We investigate the electroluminescence (EL) from light emitting diodes (LEDs) of ZnO nanowires/p-GaN structure and ZnS@ZnO core-shell nanowires/p-GaN structure. With the increase of forward bias, the emission peak of ZnO nanowires/p-GaN structure heterojunction shows a blue-shift, while the ZnS@ZnO core-shell nanowires/p-GaN structure demonstrates a changing EL emission; the ultraviolet (UV) emission at 378 nm can be observed. This discrepancy is related to the localized states introduced by ZnS particles, which results in a different carrier recombination process near the interfaces of the heterojunction. The localized states capture the carriers in ZnO nanowires and convert them to localized excitons under high forward bias. A strong UV emission due to localized excitons can be observed. Our results indicated that utilizing localized excitons should be a new route toward ZnO-based ultraviolet LEDs with high efficiency.

The grooved c-plane sapphire substrates were first systematically investigated by wet chemical etching, with H 2 SO 4 and a 3H 2 SO 4 :1H 3 PO 4 mixture as the etchants. The structural and morphological characteristics of the grooved sapphire with mask stripes along the ͗1120͘ and ͗1100͘ directions, respectively, were studied under different etching time and temperatures by scanning electron microscopy ͑SEM͒. Two kinds of groove shapes are obtained. One is a V-groove whose two sidewalls are both formed by a single facet. The other is a U-groove whose one sidewall consists of two or three facets, and the other sidewall is composed of a single facet. SEM cross-sectional images show symmetrical sidewall facets with stripes along the sapphire ͗1100͘ direction and asymmetrical sidewall facets with stripes along the sapphire ͗1120͘ direction. The etching depth is linear with the etching time. The activation energies of etching reaction are evaluated in the temperature range 340-400°C. It is confirmed that sapphire with stripes along the ͗1120͘ direction is suitable for lateral epitaxial overgrowth of low-threading-dislocation GaN films.Gallium nitride ͑GaN͒ is an important material for optoelectronic devices including blue light-emitting diodes ͑LEDs͒ and laser diodes ͑LDs͒. 1 GaN is typically grown on substrates like sapphire ͑␣-Al 2 O 3 ͒, 2 SiC, 3 and Si, 4 which exhibit a large difference in lattice constant and thermal-expansion coefficients compared to GaN. This leads to a high density of threading dislocations ͑TDs͒ which limit the performance of optoelectronic devices. 5 Epitaxial growth techniques like lateral epitaxial overgrowth ͑LEO͒ 6 and other modified LEO techniques such as pendoepitaxy ͑PE͒ 7 and facet-controlled epitaxial lateral overgrowth ͑FACELO͒ 8 are promising techniques to overcome this problem and to reduce the dislocation density in GaN layers grown by metallorganic chemical vapor deposition ͑MOCVD͒. However, LEO techniques typically require multiple MOCVD growths coupled with intermediate processing steps, and often introduce autodoping from the mask. In addition, to ensure a low threading-dislocation-density ͑TDD͒ and a flat film surface to fabricate the high-performance optoelectronic devices, more than ten-micrometer-thick GaN layers have to be grown by MOCVD. Inspiringly, these shortcomings were overcome later by cantilever epitaxy ͑CE͒, 9 which was achieved as a maskless means by which GaN films are grown from substrates with periodic grooves prepared by dry etching. 10,11 Though CE cannot reduce TDD to the order of magnitude that those conventional LEO techniques can achieve, the simplified technique is greatly effective to fabricate highperformance devices with only several-micrometer-thick GaN to form a flat film surface. 12 Nevertheless, dry etching cannot avoid damage and strain to the substrate surface, 13 which limits the further improvement of epitaxial crystal quality. Especially, it is unavoidable that TDs propagate into the top epitaxial films from GaN layers deposited on the s...

Selective growth of GaN on maskless V-grooved sapphire substrate is found by metalorganic chemical vapor deposition (MOCVD), where the V-grooved sapphire substrate is fabricated by chemical wet etching. GaN layers grow only on the (0001) mesas and no GaN growth occurs on the {110k} sidewalls of the V-grooves. The origin of the selective growth of GaN is unveiled and analyzed by energy dispersive X-ray (EDX) spectroscopy mapping. It is found that, Ga migration rates on the different facets are different clearly at high temperatures (HTs) so that the selective growth occurs. Contrary to the situation of HTs, GaN nucleates uniformly on the mesas and sidewalls at low temperatures (LTs) without selectivity. Thus, it is concluded that the selective growth of GaN on sapphire facets is a kinetic limited process. Accordingly, it is possible to fabricate GaN-based nanostructures by controlling the growth conditions.

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