Abstract
We report the results of the Monte Carlo simulation of electron dynamics in stationary and space- and time-dependent electric fields in compensated GaN samples. We have determined the frequency and wavevector dependencies of the dynamic conductivity, σω,q (i.e., the electron response to high-frequency electrical signals). We have found that the spatially dependent dynamic conductivity of the drifting electrons can be negative under stationary electric fields of moderate amplitudes, 2..5kV/cm. This effect is realized in a set of frequency windows. The low-frequency window with negative dynamic conductivity is due to the Cherenkov mechanism. For this case, the time-dependent field induces a “traveling wave” of the electron concentration in real space and a “standing wave” in the energy/momentum space. The higher frequency windows of negative dynamic conductivity are associated with the optical phonon transient time resonances. For this case, the time-dependent field is accompanied by oscillations of the electron distribution in the form of the “traveling” waves in both the real space and the energy/momentum space. We discuss the optimal conditions for the observation of these effects. We suggest that the studied negative dynamic conductivity can be used to amplify electromagnetic waves at the expense of energy of the stationary field and current.
Highlights
In polar semiconductor materials and heterostructures, such as III-V compounds, groupIII nitrides, ZnO/MgO and others, at low lattice temperature the optical phonon emission is the dominant scattering mechanism for hot electrons, which considerably suppresses their mobility
At low temperatures, when e− ̄hωop/kBT0 ≪ 1 the absorption/emission of optical phonons by the equilibrium electrons is practically absent and the electron mobility is limited only by weak quasi-elastic scattering by impurities and acoustic phonons. Under these conditions the dynamics of an electron subjected to a steady-state high electric field F0 is the following
We present a numerical study of the spatio-temporal dispersion of the HF conductivity σ(ω, q) under the optical phonon transit time (OPTT) resonance effect
Summary
In polar semiconductor materials and heterostructures, such as III-V compounds, groupIII nitrides, ZnO/MgO and others, at low lattice temperature the optical phonon emission is the dominant scattering mechanism for hot electrons, which considerably suppresses their mobility. An optical phonon emission occurs so that the electron looses practically all its energy and stops, this process is repeated again This electron dynamics gives rise to temporal and spatial modulation of the electron momentum, p, velocity, v, and concentration, ne, with characteristic time period, τF = pop/eF0, and space period, lF = eF0τF2 /2 m∗ ≡ hωop/eF0, where pop = 2m∗hωop, e is the elementary charge and m∗ is the electron effective mass. The damping of these oscillations is weak or even absent, when the frequency and/or the wavevector are multiples of ωF and/or qF = 2π/lF , respectively, i.e., under conditions of time- and/or space resonances This novel type of spatio-temporal resonant phenomena was studied analytically in[15] by using the approximation of infinitely fast emission of optical phonons by the electrons with energy exceeding hωop. We determined the {ω, q}-regions, where the real part of the HF conductivity is negative, the drifting electron gas is unstable and an external electromagnetic wave with corresponding ω and q can be amplified at the expense of the stationary field and current
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