Atomic force microscope (AFM) imaging and cross-sectional analysis were used to document the shape evolution of Ge/Si(100) islands, grown by molecular beam epitaxy, as a function of growth conditions. Growth temperatures of 450, 550, 600, and 650 °C with Ge coverages between 3.5 and 14.0 monolayers (ML) were investigated for a deposition rate of 1.4 ML/min. Low coverages produced small hut clusters which then evolved into dome clusters at higher coverages. These dome clusters eventually dislocated after further growth. Higher growth temperatures activated additional pathways for the Ge islands to relieve their strain such as Ge/Si intermixing and the formation of trenches around the islands. Our detailed AFM cross-sectional analysis indicated that dome clusters form several crystal facets in addition to those previously reported.

Ge͞Si͑100͒ island size distributions were monitored for coverages between 3.5 and 14.0 monolayers at growth temperatures from 450 to 600 ± C. Features in these distributions are correlated with characteristic island morphologies. The mean dome cluster size increased and the onset of island dislocation was delayed as the growth temperature increased. At 600 ± C, very large hut clusters are formed. This behavior is attributed to strain-assisted alloying of the Ge clusters. Energy dispersive x-ray analysis confirms Si diffusion into the Ge clusters at 600 ± C. An atomistic elastic model supports the interpretation that alloying is driven by strain energy enhancement near the island perimeters.

Strain relaxation correlated with the transport properties of AlN/GaN heterostructure grown by plasma-assisted molecular-beam epitaxy

Thin wurtzite GaN films have been grown by plasma-enhanced molecular beam epitaxy on the basal plane of 6H SiC, with and without AlN buffer layers. Threading defects, identified from high-resolution electron micrographs as double-positioning boundaries (DPBs), originate at the substrate–buffer and/or buffer–film interfaces. The density of these faults seems to be related to the smoothness of the substrate, so that their occurrence emphasizes the importance of adequate substrate preparation. Stacking faults within the GaN are often visible parallel to the SiC substrate basal plane, sometimes terminating at the DPBs. These faults are related to the particular growth conditions, with greatly decreased density obtained for lower plasma power during GaN deposition. Growth of high quality GaN without stacking faults was achieved without using AlN buffer layers by deposition directly onto a vicinal SiC surface having a miscut angle of 4°. Such stepped substrates represent a potentially useful means for controlled growth of the DPBs, which could then serve as suitable stress-relieving defects in lieu of misfit dislocations.

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α-Azidochalcones were coupled with 1/2-naphthols or 2-hydroxy-1,4-naphthoquinone using Ru­(bpy)3­(PF6)2 as a photocatalyst under blue LED light irradiation to yield 2,3-fused pyrroles in high yields (68–84%). The overall transformation involves photosensitized decomposition of α-azidochalcones into highly reactive 2H-azirines which are trapped by 1/2-naphthols or 2-hydroxy-1,4-naphthoquinone, leading to the construction of two new C–N and one new C–C bonds.