Flip-Chip GaN-Based Light-Emitting Diodes with Mesh-Contact Electrodes

GaN-based light-emitting diodes （LEDs） with mesh-contact electrodes have been developed. The p-type ohmic contact layer is composed of oxidized Ni/Au mesh and NiO overlay （20A）. An Ag （3000A） omni-directional reflector covers the p-type contact. The n-type contact is a Ti/AI planar film with a 10-μm-width Ti/AI stripe. The Ti/AI stripe surrounds the centre of LED mesa. With a 20-mA current injection, the light output power of GaN-based LEDs with mesh-contact electrodes is 23% higher than that of the conventional LEDs.     

Performance improvement of InGaN blue light-emitting diodes with several kinds of electron-blocking layers

The performance of InGaN blue light-emitting diodes(LEDs) with different kinds of electron-blocking layers is investigated numerically.We compare the simulated emission spectra,electron and hole concentrations,energy band diagrams,electrostatic fields,and internal quantum efficiencies of the LEDs.The LED using AlGaN with gradually increasing Al content from 0% to 20% as the electron-blocking layer(EBL) has a strong spectrum intensity,mitigates efficiency droop,and possesses higher output power compared with the LEDs with the other three types of EBLs.These advantages could be because of the lower electron leakage current and more effective hole injection.The optical performance of the specifically designed LED is also improved in the case of large injection current.     

The scalability of the tunnel-regenerated multi-active-region light-emitting diode structure

The scalability of the tunnel-regenerated multi-active-region （TRMAR） structure has been investigated for the application in light-emitting diodes （LEDs）. The use of the TRMAR structure was proved theoretically to have unique advantages over conventional slngle-active-layer structures in virtually every aspect, such as high quantum efficiency, high power and low leakage. Our study showed that the TRMAR LED structure could obtain high output power under low current injection and high wall-plug efficiency compared with the conventional single-active-layer LED structure.     

Low-Threshold and High-Power Oxide-Confined 850 nm AlInGaAs Strained Quantum-Well Vertical-Cavity Surface-Emitting Lasers Based on Intra-Cavity Contacted Structure

The low-threshold and high-power oxide-confined 850 nm AlInGaAs strained quantum-well （QW） vertical-cavity surface-emitting lasers （VCSELs） based on the intra-cavity contacted structure are fabricated. The threshold current of 0.1 mA for a 10-μm oxide-aperture device is obtained with the threshold current density of 0.127kA/cm^2. For a 22-μm oxide-aperture device, the peak optical output power reaches to 14.6mW at the current injection of 25 mA under the room temperature and pulsed operation with a threshold current of 2mA, which corresponds to the threshold current density of 0.526kA/cm^2. The lasing wavelength is 855.4nm. The full wave at half maximum is 2.2 nm. The analysis of the characteristics and the fabrication of VCSELs are also described.     

UV Light-Emitting Diodes at 340nm Fabricated on a Bulk GaN Substrate

We demonstrate an ultra-violet light-emitting diode （UV-LED） fabricated on a bulk GaN substrate with elec- troluminescence （EL） emission centered at about 340 nm. The UV-LED exhibits low reverse leakage current on the order of 10^-9 A under -5 V at room temperature, which can be explained by the low defect density in the epi-structure. The evolution of EL spectra as a function of injection current levels reveals the improved heat dissipation of the LEDs with vertical geometry on the bulk GaN substrate. The unusual increase of EL intensity at elevated temperatures can be explained by thermally assisted p-dopant ionization.     

Improved Light Extraction of GaN-based LEDs with Nano-roughened p-GaN Surfaces

p-GaN surfaces axe nano-roughened by plasma etching to improve the optical performance of GaN-based light emitting diodes （LEDs）. The nano-roughened GaN present a relaxation of stress. The light extraction of the LEDs with nano-roughened surfaces is greatly improved when compared with that of the conventional LEDs without nano-roughening. PL-mapping intensities of the nano-roughened LED epi-wafers for different roughening times present two to ten orders of enhancement. The light output powers are also higher for the nano-roughened LED devices, This improvement is attributed to that nano-roughened surfaces can provide photons multiple chances to escape from the LED surfaces,     

Spin Injection from Ferromagnetic Metal Directly into Non-Magnetic Semiconductor under Different Injection Currents

For ferromagnetic metal （FM）/semiconductor （SO） structure with ohmic contact, the effect of carrier polarization in the semiconductor combined with drift part of injection current on current polarization is investigated. Based on the general model we established here, spin injection efficiency under different injection current levels is calculated. Under a reasonable high injection current, current polarization in the semiconductor is actually much larger than that predicted by the conductivity mismatch model because the effect of carrier polarization is enhanced by the increasing drift current. An appreciable current polarization of 1% could be achieved for the FM/SC structure via ohmic contact, which means that efficient spin injection from FM into SC via ohmic contact is possible. The reported dependence of current polarization on temperature is verified quantitatively. To achieve even larger spin injection efficiency, a gradient doping semiconductor is suggested to enhance the drift current effect.     

InAs/GaAs submonolayer quantum-dot superluminescent diodes with active multimode interferometer configuration

With a chirped InAs/GaAs SML-QD (quantum dot) structure serving as the active region, the superluminescent diodes emitting at wavelength of around 970nm are fabricated. By using an active multimode interferometer configuration, these devices exhibit high continue-wave output powers from the narrow ridge waveguides. At continue-wave injection current of 800mA, an output power of 18.5mW, and the single Gaussian-like emission spectrum centered at 972nm with a full width at half maximum of 18nm are obtained.     

GaN-Based Thin Film Vertical Structure Light Emitting Diodes Fabricated by a Modified Laser Lift-off Process and Transferred to Cu

GaN-based thin film vertical structure light-emitting diodes （VS-LEDs） were fabricated by a modified YAG laser lift-off （LLO） process and transferred to Cu substrates. With a comparison of the electrical and optical properties of conventional LEDs on sapphire substrates and of lateral structure thin film LEDs by a KrF LLO process, the vertical structure of LLO LEDs shows obvious superiority. LLO VSLEDs made by modified YAG LLO process show less increase of leakage current than the devices made by conventional/（rE LLO process. Furthermore, owing to the well current spreading and less current path, the ideality factors and series resistance of vertical structure LEDs reduce greatly and the efficiency increases more obviously than the lateral structure LEDs, which is dso reflected on the relative L - I curves. The output power of vertical structure LEDs is over 3 times greater than that of the lateral structure LLO LEDs within 300mA.     

A low-threshold and high-power oxide-confined 850-nm AlInGaAs strained quantum-well vertical-cavity surface-emitting laser

A low-threshold and high-power oxide-confined 850-nm AlInGaAs strained quantum-well (QW) vertical-cavity surface-emitting laser (VCSEL) based on an intra-cavity contacted structure is fabricated. A threshold current of 1.5 mA for a 22 μm oxide aperture device is achieved, which corresponds to a threshold current density of 0.395 kA/cm². The peak output optical power reaches 17.5 mW at an injection current of 30 mA at room temperature under pulsed operation. While under continuous-wave (CW) operation, the maximum power attains 10.5 mW. Such a device demonstrates a high characteristic temperature of 327 K within a temperature range from -12℃ to 96℃ and good reliability under a lifetime test. There is almost no decrease of the optical power when the device operates at a current of 5 mA at room temperature under the CW injection current.     

Temperature-dependent efficiency droop behaviors of GaN-based green light-emitting diodes

The efficiency droop behaviors of GaN-based green light-emitting diodes (LEDs) are studied as a function of temperature from 300 K to 480 K. The overall quantum efficiency of the green LEDs is found to degrade as temperature increases, which is mainly caused by activation of new non-radiative recombination centers within the LED active layer. Meanwhile, the external quantum efficiency of the green LEDs starts to decrease at low injection current level (<1 A/cm2 ) with a temperature-insensitive peak-efficiency-current. In contrast, the peak-efficiency-current of a control GaN-based blue LED shows continuous up-shift at higher temperatures. Around the onset point of efficiency droop, the electroluminescence spectra of the green LEDs also exhibit a monotonic blue-shift of peak energy and a reduction of full width at half maximum as injection current increases. Carrier delocalization is believed to play an important role in causing the efficiency droop in GaN-based green LEDs.     

Output Characteristics of an InP/InGaAsP Triangle Microcavity Laser

Mode competitions between modes with different output coupling efficiencies can result in optical bistability under certain asymmetric nonlinear gain. For a GaInAsP/InP equilateral triangle microlaser with the side length of 10μm, the drop of the output power with the increase of the injection current is observed corresponding to transverse mode transitions. Furthermore, the measured laser spectra up to 270K show that lasing modes coexist with the wavelength interval of 39nm at 240K. The emission at 5.2THz can be expected by the mode frequency beating with the 39nm interval.     

Improvement of the light output and contact resistance of InGaN-based light-emitting diodes based on tantalum-doped indium tin oxide as p-type electrodes

This paper reports that highly transparent and low resistance tantalum-doped indium tin oxide (Ta-doped ITO) films contacted to p-type GaN have been prepared by the electron-beam evaporation technique. The Ta-doped ITO contacts become Ohmic with a specific contact resistance of $\sim 5.65\times 10^{ - 5}$~$\Omega \cdot$cm$^{2}$ and show the transmittance of $\sim $98% at a wavelength of 440~nm when annealed at 500~\du. Blue light emitting diodes (LEDs) fabricated with Ta-doped ITO p-type Ohmic contact layers give a forward-bias voltage of 3.21~V at an injection current of 20~mA. It further shows that the output power of LEDs with Ta-doped ITO contacts is enhanced 62% at 20~mA in comparison with that of LEDs with conventional Ni/Au contacts.     

Enhancing light extraction of GaN-based blue light-emitting diodes by a tuned nanopillar arrayEnhancing light extraction of GaN-based blue light-emitting diodes by a tuned nanopillar array

Surface patterning of p-GaN to improve the light extraction efficiency of GaN-based blue light-emitting diodes （LEDs） has been investigated. Periodic nanopillar arrays on p-GaN have been fabricated by polystyrene （PS） nanosphere lithog- raphy; the diameter of the nanopillars can be tuned to optimize the electrical and optical properties of the LEDs. The electroluminescence intensity of the nanopillar-patterned LEDs is better than that of conventional LEDs; the greatest en- hancement increased the intensity by a factor of 1.41 at a 20 mA injection current. The enhancements can be explained by a model of bilayer film on a GaN substrate. This method may serve as a practical approach to improve the efficiency of light extraction from LEDs.     

High Power 1060 nm Distributed Feedback Semiconductor Laser

A GaAs based high power distributed feedback （DFB） semiconductor laser with a second-order grating has been demonstrated. An output power of 150row at an injection current of 350mA is realized with a 1-mm cavity length. With a new design of the waveguide structure, the DFB laser maintains a stable single longitudinal mode around 106Ohm with a side mode suppression ratio of larger than 50dB.     

Performance improvement of GaN-based light-emitting diode with a p-InAIGaN hole injection layer

The characteristics of a blue light-emitting diode （LED） with a p-InA1GaN hole injection layer （HIL） is analyzed numerically. The simulation results indicate that the newly designed structure presents superior optical and electrical performance such as an increase in light output power, a reduction in current leakage and alleviation of efficiency droop. These improvements can be attributed to the p-InA1GaN serving as hole injection layers, which can alleviate the band bending induced by the polarization field, thereby improving both the hole injection efficiency and the electron blocking efficiency.     

Degradation behaviors of high power GaN-based blue light emitting diodes

The degradation mechanism of high power InGaN/GaN blue light emitting diodes(LEDs)is investigated in this paper.The LED samples were stressed at room temperature under 350-mA injection current for about 400 h.The light output power of the LEDs decreased by 35%during the first 100 h and then remained almost unchanged,and the reverse current at 5 V increased from 10 9A to 10 7A during the aging process.The power law,whose meaning was re-illustrated by the improved rate equation,was used to analyze the light output power-injection current(L–I)curves.The analysis results indicate that nonradiative recombination,Auger recombination,and the third-order term of carriers overflow increase during the aging process,all of which may be important reasons for the degradation of LEDs.Besides,simulating L–I curves with the improved rate equation reveal that higher-than-third-order terms of carriers overflow may not be the main degradation mechanism,because they change slightly when the LED is stressed.     

VCSEL with reticular electrode and larger emitting aperture and its optoelectronic characteristics

The enlargement of the emitting aperture is usually one of the important methods of increasing vertical- cavity surface-emitting laser （VCSEL） optical output power. However, in a VCSEL with a larger aperture, the inhomogeneity in the injected current often causes inhomogeneous or even no emission. To solve this problem and to increase VCSEL output power, as well as to improve its thermal characteristics, we develop a new type of injected VCSEL with a larger aperture and a reticular electrode, where the conventional circular injection electrode of the P side is turned into a reticular one, and the heat sink is on the N side. The tests of the new VCSEL show an improvement in homogeneity in not only the injected current but also the emission intensity. The optical output power is also considerably increased, and the device optoelectronic performance is improved.     

A 795 nm gain coupled distributed feedback semiconductor laser based on tilted waveguides

The 795 nm distributed feedback lasers have great application in pumping the Rb D1 transition. In this paper, in order to realize specific 795 nm lasing, we designed tilted ridge distributed feedback lasers based on purely gain coupled effect induced by periodic current injection windows through changing the angle of the tilted ridge. The fabricated devices were cleaved into 2 mm-cavity-length, including 5 tilted angles. The peak output powers of all devices were above 30 m W.Single longitudinal mode lasing was realized in all tilted Fabry–Perot cavities using periodic current injection windows,with side mode suppression ratio over 30 d B. The total wavelength range covered 8.656 nm at 20℃. It was disclosed theoretically and experimentally that the output powers, threshold currents, and central wavelengths of the tilted ridge purely gain coupled DFB lasers were relevant to the tilted angles. The results will be instructive for future design of DFB laser arrays with different central wavelengths.     

Continuous-Wave Operation of GaN Based Multi-Quantum-Well Laser Diode at Room Temperature

Room-temperature operation of cw GaN based multi-quantum-well laser diodes （LDs） is demonstrated. The LD structure is grown on a sapphire （0001） substrate by metalorganic chemical vapour deposition. A 2.5μm × 800μm ridge waveguide structure is fabricated. The electrical and optical characteristics of the laser diode under direct current injection at room temperature are investigated. The threshold current and voltage of the LD under cw operation are 110mA and 10.5V, respectively. Thermal induced series resistance decrease and emission wavelength red-shift are observed as the injection current is increased. The full width at half maximum for the parallel and perpendicular far field pattern （FFP） are 12° and 32°, respectively.