Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userCrystal Truncation Rods from Miscut Surfaces with Alternating Terminations
67
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Miscut surfaces of layered crystals can exhibit a stair-like sequence of terraces having periodic variation in their atomic structure. For hexagonal close-packed and related crystal structures with an {alpha}{beta}{alpha}{beta} stacking sequence, there have been long-standing questions regarding how the differences in adatom attachment kinetics at the steps separating the terraces affect the fractional coverage of {alpha} vs. {beta} termination during crystal growth. To demonstrate how surface X-ray scattering can help address these questions, we develop a general theory for the intensity distributions along crystal truncation rods (CTRs) for miscut surfaces with a combination of two terminations. We consider half-unit-cell-height steps, and variation of the coverages of the terraces above each step. Example calculations are presented for the GaN (0001) surface with various reconstructions. These show which CTR positions are most sensitive to the fractional coverage of the two terminations. We compare the CTR profiles for exactly oriented surfaces to those for vicinal surfaces having a small miscut angle, and investigate the circumstances under which the CTR profile for an exactly oriented surface is equal to the sum of the intensities of the corresponding family of CTRs for a miscut surface.
rate research
Read More
Crystal truncation rods calculated in the kinematical approximation are shown to quantitatively agree with the sum of the diffracted waves obtained in the two-beam dynamical calculations for different reflections along the rod. The choice and the number of these reflections are specified. The agreement extends down to at least $sim 10^{-7}$ of the peak intensity. For lower intensities, the accuracy of dynamical calculations is limited by truncation of the electron density at a mathematically planar surface, arising from the Fourier series expansion of the crystal polarizability.
Burton-Cabrera-Frank (BCF) theory has proven to be a versatile analysis to relate surface morphology and dynamics during crystal growth to the underlying mechanisms of adatom diffusion and attachment at steps. For an important class of crystal surfaces, including the basal planes of hexagonal close-packed and related systems, the steps in a sequence on a vicinal surface can exhibit properties that alternate from step to step. Here we develop BCF theory for such surfaces, to relate observables such as the alternating terrace widths as a function of growth conditions to the kinetic coefficients for adatom attachment at steps. We include the effects of step transparency and step-step repulsion. A general solution is obtained for the dynamics of the terrace widths assuming quasi-steady-state adatom distributions on the terraces, and an explicit simplified analytical solution is obtained under widely applicable approximations. We obtain expressions for the steady-state terrace fractions as a function of growth rate. Limiting cases of diffusion-limited, attachment-limited, and mixed kinetics are considered.
Small-angle X-ray scattering from GaN nanowires grown on Si(111) is studied experimentally and modeled by means of Monte Carlo simulations. It is shown that the scattering intensity at large wave vectors does not follow Porods law $I(q)propto q^{-4}$. The intensity depends on the orientation of the side facets with respect to the incident X-ray beam. It is maximum when the scattering vector is directed along a facet normal, as a reminiscence of the surface truncation rod scattering. At large wave vectors $q$, the scattering intensity is found to be decreased by surface roughness. A root mean square roughness of 0.9~nm, which is the height of just 3--4 atomic steps per micron long facet, already gives rise to a strong intensity reduction.
The exciton dynamics on flat (001) rubrene crystal surfaces have been compared with those under confined pyramidal geometry by time-resolved photoluminescence with micrometer spatial resolution. The luminescence spectra can be interpreted in terms of generation of a free and a self-trapped exciton. Their ratio depends significantly on the structural size which we explain by the optical absorption profile of the pyramids in combination with the exciton diffusion constant. For the latter a lower limit of 0.2 cm2/s at 4 K has been estimated. Temperature-dependent decay times reveal activation barriers between free and self-trapped exciton of 3 meV and 14 meV.
A method for obtaining a smooth, single crystal diamond surface is presented, whereby a sacrificial defective layer is created by implantation and graphitized by annealing before being selectively etched. We have used O+ at 240 keV, the main process variables being the ion fluence (ranging from 3x10^15 cm^-2 to 3x10^17 cm^-2) and the final etching process (wet etch, H2 plasma and annealing in air). The substrates were characterized by atomic force microscopy, optical profilometry and white beam X-ray topography. The influence of the various process parameters on the resulting lift-off efficiency and final surface roughness is discussed. An O+ fluence of 2x10^17 cm^-2 was found to result in sub-nanometre roughness over tens of um^2.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
