Excitation of Hypersound due to Coupling with Space Charge Waves of Millimeter Wave Range in Gan Films
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Excitation of Hypersound due to Coupling with Space Charge Waves of Millimeter Wave Range in Gan Films V. V. Grimalsky & S. V. Koshevaya & L. M. Gaggero-S. & F. Diaz-A.
Received: 30 January 2007 / Accepted: 31 October 2008 / Published online: 20 November 2008 # Springer Science + Business Media, LLC 2008
Abstract The excitation of hypersonic waves (f = 30–200 GHz) due to coupling with amplified space charge waves of millimeter wave range in GaN films is investigated theoretically. An amplification of space charge waves due to the negative differential conductivity in GaN films placed onto a semi-infinite substrate is considered and possible spatial increments are calculated. Both diffusion-drift equations for volume electron concentration and also an approximation of two-dimensional electron gas were used jointly with the Poisson equation for the electric field. The amplified space charge waves can excite hypersonic waves in the substrate at the same frequency due to piezoeffect and deformation potential mechanisms. It is demonstrated that the piezoeffect seems very effective for resonant excitation of hypersonic waves in the case of a full mechanic contact of GaN film and a non-piezoelectic substrate. Keywords Space charge waves . Millimeter wave range . GaN films . Negative differential conductivity . Hypersonic waves
1 Introduction There exists a problem of excitation of hypersound at the microwave frequencies f > 10 GHz where the traditional methods, like using comb-like transducers, are not effective [1]. A possible solution of this problem is the resonant coupling of acoustic waves with the microwave electric field of space charge waves (SCW) in materials possessing negative differential conductivity (NDC) (like GaAs) [2, 3]. Namely, under propagation in the bias electric field higher than the critical value for observing negative differential conductivity, the SCW is subject to amplification, and its microwave electric field can achieve high values. In turn, due to piezoeffect, deformation potential, or electrostriction, this microwave electric field excites hypersonic acoustic waves (AW). In [2, 3] it was demonstrated that this V. V. Grimalsky : L. M. Gaggero-S. Autonomous University of Morelos, Faculty of Sciences, Cuernavaca, ZP 62210 Mor., Mexico V. V. Grimalsky : S. V. Koshevaya (*) : F. Diaz-A. Autonomous University of Morelos, CIICAp, Cuernavaca, ZP 62210 Mor., Mexico e-mail: [email protected]
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J Infrared Milli Terahz Waves (2009) 30:233–242
excitation is of a resonant character with respect to the frequency and the thickness of the GaAs film. A case of acoustic waveguide was considered there. But the critical value of bias electric field in GaAs is Ec = 3.5 kV/cm that limits the maximal values of the microwave electric field of SCW and, therefore, output values of amplitudes of excited AW. Also, the frequency range of amplification of SCW in GaAs films is f < 50 GHz. To excite the powerful hypersonic AW at higher frequencies f > 50 GHz, it is rather better to use new materials possessing negat
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