48 research outputs found
Local step-flow dynamics in thin film growth with desorption
Desorption of deposited species plays a role in determining the evolution of
surface morphology during crystal growth when the desorption time constant is
short compared to the time to diffuse to a defect site, step edge or kink.
However, experiments to directly test the predictions of these effects are
lacking. Novel techniques such as \emph{in-situ} coherent X-ray scattering can
provide significant new information. Herein we present X-ray Photon Correlation
Spectroscopy (XPCS) measurements during diindenoperylene (DIP) vapor deposition
on thermally oxidized silicon surfaces. DIP forms a nearly complete
two-dimensional first layer over the range of temperatures studied (40 - 120
C), followed by mounded growth during subsequent deposition. Local
step flow within mounds was observed, and we find that there was a
terrace-length-dependent behavior of the step edge dynamics. This led to
unstable growth with rapid roughening () and deviation from a
symmetric error-function-like height profile. At high temperatures, the grooves
between the mounds tend to close up leading to nearly flat polycrystalline
films. Numerical analysis based on a 1 + 1 dimensional model suggests that
terrace-length dependent desorption of deposited ad-molecules is an essential
cause of the step dynamics, and it influences the morphology evolution.Comment: 21 pages, 9 figures, and one tabl
Wall-thickness-dependent strength of nanotubular ZnO
We fabricate nanotubular ZnO with wall thickness of 45, 92, 123 nm using nanoporous gold (np-Au) with ligament diameter at necks of 1.43 mu m as sacrificial template. Through micro-tensile and micro-compressive testing of nanotubular ZnO structures, we find that the exponent m in (sigma) over bar proportional to (rho) over bar (m), where (sigma) over bar is the relative strength and (rho) over bar is the relative density, for tension is 1.09 and for compression is 0.63. Both exponents are lower than the value of 1.5 in the Gibson-Ashby model that describes the relation between relative strength and relative density where the strength of constituent material is independent of external size, which indicates that strength of constituent ZnO increases as wall thickness decreases. We find, based on hole-nanoindentation and glazing incidence X-ray diffraction, that this wall-thickness-dependent strength of nanotubular ZnO is not caused by strengthening of constituent ZnO by size reduction at the nanoscale. Finite element analysis suggests that the wall-thickness-dependent strength of nanotubular ZnO originates from nanotubular structures formed on ligaments of np-Au