V. W. Rampton

Surface acoustic wave attenuation by localized electrons in a 2DEG at a GaAs/AlGaAs heterojunction

The surface acoustic wave (SAW) transmission has been measured at 4.2 K, as a function of magnetic field up to 5 T, for a GaAs wafer on which a heterojunction has been grown by MBE. Frequencies up to 1920 MHz were generated by interdigital SAW transducers deposited directly on the GaAs surface. At all frequencies, oscillations in the SAW attenuation, analogous to the Shubnikov-de Haas oscillations in sigma xx, can be observed due to piezoelectric coupling to the non-localized electrons of the two-dimensional electron gas (2DEG) of the heterojunction. The authors find, at frequencies of about 900 MHz and above, an additional attenuation at all magnetic fields, which increases with magnetic field. At half-integer filling factor, the increase in attenuation is initially linear with increase of field and then, at higher fields, the attenuation increases as the square root of the field. They interpret this as due to deformation potential coupling to the localized electrons of the 2DEG. A small SAW attenuation is observed at zero magnetic field which may be due to a very low density of three-dimensional electrons in the GaAs buffer layer or remaining in the doped AlGaAs layer.

Phonon studies of two-dimensional electron gases

The paper describes a number of experiments on phonon emission and scattering from a two-dimensional electron gas which have been carried out in Nottingham as part of the NUMBERS project on low-dimensional structures.

Magnetic-Field Dependence of the Phonon-Scattering and Phonon Emission by A 2Deg in A Si Mosfet

Abstract Measurements of phonon scattering by a 2DEG in a Si MOSFET show quantum oscillations as the Fermi levels is moved through the Landau level spectrum. Phonon emission by a hot 2DEG shows an increasing proportion of cyclotron emission at high fields. The emission is concentrated more closely normal to the 2DEG as the field is increased.

The Effect of a Magnetic Field on the Phonon Emission from a Hot 2-DEG in the Inversion Layer of a Silicon MOSFET

Phonon emission by a heated two dimensional electron gas in a magnetic field has been studied using heat pulse techniques. The 2-DEG was heated by applying electrical pulses to the MOSFET source-drain electrodes at constant gate voltage and the heat pulses detected by cadmium sulphide bolometers. Using constant power input to the 2-DEG the detected signal was found to decrease by up to 20% in a field of 7T. In a magnetic field phonons are emitted at the cyclotron frequency. At high fields the cyclotron phonons are severely attenuated by isotope scattering in the Si substrate.

Classical Edge Magnetoplasmon in a GaAs/AlGaAs Two-Dimensional Electron System

We have studied the edge magnetoplasmon (EMP) in a GaAs/AlGaAs two-dimensional electron gas (2DEG) system under conditions where the quantum Hall effect (QHE) is not observed; we call this a classical EMP. The classical EMPs are excited in a 2DEG disk by radio frequency electric field excitation (≤1 GHz) in a high magnetic field B (≤5 T) with the 2DEG sheet carrier density n s between 2×10 15 m -2 and 1×10 16 m -2 . The resonant frequency is found to be almost proportional to n s and inversely proportional to B , which agrees quantitatively with the theory proposed by Volkov and Mikhailov, Sov. Phys.-JETP 67 (1988) 1639 by assuming the effective dielectric constant \(\hat{\varepsilon }=(1+\varepsilon _{\rm GaAs})/2\) where e GaAs is appropriate to the GaAs. A comparison of the experimental results with the calculations revealed that the electron mobility plays an important role in the definition of the EMP frequency.

Hot electron effects and phonon emission from a two-dimensional electron gas (2 DEG)

An investigation has been made of the phonon emission from a heated 2DEG in a (100) Si MOSFET for power inputs down to 10 mu W mm-2. For sheet densities 0.7<or approximately=ns<or approximately=4.9*1016 m-2 the steady increase in phonon intensity I opposite the 2DEG as ns falls at constant power input is attributable to in-plane momentum conservation, q11<or approximately=2kF. I falls as ns is further reduced and the authors attribute this to an increase in the proportion of LA phonon emission. For ns<or approximately=3*1015 m-2 there is a significant fall in the proportion of energy emitted as phonons suggesting an increase in FIR emission. The rise in I for ns<or approximately=4.9*1016 m-2 is attributed to the occupation of a subband of excitation energy 31 meV.

Magnetic Field Dependence of Acoustic Phonon Emission and Scattering in 2D Electron Systems

Investigation of phonon emission and scattering from a 2DEG is already a rich field of study. The information it is capable of providing on the electron-phonon interaction is much more detailed than that obtainable in any other way since fewer averages are involved over phonon wave-vector and polarization. It is also clear that the techniques provide a valuable diagnostic tool for studying properties such as hot electron effects, Quantum Hall breakdown and so on. We concentrate here on magnetic field-dependent effects. A more general review of work in this area up to February 1987 is given by CHALLIS et al [1].

The acoustoelectric effect in double layer AlGaAs/GaAs 2D hole systems

The surface acoustic wave velocity shift and acoustoelectric effect have been studied in a double layer GaAs/AlGaAs 2D hole system. The transverse acoustoelectric field exhibited bipolar peaks centred at integer filling factor, in qualitative agreement with the single layer theoretical prediction. Anomalous bipolar peaks have been observed in the longitudinal acoustoelectric effect centred at ν=2 (at 0.6–0.8GHz) and ν=6 and 8 (at 0.8GHz).

Surface acoustic wave attenuation by the localized states of a two-dimensional carrier system in a magnetic field

The surface acoustic wave attenuation and dispersion have been measured as a function of magnetic field for both a two-dimensional electron system and a two-dimensional hole system at frequencies between 100 and 1100 MHz. The experiments were made at 350 mK in the quantum Hall regime. In the region of filling factor one where the quasi-dc conductivity becomes very small, an attenuation and dispersion is found which we attribute to localized carrier states. We estimate that the relaxation time of the localized electrons is about 60 ps while the relaxation time for localized holes is about 20 ps.

Amplitude-dependent effects of the acoustoelectric voltage and acoustic charge transport in GaAsAlGaAs heterostructure in the quantum Hall regime

Abstract The acoustoelectric effect induced by the coupling between a surface acoustic wave and a two-dimensional electron system is investigated on GaAs AlGaAs heterojunctions at 4.2 K in the quantum Hall effect regime. This work was performed to study two-dimensional acoustoelectric non-linearities. We report on amplitude-dependent effects observed in the acoustoelectric voltage in the open geometry. These effects are qualitatively similar to those found on a CdS thick film. We present data suggesting that an acoustic charge-transport behaviour could be present in our experiment. This charge-transport signature provides further evidence for the presence of non-linear effects since the estimated electric field threshold value for non-linear effects reproduced similar non-linear behaviour observed recently in the three-dimensional (3D) piezoelectric semiconductor InSb.