169 research outputs found
Mapping of 2+1-dimensional Kardar-Parisi-Zhang growth onto a driven lattice gas model of dimer
We show that a 2+1 dimensional discrete surface growth model exhibiting
Kardar-Parisi-Zhang (KPZ) class scaling can be mapped onto a two dimensional
conserved lattice gas model of directed dimers. In case of KPZ height
anisotropy the dimers follow driven diffusive motion. We confirm by numerical
simulations that the scaling exponents of the dimer model are in agreement with
those of the 2+1 dimensional KPZ class. This opens up the possibility of
analyzing growth models via reaction-diffusion models, which allow much more
efficient computer simulations.Comment: 5 pages, 4 figures, final form to appear in PR
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
Positron annihilation lifetime spectroscopy at a superconducting electron accelerator
The Helmholtz-Zentrum Dresden-Rossendorf operates a superconducting linear
accelerator for electrons with energies up to 35 MeV and average beam currents up to 1.6 mA.
The electron beam is employed for production of several secondary beams including X-rays
from bremsstrahlung production, neutrons, and positrons. The secondary positron beam after
moderation feeds the Monoenergetic Positron Source (MePS) where positron annihilation
lifetime (PALS) and positron annihilation Doppler-broadening experiments in materials
science are performed in parallel. The adjustable repetition rate of the continuous-wave
electron beams allows matching of the pulse separation to the positron lifetime in the sample
under study. The energy of the positron beam can be set between 0.5 keV and 20 keV to
perform depth resolved defect spectroscopy and porosity studies especially for thin films
Integrating the FHI-FEL into the FHI Research Environment - Design and Implementation Aspects
Positron annihilation analysis of nanopores and growth mechanism of oblique angle evaporated TiO2 and SiO2 thin films and multilayers
The nano-porosity embedded into the tilted and separated nanocolumns characteristic of the microstructure of evaporated thin films at oblique angles has been critically assessed by various variants of the positron annihilation spectroscopy. This technique represents a powerful tool for the analysis of porosity, defects and internal interfaces of materials, and has been applied to different as-deposited SiO and TiO thin films as well as SiO/TiO multilayers prepared by electron beam evaporation at 70° and 85° zenithal angles. It is shown that, under same deposition conditions, the concentration of internal nano-pores in SiO is higher than in TiO nanocolumns, while the situation is closer to this latter in TiO/SiO multilayers. These features have been compared with the predictions of a Monte Carlo simulation of the film growth and explained by considering the influence of the chemical composition on the growth mechanism and, ultimately, on the structure of the films
Validação de instrumento para identificar açÔes de rastreamento e detecção de neoplasia de mama
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