Matteo Buffolo

Failure causes and mechanisms of retrofit LED lamps

Abstract This paper describes one of the first studies of the degradation of retrofit light bulbs based on white GaN light emitting diodes. The results indicate that the lifetime of LED lamps depends mostly on the stability of the driver and optical elements, rather than on the degradation of the LED chips, that have a stable output over stress time. By comparing lamps from four different manufacturers stressed at room and high temperature, we found that (i) long-term stress causes a change of the chromatic properties of the lamps, which is ascribed to the degradation of the phosphors or to the inner LED reflector; (ii) during aging the LED driver may degrade gradually and/or catastrophically, causing a reduction of the output optical power, or a complete failure; (iii) proper thermal management and heat dissipation reduce the degradation rate; (iv) spectral transmissivity measurements and visual inspection reveal the degradation of the diffusive optical elements, which is induced by the short wavelength side of the LED emission spectrum.

Thermally-activated degradation of InGaN-based laser diodes: Effect on threshold current and forward voltage

This paper presents a study of the effects of high temperature stress on the electro-optical characteristics of violet InGaN-based laser diodes. The results indicate that: (i) when submitted to constant current stress (with relatively high junction temperatures), devices show a significant increase in threshold current (Ith), related to the increase in non-radiative recombination; micro-cathodoluminescence measurements indicate that the area affected by degradation is wider than the ridge; (ii) the results of purely-thermal stress test indicate that the exposure to high temperature may induce an increase in threshold current; (iii) during the first part of the stress, this mechanism is well correlated with the variation of the forward voltage, suggesting a degradation in the properties (conductivity, acceptor doping) of the p-type material; for longer stress times, a further Ith increase is detected, with a linear dependence on time.

Experimental Demonstration of Time-Dependent Breakdown in GaN-Based Light Emitting Diodes

This letter reports the first experimental demonstration of time-dependent breakdown in GaN-based light-emitting diodes (LEDs); based on a number of constant voltage stress tests, carried out below the breakdown limit of the devices, we show that: 1) when submitted to reverse-bias stress, the LEDs show a gradual increase in current, well correlated with an increase in the breakdown luminescence signal; 2) for sufficiently long stress times, the LEDs can reach a catastrophic (sudden) breakdown, which leads to the failure of the devices; 3) the breakdown process is time-dependent and the time to failure (TTF) has an exponential dependence on stress voltage; and 4) TTF is Weibull distributed. The results presented within this letter demonstrate that the GaN-based heterostructures can show a TDDB-like behavior, and can be useful for the interpretation of the degradation data of LEDs and HEMTs.

Towards high reliability GaN LEDs: Understanding the physical origin of gradual and catastrophic failure

This paper reviews the most relevant mechanisms responsible for the gradual and catastrophic failure of InGaN-based light emitting diodes, for application in general lighting. Based on recent results obtained on state-of-the-art LEDs and lamps, we discuss the following: (i) the lifetime of 800 lm retrofit lamps is still limited by the early degradation of the InGaN-LEDs, of the plastic encapsulant/reflectors, and of the driving circuitry. High temperatures and/or poor thermal management can significantly reduce the lifetime of the lamps in real-life applications. (ii) mid-power LEDs, that represent a low-cost widely adopted alternative to power devices, can suffer from both chip and package degradation, that result in a significant decrease in optical power and shift in the chromatic properties. (iii) Electrical overstress (EOS) can result in the sudden failure of power LEDs; failure can be due to the fusion of the bonding wires, to the degradation of the metal lines, or to the cracking of the semiconductor material. The results presented within this paper provide an update on the state-of-the-art of LED reliability.

Failure of High Power LEDs Submitted to EOS: Dependence on Device Layout and Pulse Properties

This paper reports an extensive analysis of the failure of high power replace with light-emitting diodes (LEDs) submitted to electrical overstress (EOS). By using a custom EOS simulator, capable of generating current pulses up to 40 A with a duration between

Aging behavior, reliability and failure physics of GaN-based optoelectronic components

50~{\mu }\text{s}

Defect-related degradation of III-V/Silicon 1.55 µm DBR laser diodes

and 5 ms, we characterized four different kinds of power LEDs having a different layout: horizontal structure with bonding wires, vertical structure with via-holes through the insulating substrate, and a flip chip-structure with isolated vias. The results described in this paper demonstrate that: 1) state-of-the-art LEDs rated for a maximum current of 1–1.5 A can withstand EOS current levels in excess of 35 A, corresponding to power densities of 350 W/mm2; 2) dependent on the layout of the devices, the failure mechanisms include: fusion of the bonding wires, migration/degradation of the metal lines, failure of the insulated vias, and cracking the semiconductor material; and 3) the EOS failure level strongly depends on the pulse duration and on the layout of the package/chip. The devices with a flip-chip structure showed the highest robustness, while the use of bonding wire was found to severely limit the stability of the devices.

Current induced degradation study on state of the art DUV LEDs

This paper critically reviews the most relevant failure modes and mechanisms of InGaN LEDs for lighting application. At chip level, both the epitaxial heterostructure and the ohmic contacts may be affected. This may result in: (i) the formation of defects within the active region, resulting in the increase of non-radiative recombination and leakage current, (ii) the reduction of the injection efficiency consequent to increased trap-assisted tunneling, (iii) the degradation of contact resistance with increase of forward voltage. Package-related failures – not described in this paper - include (iv) thermally-activated degradation processes, affecting the yellow phosphors, the plastic package or the encapsulating materials and (v) darkening of the Ag package reflective coating, the latter due to chemical reaction with contaminants as Cl or S. In order to enucleate and study the different physical failure mechanisms governing device degradation, single quantum well (SQW) blue LEDs, InGaN laser structures and commercially-available white LEDs to high temperature and/or high current density have been submitted to accelerated testing at high temperature and high current density.

Failure limits and electro-optical characteristics of GaN-based LEDs under electrical overstress

This paper reports on an extensive investigation on the degradation mechanisms that may limit the long term reliability of heterogeneous III-V/Silicon DBR laser diodes for integrated telecommunication applications in the 1.55 μm window. The devices under test, aged for up to 500 hours under different bias conditions, showed a gradual variation of both optical (L-I) and electrical (I-V, C-V) characteristics. In particular, the laser diodes exhibited an increase in the threshold current, a decrease of the turn-on voltage and an increase in the apparent charge density within the space-charge region, which was extrapolated from C-V measurements. For longer stress times, these two latter processes were found to be well correlated with the worsening of the optical parameters, which suggests that degradation occurred due to an increase in the density of defects within the active region, with consequent decrease in the non-radiative (SRH) lifetime. This conclusion is also supported by the fact that during stress the apparent charge profiles indicated a re-distribution of charge within the junction. A preliminary investigation on the physical origin of the defects responsible for degradation was carried out by DLTS measurements, which revealed the presence of five different 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 located between the In0.53AlxGa0.47-xAs SCH region and the InP layer.

Degradation mechanisms of heterogeneous III-V/Silicon loop-mirror laser diodes for photonic integrated circuits

Abstract We present the first comprehensive study of the degradation of 16 mW state of the art UVC LEDs emitting at 280 nm. The study, based on combined electrical and spectral characterization, allows to identify different degradation regimes and mechanisms, and to formulate hypotheses on their origin.