Qiang Hu

Fabrication and optical characteristics of phosphor-free InGaN nanopyramid white light emitting diodes by nanospherical-lens photolithography

A novel nanopattern technique of nanospherical-lens photolithography is introduced to fabricate the InGaN nanopyramid white (NPW) light-emitting diodes (LEDs) by selective area growth. Highly ordered NPW LED arrays are achieved after optimizing the growth conditions. It is found that the NPW LEDs vary from warm white light to cool with the increase in growth temperature. For the cool white NPW LEDs, the spectrum is similar to the conventional white LEDs obtained from the blue LEDs combined with yellow phosphors. The blue emission originates from the upper sidewalls of nanopyramids, and yellow light is mainly emitted from the lower ridges with respect to the base of nanopyramids. Furthermore, simulation shows that the light extraction efficiency of NPW LEDs is about 4 times higher compared with conventional ones, and the escape cone is as much as 85 degrees due to their three-dimensional nanopyramid structures. These observations suggest that the proposed phosphor-free NPW LEDs may have great potential for highly efficient white lighting

Mechanical Deformation Behavior of Nonpolar GaN Thick Films by Berkovich Nanoindentation

In this study, the deformation mechanisms of nonpolar GaN thick films grown on m-sapphire by hydride vapor phase epitaxy (HVPE) are investigated using nanoindentation with a Berkovich indenter, cathodoluminescence (CL), and Raman microscopy. Results show that nonpolar GaN is more susceptible to plastic deformation and has lower hardness thanc-plane GaN. After indentation, lateral cracks emerge on the nonpolar GaN surface and preferentially propagate parallel to the orientation due to anisotropic defect-related stresses. Moreover, the quenching of CL luminescence can be observed to extend exclusively out from the center of the indentations along the orientation, a trend which is consistent with the evolution of cracks. The recrystallization process happens in the indented regions for the load of 500 mN. Raman area mapping indicates that the distribution of strain field coincides well with the profile of defect-expanded dark regions, while the enhanced compressive stress mainly concentrates in the facets of the indentation.

Efficiency enhancement of Homoepitaxial InGaN/GaN light-emitting diodes on freestanding GaN substrate with double embedded SiO2 photonic crystals

Homoepitaxially grown InGaN/GaN light emitting diodes (LEDs) with SiO2 nanodisks embedded in n-GaN and p-GaN as photonic crystal (PhC) structures by nanospherical-lens photolithography are presented and investigated. The introduction of SiO2 nanodisks doesnt produce the new dislocations and doesnt also result in the electrical deterioration of PhC LEDs. The light output power of homoepitaxial LEDs with embedded PhC and double PhC at 350 mA current is increased by 29.9% and 47.2%, respectively, compared to that without PhC. The corresponding light radiation patterns in PhC LEDs on GaN substrate show a narrow beam shape due to strong guided light extraction, with a view angle reduction of about 30°. The PhC LEDs are also analyzed in detail by finite-difference time-domain simulation (FDTD) to further reveal the emission characteristics.

The improvement of GaN-based light-emitting diodes using nanopatterned sapphire substrate with small pattern spacing

Self-assembly SiO2 nanosphere monolayer template is utilized to fabricate nanopatterned sapphire substrates (NPSSs) with 0-nm, 50-nm, and 120-nm spacing, receptively. The GaN growth on top of NPSS with 0-nm spacing has the best crystal quality because of laterally epitaxial overgrowth. However, GaN growth from pattern top is more difficult to get smooth surface than from pattern bottom. The rougher surface may result in a higher work voltage. The stimulation results of finite-difference time-domain (FDTD) display that too large or too small spacing lead to the reduced light extracted efficiency (LEE) of LEDs. Under a driving current 350 mA, the external quantum efficiencies (EQE) of GaN-based LEDs grown on NPSSs with 0-nm, 50-nm, and 120-nm spacing increase by 43.3%, 50.6%, and 39.1%, respectively, compared to that on flat sapphire substrate (FSS). The optimized pattern spacing is 50 nm for the NPSS with 600-nm pattern period.

Enhancing optical power of GaN-based light-emitting diodes by nanopatterning on indium tin oxide with tunable fill factor using multiple-exposure nanosphere-lens lithography

In this study, the multiple-exposure nanosphere-lens lithography method utilizing the polystyrene nanospheres with focusing behavior is investigated and introduced to fabricate diverse photonic crystals (PCs) on indium tin oxide to enhance the optical output power of GaN-based light-emitting diode (LED). Simulated results indicate that the focused light intensity decreases with increasing tilted angle due to the shadow effect introduced by adjacent nanospheres. The fill factor of nanopattern is tunable by controlling tilted angles and exposure times. To attain quadruple PC without overlapping patterns, mathematical calculation model is used to define the optimum range of tilted angles. Angular emission patterns and three-dimensional finite-difference time domain simulated results indicate that the enhanced light extraction of PC LEDs results mainly from diffused scattering effects, and the diffraction effects of PC on light extracted efficiency increase with the increase of fill factor. Furthermore, it is confirmed that the multiple PC can extract more light from GaN into air than common PC with same period and fill factor.

Investigation of Efficiency and Droop Behavior Comparison for InGaN/GaN Super Wide-Well Light Emitting Diodes Grown on Different Substrates

In this work, efficiency droop of InGaN/GaN multiple-quantum-well LEDs with super wide well (WW) is discussed by comparing the external quantum efficiency (EQE) of GaN grown on sapphire and FS-GaN substrates. The luminescence and electrical characteristics of these WW LEDs are also experimentally and theoretically analyzed. With the increase of well width from 3 nm to 6 nm, high V-pits density and more strain relaxation are found in WW LED on sapphire, which exhibits greatly reduced peak efficiency but almost negligible droop behavior. In contrast, despite a larger polarization field, WW LED on FS-GaN shows obviously enhanced peak efficiency and comparable droop compared to the counterpart with 3-nm well. The Auger recombination probably dominates the mechanism of efficiency droop rather than defect-related nonradiative recombination or polarization effect in the WW LED on both sapphire and FS-GaN, especially at high current density.

Nanospherical-lens lithographical Ag nanodisk arrays embedded in p-GaN for localized surface plasmon-enhanced blue light emitting diodes

Large-scale Ag nanodisks (NDs) arrays fabricated using nanospherical-lens lithography (NLL) are embedded in p-GaN layer of an InGaN/GaN light-emitting diode (LED) for generating localized surface plasmon (LSP) coupling with the radiating dipoles in the quantum-well (QWs). Based on the Ag NDs with the controlled surface coverage, LSP leads to the improved crystalline quality of regrowth p-GaN, increased photoluminescence (PL) intensity, reduced PL decay time, and enhanced output power of LED. Compared with the LED without Ag NDs, the optical output power at a current of 350 mA of the LSP-enhanced LEDs with Ag NDs having a distance of 20 and 35 nm to QWs is increased by 26.7% and 31.1%, respectively. The electrical characteristics and optical properties of LEDs with embedded Ag NPs are dependent on the distance of between Ag NPs and QWs region. The LED with Ag NDs array structure is also found to exhibit reduced emission divergence, compared to that without Ag NDs.

Defect reduction in semipolar {10} GaN grown on m-sapphire via two-step nanoepitaxial lateral overgrowth

A method to obtain high quality semipolar {10} GaN grown on m-plane sapphire is presented. This method is similar to two-step nanoepitaxial lateral overgrowth (2S-NELOG) by combining a TiN interlayer and self-assembled SiO2 nanospheres. For the 2S-NELOG semi-GaN, the root-mean-square (RMS) roughness is 1.8 nm with a scan area of 5 × 5 μm2. The reduction of the defect density is demonstrated using high resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM). The full widths at half maximum (FWHMs) of the on-axis X-ray rocking curves (XRCs) are 381 and 524 arcsec, respectively rocking toward the [30] direction and the [110] direction. The anisotropy of the mosaic is lower compared to planar and TiN semi-GaN. In addition, Raman analyses also show the partial relaxation of the stress in the 2S-NELOG semi-GaN.