Vacancy complexes in nonequilibrium germanium-tin semiconductors

Understanding the nature and behavior of vacancy-like defects in epitaxial GeSn metastable alloys is crucial to elucidate the structural and optoelectronic properties of these emerging semiconductors. The formation of vacancies and their complexes is expected to be promoted by the relatively low substrate temperature required for the epitaxial growth of GeSn layers with Sn contents significantly above the equilibrium solubility of 1 at.%. These defects can impact both the microstructure and charge carrier lifetime. Herein, to identify the vacancy-related complexes and probe their evolution as a function of Sn content, depth-profiled pulsed low-energy positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy were combined to investigate GeSn epitaxial layers with Sn content in the 6.5-13.0 at.% range. The samples were grown by chemical vapor deposition method at temperatures between 300 and 330 {\deg}C. Regardless of the Sn content, all GeSn samples showed the same depth-dependent increase in the positron annihilation line broadening parameters, which confirmed the presence of open volume defects. The measured average positron lifetimes were the highest (380-395 ps) in the region near the surface and monotonically decrease across the analyzed thickness, but remain above 350 ps. All GeSn layers exhibit lifetimes that are 85 to 110 ps higher than the Ge reference layers. Surprisingly, these lifetimes were found to decrease as Sn content increases in GeSn layers. These measurements indicate that divacancies are the dominant defect in the as-grown GeSn layers. However, their corresponding lifetime was found to be shorter than in epitaxial Ge thus suggesting that the presence of Sn may alter the structure of divacancies. Additionally, GeSn layers were found to also contain a small fraction of vacancy clusters, which become less important as Sn content increases

Control of interlayer exchange coupling in Fe/Cr/Fe trilayers by ion beam irradiation

The manipulation of the antiferromagnetic interlayer coupling in the epitaxial Fe/Cr/Fe(001) trilayer system by moderate 5 keV He ion beam irradiation has been investigated experimentally. It is shown that even for irradiation with very low fluences (10^14 ions/cm^2) a drastic change in strength of the coupling appears. For thin Cr-spacers (below 0.6 - 0.7 nm) the coupling strength decreases with fluence, becoming ferromagnetic for fluences above (2x10^14 ions/cm^2). The effect is connected with the creation of magnetic bridges in the layered system due to atomic exchange events caused by the bombardment. For thicker Cr spacers (0.8 - 1.2 nm) an enhancement of the antiferromagnetic coupling strength is found. A possible explanation of the enhancement effect is given.Comment: Submitted to PR

Особенности аннигиляции позитронов в наноразмерных металлических слоях Zr/Nb после облучения ионами He+

Новые технологии получения конструкционных материалов, устойчивых к водородным и радиационным повреждениям, являются актуальными проблемами материаловедения. Водородное повреждение и радиационная деградация являются важными факторами, ограничивающими усталостную долговечность конструкционных материалов. Одной из перспективных альтернатив при разработке радиационно-водородостойких материалов с улучшенными физико-механическими свойствами является нанесение наноразмерных металлических покрытий (НМП). Настоящая работа посвящена изучению аннигиляции позитронов в НМП Zr/Nb с различной толщиной отдельных слоев Zr и Nb после облучения ионами He+ с дозами от 3⸱1016 ионов/см2 до 3⸱1017 ионов/см2

The impact of Mn nonstoichiometry on the oxygen mass transport properties of La0.8Sr0.2MnyO3±δ thin films

Oxygen mass transport in perovskite oxides is relevant for a variety of energy and information technologies. In oxide thin films, cation nonstoichiometry is often found but its impact on the oxygen transport properties is not well understood. Here, we used oxygen isotope exchange depth profile technique coupled with secondary ion mass spectrometry to study oxygen mass transport and the defect compensation mechanism of Mn-deficient La0.8Sr0.2Mn (y) O-3 +/-delta epitaxial thin films. Oxygen diffusivity and surface exchange coefficients were observed to be consistent with literature measurements and to be independent on the degree of Mn deficiency in the layers. Defect chemistry modeling, together with a collection of different experimental techniques, suggests that the Mn-deficiency is mainly compensated by the formation of La-x(Mn) antisite defects. The results highlight the importance of antisite defects in perovskite thin films for mitigating cationic nonstoichiometry effects on oxygen mass transport properties

Magnetic layer formation on plasma nitrided CoCrMo alloy

In this study structural and magnetic character of the expanded austenite phase (γN) layer formed on a medical grade CoCrMo alloy by a low-pressure Radio-Frequency plasma nitriding process was investigated. The formation of the expanded austenite phase is facilitated at a substrate temperature near 400°C for 1, 2, 4, 6 and 20h under a gas mixture of 60% N2-40% H2. The magnetic state of the γN layers was determined by a surface sensitive technique, magneto-optic Kerr effect (MOKE), and with a scanning probe microscope in magnetic force mode (MFM). Strong evidence for the ferromagnetic nature of the γN-(Co,Cr,Mo) phase is provided by the observation of stripe domain structures and the hysteresis loops. The ferromagnetic state for the γN phase observed here is mainly linked to large lattice expansions (~10%) due to high N contents (~30at.%). As an interstitial impurity, nitrogen dilates the host lattice i.e. the Co-Co (or Fe-Fe) distance is increased, which strongly influences the magnetic interactions. An analogy between the magnetic properties of the expanded phases, γN-(Fe,Cr,Ni) and γN-(Co,Cr,Mo), formed in austenitic stainless steel alloys and the CoCrMo alloy of this study is made, and it is suggested that the ferromagnetic states for the γN-(Co,Cr,Mo) and γN-(Fe,Cr,Ni) phases may be correlated with the volume dependence of the magnetic properties of fcc-Co/Co4N and fcc-Fe/Fe4N, respectively.Turkish National Science Foundation grant 107M21

Positronium Formation in Nanostructured Metals

Positronium formation in Au films was studied using a magnetically guided continuous slow positron beam with variable energy. Black Au film with porous morphology was compared with conventional smooth Au film. In the smooth Au film positronium is formed on the film surface only. The black Au film exhibits porous sub-surface region containing micro-cavities interconnected with surface. Positronium is formed on inner surfaces of micro-cavities in the sub-surface region and travels through interconnected cavities towards the surface. 3-γ annihilation of ortho-positronium leaves a clear signature in two-dimensional coincidence Doppler broadening spectra. Measures of 3-γ annihilation contribution calculated from single γ-ray and coincidence Doppler broadening spectra were calculated and compared

Positronium Formation in Nanostructured Metals

Positronium formation in Au films was studied using a magnetically guided continuous slow positron beam with variable energy. Black Au film with porous morphology was compared with conventional smooth Au film. In the smooth Au film positronium is formed on the film surface only. The black Au film exhibits porous sub-surface region containing micro-cavities interconnected with surface. Positronium is formed on inner surfaces of micro-cavities in the sub-surface region and travels through interconnected cavities towards the surface. 3-γ annihilation of ortho-positronium leaves a clear signature in two-dimensional coincidence Doppler broadening spectra. Measures of 3-γ annihilation contribution calculated from single γ-ray and coincidence Doppler broadening spectra were calculated and compared