Recently, it has been observed that ultraviolet (UV)-C light-emitting diodes (LEDs) may show an increase in optical power (OP) during the initial phase of aging (positive aging), and this has not been described so far in the literature. This article investigates for the first time the origin of the positive aging of AlGaN-based UV-C LEDs emitting at 265 nm, and proposes a physical origin by employing a modeling framework. Based on defect characterization and TCAD simulation, we show that the positive aging is caused by the generation of fixed charges in the p-n junction of the device. Moreover, we were able to reproduce different degradation trends by considering the generation of defects in two regions of the device: the interface between the interlayer and the last barrier, and the multiquantum wells. The description of positive aging given in this article provides relevant information on the defect-related processes in UV-C LEDs during operation.

In this work, we analyzed the optical degradation of 845 nm VCSELs designed for silicon photonics (SiPh) applications as a function of the oxide aperture. First, we investigated the optical degradation in relation to the stress current. The experimental results showed that devices with a larger oxide aperture exhibit better reliability. From the analysis of the degradation kinetics, we found that the current at which the highly accelerated degradation process occurs depends inversely on the aperture radius. This result was explained by showing that devices with a larger aperture have lower thermal impedance, and therefore operate at lower internal temperatures at similar bias points. This interpretation is supported by isothermal constant current stress tests, which show that when the same internal temperature is maintained during ageing, for all VCSEL geometries under study the onset of degradation occurs at the same time. The equivalent activation energy of the degradation process was found to fall between 0.43 and 0.68 eV. Our analysis demonstrated that using a larger aperture can improve device reliability.

This paper describes the physical origin of VTH instabilities in β-Ga2O3 finFETs, based on electro-optical measurements. In particular, (i) we investigated the VTH instability by means of pulsed ID–VGs, demonstrating the existence of an electron trapping process involving border states in the dielectric; then, (ii) we analyzed the charge trapping kinetics at high temperatures, identifying a temperature-dependent mechanism having activation energy of 0.44 ± 0.08 eV; and finally, (iii) we describe a new experimental methodology to investigate the optically stimulated emission of the trapped carriers. The results were employed to develop a quantitative model that identifies, as a root cause for trapping, the presence of a defect band located ∼3.3 eV below the conduction band edge of the dielectric layer.

UV-C LEDs are rapidly finding application in many fields, including sterilization and disinfection of water, air, and surfaces. The performance and reliability of these devices is strongly affected by the quality of the epitaxial layer structure, which depends on the growth parameters. Recent studies demonstrate that defects can limit the efficiency and the lifetime of UV-C LEDs. This article summarizes our most recent findings on this topic, by presenting two case studies. In the first part, we focus on the relation between defects and quantum efficiency: based on electro-optical measurements, we show that the external quantum efficiency (EQE) strongly depends on growth temperature. Our results show that the EQE is strongly correlated with the density of a deep trap, that was identified by deep-level optical spectroscopy with its incorporation depending on the temperature. The second part focuses on the physical processes responsible for device degradation. Consistent with prior reports, we demonstrate that UV-C LEDs can show a non-monotonic variation in the optical power, when they are submitted to constant current stress. Based on combined experimental analysis and TCAD (Technology Computer Aided Design) simulation, we demonstrate that: a) during a first phase, the studied devices show a positive ageing (increasing optical power), which is ascribed to the increase in trap charges at the interface between the uppermost quantum well barrier and the p-side interlayer, resulting in a change in the hole injection efficiency; b) for longer stress times (> 3-4 h), a negative ageing (decreasing optical power) is observed, which is ascribed to the increase in the density of defects within the quantum wells.

Ultrawide bandgap (UWB) semiconductors are emerging as key enablers for next-generation high-power, high-frequency electronic devices, capable of operating in extreme conditions. This review article presents a comprehensive overview of the state-of-the-art of the three most investigated UWB materials: $\beta $ -gallium oxide ( $\beta $ -Ga2O ${}_{{3}}\text {)}$ , diamond, and aluminum nitride (AlN). First, intrinsic material properties and technological details (including bulk growth and epitaxy) are analyzed and discussed. Next, recent progress in the development of diodes and transistors for power and RF applications is discussed and critically analyzed, taking into account material-specific features and challenges. Finally, a comparison of different technologies is presented, highlighting the key advantages and drawbacks of each, and providing insights into other emerging material systems.

We present a comprehensive investigation of boron-related trapping phenomena in diamond Schottky diodes based on capacitance-voltage (CV) measurements and isothermal capacitance transient spectroscopy (ICTS). The CV characteristics exhibit a pronounced hysteresis, whose magnitude increases with temperature, thus indicating the presence of temperature-activated charge trapping within the depletion region. ICTS measurements reveal the presence of two distinct trapping processes. The faster process exhibits an activation energy of (0.39 ± 0.02) eV and an unconventional apparent capture cross-section of 1.9 × 10−20 cm2. The extracted activation suggests that the trap is related to substitutional boron. By varying the filling time, we observed an emission process that follows a stretched exponential trend, which is ascribed to a distribution of trap levels. The characterization of the dynamic behavior of boron-related traps with temperature is highly relevant for studying the dynamic properties of diamond-based Schottky-barrier diodes and transistors.

We analyze and model the changes in the electrical properties of vertical GaN-on-GaN p-i-n diodes submitted to electrical stress. The analyzed devices were treated with hydrogen plasma to minimize surface leakage. Constant-current stress tests were carried out to investigate the stability of the electrical characteristics of the samples. The results indicate the existence of two phases: 1) for short stress times, we detected a lowering in the turn-on voltage (V-ON), with no significant variation in series resistance, which was ascribed to an increase in injection efficiency at the anode contact and 2) for longer stress times, the turn-on voltage stabilized, but the series resistance showed a measurable decrease (-35%). Based on TCAD simulations, the observed behavior is ascribed to the diffusion of hydrogen from the p(+) contact layer toward the junction. A model based on Fick's second law was developed, finding good agreement with the experimental data.

We evaluate the degradation of 1.3 mu m InAs quantum-dot laser diodes epitaxially grown on silicon. For the first time, the optical degradation mechanisms are investigated by evaluating the variations in the gain spectra measured during a constant-current stress test. Remarkably, the gain spectra showed that the reduction in the peak modal gain is dominant compared to the increase in internal absorption losses. Moreover, the increase in threshold current (Ith) induced by stress was found to be correlated to the gain peak reduction. This experimental evidence was investigated by modeling the peak modal gain variation through a rate equation model. The outcome of this activity confirms that the variation of both Ith and modal gain can be explained solely by the reduction in injection efficiency, caused by the stress-induced increase in non-radiative recombination centers (NRRCs). This result supports previous findings on the optical degradation of 1.3 mu m InAs quantum dot lasers, which is ultimately ascribed to the increase in concentration of NRRCs within the active region.

We investigate the robustness and charge trapping phenomena under positive and negative bias stress in Al2O3/n-GaN metal-oxide-semiconductor capacitors positive voltage stress, while ThALD results in the lowest charge trapping. We demonstrate that stacked-ALD layers presents the best trade-off to maximize robustness and minimize charge trapping phenomena, clearing the way for application in GaN MOSFET fabrication. Additionally, we compare the performance of different insitu plasma pre-treatments on the GaN surface, proving that the NH3 plasma is effective for improving the device performance in terms of trapped charge and reliability. The results confirm the beneficial role of stacked ALD deposition and surface pre-treatments on the electrical stability and reliability of oxide layers.

We report on the degradation dynamics and mechanisms of commercially available green high-power light-emitting diodes (LEDs) with a peak wavelength of 522 nm. The stress tests were carried out for up to 8800 hours with forward currents ranging from 350 mA to 1000 mA at junction temperatures between 86 C and 155 C-degrees. Two complementary test designs were used to isolate temperature- and current-driven effects. The results of the accelerated tests reveal the following key findings: 1.) A square-root-time-dependent loss in the quantum wells caused by the generation of point defects, leading to up to 90 % flux reduction within the first 500 hours at low forward currents. 2.) A logarithmic decay governed by defect-induced carrier-injection loss, evident above I-EQE,I-max and accompanied by a spectral red shift. 3.) A temperature-activated blue shift with an activation energy of E-a =0.23 eV, indicating the coexistence of competing degradation mechanisms. The interplay between different mechanisms results in an enhanced device lifetime at higher stress temperatures and stands in contrast to previous findings reported in the literature. 4.) The isothermal stress test indicates a cubic acceleration of degradation with carrier density, implicating Auger-Meitner-generated hot electrons in defect formation. These insights provide guidance for mitigating reliability issues of green high-power LEDs in future devices.

We propose a detailed approach for modeling the C-V characteristic of complex heterostructure-based devices, investigating the case of UV-C LEDs. The study is based on combined experimental measurements and TCAD simulations, and explores: i) the impact of defects at critical interfaces on the apparent charge profile; ii) the dependence of the C-V characteristic on the efficiency of carrier injection toward the QWs; iii) the impact of a non-ideal partially-rectifying p-contact on the C-V curves. By accounting for these processes and non-idealities, we were able to achieve a good reproduction of the experimental C-V characteristic and of the corresponding apparent charge profile. The result presented in this paper provide relevant information for the investigation of specific device features, based on simple electrical characterizations.

This study examines the reliability of optical materials, specifically silicone lenses, in high-power gallium nitride-based blue light-emitting diodes (LEDs) for horticulture applications. We subjected commercially available LEDs to accelerated stress tests at various temperatures $\left(45^{\circ} \mathrm{C} \text{to} 105^{\circ} \mathrm{C}\right)$ and a current 1.2 times the maximum nominal value. The results indicate a significant decrease in optical power (up to 40%) after prolonged stress, primarily due to the degradation and cracking of the silicone lens. Optical images confirmed a progressive degradation pattern. A second experiment compared a reference silicone with an optimized version. While the reference silicone showed similar degradation, the optimized silicone remained stable for nearly 5,000 hours. Raman spectroscopy revealed the reference silicone to be poly(methyl, phenyl)siloxane, while the optimized one was likely polydimethylsiloxane. The degradation in reference silicone is attributed to a combination of thermal induced chemical decomposition of silicone (possibly enhanced by high-energy blue photons) and thermomecanical stress, leading to depolymerization and cracking. This research highlights the critical role of silicone composition in the long-term reliability of high-power LEDs.

Most photovoltaic (PV) modules are guaranteed for 25-30 years. However, severe climatic events, particularly hail, can lead premature damage. In this article, a residential PV system in Padova, Italy, was studied after exposure to a severe storm with hailstones up to 16 cm in diameter, which is more than two times larger than the standard size of test stones employed for module validation (7.5 cm, as per IEC 61215-2-2021). The goals are: 1) to demonstrate the relevance of hail testing beyond what currently required by the standards; 2) to demonstrate the presence of latent damage even in the absence of broken glass or of reduced performance; and 3) to discuss the associated risks. Forward bias electroluminescence (EL) and infrared (IR) radiation investigations were conducted in dark to minimize the impact of environmental influences. In the worst case, complete glass breakage results in solar cell fragmentation, which induces nonuniformity in current flow and thermal radiation, increasing losses, compromising electrical insulation, and requiring immediate replacement. In addition, dark and outdoor light current-voltage characteristics reveal significant decrease in output power, as well as increased leakage current. Remarkably, latent or invisible damage, detectable by reduced EL intensity and higher IR radiation, poses safety issues even in modules whose protective glass withstood the mechanical impact of hail. Modules with intact glass exhibit a decreased shunt resistance, with a negligible reduction in the output power with respect to a completely intact module. The results underline the necessity of inspecting the entire PV system following hailstorms, to detect any latent damages and promptly replace the damaged modules, even in the absence of glass breakage or reduction in the output power, to ensure long-term reliability.

For the first time, we analyzed the degradation as a function of the oxide aperture in 845 nm VCSELs designed for silicon photonics (SiPh) applications. First, we evaluated the optical degradation of the devices by collecting EL images during a constant current stress. The experimental results showed an increased spreading of the optical beam of the VCSEL with increasing ageing time. Based on numerical simulations, we demonstrated that the electrical degradation (increase in series resistance) is responsible for a larger current spreading which, in turn, increases the FWHM (full width half maximum) of the optical beam. We further evaluated the series resistance variation by aging four lasers with different oxide apertures. The results of this set of experiments showed that the electrical degradation is stronger as the oxide aperture is smaller, and mostly depends on the contribution of the top DBR resistance. Thanks to our analysis we proved that the use of a larger aperture can result in a better device reliability.

This work investigates the impact of cracks on silicon heterojunction photovoltaic (PV) modules by analyzing their electrical and thermal behavior under low and high current conditions. The analysis was conducted on PV modules affected by a severe hailstorm, which produced hailstones up to 16 cm in diameter, far exceeding the standard test sizes (IEC 61215). A combination of electroluminescence (EL) and infrared (IR) thermography, along with dark and light current-voltage characterization, was employed to examine both hail and operator-induced cracks. The findings revealed that these cracks, which are latent damages not visible to the naked eye but only with EL and IR investigations, lead to localized temperature increase near open circuit voltage, and to a more uniform distributed temperature increase near short circuit conditions. A Simulink/Matlab model was developed to reproduce the thermal behavior of cracked cells in series with intact ones, to reproduce what happens in a real-world scenario. The results emphasize the importance of identifying latent defects in PV modules to ensure long-term reliability, safety, and efficiency, offering insights into their electrical and thermal behavior in low and high current regime.

Reverse-bias stability in PV devices is critical to guarantee adequate reliability during sporadic shading instances or when deliberately applying reverse-bias in photodetection applications. Testing reverse-bias stability on PSCs is crucial in providing characterizing insights both into the current state and performance of such devices and also towards their iterative improvement. This paper describes reverse-bias stability testing of semi-transparent FAPbBr(3) perovskite solar cells. Stability against reverse-bias was extensively evaluated through both reverse-bias step-stress (RBSS) tests and constant-bias stress (CBS) tests at different voltage bias intensities. During a series of 10 ks tests, cells were revealed to be stable when operated down to -1.5 V (corresponding to approximately 20 % of the breakdown voltage threshold), whereas at -3 V the observed degradation mainly consists in a decrease in open-circuit voltage (from similar to 1.5 divided by 1.6 V to as low as 0.3 V) and parallel resistance (from similar to 10(8) Omega to as low as similar to 10(2) Omega), occurring after similar to 100 s; a complete recovery is observed, if cells are left in resting conditions after removing the reverse-bias. The observed degradation is ascribed to a temporary shunt-like mechanism, triggered by ion and vacancy displacement and relocation, which causes a drastic energy-band distortion and internal potential compensation. Additional open-circuit voltage decay (OCVD) testing before and after stress reinforces this hypothesis. Reverse-bias step-stress testing until failure confirms that the mechanism occurs across the whole cell, leading to reverse-current magnitudes of over 300 mA/cm(2).

InGaN/GaN multiple quantum well (MQWs) solar cells are promising devices for application in harsh environments. However, understanding their degradation kinetics can be complicated by the high periodicity of the active region (AR). To overcome this issue, we carried out an experiment on structures with only two quantum wells, having different indium concentrations, that were submitted to an optical power step stress at 55 degrees C. Firstly, illuminated current-voltage (I-V) characterizations indicate that the QW near the p-side of the device strongly contributes to carrier collection. This is explained by the enhanced hole extraction and collection efficiency. Notably, a non-monotonic 'hump effect' emerges when the device design is suboptimal. Secondly, during optical power step stress, two key phenomena were observed: a) a reduction in short-circuit current (Isc), especially at high excitation intensities; b) an increase in current conduction below the main diode turn-on voltage, which is ascribed to the increased amount of traps in the active region of the devices. The degradation leads to a reduction in the extraction and collection efficiency of photogenerated carriers, as evidenced by the decrease in the open circuit voltage (Voc), the parameter most affected by degradation. Results give insight for the optimization of InGaN/GaN-based solar cells structure, which can be used to improve performance and reliability.