Lorenzo Roberto Trevisanello

A Review on the Reliability of GaN-Based LEDs

We review the degradation mechanisms that limit the reliability of GaN-based light-emitting diodes (LEDs). We propose a set of specific experiments, which is aimed at separately analyzing the degradation of the properties of the active layer, of the ohmic contacts and of the package/phosphor system. In particular, we show the following: 1) low-current density stress can determine the degradation of the active layer of the devices, implying modifications of the charge/deep level distribution with subsequent increase of the nonradiative recombination components; 2) high-temperature storage can significantly affect the properties of the ohmic contacts and semiconductor layer at the p-side of the devices, thus determining emission crowding and subsequent optical power decrease; and 3) high-temperature stress can significantly limit the optical properties of the package of high-power LEDs for lighting applications.

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.

A model for the thermal degradation of metal/(p-GaN) interface in GaN-based light emitting diodes

This paper analyzes the thermal degradation of GaN-based light emitting diodes with hydrogen-rich passivation layer by combined capacitance and current measurements. The decrease of optical power arising during thermal treatment at T=250°C is well related to a decrease in the high-frequency capacitance and to the generation of a further peak in the conductance/frequency curves. Deep level transient spectroscopy measurements excluded the role of further deep levels introduced/generated in the p-n junction region. Transmission line method analysis showed that stress induces nonlinearity of the characteristics of the contacts, related to the increase of Ohmic contact resistivity. All these modifications are explained using a small-signal model in which a parasitic impedance arises as a consequence of stress in the portion of metal/(p-GaN) interface exposed to diffusion of hydrogen from the passivation layer. Therefore, degradation is shown to be related to the worsening of the properties of the metal/semicon...

Stability and performance evaluation of High Brightness Light Emitting Diodes under DC and pulsed bias conditions

This paper presents an experimental analysis of high brightness light emitting diodes (HBLEDs) performance and stability under dc and pulsed current bias. Three different families of HBLEDs from three leading manufacturers have been considered. The analysis was carried out by means of current-voltage, integrated optical power and electroluminescence measurements, and failure analysis. After an initial characterization of the electrical, optical and thermal behavior of the devices, a set of ageing tests was carried out, both under dc and pulsed bias conditions. Identified degradation modes were efficiency decrease, series resistance increase, leakage current increase, and modifications of the emitted spectrum. Characterization of devices behavior during stress indicated (i) generation of non-radiative components, (ii) degradation of the anode contacts and bonding wires, (iii) degradation of the phosphorous layer conversion efficiency and (iv) of the plastic package as possible responsible of the electrical and optical degradation of the LEDs. Comparison between dc and pulsed stress carried out using the same average current level and different duty cycle values showed that the use of pulsed bias can reduce the degradation rate with respect to dc bias. However, for duty cycles lower than 20%, fast degradation and abrupt ruptures can take place, due to the high peak current levels.

Combined optical and electrical analysis of AlGaN-based deep-UV LEDs reliability

This paper describes an analysis of the reliability of AlGaN-based deep-UV Light-Emitting Diodes (LEDs) emitting in the range 280-340 nm. LEDs have been aged at their nominal operating current, and during treatment their electrical and optical characteristics have been continuously monitored. Measurement results show that (i) constant current stress can induce degradation of the optical power emitted by the devices; (ii) degradation is more prominent at low measuring current level, thus suggesting that efficiency decrease is related to the generation of non-radiative paths; (iii) degradation does not imply a significant variation of the operating voltage of the devices, thus indicating that the characteristics of the ohmic contacts are stable over stress time; (iv) optical power decrease takes place together with modifications of the C-V apparent charge profiles, indicating that the generation of non-radiative paths is related to charge instabilities in the QW region.

Electro-thermally activated degradation of blu-ray gan-based laser diodes

This paper describes an analysis of the reliability of GaN-based laser diodes, submitted to constant current, constant optical power and high temperature stress. We demonstrate that constant current and constant optical power stress induce the increase of the threshold current of the devices, that varies according to the square-root of time. The threshold current increase is found to be strongly correlated to the decrease of the sub-threshold emission of the devices, thus suggesting that stress determines the increase of the non-radiative recombination paths in the active layer. Degradation rate is found to depend on stress temperature and on current level, while it does not significantly depend on the optical field in the cavity. The evidences presented within this work support previous literature results, that attribute devices degradation to the diffusion of impurity species towards the active layer, with subsequent increase of the non-radiative recombination rate. The identified degradation process is shown to be electro-thermally activated.