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In order to justify applicability of the standard approach of perturbation theory for the description of transport phenomena in wide-band polar semiconductors with strong electron-phonon interactions, we have compared dependences of energy losses to the lattice on the electron drift velocity obtained for different materials in the frameworks of (a) a perturbative approach based on calculation of the scattering rates from Fermi's golden rule and (b) a non-perturbative approach based on the path-integral formalism of Thornber and Feynman. Our results reveal that despite strong electronphonon coupling in GaN and AlN such that intercollision times become of the order of the period of phonon oscillation, standard perturbative treatment can still be applied successfully for this type of material. Our findings also indicate possibility for unique long-distance runaway transport in nitrides which may occur at the pre-threshold electric fields. Polaron ground state energy and effective masses are calculated for GaN and AlN as well as for GaAs and Al2O3.

We analyze the Cerenkov emission of high-frequency confined acoustic phonons by drifting electrons in a quantum well. We find that the electron drift can cause strong phonon amplification (generation). A general formula for the gain coefficient α is obtained as a function of the phonon frequency and the structure parameters. The gain coefficient increases sharply in the short-wave region. For the example of a Si/SiGe/Si device it is shown that the amplification coefficients of the order of hundreds of cm −1 can be achieved in the sub-THz frequency range.PACS numbers 72.20, 68.65.+g, 63.20.Kr, 63.22. +m High-frequency lattice vibrations with a high degree of spatial and temporal coherence have been observed for a number of semiconductor materials and heterostructures. These include Si, Ge, GaAs as well as SiGe and AlGaAs superlattices. 1,2 These studies provide information on excitation mechanisms for the coherent phonons, their dynamics, electron-phonon interaction, and other important phenomena, including phonon control of the ionic motion. 3 Intense coherent phonon waves can be exploited for various applications: terahertz modulation of light, generation of high frequency electric oscillations, nondestructive testing of microstructures, etc. Usually, both optical and acoustic high-frequency coherent phonons are excited optically by ultrafast laser pulses. 1,2 The development of electrical methods of coherent phonon generation is an important problem. An electric current flowing though a semiconductor can produce high-frequency coherent acoustic phonons. Two distinct cases are possible. If the current results from transitions of carriers between bound electron states, coherent phonon generation can occur if there is a population inversion between these states. Hopping vertical transport in superlattices and three barrier structures provides examples of mechanisms for the establishment of a population inversion and for stimulated generation of terahertz phonons 4 and plasmons. 5 If the current is due to free electron motion in an electric field, phonon amplification (generation) can be achieved via the Cherenkov effect if the electron drift velocity exceeds the velocity of sound. This effect is well-known for bulk samples. 6 High drift velocities and large densities of electrons are necessary for practical use of the Cerenkov effect. Advanced technology of semiconductor heterostructures opens new possibilities to employ this effect for high-frequency phonon generation. Indeed, such phenomena as high electron mobility at large electron density and phonon confinement in a quantum well (QW) can greatly facilitate achieving phonon amplification and generation by electron drift. In this letter, we analyze the generation of high-frequency confined acoustic phonons under the electron drift in a QW layer.Consider a symmetric heterostructure shown in Fig. 1, (a) with electrons confined in the layer A of thickness 2d. Assuming isotropic elastic properties for both semiconductors A and B one can introduce the longitudin...