A. Velea

Photoexpansion and nano-lenslet formation in amorphous As2S3 thin films by 800 nm femtosecond laser irradiation

Two step laser processing has been used for the formation of nano-lenslets transmitting in red/infrared region of the optical spectrum on the surface of arsenic sulphide glass films. In the first step the films were obtained by pulsed laser deposition (248u2009nm), while in the second step the lenslets were created by low power femtosecond (800u2009nm) laser irradiation. Photoexpansion of the material along with simultaneous migration of chalcogen atoms in the irradiated area was the main phenomena involved in the generation of these structures. The maximum photoexpansion observed was 5.1%. At higher laser power, material ablation was evidenced.

In-situ characterization of the optical and electronic properties in GeTe and GaSb thin films

GeTe and GaSb thin films obtained by pulsed laser deposition were investigated by spectroscopic ellipsometry at controlled temperatures. The GeTe films were fully amorphous, while the GaSb films were partially crystalized in the as-deposited state. The Tauc-Lorentz model was employed to fit the experimental data. From the temperature study of the optical constants, it was observed the crystallization in the 150–160 °C range of GeTe amorphous films and between 230 and 240 °C of GaSb amorphous phase. A second transition in the resonance energy and the broadening parameter of the Lorentz oscillator was observed due to the crystallization of Sb after 250 °C. The temperatures of 85 °C and 130 °C are noticed as the start of the relaxation of the amorphous GeTe phase and as-deposited GaSb. The peaks of the imaginary part of the dielectric function red shifted after the phase change, while the variation with temperature of the crystalline phase follows the Varshni law. The electron-phonon coupling constants are 2.88 and 1.64 for c-GeTe and c-GaSb, respectively. An optical contrast up to 60% was obtained for GeTe films and a maximum value of 7.5% is revealed in the case GaSb, which is altered by the partial crystallinity of the as-deposited films.

In-situ crystallization of GeTe\GaSb phase change memory stacked films

Single and double layer phase change memory structures based on GeTe and GaSb thin films were deposited by pulsed laser deposition (PLD). Their crystallization behavior was studied using in-situ synchrotron techniques. Electrical resistance vs. temperature investigations, using the four points probe method, showed transition temperatures of 138 °C and 198 °C for GeTe and GaSb single films, respectively. It was found that after GeTe crystallization in the stacked films, Ga atoms from the GaSb layer diffused in the vacancies of the GeTe crystalline structure. Therefore, the crystallization temperature of the Sb-rich GaSb layer is decreased by more than 30 °C. Furthermore, at 210 °C, the antimony excess from GaSb films crystallizes as a secondary phase. At higher annealing temperatures, the crystalline Sb phase increased on the expense of GaSb crystalline phase which was reduced. Extended X-ray absorption fine structure (EXAFS) measurements at the Ga and Ge K-edges revealed changes in their local atomic environments as a function of the annealing temperature. Simulations unveil a tetrahedral configuration in the amorphous state and octahedral configuration in the crystalline state for Ge atoms, while Ga is four-fold coordinated in both as-deposited and annealed samples.

Femtosecond Laser Lithography in Organic and Non-Organic Materials

The lithography is a well established technology for fabrication of microelectronic components, integrated optics, microfluidic devices and Micro-Electro-Mechanical-Systems (MEMS) [1,2]. Using energy sources like ultra-violet (UV) photons or X-ray, various patterns are transferred from photo lithographic masks to photoresist materials. Developed for MEMS fabrications, the photoresists are light-sensitive materials. During exposure, chemical reactions are initiated in the irradiated volume, changing the chemical properties of the material. An imprinted pattern can be obtained when the exposed or unexposed material is removed by chemical solvents.

Ceramics and amorphous thin films based on gallium sulphide doped by rare-earth sulphides

Bulk ceramics of Ga2S3 and rare-earth sulfides (EuS, Gd2S3, Er2S3) as well as combinations thereof have been prepared by spark plasma sintering (SPS). The disk-shaped ceramics were used as targets for pulsed laser deposition (PLD) experiments to obtain amorphous thin films. The properties of these new bulks and amorphous thin films have been investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), optical transmission spectroscopy, and atomic force microscopy (AFM). In order to test the photoexpansion effect in Ga2S3 and the possibility to create planar arrays of microlenses, the film was irradiated with femtosecond laser pulses at different powers. For low laser power pulses (up to 100 mW power per pulse) a photoexpansion effect was observed, which leads to formation of hillocks with a height of 40–50 nm. EuS doped Ga2S3 thin film shows luminescence properties, which recommend them for optoelectronic applications.