97 research outputs found
The evidence of quasi-free positronium state in GiPS-AMOC spectra of glycerol
We present the results of processing of Age-Momentum Correlation (AMOC)
spectra that were measured for glycerol by the Gamma-induced positron
spectroscopy (GiPS) facility. Our research has shown that the shape of
experimental s(t) curve cannot be explained without introduction of the
intermediate state of positronium (Ps), called quasi-free Ps. This state yields
the wide Doppler line near zero lifetimes. We discuss the possible properties
of this intermediate Ps state from the viewpoint of developed model. The amount
of annihilation events produced by quasi-free Ps is estimated to be less than
5% of total annihilations. In the proposed model, quasi-free Ps serves as a
precursor for trapped Ps of para- and ortho-states
Chemical manipulation of hydrogen induced high p-type and n-type conductivity in Ga2O3.
Advancement of optoelectronic and high-power devices is tied to the development of wide band gap materials with excellent transport properties. However, bipolar doping (n-type and p-type doping) and realizing high carrier density while maintaining good mobility have been big challenges in wide band gap materials. Here P-type and n-type conductivity was introduced in ÎČ-Ga2O3, an ultra-wide band gap oxide, by controlling hydrogen incorporation in the lattice without further doping. Hydrogen induced a 9-order of magnitude increase of n-type conductivity with donor ionization energy of 20âmeV and resistivity of 10-4 Ω.cm. The conductivity was switched to p-type with acceptor ionization energy of 42âmeV by altering hydrogen incorporation in the lattice. Density functional theory calculations were used to examine hydrogen location in the Ga2O3 lattice and identified a new donor type as the source of this remarkable n-type conductivity. Positron annihilation spectroscopy measurements confirm this finding and the interpretation of the experimental results. This work illustrates a new approach that allows a tunable and reversible way of modifying the conductivity of semiconductors and it is expected to have profound implications on semiconductor field. At the same time, it demonstrates for the first time p-type and remarkable n-type conductivity in Ga2O3 which should usher in the development of Ga2O3 devices and advance optoelectronics and high-power devices
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
Enhanced flux pinning isotropy by tuned nanosized defect network in superconducting YBa2Cu3O6+x films
Striving to improve the critical current density Jc of superconductingYBa(2)Cu(3)O(6+x) (YBCO) thin films via enhanced vortex pinning, the interplay between film growth mechanisms and the formation of nanosized defects, both natural and artificial, is systematically studied in undoped and BaZrO3 (BZO)-doped YBCO thin films. The films were grown via pulsed laser deposition (PLD), varying the crystal grain size of the targets in addition to the dopant content. The microstructure of the PLD target has been observed to have a great impact on that of the deposited thin films, including the formation of vortex pinning centers, which has direct implications on the superconducting performance, especially on the isotropy of flux pinning properties. Based on experimentally measured angular dependencies of Jc, coupled with a molecular dynamics (MD) simulation of flux pinning in the YBCO films, we present a quantitative model of how the splay and fragmentation of BZO nanorods artifically introduced into the YBCO film matrix explain the majority of the observed critical current anisotropy
Zinc Oxide Defect Microstructure and Surface Chemistry Derived from Oxidation of Metallic Zinc: Thin-Film Transistor and Sensor Behavior of ZnO Films and Rods
Zinc oxide thin films are fabricated by controlled oxidation of sputtered zinc metal films on a hotplate in air at temperatures between 250 and 450â°C. The nanocrystalline films possess high relative densities and show preferential growth in (100) orientation. Integration in thinâfilm transistors reveals moderate charge carrier mobilities as high as 0.2â
cmâVs. The semiconducting properties depend on the calcination temperature, whereby the best performance is achieved at 450â°C. The defect structure of the thin ZnO film can be tracked by Dopplerâbroadening positron annihilation spectroscopy as well as positron lifetime studies. Comparably long positron lifetimes suggest interaction of zinc vacancies (V) with one or more oxygen vacancies (V) in larger structural entities. Such VâV defect clusters act as shallow acceptors, and thus, reduce the overall electron conductivity of the film. The concentration of these defect clusters decreases at higher calcination temperatures as indicated by changes in the S and W parameters. Such zinc oxide films obtained by conversion of metallic zinc can also be used as seed layers for solution deposition of zinc oxide nanowires employing a mild microwaveâassisted process. The functionality of the obtained nanowire arrays is tested in a UV sensor device. The best results with respect to sensor sensitivity are achieved with thinner seed layers for device construction
TiO2 phase engineering by millisecond range annealing for highly efficient photocatalysis
Air pollution and the energy crisis are the two main driving forces behind the development of alternative, environmentally friendly methods of energy production. Photoactive materials can be used both to clean the air and to produce green hydrogen for clean energy. Transition metal oxides are one of the most considered materials for high-performance photocatalysis. In this work, we investigate the effect of millisecond flash lamp annealing (FLA) of TiO2 on the degradation of methyl blue (MB) and methyl orange (MO). To reduce the energy consumption of the TiO2 deposition process, the layers were made using magnetron sputtering at room temperature followed by millisecond FLA. By controlling the flash energy input, we can tune the phase formation of TiO2 films from pure anatase to mixed anatase/rutile phases. Scanning electron microscopy, positron annihilation spectroscopy, photoluminescence, and X-ray diffraction studies show that the crystal size and film quality increase with increasing annealing temperature. Photocatalytic experiments demonstrate that FLA-treated TiO2 films are active in degrading both MB and MO. This makes them attractive not only for the production of green hydrogen but also for the purification of water from medical contaminantsTED2021-129876B-I0
Vacancy cluster in ZnO films grown by pulsed laser deposition
© 2019, The Author(s). Undoped and Ga-doped ZnO films were grown on c-sapphire using pulsed laser deposition (PLD) at the substrate temperature of 600 °C. Positron annihilation spectroscopy study (PAS) shows that the dominant V Zn -related defect in the as-grown undoped ZnO grown with relative low oxygen pressure P(O 2 ) is a vacancy cluster (most likely a V Zn -nV O complex with n = 2, 3) rather than the isolated V Zn which has a lower formation energy. Annealing these samples at 900 °C induces out-diffusion of Zn from the ZnO film into the sapphire creating the V Zn  at the film/sapphire interface, which favors the formation of vacancy cluster containing relatively more V Zn . Increasing the P(O 2 ) during growth also lead to the formation of the vacancy cluster with relatively more V Zn . For Ga-doped ZnO films, the oxygen pressure during growth has significant influence on the electron concentration and the microstructure of the V Zn -related defect. Green luminescence (GL) and yellow luminescence (YL) were identified in the cathodoluminescence study (CL) study, and both emission bands were quenched after hydrogen plasma treatment. The origin of the GL is discussed
- âŠ