Papers by Author: A.E. Romanov

Abstract: GaN epitaxial layers were successfully grown by hydride vapour phase epitaxy (HVPE) on β-Ga₂O₃ substrates produced by cleaving. The initial stages of GaN epitaxial growth on β-Ga₂O₃ were studied by scanning electron microscopy (SEM) and x-ray diffraction analysis (XRD). The nucleation and the transition from the nucleation layer to a continuous GaN film were studied. It was found that the growth starts with formation of small crystallites on the substrate surface. As the growth continues, crystallites transform into pyramidal islands which increase in size and merge together. It was found that the structural quality of the GaN layers rapidly improves with increasing thickness. The full width at half maximum of x-ray ω rocking curves for (0002) peak decreased from 1370 to 540 arcsec as the deposition time was increased from 30 to 120 sec. This corresponds to the variation of the nominal layer thickness from 250 nm to 1000 nm.

Abstract: Gradient nanomechanics is a generalized continuum mechanics framework accounting for “bulk-surface” interactions in the form of gradient terms that enter in the evolution equations of the relevant constitutive variables and/or in the governing field equations. This approach is discussed in the paper by developing appropriate differential equations for the plastic strain and/or the structural defects that bring this about. The effectiveness of the approach is illustrated by considering size-dependent stress-strain curves for nanopolycrystals with varying grain size.