Victor Karpov

(Theoretical Physics, Condensed Matter, Semiconductors, Photovoltaics, Device Physics)

Professor of Physics

Dr. of Science, 1986, Institute for Nuclear Physics, Academy of Science, Russia;

Ph.D., 1979, Polytechnic University, Russia

My current research interests are in condensed matter theory and device physics, including disordered systems, phase transitions, thin films, and photovoltaics.

One broad topic is thin-film devices, which remain poorly understood in spite of many applications in microchips, photovoltaics, and others. We have established a new area of the physics of thin photovoltaics (as opposed to the classical crystalline solar cells) encompassing many unique phenomena and helping to better engineer solar modules with improved efficiency and stability. Our contributions include the following. (1) The concept of weak micro-diodes that has now become the standard element of device operation analysis. It takes into account that different microscopic areas in thin film junctions have quite different diode characteristics. Because they are all interconnected through the device electrodes, the system operates as an array of random diodes strongly affecting each other and the system integral parameters. The related fundamental concept is the nonuniformity screening length that determines the maximum device size and its nonuniformity effects. (2) The piezo-(pyro-) photovoltaic coupling is another fundamental contribution of our group based on the realization that the standard component of thin film PV, CdS is a strong piezo-electric and its piezo-electric field strongly interacts with PV electricity leading to new physical effects. That new understanding resulted in recipes of improving the device parameters. (3) Perhaps the most known to the general photovoltaic community contribution by our group is the red wine effect, which is a practical remedy of improving the device efficiency through mitigating its nonuniformity loss (in the simplest settings achieved by applying some red wine before the final metallization). That concept was patented as an efficient self-healing remedy, and the patent exclusively licensed to a major photovoltaic company. Our photovoltaic research was funded through DOE and NSF grants. We have published a textbook Physics of thin-Film Photovoltaics (in co-authorship with Dr. Shvydka) with Wiley Publishers.

A significant effort was devoted to creating a theory of metal phase nucleation in symmetry breaking strong electric fields, which phenomenon is central to the operations of the emerging solid state memory. Our work has shown, that in strong enough, yet practically important electric fields, needle shaped conductive filaments appear in insulating hosts of glassy semiconductors. This theory triggered extensive experimental work of Intel Corporation (that funded our research) and has now become a common base in the field of solid state memory technology. That theory of field induced nucleation has been significantly extended later to explain the nonphotochemical laser induced nucleation in some solutions, anisotropic metal particle formation in insulators, and laser induced pancake shaped void formation in metal films. The fundamental nature of these results is that they reveal the possibility of symmetry breaking phase transformations (needles instead of spheres) in isotropic systems.

A more recent activity was in the field of metal whiskers that often grow across leads of electric equipment and electronic package causing current leakage or short circuits and raising significant reliability issues, especially in the case of tin that serves as a base for various soldering alloys. The nature of metal whiskers remains a mystery after several decades of research. Recently, Karpov proposed a theory of metal whiskers that for the first time provided understanding of their underlying physics and corresponding quantitative description. The theory is based on the electrostatic of imperfect metal surfaces that are capable of generating random local electric field. Simultaneously, that theory proposed practical recipes of accelerated testing of whisker growing propensity of metals and venues of whisker mitigation. This work sparked significant interest in the community of engineers and researchers and was highlighted in the media. Also, it triggered experimental research in metal whiskers revolving around the external field effects of their nucleation and growth. Our preliminary results have shown strong field effects in the capacitive field configuration and under e-beam of linear medical accelerator. This research direction is being enthusiastically developed.

Artificial Intelligence (AI) was the most recent field of my research. We developed a theory of percolation with plasticity media (PWPs) rendering properties of interest for neuromorphic computing. Unlike the standard percolation, they have multiple interfaces and exponentially large number of conductive pathways between them. These pathways consist of non-ohmic random resistors that can undergo bias induced nonvolatile modifications (plasticity). The neuromorphic properties of PWPs include multi-valued memory, high dimensionality and nonlinearity capable of transforming input data into spatiotemporal

patterns, tunably fading memory ensuring outputs that depend more on recent inputs, and no need for massive interconnects. A few conceptual examples of functionality here are random number generation, matrix-vector multiplication, and associative memory. Understanding PWP topology, statistics, and operations opens a field of its own calling upon further theoretical and

experimental insights.

My former graduate students work on different aspects of Device Physics, Photovoltaics, and Theoretical Physics:

Lilani Cooray, PhD program, theoretical physics, photovoltaics;

Mukut Mitra, PhD program, experimental physics, phase change memory;

Marco Nardone, PhD program, theoretical and device physics;

Trevor Wilson, MS program, theoretical biophysics;

Tim Muszynski, MS program, theoretical device physics;

Mark Simon, PhD program, theoretical physics, phase change memory.

Dipesh Niraula, PhD program, theoretical physics, device physics.

Osama Oudat, PhD program, experimental research in metal whiskers

Maria Patmiou. PhD program, disordered systems and AI

Selected publications (of more than 200)

Victor Karpov and Diana Shvydka, Physics of Thin-Film Photovoltaics

Book, Wiley publishers 2021 https://www.amazon.com/Physics-Thin-Film-Photovoltaics-Victor-Karpov/dp/111965100X

Vamsi Borra, Srikanth Itapu, Victor G. Karpov, Daniel G. Georgiev,

Modification of Tin (Sn) metal’s surface by Surface Plasmon Polariton Excitation,

Scripta Materialia, 208, 114357 (2022)

VG Karpov, D Shvydka, SS Bista, Threshold switching in solar cells and a no-scribe photovoltaic technology, Appl. Phys. Lett. , 119, 193904 (2021)

VG Karpov, Adiabatic theory of SET and RESET transitions

Journal of Applied Physics 129 (11), 114501 (2021)

V. G. Karpov, S. Gursel, and Maria Patmiou

Percolation with plasticity for neuromorphic systems

J. Physics: Complexity 1, 035009 (2020),

M. Patmiou, V. G. Karpov, S. Gursel, B.R. Weborg,

Numerical modeling of percolation conduction and Poole-Frenkel laws,

J. Appl. Phys. 128, 035701 (2020).

V. G. Karpov and M. Patmiou, Physics in the information age: Qualitative methods in quantum theory, Eur. J. Phys. 41 (2020) 035407

V.G. Karpov, M. Patmiou, S. Gursel, D. Shvydka

Pulse percolation conduction and multivalued memory,

AIP Advances 10, 045324 (2020)

O. Oudat, V. Arora, E. Parsai, V. G. Karpov, and D. Shvydka,

Gamma- and x-ray accelerated whisker development

J. Phys. D: Appl. Phys. 53 (2020)

V. G. Karpov and D. Niraula, OFF state conductivity in RRAM structures,

Electron Device Letters, 40, 551 (2019)

Maria Patmiou, D, Niraula, and V. G. Karpov

The Poole-Frenkel laws and a pathway to multi-valued memory,

Appl. Phys. Lett. 115, 083507 (2019);

D. Niraula and V. G. Karpov,

Comprehensive modeling of filamentary RRAM devices,

J. Appl. Phys. 124, 174502 (2018);

Osama A Oudat, Vamsi Borra, Daniel G Georgiev and Victor G Karpov,

The statistics of tin whisker diameters versus the underlying film grains,

J. Phys. D: Appl. Phys. 52 (2019) 165305

V. G. Karpov and D. Niraula, Resistive switching in nano-structures, Scientific Reports, Nature, 8, 12212 (2018).

V. Borra, D. Georgiev, V. G. Karpov, and D. Shvydka,

Microscopic structure of metal whiskers, Phys. Rev. Applied, 9, 054029 (2018)

V. G. Karpov, Dipesh Niraula, and I. V. Karpov, Thermodynamic analysis of

conductive filaments, Appl. Phys Lett. 109, 093501 (2016).

Diana Shvydka and V. G. Karpov Surface parameters determining a metal propensity for whiskers, J. Appl. Phys. 119, 085301 (2016).

D. Niraula, J. McCulloch, G. R. Warrell, R. Irving, V. G. Karpov, and Diana Shvydka, Electric Field Stimulated growth of Zn whiskers, AIP Advances, 6, 075201 (2016).

V. G. Karpov, Electrostatic Mechanism of Nucleation and Growth of Metal Whiskers, SMT Magazine, February 2015, p. 28. http://iconnect007.uberflip.com/i/455818/44

V.G.Karpov, Electrostatic Theory of Metal Whiskers, Phys. Rev. Applied, 1, 044001 (2014).

V. G. Karpov, A. Vasko, and A. Vijh, Hot spot runaway in thin film photovoltaics and related structures, Appl. Phys. Lett. 103, 074105 (2013)

M. Nardone, M. Simon, I. V. Karpov, and V. G. Karpov, Electrical Conduction in Chalcogenide Glasses of Phase Change Memory (focused review), J. Appl. Phys. 112, 071101 (2012)

V. G. Karpov, M. Nardone, and N. I. Grigorchuk, Plasmonic Mediated Nucleation of Nanoparticles, Phys. Rev. B 86, 075463 (2012)

V. G. Karpov, Coupled Electron-Heat Transport in Thin Nonuniform Films, Phys. Rev. B 86, 165317 (2012)

M. Nardone, M. Simon and V. G. Karpov, Shunting Path Formation in Thin Film Structures, Appl. Phys. Lett. 96, 163501 (2010)

M. Nardone, V. G. Karpov, D. Shvydka, and M. L. C. Attygalle, Theory of electronic transport in noncrystalline junctions, J. Appl. Phys., 106, 074503 (2009)

J. Kang, E. I. Parsai, D. Albin, V. G. Karpov, and Diana Shvydka, From photovoltaics to medical imaging: Applications of thin-film CdTe in x-ray Detection, Appl. Phys. Lett., 93, 223507 (2008)

Diana Shvydka and V. G. Karpov, Nanodipole photovoltaics, Appl. Phys. Lett. 92, 053507 (2008)

M. Mitra, J. Drayton, M. L. C. Cooray, V. G. Karpov, and Diana Shvydka, Piezo-photovoltaic coupling in CdS-based thin-film photovoltaics J. Appl. Phys. 102, 034505 (2007)

M. L. C. Cooray and V. G. Karpov, Long range fluctuations in thin-film structures, Phys. Rev. B. 75, 155303 (2007).

V. G. Karpov, Critical disorder and phase transition in random diode arrays, Phys. Rev. Lett., 91, 226806 (2003)

V. G. Karpov, A. D. Compaan, and Diana Shvydka, Random diode arrays and mesoscale physics of large-area semiconductor devices, Phys. Rev B 69, 045325 (2004).

V. G. Karpov, David W. Oxtoby, Nucleation in Disordered Systems, Phys. Rev. B 54, 9734 (1996)

M. Grimsditch, V. G. Karpov, Fluctuations During Melting, J. Phys, C, Condensed Matter, 8 L439 (1996)

V. G. Karpov, Instability in the Classic Theory of Coarsening, Phys. Rev. Lett. 74, 3185 (1995)

V. G. Karpov, Negative Diffusion and Clustering of Growing Particles, Phys. Rev. Lett. 75, 2702 (1995)