Photoresponse of a two-dimensional electron gas at the second harmonic of the cyclotron resonance

Abstract

Terahertz spectroscopy experiments at magnetic fields and low temperatures were carried out on samples of different gate shapes processed on a high electron mobility GaAs/AlGaAs heterostructure. For a given radiation frequency, multiple magnetoplasmon resonances were observed with a dispersion relation described within a local approximation of the magnetoconductivity tensor. The second harmonic of the cyclotron resonance was observed and its appearance was interpreted as resulting from a high frequency, inhomogeneous electromagnetic field on the border of a two-dimensional electron gas with a metallic gate and/or an ohmic contact. In a two-dimensional electron gas (2DEG) subject to a perpendicular magnetic field (B), electrical dipole transitions between Landau levels (LL) are restricted by the selection rule: N → N ± 1, where N = 0, 1, ... is the LL number. However, this strict selection rule was shown to be relaxed in some cases. Observations of harmonics of the cyclotron resonance (CR) were reported in a Si metal-oxide-semiconductor field-effect transistor at a sufficiently high gate polarization.1 They were explained as a result of a simultaneous action of a short-range scattering by charged traps at the Si/SiO 2 interface and the electron-electron scattering.2 Recently, Dai et al. reported on a strong response of a 2DEG at the second harmonic of the cyclotron resonance (2CR).3 The experiments were carried out at temperature (T ) between 0.3 K and 1.4 K on a GaAs/AlGaAs sample with a 2DEG mobility of 3 × 10 7 cm 2 /Vs and at radiation frequency (f ) from 60 GHz to 120 GHz. The 2CR peak was shown to be distinct from microwave-induced resistance oscillations (MIRO) 4 and disappeared at about 1.4 K. A similar observation was reported by Hatke et al. on a 2DEG in a few GaAs/AlGaAs wafers with the electron mobility of the order of 10 7 cm 2 /Vs in experiments carried out at T = 0.5 K or 1.5 K. In a series of papers 5-7 properties of a peak appearing in the photoresponse in a proximity to the 2CR were analyzed in a great detail. It was shown that the peak did not result from the MIRO and that its properties could not be explained within existing theoretical models.In the present paper we report on the observation of a photoresponse of a 2DEG at magnetic fields corresponding to the excitation of the 2CR. There are essential differences between our experiments and the experiments described in Refs. 3, 5-7. First, we investigated a 2DEG which resides in a single GaAs/AlGaAs heterostructure in comparison to modulation doped AlGaAs/GaAs/AlGaAs single quantum wells. Second, our experiments were carried out at 4.2 K and not in a mK regime. For this reason, the electron mobility in our samples was of the order of 6 × 10 5 cm 2 /Vs and not 10 7 cm 2 /Vs. This clearly indicates that the conditions of our measurements did not correspond to that of the well developed MIRO. Third, we explored both the low (120-660 GHz) and high (1.5-2.5 THz) frequency bands, which essentially enlarges the frequency range used previously, and...

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