Publications
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A First Evaluation of Thick Oxide 3C-SiC MOS Capacitors Reliability
Li F., Mawby P., Song Q., Perez-Tomas A., Shah V., Sharma Y., Hamilton D., Fisher C., Gammon P., Jennings M. IEEE Transactions on Electron Devices; 67 (1, 8935512): 237 - 242. 2020. 10.1109/TED.2019.2954911. IF: 2.913
Despite the recent advances in 3C-SiC technology, there is a lack of statistical analysis on the reliability of SiO2 layers on 3C-SiC, which is crucial in power MOS device developments. This article presents a comprehensive study of the medium-and long-term time-dependent dielectric breakdowns (TDDBs) of 65-nm-thick SiO2 layers thermally grown on a state-of-the-art 3C-SiC/Si wafer. Fowler-Nordheim (F-N) tunneling is observed above 7 MV/cm and an effective barrier height of 3.7 eV is obtained, which is the highest known for native SiO2 layers grown on the semiconductor substrate. The observed dependence of the oxide reliability on the gate active area suggests that the oxide quality has not reached the intrinsic level. Three failure mechanisms were identified and confirmed by both medium-and long-term results. Although two of them are likely due to extrinsic defects from material quality and fabrication steps, the one dominating the high field (>8.5 MV/cm) should be attributed to the electron impact ionization within SiO2. At room temperature, the field acceleration factor is found to be ≈0.906 dec/(MV/cm) for high fields, and the projected lifetime exceeds 10 years at 4.5 MV/cm. © 1963-2012 IEEE.
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Bias dependent variability of low-frequency noise in single-layer graphene FETs
Mavredakis N., Cortadella R.G., Illa X., Schaefer N., Calia A.B., Anton-Guimerà-Brunet, Garrido J.A., Jiménez D. Nanoscale Advances; 2 (11): 5450 - 5460. 2020. 10.1039/d0na00632g. IF: 0.000
Low-frequency noise (LFN) variability in graphene transistors (GFETs) is for the first time researched in this work under both experimental and theoretical aspects. LFN from an adequate statistical sample of long-channel solution-gated single-layer GFETs is measured in a wide range of operating conditions while a physics-based analytical model is derived that accounts for the bias dependence of LFN variance with remarkable performance. LFN deviations in GFETs stem from the variations of the parameters of the physical mechanisms that generate LFN, which are the number of traps (Ntr) for the carrier number fluctuation effect (ΔN) due to trapping/detrapping process and the Hooge parameter (αH) for the mobility fluctuations effect (Δμ). ΔN accounts for an M-shape of normalized LFN variance versus gate bias with a minimum at the charge neutrality point (CNP) as it was the case for normalized LFN mean value while Δμ contributes only near the CNP for both variance and mean value. Trap statistical nature of the devices under test is experimentally shown to differ from classical Poisson distribution noticed at silicon-oxide devices, and this might be caused both by the electrolyte interface in GFETs under study and by the premature stage of the GFET technology development which could permit external factors to influence the performance. This not fully advanced GFET process growth might also cause pivotal inconsistencies affecting the scaling laws in GFETs of the same process. This journal is © The Royal Society of Chemistry.
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Distortion-Free Sensing of Neural Activity Using Graphene Transistors
Garcia-Cortadella R., Masvidal-Codina E., De la Cruz J.M., Schäfer N., Schwesig G., Jeschke C., Martinez-Aguilar J., Sanchez-Vives M.V., Villa R., Illa X., Sirota A., Guimerà A., Garrido J.A. Small; 16 (16, 1906640) 2020. 10.1002/smll.201906640. IF: 11.459
Graphene solution-gated field-effect transistors (g-SGFETs) are promising sensing devices to transduce electrochemical potential signals in an electrolyte bath. However, distortion mechanisms in g-SGFET, which can affect signals of large amplitude or high frequency, have not been evaluated. Here, a detailed characterization and modeling of the harmonic distortion and non-ideal frequency response in g-SGFETs is presented. This accurate description of the input–output relation of the g-SGFETs allows to define the voltage- and frequency-dependent transfer functions, which can be used to correct distortions in the transduced signals. The effect of signal distortion and its subsequent calibration are shown for different types of electrophysiological signals, spanning from large amplitude and low frequency cortical spreading depression events to low amplitude and high frequency action potentials. The thorough description of the distortion mechanisms presented in this article demonstrates that g-SGFETs can be used as distortion-free signal transducers not only for neural sensing, but also for a broader range of applications in which g-SGFET sensors are used. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Effect of channel thickness on noise in organic electrochemical transistors
Polyravas A.G., Schaefer N., Curto V.F., Calia A.B., Guimera-Brunet A., Garrido J.A., Malliaras G.G. Applied Physics Letters; 117 (7, 073302) 2020. 10.1063/5.0019693. IF: 3.597
Organic electrochemical transistors (OECTs) have been widely used as transducers in electrophysiology and other biosensing applications. Their identifying characteristic is a transconductance that increases with channel thickness, and this provides a facile mechanism to achieve high signal amplification. However, little is known about their noise behavior. Here, we investigate noise and extract metrics for the signal-to-noise ratio and limit of detection in OECTs with different channel thicknesses. These metrics are shown to improve as the channel thickness increases, demonstrating that OECTs can be easily optimized to show not only high amplification, but also low noise. © 2020 Author(s).
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Improved metal-graphene contacts for low-noise, high-density microtransistor arrays for neural sensing
Schaefer N., Garcia-Cortadella R., Calia A.B., Mavredakis N., Illa X., Masvidal-Codina E., Cruz J.D.L., Corro E.D., Rodríguez L., Prats-Alfonso E., Bousquet J., Martínez-Aguilar J., Pérez-Marín A.P., Hébert C., Villa R., Jiménez D., Guimerà-Brunet A., Garrido J.A. Carbon; 161: 647 - 655. 2020. 10.1016/j.carbon.2020.01.066. IF: 8.821
Poor metal contact interfaces are one of the main limitations preventing unhampered access to the full potential of two-dimensional materials in electronics. Here we present graphene solution-gated field-effect-transistors (gSGFETs) with strongly improved linearity, homogeneity and sensitivity for small sensor sizes, resulting from ultraviolet ozone (UVO) contact treatment. The contribution of channel and contact region to the total device conductivity and flicker noise is explored experimentally and explained with a theoretical model. Finally, in-vitro recordings of flexible microelectrocorticography (μ-ECoG) probes were performed to validate the superior sensitivity of the UVO-treated gSGFET to brain-like activity. These results connote an important step towards the fabrication of high-density gSGFET μ-ECoG arrays with state-of-the-art sensitivity and homogeneity, thus demonstrating the potential of this technology as a versatile platform for the new generation of neural interfaces. © 2020 Elsevier Ltd
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Low-frequency noise parameter extraction method for single-layer graphene FETs
Mavredakis N., Wei W., Pallecchi E., Vignaud D., Happy H., Cortadella R.G., Schaefer N., Calia A.B., Garrido J.A., Jimenez D. IEEE Transactions on Electron Devices; 67 (5, 9042866): 2093 - 2099. 2020. 10.1109/TED.2020.2978215. IF: 2.913
In this article, a detailed parameter extraction methodology is proposed for low-frequency noise (LFN) in single-layer (SL) graphene transistors (GFETs) based on a recently established compact LFN model. The drain current and LFN of two short channel back-gated GFETs (L = 300 and 100 nm) were measured at lower and higher drain voltages, for a wide range of gate voltages covering the region away from charge neutrality point (CNP) up to CNP at p-type operation region. Current-voltage (IV) and LFN data were also available from a long-channel SL top solution-gated (SG) GFET (L = 5 μm), for both p- and n-type regions near and away CNP. At each of these regimes, the appropriate IV and LFN parameters can be accurately extracted. Regarding LFN, mobility fluctuation effect is dominant at CNP, and from there, the Hooge parameter αH can be extracted, whereas the carrier number fluctuation contribution which is responsible for the well-known M-shape bias dependence of output noise divided by squared drain current, also observed in our data, makes possible the extraction of the NT parameter related to the number of traps. In the less possible case of a Λ-shape trend, NT and αH can be extracted simultaneously from the region near CNP. Away from CNP, contact resistance can have a significant contribution to LFN, and from there, the relevant parameter SΔ R2 is defined. The LFN parameters described above can be estimated from the low drain voltage region of operation where the effect of velocity saturation (VS) mechanism is negligible. VS effect results in the reduction of LFN at higher drain voltages, and from there, the IV parameter hΩ which represents the phonon energy and is related to VS effect can be derived both from drain current and LFN data. © 1963-2012 IEEE.
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Multiplexed neural sensor array of graphene solution-gated field-effect transistors
Schaefer N., Garcia-Cortadella R., Martínez-Aguilar J., Schwesig G., Illa X., Moya Lara A., Santiago S., Hébert C., Guirado G., Villa R., Sirota A., Guimerà-Brunet A., Garrido J.A. 2D Materials; 7 (2, 025046) 2020. 10.1088/2053-1583/ab7976. IF: 7.140
Electrocorticography (ECoG) is a well-established technique to monitor electrophysiological activity from the surface of the brain and has proved crucial for the current generation of neural prostheses and brain-computer interfaces. However, existing ECoG technologies still fail to provide the resolution necessary to accurately map highly localized activity across large brain areas, due to the rapidly increasing size of connector footprint with sensor count. This work demonstrates the use of a flexible array of graphene solution-gated field-effect transistors (gSGFET), exploring the concept of multiplexed readout using an external switching matrix. This approach does not only allow for an increased sensor count, but due to the use of active sensing devices (i.e. transistors) over microelectrodes it makes additional buffer transistors redundant, which drastically eases the complexity of device fabrication on flexible substrates. The presented results pave the way for upscaling the gSGFET technology towards large-scale, high-density μECoG-arrays, eventually capable of resolving neural activity down to a single neuron level, while simultaneously mapping large brain regions. © 2020 IOP Publishing Ltd.
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P-Type Ultrawide-Band-Gap Spinel ZnGa2O4: New Perspectives for Energy Electronics
Chikoidze E., Sartel C., Madaci I., Mohamed H., Vilar C., Ballesteros B., Belarre F., Del Corro E., Vales-Castro P., Sauthier G., Li L., Jennings M., Sallet V., Dumont Y., Pérez-Tomás A. Crystal Growth and Design; 20 (4): 2535 - 2546. 2020. 10.1021/acs.cgd.9b01669. IF: 4.089
The family of spinel compounds is a large and important class of multifunctional materials of general formulation AB2X4 with many advanced applications in energy and optoelectronic areas such as fuel cells, batteries, catalysis, photonics, spintronics, and thermoelectricity. In this work, it is demonstrated that the ternary ultrawide-band-gap (∼5 eV) spinel zinc gallate (ZnGa2O4) arguably is the native p-type ternary oxide semiconductor with the largest Eg value (in comparison with the recently discovered binary p-type monoclinic β-Ga2O3 oxide). For nominally undoped ZnGa2O4 the high-temperature Hall effect hole concentration was determined to be as large as p = 2 × 1015 cm-3, while hole mobilities were found to be μh = 7-10 cm2/(V s) (in the 680-850 K temperature range). An acceptor-like small Fermi level was further corroborated by X-ray spectroscopy and by density functional theory calculations. Our findings, as an important step toward p-type doping, opens up further perspectives for ultrawide-band-gap bipolar spinel electronics and further promotes ultrawide-band-gap ternary oxides such as ZnGa2O4 to the forefront of the quest of the next generation of semiconductor materials for more efficient energy optoelectronics and power electronics. Copyright © 2020 American Chemical Society.
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Photocurrent spectroscopy of in-plane surface conductive diamond homostructures
Simon P., Beck P., Rathi A., Stutzmann M., Garrido J.A. Physical Review B; 101 (20, 205306) 2020. 10.1103/PhysRevB.101.205306. IF: 3.575
Electrical breakdown at hydrogen/oxygen interfaces is limiting the use of nanoscaled diamond devices for the control of optically active defect centers. In this work electron transport across an oxygen-terminated potential barrier in a hydrogen-terminated surface conductive diamond is investigated. We analyze temperature-dependent current-voltage characteristics for different barrier widths and report on a reduced effective barrier height compared to theoretical expectations. This is ascribed to an inhomogeneous potential landscape, as observed by Kelvin probe and conductive force microscopy. Furthermore, we use photocurrent spectroscopy to discuss possible transport processes and identify a field dependent absorption feature. A defect state involved in transport across the barrier is proposed at the hydrogen-oxygen barrier approximately 1eV above the valence band maximum. The new understanding enabled by our work may help to overcome the current limitations of diamond surface electronics. © 2020 American Physical Society.
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Production and processing of graphene and related materials
Backes C., Abdelkader A.M., Alonso C., Andrieux-Ledier A., Arenal R., Azpeitia J., Balakrishnan N., Banszerus L., Barjon J., Bartali R., Bellani S., Berger C., Berger R., Ortega M.M.B., Bernard C., Beton P.H., Beyer A., Bianco A., Bøggild P., Bonaccorso F., Barin G.B., Botas C., Bueno R.A., Carriazo D., Castellanos-Gomez A., Christian M., Ciesielski A., Ciuk T., Cole M.T., Coleman J., Coletti C., Crema L., Cun H., Dasler D., De Fazio D., Díez N., Drieschner S., Duesberg G.S., Fasel R., Feng X., Fina A., Forti S., Galiotis C., Garberoglio G., García J.M., Garrido J.A., Gibertini M., Gölzhäuser A., Gómez J., Greber T., Hauke F., Hemmi A., Hernandez-Rodriguez I., Hirsch A., Hodge S.A., Huttel Y., Jepsen P.U., Jimenez I., Kaiser U., Kaplas T., Kim H., Kis A., Papagelis K., Kostarelos K., Krajewska A., Lee K., Li C., Lipsanen H., Liscio A., Lohe M.R., Loiseau A., Lombardi L., López M.F., Martin O., Martín C., Martínez L., Martin-Gago J.A., Martínez J.I., Marzari N., Mayoral A., McManus J., Melucci M., Méndez J., Merino C., Merino P., Meyer A.P., Miniussi E., Miseikis V., Mishra N., Morandi V., Munuera C., Muñoz R., Nolan H., Ortolani L., Ott A.K., Palacio I., Palermo V., Parthenios J., Paste 2D Materials; 7 (2, 022001) 2020. 10.1088/2053-1583/ab1e0a. IF: 7.140
We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results. Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach. The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step. Sections IV-VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning. Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V. Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resourceconsuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer. There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown. © 2020 The Author(s).
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Switchless multiplexing of graphene active sensor arrays for brain mapping
Garcia-Cortadella R., Schäfer N., Cisneros-Fernandez J., Ré L., Illa X., Schwesig G., Moya A., Santiago S., Guirado G., Villa R., Sirota A., Serra-Graells F., Garrido J.A., Guimerà-Brunet A. Nano Letters; 20 (5): 3528 - 3537. 2020. 10.1021/acs.nanolett.0c00467. IF: 11.238
Sensor arrays used to detect electrophysiological signals from the brain are paramount in neuroscience. However, the number of sensors that can be interfaced with macroscopic data acquisition systems currently limits their bandwidth. This bottleneck originates in the fact that, typically, sensors are addressed individually, requiring a connection for each of them. Herein, we present the concept of frequency-division multiplexing (FDM) of neural signals by graphene sensors. We demonstrate the high performance of graphene transistors as mixers to perform amplitude modulation (AM) of neural signals in situ, which is used to transmit multiple signals through a shared metal line. This technology eliminates the need for switches, remarkably simplifying the technical complexity of state-of-the-art multiplexed neural probes. Besides, the scalability of FDM graphene neural probes has been thoroughly evaluated and their sensitivity demonstrated in vivo. Using this technology, we envision a new generation of high-count conformal neural probes for high bandwidth brain machine interfaces. © 2020 American Chemical Society.
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Ultra-high critical electric field of 13.2 MV/cm for Zn-doped p-type β-Ga2O3
Chikoidze E., Tchelidze T., Sartel C., Chi Z., Kabouche R., Madaci I., Rubio C., Mohamed H., Sallet V., Medjdoub F., Perez-Tomas A., Dumont Y. Materials Today Physics; 15 (100263) 2020. 10.1016/j.mtphys.2020.100263. IF: 10.443
Which the actual critical electrical field of the ultra-wide bandgap semiconductor β-Ga2O3 is? Even that it is usual to find in the literature a given value for the critical field of wide and ultra-wide semiconductors such as SiC (3 MV/cm), GaN (3.3 MV/cm), β-Ga2O3 (~8 MV/cm) and diamond (10 MV/cm), this value actually depends on intrinsic and extrinsic factors such as the bandgap energy, material residual impurities or introduced dopants. Indeed, it is well known from 1950's that reducing the residual doping (NB) of the semiconductor layer increases the breakdown voltage capability of a semiconductor media (e.g. as NB−3/4 by using the Fulop's approximation for an abrupt junction). A key limitation is, therefore, the residual donor/acceptor concentration generally found in these materials. Here, we report that doping with amphoteric Zinc a p-type β-Ga2O3 thin films shortens free carrier mean free path (0.37 nm), resulting in the ultra-high critical electrical field of 13.2 MV/cm. Therefore, the critical breakdown field can be, at least, four times larger for the emerging Ga2O3 power semiconductor as compared to SiC and GaN. We further explain these wide-reaching experimental facts by using theoretical approaches based on the impact ionization microscopic theory and thermodynamic calculations. © 2020