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A set of GaN layers prepared by metalorganic vapor phase epitaxy under different technological conditions (growth temperature carrier gas type and Ga precursor) were investigated by varia-ble energy positron annihilation spectroscopy (VEPAS) to find a link between technological conditions, GaN layer properties and the concentration of galium vacancies (VGa). Different cor-relations between technological parameters and VGa concentration were observed for layers grown from triethyl gallium (TEGa) and trimethyl gallium (TMGa) precursors. In case of TEGa formation of VGa was significantly influenced by type of reactor atmosphere (N2 or H2), while no similar behaviour was observed for growth from TMGa. Formation of VGa was suppressed with increasing temperature for the growth from TEGa. On the contrary, enhancement of VGa concen-tration was observed for growth from TMGa with cluster formation for the highest temperature of 1100°C. From the correlation of photoluminescence results with VGa concentration determined by VEPAS it can be concluded, that yellow band in GaN is likely not connected with VGa and ad-ditionally, increased VGa concentration enhances excitonic luminescence. Probable explanation is that VGa prevents formation of some other highly efficient nonradiative defects. Possible types of such defects are suggested.

The aim of this work is to compare and improve optical and structural properties of GaN layers prepared using TMGa or TEGa precursors. MOVPE grown GaN buffer layers on sapphire substrates are usually grown from TMGa precursor at the temperatures above 1000 °C. These layers contain deep and shallow acceptor levels which are responsible for blue and yellow defect bands in luminescent spectra. Both defect bands are detrimental for all major nitride device applications. Especially n-doped GaN layers suffer from strong yellow defect bands. In this work, it is shown that yellow band photoluminescence intensity can be suppressed by using TEGa precursor during the growth of n–doped GaN layers. Different kinds of growth parameters, such as growth temperature or growth rate, have been studied. It is also shown that the change of carrier gas (H2 or N2) has very strong influence on the layer quality. H2 carrier gas increased intensity of yellow band in sample grown from TEGa precursor while N2 carrier gas had the same effect for sample grown from TMGa precursor. Variable energy positron annihilation spectroscopy showed creation of single VGa in H2 atmosphere and clustering of VGa to big complexes ((VGa)3(VN)n) in N2 atmosphere.

This work suggests new alternative explanation why a single InGaN quantum well (QW) or the deepest QWs in the multiple quantum well (MQW) structures suffer with a high non-radiative recombination rate. According to SIMS results, positron annihilation spectroscopy and photoluminescence measurements we suggest that vacancy of Ga in complex with hydrogen atoms can play a dominant role in non-radiative Shockley-Read-Hall recombination of the deepest QWs in InGaN/GaN MQW structures. Vacancy of gallium originate dominantly in GaN buffer layers grown at higher temperatures in H2 atmosphere and are transported to the InGaN/GaN MQW region by diffusion, where they are very effectively trapped in InGaN layers and form complex defects with hydrogen atoms during epitaxy of InGaN layers. Trapping of gallium vacancies is another suggested mechanism explaining why the widely used In containing prelayers help to increase the luminescence efficiency of the InGaN/GaN MQW active region grown above them. Understanding the mechanism why the luminescence efficiency is suppressed in deeper QWs may be very important for LED community and can help to develop new improved technologies for the growth of InGaN/GaN MQW active region.

Nanostructured metals containing nano- and micro-cavities can be prepared by various methods. Morphology of cavities can be controlled by varying the parameters of preparation. This enables fabrication of nanostructured metals with properties tailored for particular applications, e.g. nanostructured metals containing fractal-like cavities with a wide size distribution are used as omnidirectional absorbers of light from the visible into the infrared spectral region. Positronium (Ps) is a non-destructive probe of nanoscopic cavities capable of precise determination of their size distribution. In conventional metals Ps does not form since any bound state of positron and electron is quickly destroyed by the screening of conduction electrons. However, a thermalized positron escaping from a metal through inner surface into a cavity may form Ps by picking an electron on the surface. This process was examined in the present work on nanostructured metals prepared three various methods: (i) thin films of black metals (Au and Al) evaporated in N2 atmosphere; (ii) nano-porous bulk Pd prepared by electrochemical etching of PdCo alloy; (iii) nanostructured Gd prepared by selective evaporation of Mg from MgGd alloy. Our investigations confirmed that Ps was formed in nanostructured metals. The the size distribution of nano-pores in the samples has been determined. The mechanism of Ps formation in these samples is discussed in the paper.

Titanium readily absorbs hydrogen and undergoes phase transition into the hydride phase (TiH2). In the hydride phase Ti is able to absorb the hydrogen concentration as high as 1.4 wt.%. These properties make Ti and Ti-based alloys attractive for hydrogen storage applications. Hydrogen absorption in titanium matrix may introduce open volume defects since the volume of TiH2 phase exceeds that of titanium matrix. Absorbed hydrogen may segregate at these defects forming defect-hydrogen complexes. In the present work positron annihilation spectroscopy was employed for characterization of hydrogen-induced defects in titanium. Defects created by hydrogen loading from the gas phase were compared with those introduced by electrochemical hydrogen charging. In general hydrogen loading introduces a high density of dislocations and vacancy clusters created by agglomeration of hydrogen-induced vacancies. The mean size of vacancy clusters depends on the hydrogen absorption temperature. Thermal stability of hydrogen absorbed in titanium and recovery of hydrogen-induced defects were studied by positron lifetime spectroscopy combined with in-situ X-ray diffraction and thermal desorption spectroscopy. Fig. 1 shows the temperature dependence of positron lifetimes and relative intensities of individual components for hydrogen gas loaded titanium. The decomposition of TiH2 phase is accompanied with introduction of additional vacancies agglomerating into vacancy clusters. Further annealing of the sample above 500 °C leads to recovery of dislocations.

Palladium is well known for its excellent hydrogen absorption kinetics. The gravimetric hydrogen absorption capacity of Pd is however only 0.93 wt. %. Magnesium exhibits a high hydrogen absorption capacity up to 7.6 wt. %, however the hydrogen absorption kinetics is slow. The aim of this work was to create thin Pd-Mg multilayered films combining positive hydrogen absorption properties of both elements. Pd-Mg multilayers were deposited by RF magnetron sputtering on fused silica substrates coated with 100 nm thick Pd wetting layer. The multilayers consist of alternating Pd and Mg layers (3, 12 and 60) of the same thickness. Three types of Pd-Mg multilayers were compared: (i) as deposited samples, (ii) hydrogen gas loaded samples at room temperature and H2 pressure of 4000 Pa for 2 h, (iii) samples annealed up to 450°C under Ar atmosphere. Defect structure of Pd-Mg multilayers was characterized using variable energy positron annihilation spectroscopy. Doppler broadening of the annihilation photopeak was analyzed using the S and W line-shape parameters and the measured S(E) curves were fitted using the VEPFIT code. The development of the structure during the annealing of the films was monitored by in-situ X-ray diffraction. Atomic force microscopy was employed for the study of the surface morphology. All films were characterized by nanocrystalline structure with a high density of grain boundaries with open-volume defects capable of positron trapping. The density of grain boundaries is determined by the mean grain size which increases with increasing thickness of a single phase layer. Hydrogen loading led to buckling of the film and introduced additional defects into the film. Annealing of the multilayers leads to diffusion of Mg atoms into the Pd layers and precipitates of Mg-Pd phase are formed.

In the present work point defects in ZnO crystals were characterized by positron lifetime spectroscopy combined with back-diffusion measurement of slow positrons. Defects in ZnO crystals grown by various techniques were compared. Hydrothermally grown ZnO crystals contain defects characterized by lifetime of ≈181 ps. These defects were attributed to Zn vacancies associated with hydrogen. ZnO crystals prepared by other techniques (Bridgman, pressurized melt growth, and seeded chemical vapour transport) exhibit shorter lifetime of ≈165 ps. Positron back-diffusion studies revealed that hydrothermally grown ZnO crystals contain higher density of defects than the crystals grown by other techniques. The lowest concentration of defects was detected in the crystal grown by seeded chemical vapor transport.

Positron annihilation spectroscopy (PAS) was employed for characterization of defects in the hydrothermally (HT) grown zinc oxide single crystals irradiated by high energy ions. Defects created in ZnO crystals by 2.5 MeV protons, 7.5 MeV N3+ and 167 MeV Xe26+ ions were compared. The virgin ZnO crystals contain Zn-vacancies associated with hydrogen. Ion implantation introduced additional defects, namely Zn+O di-vacancies in crystals irradiated by protons and small vacancy clusters in samples implanted by N and Xe ions.