Giovanna Mura

A Review on the Physical Mechanisms That Limit the Reliability of GaN-Based LEDs

We review the failure modes and mechanisms of gallium nitride (GaN)-based light-emitting diodes (LEDs). A number of reliability tests are presented, and specific degradation mechanisms of state-of-the-art LED structures are analyzed. In particular, we report recent results concerning the following issues: 1) the degradation of the active layer induced by direct current stress due to the increase in nonradiative recombination; 2) the degradation of LEDs submitted to reverse-bias stress tests; 3) the catastrophic failure of advanced LED structures related to electrostatic discharge events; 4) the degradation of the ohmic contacts of GaN-based LEDs; and 5) the degradation of the optical properties of the package/phosphors system of white LEDs. The presented results provide important information on the weaknesses of LED technology and on the design of procedures for reliability evaluation. Results are compared with literature data throughout the text.

Accelerated Life Test of High Brightness Light Emitting Diodes

Short-term accelerated life test activity on high brightness light emitting diodes is reported. Two families of 1-W light-emitting diodes (LEDs) from different manufacturers were submitted to distinct stress conditions: high temperature storage without bias and high dc current test. During aging, degradation mechanisms like light output decay and electrical property worsening were detected. In particular, the degradation in light efficiency induced by thermal storage was found to follow an exponential law, and the activation energy of the process was extrapolated. Aged devices exhibited a modification of the package epoxy color from white to brown. The instability of the package contributes to the overall degradation in terms of optical and spectral properties. In addition, an increase in thermal resistance was detected on one family of LEDs. This increase induces higher junction temperature levels during operative conditions. In order to correlate the degradation mechanisms and kinetics found during thermal stress, a high dc current stress was performed. Results from this comparative analysis showed similar behavior, implying that the degradation process of dc current aged devices is thermal activated due to high temperatures reached by the junction during stress. Finally, the different effects of the stress on two families of LEDs were taken into account in order to identify the impact of aging on device structure.

High temperature electro-optical degradation of InGaN/GaN HBLEDs

This paper presents a study of the high temperature degradation of high brightness light emitting diodes (HBLEDs) on gallium nitride. Two different families of devices, from two leading manufacturers, have been submitted to thermal stress: during treatment, the optical and electrical characteristics of the devices have been analyzed. Degradation modes detected after stress have been (i) operating voltage increase, (ii) output power decrease, (iii) modifications of the spectral properties. The degradation of the electrical and optical characteristics of the devices were found to have different kinetics: this fact indicates that optical power (OP) loss is not strongly related to the degradation of the electrical parameters of the LEDs. On the other hand, spectral analysis indicated that OP loss is strongly related to the decrease of the phosphors-related yellow emission band. Microscopic analysis showed that this effect can be ascribed to the carbonization of the package and phosphorous material. A degradation of the transparency of the top-side ohmic contact has been also detected after stress: these mechanisms are thought to be responsible for the detected OP decrease. OP decay process has been found to be thermally activated, with activation energy equal to 1.5 eV.

Failure Analysis-assisted FMEA

In this paper, ordinary FMEA (Failure Mode and Effects Analysis) was applied during the design phase of an electric motor control system for vehicle HVAC (Heating/Ventilation/Air Conditioning). The analysis of the field data from the second year forces to review FMEA. The corrective actions, planned on the basis of the sole failure mode, as usual in FMEA, proved to be inadequate and Failure Analysis was performed to understand the failure mechanism of the indicted component and integrate. New proper corrective actions were devised and successfully implemented.

Thermal stability analysis of high brightness LED during high temperature and electrical aging

In this paper we report the analysis of thermal stability of High Brightness Light Emitting Diode subjected to thermal and bias ageing. The degradation mechanisms of several families of commercial available devices were investigated. In the first part of the work we estimated thermal resistance and thermal behaviour under dc bias condition. After this thermal characterisation two different ageing tests were carried out on devices: thermal aging at high temperature levels without biasing the devices and accelerated dc stress at nominal current value (400mA). At each step a complete electrical and optical characterisation of aged devices was performed, in order to find a correlation between different aging and a better understanding of degradation mechanism. This characterisation included I-V measurements, optical power vs current characteristics and spectral analysis. During thermal stress we observed the increase of forward voltage at nominal current and the degradation of optical power with nearly exponential kinetics. We found that lifetimes were well correlated with stress temperature: therefore it was possible to find an activation energy of degradation mechanism of about 1.5eV. Moreover, modifications of spectral properties during electrical and thermal stress were found. Thus, a package level analysis was carried out in order to clarify the role of modification in optical properties of reflector cup and the efficiency of phosphors. Finally, evaluation of differential structure functions indicated that stress induces also the worsening of the properties of the chip-to-package thermal path: this phenomenon has been attributed to the partial detachment and degradation of the ohmic contacts.

Failure modes and mechanisms of DC-aged GaN LEDs

This paper presents failure modes observed in long-term aging of high-brightness GaN/InGaN LEDs. The blue LEDs submitted to DC aging test present large decrease of emitted optical power and increase of diode reverse leakage current. Increase of parasitic series resistance, suggesting contact degradation, has also been found in stressed sample, together with apparent carrier density increases and reduction of the junction depletion width. Furthermore stressed LEDs present modification of a specific trap property: trap activation energy decreases from 340 meV in the virgin sample down to 75 meV in the stressed sample. Generation of non-radiative recombination centers seems to be one of the dominant failure mechanisms responsible for the observed electrical and optical LED degradations.

Chip and package-related degradation of high power white LEDs

With this paper we present an analysis of the degradation of state-of-the-art high power LEDs. Three different kinds of commercially available samples, from leading manufacturers, were submitted to stress under various current and temperature levels. Based on an accurate estimation of the thermal resistance of the devices, iso-thermal and iso-current stress tests have been carried out, with the aim of separately evaluating the role of current and temperature in determining the degradation of the LEDs. Results indicate that state-of-the-art LEDs can show a significant degradation of their electrical and optical characteristics, when they are operated close to their current/temperature limits. In particular, data reveal the presence of two different degradation mechanisms: (i) the degradation of the blue semiconductor chip, due to the increase in non-radiative recombination, or to the decrease in the acceptor dopant concentration at the p-side of the diodes; (ii) the chemical degradation of the package, with subsequent worsening of its optical properties. Results suggest that even high-performance LEDs can suffer from limited lifetime: thermal management and driving conditions must be carefully optimized with the aim of achieving high reliability for LEDs to be adopted in high efficiency lighting systems.

Sulfur-contamination of high power white LED.

Silver–sulfur compounds have been detected at the internal lead frame of several commercial plastic packaged white LEDs that passed a standard incoming lot inspection but resulted all electrically open or degraded at the switching-on after mounting on the final boards.

Influence of Shunt Resistance on the Performance of an Illuminated String of Solar Cells: Theory, Simulation, and Experimental Analysis

This paper presents an extensive study of how a solar cell with low shunt resistance can affect the performance and reliability of a solar panel. The analysis is based on both simulations and experimental tests and provides the following results: 1) the cell with low shunt resistance significantly reduces the efficiency of a panel; 2) this is particularly pronounced if the shunted cell is partially shaded: in this case, the shunt resistance of the cell acts as a load for the entire panel; 3) in these conditions, the shunted cell can significantly degrade: in fact, the small-size shunt paths are crossed by a high current density, generated by the other cells in the panel, thus reaching high temperature levels. Stress tests have been also carried out to fully characterize the degradation process and its dynamics and to understand the physical origin of the failure of shunted cells.

Reliability predictions in electronic industrial applications

The paper describes a reliability prediction methodology that may be used to evaluate the reliability of electronics systems for industrial applications. The proposed methodology takes advantage of the potentiality of different reliability approaches. The aim of this new methodology is to minimize the deficiencies of the traditional reliability prediction methods calculating a corrective factor using the available field return data. In this way is possible to realize more realistic reliability assessment also in the case of new products or products without historic data. Applications of this prediction methodology on real electronic industrial systems are presented.