V. Craciun

Characteristics of high quality ZnO thin films deposited by pulsed laser deposition

This paper show that under optimized deposition condition, films can be grown having a full width at half maximum (FWHM) value of the (002) x-ray diffraction (XRD) line a factor of 4 smaller than the previously published results using PLD and among the best reported so far by any technique. Under optimized conditions, c-axis oriented ZnO films having a FWHM value of the (002) XRD reflection line less than 15°, electrical resistivities around 5 × 10-2 Ω cm and optical transmittance higher than 85% in the visible region of the spectrum were obtained. Refractive index was around 1.98 and the Eg = 3.26 eV, values characteristic of very high quality ZnO thin films.

Growth of ZnO thin films on GaAs by pulsed laser deposition

ZnO thin films have been grown on GaAs substrates using the pulsed laser deposition technique with or without a photodeposited SiO2 buffer layer. The presence of the SiO2 layer has a beneficial effect on the crystalline quality of the grown ZnO films. Highly c-axis oriented ZnO films having a full width at half maximum value of the (002) X-ray diffraction line of less than 0.13 ° have been grown on such buffer layers at a substrate temperature of only 350 °C.

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.

Light emission from germanium nanoparticles formed by ultraviolet assisted oxidation of silicon-germanium

Nanocrystalline Ge particles embedded in a SiO2 layer, directly formed by an excision process from a SiGe strained layer during low temperature ultraviolet‐assisted oxidation are shown to exhibit visible photoluminescence. The emission maxima of the photoluminescence spectra are situated at around 2.18 eV, a value that corresponds, according to recent data to an average particle size of 5 nm, in excellent agreement with our previous Raman and transmission electron microscopy measurements of particle size. It is proposed that stress effects associated with the oxidation of small spherical particles allow the formed nanocrystalline Ge to survive during prolonged ultraviolet oxidation of the SiGe layer.

Characteristics of dielectric layers grown on Ge by low temperature vacuum ultraviolet-assisted oxidation

The growth of thin dielectric layers on (100) Ge samples at temperatures lower than 450u200a°C by photoassisted oxidation with vacuum ultraviolet radiation emitted by a Xe silent discharge lamp has been investigated. The thickness of the grown layers increased with both the oxidation time and processing temperature. Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy (XPS) indicated that the layers are mainly stoichiometric GeO2. XPS investigations also indicated the presence of a several-nanometer-thick substoichiometric oxide layer at the interface region for samples irradiated for short periods of time. Capacitance– and current–voltage measurement showed that layers thicker than 15 nm exhibited good electrical characteristics.

Vacuum-Ultra-Violet and Ozone Induced Oxidation of Silicon and Silicon-Germanium

UV radiation has been used to induce low temperature oxidation of crystalline silicon (c-Si) surfaces and silicon-germanium (Si-Ge) strained layers on Si. For c-Si at 550°C, growth rate enhancements of up to 50 times the dry thermal oxidation rate are achieved, equivalent to what is normally only obtainable thermally at 900°C. When irradiated under identical conditions by the UV light, SiGe layers were found to oxidise two times faster than c-Si. Prolonged oxidation of SiGe produced a dielectric structure consisting of a pure SiO2 film on top of a SiO2 layer containing several percent of trapped Ge, with the remaining SiGe layer snow-ploughed at the Si interface but retaining its epitaxial structure. Simple modelling qualitatively explains the observed features of the silicon oxidation and has led to further studies of ozone oxidation of c-Si.

MICROSTRUCTURE OF OXIDIZED LAYERS FORMED BY THE LOW-TEMPERATURE ULTRAVIOLET-ASSISTED DRY OXIDATION OF STRAINED SI0.8GE0.2 LAYERS ON SI

Ultraviolet‐assisted low‐temperature (550u2009°C) dry oxidation of Si0.8Ge0.2 strained layers on (100)Si has been studied. The oxidation rate of this material was found to be a factor of 2 greater than that of pure Si oxidation under identical irradiation conditions. Initially, the structure of the oxidized material consists of a SiO2 layer on top of a strained Si1−xGex layer with a Ge concentration significantly higher (x≳0.2) than the initial value. Increasing the oxidation time produces more SiO2 and a Si1−xGex layer further enriched with Ge. However, the oxidation rate is reduced and some of the Ge becomes trapped inside the growing SiO2 layer. For a prolonged irradiation time (≳5 h) SiGe oxidation still continues, unlike the case for pure Si, while the Ge trapped inside the SiO2 forms isolated microcrystalline regions.

Low temperature synthesis of Ge nanocrystals in SiO2

A novel and simple technique for the synthesis of Ge nanocrystals embedded in SiO2 is reported. The method is fully compatible with silicon microelectronic technology and relies solely upon low temperature (only 550u2009°C) ultraviolet oxidation of Si0.8Ge0.2 strained layers. This temperature is significantly lower than that usually used for the formation of Ge nanocrystals from SiGe oxides by H2 reduction.

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 (450u200a°C) upon thin hydroxyapatite (HA) films grown by the pulsed laser deposition technique was investigated. The HA layers were deposited at 650u200a°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...