Chew Beng Soh

Cool white III-nitride light emitting diodes based on phosphor-free indium-rich InGaN nanostructures

Phosphor-free cool white emitting light emitting diodes (LEDs) have been fabricated using a dual stacked InGaN∕GaN multiple quantum wells (MQWs) comprising of a lower set of MQWs emitting yellow and an upper set of MQWs emitting blue. The lower set of MQWs incorporates indium-rich InGaN connected-dot nanostructures with a height of ∼1.0nm in the well. The well is first grown with an InGaN layer serving as the wetting layer, then treated with trimethylindium (TMIn) to initiate nanostructure growth of another InGaN layer to complete the well layer. This gives a broadened yellow emission peak. With the combination of emission from the upper blue emitting InGaN∕GaN MQWs subsequently grown, cool white light emission is achieved. The In-rich nanostructures formed during TMIn treatment enhance indium incorporation in InGaN well and also act as effective radiative recombination sites for carriers at the lower set of MQWs.

Enhanced luminescence efficiency due to carrier localization in InGaN/GaN heterostructures grown on nanoporous GaN templates

Low defect density GaN was achieved through dislocation annihilation by regrowing GaN on strain relaxed nanoporous GaN template formed by UV-enhanced electrochemical etching. The InGaN∕GaN single and multiple quantum wells grown on this nanoporous GaN template show enhanced indium incorporation due to strain relaxation. The step edges of regrown GaN on these nanoporous GaN act as effective nucleation sites for impinging indium atoms during growth. Evidence shows fluctuation in the quantum well width caused by indium segregation leading to carrier localization. A higher luminescence efficiency of InGaN∕GaN quantum wells is achieved through a combination of excitons localization, higher energy barrier for nonradiative recombination of carriers with dislocations and the reduction in defect density of the materials grown on the nanoporous GaN template.

Influence of the alignment of ZnO nanorod arrays on light extraction enhancement of GaN-based light-emitting diodes

ZnO nanorod arrays (ZNAs) were hydrothermally synthesized on the surface of patterned indium-doped tin oxide p-type contact (PIPC) of GaN-based light-emitting diodes (GaN-LEDs) for enhancing the light extraction efficiency (LEE). It was found that the alignment of the ZnO nanorod arrays in the grooves of the PIPC was poorer than these grown on the ridges of the PIPC. By comparing the light output of the GaN-LEDs with and without ZNAs grown in the grooves of PIPC, the influence of the alignment of ZNAs on the LEE of GaN-LEDs was revealed and investigated. Numerical analysis based on the finite difference of time domain (FDTD) method suggested that the poorer alignment of ZNAs grown on GaN-LEDs resulted in more energy reflected back into GaN-LEDs and lower light extraction efficiency.

Exciton recombination in ZnO nanorods grown on GaN/sapphire template

The authors have employed variable temperature photoluminescence (PL) and time-resolved PL spectroscopy to probe the exciton recombination in high density and vertically aligned ZnO nanorods grown on p-type GaN/sapphire template. The low-temperature PL characterizes the dominant near-band-edge excitonic emissions from such nanorod arrays. At 4.3 K, a PL decay time of 432 ps reveals improved crystalline quality. The PL decay time shows irregular behavior due to different types of excitonic transitions dominating the PL spectra at different temperatures and a competitive effect of radiative recombination and nonradiative relaxation processes.

Generation of amber III-nitride based light emitting diodes by indium rich InGaN quantum dots with InGaN wetting layer and AlN encapsulation layer

Indium rich InGaN nanostructures grown by metalorganic chemical vapor deposition were incorporated in InGaN/GaN quantum wells for long wavelength generation. These results were achieved by optimizing the growth temperature of the nanostructures, InGaN quantum well, the AlN capping layer and the GaN barrier layers. Before the growth of nanostructures, a thin InGaN wetting layer was included to reduce the lattice mismatch as well as to enhance the deposition of indium-rich InGaN nanostructures These individual quantum wells were each subsequently capped with an AlN layer which better preserved the In-rich phase in the nanostructures and prevented the indium interdiffusion between the InGaN/GaN heterojunctions. The AlN capping layer also reduces the effect of piezeoelectric field in the active layers of the light emitting diodes as seen from the reduction in the blueshift in the electroluminescence peaks with higher injection currents. The energy band profile of such a structure is discussed.

Phosphor-Free Apple-White LEDs with Embedded Indium-Rich Nanostructures Grown on Strain Relaxed Nano-epitaxy GaN

Phosphor-free apple-white light emitting diodes have been fabricated using a dual stacked InGaN/GaN multiple quantum wells comprising of a lower set of long wavelength emitting indium-rich nanostructures incorporated in multiple quantum wells with an upper set of cyan-green emitting multiple quantum wells. The light-emitting diodes were grown on nano-epitaxially lateral overgrown GaN template formed by regrowth of GaN over SiO2 film patterned with an anodic aluminum oxide mask with holes of 125 nm diameter and a period of 250 nm. The growth of InGaN/GaN multiple quantum wells on these stress relaxed low defect density templates improves the internal quantum efficiency by 15% for the cyan-green multiple quantum wells. Higher emission intensity with redshift in the PL peak emission wavelength is obtained for the indium-rich nanostructures incorporated in multiple quantum wells. The quantum wells grown on the nano-epitaxially lateral overgrown GaN has a weaker piezoelectric field and hence shows a minimal peak shift with application of higher injection current. An enhancement of external quantum efficiency is achieved for the apple-white light emitting diodes grown on the nano-epitaxially lateral overgrown GaN template based on the light -output power measurement. The improvement in light extraction efficiency, ηextraction, was found to be 34% for the cyan-green emission peak and 15% from the broad long wavelength emission with optimized lattice period.

The effects of cap layers on electrical properties of indium nitride films

The unintentional n-type doping in the indium nitride thin films was investigated. The electron density decreases from 3.5×1019 to 9×1018 cm−3 and the mobility increases from 4 to 457 cm2 V−1 s−1 when the thickness increases from 50 to 350 nm. This can be explained by assuming the film consists of a surface accumulation layer and a bulk layer. It was found that the accumulation layer can be eliminated by capping the surface with silicon nitride, GaN or zinc nitride of 2 nm each, respectively; while an AlN cap layer will cause the formation of two-dimensional electron gas at the AlN/InN interface.

Tapered and aperiodic silicon nanostructures with very low reflectance for solar hydrogen evolution

We introduce a facile method to generate silicon nanostructures with superior anti-reflectance (AR) properties. These nanostructures possess the ideal tapered structure and aperiodic distribution required for low reflectance over a broad range of wavelengths. Consideration of effective medium theories might explain the behavior between structure and reflectance, and suggest the advantage of aperiodicity in affording the material broadband AR properties. We then implemented these nanostructures as photocathodes to drive the hydrogen evolution reaction in AM 1.5 illumination. These nanostructures showed a significant improvement in photoelectrochemical performance over their planar counterpart, with the best performances corresponding to nanostructures which possessed AR properties matching the solar spectrum output.

Crystallographically tilted and partially strain relaxed GaN grown on inclined {111} facets etched on Si(100) substrate

High resolution X-ray diffractometry (HR-XRD), Photoluminescence, Raman spectroscopy, and Transmission electron microscope measurements are reported for GaN deposited on a conventional Si(111) substrate and on the {111} facets etched on a Si(100) substrate. HR-XRD reciprocal space mappings showed that the GaN(0002) plane is tilted by about 0.63° ± 0.02° away from the exposed Si{111} growth surface for GaN deposited on the patterned Si(100) substrate, while no observable tilt existed between the GaN(0002) and Si(111) planes for GaN deposited on the conventional Si(111) substrate. The ratio of integrated intensities of the yellow to near band edge (NBE) luminescence (IYL/INBE) was determined to be about one order of magnitude lower in the case of GaN deposited on the patterned Si(100) substrate compared with GaN deposited on the conventional Si(111) substrate. The Raman E2(high) optical phonon mode at 565.224 ± 0.001 cm−1 with a narrow full width at half maximum of 1.526 ± 0.002 cm−1 was measured, for GaN deposited on the patterned Si(100) indicating high material quality. GaN deposition within the trench etched on the Si(100) substrate occurred via diffusion and mass-transport limited mechanism. This resulted in a differential GaN layer thickness from the top (i.e., 1.8 μm) of the trench to the bottom (i.e., 0.3 μm) of the trench. Mixed-type dislocation constituted about 80% of the total dislocations in the GaN grown on the inclined Si{111} surface etched on Si(100).

Reduction of V-pit and threading dislocation density in InGaN/GaN heterostructures grown on cracked AlGaN templates

The high density of threading dislocations, often leading to the formation of inverted hexagonal pits in InGaN/GaN heterostructures on sapphire substrates, lowers the radiative efficiency of light emitting devices. In this study, a cracked AlGaN template has been implemented as a strain-relaxed layer for subsequent growth of InGaN/GaN heterostructures. The detailed electron microscopy and surface topographic analyses show that such a template has led to a reduction of threading dislocation density especially for screw dislocations and V-pits in the overgrown InGaN/GaN layers. The relaxed regrowth of such heterostructures also leads to an improved crystalline quality and a higher In incorporation in InGaN. The improvement in the optical and structural quality of these InGaN/GaN layers is investigated by means of photoluminescence spectroscopy and transmission electron microscopy.