Source: http://web-ext.u-aizu.ac.jp/official/researchact/annual-review/2011/computerScience/cnl.html
Timestamp: 2019-04-24 10:09:53+00:00

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The research activity of the Computational Nano-Electronics Laboratory is focused on theoretical studies and computer modeling of physical processes in semiconductor microand nanostructures and in novel electronic and optoelectronic classical and quantum devices based on such structures. Since the laboratory establishment in 1993, its members have published more than 200 journal articles and made more than 200 presentations at international conferences.
Original research results obtained in this academic year have been published in refereed journal papers (Physical Review B, Journal of Applied Physics, Applied Physics Letters, Japanese Journal of Applied Physics, and others) and presented at different international conferences.
R. Yamasea, T. Maeda, I. Khmyrova, E. Shestakova, E. Polushkin, A. Kovalchuk, and S. Shapoval. Study of Fringing Effects in Multi-Cantilever HEMT-Based Resonant MEMS. Int. J. Appl. Electromagnetics and Mechanics, 38(2-3):93-100, 2012.
An analytical model of a resonant micro-electromechanical system (MEMS) with a high-electron mobility transistor (HEMT)-like structure and an array of resonant cantilevers over its two-dimensional electron gas (2DEG) channel is developed. To account for impact of fringing, spatial distributions of the electric field and sheet electron density at the surface of the HEMT channel are calculated in parametric form. Resistance of the 2DEG channel and frequency-dependent ac amplitude of the source-drain current are derived. The developed model allows evaluation of impact of fringing electric field on output source-drain current for any different number of cantilevers in the array at different spacing between them and can be used for device optimization.
I. Khmyrova, R. Yamase, N. Watanabe, T. Maeda, E. Shestakova, E. Polushkin, A. Kovalchuk, and S. Shapoval. Analysis of resonant MEMS based on high-electron mobility transistor-like structure. Physica Status Solidi C, C9(2):399-402, 2012.
We develop an analytical model for the resonant MEMS with electrical readout in which electrostatically actuated micromashined cantilever oscillates over the twodimensional electron gas (2DEG) channel and controls the current flowing along it. The model accounts for the impact of electric field fringing on spatial distribution of sheet electron density along the 2DEG channel.
V. Ryzhii, M. Ryzhii, A. Satou, T. Otsuji, and V. Mitin. Analytical device model for graphene bilayer field-effect transistors using weak nonlocality approximation. Journal of Applied Physics, 109:064508 (10), 2011.
We develop ananalytical device model for graphene bilayer field-effect transistors (GBL-FETs) with the back and top gates. The model is based on the Boltzmann equation for the electron transport and the Poisson equation in the weak nonlocality approximation for the potential in the GBL-FET channel. The potential distributions in the GBL-FET channel are found analytically. The source-drain current in GBL-FETs and their transconductance are expressed in terms of the geometrical parameters and applied voltages by analytical formulas in the most important limiting cases. These formulas explicitly account for the short-gate effect and the effect of drain-induced barrier lowering. The parameters characterizing the strength of these effects are derived. It is shown that the GBL-FET transconductance exhibits a pronounced maximum as a function of the top-gate voltage swing. The interplay of the short-gatee ffect and the electron collisions results in a nonmonotonic dependence of the transconductance on the top-gate length.
V. Ryzhii, M. Ryzhii, N. Ryabova, V. Mitin, and T. Otsuji. Terahertz and infrared detectors based on graphene structures. Infrared Physics and Technology, 54(3):302-305, 2011.
We consider newly proposed terahertz and infrared interband detectors based on multiple-graphene-layer structures with pin junctions. Using the developed device model, we calculate the photodetector characteristics (responsivity and dark current limited detectivity) and compare them with the characteristics of other photodetectors. It is shown that due to relatively high quantum efficiency and weakened thermogeneration processes, the detectors under consideration can exhibit superior performance.
M. Ryzhii, T. Otsuji, V. Mitin, and V.Ryzhii. Characteristics of pin terahertz and infrared photodiodes based on multiple graphene layer structures. Japanese Journal of Applied Physics, 50:070117 (6), 2011.
We calculate the responsivity and dark-current-limited detectivity of terahertz and infrared interband detectors based on pin junctions in the multiple graphene layer structures proposed recently. It is demonstrated that the interband tunneling can be an essential limiting mechanism of the ultimate values of detectivity. To achieve the ultimate detectivity, the optimization of the device structure and the proper choice of the temperature and the bias voltage are required. We show that owing to high values of the quantum efficiency and relatively low rates of thermogeneration, the detectors under consideration can exhibit high responsivity and detectivity at elevated temperatures (in particular, at room temperature) in a wide radiation spectrum and can substantially surpass other detectors.
V. Ryzhii, M. Ryzhii, V. Mitin, and T. Otsuji. Toward the creation of terahertz graphene injection laser. Journal of Applied Physics, 110:094503 (9), 2011.
We study the effect of population inversion associated with the electron and hole injection in graphene p-i-n structures at the room and slightly lower temperatures. It is assumed that the recombination and energy relaxation of electrons and holes are associated primarily with the interband and intraband processes assisted by optical phonons. The dependences of the electron-hole and optical phonon effective temperatures on the applied voltage, the current-voltage characteristics, and the frequency-dependent dynamic conductivity are calculated. In particular, we demonstrate that at low and moderate voltages, the injection can lead to a pronounced cooling of the electron-hole plasma in the device i-section to the temperatures below the lattice temperature. However at higher voltages, the voltage dependences can be ambiguous exhibiting the S-shape. It is shown that the frequency-dependent dynamic conductivity can be negative in the terahertz (THz) range of frequencies at certain values of the applied voltage. The electron-hole plasma cooling substantially reinforces the effect of negative dynamic conductivity and promotes the realization of terahertz lasing. On the other hand, the heating of optical phonon system can also be crucial affecting the realization of negative dynamic conductivity and terahertz lasing at the room temperatures.
V. Ryzhii, M. Ryzhii, and T. Otsuji. Tunneling recombination in optically pumped graphene with electron-hole puddles. Applied Physics Letters, 99:173504 (3), 2011.
We evaluate recombination of electrons and holes in optically pumped graphene associated with the interband tunneling between electron-hole puddles and calculate the recombination rate and time. It is demonstrated that this mechanism can be dominant in a wide range of pumping intensities. We show that the tunneling recombination rate and time are nonmonotonic functions of the quasi-Fermi energies of electrons and holes and optical pumping intensity. This can result in hysteresis phenomena.
V. Ryzhii, M. Ryzhii, V. Mitin, A. Satou, and T. Otsuji. Effect of heating and cooling of photogenerated electronhole plasma in pptically pumped graphene on population inversion. Japanese Journal of Applied Physics, 50:094001 (9), 2011.
We study the characteristics of photogenerated electronhole plasma in optically pumped graphene layers at elevated (room) temperatures when the interband and intraband processes of emission and absorption of optical phonons play a crucial role. The electronhole plasma heating and cooling as well as the effect of nonequilibrium optical phonons are taken into account. The dependences of the quasi-Fermi energy and effective temperature of optically pumped graphene layers on the intensity of pumping radiation are calculated. The variation of the frequency dependences dynamic conductivity with increasing pumping intensity as well as the conditions when this conductivity becomes negative in a certain range of frequencies are considered. The effects under consideration can markedly influence the achievement of the negative dynamic conductivity in optically pumped graphene layers associated with the population inversion and, hence, lead to the in-depth understanding of the experimental results and the realization graphene-based terahertz and infrared lasers operating at room temperatures.
V. Ryzhii, N. Ryabova, M. Ryzhii, N.V. Baryshnikov, V.E. Karasik, V. Mitin, and T. Otsuji. Terahertz and infrared photodetectors based on multiple graphene layerand nanoribbon structures. Opto-electronics Review, 20(1):15-20, 2012.
We consider new concepts of terahertz and infrared photodetectors based on multiple graphene layer and multiple graphene nanoribbon structures and we evaluate their responsivity and detectivity. The performance of the detectors under consideration is compared with that of photodetectors made of the traditional structures. We show that due to high values of the quantum efficiency and relatively low rates of thermogeneration, the graphene – based detectors can exhibit high responsivity and detectivity at elevated temperatures in a wide radiation spectrum and can substantially surpass other detectors. The detector being discussed can be used in different wide – band and multi – colour terahertz and infrared systems.
I. Khmyrova, R. Yamase, T. Maeda, E. Polushkin, A. Kovalchuk, and S. Shapoval. Resonant MEMS with two-dimensional electron gas system. In Proc. of the 35th Workshop on Compound Semiconductor Devices and Integr ated Circuits (WOCSDICE 2011), pages 201-202, Catania, Italy, May-June 2011.
In the paper we develop a model accounting for the influence of the electric field fringing on the output source-drain current in the 2DEG channel of the high-electron mobility transistor (HEMT)-based resonant MEMS with micromachined cantilever serving as a floating gate.
E. Polushkin, R. Yamase, T. Maeda, I. Khmyrova, A. Kovalchuk, S. Shapoval, and E. Shestakova. Impact of fringing on resonant sensor with electrical readout for multi-target detection. In EUROSENSORS XXV, pages A5-05, Athens, Greece, Sept. 2011.
The paper addresses a resonant sensor with array of micromachined cantilevers oscillating over the channel of high-electron-mobility transistor (HEMT)-like structure and controlling source-drain current flowing along it. Electrical readout is realized via direct modulation of the source-drain current by electrostatically actuated cantilevers. Simple analytical model accounting for the impact of electric field fringing on the performance of the HEMT-based resonant sensor is developed.
V. Ryzhii, M. Ryzhii, A. Satou, T. Otsuji, and V. Mitin. Nonequilibrium carriers and phonons in optically excited graphene: heating and cooling. In EDISON17: 17th Int. Conf. on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures, pages P2.8, Virginia Tech, VA, USA, August 2011.
In this communication , we report on the effect of the carrier heating and cooling in optically excited graphene layers (GLs) at elevated temperatures, when the interaction with optical phonons is the main mechanism of the recombination and energy relaxation, on its characteristics, particularly on its dynamic conductivity at the terahertz frequencies. The deviation of the optical phonon system from equilibrium (optical phonon heating) is also considered.
M. Ryzhii, V. Ryzhii, N.V. Baryshnikov, V.E. Karasik, and T. Otsuji. Interband detectors of terahertz and infrared radiation based on graphene p-i-n structures. In The Progress in Electromagnetics Research Symposium (PIERS 2012), Kuala Lumpur, Malaysia, March 2012.
The Electromagnetics Academy. In this communication, we consider the concepts of THz/IR photodetectors utilizing multiple graphene layers (MGL) and graphene nanoribbons (GNR) structures: the MGL p-i-n photodiode considered previously, and the newly proposed GNR p-i-n photodiode.
V. Ryzhii, M. Ryzhii, A.A. Dubinov, V.Ya. Aleshkin, V. Mitin, M.S. Shur, A. Satou, and T. Otsuji. Concepts of terahertz and infrared devices based on graphene structures. In 36th Int. Conf. on Infrared, Millimeter and Terahertz Waves Digest, pages Tu3C.1, Houston, TX, USA, October 2011.
IEEE. We overview novel concepts of terahertz and infrared interband detectors and lasers based on single- and multiple-graphene layer structures and discuss the features of their operation and feasibility of realization.
T. Otsuji, S.A.B. Tombet, S. Chan, T. Watanabe, A. Satou, M. Ryzhii, and V. Ryzhii. Terahertz light amplification of stimulated emission of radiation in optically pumped graphene. In XXXth General Assembly and Scientific Symposium, URSI GASS, Istanbul, Turkey, August 2011.
International Union of Radio Science. The gapless and linear energy spectra of electrons and holes in graphene lead to nontrivial features such as negative dynamic conductivity in the terahertz spectral range. This paper reviews recent advances in theoretical and experimental study on terahertz light amplification by stimulated emission of radiation in optically pumped graphene.
M.Ryzhii, V. Mitin, A. Satou, T. Otsuji, and V. Ryzhii. Modeling of short-gate and drain-induced barrier lowering effects in graphene bilayer field-effect transistors: weak nonlocality approximation. In SISPAD: 2011 International Conference on Simulation of Semiconductor Processes and Devices Digest, pages P22, Osaka, Japan, September 2011.
In this communication, we report the development of an analytical device model for graphene field-effect transistors with the back and top gates, which explicitly takes into account the short-gate and drain-induced barrier lowering effects.
V.Ryzhii, M. Ryzhii, A. Satou, T. Otsuji, and V. Mitin. Nonequilibrium electrons, holes, and phononsin graphene under optical and injection pumping: model and analysis. In SISPAD: 2011 International Conference on Simulation of Semiconductor Processes and Devices Digest, Osaka, Japan, September 2011.
In the present paper, we propose a model for the nonequilibrium electron-hole plasma and optical phonon system in graphene layers (GLs) under optical and injection pumping. This model is used for the calculations of the effective temperatures and quasi-Fermi energy of electron-hole plasma and the frequency dependences of the GL dynamic conductivity.
V. Ryzhii, M. Ryzhii, A. Satou, T. Otsuji, and V. Mitin. Optical excitation of graphene, population inversion, and terahertz lasing. In 15th Int. Conf. on Narrow Gap Systems (NGS15), AIP Conf. Proc. 1416, pages 26-30, Virginia Tech, VA, USA, August 2011. AIP.
We overview effects associated with the optical (infrared) pumping of graphene. Different scenarios of the photogenerated electrons and holes electron relaxation are considered.
Akihiro Konishi. Calculation of electric field distribution in multigate structures by conformal mapping technique. Graduation thesis, School of Computer Science and Engineering, 2012.
Ryosuke Yamase. Analysis of terahertz plasma resonances in HEMT-like structures with interdigitated gate. Graduation thesis, School of Computer Science and Engineering, 2012.
Norikazu Watanabe. Model for electric field and charge distribution in multi-gate structures with two-dimensional electron gas. Graduation thesis, School of Computer Science and Engineering, 2012.

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