Surface profile gradient in amorphous Ta2O5 semi conductive layers regulates nanoscale electric current stability

Significance Statement

The structure of amorphous semiconductors, such as tantalum oxide, show different kinetic and electrical characteristics compared with the crystalline structures, and even then, there is a huge variation in properties within the same amorphous material structure which is not properly understood at the nanoscale level. Some of the differences include reduced leakage currents, grouping of electrons between the interface of nanodomains, and increased density of faulty electronic states.

In a recent paper published in Applied Surface Science Professor A.C Cefalas and colleagues demonstrated that the surface structure of semi-conductive layers of amorphous tantalum oxide at the nanoscale level is closely connected with both the thermal and electric properties, and that it regulates the stability of electric current.

 Analyzing conductivity maps it was evident that when positive voltages are low, most layer areas are non-conductive with a small portion displaying low-conductivity, while at increased voltages the conductivity becomes saturated. In most layer areas, there is negligible current at negative voltages. An analysis showed that there was a diverting I-V response for different amorphous tantalum oxides, and the average threshold conductive voltage was approximately 3.4V. Most areas remained non-conductive which was evidence that there was no continuity and surface homogeneity.

The research team interestingly observed that some points responded to only positive or negative bias voltages, and other points responded to both positive and negative voltages as a result of either interactive or non-interactive electron Coulombic trapping sites from impurities or oxygen vacancies. From the electrostatic force microscopy phase shift spectrum, the authors observed no phase contrast at zero voltage but at higher voltages there was a substantial phase shift contrast. This is an indication that induced electric charges at increased bias voltages are widely dispersed and separated by a potential barrier between adjacent conductive channels. A mapping of the bias voltages and phase differences indicates zero phase difference at zero voltage which confirms that the surface of the amorphous tantalum oxide is non-polarized.

From the scanning force, thermal and electrical microscopy analysis, the authors noted a concealed spatial gradient degeneracy of observables as a result of the occurrence of opposite algebraic spatial gradients signs along opposite conductive paths, which ceases for increased path length where the spatial profile gradient approaches zero. The entropic and electric current linking at the nanoscale elevates the degeneracy of the gradient through either amplified or dumping current fluctuations which is because surface morphological gradients reflect the electron distribution gradient on the surface.

The study demonstrated that structural features of the amorphous tantalum oxide regulate the directional stability of current at the nanoscale level, and they are correlated with the thermal and electric responses along the conductive paths.

 It was also evident that for long conductive paths, the I-V curves exhibit bidirectional current stability while there were different I-V curve stability responses for short conductive paths, because of  the unique transformation properties of the current density and the surface profile gradient pseudo-vectors upon reflections that are different for those of proper vectors.  Moreover, the interaction between entropic and electric production currents together with the symmetries of surface profile gradients at the nanoscale level affected the current stability response.

Surface profile gradient in amorphous Ta2O5 semi conductive layers regulates nanoscale electric current stability- Advances in Engineering

About the author

Alkiviadis-Constantinos Cefalas is a director of Research at the National Hellenic Research Foundation (NHRF), Theoretical and Physical Chemistry Institute (TPCI), Athens Greece and a visiting Professor at Kazan Federal University, Russia. He studied Physics at the National and Kapodistrian University of Athens (B.Sc. 1978) and the Victorian University of Manchester, UK (M.Sc. 1980, PhD. 1983).

His current research activities include fundamental interactions at the nanoscale level, nano-thermodynamics, complexity and biophysics. He was a national expert on nanotechnology in the European Commission from 2004-2006, National Representative of Greece for Nanotechnology in the European Commission from 2006-2010 and a visiting professor of quantum and nonlinear optics at the Postgraduate School of Josef Stefan Institute, Ljubljana, Slovenia. He is the author of 130 published papers at high reputed international scientific journals and he was participating as a principal investigator in different international research projects, (EU, ESA, NATO).

About the author

Zoe Kollia is a researcher at the Photonics for Nano Applications Laboratory (NHRF/TPCI), Athens, Greece. She received the B.Sc in Physics from the University of Crete, Greece and her Ph.D. thesis, from the National Technical University of Athens, on Laser Physics and material science.  She works at Short Light Wavelengths and Nano-applications Laboratory. She is the co-author of 71 scientific publications in international refereed journals, 22 publications in conference proceedings. She has participated in more than 110 international conferences. She has been involved in various international research projects (FP5, FP6, FP7, NATO, and ESA).

Her main research activities include electronic, optical and magnetic properties of nanocomposites,  high-resolution imaging techniques (SPM & AFM) and surface modification and functionalization of organic thin films with high energy photons.

About the author

Vassilios E. Gavriil awarded a B.Sc. honors degree in Electrical and Computer Engineering from the National Technical University of Athens in 2009 and an M.Sc on Microsystems and Nanodevices at the same University. Today he is a Ph.D. student working on electric and optic properties of nanomaterials at the Aristotle University of Thessaloniki and the National Hellenic Research Foundation, Theoretical and Physical Chemistry Institute, Athens, Greece. His research interest involves renewable energy, nanotechnology, optics, imaging, bio-memetics.

About the author

Dimitris Christofilos obtained his degree in Physics in 1990 from the Aristotle University of Thessaloniki (AUTh), Greece and subsequently his master’s degree in Electronic Physics (Radioelectrology) and his Ph.D. degree in Physics in 1997 from the same University. He subsequently worked as a post-doctoral fellow in Ecole Polytechnique, Palaiseau, France (1998-1999), as an assistant professor at the Technological Educational Institute of Thessaloniki and as a researcher in AUTh (2000-2001), and as an invited CNRS researcher at the University of Bordeaux, France (2002). Since 2003 he holds a position in the Faculty of Engineering of AUTh, currently as Associate Professor in the School of Chemical Engineering, while he has been a visiting professor/researcher at Lyon I University/CNRS on several occasions (2008, 2009, 2012).

His research interests include high-pressure physics of condensed matter, Raman, absorption and time-resolved spectroscopy while in recent years he has focused his research activities on the investigation of nanostructures, among others, graphene, carbon nanotubes and metallic nanoparticles.  

About the author

Gerasimos A. Kourouklis is a Professor (Emeritus),  at the Department of Chemical Engineering, School of Technology, Aristotle University, Thessaloniki, Greece. He received a  Diploma Degree  in Physics,  from the National University of Athens, GREECE in 1972 and his Ph.D. from the University of Tennessee, Knoxville,Tennessee,U.S.A. He was  an Assistant Professor at the Department of Physics, University of Ioannina, Ioannina, Greece(1975-1978),  Graduate Teaching Assistant. Department of Physics, The University of Tennessee,Knoxville, and Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A., (1978-1982), Postdoctoral Research Fellow. Oak Ridge National Laboratory, and Department of Physics, The University of Tennessee Knoxville, Tennessee, U.S.A (1982-1983),  Lecturer. Department of Physics, National Technical University, Athens, Greece (1983-1988),  Associate Professor. Physics Division, School of Technology, University of Thessaloniki, Thessaloniki, Greece (1988-1993), Professor. Physics Division, School of  Technology, Aristotle University, Thessaloniki, Greece (1993-Today).

He is a member of  Society of Greek Physicists, Optical Society of America,  European High-Pressure Research Group (EHPRG), Elected Member of the organizing committee of the EHPRG,( 1985-1988) and Elected Member of the organizing committee of the EHPRG, ( 1990-1993). His research Interests include Optical properties of materials, Properties of materials under extreme conditions of pressure and temperature, Properties of Fullerenes and carbon nanotubes, Phase transitions of materials induced by high pressure, Optical spectroscopy, Raman spectroscopy.

About the author

Vadim Semashko is a professor and a leading researcher in the Department of quantum electronics and radio spectroscopy of the Institute of Physics at Kazan Federal University. He studied radiophysics at Kazan State University and then completed PhD degree in solid-state physics. His research areas include optical spectroscopy of doped dielectric materials, laser spectroscopy, laser physics, nonlinear optics, properties of nanosized objects, crystal, ceramic and composite functional materials, the design of new materials, biophotonic.

His work has been recognized by numerous awards, including the International Science Foundation individual grant (1991), Diploma of Russian Federation Ministry of Education for the scientific supervising of student’s research (2000), Certificates of honor for scientific activity & innovations (2004, 2005, 2014, 2015, 2016). His published papers have been cited more than 1120 times.

About the author

Vitaly Pavlov is a junior researcher and an assistant lecturer in the Department of quantum electronics and radio spectroscopy of the Institute of Physics at Kazan Federal University. He received his Ph.D. from Kazan Federal University (2016).

His research is focused on the optical spectroscopy of rare-earth doped crystals, the photoconductivity measurements of dielectric crystals, semiconductors thin films and nanoparticles with the development of novel microwave resonant technique and the room-temperature ferromagnetism in oxide nanoparticles. His research has been supported by the Russian Foundation for Basic Research (2012-2013).

About the author

Evangelia Sarantopoulou is a Senior Researcher at Photonics for Nano Applications Laboratory, National Hellenic Research Foundation, Theoretical and Physical Chemistry Institute, Athens, Greece and a visiting Professor of Physics at Kazan Federal University, Russia. She graduated from the National and Kapodistrian University of Athens and she defended her Ph.D. thesis at the same University in 1996.   She supervised several Ph.D., M.Sc and Diploma thesis and participated in various national and international research projects. She was a scientist in charge in European Space Agency projects, EU-FP7, and bilateral research projects.

Her current research interests are focused on the photonic synthesis of novel nanostructures, photonic surface processing, physics of interphases, electronic properties of nanocomposites, nanotoxicology, and space sciences including materials and biological effects. She is a member of the Editorial Board of the Heliyon- ELSEVIER Journal and of Nanosafety Cluster. Also, she is a regular reviewer for scientific journals, evaluator and scientific rapporteur in research projects.


A.C. Cefalas, Z. Kollia, N. Spyropoulos-Antonakakis, V. Gavriil, D. Christofilos, G. Kourouklis, V.V Semashko, V. Pavlov, E. Sarantopoulou. Surface profile gradient in amorphous Ta2O5 semi conductive layers regulates nanoscale electric current stability. Applied Surface Science 396 (2017) 1000-1019.

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