We investigate the nonlinear drift response of electrons in Si, GaAs, and InP crystals to high-power electromagnetic waves by means of a Monte Carlo technique, with the aim of developing an efficient frequency converter for 1 THz output radiation. Drift velocity amplitudes and phases determining the conversion efficiency are calculated for the first, third, and fifth harmonics in the pumping wave amplitude range of 10<E1<100 kV/cm, for frequencies between 30 and 500 GHz, and at the lattice temperatures of 80, 300, and 400 K. It is found that the efficiency is a maximum at the pumping wave amplitude of the order of 10 kV/cm depending on the intervalley electron scattering parameters and the lattice temperature. Cooling the nonlinear crystal down to the liquid-nitrogen temperature enhances the efficiency several times in Si and by orders of magnitude in GaAs and InP. This is promising for obtaining a 10% conversion efficiency.
Electronic properties of InSb and InAs are sensitive to electric field due to their narrow forbidden energy gaps and big difference in effective masses of electrons in different conduction band valleys. Here we report impact ionization processes and redistribution of electrons between the Γ, L and X valleys induced by a single ultrashort terahertz (THz) pulse at 80 K temperature. Monte Carlo simulation revealed that electron motion in this case has near ballistic character. The threshold electric field of impact ionization increases as the THz pulse gets shorter, and the process of impact ionization essentially raises cooling rate of hot electrons. The L valley gets mainly occupied by electrons in InSb while the X valley holds the majority of electrons in InAs at strong electric fields, respectively above 20 kV/cm and 90 kV/cm. The calculated results are in good agreement with the available experimental data.
Conventional models of electron transport in hexagonal GaN crystals predicting electron drift velocity peak value up to 3.2×107cm∕s at 140–220kV∕cm and a pronounced negative differential mobility at higher fields are revised. The new model is suggested accounting for the additional low-energy optical phonon modes (∼26meV) and the satellite valley location close (400meV) to the conduction band bottom. Electron scattering on these and conventional (∼92meV) LO-phonon modes together with the fast intervalley exchange is shown to limit electron drift velocity (<1.9×107cm∕s at T=300K), in excellent agreement with the time-of-flight experiment.
Monte Carlo computer simulations of electron impact ionization in InSb crystal are carried out for both instantly switched on dc and high-frequency electric fields. It is established that the rate of generation of electron-hole pairs decreases with the increase of electric field frequency, due to the inertia of electron heating by high-frequency electric field. For fields oscillating at frequencies much higher than the reciprocal momentum relaxation time, the impact ionization threshold field is found to be a linear function of frequency. Good agreement between calculations and available experimental data has been obtained.
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