucharest-Magurele
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