Due to the weak thermal and chemical stability of organic resins which are used for conventional white LEDs to embed phosphors, inorganic color converters such as phosphor ceramics and phosphor-in-glasses are currently being used to replace conventional color converters based on organic materials, especially for high power and high brightness applications. In this paper we report on the study of sintered glass ceramics based on low melting glass in which commercial YAG:Ce3+ phosphors are embedded. A low Tg is necessary to avoid high temperature sintering which can damage the optical properties of the embedded phosphors. Two different types of glass have been studied: borosilicate and tellurite. The compositions have been optimized in terms of stability, sintering efficiency and thermal conductivity. Selected samples were optical charterized using a GaN high power multimode 450 nm Laser Diode, with a maximum output power of 1.6 W at 1.5 A.
We investigated both the effect of high irradiation density and high operating temperature, as well as their color-rendering index. The sintered glass ceramic based on borosilicate glass showed better high power stability because of its higher thermal conductivity.
Avalanche generation is a physical mechanism responsible for the breakdown at extremely high field, such as in the reverse bias conditions typical of ESD discharges. In this work, for the first time we provide experimental evidence that avalanche generation can take place in state-of-the-art InGaN-based blue LEDs. We measured the current-voltage and electroluminescence curves of the devices while pulsing them with increasing reverse voltages. We investigated a wide span of temperatures (from cryogenic to room temperature) in order to verify that the increase in leakage current detected below -80 V is related to avalanche generation (positive temperature-coefficient). Numerical simulations show that in this bias condition the band-to-band tunneling barrier thickness is low, leading to the possible injection of highly-energetic electrons from the p-side to the n-side that can start the avalanche process. The spectral shape shows a broad emission, covering the spectral range between 1.25 and 3.5 eV; the low energy side slowly decreases below 2.2 eV, and two sharp edges are seen at the high-energy side. Since an avalanche generation process is present, we can interpret the spectrum as follows: (i) hole and electron pairs generated by the avalanche process recombine, emitting photons; (ii) high-energy side: reabsorption of the emitted photons in the In-containing layers and nGaN side, confirmed by the red-shift at higher temperature; (iii) low-energy side: internal photoluminescence of the defects in the n-GaN layer, confirmed by PL measurements with external excitation. A theoretical computation based on this model is able to reproduce the experimental data.
We investigate the electroluminescence of blue LEDs in low bias (500 pA – 9 μA) at different temperature (15°C – 75°C). From 500 pA to 100 nA, the OP increases with bias current up to 10nA, and is stronger at higher temperature, as expected by radiative recombination through deep levels. The stronger contribution is at λ > 800 nm, i.e. at energies lower than the QW and GaN barrier midgap (720 nm). Above 100 nA the OP increases with current, and is compatible with QW emission. Its intensity decreases at higher temperature, as expected for non-radiative recombination. The experimental findings indicate that radiative recombination through deep levels can significantly influence the low current characteristics of the devices, even when those states are not at midgap.
This paper reviews the most relevant mechanisms responsible for the degradation of GaN-based lateral and vertical electron devices. These components are almost ideal for application in power electronics, but the presence of semiconductor defects and the existence of degradation processes may limit their stability and lifetime. In this paper we focus on the following aspects: (i) the degradation processes induced by off-state conditions and leading to a time-dependent and/or catastrophic breakdown of the devices; (ii) the stability of the gate stack; (iii) the degradation of the electrical performance of vertical GaN transistors and diodes. To discuss these topics, we refer to case studies carried out in our laboratories.
This paper reports on a preliminary investigation of the gradual degradation processes that may affect the lifetime of InAs quantum dot (QD) lasers epitaxially grown on silicon substrates. To this aim, a series of identical Fabry-Pérot lasers emitting at 1.31 μm have been subjected to current step-stress and constant-current aging experiments at an ambient temperature of 35°C. With the adopted stress conditions, the optical characteristics of the devices exhibited an increase in the threshold-current and a decrease in the slope efficiency. This latter process was found to be well correlated with the variation in the threshold current, suggesting that this specific degradation mode may be ascribed to a stress-induced reduction in the injection efficiency. Moreover, the linear dependence of the threshold-current variation on the square root of time observed for longer stress time highlighted the possible role of a charge/defects diffusion process in the optical degradation of the devices. Consistent with this hypothesis, the electrical characteristics of the devices exhibited an increase of the forward leakage current in the bias regime dominated by defect-assisted current conduction mechanisms. The degradation process was found to be heavily accelerated for bias values allowing excited-state operation: this peculiar behavior was ascribed to the higher rate of carriers escaping from the quantum dots that undergo Recombination Enhanced Defect Reactions (REDR) in proximity of the active region of the device.
Within this paper, we summarize some of the degradation mechanism that still affect GaN-based optoelectronic devices. The most common source of the degradation is the creation of lattice defects, which lower the optical efficiency due to their role as non-radiative recombination centers, as proven in the case of UV-B LEDs. The local generation of defects is not the only possibility, with diffusion of impurities (possibly hydrogen from the p-side) being shown to be the limiting factor in the case of green laser diodes. Under extreme bias conditions, such as the EOS events, the robustness of the current carriers and spreading structures is critical, as shown by failure of bonding wires, metal lines and vias in white LEDs. In every optoelectronic device photons themselves possess an energy at least equal to the bandgap, and can be an additional source of degradation that cannot be eliminated.
GaN-based multi-quantum well devices are promising candidates as photodetectors in the UV to visible spectral range. Their complex structure and the extreme input optical power density still poses problems of reliability. In the devices under test, degradation takes place when the optical power density reaches values higher than 44 W/cm<sup>2</sup> , and consists in a reduction in the efficiency of the device and in its output current. This degradation process is not sudden and is caused by a gradual increase in the defect concentration, detected by means of photocurrent spectroscopy experiments, that suggest the role of gallium vacancies and/or their complexes as the physical origin. A secondary effect is the reduction in open circuit voltage, likely originating from an improvement in dopant and/or contact quality.
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 In<sub>0.53</sub>Al<sub>x</sub>Ga<sub>0.47-x</sub>As SCH region and the InP layer.
The aim of this work is to analyze the degradation in (In)AlGaN-based UV-B LEDs, with a nominal emission wavelength of 310 nm, submitted to constant current stress at a high current density of 350 A/cm<sup>2</sup>. We observed two main degradation mechanisms that were studied by investigating the evolution of the main emission peak from the quantum well (QW) and of a parasitic peak centered at 340 nm. In the first 50 hours of stress the main peak decreases and the parasitic peak (probably related to radiative recombination in the quantum barrier next to the electron blocking layer) increases at drive currents between 5 mA and 50 mA. Secondly, after 50 hours of stress both the main and the parasitic peak decrease. The first degradation mode could be related to carrier escape from the QWs, since the increase in the parasitic peak is correlated with the decrease in the main peak. After 50 hours of stress, we observed that the current below the turn-on voltage at V = 2 V increases with a square-root of time dependence. This behavior indicates the presence of a diffusion process, probably by point defects causing an increase of non-radiative recombination in the LED.
METIS is one of the remote sensing instrument on the Solar Orbiter mission. It will acquire coronal images from distances from the Sun as close as 0.28 AU. The mission innovations rely not only in the spacecraft orbit; METIS introduces many technical breakthroughs in the optical layout and in many other areas, mainly the inverted external occulter and the visible light (VL) polarimeter.
This paper demonstrates that when InGaN LEDs are submitted to a constant reverse bias, they can show a time-dependent breakdown, that leads to the catastrophic failure of the devices. By submitting green and blue LEDs to constant voltage stress in the range between -40 V and -60 V we demonstrate that: (i) under reverse bias conditions, current is focused on localized paths, whose positions can be identified by electroluminescence measurements, and that originate from the presence of extended defects; (ii) during a constant voltage stress, the reverse current of the LEDs gradually increases; (iii) for longer stress times, all devices show a time-dependent breakdown; (iv) time-to-failure has an exponential dependence on stress voltage, and is Weibull-distributed.
The paper reports the analysis of (In)AlGaN-based UV-B LEDs degradation under constant current stress, and investigates the impact of defects in changing the devices electro-optical performance. The study is based on combined electro-optical characterization, deep-level transient- (DLTS) and photocurrent spectroscopy. UV-B LEDs show a decrease of the optical power during stress, more pronounced at low measuring current levels, indicating that the degradation is related to an increase of Shockley-Read-Hall (SRH) recombination. DLTS measurements allowed the identification of three defects, in particular one ascribed to Mg-related acceptor traps presence. Photocurrent spectroscopy allows the localization of the defects close to the mid-gap.
GaN-HEMTs with p-GaN gate have recently demonstrated to be excellent normally-off devices for application in power conversion systems, thanks to the high and robust threshold voltage (V<sub>TH</sub>>1 V), the high breakdown voltage, and the low dynamic Ron increase. For this reason, studying the stability and reliability of these devices under high stress conditions is of high importance. This paper reports on our most recent results on the field- and time-dependent degradation of GaN-HEMTs with p-GaN gate submitted to stress with positive gate bias. Based on combined step-stress experiments, constant voltage stress and electroluminescence testing we demonstrated that: (i) when submitted to high/positive gate stress, the transistors may show a negative threshold voltage shift, that is ascribed to the injection of holes from the gate metal towards the p-GaN/AlGaN interface; (ii) in a step-stress experiment, the analyzed commercial devices fail at gate voltages higher than 9-10 V, due to the extremely high electric field over the p-GaN/AlGaN stack; (iii) constant voltage stress tests indicate that the failure is also time-dependent and Weibull distributed. The several processes that can explain the time-dependent failure are discussed in the following.
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.
The thermal droop (reduction of the optical power when the temperature is increased) is a phenomenon that strongly
limits the efficiency of InGaN-based light-emitting diodes. In this paper we analyze the role of Shockley-Read-Hall
(SRH) recombination and of the electron blocking layer (EBL) in the process by using numerical simulations and
literature data. The benefic impact of EBL suggests that carrier escape from the quantum wells gives a significant
contribution to the thermal droop, therefore we review some of the mechanisms described in the literature (thermionic
emission, phonon-assisted tunneling, thermionic trap-assisted tunneling). Since no formulation is able to fit the behavior
of the measured SQW devices, we develop a new model based on two phonon-assisted tunneling steps through a
defective state, extended in order to take into account zero-field emission. By using experimental data, material constants
from the literature and only two fitting parameters the model is able to reproduce the experimental behavior.
We discuss some of the key issues to be addressed along the way to complement, and possibly to replace, the standard semiclassical Boltzmann picture with genuine quantum approaches for the simulation of carrier transport and recombination in GaN-based LEDs, with the goal of gradually removing the fitting parameters presently required by semiempirical "quantum corrections" and to better understand the processes responsible for the efficiency droop. As examples of augmented semiclassical models, we present a three-step description of trap-assisted tunneling, especially relevant below the optical turn-on, and a carrier-density-dependent estimate of the phonon-assisted capture rate from bulk states to quantum wells (QWs). Moving to genuine quantum models, we solve the semiconductor Bloch equations to calculate the gain/absorption spectra of AlGaN/GaN QWs, and we discuss our first simulations of spatially and energetically resolved currents across the active region of a single-QW LED based on the nonequilibrium Green’s function approach.
III-N nanowires (NWs) are an attractive alternative to conventional planar layers as the basis for light-emitting diodes (LEDs). In fact, the NW geometry enables the growth of (In,Ga)N/GaN heterostructures with high indium content and without extended defects regardless of the substrate. Despite these conceptual advantages, the NW-LEDs so far reported often exhibit higher leakage currents and higher turn-on voltages than the planar LEDs. <p> </p>In this work, we investigate the mechanisms responsible for the unusually high leakage currents in (In,Ga)N/GaN LEDs based on self-induced NW ensembles grown by molecular beam epitaxy on Si substrates. The temperature-dependent current-voltage (I-V) characteristics, acquired between 83 and 403 K, reveal that temperatures higher than 240 K may activate a further conduction process, which is not present in the low temperature range. Deep level transient spectroscopy (DLTS) measurements show the presence of electron traps, which are activated in the same temperature interval. A detailed analysis of the DLTS signal reveals the presence of two distinct deep levels with apparent activation energies close to E<sub>c</sub>-570 meV and E<sub>c</sub>-840 meV, and capture cross sections of about 1.0x10<sup>-15</sup> cm<sup>2 </sup>and 2x10<sup>-14</sup> cm<sup>2</sup>, respectively. These results suggest that the leakage process might be related to trap-assisted tunneling, possibly produced by point defects located in the core and/or on the sidewalls of the NWs.
This paper reviews our recent results on the impact of iron and carbon compensation on the dynamic performance of GaN-HEMTs; based on pulsed and transient characterization, we demonstrate that: (i) the use of Fe-doping may lead to a significant current collapse, due to the presence of a trap with activation energy Ea=0.6eV. We discuss the properties of this trap and its physical origin; (ii) high C-doping levels may favor dynamic Ron increase, due to the presence of a trap level located at Ev+0.84 eV. The effect of this trap can be significantly reduced through the use of a double heterostructure.
This paper reviews the properties of the defects which limit the performance and the reliability of LEDs based on InGaN. More specifically we discuss: (i) the origin and properties of the defects responsible for SRH recombination; (ii) the role of defects in favoring the degradation of InGaN-based LEDs. Original data are compared to previous literature reports to provide a clear understanding of the topic.
Recent studies demonstrated that degradation of InGaN-based laser diodes is due to an increase in non-radiative
recombination rate within the active layer of the devices, due to the generation of defects.
The aim of this paper is to show - by DLTS - that the degradation of InGaN-based laser diodes is strongly correlated to
the increase in the concentration of a deep level located within the active region. The activation energy of the detected
deep level is 0.35-0.45 eV. Hypothesis on the nature of this deep level are presented in the paper.