Microwave radiation induced magneto-oscillations in the longitudinal and transverse resistance of a two dimensional electron gas

We confirm the existance of magneto-resistance oscillations in a microwave-irradiated two-dimensional electron gas, first reported in a series of papers by Zhudov et al. and Mani et al. In our experiments, on a sample with a more moderate mobility, the microwave induced oscillations are observed not only in the longitudinal - but also in the transverse-resistance (Hall resistance). The phase of the oscillations is such that the decrease (increase) in the longitudinal resistance is accompanied by an increase (decrease) in the absolute value of the Hall resistance. We believe that these new results provide valuable new information to better understand the origin of this interesting phenomenon.Comment: Accepted for publication in journal of Solid State Comunication

Induced currents in the quantum Hall regime: energy storage, persistence, and I-V characteristics

Copyright © 2012 American Physical SocietyInduced currents associated with the quantum Hall effect are studied in the temperature range 39 mK to 1.6 K, and at Landau-level filling factors ν=1,2,3,4, and 6, using torsion-balance magnetometry. A quantitative link is demonstrated between (nonlinear induced current) vs (inducing electromotive force) curves, and the subexponential decay of the induced current in a static magnetic field. The energy storage in the induced currents is reexamined with the conclusion that the predominant mechanism for storage is inductive, through the mutual inductance between the sample and the magnet, not capacitive as previous reports have assumed. The temperature dependencies of the currents are consistent with previous models, except for a low-temperature saturation at filling factors ν=1 and ν=2, which we attribute to electron heating

The microwave induced resistance response of a high mobility 2DEG from the quasi-classical limit to the quantum Hall regime

Microwave induced resistance oscillations (MIROs) were studied experimentally over a very wide range of frequencies ranging from ~20 GHz up to ~4 THz, and from the quasi-classical regime to the quantum Hall effect regime. At low frequencies regular MIROs were observed, with a periodicity determined by the ratio of the microwave to cyclotron frequencies. For frequencies below 150 GHz the magnetic field dependence of MIROs waveform is well described by a simplified version of an existing theoretical model, where the damping is controlled by the width of the Landau levels. In the THz frequency range MIROs vanish and only pronounced resistance changes are observed at the cyclotron resonance. The evolution of MIROs with frequency are presented and discussed.Comment: 4 pages, presented at EP2DS, to be published in Physica

Effects of inhomogeneous broadening on reflection spectra of Bragg multiple quantum well structures with a defect

The reflection spectrum of a multiple quantum well structure with an inserted defect well is considered. The defect is characterized by the exciton frequency different from that of the host's wells. It is shown that for relatively short structures, the defect produces significant modifications of the reflection spectrum, which can be useful for optoelectronic applications. Inhomogeneous broadening is shown to affect the spectrum in a non-trivial way, which cannot be described by the standard linear dispersion theory. A method of measuring parameters of both homogeneous and inhomogeneous broadenings of the defect well from a single CW reflection spectrum is suggested.Comment: 27 pages, 6 eps figures; RevTe

Effects of biased and unbiased illuminations on two-dimensional electron gases in dopant-free GaAs/AlGaAs

Illumination is performed at low temperature on dopant-free two-dimensional electron gases (2DEGs) of varying depths, under unbiased (gates grounded) and biased (gates at a positive or negative voltage) conditions. Unbiased illuminations in 2DEGs located more than 70 nm away from the surface result in a gain in mobility at a given electron density, primarily driven by the reduction of background impurities. In 2DEGs closer to the surface, unbiased illuminations result in a mobility loss, driven by an increase in surface charge density. Biased illuminations performed with positive applied gate voltages result in a mobility gain, whereas those performed with negative applied voltages result in a mobility loss. The magnitude of the mobility gain (loss) weakens with 2DEG depth, and is likely driven by a reduction (increase) in surface charge density. Remarkably, this mobility gain/loss is fully reversible by performing another biased illumination with the appropriate gate voltage, provided both Formula Presented-type and Formula Presented-type Ohmic contacts are present. Experimental results are modeled with Boltzmann transport theory, and possible mechanisms are discussed

Phonon and polaron enhanced IR-THz photodetectors

Thanks to the modern compound semiconductor growth and processing technologies, quantum wells and related semiconductor nanostructures have been widely investigated for infrared-terahertz devices. Here we propose a new general approach to make use of polar optical phonons in quantum wells for infrared (IR) and terahertz (THz) detection. Polar optical phonons strongly couple with both electrons and photons, and hence are potentially useful for photonic devices. As the first example, we show the coupling of phonon and intersubband transition leading to Fano resonance in photocurrent spectra. We investigate the phenomenon experimentally in specially designed GaAs/AlGaAs quantum well infrared photodetectors. Finally, we discuss the future research and potentials. Strongly coupled systems of electrons and phonons, i.e., polarons, may lead to new IR-THz photodetectors. \ua9 2011 SPIE.Peer reviewed: YesNRC publication: Ye

An asymmetric quantum well infrared photodetector with voltage-tunable narrow and broad-band response

We describe a 9 \ufffdm AlGaAs/GaAs asymmetric quantum well infrared photodetector with voltage tunable spectral bandwidth. A very narrow spectral response of 9.2 meV (0.6 \ufffdm) full width half maximum is observed for an applied electric field of 28 kV/cm. The linewidth quadruples when the bias polarity is reversed, with very little shift in the peak detection wavelength. This structure is based on a conventional intersubband photodetector modified by using AlGaAs barriers that are graded in Al content and by adding a thin AlGaAs confinement layer on one side of the well. The asymmetry in the barriers is shown to give rise to the dependence of the spectral linewidth on applied bias. As well, a series of unusually well-resolved and intense bound-to-continuum transitions are observed at low bias, that may indicate that the unique barrier shape also leads to enhanced electron interference effects at the well/barrier interfaces.Peer reviewed: YesNRC publication: Ye

Nitride-based laser diodes by plasma-assisted MBE\u2014From violet to green emission

We present recent progress in growth of nitride-based laser diodes (LDs) and efficient light-emitting diodes (LEDs) made by plasma-assisted MBE (PAMBE). This technology is ammonia free, and nitrogen for growth is activated by RF plasma source from nitrogen molecules. The recent demonstration of CW blue InGaN LDs has opened a new perspective for PAMBE in optoelectronics. The LDs were fabricated on low threading dislocation density (TDD) bulk GaN substrates at low growth temperatures 600\u2013700 \ub0C. In this work, we describe the nitride growth fundamentals, the influence of the TDD on the layer morphology, the peculiarities of InGaN growth as well as properties of LEDs and LDs made by PAMBE.Peer reviewed: NoNRC publication: Ye

Electron-phonon coupling and phonon-drag thermopower of a very low mobility 2DEG

It has been predicted that the phonon drag thermopower will be enhanced in very low mobility samples. The effect occurs when ql < 1 where q is the phonon wave number and l the electron mean free path. We present experimental data and detailed numerical calculations which confirm this prediction. © 2002 Elsevier Science B.V. All rights reserved

Scanning voltage microscopy study of lasing and non-lasing terahertz quantum cascade lasers

Scanning voltage microscopy results clearly show that the formation of electric field domains is responsible for the missing of lasing operation in a resonant-phonon based terahertz quantum cascade laser with a highly diagonal transition.Peer reviewed: YesNRC publication: Ye