White light-emitting InGaN∕GaN diode with an InGaN underlying layer grown on the (0001) sapphire substrate was fabricated by low pressure metal-organic vapor phase epitaxy. The electroluminescence measurements show that the emitted white light is composed of blue and yellow lights, centered at around 440 and 570nm, respectively, for an injection current of 20mA. Cross-sectional transmission electron microscopy reveals that In-rich quantum dots were formed in InGaN wells due to phase separation of indium. It is suggested that the yellow and blue lights come from In-rich quantum dots and the low-indium regions, respectively, in InGaN quantum wells.

Light emission from green to white in a single-chip light emitting diode is modulated by adjusting the strain in InGaN underlying layer (UL) embedded below an active layer of InGaN/GaN multiple quantum wells. Transmission electron microscopy combined with x-ray reciprocal space mapping reveals that indium phase separation in InGaN quantum well active layer is enhanced by using a partly relaxed InGaN UL and In-rich quantum dots with different size and indium composition are formed. They emit multicolor lights whose mixing produces white light. Quality of the white light could be controlled by modulation on relaxation degree of the InGaN UL.

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...