Staff directory Amador Pérez Tomás

Amador Pérez Tomás

Senior Postdoctoral Researcher
Advanced Electronic Materials and Devices



  • Electrical properties of p -type Zn:Ga2O3thin films

    Chikoidze E., Sartel C., Yamano H., Chi Z., Bouchez G., Jomard F., Sallet V., Guillot G., Boukheddaden K., Pérez-Tomás A., Tchelidze T., Dumont Y. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films; 40 (4, 043401) 2022. 10.1116/6.0001766.

    Ultra-wide bandgap gallium oxide (∼5 eV) has emerged as a novel semiconductor platform for extending the current limits of power electronics and deep ultraviolet optoelectronics at a predicted fraction of cost. Finding effective acceptor dopant for gallium oxide is a hot issue. One element that quite often is considered as a potential candidate is zinc. A number of experimental works have reported the signature of Zn-acceptor, but the direct evidence of hole conductivity was missing. In this work, p-type Zn-doped Ga2O3 thin films were grown by the metal-organic chemical vapour deposition technique on sapphire substrates. By high-temperature Hall effect measurements, Zn related acceptor level ionization energy as 0.77 eV above the valence band maximum was determined. Additionally, we have carried out the simulation study regarding the application of the Zn:Ga2O3 semi-insulating material, to be used as a guard ring for improving the high voltage performance of the Schottky diode structure. © 2022 Author(s).


  • A study on free-standing 3C-SiC bipolar power diodes

    Li F., Renz A.B., Pérez-Tomás A., Shah V., Gammon P., Via F.L., Jennings M., Mawby P. Applied Physics Letters; 118 (24, 242101): 1ENG. 2021. 10.1063/5.0054433. IF: 3.791

    A low p-n built-in potential (1.75 V) makes 3C-SiC an attractive choice for medium voltage bipolar or charge balanced devices. Until recently, most 3C-SiC had been grown on Si, and power device fabrication had, therefore, been hindered by issues, such as high defect density and limited processing temperature, while devices were necessarily limited to lateral structures. In this work, we present the fabrication and characterization of a vertical PiN diode using bulk 3C-SiC material. A p-type ohmic contact was obtained on Al implanted regions with a specific contact resistance ∼10−3 Ω cm2. The fabricated PiN diode has a low forward voltage drop of 2.7 V at 1000 A/cm2, and the on-off ratio at ±3 V is as high as 109. An ideality factor of 1.83-1.99 was achieved, and a blocking voltage of ∼110 V was observed using a single-zone junction termination design. © 2021 Author(s).

  • Bipolar self-doping in ultra-wide bandgap spinel ZnGa(2)O4

    Chi Z, Tarntair FG, Fregnaux M, Wu WY, Sartel C, Madaci I, Chapon P, Sallet V, Dumont Y, Perez-Tomas A, Horng RH, Chikoidze E Materials Today Physics; 20 (100466) 2021. 10.1016/j.mtphys.2021.100466. IF: 9.298

    The spinel group is a growing family of materials with general formulation AB2X4 (the X anion typically being a chalcogen like O and S) with many advanced applications for energy. At the time being, the spinel zinc gallate (ZnGa2O4) arguably is the ternary ultra-wide bandgap bipolar oxide semiconductor with the largest bandgap (∼5eV), making this material very promising for implementations in deep UV optoelectronics and ultra-high power electronics. In this work, we further demonstrate that, exploiting the rich cation coordination possibilities of the spinel chemistry, the ZnGa2O4 intrinsic conductivity (and its polarity) can be controlled well over 10 orders of magnitude. p-type and n-type ZnGa2O4 epilayers can be grown by tuning the pressure, oxygen flow and cation precursors ratio during metal-organic chemical vapor deposition. A relatively deep acceptor level can be achieved by promoting antisites (ZnGa) defects, while up to a (n > 1019 cm−3) donor concentration is obtained due to the hybridization of the Zn–O orbitals in the samples grown in Zn-rich conditions. Electrical transport, atomic and optical spectroscopy reveal a free hole conduction (at high temperature) for p-ZnGa2O4 while for n-ZnGa2O4 a (Mott) variable range hopping (VRH) and negative magnetoresistance phenomena take place, originated from “self-impurity” band located at Ev+ ∼3.4 eV. Among arising ultra-wide bandgap semiconductors, spinel ZnGa2O4 exhibit unique self-doping capability thus extending its application at the very frontier of current energy optoelectronics.

  • Carbon Incorporation in MOCVD of MoS2Thin Films Grown from an Organosulfide Precursor

    Schaefer C.M., Caicedo Roque J.M., Sauthier G., Bousquet J., Hébert C., Sperling J.R., Pérez-Tomás A., Santiso J., Del Corro E., Garrido J.A. Chemistry of Materials; 33 (12): 4474 - 4487. 2021. 10.1021/acs.chemmater.1c00646. IF: 9.811

    With the rise of two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors and their prospective use in commercial (opto)electronic applications, it has become key to develop scalable and reliable TMD synthesis methods with well-monitored and controlled levels of impurities. While metal-organic chemical vapor deposition (MOCVD) has emerged as the method of choice for large-scale TMD fabrication, carbon (C) incorporation arising during MOCVD growth of TMDs has been a persistent concern-especially in instances where organic chalcogen precursors are desired as a less hazardous alternative to more toxic chalcogen hydrides. However, the underlying mechanisms of such unintentional C incorporation and the effects on film growth and properties are still elusive. Here, we report on the role of C-containing side products of organosulfur precursor pyrolysis in MoS2 thin films grown from molybdenum hexacarbonyl Mo(CO)6 and diethyl sulfide (CH3CH2)2S (DES). By combining in situ gas-phase monitoring with ex situ microscopy and spectroscopy analyses, we systematically investigate the effect of temperature and Mo(CO)6/DES/H2 gas mixture ratios on film morphology, chemical composition, and stoichiometry. Aiming at high-quality TMD growth that typically requires elevated growth temperatures and high DES/Mo(CO)6 precursor ratios, we observed that temperatures above DES pyrolysis onset (â 600 °C) and excessive DES flow result in the formation of nanographitic carbon, competing with MoS2 growth. We found that by introducing H2 gas to the process, DES pyrolysis is significantly hindered, which reduces carbon incorporation. The C content in the MoS2 films is shown to quench the MoS2 photoluminescence and influence the trion-To-exciton ratio via charge transfer. This finding is fundamental for understanding process-induced C impurity doping in MOCVD-grown 2D semiconductors and might have important implications for the functionality and performance of (opto)electronic devices. ©

  • Direct Visualization of Anti-Ferroelectric Switching Dynamics via Electrocaloric Imaging

    Vales-Castro P., Vellvehi M., Perpiñà X., Caicedo J.M., Jordà X., Faye R., Roleder K., Kajewski D., Perez-Tomas A., Defay E., Catalan G. Advanced Electronic Materials; 7 (12, 2100380) 2021. 10.1002/aelm.202100380. IF: 7.295

    The large electrocaloric coupling in PbZrO3 allows using high-speed infrared imaging for visualizing anti-ferroelectric switching dynamics via the associated temperature change. It is found that in ceramic samples of homogeneous temperature and thickness, switching is fast due to the generation of multiple nucleation sites, with devices responding in the millisecond range. By introducing gradients of thickness, however, it is possible to change the dynamics to propagation limited, whereby a single-phase boundary sweeps across the sample like a cold front, at a speed of ≈20 cm s−1. Additionally, introducing thermostatic temperature differences between two sides of the sample enables the simultaneous generation of a negative electrocaloric effect on one side and a positive one on the other, yielding a Janus-like electrocaloric response. © 2021 Wiley-VCH GmbH

  • Origin of large negative electrocaloric effect in antiferroelectric PbZr O3

    Vales-Castro P., Faye R., Vellvehi M., Nouchokgwe Y., Perpiñà X., Caicedo J.M., Jordà X., Roleder K., Kajewski D., Perez-Tomas A., Defay E., Catalan G. Physical Review B; 103 (5, 054112) 2021. 10.1103/PhysRevB.103.054112. IF: 4.036

    We have studied the electrocaloric response of the archetypal antiferroelectric PbZrO3 as a function of voltage and temperature in the vicinity of its antiferroelectric-paraelectric phase transition. Large electrocaloric effects of opposite signs, ranging from an electrocooling of -3.5 K to an electroheating of +5.5K, were directly measured with an infrared camera. We show by calorimetric and electromechanical measurements that the large negative electrocaloric effect comes from an endothermic antiferroelectric-ferroelectric switching, in contrast to dipole destabilization of the antiparallel lattice, previously proposed as an explanation for the negative electrocaloric effect of antiferroelectrics. © 2021 American Physical Society.

  • Status and prospects of cubic silicon carbide power electronics device technology

    Li F., Roccaforte F., Greco G., Fiorenza P., La Via F., Pérez-Tomas A., Evans J.E., Fisher C.A., Monaghan F.A., Mawby P.A., Jennings M. Materials; 14 (19, 5831) 2021. 10.3390/ma14195831. IF: 3.623

    Wide bandgap (WBG) semiconductors are becoming more widely accepted for use in power electronics due to their superior electrical energy efficiencies and improved power densities. Although WBG cubic silicon carbide (3C-SiC) displays a modest bandgap compared to its commercial counterparts (4H-silicon carbide and gallium nitride), this material has excellent attributes as the WBG semiconductor of choice for low-resistance, reliable diode and MOS devices. At present the material remains firmly in the research domain due to numerous technological impediments that hamper its widespread adoption. The most obvious obstacle is defect-free 3C-SiC; presently, 3C-SiC bulk and heteroepitaxial (on-silicon) display high defect densities such as stacking faults and antiphase boundaries. Moreover, heteroepitaxy 3C-SiC-on-silicon means low temperature processing budgets are imposed upon the system (max. temperature limited to ~1400 °C) limiting selective doping realisation. This paper will give a brief overview of some of the scientific aspects associated with 3C-SiC processing technology in addition to focussing on the latest state of the art results. A particular focus will be placed upon key process steps such as Schottky and ohmic contacts, ion implantation and MOS processing including reliability. Finally, the paper will discuss some device prototypes (diodes and MOSFET) and draw conclusions around the prospects for 3CSiC devices based upon the processing technology presented. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.


  • 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.

  • 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.

  • 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


  • Electrical characterisation of thick 3C-SiC layers grown on off-axis 4H-SiC substrates

    Li F., Jokubavicius V., Jennings M., Yakimova R., Pérez-Tomás A., Russell S., Sharma Y., Roccaforte F., Mawby P., Lavia F. Materials Science Forum; 963 MSF: 353 - 356. 2019. 10.4028/

    300 μm thick 3C-SiC epilayer was grown on off-axis 4H-SiC(0001) substrate with a high growth rate of 1 mm/hour. Dry oxidation, wet oxidation and N2O anneal were applied to fabricate lateral MOS capacitors on these 3C-SiC layers. MOS interface obtained by N2O anneal has the lowest interface trap density of 3~4x1011 eV-1cm-2. Although all MOS capacitors still have positive net charges at the MOS interface, the wet oxidised sample has the lowest effective charge density of ~9.17x1011 cm-2. © 2019 Trans Tech Publications Ltd, Switzerland.

  • Enhancing the intrinsic p-type conductivity of the ultra-wide bandgap Ga2O3 semiconductor

    Chikoidze E., Sartel C., Mohamed H., Madaci I., Tchelidze T., Modreanu M., Vales-Castro P., Rubio C., Arnold C., Sallet V., Dumont Y., Perez-Tomas A. Journal of Materials Chemistry C; 7 (33): 10231 - 10239. 2019. 10.1039/c9tc02910a. IF: 6.641

    While there are several n-type transparent semiconductor oxides (TSO) for optoelectronic applications (e.g. LEDs, solar cells or display TFTs), their required p-type counterpart oxides are known to be more challenging. At this time, the n-type TSO with the largest bandgap (∼5 eV) is Ga2O3 that holds the promise of extending the light transparency further into the deep ultraviolet. In this work, it is demonstrated that strongly compensated Ga2O3 is also an intrinsic (or native) p-type TSO with the largest bandgap for any reported p-type TSO (e.g. NiO, SnO, delafossites, oxychalcogenides). The achievement of hole mobility in excess of 10 cm2 V-1 s-1 and (high temperature) free hole concentrations in the ∼1017 cm-3 range challenges the current thinking about achieving p-type conductivity in Ga2O3 being "out of the question". The results presented in this paper therefore further clarify that p-type Ga2O3 is possible, although more research must be conducted to determine what are the real prospects for Ga2O3 solar blind bipolar optoelectronics and ultra-high power electronics based on p-n homojunctions. © The Royal Society of Chemistry 2019.

  • Functional Oxides for Photoneuromorphic Engineering: Toward a Solar Brain

    Pérez-Tomás A. Advanced Materials Interfaces; 6 (15, 1900471) 2019. 10.1002/admi.201900471. IF: 4.713

    New device concepts and new computing principles are needed to balance our ever-growing appetite for data and information with the realization of the goals of increased energy efficiency, reduction in CO2 emissions, and the circular economy. Neuromorphic or synaptic electronics is an emerging field of research aiming to overcome the current computer's Von-Neumann bottleneck by building artificial neuronal systems to mimic the extremely energy efficient biological synapses. The introduction of photovoltaic and/or photonic aspects into these neuromorphic architectures will produce self-powered adaptive electronics but may also open new possibilities in artificial neuroscience, artificial neural communications, sensing, and machine learning which would enable, in turn, a new era for computational systems owing to the possibility of attaining high bandwidths with much reduced power consumption. This perspective is focused on recent progress in the implementation of functional oxide thin-films into photovoltaic and neuromorphic applications toward the envisioned goal of self-powered photovoltaic neuromorphic systems or a solar brain. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

  • Giant bulk photovoltaic effect in solar cell architectures with ultra-wide bandgap Ga2O3 transparent conducting electrodes

    Pérez-Tomás A., Chikoidze E., Dumont Y., Jennings M.R., Russell S.O., Vales-Castro P., Catalan G., Lira-Cantú M., Ton –That C., Teherani F.H., Sandana V.E., Bove P., Rogers D.J. Materials Today Energy; 14 (100350) 2019. 10.1016/j.mtener.2019.100350. IF: 0.000

    The use of ultra-wide bandgap transparent conducting beta gallium oxide (β-Ga2O3) thin films as electrodes in ferroelectric solar cells is reported. In a new material structure for energy applications, we report a solar cell structure (a light absorber sandwiched in between two electrodes - one of them - transparent) which is not constrained by the Shockley–Queisser limit for open-circuit voltage (Voc) under typical indoor light. The solar blindness of the electrode enables a record-breaking bulk photovoltaic effect (BPE) with white light illumination (general use indoor light). This work opens up the perspective of ferroelectric photovoltaics which are not subject to the Shockley-Queisser limit by bringing into scene solar-blind conducting oxides. © 2019 Elsevier Ltd

  • PbZrTiO 3 ferroelectric oxide as an electron extraction material for stable halide perovskite solar cells

    Pérez-Tomas A., Xie H., Wang Z., Kim H.-S., Shirley I., Turren-Cruz S.-H., Morales-Melgares A., Saliba B., Tanenbaum D., Saliba M., Zakeeruddin S.M., Gratzel M., Hagfeldt A., Lira-Cantu M. Sustainable Energy and Fuels; 3 (2): 382 - 389. 2019. 10.1039/c8se00451j. IF: 4.912

    State-of-the-art halide perovskite solar cells employ semiconductor oxides as electron transport materials. Defects in these oxides, such as oxygen vacancies (O vac ), act as recombination centres and, in air and UV light, reduce the stability of the solar cell. Under the same conditions, the PbZrTiO 3 ferroelectric oxide employs O vac for the creation of defect-dipoles responsible for photo-carrier separation and current transport, evading device degradation. We report the application of PbZrTiO 3 as the electron extraction material in triple cation halide perovskite solar cells. The application of a bias voltage (poling) up to 2 V, under UV light, is a critical step to induce charge transport in the ferroelectric oxide. Champion cells result in power conversion efficiencies of ∼11% after poling. Stability analysis, carried out at 1-sun AM 1.5 G, including UV light in air for unencapsulated devices, shows negligible degradation for hours. Our experiments indicate the effect of ferroelectricity, however alternative conducting mechanisms affected by the accumulation of charges or the migration of ions (or the combination of them) cannot be ruled out. Our results demonstrate, for the first time, the application of a ferroelectric oxide as an electron extraction material in efficient and stable PSCs. These findings are also a step forward in the development of next generation ferroelectric oxide-based electronic and optoelectronic devices. © 2019 The Royal Society of Chemistry.

  • Puzzling robust 2D metallic conductivity in undoped β-Ga 2 O 3 thin films

    Chikoidze E., Rogers D.J., Teherani F.H., Rubio C., Sauthier G., Von Bardeleben H.J., Tchelidze T., Ton-That C., Fellous A., Bove P., Sandana E.V., Dumont Y., Perez-Tomas A. Materials Today Physics; 8: 10 - 17. 2019. 10.1016/j.mtphys.2018.11.006. IF: 0.000

    Here, we report the analogy of an extremely stable topological-like ultra-wide bandgap insulator, a solid that is a pure insulator in its bulk but has a metallic conductive surface, presenting a two-dimensional conductive channel at its surface that challenges our current thinking about semiconductor conductivity engineering. Nominally undoped epitaxial β-Ga 2 O 3 thin films without any detectable defect (after a range of state-of-the-art techniques) showed the unexpectedly low resistivity of 3 × 10 −2 Ωcm which was found to be also resistant to high dose proton irradiation (2 MeV, 5 × 10 15 cm −2 dose) and was largely invariant (metallic) over the phenomenal temperature range of 2 K up to 850 K. The unique resilience and stability of the electrical properties under thermal and highly ionizing radiation stressing, combined with the extended transparency range (thanks to the ultra-wide bandgap) and the already known toughness under high electrical field could open up new perspectives for use as expanded spectral range transparent electrodes (e.g., for UV harvesting solar cells or UV LEDs/lasers) and robust Ohmic contacts for use in extreme environments/applications and for novel optoelectronic and power device concepts. © 2018 Elsevier Ltd


  • A Solar Transistor and Photoferroelectric Memory

    Pérez-Tomás A., Lima A., Billon Q., Shirley I., Catalan G., Lira-Cantú M. Advanced Functional Materials; 28 (17, 1707099) 2018. 10.1002/adfm.201707099. IF: 13.325

    This study presents a new self-powered electronic transistor concept “the solar transistor.” The transistor effect is enabled by the functional integration of a ferroelectric-oxide thin film and an organic bulk heterojunction. The organic heterojunction efficiently harvests photon energy and splits photogenerated excitons into free electron and holes, and the ferroelectric film acts as a switchable electron transport layer with tuneable conduction band offsets that depend on its polarization state. This results in the device photoconductivity modulation. All this (i.e., carrier extraction and poling) is achieved with only two sandwiched electrodes and therefore, with the role of the gating electrode being taken by light. The two-terminal solar-powered phototransistor (or solaristor) thus has the added advantages of a compact photodiode architecture in addition to the nonvolatile functionality of a ferroelectric memory that is written by voltage and nondestructively read by light. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

  • Metal Oxides in Photovoltaics: All-Oxide, Ferroic, and Perovskite Solar Cells

    Pérez-Tomás A., Mingorance A., Tanenbaum D., Lira-Cantú M. The Future of Semiconductor Oxides in Next-Generation Solar Cells; : 267 - 356. 2018. 10.1016/B978-0-12-811165-9.00008-9.

    Semiconductor oxides have been applied in photovoltaic technologies for many years. The remarkable versatility of their properties and the feasibility to be fabricated by simple, low-cost and easily scalable fabrication methods confers oxides a unique place in next generation photovoltaics (NGPVs). Their outstanding ability to preserve or improve device characteristics, even as a noncrystalline (amorphous) material, allows their application in flexible and semitransparent PVs devices and printed electronics. Basic (doped and undoped) semiconductor oxides have demonstrated to provide enhanced lifetime stability to state-of-the-art PVs such as Organic (OPV) and halide perovskite solar cells (PSCs), which is a significant step toward NGPVs industrialization and commercialization. But semiconductor oxides, in their more complex form, can also provide properties like magnetism, ferroelectricity, or pyroelectricity (among others), that collectively with the most classical materials, can deliver novel and innovative features. This chapter documents the most recent results observed when semiconductor oxides are applied in different NGPVs technologies. The chapter covers technologies like all-oxide solar cells where the oxide is not only part of the device but also acts as the main light harvesting material. The chapter also describes the application of oxides as part of OPV and PSCs where semiconductor oxides are mostly applied as barrier layers eliminating the use of expensive and unstable organic semiconductors, enhancing device lifetime. © 2018 Elsevier Inc. All rights reserved.

  • Wide and ultra-wide bandgap oxides: Where paradigm-shift photovoltaics meets transparent power electronics

    Pérez-Tomás A., Chikoidze E., Jennings M.R., Russell S.A.O., Teherani F.H., Bove P., Sandana E.V., Rogers D.J. Proceedings of SPIE - The International Society for Optical Engineering; 10533 ( 105331Q) 2018. 10.1117/12.2302576. IF: 0.000

    Oxides represent the largest family of wide bandgap (WBG) semiconductors and also offer a huge potential range of complementary magnetic and electronic properties, such as ferromagnetism, ferroelectricity, antiferroelectricity and high-temperature superconductivity. Here, we review our integration of WBG and ultra WBG semiconductor oxides into different solar cells architectures where they have the role of transparent conductive electrodes and/or barriers bringing unique functionalities into the structure such above bandgap voltages or switchable interfaces. We also give an overview of the state-of-the-art and perspectives for the emerging semiconductor β- Ga2O3, which is widely forecast to herald the next generation of power electronic converters because of the combination of an UWBG with the capacity to conduct electricity. This opens unprecedented possibilities for the monolithic integration in solar cells of both self-powered logic and power electronics functionalities. Therefore, WBG and UWBG oxides have enormous promise to become key enabling technologies for the zero emissions smart integration of the internet of things. © Copyright 2018 SPIE.


  • Functional oxide as an extreme high-k dielectric towards 4H-SiC MOSFET incorporation

    Russell S.A.O., Jennings M.R., Dai T., Li F., Hamilton D.P., Fisher C.A., Sharma Y.K., Mawby P.A., Pérez-Tomás A. Materials Science Forum; 897 MSF: 155 - 158. 2017. 10.4028/ IF: 0.399

    MOS Capacitors are demonstrated on 4H-SiC using an octahedral ABO3 ferroic thin-film as a dielectric prepared on several buffer layers. Five samples were prepared: ABO3 on SiC, ABO3 on SiC with a SiO2 buffer (10 nm and 40 nm) and ABO3 on SiC with an Al2O3 buffer (10nm and 40 nm). Depending on the buffer material the oxide forms in either the pyrochlore or perovskite phase. A better lattice match with the Al2O3 buffer yields a perovskite phase with internal switchable dipoles. Hysteresis polarization-voltage loops show an oxide capacitance of ~ 0.2 μF/cm2 in the accumulation region indicating a dielectric constant of ~120. © 2017 Trans Tech Publications, Switzerland.

  • Heteroepitaxial Beta-Ga2O3 on 4H-SiC for an FET with Reduced Self Heating

    Russell S.A.O., Perez-Tomas A., McConville C.F., Fisher C.A., Hamilton D.P., Mawby P.A., Jennings M.R. IEEE Journal of the Electron Devices Society; 5 (4, 7932063): 256 - 261. 2017. 10.1109/JEDS.2017.2706321. IF: 3.141

    A method to improve thermal management of ${\beta }$ -Ga2O3 FETs is demonstrated here via simulation of epitaxial growth on a 4H-SiC substrate. Using a recently published device as a model, the reduction achieved in self-heating allows the device to be driven at higher gate voltages and increases the overall performance. For the same operating parameters an 18% increase in peak drain current and 15% reduction in lattice temperature are observed. Device dimensions may be substantially reduced without detriment to performance and normally off operation may be achieved. © 2013 IEEE.

  • High-Temperature Electrical and Thermal Aging Performance and Application Considerations for SiC Power DMOSFETs

    Hamilton D.P., Jennings M.R., Perez-Tomas A., Russell S.A.O., Hindmarsh S.A., Fisher C.A., Mawby P.A. IEEE Transactions on Power Electronics; 32 (10, 7776925): 7967 - 7979. 2017. 10.1109/TPEL.2016.2636743. IF: 7.151

    The temperature dependence and stability of three different commercially-available unpackaged SiC Dmosfets have been measured. On-state resistances increased to 6 or 7 times their room temperature values at 350 °C. Threshold voltages almost doubled after tens of minutes of positive gate voltage stressing at 300 °C, but approached their original values again after only one or two minutes of negative gate bias stressing. Fortunately, the change in drain current due to these threshold instabilities was almost negligible. However, the threshold approaches zero volts at high temperatures after a high temperature negative gate bias stress. The zero gate bias leakage is low until the threshold voltage reduces to approximately 150 mV, where-after the leakage increases exponentially. Thermal aging tests demonstrated a sudden change from linear to nonlinear output characteristics after 24-100 h air storage at 300 °C and after 570-1000 h in N2 atmosphere. We attribute this to nickel oxide growth on the drain contact metallization which forms a heterojunction p-n diode with the SiC substrate. It was determined that these state-of-the-art SiC mosfet devices may be operated in real applications at temperatures far exceeding their rated operating temperatures. © 1986-2012 IEEE.

  • Physical characterisation of 3C-SiC(001)/SiO2 interface using XPS

    Li F., Vavasour O., Walker M., Martin D.M., Sharma Y., Russell S., Jennings M., Pérez-Tomás A., Mawby P.A. Materials Science Forum; 897 MSF: 151 - 154. 2017. 10.4028/ IF: 0.399

    Normally-on MOSFETs were fabricated on 3C-SiC epilayers (Si face) using high temperature (1300 °C) wet oxidation. XPS analysis found little carbon at the MOS interface yet the channel mobility (60 cm2/V.s) is considerably low. Si suboxides (SiOx, x<2) exist at the wet oxidised 3C-SiC/SiO2 interface, which may act as interface traps and degrade the conduction performance. © 2017 Trans Tech Publications, Switzerland.


  • 3C-SiC Transistor with Ohmic Contacts Defined at Room Temperature

    Li F., Sharma Y., Walker D., Hindmarsh S., Jennings M., Martin D., Fisher C., Gammon P., Pérez-Tomás A., Mawby P. IEEE Electron Device Letters; 37 (9, 7518645): 1189 - 1192. 2016. 10.1109/LED.2016.2593771. IF: 2.528

    Among all SiC polytypes, only 3C-SiC has a cubic structure and can be hetero-epitaxial grown on large area Si substrate, thus providing an alternative choice for fabricating cheap wide bandgap power devices. Here, we present a low resistivity (~3 × 10-5Ω cm2) ohmic contact formed by directly depositing a Ti/Ni metal stack on n-type 3C-SiC without any extra annealing. For the first time, 3C-SiC lateral MOSFETs with asdeposited ohmic contacts were fabricated, and it turned out not only the ohmic contact is free from any interface voids, but also a higher field-effect mobility value (~80 cm2/V · s) was achieved compared with the annealed devices. © 1980-2012 IEEE.

  • Above-Bandgap Photovoltages in Antiferroelectrics

    Pérez-Tomás A., Lira-Cantú M., Catalan G. Advanced Materials; 28 (43): 9644 - 9647. 2016. 10.1002/adma.201603176. IF: 18.960

    The closed circuit photocurrent and open circuit photovoltage of antiferroelectric thin films were characterized both in their ground (antipolar) state and in their polarized state. A sharp transition happens from near zero to large photovoltages as the polarization is switched on, consistent with the activation of the bulk photovoltaic effect. The AFE layers have been grown by a solution processing method (sol?gel synthesis followed by spin coating deposition) onto fluorine-doped tin oxide (FTO), a transparent conducting oxide with low sheet resistance and a higher resilience to high-temperature processing than indium tin oxide and a standard for solar cells such as organometal trihalide perovskites. Light absorption confirmed that the PZO films are, as expected, wide-band gap semiconductors with a gap of 3.7.8 eV and thus highly absorbing in the near-ultraviolet range. On a virgin sample, there is no shortcircuit photocurrent, consistent with the antipolar nature of the ground state. As an external bias voltage is applied, the current remains negligible until suddenly, at the coercive voltage, a spike is observed, corresponding to the transient displacement current caused by the onset of polarization.

  • Improved channel mobility by oxide nitridation for n-channel MOSFET on 3C-SiC(100)/Si

    Li F., Sharma Y.K., Jennings M.R., Pérez-Tomás A., Shah V.A., Rong H., Russell S.A.O., Martin D.M., Mawby P.A. Materials Science Forum; 858: 667 - 670. 2016. 10.4028/ IF: 0.000

    In this work we studied the gate oxidation temperature and nitridation influences on the resultant 3C-SiC MOSFET forward characteristics. Conventional long channel lateral MOSFETs were fabricated on 3C-SiC(100) epilayers grown on Si substrates using five different oxidation processes. Both room temperature and high temperature (up to 500K) forward IV performance were characterised, and channel mobility as high as 90cm2/V.s was obtained for devices with nitrided gate oxide, considerable higher than the ones without nitridation process (~70 cm2/V.s). © 2016 Trans Tech Publications, Switzerland.

  • Performance and stability of mixed FAPbI3(0.85)MAPbBr3(0.15) halide perovskite solar cells under outdoor conditions and the effect of low light irradiation

    Reyna Y., Salado M., Kazim S., Pérez-Tomas A., Ahmad S., Lira-Cantu M. Nano Energy; 30: 570 - 579. 2016. 10.1016/j.nanoen.2016.10.053. IF: 11.553

    We demonstrate for the first time, the real lifetime response of mixed halide perovskite solar cells (PSCs) for >1000 h under outdoor conditions and the exceptional photoresponse observed at low-light intensities attributed to the ionic-electronic nature of the material. The investigated devices were fabricated by utilizing mixed perovskites containing formamidinium (FA) and methylammonium (MA) cations, in a one step solution-process method through a solvent engineering approach. The devices’ architecture is FTO/TiO2 (blocking layer) TiO2 (mesoporous)/FAPbI3(0.85)MAPbBr3(0.15)/Spiro-OMeTAD/Au. Notably, low short circuit current (Jsc) was observed at low light intensities (<50 W/m2) together with high open circuit potential build-up, which resulted in high PCEs. This response is in agreement with a “double electronic-ionic transport” model of the halide perovskite where the ionic component dominates at low light intensities and the electronic component dictates at high light irradiances. Our results highlight the exceptional stability of mixed MA/FA mesoscopic PSCs when operated for >1000 h under real outdoor conditions and the strong ionic component observed at low light irradiation. © 2016 Elsevier Ltd


  • Electrical activation of nitrogen heavily implanted 3C-SiC(1 0 0)

    Li F., Sharma Y., Shah V., Jennings M., Pérez-Tomás A., Myronov M., Fisher C., Leadley D., Mawby P. Applied Surface Science; 353: 958 - 963. 2015. 10.1016/j.apsusc.2015.06.169. IF: 2.711

    A degenerated wide bandgap semiconductor is a rare system. In general, implant levels lie deeper in the band-gap and carrier freeze-out usually takes place at room temperature. Nevertheless, we have observed that heavily doped n-type degenerated 3C-SiC films are achieved by nitrogen implantation level of ∼6 × 1020 cm-3 at 20 K. According to temperature dependent Hall measurements, nitrogen activation rates decrease with the doping level from almost 100% (1.5 × 1019 cm-3, donor level 15 meV) to ∼12% for 6 × 1020 cm-3. Free donors are found to saturate in 3C-SiC at ∼7 × 1019 cm-3. The implanted film electrical performances are characterized as a function of the dopant doses and post implantation annealing (PIA) conditions by fabricating Van der Pauw structures. A deposited SiO2 layer was used as the surface capping layer during the PIA process to study its effect on the resultant film properties. From the device design point of view, the lowest sheet resistivity (∼1.4 mΩ cm) has been observed for medium doped (4 × 1019 cm-3) sample with PIA 1375 °C 2 h without a SiO2 cap. Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.

  • Fabrication of 3C-SiC MOS capacitors using high-temperature oxidation

    Sharma Y.K., Li F., Fisher C.A., Jennings M.R., Hamilton D., Thomas S.M., Pérez-Tomás A., Mawby P.A. Materials Science Forum; 821-823: 464 - 467. 2015. 10.4028/

    A systematic study on the 3C-SiC/SiO2 interface has been done. 3C-SiC epilayers have been grown on as Si (001) substrate. Results obtained from room temperature conductance-voltage (G-V) and hi-low capacitance-voltage (C-V) on n-type 3C-SiC/SiO2 metal-oxide-semiconductor capacitors (MOS-Cs) have been reported using various types of oxides. The oxides used in these studies have been thermally grown at different oxidation temperatures - 1200°C, 1300°C and 1400°C. Also, the interface trap density (Dit) of as-grown MOS-C is compared with nitrided (thermally grown oxide + N2O post-oxidation annealing) oxides. Oxide grown at 1300°C followed by N2O-passivation at the same temperature gives the lowest Dit of 6x1011 cm-2eV-1 at 0.2eV from the conduction band (CB) edge. © (2015) Trans Tech Publications, Switzerland.

  • High-Temperature (1200–1400°C) Dry Oxidation of 3C-SiC on Silicon

    Sharma Y.K., Li F., Jennings M.R., Fisher C.A., Pérez-Tomás A., Thomas S., Hamilton D.P., Russell S.A.O., Mawby P.A. Journal of Electronic Materials; 44 (11): 4167 - 4174. 2015. 10.1007/s11664-015-3949-4. IF: 1.798

    In a novel approach, high temperatures (1200–1400°C) were used to oxidize cubic silicon carbide (3C-SiC) grown on silicon substrate. High-temperature oxidation does not significantly affect 3C-SiC doping concentration, 3C-SiC structural composition, or the final morphology of the SiO2 layer, which remains unaffected even at 1400°C (the melting point of silicon is 1414°C). Metal-oxide-semiconductor capacitors (MOS-C) and lateral channel metal-oxide-semiconductor field-effect-transistors (MOSFET) were fabricated by use of the high-temperature oxidation process to study 3C-SiC/SiO2 interfaces. Unlike 4H-SiC MOSFET, there is no extra benefit of increasing the oxidation temperature from 1200°C to 1400°C. All the MOSFET resulted in a maximum field-effect mobility of approximately 70 cm2/V s. © 2015, The Minerals, Metals & Materials Society.

  • Multiwavelength excitation Raman scattering analysis of bulk and two-dimensional MoS2: Vibrational properties of atomically thin MoS2 layers

    Placidi M., Dimitrievska M., Izquierdo-Roca V., Fontané X., Castellanos-Gomez A., Pérez-Tomás A., Mestres N., Espindola-Rodriguez M., López-Marino S., Neuschitzer M., Bermudez V., Yaremko A., Pérez-Rodríguez A. 2D Materials; 2 (3, 035006) 2015. 10.1088/2053-1583/2/3/035006.

    In order to deepen the knowledge of the vibrational properties of two-dimensional (2D) MoS2 atomic layers, a complete and systematic Raman scattering analysis has been performed using both bulk single-crystal MoS2 samples and atomically thin MoS2 layers. Raman spectra have been measured under non-resonant and resonant conditions using seven different excitation wavelengths from near-infrared (NIR) to ultraviolet (UV). These measurements have allowed us to observe and identify 41 peaks, among which 22 have not been previously experimentally observed for this compound, and characterize the existence of different resonant excitation conditions for the different excitation wavelengths. This has also included the first analysis of resonant Raman spectra that are achieved using UV excitation conditions. In addition, the analysis of atomically thin MoS2 layers has corroborated the higher potential of UV resonant Raman scattering measurements for the non-destructive assessment of 2D MoS2 samples. Analysis of the relative integral intensity of the additional first- and second-order peaks measured under UV resonant excitation conditions is proposed for the non-destructive characterization of the thickness of the layers, complementing previous studies based on the changes of the peak frequencies. © 2015 IOP Publishing Ltd.

  • Nanoscale conductive pattern of the homoepitaxial AlGaN/GaN transistor

    Pérez-Tomás A., Catalàn G., Fontserè A., Iglesias V., Chen H., Gammon P.M., Jennings M.R., Thomas M., Fisher C.A., Sharma Y.K., Placidi M., Chmielowska M., Chenot S., Porti M., Nafría M., Cordier Y. Nanotechnology; 26 (11, 115203) 2015. 10.1088/0957-4484/26/11/115203. IF: 3.821

    The gallium nitride (GaN)-based buffer/barrier mode of growth and morphology, the transistor electrical response (25-310 C) and the nanoscale pattern of a homoepitaxial AlGaN/GaN high electron mobility transistor (HEMT) have been investigated at the micro and nanoscale. The low channel sheet resistance and the enhanced heat dissipation allow a highly conductive HEMT transistor (Ids>1 A mm-1) to be defined (0.5 A mm-1 at 300 C). The vertical breakdown voltage has been determined to be ∼850 V with the vertical drain-bulk (or gate-bulk) current following the hopping mechanism, with an activation energy of 350 meV. The conductive atomic force microscopy nanoscale current pattern does not unequivocally follow the molecular beam epitaxy AlGaN/GaN morphology but it suggests that the FS-GaN substrate presents a series of preferential conductive spots (conductive patches). Both the estimated patches density and the apparent random distribution appear to correlate with the edge-pit dislocations observed via cathodoluminescence. The sub-surface edge-pit dislocations originating in the FS-GaN substrate result in barrier height inhomogeneity within the HEMT Schottky gate producing a subthreshold current. © 2015 IOP Publishing Ltd.

  • Simulations of a lateral PiN diode on Si/SiC substrate for high temperature applications

    Chan C.W., Gammon P.M., Shah V.A., Chen H., Jennings M.R., Fisher C.A., Pérez-Tomás A., Myronov M., Mawby P.A. Materials Science Forum; 821-823: 624 - 627. 2015. 10.4028/

    Simulations are presented of a lateral PiN power diode on a Si/SiC substrate for harsh environment, high temperature applications. Thermal simulations compare the Si/SiC solution to SOI, Si/SiO2/SiC, bulk Si and SiC, showing that the Si/SiC architecture, with its thin Si film intimately formed on SiC, displays significant thermal advantages over any other Si solution, and is comparable to bulk SiC. Detailed electrical simulations show that in comparison to the same device in SOI, a Si/SiC PiN diode offers no deterioration of the on-state performance, improved self-heating effects at increased current and can potentially support higher breakdown voltages. © (2015) Trans Tech Publications, Switzerland.


  • Enhanced field effect mobility on 4H-SiC by oxidation at 1500°C

    Thomas S.M., Sharma Y.K., Crouch M.A., Fisher C.A., Perez-Tomas A., Jennings M.R., Mawby P.A. IEEE Journal of the Electron Devices Society; 2 (5, 6849425): 114 - 117. 2014. 10.1109/JEDS.2014.2330737.

    A novel 1500°C gate oxidation process has been demonstrated on Si face of 4H-SiC. Lateral channel metal-oxide-semiconductor-field-effect-transistors (MOSFETs) fabricated using this process have a maximum field effect mobility of approximately 40 cm\2 V-1 s-1 without post oxidation passivation. This is substantially higher than other reports of MOSFETs with thermally grown oxides (typically grown at the standard silicon temperature range of 1100-1200°C). This result shows the potential of a high temperature oxidation step for reducing the channel resistance (thus the overall conduction loss), in power 4H-SiC MOSFETs. © 2014 IEEE.

  • Enhanced forward bias operation of 4H-SiC PiN diodes using high temperature oxidation

    Fisher C.A., Jennings M.R., Sharma Y.K., Hamilton D.P., Thomas S.M., Li F., Gammon P.M., Pérez-Tomás A., Burrows S.E., Mawby P.A. Materials Research Society Symposium Proceedings; 1693 2014. 10.1557/opl.2014.714.

    In this paper, high temperature (>1400°C) thermal oxidation has been applied, for the first time, to 4H-SiC PiN diodes with thick (110 μm) drift regions, for the purpose of increasing the carrier lifetime in the semiconductor. PiN diodes were fabricated using 4H-SiC material that had undergone thermal oxidation performed at 1400°C, 1500°C and 1600°C, then were electrically characterized. Forward current-voltage (I-V) measurements showed that thermally oxidized PiN diodes exhibited considerably improved electrical characteristics, with devices oxidized at 1500°C having a forward voltage drop (V F) of 4.15 V and a differential on-resistance (R on,diff) of 8.9 mΩ-cm2 at 100 A/cm2 and 25°C. Compared to typical control sample PiN diode characteristics, this equated to an improvement of 8% and 23% for V F and R on,diff, respectively. From analysis of the reverse recovery characteristics, the carrier lifetime of the PiN diodes oxidized at 1500°C was found to be 1.05 μs, which was an improvement of around 30% compared to the control sample PiN diodes. Copyright © 2014 Materials Research Society.

  • Improved performance of 4H-SiC PiN diodes using a novel combined high temperature oxidation and annealing process

    Fisher, C.A.; Jennings, M.R.; Sharma, Y.K.; Hamilton, D.P.; Gammon, P.M.; Pérez-Tomás, A.; Thomas, S.M.; Burrows, S.E.; Mawby, P.A. IEEE Transactions on Semiconductor Manufacturing; 27 (3): 443 - 451. 2014. 10.1109/TSM.2014.2336701. IF: 0.977

  • On the Schottky barrier height lowering effect of Ti3SiC2 in ohmic contacts to p-type 4H-SiC

    C. A. Fisher; M. R. Jennings; Y. K. Sharma; A. Sanchez; D. Walker; P. M. Gammon; A. Pérez-Tomás; S. M. Thomas; S. E. Burrows; P. A. Mawby International Journal of Fundamental Physical Sciences (IJFPS); 4 (3): 95 - 100. 2014. 10.14331/ijfps.2014.330071. IF: 0.000