Simone Levada

Optical evidence of an electrothermal degradation of InGaN-based light-emitting diodes during electrical stress

The optical properties of blue InGaN-based light-emitting diodes aged at high current levels have been studied by electroluminescence and cathodoluminescence. The onset of a broad optical band peaked at about 3.1 eV in devices aged without a heat sink (junction temperature higher than 300 °C) has been correlated to an electrothermal threshold effect. The band is attributed to the dissociation of Mg–H complexes inside the p-type layers and to the consequent formation of Mg-related metastable complexes acting as acceptors. Subsequent electron-beam irradiation determines the almost complete quenching of the band.

Analysis of DC current accelerated life tests of GaN LEDs using a Weibull-based statistical model

Gallium-nitride-based light-emitting diode (LED) accelerated life tests were carried out over devices adopting two different packaging schemes (i.e., with plastic transparent encapsulation or with pure metallic package). Data analyses were done using a Weibull-based statistical description with the aim of estimating the effect of high current on device performance. A consistent statistical model was found with the capability to estimate the mean time to failure (MTTF) of devices during DC current stress and the accelerating factors of high current stresses.

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.

Degradation mechanisms of GaN-based LEDs after accelerated DC current aging

This work presents the results of an extensive DC current aging and failure analysis carried out on blue InGaN/GaN LEDs which identify failure mechanisms related to package degradation, changes in effective doping profile, and generation of deep levels. DLTS and photocurrent spectra indicate the creation of extended defects in devices aged at very high current density.

Failure mechanisms of gallium nitride LEDs related with passivation

This paper analyzes the thermally-activated failure mechanisms of GaN LEDs under thermal overstress related with the presence of a PECVD SiN passivation layer. It is shown that the properties of the passivation layer can remarkably affect devices reliability during high-temperature stress: degradation mechanisms identified consist in emission crowding and series resistance increase, attributed to the thermally-activated indiffusion of hydrogen from the passivation to the p-layer, and subsequent p-doping compensation

High Brightness Ingan Leds Degradation at High Injection Current Bias

In this work, we report on the results of a reliability evaluation, in terms of electrical characteristics, stability and optical output power maintenance carried out on high brightness GaN/InGaN blue LEDs. Constant and pulsed bias where investigated over devices with and without heat sink in order to identify the influence of self-heating on the devices reliability. Several degradation modes have been identified such as increase of reverse and generation/recombination current, increase of the series resistance and decrease in the optical emitted power

Failure mechanisms of GaN-based LEDs related with instabilities in doping profile and deep levels

In this work we report on a reliability study carried out over InGaN/GaN LEDs. Accelerated life tests have been carried out at various biasing current levels and at various temperature. Failure modes have been identified and correlated with the instability of Magnesium as dopant in the p-layer and with the worsening of the efficiency of the quantum well in active layer.

Accelerated Aging of GaN Light Emitting Diodes Studied by 1/f and RTS Noise

We study variations of 1/f and random telegraph signal (RTS) noise with accelerated aging in blue GaN light emitting diodes. RTS noise is dominant at low currents and its parameters vary with the aging. 1/f noise has a junction component dominant at medium currents and a series resistance component dominant at elevated currents. While the junction noise is relatively non‐sensitive to aging, the series resistance contact noise is strongly aging‐dependent. The contact noise increases by the factor of 60–800 after aging and is found to be a good reliability indicator. The physical origin of degradation is discussed.

Study of short-term instabilities of InGaN/GaN light-emitting diodes by means of capacitance-voltage measurements and deep-level transient spectroscopy

In this paper we present a combined current-voltage, capacitance-voltage, Deep Level Transient Spectroscopy and electroluminescence study of short-term instabilities of InGaN/GaN LEDs submitted to forward current aging tests at room temperature. In the early stages of the aging tests at low forward current levels (15-20 mA), LEDs present a decrease in optical power, which stabilizes within the first 50 hours and never exceeds 10% (measured at 20 mA). The spectral distribution of the electroluminescence intensity does not change with stress, while C-V profiles detect changes consisting in apparent doping and/or charge concentration increase within quantum wells. This increase is correlated with the decrease in optical power. Capacitance Deep Level Transient Spectroscopy has been carried out to clarify the DC aging induced generation/modification of the energy levels present in the devices. Remarkable changes occur after the stress, which can be related to the doping/charge variation and thus to the efficiency loss.

Study of short-term instabilities in InGaN/GaN light-emitting diodes by means of capacitance voltage measurements and deep-level transient spectroscopy

In this paper we present a combined current-voltage, capacitance-voltage, Deep Level Transient Spectroscopy and electroluminescence study of short-term instabilities of InGaN/GaN LEDs submitted to forward current aging tests at room temperature. In the early stages of the aging tests at low forward current levels (15-20 mA), LEDs present a decrease in optical power, which stabilizes within the first 50 hours and never exceeds 10% (measured at 20 mA). The spectral distribution of the electroluminescence intensity does not change with stress, while C-V profiles detect changes consisting in apparent doping and/or charge concentration increase within quantum wells. This increase is correlated with the decrease in optical power. Capacitance Deep Level Transient Spectroscopy has been carried out to clarify the DC aging induced generation/modification of the energy levels present in the devices. Remarkable changes occur after the stress, which can be related to the doping/charge variation and thus to the efficiency loss.