D. Craciun

Low temperature growth of highly transparent c-axis oriented ZnO thin films by pulsed laser deposition

The effects of the oxygen partial pressure, substrate temperature and laser wavelength on the structural and optical properties of thin films of ZnO grown on silicon and glass substrates by pulsed laser deposition have been studied. Regardless of thickness, all the grown layers are c-axis oriented and optically transparent. At substrate temperatures as low as 300°C, featureless layers with a FWHM value for the (002) XRD reflection less than 0.18° and exhibiting an optical transmission higher than 80% in the visible region were produced. For otherwise identical deposition conditions, the KrF excimer laser (at 248 nm) was always found to produce better quality thin films than the frequency-doubled Nd: YAG laser (532 nm). This is explained by the large difference between the optical absorption coefficients of ZnO at the two wavelengths employed, which play a keu role in the laser-target interaction. SEM investigation of the target surface after deposition revealed very different surface morphologies for the two wavelengths employed supporting this assumption.

EFFECTS OF LASER WAVELENGTH AND FLUENCE ON THE GROWTH OF ZNO THIN-FILMS BY PULSED-LASER DEPOSITION

Transparent, electrically conductive and c-axis oriented ZnO thin films have been grown by the pulsed laser deposition (PLD) technique on silicon and Corning glass substrates employing either a KrF excimer laser (? = 248 nm) or a frequency-doubled Nd:YAG laser (? = 532 nm). The crystalline structure, surface morphology, optical and electrical properties of the deposited films were found to depend not only on the substrate temperature and oxygen partial pressure, but also on the irradiation conditions. The quality of the ZnO layers grown by the shorter wavelength laser was always better than that of the layers grown by the longer wavelength, under otherwise identical deposition conditions. This behaviour was qualitatively accounted for by the results of the numerical solution of a one-dimensional heat diffusion equation which indicated a strong superheating effect of the melted target material for the case of frequency-doubled Nd:YAG laser irradiations. By optimizing the deposition conditions we have grown, employing the KrF laser, very smooth c-axis oriented ZnO films having a full-width at half-maximum value of the (002) X-ray diffraction value less than 0.16° and optical transmittance around 85% in the visible region of the spectrum at a substrate temperature of only 300°C.

Evidence for volume boiling during laser ablation of single crystalline targets

The surface morphology of monocrystalline Ge and Si samples irradiated by the fundamental, second and forth harmonic of a pulsed Nd:YAG laser at fluences typically used for laser ablation has been investigated by scanning electron microscopy. Whenever the samples were irradiated by the 1064 nm radiation, which is poorly absorbed by the solid targets, round-shaped micrometer-sized cavities were found on the irradiated area even after one single laser pulse. Irradiation by the 532 nm or 266 nm radiation resulted in a smooth, featureless surface. It is shown here that the only possible cause for the cavities formation during irradiation by the 1064 nm radiation is the subsurface, explosive, volume boiling. Numerical estimations of the temperature depth-profiles evidenced the formation in this case of a thick and superheated liquid layer, a condition which favours the occurrence of volume boiling.

Vacuum ultraviolet annealing of hydroxyapatite films grown by pulsed laser deposition

The effect of a post-deposition vacuum ultraviolet (VUV) radiation-assisted annealing treatment performed under 1 bar of oxygen at moderate temperatures (450 °C) upon thin hydroxyapatite (HA) films grown by the pulsed laser deposition technique was investigated. The HA layers were deposited at 650 °C under different partial oxidizing pressures without any water vapor and exhibited, besides the HA crystalline phase, tetracalcium phosphate and calcium oxide phases, more so for the films grown at lower oxidizing pressures. After the VUV-assisted anneal the layers were transformed into high quality crystalline HA films, exhibiting Ca/P ratio values closer to 1.67, the value for stoichiometric HA. The content of the other crystalline phases initially present was reduced significantly. Infrared spectroscopy also showed that the amount of OH− in the films increased after the treatment. The combination of these two low temperature techniques opens the possibility of growing high quality HA layers without signific...

Excimer laser synthesis of thin AlN coatings

Abstract A new technique for the production of thin AlN films on Al samples by direct laser synthesis is reported. This has the advantages of good adhesion, low temperature deposition and localisation. A metallic, pure Al sample is irradiated by an excimer laser under a nitrogen containing atmosphere at high intensity levels such that a plasma is formed above the sample. The active nitrogen atoms and ions contained in this plasma react with the hot melted Al surface resulting in the synthesis of an AlN layer. The influence of the incident laser fluence, nature and pressure of the ambient atmosphere and of the number of laser pulses on the chemical composition and crystalline structure of the synthesised layers is reported. Under optimised conditions a few μm thick polycrystalline AlN layer, containing only a few percent oxygen and exhibiting high hardness and very good adhesion, has been prepared on Al.

Growth of highly transparent oxide layers by pulsed laser deposition: reduction of droplet density

Thin layers of ZnO and CeO2 have been grown on Corning glass and Si substrates by pulsed laser deposition (PLD) method. Under optimised conditions, films exhibiting absorption coefficients in the visible region as low as 4×102 cm−1 and 3×103 cm−1 for ZnO and CeO2, respectively, have been deposited at a substrate temperature of only 350°C. When using the PLD technique, the grown layers very often exhibit some micrometer sized droplets. Although the surface density of these can be dramatically reduced, there is still much effort being directed at completely eliminating their presence, which could clearly restrict the applications. There is still much controversy concerning the mechanisms of droplet formation, whether they are caused by a sub-surface super-heating effect or by explosive boiling. Careful scanning electron microscopy investigations of various target surfaces after the laser ablation process and numerical simulations of the temperature distribution inside these targets during the action of the laser pulse tend to support the sub-surface super-heating effect as one of the main causes of droplet emission.

Droplet formation during extended time pulsed laser deposition of La0.5Sr0.5CoO3 thin layers

The appearance of a strong splashing effect during extended time pulsed laser deposition of La0.5Sr0.5CoO3 (LSCO) films has been investigated. For this aim, films were grown in a base vacuum (4×10−7 Torr) from targets ablated for prolonged time duration or under an oxidizing atmosphere of 10−2 Torr of N2O from stoichiometric, as-prepared, fresh targets. The films grown under vacuum were found by Rutherford backscattering spectrometry to be oxygen deficient and transparent, while those grown under the oxidizing atmosphere were stoichiometric and opaque. The measured optical absorption coefficient (α) at the laser wavelength used for the ablation process (KrF, λ=248 nm) for these two series of films was found to be around 1–2×104 cm−1 and 2–3×105 cm−1, respectively. It is suggested here that, because of preferential oxygen evaporation from the surface layer of the LSCO target during prolonged laser ablation time, the value of α decreased, inducing a volume absorption of the laser radiation. This favored the...

Growth of thin transparent titanium nitride layers by reactive laser ablation

Abstract Transparent and conductive thin layers of TiN have been grown on Corning glass and silicon substrates by the reactive pulsed laser deposition method. An excimer laser (KrF, λ=248 nm, 4.0 J/cm2) was used to ablate a massive, metallic Ti target in a N2 atmosphere. Under optimised conditions, continuous polycrystalline films of fcc TiN exhibiting a lattice parameter a=0.4242 nm very close to the bulk value, an optical transmittance higher than 70% in the 350–1100 nm range, a flat morphology and an electrical conductivity around 550 μΩ cm have been deposited at a substrate temperature of only 400°C. The grown films also posses a good chemical and wear resistance as their properties have not changed after exposure to the ambient atmosphere for 6 months.

Oxygen trapping during pulsed laser deposition of oxide films

Abstract Ba0.5Sr0.5TiO3, indium tin oxide (ITO), ZnO, and ZrO2 thin films were grown directly on Si substrates by the pulsed laser deposition technique. X-ray photoelectron spectroscopy investigations showed that these films contain oxygen species that are loosely bound, corresponding to what can be described as physisorbed oxygen. This oxygen is responsible for the formation of an interfacial layer at the interface between the Si substrate and the deposited oxide layer during the deposition process. The chemical composition of this interfacial layer consists of SiOx partially mixed with the grown oxide. The trapped oxygen can ensure further growth of the interfacial layer during any post-deposition anneals even when performed in vacuum or inert atmospheres.

Microstructure of hydroxyapatite thin layers grown by pulsed laser deposition

Thin films of hydroxyapatite (HAp) have been grown on Si, quartz, Ti, and Ge substrates by the pulsed laser deposition (PLD) method employing a KrF excimer laser (wavelength (lambda) equals 248 nm, pulsed duration (tau) FWHM equals 20 ns). The influence of the laser deposition parameters on the properties of the grown layers was investigated in order to optimize the Ca/P ratio and the crystalline structure. It was found that the optimum conditions for preserving the Ca/P ratio i.e. high oxygen pressures and low substrate temperatures do not coincide with those for obtaining adherent and crystalline layers i.e. low oxygen pressures and high substrate temperatures. For films deposited onto Ti substrates it was also found that high substrate temperatures promote the diffusion of Ti through the depositing film up to the surface where it gets oxidized. Further investigations are required before high quality HAp-coated Ti implants by PLD can be obtained.