Degradation Of Optical Power Research Articles

Study on the Degradation Performance of AlGaN-Based Deep Ultraviolet LEDs under Thermal and Electrical Stress

AlGaN-based deep-ultraviolet (DUV) LEDs could realize higher optical power output when adopting a p-AlGaN contact layer instead of a p-GaN contact layer. However, this new type DUV LEDs exhibit poor reliability. Thus, this study thoroughly investigates the degradation behaviors of AlGaN-based DUV LEDs with a p-AlGaN contact layer through different aging tests, including single thermal stress, single electrical stress with air-cooling, single electrical stress, and thermoelectric complex stress. It can be found that both high temperature and large working current play crucial roles in accelerating the degradation of optoelectronic properties of the DUV LEDs, and the single high thermal stress without electrical stress can also bring obvious performance degradation to the DUV LEDs, which is a significantly different finding from previous studies. This is because thermal stress on DUV LED could bring some metal electrode elements entering the p-AlGaN layer. Thus, the degradation of optical and electrical properties under the thermal and electrical stress could be not only attributed to the degradation of the device’s ohmic contacts, but also due to the metal electrode elements entering the p-AlGaN layer through thermal diffusion, leading to the generation of tunneling current and the generation of defects within or around the active region. Despite that the peak wavelengths of the DUV LEDs remained stable, the turn-on voltage and series resistance increased. Particularly worth mentioning is that the value of the optical power degradation under thermoelectric conditions is larger than the sum of the single thermal and single electrical optical power degradation, which is a result of the mutual reinforcement of thermal and electrical stresses to exacerbate the defect generation and ohmic contact degradation. Based on the study above, preparing p-AlGaN layers with hyperfine gradient aluminum fractions and reducing the junction temperature may help to improve the reliability of AlGaN-based DUV LEDs with the p-AlGaN contact layer.

Degradation- and Thermal-Related Changes in Selected Electro-Optical Parameters of High-Power 270–280 nm LEDs

Recently, the rapid development of LED sources emitting high-power radiation in the UVC range has been observed, and there is a growing interest in using these LED sources in practical solutions. The innovative constructions of disinfection and sterilization devices depend on the effectiveness and reliability of UVC radiation sources. At the same time, the literature reports that deep experimental analysis of degradation of high-power LEDs is limited. The aim of this research is to contribute to existing knowledge through a comparative assessment of the changes in optical power, spectral power distribution, and forward voltage drop in time and temperature of exemplary high-power UVC LEDs. For this purpose, a controlled 1500 h degradation of six different high-power UVC LEDs was performed, based on which we determined their expected lifetimes L70, L80, and L90. According to our results, the L80 varies from 180 h to 1500 h. Stronger degradation of optical power was observed with lower current. No significant impact on the spectral parameters was observed. The results also indicate the low influence of temperature on the voltage (<0.12%/°C), optical power (<0.22%/°C), and spectral parameters (peak wavelength Δλ and full width at half maximum ΔFWHM < 0.025 nm/°C).

Optical power degradation mechanisms in 271 nm AlGaN-based deep ultraviolet light-emitting diodes.

The degradation of AlGaN-based UVC LEDs under constant temperature and constant current stress for up to 500 hrs was analyzed in this work. During each degradation stage, the two-dimensional (2D) thermal distributions, I-V curves, optical powers, combining with focused ion beam and scanning electron microscope (FIB/SEM), were thoroughly tested and analyzed the properties and failure mechanisms of UVC LEDs. The results show that: 1) the opto-electrical characteristics measured before/during stress indicate that the increased leakage current and the generation of stress-induced defects increase the non-radiative recombination in the early stress stage, resulting in a decrease in optical power; 2) the increase of temperature caused by the deterioration of the Cr/Al layer of p-metal after 48 hrs of stress aggravates the optical power in UVC LEDs. The 2D thermal distribution in conjunction with FIB/SEM provide a fast and visual way to precisely locate and analyze the failure mechanisms of UVC LEDs.

The model of degradation of an InGaN/GaN LED during current tests taking into account the inhomogeneous distribution of the defects density in the heterostructure

A model of the optical power degradation of an InGaN/GaN LED during testing under direct current, which takes into account the inhomogeneous distribution of the defects density in the heterostructure, is presented. According to the simulation results, the rate of degradation of the LED optical power significantly depends on the degree of inhomogeneity of the defects density distribution profile. Experimental testing of the model has been carried out. The proposed model makes it possible to predict the rate of degradation of InGaN-based LEDs with varying degrees of inhomogeneity of the defects density distribution profile and can be used to develop a technique for rejecting defective and potentially unreliable LEDs.

Модель деградации InGaN/GaN светодиода при токовых испытаниях с учетом неоднородного распределения температуры и плотности тока в гетероструктуре

A diffusion kinetic model of the optical power degradation of an LED based on a double InGaN/GaN heterostructure during direct current tests is presented. According to the model, the main process causing a decrease in the optical power of the LED is the diffusion of Mg impurity atoms from the p-layer barrier of the heterostructure into the active region. The model takes into account the effect of non-uniform current distribution due to non-uniform heating of the chip by high-density current and can be used to predict the lifetime of the LED when operating in continuous and pulsed mode.

Improved Reliability of InGaN-Based Light-Emitting Diodes by HfO2 Passivation Layer.

We utilized a passivation layer to improve the leakage current and reliability characteristics of GaN-based light-emitting diodes. The electrical and optical characteristics of the fabricated LEDs were characterized by current-voltage and optical power measurements. The HfO2 passivation layer showed no optical power degradation and suppressed leakage current. The low deposition temper- ature of sputtered HfO2 is responsible for the improved reliability of the LEDs because it suppresses the diffusion of hydrogen plasma into GaN to form harmful Mg-H complexes.

Pulse Amplitude and Delay Modulation: Design and performance analysis

Power efficient modulation techniques have previously been proposed to provide the uplink in visible light communication systems. However, such techniques have poor bandwidth utilization as multiple bits are mapped to single narrow pulse. When the bandwidth is limited, it has been found that degradation in optical power becomes very high and data rate poor. In this paper we introduce a new modulation technique called Pulse Amplitude and Delay Modulation (PADM). We compare its performance with Dual Header Pulse Interval Modulation (DH-PIM) that has the best reported bandwidth efficiency. Experiment results show that the data rate could be enhanced from 3.2kps to 4.3kbs using a red link (640nm) under same error rate. This suggests PADM has better bandwidth efficiency than DH-PIM.

Research on Useful Life of Low Power LED Illumination Sources

This paper reports experimental and simulation studies on degradation of optical power of low power LEDs due to current stress and temperature variation. The goal of this study was to study the effect of aging and ambient temperature on optical output power of light emitting diodes and to determine the effective lifespan of LED under stress condition. The LED samples were stressed at four levels of operating current 20mA, 30mA, 40mA, and 45mA with stress time up to 1000h. We find that degradation rate increases with increasing operating current. When LEDs are overstressed by driving at higher current the optical power degraded with stress time. A Mathematical model is developed to predict the behavior of LED at different operating current. Simulations are carried out with this model using MATLAB. Using degradation pattern at higher operating current the lifespan of LEDs was predicted at normal operating current.

Equivalence of displacement radiation damage in superluminescent diodes induced by protons and heavy ions

The degradation of optical power for superluminescent diodes is in situ measured under exposures of protons with various energies (170keV, 3MeV and 5MeV), and 25MeV carbon ions for several irradiation fluences. Experimental results show that the optical power of the SLDs decreases with increasing fluence. The protons with lower energies cause more degradation in the optical power of SLDs than those with higher energies at a given fluence. Compared to the proton irradiation with various energies, the 25MeV carbon ions induce more severe degradation to the optical power. To characterize the radiation damage of the SLDs, the displacement doses as a function of chip depth in the SLDs are calculated by SRIM code for the protons and carbon ions. Based on the irradiation testing and calculation results, an approach is given to normalize the equivalence of displacement damage induced by various charged particles in SLDs.

GaN基蓝绿光LED电应力老化分析

The behavior of the related reliability of InGaN-based blue and green LED chips were investigated with 20,40,60 mA constant current stress up to 424 h at room temperature. Under 60mA constant current stress,the green LED chip showed more prominent optical power degradation at low measuring current levels than high measuring current levels. While,the blue LED chip showed the consistent optical power decrease at different measuring current levels. It should be pointed out that,at high measured current levels( 20 mA),blue and green LED chips expressed similar optical degradation( 18%) after the aging tests. Meanwhile,the aging stress does not significant affect the forward-bias electrical characteristics of blue and green LED chips,which indicated the similar degradation process in some aging tests. It is believed that the induced defects in blue LED by electrical stress mainly contribute to the nonradiative recombination centers,while in green LED chip the defects contribute to the localized leakage paths rather than the nonradiative recombination centers.Based on the mechanism analysis of blue and green LED,the design of GaN based epitaxial structure was optimized,and the thickness of quantum barrier was reduced to weaken the internal electrical field in the active layer to improve the reliability of LED.

Failure Analysis of InGaN/GaN High Power Light-Emitting Diodes Fabricated with ITO Transparent <I>p</I>-Type Electrode During Accelerated Electro-Thermal Stress

We have investigated the high-temperature degradation of optical power as well as electrical properties of InGaN/GaN light-emitting diodes (LEDs) fabricated with ITO transparent p-electrode during accelerated electro-thermal stress. As the thermal stress increased from 150 degrees C to 250 degrees C at a electrical stress of 200 mA, the optical power of the LEDs was significantly reduced. Degradation of the optical power was thermally activated, with the activation of 0.9 eV. In addition, the activation energy of the degradation of optical power was fairly similar to that of the degradation of series resistance of the LEDs, 1.0 eV, which implies that the increase in the series resistance may result in the severe degradation of optical power. We also showed that the increase in the series resistance of the LEDs during the accelerated electro-thermal stress can be attributed to reduction of the active acceptor concentration in the p-type semiconductor layers and local joule heating due to the current crowding.

Experimental and simulation studies on useful life of low power LED illumination sources

This paper reports experimental and simulation studies on degradation of optical power of low power LEDs due to current stress and temperature variation. The goal of this study was to study the effect of aging and ambient temperature on optical output power of light emitting diodes and to determine the effective life span of LED under stress condition. The LED samples were stressed at four levels of operating current (recommended current 20 mA) 20 mA, 30 mA, 40 mA, and 45 mA with stress time up to 1000 h. We find that degradation rate increases with increasing operating current. When LEDs are overstressed by driving at higher current the optical power degraded with stress time. A Mathematical model is developed to predict the behavior of LED at different operating current. Simulations are carried out with this model using MATLAB. Using degradation pattern at higher operating current the life span of these LEDs was predicted at normal operating current. To verify the simulation results we have used LED life data from different research papers. The authors believe that the model is useful for predicting working life of AlGaInP /AlGaAs / GaAs / GaInN / InGaAsP / InP LED.

Secondary ions mass spectroscopy measurements of dopant impurities in highly stressed InGaN laser diodes

We performed a systematic secondary ions mass spectroscopy (SIMS) study of dopant impurities in life-time stressed InGaN laser devices in order to investigate the main degradation mechanism that is observed in nitride laser diodes. A continuous wave (cw) current density of 3 kA/cm2 was applied to InGaN laser diodes over an extended period of time and we observed the characteristic square root degradation of optical power. We compared the SIMS profiles of Mg, H, and Si impurities in the aged devices and observe that the impurities are remarkably stable over 10 000 h of cw operation. Nor is there any SIMS evidence of p-contact metals penetrating into the semiconductor material. Thus our SIMS results are contrary to what one would expect for impurity diffusion causing the observed square root degradation characteristic.

Design of copper/carbon-coated fiber Bragg grating acoustic sensor net for integrated health monitoring of nuclear power plant

A nuclear power plant (NPP) is a harsh environment that gives rise to age-related degradation of the plant structures, and eventually leads to radiation leakage that threatens humans. Integrated structural health monitoring (ISHM) technology is a strong candidate for the prevention of the NPP accidents during operation. Prior studies have shown that fiber Bragg gratings (FBGs) and metal-coated fibers have good radiation and high temperature resistance. In this study, a FBG acoustic sensor using a metallic adhesive for installation and a relatively economical copper/carbon (Cu/C)-coated fiber is developed for ISHM of high temperature NPP structures. A chemical method is proposed to remove the Cu/C coating. A 5 mm FBG was successfully inscribed in a Ge-doped silica core through a 7 mm-long silica section with the coating removed. The Cu/C-coated fiber with the same core/clad structure as the standard SMF allowed no-loss fusion splicing, and showed good adaptability to the economical standard fiber, adaptor, connector, and instruments. It showed also good thermal resistance (<345 °C) with no degradation in optical power during the optical transmission. The metallic adhesive used to install the FBG in a one-end-free configuration showed superior bonding reliability during temperature cycles ranging from 25 °C to 345 °C. The FBG reflectivity was stabilized at a 58% drop from the initial reflectivity, and the Cu/C-coated FBG sensor using the metallic adhesive could successfully detect the acousto-ultrasonic waves generated by pencil lead breaking and laser beam excitation.

The effect of proton radiation on a superluminescent diode (SLD)

The effect of proton radiation on a superluminescent diode (SLD) was studied, and the radiation damage from different energies was compared. The results reveal that the optical power degradation is greater from 350 KeV protons than from 1 MeV protons. Analysis of SLD characteristics after irradiation shows that the main effect of radiation is damage within the active region. At the same time, the results also show that quantum-well (QW) SLDs are far less sensitive to radiation than double-heterojunction (DH) SLDs.

Improvement in performance of GaN-based light-emitting diodes with indium tin oxide based transparent ohmic contacts

InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) with indium tin oxide (ITO) and Ni/Au p-contacts were fabricated. ITO (500 nm) and Ni/Au (2 nm/9 nm) films were deposited onto p-GaN epitaxial layers by an e-beam evaporation system. For the LEDs using in situ annealed ITO and Ni/Au films as p-contacts, the forward voltage at 20 mA was 3.5 and 3.2 V, respectively. Under the same amount of injection current, the LED with in situ annealed ITO p-contact had higher output electroluminescence (EL) intensity and larger light output power. The EL intensities and the light output power of ITO LEDs were enhanced by 85% and 60%, respectively, at 20 mA. As a result, the light output and power conversion efficiency of ITO LEDs on GaN were greatly improved at high injection currents. The fabricated LEDs were subjected to a stress test at 30 mA and 55 °C and showed a very small degradation of optical power (<1% decrease) for 24 h. The light output of MQW LEDs keeps 80% of the original value after 1000 h stressing. Therefore, the fabricated LED devices have demonstrated a good reliability.

Optical power degradation mechanisms in AlGaN-based 280nm deep ultraviolet light-emitting diodes on sapphire

We present a study of reliability of AlGaN-based 280nm deep ultraviolet light-emitting diodes on sapphire substrate grown by migration-enhanced metal-organic chemical vapor deposition. Two modes of optical power degradation were observed: catastrophic and gradual. The catastrophic degradation is believed to be due to metal alloying at macroscopic defects in the top p layers of the light-emitting diode structure. For the gradual power degradation, two time constants were determined, which were temperature and bias dependent. For the temperature-dependent part, the values of the activation energies and room-temperature degradation rates at dc currents of 10 and 20mA were determined to be 0.23 and 0.27eV and 1.31×10−3and5.93×10−3h−1, respectively.

Luminescence degradation of InGaN/GaN violet LEDs

The changes of optical output power and electrical property of InGaN/GaN violet light-emitting diodes (LEDs) with different indium compositions have been investigated, under injection current densities of 26.5 and 79.5 A/cm 2. When injection current density was 26.5 A/cm 2, output power of LEDs with low indium composition (2%) degraded more rapidly than the ones of 10% indium inside. At a large injection level of 79.5 A/cm 2, the difference in optical power degradation between LEDs of 10% and 2% indium composition became small. A discussion based on the results of current–voltage curves , L– I curves, as well as optical power degradation with stress time indicates that LEDs with 10% indium composition have better performance than LEDs with 2% indium composition. It is believed that the indium enhancement of radiative recombination helps preventing defect generation during electrical stress.

High-power and reliable operation of vertical light-emitting diodes on bulk GaN

In Ga N ∕ Ga N light-emitting diodes (LEDs) with lateral and vertical geometries have been fabricated on free-standing GaN substrates. Current spreading was significantly enhanced in the vertical LED, leading to a reduced series resistance of 7Ω compared to 12.2 and 14.2Ω for the lateral LEDs on GaN and sapphire, respectively. As a result, the light output and power conversion efficiency of the vertical LED on GaN were greatly improved at high injection currents. The vertical LED was subjected to a stress test at 400mA and showed minimal degradation of optical power, whereas the same stress resulted in the destruction of the lateral LED on sapphire due to increased current crowding and self-heating. However, lateral LEDs on sapphire with optimized current spreading exhibited excellent reliability, indicating the presence of a high density of dislocations (∼109cm−2) in the heteroepitaxial device does not accelerate LED degradation at current densities up to 700A∕cm2.

Annealing effects on multi-quantum well laser diodes after proton irradiation

The threshold current of multi-quantum well laser diodes increased by about 1.5 mA at a proton fluence of 6/spl times/10/sup 12/ p/cm/sup 2/. It recovered gradually due to forward-bias annealing at I/sub bias/=35 mA and became about 0.8 mA less than before irradiation after 86 h of forward bias annealing. At the same time, the optical power at a given bias became greater than before irradiation. There is a fast recovery during the first 100 s after proton irradiation. Increasing the forward bias current during annealing speeds up the annealing process. The degradation of optical power at a proton flux of 1.87/spl times/10/sup 9/ p/cm/sup 2//s (4.7% at a fluence of 6/spl times/10/sup 12/ p/cm/sup 2/) is less than that at a proton flux of 1.45/spl times/10/sup 10/ p/cm/sup 2//s (10.5% at a fluence of 6/spl times/10/sup 12/ p/cm/sup 2/) due to the in-situ forward-bias annealing effects.