Degradation mechanisms of devices for optoelectronics and power electronics based on Gallium Nitride heterostructures

Gallium Nitride is rapidly emerging as a promising material for electronic devices in various fields. Since it is a direct bandgap semiconductor it can be used for highly efficient light emitting devices (Light Emitting Diodes and Laser Diodes) and the possibility of growing alloys containing Aluminum and Indium allow for the selection of the peak wavelength along the whole UV-green part of the radiation spectrum. Moreover, the high electron mobility, the ability of withstand high electric fields and the good thermal dissipation make GaN-based diodes and transistors devices with a good potential for high frequency and power applications. Before final products containing Gallium Nitride devices can permeate the international market, it is required to guarantee that they are reliable enough to have long lifetimes to appeal potential customers, and that their performance/cost relationship is superior compared to other competitors, at least in some specific fields of application. Aim of this thesis is to investigate the strong points of Gallium Nitrides by means of characterization and reliability tests on various different structures (LEDs, laser diodes, blocking diodes, HEMTs, GITs, MISs), in order to analyze the behavior of the material from different points of view. Within this work is reported a detailed study of the gradual degradation of InGaN-based laser diodes and Light-Emitting Diodes submitted to electro-thermal stress. The purpose is to compare the behavior of the two devices by means of electro-optical measurements, electroluminescence characterization, near field emission measurements and Deep-Level Transient Spectroscopy (DLTS) investigation in order to give a deeper understanding of the mechanisms involved in LD degradation. Particular attention is given to the role of injection efficiency decrease and non-radiative recombination. The comparison of the degradation kinetics and an analysis of the degradation modes of the two device structures allowed a complete study of the physical mechanisms responsible for the degradation. It was found that the degradation of the devices can be ascribed to an increase of the defect density, which has a strong impact on non radiative recombination kinetics. The activation energy of the detected deep level is 0.35 - 0.45 eV. As an effect of combined electrical and thermal stress tests on commercially-available InGaN-based blue laser diodes, it has been found that sometimes there is an initial decrease of the threshold current, which is ascribed to the increase of the activation of p-type dopant, promoted by the temperature and the flow of minority carriers. In order to investigate the effects of the creation of defects, two different commercial blue InGaN-based LEDs were submitted to 3 MeV proton irradiation at various fluencies (10^11, 10^12 and 10^13 p/cm2). The degradation process was characterized by combined current-voltage (I - V), optical power-current (L - I) and capacitance-voltage (C - V) measurements, in order to investigate the changes induced by the irradiation and the recovery after annealing time at high temperature (150 °C). The experimental data suggest the creation of non-radiative recombination centers near or into the active region of the LEDs, due to atomic displacement. This hypothesis is confirmed by the results of the recovery tests: the increase of the optical power and its correlation with the recovery of the forward current is consistent with the annealing of those defects. Part of the activity on high electron mobility transistors was devoted to the realization of measurement setups in order to carry out novel characterization techniques. Were analyzed the advantages and limitations of the current-transient method used for the study of the deep levels in GaN-based high electron mobility transistors (HEMTs), by evaluating how the procedures adopted for measurement and data analysis can influence the results of the investigation. The choice of the measurement parameters (such as the voltage levels used to induce the trapping phenomena and monitor the current transients and the duration of the filling pulses) and of the analysis procedure (the method used for the extrapolation of the time constants of the processes) can influence the results of the drain current transient investigation and can provide information on the location of the trap levels responsible for current collapse. Moreover, was collected a database of defects described in more than 60 papers on GaN and its compounds, which can be used to extract information on the nature and origin of the traps in AlGaN/GaN HEMTs. Using this newly developed technique and other more common tests, several reliability and lifetime test were carried out on various structures, in order to gain a better understanding of their problematic aspects and possible improvements. One potential variation is the composition of the gate stack. Degradation tests were performed at Vgs = -5 V and increasing Vds levels on GaN HEMTs with different gate materials: Ni/Au/Ni, ITO and Ni/ITO. At each step of the stress experiment, the electrical and optical characteristics of the transistors were measured in order to analyze the degradation process. It was found that stress induces a permanent degradation of the gate diode, consisting in an increase in the leakage current. This change is due to the generation of parasitic conductive paths, as suggested by electroluminescence (EL) mapping, and devices based on ITO showed higher reliability. These data strongly support the hypothesis that the robustness is influenced by processing parameters and/or by the gate material, since all analyzed devices come from the same epitaxial wafer. Other than varying the gate material, it is possible to add a p-type layer under the gate in order to achieve normally-off operation. This change produces a benefit in terms of performances, but can give birth to unusual trapping phenomena. It was carried out an extensive analysis of the time and field-dependent trapping processes that occur in GaN-based gate injection transistors exposed to high drain voltage levels. Results indicate that, even if the devices do not suffer from current collapse, continuous exposure to high drain voltages can induce a remarkable increase in the on-resistance (Ron). The increase in Ron can be recovered by leaving the device in rest conditions. Temperature-dependent analysis indicates that the activation energy of the detrapping process is equal to 0.47 eV. By time-resolved electroluminescence characterization, it is shown that this effect is related to the capture of electrons in the gate - drain access region. This is further confirmed by the fact that charge emission can be significantly accelerated through the injection of holes from the gate. A first-order model was developed to explain the time dependence of the trapping process. Using other deep levels characterization techniques, such as drain current transients, gate frequency sweeps and backgating, several other trap states were identified in these devices. Their activation energies are 0.13, 0.14, 0.25, 0.47 and 0.51 eV. During the accelerated lifetime tests of these devices, it was found a variation of the relative amplitude of the transconductance peaks, well correlated with the increase of the electroluminescence. This effect can be explained by the activation of the p-type dopant, a phenomenon which was detected also in laser diodes. It is possible to develop diodes able to withstand very high reverse voltages using a similar structure, deprived of the gate region and with an additional Schottky diode (Natural superjunction). In this case, the activation energies of the detected deep levels were 0.35, 0.36, 0.44 and 0.47 eV. These values are very similar to the ones found in GITs, and this fact, along with the presence of the p-dopant activation in very different devices, confirms that it is useful to study different structures based on the same material in order to gain more knowledge on its performances, possibilities and reliability aspects

Nonequilibrium Green's Function Modeling of Trap-Assisted Tunneling in In_{x}Ga_{1-x}N/GaN Light-Emitting Diodes

This work presents an investigation of carrier transport in GaN-based light-emitting diodes in the subthreshold forward-bias regime where tunneling processes are relevant. A quantum kinetic theory of trap-assisted tunneling is developed within the framework of the nonequilibrium Green’s function formalism. Based on fully nonlocal scattering self-energies computed in the self-consistent Born approximation and a multiband description of the electronic structure, the model provides access to spectral quantities, such as the local density of states and the current density, which are essential to understand the nature of the tunneling process. The quantum nonradiative recombination rates can be reproduced by the conventional Shockley-Read-Hall theory, provided that the classical charge is replaced with the correct quantum charge, which means that trap-assisted tunneling can be described with drift-diffusion solvers complemented with appropriate quantum corrections for the calculation of the local density of states. The subthreshold I-V characteristics and ideality factors predicted by the quantum kinetic model are in agreement with measurements

Rappresentazioni planimetriche, vedutistiche e tridimensionali per la fortificazione di due isole del Mediterraneo occidentale: Elba e Palmaria (secolo XIX)

[EN] The French expansion and domination in Italy between the Revolutionary Age and the Empire based on a widespread activity of territorial knowledge, which rested in the Corps of Engineers-Geographers and in the Military Genius the main actors. The paper summarizes the results of long research on this activity, carried out in the islands of Elba (Tuscany) and Palmaria (Liguria): two strategic islands in the western Mediterranean. The need to equip the territories dominated by the French with increasingly functional defenses, gave a strong impulse to the renewal of surveying and cartography, with the use of geodetic projections, views and three-dimensional models. Elba example is significant for the complete triangulation of the island connected to the Corsica one (with part of Sardinia and the smaller islands of the Tuscan archipelago). Geographer engineers such as Tranchot, Simonel, Moynet, Puissant worked on these activities and produced some maps and a small model of part of Elba. In the Palmaria example the threedimensional reproduction (plan-relief) was contextual to the work of Genius engineers who produced a vast and organic corpus of maps of various scales, views, sketches and watercolors, suitable to represent the most complete visualization of the landscapes where to insert defensive buildings. The collaboration between French and Italian engineers took advantage of this first experience in designing some batteries. However, it was the post-Napoleonic decades that made Palmaria island a powerful “fortress island” to defend the entrance to the Gulf of La Spezia, where the military arsenal (commissioned by Cavour and built by Domenico Chiodo) arose.De Santi, V.; Gemignani, C.; Guarducci, A.; Rossi, L. (2020). Rappresentazioni planimetriche, vedutistiche e tridimensionali per la fortificazione di due isole del Mediterraneo occidentale: Elba e Palmaria (secolo XIX). Editorial Universitat Politècnica de València. 751-758. https://doi.org/10.4995/FORTMED2020.2020.11497OCS75175

Use of Bilayer gate insulator in GaN-on-Si Vertical Trench MOSFETs : impact on performance and reliability

We propose to use a bilayer insulator (2.5 nm Al2O3 + 35 nm SiO2) as an alternative to a conventional uni-layer Al2O3 (35 nm), for improving the performance and the reliability of GaN-on-Si semi vertical trench MOSFETs. This analysis has been performed on a test vehicle structure for module development, which has a limited OFF-state performance. We demonstrate that devices with the bilayer dielectric present superior reliability characteristics than those with the uni-layer, including: (i) gate leakage two-orders of magnitude lower; (ii) 11 V higher off-state drain breakdown voltage; and (iii) 18 V higher gate-source breakdown voltage. From Weibull slope extractions, the uni-layer shows an extrinsic failure, while the bilayer presents a wear-out mechanism. Extended reliability tests investigate the degradation process, and hot-spots are identified through electroluminescence microscopy. TCAD simulations, in good agreement with measurements, reflect electric field distribution near breakdown for gate and drain stresses, demonstrating a higher electric field during positive gate stress. Furthermore, DC capability of the bilayer and unilayer insulators are found to be comparable for same bias points. Finally, comparison of trapping processes through double pulsed and V-th transient methods confirms that the V-th shifts are similar, despite the additional interface present in the bilayer devices

Clinical relevance of genetic variants of gonadotrophins and their receptors in controlled ovarian stimulation: a systematic review and meta-analysis

Genotype has been implicated in the outcome of ovarian stimulation. The analysis of patient-specific genotypes might lead to an individualized pharmacogenomic approach to controlled ovarian stimulation (COS). However, the validity of such an approach remains to be established

Modeling of gate capacitance of GaN-based trench-gate vertical metal-oxide-semiconductor devices

We propose a model for the gate capacitance of GaN-based trench-gate metal-oxide-semiconductor transistors, based on combined measurements, analytical calculations and TCAD simulations. The trench capacitance is found to be equivalent to four different capacitors, used to model the various regions with different doping and orientation of the semiconductor/dielectric interface. In addition, we demonstrate and explain the characteristic double-hump behavior of the G-D and G-DS capacitance of trench-MOSFETs. Lastly, a TCAD simulation results accurately reproduce the experimental data, thus confirming the interpretation on the double hump behavior, and providing insight on the electron density at the gate interface. (C) 2020 The Japan Society of Applied Physic

Mechanisms of Step-Stress Degradation In Carbon-Doped 0.15 μm AlGaN/GaN HEMTs for Power RF Applications

We discuss the degradation mechanisms of C-doped 0.15-μm gate AlGaN/GaN HEMTs tested by drain step-stress experiments. Experimental results show that these devices exhibit cumulative degradation effects during the step stress experiments in terms of either (i) transconductance (gm) decrease without any threshold-voltage (VT) change under OFF-state stress, or (ii) both VT and gm decrease under ON-state stress conditions. To aid the interpretation of the experiments, two-dimensional hydrodynamic device simulations were carried out. Based on obtained results, we attribute the gm decrease accumulating under OFF-state stress to hole emission from CN acceptor traps in the gate-drain access region of the buffer, resulting in an increase in the drain access resistance. On the other hand, under ON-state stress, channel hot electrons are suggested to be injected into the buffer under the gate and in the gate-drain region where they can be captured by CN traps, leading to VT and gm degradation, respectively

GaN-based power devices: Physics, reliability, and perspectives

Over the last decade, gallium nitride (GaN) has emerged as an excellent material for the fabrication of power devices. Among the semicon- ductors for which power devices are already available in the market, GaN has the widest energy gap, the largest critical field, and the highest saturation velocity, thus representing an excellent material for the fabrication of high-speed/high-voltage components. The presence of spon- taneous and piezoelectric polarization allows us to create a two-dimensional electron gas, with high mobility and large channel density, in the absence of any doping, thanks to the use of AlGaN/GaN heterostructures. This contributes to minimize resistive losses; at the same time, for GaN transistors, switching losses are very low, thanks to the small parasitic capacitances and switching charges. Device scaling and monolithic integration enable a high-frequency operation, with consequent advantages in terms of miniaturization. For high power/high- voltage operation, vertical device architectures are being proposed and investigated, and three-dimensional structures—fin-shaped, trench- structured, nanowire-based—are demonstrating great potential. Contrary to Si, GaN is a relatively young material: trapping and degradation processes must be understood and described in detail, with the aim of optimizing device stability and reliability. This Tutorial describes the physics, technology, and reliability of GaN-based power devices: in the first part of the article, starting from a discussion of the main proper- ties of the material, the characteristics of lateral and vertical GaN transistors are discussed in detail to provide guidance in this complex and interesting field. The second part of the paper focuses on trapping and reliability aspects: the physical origin of traps in GaN and the main degradation mechanisms are discussed in detail. The wide set of referenced papers and the insight into the most relevant aspects gives the reader a comprehensive overview on the present and next-generation GaN electronics

III-N optoelectronic devices: understanding the physics of electro-optical degradation

III-N optoelectronic devices are of great interest for many applications. Visible emitters (based on InGaN) are widely used in the lighting, display and automotive fields. Ultraviolet LEDs (based on AlGaN) are expected to be widely used for disinfection, medical treatments, surface curing and sensing. Photodetectors and solar cells based on InGaN are also of interest, thanks to their great robustness and wavelength tunability. III-N semiconductors are expected to be robust, thanks to the wide bandgap (allowing high temperature operation) and to the high breakdown field (favoring the robustness against electrostatic discharges and electrical overstress). However, InGaN- and AlGaN-based devices can show a significant degradation when submitted to long-term ageing. Several driving forces can contribute to the worsening of the electrical and optical characteristics, including the operating temperature, the current, and the rate of non-radiative recombination in the quantum wells. The goal of this paper is to discuss the physics of degradation of III-V devices, by presenting a set of recent case studies, evaluated in our laboratories