J. Kuriplach

Defect studies of ZnO single crystals electrochemically doped with hydrogen

Various defect studies of hydrothermally grown (0001) oriented ZnO crystals electrochemically doped with hydrogen are presented. The hydrogen content in the crystals is determined by nuclear reaction analysis and it is found that already 0.3at.% H exists in chemically bound form in the virgin ZnO crystals. A single positron lifetime of 182ps is detected in the virgin crystals and attributed to saturated positron trapping at Zn vacancies surrounded by hydrogen atoms. It is demonstrated that a very high amount of hydrogen (up to ∼30at.%) can be introduced into the crystals by electrochemical doping. More than half of this amount is chemically bound, i.e., incorporated into the ZnO crystal lattice. This drastic increase of the hydrogen concentration is of marginal impact on the measured positron lifetime, whereas a contribution of positrons annihilated by electrons belonging to O–H bonds formed in the hydrogen doped crystal is found in coincidence Doppler broadening spectra. The formation of hexagonal shape ...

Crystal field in rare earth intermetallics with CsCl structure

Abstract Electronic structure of rare earth intermetallics REX (RE  Y, Er; X  Rh, Cu, Ag, Zn, and Mg) was calculated using the full potential LAPW method in the LDA (LSDA) approximation. The crystal field (CF) splittings of RE 3+ energy levels were obtained from the aspherical components of the self-consistent potential from the region inside the RE muffin-tin sphere as well as from the interstitial region. We show that the calculated CF parameters account for the experimentally observed trend in sign and even values of CF parameter A 4 0 〈 r 4 〉. As the charge density of the whole crystal contributes to CF in general, we provide the discussion of contributions to CF from the different regions of REX crystals.

Positron-defect interactions in complex systems

Positron annihilation (PA) spectroscopy (PAS) is successfully used to study various types of defects in many kinds of materials. As experimental techniques become more precise and sophisticated, finer details about studied defects/materials can be obtained and, consequently, requirements for the quality of the theoretical description of the positron annihilation process in a given material also increases. In this paper, a short review of recent developments in PA defect studies is given. In particular, several examples of defect studies in metallic, semiconducting and insulating systems are discussed. Finally, new perspectives and problems for the theoretical description of positron-defect interactions are outlined. A tight cooperation between experiment and theory in PA studies is pointed out.

Electronic structure and crystal field in REMg and RERh (RE = rare earth) intermetallics

Abstract Electronic structures of the rare earth intermetallics YRh, YMg, GdRh and GdMg were calculated using the full potential LAPW method at LDA approximation. The crystal field splittings of RE3+ energy levels were obtained from the aspherical components of the self-consistent potential. Our method is shown to describe the trends in experimental values of the crystal field parameters for RERh and REMg systems correctly.

Activities towards p-type doping of ZnO

Zinc oxide (ZnO) is an interesting and promising semiconductor material for many potential applications, e.g. in opto-electronics and for sensor devices. However, its p-type doping represents a challenging problem, and the physical reasons of its mostly n-type conductivity are not perfectly clear at present. Efforts to achieve p-type conductivity by ion implantation are reviewed, and ways to achieve p-type ZnO nanorods and thin films through various growth conditions are summarized. Then, issues associated with the preparation of Schottky contacts is discussed in some detail as this is a requirement of the device formation process. Finally, the possible incorporation of hydrogen and nitrogen into structural defects, which can act as trapping sites for positrons, is discussed in the context of experimental and theoretical positron results and the estimated H and N content in a variety of ZnO materials.

Improved generalized gradient approximation for positron states in solids

Several first-principles calculations of positron-annihilation characteristics in solids have added gradient corrections to the local-density approximation within the theory by Arponen and Pajanne [Ann. Phys. (N.Y.) 121, 343 (1979)] since this theory systematically overestimates the annihilation rates. As a further remedy we propose to use gradient corrections for other local density approximation schemes based on perturbed hypernetted-chain and on Quantum Monte Carlo results. Our calculations for various metals and semiconductors show that the proposed schemes generally improve the positron lifetimes when they are confronted with experiment. We also compare the resulting positron affinities in solids with data from slow-positron measurements.

Positron characteristics of various SiO2 polymorphs

Abstract Theoretical calculations of positron lifetimes, affinities and core electron contributions to Doppler broadening spectra are carried out for several phases (polymorphs) of SiO 2 . An interesting feature of SiO 2 is that not all polymorphs have the same coordination of Si and O atoms, which also affects positron properties. The calculated quantities are compared with available experimental data and discussed with reference to SiO 2 /Si interfaces.

Characterization of microstructural defects in melt grown ZnO single crystals

Various nominally undoped, hydrothermally or melt grown (MG) ZnO single crystals have been investigated by standard positron lifetime measurements. Furthermore, optical transmission measurements and structural characterizations have been performed; the content of hydrogen in the bound state was determined by nuclear reaction analysis. A positron lifetime of 165-167 ps, measured for a brownish MG ZnO sample containing (0.30 ± 0.03) at.-% of bound hydrogen, matches perfectly the value found for colorless MG ZnO crystals. The edge shift, observed in the “blue light domain” of the optical absorption for the former sample with respect to the latter samples, is estimated to be 0.70 eV, and found equal to a value reported previously. The possible role of zinc interstitials is considered and discussed. Microstructure analysis by X-ray diffraction and transmission electron microscopy revealed the presence of stacking faults in MG crystals in a high concentration, which suggests these defects to be responsible for ...

Hydrogen implantation effect in copper alloys selected for ITER investigated by positron annihilation spectroscopy

Defects in the form of vacancies (loops, voids, etc) created by hydrogen implantation into selected Cu alloys foreseen for the International Thermonuclear Experimental Reactor (ITER) first wall cladding were studied using positron annihilation spectroscopy. The pulsed low-energy positron system, which enables depth profiling of the positron lifetime spectra in the near-surface region (20–460 nm) of hydrogen-implanted copper alloys, was applied, and its results were compared with TRIM calculations and transmission electron microscopy studies. The selected specimens were implanted in the Ion Beam Laboratory of FEI STU Bratislava. The energy of implantation was EH = 2 × 95 keV for the molecular ion beam. The temperature during this process was lower than 90°C. Two implantation doses were chosen for both the alloys: 1.3 × 1019 ions cm−2 (1.1 C cm−2) and 5 × 1018 ions cm−2 (0.4 C cm−2). Although the influences of neutrons with energy 14 MeV and protons with energy 95 keV are not the same (differences in energy and existence of proton charge), experimental simulation of radiation damage of ITER construction materials was successfully performed. The results are discussed in terms of microstructural changes of the studied materials upon irradiation. The CuAl25 alloy seems to be more resistant to proton bombardment than CuCrZr.

Gradient correction scheme for bulk and defect positron states in materials: New developments

As local density approximation positron calculations systematically underestimate positron lifetimes when they are compared with their experimental counterparts, the generalized gradient approximation (GGA) for positrons was introduced in the 1990s, in analogy with the GGA for electrons. New developments in the GGA for positrons are summarized and presented here and it is also discussed how they affect and possibly improve calculated positron lifetimes. In particular, these new GGA approaches are based on the recent perturbed hypernetted-chain and quantum Monte Carlo results.