Single crystal silicon nanowires (SiNWs) were synthesized with silane reactant using Au nanocluster-catalyzed one-dimensional growth. We have shown that under our experimental conditions, SiNWs grown epitaxially on Si(111) via the vapor-liquid-solid growth mechanism change their growth direction as a function of the total pressure. Structural characterization of a large number of samples shows that SiNWs synthesized at a total pressure of 3 mbar grow preferentially in the 111 direction, while the one at 15 mbar favors the 112 direction. Specifically by dynamically changing the system pressure during the growth process morphological changes of the NW growth directions along their length have been demonstrated.
We present a Ga adsorption study of both polar GaN ͑0001͒ and (0001 ) surfaces using line-of-sight quadrupole mass spectrometry as a quantitative in situ method. Monitoring the desorbing Ga atoms, two characteristic desorption regimes ͑exponential and steady-state regimes͒ were found that are assigned to the formation of a thin equilibrium Ga adlayer and Ga droplets on top of it. The Ga adlayer coverage differs substantially between the two surface polarities, being 1.1 monolayers on (0001 ) GaN and 2.4 monolayers on ͑0001͒ GaN. Additional temperature-dependent measurements of the surface lifetime of Ga adatoms unveil fundamental differences in the adsorbate-substrate binding energetics both for the Ga adlayers on the two surface polarities and for the Ga droplets.
A growth diagram for molecular beam epitaxy of AlN on sapphire and 6H–SiC was established using reflection high energy electron diffraction, atomic force microscopy, and Rutherford backscattering spectrometry. In varying the Al/N ratio and growth temperature, distinctive surface morphologies emerge, which are assigned to three regimes of growth, one N-rich (Al/N<1) and two Al-rich (Al/N>1) regimes. Under N-rich conditions, AlN films exhibit rough surface morphologies. In contrast, Al-rich conditions produce excellent smooth surface morphologies, but with the constraint of Al droplet formation at very high Al/N ratios and low temperatures. The differentiation between N-rich and Al-rich regimes is given only by the Al/N ratio, while the two Al-rich regimes (intermediate self-regulated and droplet regime) are separated by the boundary line of Al droplet formation. For this boundary an Arrhenius dependence of growth temperature was found, yielding an activation energy of 3.4±0.1 eV. The observed morphology transitions are attributed to varying surface adatom mobilities present under the different Al/N ratios.
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