InGaN LEDs Research Articles

(Invited) Monolithic Integration of Si-CMOS and III-V-on-Si through Direct Wafer Bonding Process

Integration of Si-CMOS and III-V compound semiconductors (with device structures of either InGaAs HEMT, AlGaInP LED, GaN HEMT or InGaN LED) on a common Si substrate is demonstrated. The Si-CMOS layer is temporarily bonded on a Si handle wafer. Another III-V/Si substrate is then bonded to the Si-CMOS containing handle wafer. Finally, the handle wafer is released to realize the Si-CMOS on III-V/Si substrate. For GaN HEMT or LED on Si substrate, additional wafer bonding step is required to replace the fragile Si (111) substrate after high temperature GaN growth with a new Si (001) wafer to improve the robustness of the GaN/Si wafers. Through this substrate replacement step, the bonded wafer pair can survive the subsequent processing steps. The monolithic integration of Si-CMOS + III-V devices on a common Si platform enables new generation of systems with more functionality, better energy efficiency, and smaller form factor.

(Invited) Monolithic Integration of Si-CMOS and III-V-on-Si through Direct Wafer Bonding Process

Integration of Si-CMOS and III-V compound semiconductors (with device structures of either InGaAs HEMT, InGaP LED, GaAs HBT, GaN HEMT or InGaN LED) on a common Si substrate is demonstrated. The Si-CMOS layer is temporarily bonded on a Si handle wafer. Another III-V/Si substrate is then bonded to the Si-CMOS containing handle wafer. Finally, the handle wafer is released to realize the Si-CMOS on III-V/Si substrate. For GaN LED or HEMT on Si substrate, additional wafer bonding step is required to replace the fragile Si (111) substrate after high temperature GaN growth with a new Si (001) wafer to improve the robustness of the GaN/Si wafers. Through this substrate replacement step, the bonded wafer pair can survive the subsequent processing steps. The monolithic integration of Si-CMOS + III-V devices on a common Si platform enables new generation of systems with more functionality, better energy efficiency, and smaller form factor.

Low-frequency noise characteristics of high-power white LED during long-term aging experiment

Low-frequency electrical and optical noise characteristics of a high power white InGaN LED and the cross-correlation coefficient between these fluctuations are investigated during the long-term ageing experiment (28000 h). The analysis of cross-correlation shows that the correlated part of the low-frequency electrical noise, which indicates physical processes in the active area of the LED, varies during the ageing experiment. Two main stages are distinguished considering the rapid changes of the noise characteristics. The initial stage (after the first 1100 h) is followed by the decrease in the noise power at low forward currents and the increase in the light output. The final stage (after 28000 h) is characterized by a strong growth of the low-frequency noise, the decrease of the light output and the increase of the correlated part of electrical and optical fluctuations. These changes mean a failure in the active layer of the LED. The optical output decomposition into two spectral parts, the radiation generated by the LED chip and the radiation generated by the phosphor layer, enables evaluation of the phosphor layer influence on the degradation process of device optical characteristics. Despite the fact that this layer contributes to the variation of the LED chromatic properties, the cross-correlation analysis has shown that the phosphor layer does not act as a possible noise source during the ageing experiment.

Zinc phosphate glasses activated with Dy3+/Eu3+/Sm3+ and Tb3+/Eu3+/Sm3+ for reddish-orange and yellowish white phosphor applications

Spectroscopic evaluations of Dy3+/Eu3+/Sm3+ and Tb3+/Eu3+/Sm3+ doped zinc phosphate glasses, based on excitation and emission spectra, and emission decay measurements, are particularly focused on potential white light-emitting diodes applications. All the excitation wavelengths located in the 337–382 nm range, match with the emissions of AlGaN, GaN and InGaN LEDs. The Dy3+/Eu3+/Sm3+ doped zinc phosphate glass excited at 347 nm displays yellowish white tonality according with the x = 0.396 and y = 0.408 CIE1931 chromaticity coordinates and correlated color temperature (CCT) value of 3837 K, whereas under 362, 374 and 382 nm excitations, it displays reddish-orange tonality with CIE1931 chromaticity coordinates (and CCT values): x = 0.503 and y = 0.398 (2075 K), x = 0.570 and y = 0.388 (1640 K), and x = 0.527 and y = 0.386 (1804 K), respectively, with color purities higher than 72%. The Dy3+ and Sm3+ emission decay analysis suggests that non-radiative energy transfer processes from Dy3+ to Eu3+ and/or Sm3+ and Sm3+ to Eu3+ take place with efficiencies of 0.09 ± 0.04 and 0.15 ± 0.04, respectively. The Dy3+ and Sm3+ emission decay fitting by the Inokuti-Hirayama model, indicates that electric dipole-quadrupole and quadrupole-quadrupole interactions might respectively mediate the energy transfer processes inside the Dy3+-Sm3+-Eu3+ clusters. The Tb3+/Eu3+/Sm3+ doped zinc phosphate glasses only exhibits reddish-orange emission tonality with CIE1931 chromaticity coordinates and (CCT values): x = 0.510 and y = 0.425 (2210 K), x = 0.549 and y = 0.399 (1754 K), x = 0.510 and y = 0.411 (2108 K), and x = 0.544 and y = 0.385 (1710 K), under 337, 361, 374 and 380 nm excitations, respectively, with color purities higher than 79% in all cases. The Tb3+ and Sm3+ emission decay shortening in presence of Sm3+ and Eu3+, and Tb3+ and Eu3+, respectively, points out Tb3+→ Eu3+ and/or Sm3+ and Sm3+ → Eu3+ non-radiative energy transfers, with efficiencies of 0.08 ± 0.04 and 0.04 ± 0.05, respectively. The Inokuti-Hirayama model suggests that the Tb3+→ Eu3+ and/or Sm3+ and Sm3+ → Eu3+ energy transfer processes might be, respectively, dominated by electric dipole-quadrupole and quadrupole-quadrupole interactions inside Tb3+-Sm3+-Eu3+ clusters.

Efficient and stable green-emitting CsPbBr3 perovskite nanocrystals in a microcapsule for light emitting diodes

Fully inorganic lead halide perovskite materials have attracted tremendous attention because of their practical applications in optoelectronic devices, such as light emitting diodes. In this work, we report the first development of highly efficient, stable and green-emitting CsPbBr3 perovskite NCs with a PE microcapsule. The CsPbBr3 perovskite NCs were perfectly incorporated inside the PE microcapsule with a multi-lamellar structure, which shows better luminescence characteristics such as a high PLQY and thermal stability Furthermore, our results revealed the high luminous performance with a white conversion efficacy of 62.4 lm W−1, EQE of 31.8%, CRI of 37.01, and CCT of 6028 K on blue emitting InGaN LED chip. We suggest that our material will be one of the next generation candidate luminescence materials for application in optoelectronic devices.

Monolithic Integration of Si-CMOS and III-V-on-Si Through Direct Wafer Bonding Process

Integration of silicon-complementary metal oxide-semiconductor (Si-CMOS) and III-V compound semiconductors (with device structures of either InGaAs HEMT, AlGaInP LED, GaN HEMT, or InGaN LED) on a common Si substrate is demonstrated. The Si-CMOS layer is temporarily bonded on a Si handle wafer. Another III-V/Si substrate is then bonded to the Si-CMOS containing handle wafer. Finally, the handle wafer is released to realize the Si-CMOS on III-V/Si substrate. For GaN HEMT or LED on Si substrate, additional wafer bonding step is required to replace the fragile Si (111) substrate after high temperature GaN growth with a new Si (001) wafer to improve the robustness of the GaN/Si wafers. Through this substrate replacement step, the bonded wafer pair can survive the subsequent processing steps. The monolithic integration of Si-CMOS + III-V devices on a common Si platform enables new generation of systems with more functionality, better energy efficiency, and smaller form factor.

Weak thermal quenching of the luminescence in the Ca3Sc2Si3O12:Ce3+garnet phosphor

We report results of the luminescence properties of the three garnet type phosphors Ce3+-doped Ca3Sc2Si3O12(CSSO:Ce3+), Sr3Y2Ge3O12(SYG:Ce3+) and Y3Al5O12(YAG:Ce3+), investigated using optical spectroscopy techniques and vacuum referred binding energy (VRBE) diagram analysis.

Luminescent Properties of a Cs3AlF6:Mn4+ Red Phosphor for Warm White Light-Emitting Diodes

Mn4+ activated fluoride phosphors have been considered as highly promising red component for warm white light-emitting diodes (WLEDs) in recent years. In this work, a novel nonequivalent doping Cs3AlF6:Mn4+ red-emitting phosphor were synthesized via a co-precipitation method and its crystal structure, morphology, and luminescent properties were investigated in detail. The Cs3AlF6:Mn4+ phosphor with a sheet morphology have broad band absorption in blue light region (∼467 nm) and narrow bandwidth emissions in red region (∼634 nm). The WLED fabricated with Cs3AlF6:Mn4+ red phosphor, YAG:Ce3+ yellow phosphor and a blue InGaN LED chip demonstrates intense warm light with good luminescence performance of low related color temperature (CCT, Tc = 3659 K) and high color rendering index (CRI, Ra = 86.2). These results indicate the Cs3AlF6:Mn4+ red phosphor can be a potential candidate for warm WLEDs.

Synthesis and improved photoluminescence of hexagonal crystals of Li2ZrF6:Mn4+ for warm WLED application.

A series of red-emitting phosphors composed of Li2ZrF6:Mn4+ (LZF:Mn) have been synthesized via an ionic-exchange reaction route at room temperature. The microstructure and optical characterizations have been investigated according to the detailed experiments. The morphology of the phosphor LZF:Mn changes with the concentration of HF, the reaction time and temperature. The uniform crystals of LZF:Mn with regular hexagonal tablets have been obtained in 30-40 wt% HF solutions by conducting the reaction at room temperature for about 8 h. The influences of reaction parameters on the morphology and photoluminescence properties of LZF:Mn have been systematically investigated. The luminescence intensity of LZF:Mn has been optimized by controlling the synthesis procedure and parameters. The white light-emitting diode (WLED) fabricated with LZF:Mn and Y3Al5O12:Ce3+ (YAG:Ce) on InGaN LED chip displays a warm white light with correlated color temperature (CCT) at 3789.4 K and color rendering index (CRI) of 91.

Polarization-enhanced InGaN/GaN-based hybrid tunnel junction contacts to GaN p–n diodes and InGaN LEDs

Improved turn-on voltages and reduced series resistances were realized by depositing highly Si-doped n-type GaN using molecular beam epitaxy on polarization-enhanced p-type InGaN contact layers grown using metal–organic chemical vapor deposition. We compared the effects of different Si doping concentrations and the addition of p-type InGaN on the forward voltages of p–n diodes and light-emitting diodes, and found that increasing the Si concentrations from 1.9 × 1020 to 4.6 × 1020 cm−3 and including a highly doped p-type InGaN at the junction both contributed to reductions in the depletion width, the series resistance of 4.2 × 10−3–3.4 × 10−3 Ω·cm2, and the turn-on voltages of the diodes.

Threading dislocation density effect on the electrical and optical properties of InGaN light-emitting diodes

The electrical and optical properties of InGaN LEDs with different threading dislocation densities (TDDs) were investigated. LEDs with low TDDs exhibited low forward voltage and high injection efficiency compared with LEDs with high TDDs. The effect of TDDs on the electrical properties of InGaN LEDs was attributed to efficient carrier injection into the QWs brought about by increased transverse carrier mobility in the InGaN layer resulting from reductions in carrier scattering around dislocation cores. In terms of optical properties, LEDs with low TDDs exhibited high peak efficiency and substantial efficiency droops under increased current densities, whereas LEDs with high TDDs showed low peak efficiency and minimal droops under the same condition. These trends can be explained by the correlations of high TDDs with increased dominance of nonradiative recombination and pronounced suppression of peak efficiency under low current densities.

Investigation of sensing properties of sol–gel processed 4 at%Sb:SnO2/TiO2 thin films

In this work we investigate the gas and photo sensing properties of the antimony doped tin oxide and titanium oxide (4 at%Sb:SnO2/TiO2) nanocrystalline thin films deposited by sol–gel dip-coating. Photoconductivity measurements are carried out under solar light spectra irradiation at different powers. These results show a photo sensitivity of the films in a lateral junction due to interfacial defects. Gas sensitivity was studied at different pressures, and higher conductivity is presented at lower pressure compared to oxygen-rich atmosphere. It occurs due to absence of oxygen adsorption on the semiconductors surface. TiO2 films are also investigated concerning its properties to gas sensing under photo-excitation with InGaN LED light source with wavelength centered in 450 nm. The decay of photo-induced current evaluated under O2 and vacuum atmospheres shows that the sample illumination may contribute to higher gas-sensitivity. This measurement allows determining the charge carrier capture energy, that is related to trapping dominated by distinct defects in each atmosphere.

Characterisation of defects generated during constant current InGaN-on-silicon LED operation

We studied the degradation of MOCVD-grown InGaN LEDs on Si substrates under constant current stressing. Characterisations using Deep Level Transient Spectroscopy and Electron Energy Loss Spectroscopy on active areas showed that the stressing had generated defects that have trap states at 0.26eV below the conduction band edge (Ec – 0.26eV) and that correlated with the active area's lower nitrogen content as compared to unstressed samples. The combination of Current-Voltage, Electroluminescence, Cathodoluminescence, and device simulations indicate that an increase in the density of these defects is correlated with an increase in the non-radiative carrier recombination that causes degradation in light emission. Preventing formation of these defects will be critical for improving InGaN-on-silicon LED reliability.

White light emissive bipolar ligand and their Eu III complex for white/red light emitting diodes

Abstract Fluorene integrated phenanthro-imidazole (phenyl factionalized) bipolar ligand and their corresponding EuIII complex was successfully synthesized and studied their photophysical properties. The newly synthesized ligand shown strong absorption in the near UV region and emit white light by covering the whole visible region (400–700 nm). The corresponding Eu-complex (Eu(TTA)3Phen-Fl-Ph) shown red emission with strong electric dipole transition at 612 nm (5D0 → 7F2). Efficient energy transfer from ligand to Eu metal ion was confirmed by combined experimental and time-dependent density functional theory (TD-DFT). The absolute quantum yield (QY) of the complex found to be 57.3 (0.5%) in PMMA matrix. The ligand integrated with InGaN LED chip (395 nm, forward bias 20 mA) shown the potentiality of the ligand and displayed white emission. Whereas, the Eu-complex conjugated LED was shown red emission. The obtained results are indicating that the currently synthesized ligand and the complex were potential to be used as single molecular white emission and red emitting component LEDs, respectively.

Solid-state source of intense yellow light based on a Ce:YAG luminescent concentrator.

A luminescent concentrator functioning as a bright source of yellow light is reported. It comprises a waveguide made of cerium-doped YAG crystal, in the form of a long-thin rectangular strip, surrounded by flowing air and optically pumped from both sides with blue light from arrays of high-efficiency InGaN LEDs. Phosphor-converted yellow light, generated within the strip, is guided to a glass taper that is butt-coupled to one of the strip's end faces. Up to 20 W of optical power, centered on 575 nm with a linewidth of 76 nm, can be continuously radiated into air from the taper's 1.67 mm × 1.67 mm square output aperture. The intensity of the outputted light is significantly greater than what any yellow (AlGaInP) LED can directly produce (either singly or arrayed), with only a modest increase in linewidth. Furthermore, the wall-plug efficiency of the source exceeds that of any yellow laser. The concept allows for further substantial increases in intensity, total output power and wall-plug efficiency through scaling-up and engineering refinements.

Impact of carrier localization and diffusion on photoluminescence in highly excited cyan and green InGaN LED structures

Localization of charge carriers is of crucial importance in InGaN light emitting devices since it governs carrier transport and ensures high radiative efficiency. In this work, we observe the dynamics of carrier localization from an untypical redshift of photoluminescence spectra. We investigate two light emitting diode structures grown by MOCVD on c-plane sapphire and emitting at 500nm (cyan) or 530nm (green). For the study, we employ photoluminescence, differential transmission, and light induced transient gratings techniques. We observe non-monotonous dynamics of photoluminescence peak position: a blueshift at short delay times (<10ns) is later replaced by a redshift, which increases both with time and excitation. The redshift values as large as 30meV in the cyan and 20meV in the green structure were observed at 40ns delay. We attribute this redshift to density-dependent and diffusion-driven carrier redistribution between the shallower and deeper localized states. The carrier delocalization with increasing density is confirmed by growing diffusivity of carriers with excitation. We also demonstrate a correlation between the growth of diffusion coefficient and the onset of efficiency droop.

A chromaticity-tunable white LED by screen-printing red phosphor coating on PiG plates

Currently, phosphor-in-glass (PiG) approach draws great attention because of its excellent thermal resistance and facile process in w-LED fabrication. However, the absence of red emissive component results in poor chromaticity quality, which greatly restricts its application in the high-quality indoor lighting. In the present work, a transparent garnet-based PiG was successfully prepared by introducing the Y3Al4.5Ga0.5O12:Ce3+(YAGG) green phosphor into TeO2-based glass matrix, and we demonstrated the possibility offered by the screen-printing technique for warm w-LED applications by coupling the PiG plate stacked with a red phosphor coating on commercial InGaN LED. The microstructures and luminescent properties of them were investigated in detail. After simply varying the red phosphor concentration as well as the YAGG PiG thickness, a facile chromaticity tuning for the thus-fabricated LED was achieved in the range changing from cool white to warm white. Moreover, this developed luminescent material presented an excellent heat-resistance performance. Hopefully, the red phosphor coated PiG color converter is applicable in the long-lifetime high-power color-tunable w-LEDs.

Experimental observation of TDDB-like behavior in reverse-biased green InGaN LEDs

This paper reports the outcome of a series of reverse-bias experiments performed on commercial GaN-based green LEDs. The experimental results showed that green LEDs submitted to reverse bias i) show a time-dependent failure when they are submitted to constant (reverse) voltage stress, at a bias point smaller than the BDV; ii) experience an increase of the reverse-bias electro-luminescence signal, well-correlated with the increase of the reverse leakage current; iii) the TTF (Time-To-Failure) related to the time-dependent breakdown process has an exponential dependence on stress voltage; iv) the TTF is Weibull distributed. This work provides the first experimental demonstration of time-dependent failure of GaN LEDs.

Investigation of blue InGaN light-emitting diodes with gradual wide wells and thin barriers

In this study, the blue InGaN LEDs, employing gradual wide wells and thin barriers, is proposed. The carrier distribution, hole injection efficiency, electron leakage, radiative recombination rates, and Auger recombination rates are investigated numerically. The simulation results show that the newly designed LEDs have better performance over its conventional counterpart due to the successful enhancement of hole injection efficiency and suppression of Auger recombination. The simulation results also suggest that under relatively high current injection, the internal quantum efficiency (IQE), efficiency droop and light output power are markedly improved.

Evaluation of the quality of green InGaN LEDs by values of the threshold current

The article shows the possibility of using the threshold current for evaluating the quality of green InGaN/GaN LEDs. It was determined that the current threshold correlated with the position of the maximum of the current dependence of external quantum efficiency, and a concentration gradient of charge carriers in the heterostructure.