Ivars Tale

Nature of the blue luminescence bands in PbWO4

Abstract The photoluminescence spectrum of PbWO 4 is composed of blue and green bands, previously attributed to the regular WO 4 group and to the defect-related WO 3 group, respectively. Untill now only green emission was observed in the thermostimulated luminescence (TSL) above 77 K. Investigation of the TSL spectra starting from 20 K indicates that the blue band, being definitely present at least in the 28 K TSL peak, is also due to recombination emission at defect sites.

Crystallization processes of amorphous Si by thermal annealing and pulsed laser processing

Amorphous Si thin films deposited on oxidized crystalline Si surface (111) were crystallized by thermal annealing and nanosecond green laser pulses. The Raman scattering spectra show that thermal annealing can provide nearly fully crystallized poly-Si film. Laser crystallization of amorphous Si is more flexible crystallization method, but it is more difficult to reach high levels of crystallinity. Depth studies of laser treated samples reveal a thin amorphous-like interlayer between substrate surface and crystallized Si film.

Work function of colloidal semiconducting nanocrystals measured by Kelvin probe

Colloidal suspensions of CdS, PbS, Cu2S, Ag2S and ZnO nanocrystals were prepared by chemical route in presence of organic capping ligands. The formation of nanocrystals was studied by using UV-visible absorption spectroscopy. Kelvin probe measurements of work function were performed on nanocrystals thin film on ITO (indium tin oxide) coated glass.

Thermostimulated processes in dynamically disordered ionic crystals

Abstract Thermostimulated electronic recombination processes have been studied by means of the fractional glow technique in KCl-T1, CaF2, LiF-Mg, Ti and Al2O3 crystals in the temperature range where the ionic motion arises. A novel model concerning dynamic disordering for describing the kinetics of thermostimulated luminescence is developed, which takes into account the fluctuational changes of the trap spatial structure as well as its interaction with mobile ions and/or vacancies.

Laser ablation for analysis of nanoscale layers

Deposition of carbon layer, tungsten and carbon migration, as well as retention, co-deposition and diffusion of hydrogen isotopes are the major concerns in fusion devices. We propose laser ablation spectroscopy as an effective method of rapid qualitative and quantitative surface mapping. An average growth of the deposited carbon layer in divertor region of a tokamak is about 0.2 nm/s. Depending on the number of discharges, the thickness of a layer can reach 200–400 nm. The information about the layer must be gained within this range of thickness. Obtained results show that proposed method can be used for analysis of layers in the range of thickness of several hundreds of nanometres.

Annealing of radiation defects in x-irradiated LiBaF3

Results of the glow rate technique application for analysis of the activation energy of thermostimulated annealing of X-ray created F-type color centers in pure and containing oxygen centers LiBaF3 crystals are presented. It is shown that depending on the impurity composition two alternative mechanisms could be involved in the annealing of color centers. It is proposed that either the anion vacancy governed migration of F-centers resulting in recombination with complementary defects, or the thermal delocalization of radiation created fluorine (Fi) interstitials captured by anti-structure defects followed by recombination with all kinds of complementary F-type centers are responsible for the recombination of radiation defects above RT.

Characterization of Crystalline Structure and Morphology of Ga2O3 Thin Film Grown by MOCVD Technique

Growth of gallium oxide thin film was realized with MOCVD on (0001) sapphire substrate. Structural and compositional properties of thin film were studied employing trimethylgallium and water as precursors, carrier gases were H2 and N2. Obtained film is polycrystalline and predominantly consisted of (201) oriented β-Ga2O3. Sample exhibited blue luminescence which is attributed to oxygen vacancies. H2 gas proved to have beneficial effect on film quality and overall growth process.

Laser scribing on HOPG for graphene stamp printing on silicon wafer

Highly oriented pyrolytic graphite (HOPG) was scribed by pulsed laser beam to produce square patterns. Patterning of HOPG surface facilitates the detachment of graphene layers during contact printing. Direct HOPG-to-substrate and glue-assisted stamp printing of a few-layers graphene was compared. Printed graphene sheets were visualized by optical and scanning electron microscopy. The number of graphene layers was measured by atomic force microscopy. Glue-assisted stamp printing allows printing relatively large graphene sheets (40×40 μm) onto a silicon wafer. The presented method is easier to implement and is more flexible than the majority of existing ways of placing graphene sheets onto a substrate.

Processing of amorphous Si by pulsed laser irradiation at different wavelengths

Amorphous Si thin films deposited on thermally oxidized Si wafers have been processed by the 2 nd and 3 rd harmonics of Nd:YAG laser. Surface modification of amorphous silicon layers have been investigated by scanning electron microscopy before and after chemical etching of processed silicon films. The super-lateral crystal growth regime was achieved with laser fluence of 280 mJ/cm 2 for the 2 nd harmonics and 155 mJ/cm 2 for the 3 rd harmonics. The grain size in polycrystalline Si samples prepared by successive crystallization in the lateral growth regime is about 0.5 - 1 μm.

Ex situ investigations of MOCVD-grown gallium nitride nanowires using reflection high energy electron diffraction

Vertically oriented nanowires (NWs) of single-crystalline wurtzite GaN have been fabricated on sapphire substrates, via metal organic chemical vapor deposition (MOCVD). We present ex situ investigations on orientation and structure of grown GaN nanowires on GaN(0001) surface using reflection high energy electron diffraction (RHEED). Both ordered and randomly oriented GaN crystalline structures have been detected.