Progress in research of GaN-based LEDs fabricated on SiC substrate

The influence of buffer layer growth conditions on the crystal quality and residual stress of GaN film grown on silicon carbide substrate is investigated. It is found that the A1GaN nucleation layer with high growth temperature can efficiently decrease the dislocation density and stress of the GaN film compared with A1N buffer layer. To increase the light extraction efficiency of GaN-based LEDs on SiC substrate, flip-chip structure and thin film flip-chip structure were designed and optimized. The fabricated blue LED had a maximum wall-plug efficiency of 72% at 80 mA. At 350 mA, the output power, the Vf, the dominant wavelength, and the wall-plug efficiency of the blue LED were 644 roW, 2.95 V, 460 nm, and 63%, respectively.

Efficiency droop in InGaN/GaN-based LEDs with a gradually varying In composition in each InGaN well layer

Temperature-dependent and driving current-dependent electroluminescence spectra of two different InGaN/GaN multiple quantum well structures SA and SB are investigated,with the In composition in each well layer(WL)along the growth direction progressively increasing for SA and progressively decreasing for SB.The results show that SB exhibits an improved efficiency droop compared with SA.This phenomenon can be explained as follows:owing to the difference in growth pattern of the WL between these two samples,the terminal region of the WL in SB contains fewer In atoms than in SA,and therefore the former undergoes less In volatilization than the latter during the waiting period required for warming-up due to the difference in the growth temperature between well and barrier layers.This results in SB having a deeper triangular-shaped potential well in its WL than SA,which strongly confines the carriers to the initial region of the WL to prevent them from leaking to the p-GaN side,thus improving the efficiency droop.Moreover,the improvement in the efficiency droop for SB is also partly attributed to its stronger Coulomb screening effect and carrier localization effect.

Combined effects of carrier scattering and Coulomb screening on photoluminescence in InGaN/GaN quantum well structure with high In content

Photoluminescence(PL) spectra of two different green InGaN/GaN multiple quantum well(MQW) samples S1 and S2,respectively with a higher growth temperature and a lower growth temperature of InGaN well layers are analyzed over a wide temperature range of 6 K-3 30 K and an excitation power range of 0.001 mW-75 mW.The excitation power-dependent PL peak energy and linewidth at 6 K show that in an initial excitation power range,the emission process of the MQW is dominated simultaneously by the combined effects of the carrier scattering and Coulomb screening for both the samples,and both the carrier scattering effect and the Coulomb screening effect are stronger for S2 than those for S1;in the highest excitation power range,the emission process of the MQWs is dominated by the filling effect of the high-energy localized states for S1,and by the Coulomb screening effect for S2.The behaviors can be attributed to the fact that sample S2 should have a higher amount of In content in the InGaN well layers than S1 because of the lower growth temperature,and this results in a stronger component fluctuation-induced potential fluctuation and a stronger well/barrier lattice mismatchinduced quantum-confined Stark effect.This explanation is also supported by other relevant measurements of the samples,such as temperature-dependent peak energy and excitation-power-dependent internal quantum efficiency.

Picosecond dissipative soliton generation from an ytterbium‑doped fber laser based on a BP/SnSe_(2)‑PVA mixture saturable absorber

Stable picosecond dissipative soliton pulses were observed in an ytterbium-doped fber laser employing a high-quality mixture of BP/SnSe_(2)-PVA saturable absorber(SA).The modulation depth,saturation intensity,and non-saturable loss of the mixture of BP/SnSe_(2)-PVA SA were measured with values of 5.98%,18.37 MW/cm2,and 33%,respectively.Within the pump power range of 150–270 mW,stable dissipative soliton pulses were obtained with an output power of 1.68–4 mW.When the minimum pulse duration is 1.28 ps,a repetition rate of 0.903 MHz,center wavelength of 1064.38 nm and 3 dB bandwidth of 2 nm were obtained.The maximum pulse energy of 4.43 nJ and the signal-to-noise ratio up to 72 dB were achieved at pump power of 270 mW.The results suggest that the BP/SnSe_(2)-PVA mixture SA has outstanding nonlinear saturable absorption characteristics and broad ultrafast laser applications.

Two types of ultrafast mode‑locking operations from an Er‑doped fber laser based on germanene nanosheets

As a member of Xenes family,germanene has excellent nonlinear saturable absorption characteristics.In this work,we prepared germanene nanosheets by liquid phase exfoliation and measured their saturation intensity as 0.6 GW/cm^(2) with a modulation depth of 8%.Then,conventional solitons with a pulse width of 946 fs and high-energy noise-like pulses with a pulse width of 784 fs were obtained by using germanene nanosheet as a saturable absorber for a mode-locked Erbium-doped fber laser.The characteristics of the two types of pulses were investigated experimentally.The results reveal that germanene has great potential for modulation devices in ultrafast lasers and can be used as a material for creation of excellent nonlinear optical devices to explore richer applications in ultrafast photonics.

MOCVD growth of AlGaInP/GaInP quantum well laser diode with asymmetric cladding structure for high power applications

In order to improve the characteristics of the general broad-waveguide 808-nm semiconductor laser diode （LD）, we design a new type quantum well LD with an asymmetric cladding structure. The structure is grown by metal organic chemical vapor deposition （MOCVD）. For the devices with 100-ttm-wide stripe and 1000-/zm-long cavity under continuous-wave （CW） operation condition, the typical threshold current is 190 mA, the slope efficiency is 1.31 W/A, the wall-plug efficiency reaches 63%, and the maximum output power reaches higher than 7 W. And the internal absorption value decreases to 1.5 cm^-1.

Influence of the upper waveguide layer thickness on optical field in asymmetric heterostructure quantum well laser diode

Asymmetric broad-waveguide separate-confinement heterostructure（BW-SCH） quantum well（QW） laser diode emitting at 808 nm is analyzed and designed theoretically.The dependence of the optical field distribution, vertical far-field angle,and internal loss on different thicknesses of the upper waveguide layer is calculated and analyzed.Calculated results show that when the thicknesses of the lower and upper waveguide layers are 0.45 and 0.3μm,respectively,for the devices with 100-μm-wide stripe and 1000-μm-long cavity,an output power of 7.6 W at 8 A,a vertical far-field angle of 37°,a slope efficiency of 1.32 W/A, and a threshold current of 189 mA can be obtained.

Influence of initial growth conditions and Mg-surfactant on the quality of GaN film grown by MOVPE

The initial growth conditions of a 100 nm thick GaN layer and Mg-surfactant on the quality of the GaN epilayer grown on a 6H-SiC substrate by metal-organic vapor phase epitaxy have been investigated in this research. Experimental results have shown that a high V/III ratio and the initially low growth rate of the GaN layer are favorable for two-dimension growth and surface morphology of GaN and the formation of a smoother growth surface. Mg-surfactant occurring during GaN growth can reduce the dislocations density of the GaN epilayer but increase the surface RMS, which are attributed to the change of growth mode.

Fluorescence spectrum, thermal properties, and continuous-wave laser performance of the mixed crystal Nd:Lu_(0.99)La_(0.01)VO_4

The fluorescence spectrum and thermal properties of the mixed crystal Nd：Luo.gvLa~.o1VO4 are determined. The strongest emission peak located at 1065.6 nm had a full width at half maximum （FWHM） of 2.1 nm. Continuous-wave （CW） laser performance is demonstrated by a compact planar planar cavity that is end- pumped by a diode laser. The laser output characteristics are investigated by using output couplers with different transmissions. A maximum CW output power of 8.09 W was obtained at an incident pump power of 19.4 W, which corresponds to an optical-to-optical conversion efficiency of 41.7% and a slope efficiency of 54.6%. The dependence of optimum transmission on pump power is calculated theoretically and is found to be consistent with experimental results.

Growth of strain-compensated InGaAs/GaAsP multiple quantum wells by MOVPE

InGaAs/GaAsP strain-compensated multiple quantum wells (SCMQWs) and strained InGaAs/GaAsmultiple quantum wells (MQWs) were grown on GaAs substrates by metal organic vapor phase epitaxy(MOVPE). The results of double crystal X-ray diffraction (DCXRD) revealed that strain relief had beenpartly accommodated by the misfit dislocation formation in the strained MQW material. It led to thatthe full width half maximums (FWHMs) of superlattice satellite peaks are broader than those of SCMQWstructures, and there was no detectable room temperature photoluminecence(RT-PL)for the strained

Wafer-bonding AlGaInP light emitting diodes with pyramidally patterned metal reflector

We demonstrate and introduce here a pyramidally patterned metal reflector into wafer-bonding AlGaInP light emitting diodes （LEDs） to improve the light extraction efficiency by using a photo-assisted chemical etched GaP：Mg layer. The pyramid patterns were fabricated employing a HF and H202 mixed solution in combination with a 532 nm laser on a GaP：Mg surface firstly, and then a gold reflector layer was evaporated onto the patterned GaP：Mg surface. After the whole chip process, the patterned gold reflector structure was confirmed to be efficient for light extraction and a 18.55% enhancement of the electroluminescent flux has been obtained by an integrating sphere, compared to the surface textured LEDs with flat reflectors.

Voltage reduction of 808 nm GaAsP/(Al)GaInP laser diodes with GaInAsP intermediate layer

Ga In As P layers and Ga As P/（Al） Ga In P laser diodes（LDs） have been grown on Ga As substrates by metalorganic chemical vapor deposition. The Ga In As P layer, which is lattice matched to Ga As, has an intermediate band gap between Ga0.5In0.5P and Ga As. The Ga In P/Ga As heterojunction spikes, especially in the valence band,can be suppressed by introducing this thin Ga In As P layer into the heterostructure interface. The 808 nm Ga As P/（Al）Ga In P LDs with Ga In As P intermediate layer show a reduced operating voltage compared to the conventional LDs with abrupt Ga In P/Ga As interface due to the enhanced hole injection. As a result, the power conversion efficiency is improved from 52% to 60% at 350 m W output power. At high current injection, the LD with Ga In As P intermediate layer has higher light power owing to the decreased joule heating.