1,213 research outputs found
The Morphology and Misfit Dislocation Formation Characteristics of Strained Heteroepitaxial Layers: Ex Situ and In Situ Growth Studies
Under certain regimes of heteroepitaxial layer growth, misfit stresses can lead to very significant distortions in interface morphology, which can influence strain relief and subsequent misfit dislocation introduction. These phenomena have been clearly demonstrated in the case of SiGe/Si heteroepitaxy and the way in which surface SiGe growth ripples are accompanied by strain waves has been established. The ripples provide partial elastic relief of the layer misfit stress in a manner which has been correlated with theoretical expectations. The local stress variations ultimately may influence the formation and disposition of misfit dislocations in the strained layer structures. The present paper reviews primarily our understanding of these growth characteristics. It also outlines current in situ synchrotron growth studies which exploit real-time X-ray topography and diffraction to probe heteroepitaxial strain-relief processes
Seventy years of sex education in Health Education Journal: a critical review
This paper examines key debates and perspectives on sex education in Health Education Journal (HEJ), from the date of the journal’s first publication in March 1943 to the present day. Matters relating to sexuality and sexual health are revealed to be integral to HEJ’s history. First published as Health and Empire (1921 – 1942), a key purpose of the journal since its inception has been to share information on venereal disease and its prevention within the UK and across the former British Empire. From 1943 to the present day, discussions on sex education in the newly-christened HEJ both reflect and respond to evolving socio-cultural attitudes towards sexuality in the UK. Changing definitions of sex education across the decades are examined, from the prevention of venereal disease and moral decline in war-time Britain in the 1940s, to a range of responses to sexual liberation in the 1960s and 1970s; from a focus on preventing sexually-transmitted infections, teenage pregnancy and HIV in the 1980s, to the provision of sexual health services alongside sex education in the 2000s. Over the past 70 years, a shift from prevention of pre-marital sexual activity to the management of its outcomes is apparent; however, while these changes over time are notable, perhaps the most striking findings of this review are the continuities in arguments for and against the discussion of sexual issues. After more than 70 years of debate, it would seem that there is little consensus concerning motivations for and the content of sex education
Optimizing Umkehr Ozone Profile Retrievals
NOAA Dobson Umkehr ozone profile records have been collected since the 1970s. Umkehr ozone profiles are used to monitor stratospheric ozone recovery predicted to occur by the 2050s. Current operational Dobson Umkehr profile algorithms produce data that have uncertainty on the order of ~ 5 % in the stratosphere. However, when large volcanic eruptions inject aerosols into the stratosphere, the errors can be as large as 70 %. In order to evaluate Umkehr records for aerosol-related and instrumental artifacts, we compare observations with a Hindcast simulation of the NASA Merra-2 Global Modeling Initiative (GMI) Replay (M2GMI, Orbe et al, 2017; Wargan et al, 2018) and Chemistry Transport Model (GMI CTM, Strahan et al, 2013, Strahan et al, 2016). The biases found between the models and observations are summarized for each Dobson calibration and volcanic eruption period, thus providing a reference tool for homogenization of the Umkehr time series and removal of volcanic aerosol errors
Interwell relaxation times in p-Si/SiGe asymmetric quantum well structures: the role of interface roughness
We report the direct determination of nonradiative lifetimes in Si∕SiGe asymmetric quantum well structures designed to access spatially indirect (diagonal) interwell transitions between heavy-hole ground states, at photon energies below the optical phonon energy. We show both experimentally and theoretically, using a six-band k∙p model and a time-domain rate equation scheme, that, for the interface quality currently achievable experimentally (with an average step height ⩾1 Å), interface roughness will dominate all other scattering processes up to about 200 K. By comparing our results obtained for two different structures we deduce that in this regime both barrier and well widths play an important role in the determination of the carrier lifetime. Comparison with recently published experimental and theoretical data obtained for mid-infrared GaAs∕AlxGa1−xAs multiple quantum well systems leads us to the conclusion that the dominant role of interface roughness scattering at low temperature is a general feature of a wide range of semiconductor heterostructures not limited to IV-IV material
Energetics and atomic mechanisms of dislocation nucleation in strained epitaxial layers
We study numerically the energetics and atomic mechanisms of misfit
dislocation nucleation and stress relaxation in a two-dimensional atomistic
model of strained epitaxial layers on a substrate with lattice misfit.
Relaxation processes from coherent to incoherent states for different
transition paths are studied using interatomic potentials of Lennard-Jones type
and a systematic saddle point and transition path search method. The method is
based on a combination of repulsive potential minimization and the Nudged
Elastic Band method. For a final state with a single misfit dislocation, the
minimum energy path and the corresponding activation barrier are obtained for
different misfits and interatomic potentials. We find that the energy barrier
decreases strongly with misfit. In contrast to continuous elastic theory, a
strong tensile-compressive asymmetry is observed. This asymmetry can be
understood as manifestation of asymmetry between repulsive and attractive
branches of pair potential and it is found to depend sensitively on the form of
the potential.Comment: 11 pages, 9 figures, to appear in Phys. Rev.
Thermal effects on electron-phonon interaction in silicon nanostructures
Raman spectra from silicon nanostructures, recorded using excitation laser
power density of 1.0 kW/cm^2, is employed here to reveal the dominance of
thermal effects at temperatures higher than the room temperature. Room
temperature Raman spectrum shows only phonon confinement and Fano effects.
Raman spectra recorded at higher temperatures show increase in FWHM and
decrease in asymmetry ratio with respect to its room temperature counterpart.
Experimental Raman scattering data are analyzed successfully using theoretical
Raman line-shape generated by incorporating the temperature dependence of
phonon dispersion relation. Experimental and theoretical temperature dependent
Raman spectra are in good agreement. Although quantum confinement and Fano
effects persists, heating effects start dominating at higher temperatures than
room tempaerature.Comment: 9 Pages, 3 Figures and 1 Tabl
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