Monolithic stacked blue light-emitting diodes with polarization-enhanced tunnel junctions.

Monolithic stacked InGaN light-emitting diode (LED) connected by a polarization-enhanced GaN/AlN-based tunnel junction is demonstrated experimentally in this study. The typical stacked LEDs exhibit 80% enhancement in output power compared with conventional single LEDs because of the repeated use of electrons and holes for photon generation. The typical operation voltage of stacked LEDs is higher than twice the operation voltage of single LEDs. This high operation voltage can be attributed to the non-optimal tunneling junction in stacked LEDs. In addition to the analyses of experimental results, theoretical analysis of different schemes of tunnel junctions, including diagrams of energy bands, diagrams of electric fields, and current-voltage relation curves, are investigated using numerical simulation. The results shown in this paper demonstrate the feasibility in developing cost-effective and highly efficient tunnel-junction LEDs.

GaN-based ultraviolet light-emitting diodes with AlN/GaN/InGaN multiple quantum wells.

We demonstrate indium gallium nitride/gallium nitride/aluminum nitride (AlN/GaN/InGaN) multi-quantum-well (MQW) ultraviolet (UV) light-emitting diodes (LEDs) to improve light output power. Similar to conventional UV LEDs with AlGaN/InGaN MQWs, UV LEDs with AlN/GaN/InGaN MQWs have forward voltages (V(f)'s) ranging from 3.21 V to 3.29 V at 350 mA. Each emission peak wavelength of AlN/GaN/InGaN MQW UV LEDs presents 350 mA output power greater than that of the corresponding emission peak wavelength of AlGaN/InGaN MQW UV LEDs. The light output power at 350mA of AlN/GaN/InGaN MQWs UV LEDs with 375 nm emission wavelength can reach around 26.7% light output power enhancement in magnitude compared to the AlGaN/InGaN MQWs UV LEDs with same emission wavelength. But 350mA light output power of AlN/GaN/InGaN MQWs UV LEDs with emission wavelength of 395nm could only have light output power enhancement of 2.43% in magnitude compared with the same emission wavelength AlGaN/InGaN MQWs UV LEDs. Moreover, AlN/GaN/InGaN MQWs present better InGaN thickness uniformity, well/barrier interface quality and less large size pits than AlGaN/InGaN MQWs, causing AlN/GaN/InGaN MQW UV LEDs to have less reverse leakage currents at -20 V. Furthermore, AlN/GaN/InGaN MQW UV LEDs have the 2-kV human body mode (HBM) electrostatic discharge (ESD) pass yield of 85%, which is 15% more than the 2-kV HBM ESD pass yield of AlGaN/InGaN MQW UV LEDs of 70%.

White emission from non-planar InGaN/GaN MQW LEDs grown on GaN template with truncated hexagonal pyramids.

Non-planar InGaN/GaN multiple quantum well (MQW) structures are grown on a GaN template with truncated hexagonal pyramids (THPs) featuring c-plane and r-plane surfaces. The THP array is formed by the regrowth of the GaN layer on a selective-area Si-implanted GaN template. Transmission electron microscopy shows that the InGaN/GaN epitaxial layers regrown on the THPs exhibit different growth rates and indium compositions of the InGaN layer between the c-plane and r-plane surfaces. Consequently, InGaN/GaN MQW light-emitting diodes grown on the GaN THP array emit multiple wavelengths approaching near white light.

Effects of InGaN layer thickness of AlGaN/InGaN superlattice electron blocking layer on the overall efficiency and efficiency droops of GaN-based light emitting diodes.

The operating voltage, light output power, and efficiency droops of GaN-based light emitting diodes (LEDs) were improved by introducing Mg-doped AlGaN/InGaN superlattice (SL) electron blocking layer (EBL). The thicker InGaN layers of AlGaN/InGaN SL EBL could have a larger effective electron potential height and lower effective hole potential height than that of AlGaN EBL. This thicker InGaN layer could prevent electron leakage into the p-region of LEDs and improve hole injection efficiency to achieve a higher light output power and less efficiency droops with the injection current. The low lateral resistivity of Mg-doped AlGaN/InGaN SL would have superior current spreading at high current injection.

GaN-based light-emitting diodes with graphene/indium tin oxide transparent layer.

We have demonstrated a gallium nitride (GaN)-based green light-emitting diode (LED) with graphene/indium tin oxide (ITO) transparent contact. The ohmic characteristic of the p-GaN and graphene/ITO contact could be preformed by annealing at 500 °C for 5 min. The specific contact resistance of p-GaN/graphene/ITO (3.72E-3 Ω·cm²) is one order less than that of p-GaN/ITO. In addition, the 20-mA forward voltage of LEDs with graphene/ITO transparent (3.05 V) is 0.09 V lower than that of ITO LEDs (3.14 V). Besides, We have got an output power enhancement of 11% on LEDs with graphene/ITO transparent contact.

Optoelectrical characteristics of green light-emitting diodes containing thick InGaN wells with digitally grown InN/GaN.

Compared with conventionally grown thin InGaN wells, thick InGaN wells with digitally grown InN/GaN exhibit superior optical properties. The activation energy (48 meV) of thick InGaN wells (generated by digital InN/GaN growth from temperature-dependent integrated photoluminescence intensity) is larger than the activation energy (25 meV) of conventionally grown thin InGaN wells. Moreover, thick InGaN wells with digitally grown InN/GaN exhibit a smaller σ value (the degree of localization effects) of 19 meV than that of conventionally grown thin InGaN wells (23 meV). Compared with green light-emitting diodes (LEDs) with conventional thin InGaN wells, the improvement in 20-A/cm² output power for LEDs containing thick InGaN wells with digitally grown InN/GaN is approximately 23%.

Terahertz plasmonic waveguide based on metal rod arrays for nanofilm sensing.

A high-aspect-ratio metallic rod array is demonstrated to generate and propagate highly confined terahertz (THz) surface plasmonic waves under end-fire excitation. The transverse modal power distribution and spectral properties of the bound THz plasmonic wave are characterized in two metallic rod arrays with different periods and in two configurations with and without attaching a subwavelength superstrate. The integrated metallic rod array-based waveguide can be used to sense the various thin films deposited on the polypropylene superstrate based on the phase-sensitive mechanism. The sensor exhibits different phase detection sensitivities depending on the modal power immersed in the air gaps between the metallic rods. Deep-subwavelength SiO(2) and ZnO nanofilms with an optical path difference of 252 nm, which is equivalent to λ/3968 at 0.300 THz, are used as analytes to test the integrated plasmonic waveguide. Analysis of the refractive index and thickness of molecular membranes indicates that the metallic rod array-based THz waveguide can integrate various biochip platforms for minute molecular detection, which is extremely less than the coherent length of THz waves.

Short-term glutamine supplementation decreases lung inflammation and the receptor for advanced glycation end-products expression in direct acute lung injury in mice.

BACKGROUND: Glutamine (GLN) has been reported to improve clinical and experimental sepsis outcomes. However, the mechanisms underlying the actions of GLN remain unclear, and may depend upon the route of GLN administration and the model of acute lung injury (ALI) used. The aim of this study was to investigate whether short-term GLN supplementation had an ameliorative effect on the inflammation induced by direct acid and lipopolysaccharide (LPS) challenge in mice. METHODS: Female BALB/c mice were divided into two groups, a control group and a GLN group (4.17% GLN supplementation). After a 10-day feeding period, ALI was induced by intratracheal administration of hydrochloric acid (pH 1.0; 2 mL/kg of body weight [BW]) and LPS (5 mg/kg BW). Mice were sacrificed 3 h after ALI challenge. In this early phase of ALI, serum, lungs, and bronchoalveolar lavage fluid (BALF) from the mice were collected for further analysis. RESULTS: The results of this study showed that ALI-challenged mice had a significant increase in myeloperoxidase activity and expression of interleukin (IL)-1ß, IL-6, and tumor necrosis factor-α in the lung compared with unchallenged mice. Compared with the control group, GLN pretreatment in ALI-challenged mice reduced the levels of receptor for advanced glycation end-products (RAGE) and IL-1ß production in BALF, with a corresponding decrease in their mRNA expression. The GLN group also had markedly lower in mRNA expression of cyclooxygenase-2 and NADPH oxidase-1. CONCLUSIONS: These results suggest that the benefit of dietary GLN may be partly contributed to an inhibitory effect on RAGE expression and pro-inflammatory cytokines production at an early stage in direct acid and LPS-induced ALI in mice.

Optoelectrical and low-frequency noise characteristics of flexible ZnO-SiO2 photodetectors with organosilicon buffer layer.

The present study demonstrates the optoelectrical and low-frequency noise characteristics of ZnO-SiO(2) nanocomposite solar-blind metal-semiconductor-metal photodetectors (MSM PDs) on flexible polyethersulfone (PES) substrate with and without an organosilicon [SiO(x)(CH(3))] buffer layer. For a given bandwidth of 100 Hz and a -5 V applied bias, the noise equivalent powers of the ZnO-SiO(2) nanocomposite MSM PD on PES with and without the SiO(x)(CH(3)) buffer layer were 1.39 × 10(-14) and 5.72 × 10(-14) W at 240nm, respectively, corresponding to the normalized detectivities of 5.04 × 10(14) and 1.22 × 10(14) Hz(0.5) W(-1), respectively. These findings indicate that a lower noise level and a higher detectivity can be achieved for ZnO-SiO(2) nanocomposite MSM PDs on PES by introducing a SiO(x)(CH(3)) buffer layer.

Dual-wavelength GaN-based LEDs grown on truncated hexagonal pyramids formed by selective-area regrowth on Si-implanted GaN templates.

GaN-based blue light-emitting diodes (LEDs) with micro truncated hexagonal pyramid (THP) array were grown on selective-area Si-implanted GaN (SIG) templates. The GaN epitaxial layer regrown on the SIG templates exhibited selective growth and subsequent lateral growth to form the THP array. The observed selective-area growth was attributed to the different crystal structures between the Si-implanted and implantation-free regions. Consequently, LEDs grown on the GaN THP array emitted broad electroluminescence spectra with multiple peaks. Spatially resolved cathodoluminescence revealed that the broad spectra originated from different areas within each THP. Transmission electron microscopy showed the GaN-based epitaxial layers, including InGaN/GaN multi-quantum wells regrown at different growth rates (or with different In content in the InGaN wells) between the semi-polar and c-face planes of each THP.

Numerical study of the suppressed efficiency droop in blue InGaN LEDs with polarization-matched configuration.

In blue InGaN light-emitting diodes (LEDs), the intuitive approaches to suppress Auger recombination by reducing carrier density, e.g., increasing the number of quantum wells (QWs) and thickening the width of wells, suffer from nonuniform carrier distribution and more severe spatial separation of electron and hole wave functions. To resolve this issue, LED structures with thick InGaN wells and polarization-matched AlGaInN barriers are proposed theoretically. Furthermore, the number of QWs is reduced for the purpose of mitigating the additional compressive strain in AlGaInN barriers. Simulation results reveal that, in the proposed structures, the quantum-confined Stark effect in strained wells is nearly eliminated through the utilization of polarization-matched barriers, which efficiently promotes internal quantum efficiency. Furthermore, the phenomenon of efficiency droop is also markedly improved because of the uniformly distributed or dispersed carriers, and accordingly the suppressed Auger recombination.

Investigation of Different Oxygen Partial Pressures on MgGa2O4-Resistive Random-Access Memory.

Different oxygen partial-pressure MgGa2O4-resistive RAMs (RRAMs) are fabricated to investigate the resistive switching behaviors. The X-ray photoelectron spectroscopy results, set voltage, reset voltage, cycling endurance, and retention time are drawn for comparison. With the increasing oxygen ratio gas flow, the resistive switching characteristics of MgGa2O4 RRAM are drastically elevated by changing the fabrication conditions of the RS layer. Moreover, we portray a filament model to explain the most likely mechanism associated with the generation and rupture of conductive filaments composed of oxygen vacancies. The formation of the interfacial layer (AlO x ) and the participation of the Joule heating effect are included to explain the highly distributed high-resistance state (HRS). The high randomness among switching cycles for memory application should be prevented, but it is suitable for the physical unclonable function. The relationship between HRS and the next time set voltage shows a strong correlation, and the conduction mechanisms of the low-resistance state (LRS) and HRS correspond to ohmic conduction and space charge-limited conduction, respectively. Meanwhile, the RRAM undergoes 10,000 s retention tests, and the two resistance states can be distinguished without obvious alternation or degradation. A favorable cycling endurance and retention time achieved by optimizing the fabrication parameters of Al/MgGa2O4/Pt RRAM have the potential for nonvolatile memristors and information security applications.

The influences of AlGaN barrier epitaxy in multiple quantum wells on the optoelectrical properties of AlGaN-based deep ultra-violet light-emitting diodes.

The growth conditions of the AlGaN barrier in AlGaN/AlGaN deep ultra-violet (DUV) multiple quantum wells (MQWs) have crucial influences on the light output power of DUV light-emitting diodes (LEDs). The reduction of the AlGaN barrier growth rate improved the qualities of AlGaN/AlGaN MQWs, such as surface roughness and defects. The light output power enhancement could reach 83% when the AlGaN barrier growth rate was reduced from 900 nm h-1 to 200 nm h-1. In addition to the light output power enhancement, lowering the AlGaN barrier growth rate altered the far-field emission patterns of the DUV LEDs and increased the degree of polarization in the DUV LEDs. The enhanced transverse electric polarized emission indicates that the strain in AlGaN/AlGaN MQWs was modified by lowering the AlGaN barrier growth rate.

Laser-induced periodic structures for light extraction efficiency enhancement of GaN-based light emitting diodes.

The laser-induced periodic surface structure technique was used to form simultaneously dual-scale rough structures (DSRS) with spiral-shaped nanoscale structure inside semi-spherical microscale holes on p-GaN surface to improve the light-extraction efficiency of light-emitting diodes (LEDs). The light output power of DSRS-LEDs was 30% higher than that of conventional LEDs at an injection current of 20 mA. The enhancement in the light output power could be attributed to the increase in the probability of photons to escape from the increased surface area of textured p-GaN surface.

Vertical InGaN light-emitting diode with a retained patterned sapphire layer.

We present an efficient vertical InGaN light-emitting diode (LED) in which the proposed vertical LEDs were fabricated with patterned sapphire substrates (PSS) using thinning techniques. After the thinning of sapphire substrate, selective dry etching process was performed on the remainder sapphire layer to expose the n-GaN contact layer instead of removing the sapphire substrate using the laser lift-off technique. These processes feature the LEDs with a sapphire-face-up structure and vertical conduction property. The PSS was adopted as a growth substrate to mitigate the light-guided effect, and thereby increase the light-extraction efficiency. Compared with conventional lateral GaN LEDs grown on PSS, the proposed vertical LEDs exhibit a higher light output power and less power degradation at a high driving current. This could be attributed to the fact that the vertical LEDs behave in a manner similar to flip-chip GaN/sapphire LEDs with excellent heat conduction.

Vertical InGaN light-emitting diode with a retained patterned sapphire layer.

We present an efficient vertical InGaN light-emitting diode (LED) in which the proposed vertical LEDs were fabricated with patterned sapphire substrates (PSS) using thinning techniques. After the thinning of sapphire substrate, selective dry etching process was performed on the remainder sapphire layer to expose the n-GaN contact layer instead of removing the sapphire substrate using the laser lift-off technique. These processes feature the LEDs with a sapphire-face-up structure and vertical conduction property. The PSS was adopted as a growth substrate to mitigate the light-guided effect, and thereby increase the light-extraction efficiency. Compared with conventional lateral GaN LEDs grown on PSS, the proposed vertical LEDs exhibit a higher light output power and less power degradation at a high driving current. This could be attributed to the fact that the vertical LEDs behave in a manner similar to flip-chip GaN/sapphire LEDs with excellent heat conduction.

Sputtered ZnO-SiO2 nanocomposite light-emitting diodes with flat-top nanosecond laser treatment.

Sputtered ZnO-SiO2 nanocomposite light-emitting diodes (LEDs) were treated using a flat-top nanosecond laser (FTNL) under room temperature. The intensity of the 376 nm electroluminescence (EL) emission of ZnO-SiO2 nanocomposite LEDs at a current of 9 mA with FTNL treatment was approximately 1.4 times greater than LEDs without FTNL treatment. Furthermore, the FTNL-treated LEDs indicated a narrower full width at half maximum of the 376 nm EL emission than those of LEDs without FTNL treatment. Thus, FTNL treatment of ZnO-SiO2 nanocomposite LEDs could induce the recrystallization of distributed ZnO nanoclusters and reduce the defects in ZnO-SiO2 nanocomposite layers.

Vertical InGaN light-emitting diodes with a sapphire-face-up structure.

Vertical GaN-based light-emitting diodes (LEDs) were fabricated with a Si substrate using the wafer-bonding technique. Lapping and dry-etching processes were performed for thinning the sapphire substrate instead of removing this substrate using the laser lift-off technique and the thinning process associated with the wafer-bonding technique to feature LEDs with a sapphire-face-up structure and vertical conduction property. Compared with conventional lateral GaN/sapphire-based LEDs, GaN/Si-based vertical LEDs exhibit higher light output power and less power degradation at a high driving current, which could be attributed to the fact that vertical LEDs behave in a manner similar to flip-chip GaN/sapphire LEDs with excellent heat conduction. In addition, with an injection current of 350 mA, the output power (or forward voltage) of fabricated vertical LEDs can be enhanced (or reduced) by a magnitude of 60% (or 5%) compared with conventional GaN/sapphire-based LEDs.

AlGaN-Based Deep Ultraviolet Light-Emitting Diodes with Thermally Oxidized Al x Ga2-x O3 Sidewalls.

AlGaN and GaN sidewalls were turned into Al x Ga2-x O3 and Ga2O3, respectively, by thermal oxidation to improve the optoelectrical characteristics of deep ultraviolet (DUV) light-emitting diodes (LEDs). The thermally oxidized Ga2O3 is a single crystal with nanosized voids homogenously distributed inside the layer. Two oxidized Al x Ga2-x O3 layers were observed on the sidewall of the AlGaN layer in transmission electron microscopy images. The first oxidized Al x Ga2-x O3 layer is a single crystal, while the second oxidized Al x Ga2-xO3 layer is a single crystal with numerous nanosized voids inside. The composition of Al in the first oxidized Al x Ga2-x O3 layer is higher than that in the second one. The thermal oxidation at high temperature degrades the quality of the p-GaN layer and increases the forward voltage from 8.18 to 11.36 V. The thermally oxidized Al x Ga2-x O3 sidewall greatly enhances the light extraction efficiency of the lateral light of the DUV LEDs by combined mechanisms of holey structure, graded refractive index, high transparency, and tensile stress. Consequently, the light output power of the DUV LEDs increases from 0.69 to 0.88 mW by introducing a 420 nm thick Al x Ga2-x O3 sidewall oxidized at 900 °C, in which the enhancement of light output power can reach 27.5%.

Electroluminescence of ZnO nanocrystal in sputtered ZnO-SiO2 nanocomposite light-emitting devices.

We have demonstrated the electroluminescence (EL) of Ga:ZnO/i-ZnO-SiO2 nanocomposite/p-GaN n-i-p heterostructure light-emitting devices (LEDs). ZnO nano-clusters with sizes distributing from 2 to 7nm were found inside the co-sputtered i-ZnO-SiO2 nanocomposite layer under the observation of high-resolution transparent electron microscope. A clear UV EL at 376 nm from i-ZnO-SiO2 nanocomposite in these p-i-n heterostructure LEDs was observed under the forward current of 9 mA. The EL emission peak at 376 and 427nm of the Ga:ZnO/i-ZnO-SiO2 nanocomposite/p-GaN n-i-p heterostructure LEDs were attributed to the radiative recombination from the ZnO clusters and the Mg acceptor levels in the p-GaN layer, respectively.