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Here we demonstrate high-brightness InGaN/GaN green light emitting diodes (LEDs) with in-situ low-temperature GaN (LT-GaN) nucleation layer (NL) and ex-situ sputtered AlN NL on 4-inch patterned sapphire substrate. Compared to green LEDs on LT-GaN (19 nm)/sapphire template, green LEDs on sputtered AlN (19 nm)/template has better crystal quality while larger in-plane compressive strain. As a result, the external quantum efficiency (EQE) of green LEDs on sputtered AlN (19 nm)/sapphire template is lower than that of green LEDs on LT-GaN (19 nm)/sapphire template due to strain-induced quantum-confined Stark effect (QCSE). We show that the in-plane compressive strain of green LEDs on sputtered AlN/sapphire templates can be manipulated by changing thickness of the sputtered AlN NL. As the thickness of sputtered AlN NL changes from 19 nm to 40 nm, the green LED on sputtered AlN (33 nm)/sapphire template exhibits the lowest in-plane compressive stress and the highest EQE. At 20 A/cm2, the EQE of 526 nm green LEDs on sputtered AlN (33 nm)/sapphire template is 36.4%, about 6.1% larger than that of the green LED on LT-GaN (19 nm)/sapphire template. Our experimental data suggest that high-efficiency green LEDs can be realized by growing InGaN/GaN multiple quantum wells (MQWs) on sputtered AlN/sapphire template with reduced in-plane compressive strain and improved crystal quality.

The isoelectronic doping in semiconductors is an effective method for improving device performance. Here we have investigated effects of isoelectronically Al-doped GaN buffer layer on optical and electrical properties of 365 nm InGaN/AlGaN ultraviolet light-emitting diodes (UV LEDs) grown by metal-organic chemical vapor deposition (MOCVD) on patterned sapphire substrate. In situ reflectance measurements revealed that the transition time from three-dimensional (3D) grain to step-flow two-dimensional (2D) coalesced growth mode was extended in the isoelectronically Al-doped GaN buffer layer as compared to undoped GaN buffer layer. The improved crystal quality, in terms of threading dislocation reduction, of the isoelectronically Al-doped GaN buffer layer was determined by cross sectional transmission electron microscopy studies and showed that the screw-type threading dislocation density was reduced. The light output power of UV LED was enhanced by 7.6% by incorporating optimum isoelectronic Al doping concentration in GaN buffer layer.

The paper proposes that usability, usefulness and service quality will respectively exercise a positive impact on college students'satisfaction with running APP based on computer information technology. There are four theoretical assumptions on the positive effect of user satisfaction on loyalty, which have established a structural equation model on college students' satisfaction with running APP based on computer information technology with model fitting and validation performed. The results show that the actual conditions of five dimensions can be objectively reflected with tenable research assumptions, reasonable questionnaire design and measurement indicators. Further statistical analysis of the questionnaire data suggests that users have a high degree of satisfaction with the usability of running APP, followed by usefulness and service quality. The lowest satisfaction rates in such aspects of usability, usefulness and service quality cover positioning (Q4), offer professional guidance (Q6) and backstage service (Q10). It is recommended that running APP should also improve the accuracy of positioning, enrich content, perform well in connotation building, enhance the level of backstage intelligence and improve backstage service. The overall satisfaction and loyalty of college students for running APP still need to be further improved.

The realization of efficient III-nitride emitters in the green-to-amber region is fundamental to the monolithic integration of multicolor emitters and the development of III-nitride-based full-color high-resolution displays. A hybrid nucleation layer, which includes sputtered AlN and mid-temperature GaN components, was proposed for the development of efficient III-nitride emitters in the green-to-amber region. The mid-temperature GaN component in the hybrid nucleation layer induced the formation of a stacking fault band structure, which effectively relaxed the misfit stress at the GaN/sapphire interface. A reduced dislocation density and in-plane compressive stress in InGaN/GaN multiple quantum wells were obtained on the hybrid nucleation layer in comparison with the conventional sputtered AlN nucleation layer. Consequently, a significantly enhanced internal quantum efficiency and improved light output power were achieved for the LEDs grown on the hybrid nucleation layer. This gain is attributed to the increased localization depth and spatial overlapping of the electron and hole wave functions. In the present study, the hybrid nucleation layer provides a promising approach for the pursuit of efficient III-nitride emitters in the green-to-amber region.

A ring-shaped SiO2 CBL underneath the p-electrode was employed to enhance current spreading of GaN-based light-emitting diodes (LEDs). Effects of ring-shaped SiO2 current blocking layer (CBL) thickness on optical and electrical characteristics of high power LEDs were investigated. A 190-nm-thick ring-shaped SiO2 CBL with inclined sidewalls was obtained using a combination of a thermally reflowed photoresist technique and an inductively coupled plasma (ICP) etching process, allowing for the deposition of conformal indium tin oxide (ITO) transparent conductive layer on sidewalls of ring-shaped SiO2 CBL. It was indicated that the external quantum efficiency (EQE) of high power LEDs increased with increasing thickness of ring-shaped SiO2 CBL. The EQE of high power LED with 190-nm-thick ring-shaped SiO2 CBL was 12.7% higher than that of high power LED without SiO2 CBL. Simulations performed with commercial SimuLED software package showed that the ring-shaped SiO2 CBL could significantly alleviate current crowding around p-electrode, resulting in enhanced current spreading over the entire high power LED structure.

High-quality and crack-free aluminum nitride (AlN) film on sapphire substrate is the foundation for high-efficiency aluminum gallium nitride (AlGaN)-based deep ultraviolet light-emitting diodes (DUV LEDs). We reported the growth of high-quality and crack-free AlN film on sapphire substrate with a nanometer-scale-thick AlN nucleation layer (NL). Three kinds of nanometer-scale-thick AlN NLs, including in situ low-temperature AlN (LT-AlN) NL, oxygen-undoped ex situ sputtered AlN NL, and oxygen-doped ex situ sputtered AlN NL, were prepared for epitaxial growth of AlN films on sapphire substrates. The influence of nanoscale AlN NL thickness on the optical transmittance, strain state, surface morphology, and threading dislocation (TD) density of the grown AlN film on sapphire substrate were carefully investigated. The average optical transmittance of AlN film on sapphire substrate with oxygen-doped sputtered AlN NL was higher than that of AlN films on sapphire substrates with LT-AlN NL and oxygen-undoped sputtered AlN NL in the 200&ndash, 270 nm wavelength region. However, the AlN film on sapphire substrate with oxygen-undoped sputtered AlN NL had the lowest TD density among AlN films on sapphire substrates. The AlN film on sapphire substrate with the optimum thickness of sputtered AlN NL showed weak tensile stress, a crack-free surface, and low TD density. Furthermore, a 270-nm AlGaN-based DUV LED was grown on the high-quality and crack-free AlN film. We believe that our results offer a promising and practical route for obtaining high-quality and crack-free AlN film for DUV LED.

We report the metal-organic chemical vapor deposition growth of high-quality AlN on sapphire enabled by an alternating V/III ratio AlN superlattice. We demonstrated that the insertion of an alternating V/III ratio AlN superlattice facilitates the relaxation of tensile stress during growth and assists the annihilation of threading dislocations. Dislocation inclination was proposed to play a major role in the stress relaxation and dislocation reduction, which not only provides an effective misfit-dislocation component but also increases the dislocation reaction probability. By this method, crack-free 3.25 μm-thick AlN films were grown on flat sapphire substrate (FSS) and nano-patterned sapphire substrate (NPSS) at 1180 °C, and the full width at half maximum of (002)/(102) rocking curves was 204/408 and 152/323 arcsec for such AlN/FSS and AlN/NPSS templates.

A strategically constructed substrate, patterned sapphire with silica array (PSSA), was developed to boost the efficiency of patterned sapphire substrate (PSS) in GaN-based light-emitting diodes (LEDs) application. The light output power of a flip-chip LED on PSSA improved by 16.5% at 120 mA than that of device grown on PSS. The XRD and STEM measurements revealed that the GaN epilayer grown on PSSA had better crystalline quality compared to the epilayer grown on PSS, which was the result of decreased misfit at coalescence boundary in the PSSA case. Moreover, the light extraction efficiency of the flip-chip LED on PSSA was significantly enhanced, benefiting from the small refractive-index contrast between the patterned silica array and air. This small refractive-index contrast also contributed to a more convergent emission pattern for the flip-chip LED on PSSA, as demonstrated by the far-field radiation pattern measurements. The discovery that PSSA could excel at defect suppression and light extraction revealed a new substrate platform for III-nitride optoelectronic devices.

We demonstrate the achieving of high-quality AlN films on flat sapphire substrate (FSS) by introducing voids during growth. Voids are embedded into AlN epilayers through a growth-mode transition from island growth to step flow growth. Such voids significantly facilitated the underlying dislocations annihilation as demonstrated by the transmission electron microscopy (TEM) image. For the 3 μm-thick AlN film grown on FSS, the full width at half maximum of the X-ray rocking curve was 57/260 arcsec for (0 0 2)/(1 0 2) reflection and a threading dislocation density of 1.7 × 108 cm−2 was determined from plain-view TEM image. Moreover, the voids provided an additional stress relief channel in the AlN film grown on FSS, resulting in a tensile stress comparable to that of grown on nano-patterned sapphire substrate (NPSS). The measured lattice constants and Raman shift of AlN-E2 (high) peak verified the 3 μm-thick AlN film grown on FSS is nearly stress free at room temperature. Taking advantages of the deliberately embedded voids, a crack-free and atomically flat AlN film was grown on FSS. The strategy put forward in this work to obtaining high-quality AlN films on FSS is much cost-efficient, which is believed to hold great promise for commercialization in AlN-based devices.

We demonstrate a GaN-based flip-chip LED (FC-LED) with a highly reflective indium-tin oxide (ITO)/distributed Bragg reflector (DBR) ohmic contact. A transparent ITO current spreading layer combined with Ta2O5/SiO2 double DBR stacks is used as a reflective p-type ohmic contact in the FC-LED. We develop a strip-shaped SiO2 current blocking layer, which is well aligned with a p-electrode, to prevent the current from crowding around the p-electrode. Our combined numerical simulation and experimental results revealed that the FC-LED with ITO/DBR has advantages of better current spreading and superior heat dissipation performance compared to top-emitting LEDs (TE-LEDs). As a result, the light output power (LOP) of the FC-LED with ITO/DBR was 7.6% higher than that of the TE-LED at 150 mA, and the light output saturation current was shifted from 130.9 A/cm2 for the TE-LED to 273.8 A/cm2 for the FC-LED with ITO/DBR. Owing to the high reflectance of the ITO/DBR ohmic contact, the LOP of the FC-LED with ITO/DBR was 13.0% higher than that of a conventional FC-LED with Ni/Ag at 150 mA. However, because of the better heat dissipation of the Ni/Ag ohmic contact, the conventional FC-LED with Ni/Ag exhibited higher light output saturation current compared to the FC-LED with ITO/DBR.

We report on the demonstration of GaN-based ultraviolet light-emitting diodes (UV LEDs) emitting at 375 nm grown on patterned sapphire substrate (PSS) with in-situ low temperature GaN/AlGaN nucleation layers (NLs) and ex-situ sputtered AlN NL. The threading dislocation (TD) densities in GaN-based UV LEDs with GaN/AlGaN/sputtered AlN NLs were determined by high-resolution X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM), which revealed that the TD density in UV LED with AlGaN NL was the highest, whereas that in UV LED with sputtered AlN NL was the lowest. The light output power (LOP) of UV LED with AlGaN NL was 18.2% higher than that of UV LED with GaN NL owing to a decrease in the absorption of 375 nm UV light in the AlGaN NL with a larger bandgap. Using a sputtered AlN NL instead of the AlGaN NL, the LOP of UV LED was further enhanced by 11.3%, which is attributed to reduced TD density in InGaN/AlInGaN active region. In the sputtered AlN thickness range of 10–25 nm, the LOP of UV LED with 15-nm-thick sputtered AlN NL was the highest, revealing that optimum thickness of the sputtered AlN NL is around 15 nm.

GaN-based ultraviolet-light-emitting diodes (UV LEDs) with 375 nm emission were grown on different-sized patterned sapphire substrates (PSSs) with ex situ 15-nm-thick sputtered AlN nucleation layers by metal–organic chemical vapor deposition (MOCVD). It was observed through in situ optical reflectance monitoring that the transition time from a three-dimensional (3D) island to a two-dimensional (2D) coalescence was prolonged when GaN was grown on a larger PSS, owing to a much longer lateral growth time of GaN. The full widths at half-maximum (FWHMs) of symmetric GaN(002) and asymmetric GaN(102) X-ray diffraction (XRD) rocking curves decreased as the PSS size increased. By cross-sectional transmission electron microscopy (TEM) analysis, it was found that the threading dislocation (TD) density in UV LEDs decreased with increasing pattern size and fill factor of the PSS, thereby resulting in a marked improvement in internal quantum efficiency (IQE). Finite-difference time-domain (FDTD) simulations quantitatively demonstrated a progressive decrease in light extraction efficiency (LEE) as the PSS size increased. However, owing to the significantly reduced TD density in InGaN/AlInGaN multiple quantum wells (MQWs) and thus improved IQE, the light output power of the UV LED grown on a large PSS with a fill factor of 0.71 was 131.8% higher than that of the UV LED grown on a small PSS with a fill factor of 0.4, albeit the UV LED grown on a large PSS exhibited a much lower LEE.

Distributed Bragg reflectors (DBR) with metal film on the bottom have been demonstrated to further improve the light output power of GaN-based light-emitting diodes (LEDs). Periods of TiO2/SiO2 stacks, thickness of metal film, and material of metallic reflector were designed and optimized in simulation software. The maximal bandwidth of double-DBR stacks have reached up to 272 nm, which was 102 nm higher than a single-DBR stack. The average reflectance of LEDs with wavelength from 380 nm to 780 nm in double-DBR stacks is 95.09% at normal incident, which was much higher than that of a single-DBR stack whose average reflectance was 91.38%. Meanwhile, maximal average reflectance of LEDs for double-DBR stacks with an incident angle from 0 to 90° was 97.41%, which was 3.2% higher than that of a single-DBR stack with maximal average reflectance of 94.21%. The light output power of an LED with double-DBR stacks is 3% higher than that of an LED with a single-DBR stack, which was attributed to high reflectance of double-DBR stacks.