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Reliability of III-V laser diodes and LEDs for lighting and telecommunication applications
Full text linkThe research in optoelectronics has recently shown impressive advancements, thanks to the development of disruptive technologies, such as Gallium Nitride LEDs and lasers - that are enabling novel lighting devices - and Silicon photonics - that will change the paradigm in broadband data transmission. Industrial laboratories and academic researchers are working towards the improvement of device performance and reliability. This goal is complicated by the complex nature of optoelectronic devices and systems: a deep knowledge on material properties, device characteristics and system optimization is needed to identify the physical mechanisms that limit the lifetime of the devices, and to address possible solutions. The aim of this thesis is to report on a detailed study of the physical factors that limit the performance and reliability of innovative optoelectronic technologies based on nitrides and arsenides. The investigation was conducted by means of purposely planned Accelerated Life Test (ALT) experiments and short-term stress tests on selected state-of-the-art devices, which let us identify the main physical processes responsible for the long-term degradation of the devices, and pinpointed the weaknesses of modern LEDs when temporary operated outside their Safe Operating Area (SOA) as a consequence of Electrical Over-Stress (EOS) events.
With regard to Gallium Nitride (GaN)-based white and visible monochromatic LEDs, we found that under constant near-breakdown reverse bias the devices exhibit a time-dependent degradation phenomenon, which promotes the increase of the leakage current of the device, eventually leading to its catastrophic failure. The peculiar failure statistics, as well as the electro-luminescence and leakage-current data acquired during stress, could be interpreted by considering that highly-depleted GaN under reverse bias may behave like a partially-leaking dielectric that degrades over time due to a defect percolation process, similar to the dielectric degradation-driven breakdown process affecting Silicon MOS devices.
On the other hand, high forward-current short-term step-stress on high-power LEDs revealed that sustained operation at driving currents above the maximum rating of the devices can rapidly induce failure in correspondence of a major current injection point, dependent on the specific chip structure, as a consequence of the localized power dissipation and temperature reached due to extreme current crowding effects and to the degradation of the conductivity of extended device regions.
A clear dependence of the failure mode on the chip structure and device layout could also be found during the investigation on the effects of single short EOS events of increasing amplitude on high-power white LEDs. The experimental data helped identifying the best LED design to be employed in an electrically critical environment, also showing that EOS-related reliability issues tend to arise more from extrinsic elements of the LED system rather than from the semiconductor chip.
Unlike High-Brightness (HB) state-of-the-art LEDs, whose main reliability concern is represented by EOS events, cost-effective mid-power LEDs for lighting applications were found to suffer from gradual degradation processes impacting in the long term on both the electrical an optical characteristics of the device. Moreover, the results of the ALTs highlighted the role of the plastic package in the degradation of the optical properties of the emitted light.
The long-term reliability of mid-power LEDs was further investigated at system level by performing a lifetime analysis of commercial LED bulbs employing these devices as primary light sources. The increased complexity of the system under stress negatively impacted on the stability over time of the luminous performances of the luminaries, which was severely affected by the degradation of extrinsic elements like the diffusing dome or the current driver.
In Part II of this work, our system-level analysis continued with an extensive investigation on the reliability of blue-emitting phosphors for near-UV laser excitation, as part of a research project performed in collaboration with the New Energy and Industrial Technology Development Organization (NEDO), Japan. By means of a series of pure thermal stress experiments and stress under high levels of optical excitation, we have been able to identify the physical process responsible for the degradation of the phosphors under extreme and more conservative operating conditions. In particular, while the phosphors demonstrated good stability during pure thermal treatment in air up to 300 °C, for temperatures equal to or greater than 450 °C the material exhibited a time-dependent drop in the photo-luminescence, which was attributed to the thermally-induced autoionization of the Eu2+ optically active centers. By means of different material characterization techniques, evidence of this degradation process was also found on samples stressed under moderate 3 W/mm2 – 405 nm optical excitation. This indicated that the optically (and thermally) induced ionization of the optically-active species is the most critical degradation process for this family of phosphorescent material.
The operating limits of an improved second generation luminescent material were also investigated by means of short term stress under 405 nm optical excitation. The experimental data showed that, for a given deposition condition, a threshold excitation intensity for continuous pumping exists. Above this threshold, decay of the steady-state photo-luminescence performances and degradation of the material were found to take place, which suggested that the material was being operated in an unuseful excitation regime, mainly limited by the thermal management capabilities of the carrier substrate employed for our experimental purposes rather than from intrinsic properties of the phosphor.
Part III of this thesis is devoted to the analysis of the degradation processes of heterogeneous III-V/Silicon infrared (IR) laser diodes designed for integrated telecommunications and interconnects. By submitting the devices to a series of constant current stress tests, a gradual degradation of the main device parameters was observed. In particular, in every stress scenario the devices under test showed (i) an increase in the threshold current, (ii) a decrease in the turn-on voltage, and (iii) an increase in the apparent carrier concentration within the space charge region. The variation of the electrical parameters was found to be significantly correlated to the optical degradation for long stress times; the results support the hypothesis that degradation originates from an increase in the non-radiative recombination rate, possibly due to the diffusion of defects towards the active region of the devices.
In order to further investigate the physical origin of the diffusing defects, capacitance Deep-Level Transient Spectroscopy (C-DLTS) analysis was performed. The results indicate the presence of several deep levels, with a main trap located around 0.43 eV above the valence band energy. This trap was found to be compatible with an interface defect characteristic of the quaternary material employed to grow the active region of the device
EOS-related failures of modern high-brightness white LEDs: failure limits and correlation with device structure
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Analysis of Current Transport Layer Localized Resistivity Increase After High Stress on InGaN LEDs
Full text linkThis work reports on the effects of high current stress on InGaN-based light-emitting diodes (LEDs.) The analyzed devices are high power blue LEDs with an emitting area of 1 mm2 subjected to a constant current stress at increasing current until the catastrophic device failure. By means of optical, electrical and photoluminescence analysis we hypothesize that the optical power decrease is related to a degradation in the current transport layers of the devices. The effect has been modelized as a gradual and localized increase in the current transport layer resistance; the model has been validated by means of SPICE electrical simulations. The results clearly indicate that, to extend the operating range of high-power devices, it is crucial to control the current crowding effect, which is here considered to play a significant role in the degradation in the current transport layer
Long-term degradation mechanisms of mid-power LEDs for lighting applications
No full textThis paper reports the results of a reliability investigation performed on four different groups of commercially available mid-power white LEDs. In order to determine the robustness of this continuously growing class of lighting devices we arranged an experiment of accelerated aging: the four groups of LEDs (from different manufacturers) were submitted to a series of stress tests in environmental chambers with set-point temperatures ranging from 45 °C to 105 °C, in accordance to the IES LM-80-08 lumen maintenance measurement standard. The experimental data gathered all along the 4000 h of stress accumulated up to now suggest the presence of multiple degradation mechanisms that may limit the useful lifespan of the light-emitting diodes under test. In particular we observed the following phenomena: i) a decay of the luminous flux; ii) an increase in the reverse and forward leakage current; iii) the worsening of the chromatic properties of the emitted light; and iv) the presence of a thermally activated degradation mechanism. The results provide a first insight into the reliability of those widely used LEDs; the results on the temperature-dependence of the degradation kinetics can be used as a guideline for the thermal design of modern distributed-light lamps. © 2015 Elsevier Ltd
Experimental Demonstration of Time-Dependent Breakdown in GaN-Based Light Emitting Diodes
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Injection-limited efficiency of InGaN LEDs and impact on electro-optical performance and ageing: a case study
No full textThis paper investigates the factors that influence the efficiency and reliability of InGaN based visible light emitting diodes with non-optimized carrier injection. The devices under test are LEDs with a single quantum well with a nominal emission wavelength of 495 nm at I = 100 mA. We stressed the devices with a constant current density of J = 80 A/cm2, at room temperature, for 25000 minutes. We monitored the optical performance of the devices before and during stress. From the preliminary characterization we observed an increment in the optical power followed by a blue shift as a function of current. Simulation results highlight an asymmetric carrier injection, in particular a lack of electrons in the low bias regime. The low injection efficiency is also confirmed by temperature-dependent measurements, where we observed an increment in the OP with increasing temperature. During an ageing experiment, we observed an increment in OP for high injection level, accompanied with a blue shift in the peak emission wavelength. This result suggests an improvement in the injection efficiency and or a better carrier confinement. In this regard, we performed photoluminescence measurements during stress, which confirm the hypothesis of a better carrier confinement. In particular, PL signal show an increasing trend during the ageing process, which can be ascribed to the generation of negatively charged defects in the quantum barrier, with consequent impact on carrier confinement
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