High-quality coalescence overgrowth of patterned-grown GaN nanocolumns on c-plane sapphire substrate with metal organic chemical vapor deposition is demonstrated. Although domain structures of a tens of micron scale in the overgrown layer can be identified with cathodoluminescence measurement, from atomic force microscopy ͑AFM͒ measurement, the surface roughness of the overgrown layer in an area of 5 ϫ 5 m 2 is as small as 0.411 nm, which is only one-half that of the high-quality GaN thin-film template directly grown on sapphire substrate ͑the control sample͒. Based on the AFM and depth-dependent x-ray diffraction measurements near the surface of the overgrown layer, the dislocation density is reduced to the order of 10 7 cm −2 , which is one order of magnitude lower than that of the control sample and two to three orders of magnitude lower than those of ordinary GaN templates for fabricating light-emitting diodes. Also, the lateral domain size, reaching a level of ϳ2.7 m, becomes three times larger than the control sample. Meanwhile, the ratio of photoluminescence intensity at room temperature over that at low temperature of the overgrown sample is at least six times higher than that of the control sample. Although the strain in nanocolumns is almost completely released, a stress of ϳ0.66 GPa is rebuilt when the coalescence overgrowth is implemented.

High-phase-purity zinc-blende ͑zb͒ InN thin film has been grown by plasma-assisted molecular-beam epitaxy on r-plane sapphire substrate pretreated with nitridation. X-ray diffraction analysis shows that the phase of the InN films changes from wurtzite ͑w͒ InN to a mixture of w-InN and zb-InN, to zb-InN with increasing nitridation time. High-resolution transmission electron microscopy reveals an ultrathin crystallized interlayer produced by substrate nitridation, which plays an important role in controlling the InN phase. Photoluminescence emission of zb-InN measured at 20 K shows a peak at a very low energy, 0.636 eV, and an absorption edge at ϳ0.62 eV is observed at 2 K, which is the lowest bandgap reported to date among the III-nitride semiconductors.

The authors demonstrate the coalescence overgrowth of GaN nano-columns on a (111) Si substrate with metalorganic chemical vapor deposition to show high-quality optical properties in the overgrown film. Plan-view scanning electron microscopy (SEM) shows coalesced surface morphology, although hexagonal structures are still visible in the images. The cross-section cathodoluminescence (CL) image shows more efficient emission in the overgrowth layer than from the nano-column layer. The plan-view CL image demonstrates that the emitted light is mainly from the hexagonal structures. The photoluminescence measurement result indicates that the emission efficiency of the overgrown layer is even higher than that of an undoped GaN thin film of high quality. The presence of hexagonal structures correlates to surface roughness values in the range of several nanometres.

No abstract