Computer Solid State Physics Laboratory

Victor I. Ryzhii Professor	Irina I. Khmyrova Assistant Professor	Maxim V. Ryzhii Assistant Professor	Fedir Vasko Visiting Researcher

The research activity of the Computer Solid State Physics Laboratory is focused on theoretical studies and computer modeling of physical processes in semiconductor micro- and nanostructures and in novel electronic and optoelectronic classical and quantum devices based on such structures. A substantial part of activity of the laboratory is associated with the development of interactive educational resources and materials. Since the laboratory establishment in 1993, its members have published more than 130 journal articles and made more than 140 presentations at international conferences. One of the members of the laboratory is a Fellow of the Institute of Electrical and Electronics Engineers (IEEE), Fellow of the American Physical Society, and Corresponding member of the Russian (national) Academy of Sciences. Two professors are the IEEE Senior Members. The research efforts of the laboratory members are now centered on theory andcomputer modeling of: Semiconductor classical and quantum heterostructure devices operating in the terahertz range of frequencies (terahertz detectors, mixers, generators, and photomixers). The theory and computer modeling of graphene-based devices - the basis of future Carbon electronics. Quantum-well and quantum-dot infrared photodetectors (QWIPs and QDIPs), QWIP- and QWIP-based functional devices. Theory and computer modeling of zero-resistance and zero-conductance states in two-dimensional electron systems irradiated with microwaves and related phenomena. Original research results obtained in this academic year have been published in refereed journal papers (published in Journal of Applied Physics, Applied Physics Letters, Japanese Journal of Applied Physics, and others) and presented at different international conferences. The computer Solid State Physics Laboratory conducts research in cooperation with leading research groups at: Rensselaer Polytechnic Institute (Troy, USA) Tohoku University (Sendai, Japan), University at Buffalo (Buffalo, USA), Moscow Institute of Physics and Technology, Ioffe Physical-Technical Institute (St. Petersburg, Russia), Gothenburg University (Gothenburg, Sweden), and others.

Refereed Journal Papers

[khmyrova-01:2007]	I. Khmyrova and Yu. Seijyou. Analysis of plasma oscillations in high-electron mobility transistorlike structures: Distributed circuit approach. Appl. Phys. Lett., 91:143515, 2007.
	We develop simple distributed circuit model of the high-electron mobility transistor (HEMT)-like structure for the analysis of the effects associated with plasma oscillations excited in its two-dimensional electron gas (2DEG) channel. Circuit components of the model are related to physical and geometrical parameters of the structure. Developed model accounts for dependence of resistance and inductance of 2DEG channel gated region on gate voltage. Such an approach facilitates and improves understanding of HEMT-like structures' behavior in the regime of excitation of plasma oscillations and is applicable for their performance evaluation and optimization as well.
[m-ryzhii-01:2007, v-ryzhii-01]	V. Ryzhii, M. Ryzhii, Y. Hu, I. Hagiwara., and M. S. Shur. Resonant detection of modulated terahertz radiation in micromachined high-electron-mobility transistor. Appl. Phys. Lett., 90(20):203503(3), 2007.
	The authors develop a device model for a resonant detector of modulated terahertz radiation based on a micromachined high-electron-mobility transistor with the microcantilever serving as the gate. The device model accounts for mechanical motion of the microcantilever and plasma effects of the two-dimensional electron channel. It is demonstrated that at a combined resonance when the carrier terahertz frequency and the modulation frequency coincide with the plasma resonant frequency and the mechanical resonant frequency, respectively, the amplitude of the output gate and drain ac currents and, hence, the detector resonsivity exhibit sharp and high maximum.
[m-ryzhii-02:2007]	V. Ryzhii, M. Ryzhii, and T. Otsuji. Tunneling current-voltage characteristics of graphene field-effect transistor. Appl. Phys. Express, 1:013001(3), 2008.
	We develop an analytical device model for a graphene field-effect transistor. Using this model, we calculate its current-voltage characteristics at sufficiently large gate voltages when in the transistor channel a n-p-n (p-n-p)lateral junction is formed and the source-drain current is associated with the interband tunneling through this junction.
[m-ryzhii-03:2007, v-ryzhii-02:2007]	V. Ryzhii, M. Ryzhii, and T. Otsuji. Negative dynamic conductivity of graphene with optical pumping. J. Appl. Phys., 101(8):083114(1-4), 2007.
	We study the dynamic ac conductivity of a nonequilibrium two-dimensional electronhole system in optically pumped graphene. Considering the contribution of both interband and intraband transitions, we demonstrate that at sufficiently strong pumping the population inversion in graphene can lead to the negative net ac conductivity in the terahertz range of frequencies. This effect might be used in graphene-based coherent sources of terahertz radiation.
[m-ryzhii-04:2007]	V. Ryzhii, M. Ryzhii, and T. Otsuji. Thermionic and tunneling transport mechanisms in graphene field-effect transistors. Phys. Stat. Sol. (a), accepted for publication, 2008.
	We present an analytical device model for a graphene field-effect transistor (GFET) on a highly conducting substrate, playing the role of the back gate, with relatively short top gate which controls the source-drain current. The equations of the GFET device model include the Poisson equation in the weak nonlocality approximation. Using this model, we find explicit analytical formulae for the spatial distributions of the electric potential along the channel and for the voltage dependences of the thermionic and tunneling currents.
[m-ryzhii-05:2007]	V. Ryzhii, M. Ryzhii, A. Satou, and T. Otsuji. Current-voltage characteristics of a graphene nanoribbon field-effect transistor. J. Appl. Phys., accepted for publication, 2008.
	We calculate the high-frequency response of a graphene nanoribbon field-effect transistor (GNR-FET) using the developed analytical model. The GNR-FET under consideration is based on a heterostructure which consists of an array of nanoribbons clad between the highly conducting substrate (the back gate) and the top gate controlling the dc and ac source-drain currents. We derive explicit analytical formulas for the GNR-FET transconductance as a function of the signal frequency, collision frequency of electrons, and the top gate length. The transition from the ballistic and to strongly collisional electron transport is considered.
[m-ryzhii-06:2007]	Y. Hu, M. Ryzhii, I. Hagiwara, MS. Shur, and V. Ryzhii. Combined resonance and resonant detection of modulated terahertz radiation in a micromachined high-electron mobility transistor. Phys. Stat. Sol. (c), 5:277-281, 2008.
	We analyze the operation of a resonant detector of modulated terahertz (THz) radiation based on a micromachined high-electron mobility transistor (HEMT) with a microcantilever (MC) as mechanically oating gate. Using the device model, which describes both MC mechanical motion and plasma effects in the HEMT twodimensional electron channel, we calculate the output gate and source-drain ac currents and the detector responsivity as functions of the modulation and carrier frequencies in linear and saturation regimes of the HEMT operation. It is shown that the THz detector responsivity exhibit sharp and high maxima under the conditions of both mechanical and plasma resonances.
[m-ryzhii-07:2007]	V. Ryzhii, I. Khmyrova, M. Ryzhii, A. Satou, T. Otsuji, V. Mitin, and M.S. Shur. Terahertz plasma waves in two-dimensional electron systems and their device applications. Int. J. High Speed Electron. Syst., 17(3):521-538, 2007.
	We overview the plasma properties of a two-dimensional electron gas in semiconductor heterostructures as well as new concepts and proposals of novel terahertz devices (detectors, sources utilizing plasma instabilities, and photomixers) based on these heterostructures.
[m-ryzhii-08:2007]	V. Ryzhii, M. Ryzhii, and T. Otsuji. Population inversion of phoexcited electrons and holes in graphene and its negative terahertz conductivity. Phys. Stat. Sol. (c), 5:261-264, 2008.
	We demonstrate that sufficiently strong optical excitation may result in the population inversion in graphene, so that the real part of the ac conductivity can be negative in the terahertz range of frequencies. We study also how the heating of the electron-hole system in uences the effect of negative ac conductivity. The effect of population inversion and negative ac conductivity might be used in graphene-based coherent sources of terahertz electromagnetic radiation.
[m-ryzhii-09:2007]	M. Ryzhii and V. Ryzhii. Population inversion in electrically and optically pumped graphene. Physica E, 40:317-320, 2007.
	We find the electron and hole distribution functions and calculate the dynamic conductivity of nonequilibrium two-dimensional (2D) electron-hole systems in graphenebased structures under the injection or optical pumping. Both the interband transitions and intraband transitions associated with the electron and hole scattering on impurities and acoustic and optical phonons are considered. It is shown that under sufficiently strong pumping, the population inversion may result in the negative net dynamic AC conductivity in a certain range of photon frequencies. Since the frequencies corresponding to the negative conductivity fall into the terahertz range, the predicted effects might be used in coherent sources of terahertz radiation.
[v-ryzhii-03:2007]	V. Ryzhii, A. Satou, T. Otsuji, and M.S. Shur. Plasma mechanisms of resonant terahertz detection in a two-dimensional electron channel with split gates. J. Appl. Phys., 103:014504(1-6), 2008.
	We analyze the operation of a resonant detector of terahertz (THz) radiation based on a two-dimensional electron gas (2DEG) channel with split gates. The side gates are used for the excitation of plasma oscillations by incoming THz radiation and control of the resonant plasma frequencies. The central gate provides the potential barrier separating the source and drain portions of the 2DEG channel. Two possible mechanisms of the detection are considered: (1) modulation of the ac potential drop across the barrier and (2) heating of the 2DEG due to the resonant plasma-assisted absorption of THz radiation followed by an increase in thermionic dc current through the barrier. Using the device model, we calculate the frequency and temperature dependences of the detector responsivity associated with both dynamic and heating (bolometric) mechanisms. It is shown that the dynamic mechanism dominates at elevated temperatures, whereas the heating mechanism provides larger contribution at low temperatures, T=35-40 K.
[v-ryzhii-04:2007]	I. Semenikhin, V. Ryzhii, E. Vostrikova, and A. Ivanov. Electrical excitation of shock and soliton-like waves in high-electron-mobility transistor structures. Phys.Stat.Sol.(c), 5(1):61-65, 2008.
	We study electrical excitation of nonlinear plasma waves in heterostructures with two-dimensional electron channels and with split gates, and the propagation of these waves using hydrodynamic equations for electron transport coupled with twodimensional Poisson equation for self-consistent electric potential. The term related to electron collisions with impurities and phonons as well as the term associated with viscosity are included into the hydrodynamic equations. We demonstrate the formation of shock and soliton-like waves as a result of the evolution of strongly nonuniform initial electron density distribution. It is shown that the shock wave front and the shape of soliton-like pulses pronouncedly depend on the coefficient of viscosity, the thickness of the gate layer and the nonuniformity of the donor distribution along the channel. The electron collisions result in damping of the shock and soliton-like waves, while they do not markedly affect the thickness of the shock wave front.
[v-ryzhii-05:2007]	F.T. Vasko and V. Ryzhii. Voltage and temperature dependences of conductivity in gated graphene. Phys. Rev. B, 76:233404(1-4), 2007.
	The resistivity of gated graphene is studied taking into account electron and hole scattering by short- and long-range structural imperfections (the characteristics of disorder were taken from the scanning tunneling microscopy data) and by acoustic phonons. The calculations are based on the quasiclassical kinetic equation with the normalization condition fixed by surface charge. The gate-voltage and temperature effects on the resistance peak, which is centered at the point of intrinsic conductivity, are found to be in agreement with the transport measurements.
[v-ryzhii-06:2007]	V. Vyurkov and V. Ryzhii. Why graphene conductivity is constant: scaling theory consideration. LANL preprint arXiv:0803.3926, 2008.
[v-ryzhii-07:2007]	A. Satou, V. Ryzhii, T. Otsuji, and M.S. Shur. Resonant terahertz detection antenna utilizing plasma oscillations in lateral Schottky diode. Int. J. High Speed Electron., 17(3):539-546, 2007.
	In this paper, we extend our theory of the THz detector with LSJ by accounting for the effects of the contact pads serving as an antenna.
[v-ryzhii-8:2007]	V. Ryzhii, A. Satou, and T. Otsuji. Plasma waves in two-dimensional electron-hole system in gated graphene heterostructures. J. Appl. Phys., 101:024509(1-5), 2007.
	Plasma waves in the two-dimensional electron-hole system in a graphene-based heterostructure controlled by a highly conducting gate are studied theoretically. The energy spectra of two-dimensional electrons and holes are assumed to be conical (neutrinolike),i.e., corresponding to their zero effective masses. Using the developed model, we calculate the spectrum of plasma waves (spatio-temporal variations of the electron and hole densities and the self-consistent electric potential). We find that the sufficiently long plasma waves exhibit a linear (soundlike) dispersion, with the wave velocity determined by the gate layer thickness, the gate voltage, and the temperature. The plasma wave velocity in graphene heterostructures can significantly exceed the plasma wave velocity in the commonly employed semiconductor gated heterostructures. The gated graphene heterostructures can be used in different voltage tunable terahertz devices which utilize the plasma waves.

Refereed Proceeding Papers

[khmyrova-02:2007]	I.Khmyrova. Plasma effects in HEMT-like structures: Equivalent circuit model and simulation. In Techn. Digest of 2007 Korea-Japan Mi- croWave Conference, pages 197-200, Naha, Okinawa, Nov. 2007.
	An electric equivalent circuit is developed to study the manifestation of the plasma oscillations excited in the two-dimensional electron (2DE) channel of high-electron mobility transistor (HEMT)-like structure implementing IsSpice simulation. The components of the equivalent circuit model are related to physical and geometrical parameters of the structure. The distinctive feature of the developed model is the distributed circuit representation of the gated portion of the 2DEG channel with gate voltage dependent resistance and inductance. The simulated HEMT frequency performance reveals resonant behavior. The effect of "load" impedance mismatch is also discussed. The reported equivalent circuit approach invoking IsSpice simulation can be used for the evaluation and optimization of the performance of HEMT-like structures which operation is based on plasma wave excitation.
[khmyrova-03:2007]	I.Khmyrova. Distributed circuit analysis of plasma effects in HEMTlike structures for THz applications. In 14th Electronic Devices and Systems IMAPS CS Int. Conference (EDS07), pages 112-116, Brno, Czech Republic, Sep. 2007.
	A distributed circuit model to study the manifestation of plasma oscillations excited in the channel of HEMT-like structures is presented. The components of the equivalent circuit model are related to physical and geometrical parameters of the structure. The gate voltage dependent resistance and inductance of the gated portion of the 2DEG channel is the distinctive feature of the develop ed model. The equivalent circuit model has been used for the IsSpice analysis of plasma effects in the HEMT-like structure. The effect of 'load' impedance mismatch has been discussed.
[m-ryzhii-10:2007, v-ryzhii-09:2007]	F. Vasko, M. Ryzhii, and V. Ryzhii. Relaxation and recombination of nonequilibrium electrons and holes in optically pumped graphene. In 13th Int. Symposium on Ultrafast Phenomena in Semiconductors, Vilnius, Lithuania, Aug. 2007.
	We study the dynamics of nonequilibrium electrons and holes in graphene photogenerated by optical radiation and calculate the steady-state electron and hole distribution function.
[m-ryzhii-11:2007, v-ryzhii-10:2007]	V. Mitin, N. Ryabova, M. Ryzhii, T. Otsuji, and V. Ryzhii. Graphene nanoribbon phototransistor: Principle of operation, device model, and characteristics. In The 29th Int. Conference on the Physics of Semiconductors, page accepted, Rio de Janeiro, Brazil, 2008.
	We propose of a graphene nanoribbon phototransistor (GNR-PT), consider its operational principle, and develop its physical and mathematical models.
[m-ryzhii-12:2007]	M. Ryzhii, V. Ryzhii, Y. Hu, I. Hagiwara, and M.S. Shur. Detector of modulated terahertz radiation based on HEMT with mechanically oating gate. In The Joint 32nd Int. Conference on Infrared & Millimetre Waves and 15th Int. Conference on Terahertz Electronics, Cardiff, UK, Sep. 2007. IEEE.
	We consider a concept of a resonant detector of terahertz radiation based on a micromachined high-electron-mobility transistor (HEMT) with a microcantilever serving as the mechanically oating gate. The device can exhibit both the plasma (in the terahertz range) and mechanical (in megahertz or gigahertz range) resonances.
[m-ryzhii-13:2007]	M. Ryzhii, V. Ryzhii, Y. Hu, I. Hagiwara, , and M.S. Shur. Detection of modulated terahertz radiation using electromechanical and plasma resonances in two-dimensional electron systems with mechanically oating gates. In Photonics Asia 2007, Beijing, China, Nov. 2007. SPIE, SPIE.
	We study plasma effects in a micromachined high-electron mobility transistor (HEMT) with the microcantilever serving as the gate using the developed a model. The model accounts for mechanical motion of the microcantilever and spatiotemporal variations (plasma effects) of the two-dimensional electron gas (2DEG) system in the transistor channel. The microcantilever mechanical motion is described in the point-mass approximation. The hydrodynamic electron transport model is used to describe distributed electron plasma phenomena in the 2DEG system. Using the developed model, we calculated the response function characterizing the amplitude microcantilever oscillations and the output electric signal as functions of the signal frequency and the bias voltage or the devices with different parameters. We find the voltage dependences of the frequency of the mechanical resonance and its damping. In particular, it is demonstrated that the amplitudes of the mechanical oscillations and output electric signal exhibit pronounced maxima at the bias voltages close to the voltage of the 2DEG channel depletion followed by a steep drop with further increase in the bias voltage.
[[m-ryzhii-14:2007, v-ryzhii-11:2007]	V. Ryzhii, A. Satou, M. Ryzhii, T. Otsuji, and M.S. Shur. Excitation of terahertz plasma oscillations in two-dimensional electron channel by optical signals. In 31th Int. Workshop on Compound Semiconduc- tor Devices and Integrated Circuits (WOCSDICE 2007), Venice, Italy, May 2007. University of Padova.
	In this paper, we discuss a concept of a resonant detector of terahertz (THz) radiation based on a micromachined HEMT exhibiting both the plasma and mechanical resonances and substantiate this concept using the developed device model.
[m-ryzhii-15:2007, v-ryzhii-12:2007]	Y. Hu, M. Ryzhii, I. Hagiwara, M.S. Shur, and V. Ryzhii. Combined resonance and resonant detection of modulated terahertz radiation in a micromachined high-electron mobility transistor. In 15th Int. Conference on Nonequilibrium Carrier Dynamics in Semiconductors (HCIS15), Tokyo, Japan, July 2007. JSAP, IEICE.
	We propose and substantiate the operation of a micromachined high-electron mobility transistor (MMHEMT) as a detector of modulated terahertz (THz) radiation.

Unrefereed Papers

[khmyrova-04:2007]

I.Khmyrova. IsSpice modeling of micromachined HEMT with resonant cantilever gate. In IEICE Technical committee meeting, IEICE Techn. Rep., volume EMCJ - 53-79, pages 37-40, Sendai, Japan, Oct. 2007.

In this paper we developed lumped equivalent circuit model for the resonant sensor based on micromachined high-electron mobility transistor (HEMT) in which suspended resonant cantilever plays a role of a oating gate. Mechanical oscillations of the cantilever and electromechanical transducer are represented by relevant circuit components. The developed equivalent circuit has been used in IsSpice simulation of the performance of the micromachined HEMT under consideration. The simulated source-drain current reveals a peak at a frequency corresponding to resonant frequency of cantilever mechanical oscillations. The effect of damping of such oscillations and contribution of ungated parts of 2DEG channel to the HEMT performance have been demonstrated. The proposed approach can be used for optimization of resonant sensors based on micromacined HEMT.

Books

[m-ryzhii-16:2007]

M. Ryzhii and V. Ryzhii. Physics and modeling of tera- and nano- devices. World Scientific Publishing, Singapore, 2008.

Grants

[m-ryzhii-17:2007]	M. Ryzhii. Subsidy of Fukushima Prefectural Foundation for Advancement of Science and Education, 2007-2008.
[v-ryzhii-13:2007]	V. Ryzhii. Grant-in-Aid (S) from the Japan Society for Promotion of Science (JSPS): Terahertz plasma wave nanoelectronic devices, 2006-2010.
[v-ryzhii-14:2007]	V. Ryzhii. Grant from the Japan Science and Technology Agency, CREST: Development of Graphene on Silicon (GOS) device modeling technology, 2007-2012.
[v-ryzhii-15:2007]	V. Ryzhii. Nation-wide Research Grant from RIEC, 2006-2008.

Academic Activities

[khmyrova-05:2007]	Irina Khmyrova, 2007. Senior Member
[khmyrova-06:2007]	Irina Khmyrova, 2007. Member
[m-ryzhii-18:2007]	M. Ryzhii, Oct. 2006. Organizer of International Symposium Moscow-Aizu (ISMA 07), University of Aizu, Oct. 29-30, 2007.
[m-ryzhii-19:2007]	M. Ryzhii, July 1995-. Member (lifelong)
[m-ryzhii-20:2007]	M. Ryzhii. Reviewer for The Journal of Physics D: Applied Physics
[m-ryzhii-21:2007]	M. Ryzhii. Reviewer for Semiconductor Science and Technology Journal
[m-ryzhii-22:2007]	M. Ryzhii, Dec. 2002-. Senior Member
[v-ryzhii-16:2007]	V. Ryzhii. Reviewer for Physical Review Letters Journal
[v-ryzhii-17:2007]	V. Ryzhii. Fellow Member (1994-present)
[v-ryzhii-18:2007]	V. Ryzhii. Reviewer for Semiconductor Science and Technology Journal
[v-ryzhii-19:2007]	V. Ryzhii. Fellow Member (1995, lifelong)
[v-ryzhii-20:2007]	V. Ryzhii, 2006-2008. Member of the Int. Advisory Committee of the Optoelectronics Review Int. Journal
[v-ryzhii-21:2007]	V. Ryzhii. Reviewer for Applied Physics Letters Journal
[v-ryzhii-22:2007]	V. Ryzhii. Reviewer for Physical Review B Journal
[v-ryzhii-23:2007]	V. Ryzhii. Reviewer for Nanotechnology Journal
[v-ryzhii-24:2007]	V. Ryzhii. Reviewer for Solid-State Electronics Journal
[v-ryzhii-25:2007]	V. Ryzhii. Reviewer for Journal of Physics Condensed Matter
[v-ryzhii-26:2007]	V. Ryzhii. Reviewer for Journal of Applied Physics
[v-ryzhii-27:2007]	V. Ryzhii. Corresponding Member (1987, lifelong)

Ph.D and Other Theses

[khmyrova-07:2007]	Daisuke Satou. Graduation Thesis: Some aspects of the HEMT distributed circuit model, University of Aizu, 2007. Thesis Advisor: I. Khmyrova
[m-ryzhii-23:2007]	Takeshi Yabe. Graduation Thesis: Mobile learning for natural science education: Kepler's laws, University of Aizu, 2008. Thesis Advisor: Ryzhii, M.
[m-ryzhii-24:2007]	Ryosuke Serizawa. Graduation Thesis: Developing mobile phone software for physics education with DoJa, University of Aizu, 2008. Thesis Advisor: Ryzhii, M.
[m-ryzhii-25:2007]	Takeshi Yabe. Graduation Thesis: Mobile learning for natural science education: Kepler's laws, University of Aizu, 2008. Thesis Advisor: Ryzhii, M.
[m-ryzhii-26:2007]	Ken Watanabe. Graduation Thesis: Mobile learning for natural science education: Momentum of inertia, University of Aizu, 2008. Thesis Advisor: Ryzhii, M.
[m-ryzhii-27:2007]	Shinichi Ohkouchi. Graduation Thesis: Mobile learning for natural science education: Momentum of a body, University of Aizu, 2008. Thesis Advisor: Ryzhii, M.

2009 THE UNIVERSITY OF AIZU ALL RIGHTS RESERVED