Far-infrared Cyclotron Resonance Research Articles

Optically detected far-infrared cyclotron resonance of two-dimensional electrons in a single GaAs/(Al,Ga)As heterojunction

Far-infrared (FIR) cyclotron resonance of a high-mobility two-dimensional electron gas (2DEG) in a GaAs/(Al,Ga)As single heterojunction is detected by resonant modulation of the photoluminescence. This optically detected cyclotron resonance (ODCR) of the 2DEG occurs in two photoluminescence (PL) bands originating from spatially separated sample regions: The exciton emission from the wide GaAs buffer layer is enhanced, while the photoluminescence of the 2D electrons with itinerant holes is suppressed when the 2D electrons resonantly absorb FIR radiation. The ODCR mechanism is attributed to the interaction of excitons with ballistically propagating acoustic phonons (phonon wind) emitted upon energy relaxation of 2D electrons heated by FIR. The interaction results in an effective long-range 2DEG-exciton repulsion that hinders dissociation of free excitons in the 2DEG vicinity. We show that the FIR ODCR lineshape is well described in the frame of the Drude model accounting for FIR reflection from the 2DEG. The significant difference between this FIR-induced ODCR and previously studied microwave-induced ODCR is discussed.

Cyclotron resonance photoconductivity of a two-dimensional electron gas in HgTe quantum wells

Far-infrared cyclotron resonance photoconductivity (CRP) is investigated in HgTe quantum wells (QWs) of various widths grown on (0 1 3) oriented GaAs substrates. It is shown that CRP is caused by the heating of two-dimensional electron gas (2DEG). From the resonance magnetic field strength effective masses and their dependence on the carrier concentration is obtained. We found that the effective mass in each sample slightly increases from the value (0.0260±0.0005) m 0 at N s=2.2×10 11 cm −2 to (0.0335±0.0005) m 0 at N s=9.6×10 11 cm −2. Compared to determination of effective masses by the temperature dependence of magnitudes of the Shubnikov-de Haas (SdH) oscillations used so far in this material our measurements demonstrate that the CRP provides a more accurate (about few percents) tool. Combining optical methods with transport measurements, we found that the transport time substantially exceeds the cyclotron resonance lifetime as well as the quantum lifetime which is the shortest.

CYCLOTRON RESONANCE IN COUPLED BILAYERS IN HIGH MAGNETIC FIELDS

We present far-infrared cyclotron resonance measurements on a strongly coupled symmetric GaAs/GaAlAs double quantum well sample. Cyclotron resonance is measured at several discrete wavelengths and tilt angles of the sample with respect to the magnetic field. The width and strength of the resonance peaks depend strongly on the tilt angle and the laser wavelength, demonstrating the complexity of this system.

Filling factor dependence of electron effective mass in Si/SiGe quantum wells at high magnetic field

Far-infrared cyclotron resonance of a two-dimensional electron gas in strained Si/SiGe single quantum wells was studied at low temperatures (1.5–25K) in high magnetic fields up to 40T. We found a remarkable Landau-level filling factor (ν) dependence of the effective mass with prominent peaks at around ν=1 and 2. The carrier scattering time deduced from the line-width was found to oscillate also as a function of ν. The result is explained by screening of impurity potentials that depends on the filling of the Landau levels.

Photoluminescence of two-dimensional electron system excited by far-infrared cyclotron resonance in GaAs/AlGaAs double heterostructure

We report the H band luminescence due to an indirect exciton composed of a two-dimensional electron and a hole in a modulation-doped GaAs/AlGaAs double heterostructure. The two-dimensional feature of the H band was clearly revealed by far-infrared optically detected cyclotron resonance in tilted magnetic fields. In addition, the Fermi-edge singularity was observed in the H band spectra and the characteristics of excitons have been confirmed by the magneto-oscillatory spectra.

Properties of two-dimensional electron gas containing self-organized quantum antidots

A nonuniform two-dimensional electron gas in a heterojunction with inserted self-organized electrically inactive dots (acting as antidots) has been fabricated by molecular-beam epitaxy of AlGaAs/AlInAs/GaAs layer sequences. Transport measurements give the ratio of the transport mobility to the quantum mobility less than four, which suggests that the dominant scattering at low magnetic fields is the short-range scattering from the lateral potential of the antidots. Far-infrared cyclotron resonance (CR) spectra show an absorption mode as narrow as 0.5 cm−1 at high magnetic fields associated with the high-mobility electron gas formed between the antidot islands and confined in the lateral directions. The confinement energy of 14 cm−1 is derived from the CR spectra.

Electron dynamics of a two-dimensional electron gas with a random array of InAs quantum dots

We show that the presence of InAs dots embedded in a host GaAs quantum well containing a two-dimensional electron gas dramatically modifies the cyclotron resonance (CR). Far-infrared CR measurements show two modes with different dispersions with applied magnetic field B. The lower-frequency mode, with a sublinear dependence on B, is identified as a CR at low B, developing into a skipping orbit around the dot perimeters at higher B; this has not been previously observed for a system with randomly distributed scatterers. The higher-frequency mode is identified as a magnetoplasmon localized by the confining effect of the arrays of repulsive potentials due to the dots in the well. The linewidths of these modes, despite the broad size distribution of the dots, are narrow, suggesting that Coulombic interactions are important.

The far-infrared magneto-optical response of strongly coupled 2DEGs near the quantum and semi-classical limits

Far-infrared cyclotron resonance (CR) spectroscopy was used to study a pair of strongly coupled two-dimensional electron gases (2DEGs) formed in two GaAs quantum wells separated by a thin AlGaAs barrier. The degree of wavefunction hybridisation, along with the effect of a magnetic field parallel to the plane of the electron gas, were investigated near both the quantum and semi-classical limits, corresponding to low and high filling factors, respectively. Results in both limits are compared with predictions from self-consistent solutions of the Poisson and Schrödinger equations. Near the quantum regime, the CR transitions in a small parallel field reveal anticrossing between the Landau levels associated with different hybridised subbands. The energies and intensities of these transitions change with front gate voltage, yielding information on the bias dependence of the wavefunction hybridisation and the subband energy splitting. Near the semi-classical limit and with strong parallel magnetic fields, two CR peaks were observed. The corresponding cyclotron masses are compared to those expected for non-circular Fermi contours created by anticrossing of the parabolic dispersion curves associated with the coupled 2DEGs.

Far-infrared cyclotron resonance study of the effect of strain and localisation in Si/SiGe two dimensional electron gases

Far infrared cyclotron resonance measurements have been used to investigate the effective mass ( m*) in the strained silicon channel of modulation-doped, two dimensional electron gases grown on relaxed Si 1− x Ge x . Samples with germanium fractions from 24% to 31% were measured to investigate the influence of strain on m*. Little variation as a function of strain was observed, but for one sample, the resonance position was shifted up in frequency due to localisation effects. This persisted up to higher Landau level filling factors than has been observed previously in other materials systems and was accompanied by a large enhancement in the quantum lifetime.

Cyclotron resonance studies of strongly coupled double quantum wells in tilted magnetic fields near the quantum and semi-classical limits

Far-infrared cyclotron resonance (CR) spectroscopy has been used to study a pair of strongly coupled two-dimensional electron gases (2DEGs) formed in two GaAs quantum wells separated by a thin AlGaAs barrier. The degree of wave function hybridisation, along with the effect of a magnetic field parallel to the plane of the electron gas, have been investigated near both the quantum and semi-classical limits, corresponding to low and high filling factors, respectively. Near the quantum regime, the CR transitions in a small parallel field reveal anticrossing between the Landau levels associated with different hybridised subbands. The energies and intensities of these transitions change with front gate bias, yielding information on the bias dependence of the wave function hybridisation and the subband energy splitting. Near the semi-classical limit and with strong parallel magnetic fields, two CR peaks are observed. The corresponding cyclotron masses are compared to those expected for non-circular Fermi contours created by anticrossing of the parabolic dispersion curves associated with the coupled 2DEGs. Experimental results in both limits are compared with predictions from self-consistent solutions of Poisson's and Schrödinger's equations.

Plasmons in spatially non-uniform magnetic fields

A non-planar two-dimensional electron gas (2DEG) has been realised by using molecular beam epitaxy to grow a high mobility heterostructure on a (1 0 0) n +-GaAs layer selectively etched to create a two-dimensional array of cavities through the n +-GaAs which are bounded by higher-index facets. Far-infrared (FIR) cyclotron resonance spectra show one absorption mode associated with an electron gas formed inside the cavities and confined in both lateral directions. Typical confinement energies of 23 cm −1 and widths of 1000 nm are derived from the FIR spectra and magneto resistance measurements. A second mode occurs at a frequency lower than that measured for a planar 2DEG. Its origin is discussed in terms of magneto-plasmons excited across a non-planar region of the electron gas in a spatially non-uniform magnetic field. Combining this information with atomic force microscopy images provides a comprehensive picture of the nature of the lateral confinement in this structure.

Dynamical properties of excitons interacting with electrons heated by far-infrared cyclotron resonance in InGaAs

Optically detected cyclotron resonance (ODCR) which combines photoluminescence (PL) and cyclotron resonance (CR), is applied to a study of excitons in InGaAs layers. By comparison with the temperature dependence of PL spectra, it is concluded that hot electrons excited by CR play an important role.

Cyclotron resonance measurements of Si/SiGe two-dimensional electron gases with differing strain

Far-infrared cyclotron resonance measurements have been used to investigate the effective mass in the strained silicon channels of modulation-doped, two-dimensional electron gases grown on relaxed Si1−xGex. By using a range of Ge fractions x, the effect of strain was investigated. Consistent results were obtained when the resonance positions were fitted to a model for zero-dimensional confinement, yielding m*≈0.196 me for most samples. The use of this formula was justified by invoking electron localization due to a disorder potential. The observed confinement effect was strongest in two samples where the Si channel was partially relaxed, suggesting this to be a possible mechanism. Qualitatively different results were obtained for a sample with a high background concentration of donor impurities, indicating that the type of disorder present can affect the nature of the resonances.

Cyclotron-resonance studies of strongly coupled double quantum wells in tilted magnetic fields near the quantum and semiclassical limits

Far-infrared cyclotron-resonance (CR) spectroscopy has been used to study a pair of strongly coupled two-dimensional electron gases (2DEG's) which were formed in two GaAs quantum wells and separated by a thin ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ barrier. The degree of wave-function hybridization, along with the effect of a magnetic field parallel to the plane of the electron gas, have been investigated near both the quantum and semiclassical limits, corresponding to low and high filling factors, respectively. Near the quantum regime, the CR transitions in the presence of a small parallel field reveal anticrossing between the Landau levels associated with different hybridized subbands. The energies and intensities of these transitions change with front gate bias, yielding information on the bias dependence of the wave-function hybridization and the subband energy splitting. Close to the semiclassical limit and with strong parallel magnetic fields, two CR peaks are observed. The corresponding cyclotron masses are compared to those expected for noncircular Fermi contours created by anticrossing of the parabolic dispersion curves associated with the coupled 2DEG's. Experimental results in both limits are discussed in the light of predictions from self-consistent solutions of Poisson's and Schroedinger's equations.

Far-infrared study of a laterally confined electron gas formed by molecular beam epitaxial regrowth on a patterned (100) n+-GaAs substrate

A method of producing lateral confinement of an electron gas has been realized by using molecular beam epitaxy to grow a high mobility heterostructure on a (100) n+-GaAs layer selectively etched to create a two-dimensional array of cavities through the n+-GaAs, which are bound by higher index facets. Far-infrared cyclotron resonance (CR) spectra unambiguously demonstrate that the electron gas formed inside the cavities is confined in both lateral directions. Typical confinement energies of 30 cm−1 and widths of 2000 nm are derived from the spectra and magnetoresistance measurements. The effect of different n+-GaAs backgate biases is also investigated. Combining information from CR spectra with atomic force microscopy images provides a picture of the nature of the lateral confinement in this structure.

Measuring electron transfer in real space in biased asymmetric double quantum wells using far-infrared cyclotron resonance

We report measurements of electron transfer in real space in GaAs/AlxGa1−xAs asymmetric double quantum wells under an electric field by far-infrared cyclotron resonance (CR). Due to nonparabolicity, the asymmetric quantum structure results in well-resolved CR lines, which allow contactless measurements of the electron density in each well. Our results show that electrons tunnel through the barrier from one well to the other at the level anticrossing of their ground states.

An investigation of the multicarrier transport properties of -doped InSb at high temperatures using a mobility spectrum technique

Transport properties of Si--doped InSb grown by molecular beam epitaxy are studied. The mobility spectrum (MS) analysis made over a wide temperature range reveals three species of carriers: those associated with the bulk of the film, and electrons in quantized subbands in the -layer. At low temperatures the MS data are consistent with self-consistent calculations and measurements of the Shubnikov - de Haas oscillations. The far-infrared cyclotron resonance lineshapes are well described by the Drude approximation using the obtained values of subband mobilities, concentrations and effective masses.

Far-infrared study of a quasi-one-dimensional electron gas formed on (100)GaAs facets with hole gas sidegates on a (311)A GaAs substrate

A new type of quasi-one-dimensional electron gas has been realized by using molecular beam epitaxy to grow a high mobility heterostructure on a (311)A GaAs substrate selectively etched to expose (100) facets. The electron gas formed on the (100) facets is confined in one lateral dimension by the p–n junctions formed with the adjacent two-dimensional hole gases on (311)A, thereby forming a p–n–p structure. Far-infrared cyclotron resonance spectra demonstrate the dimensionality of such structures and yield typical lateral confinement energies of 22.3 cm−1 and electronic widths of ∼900 nm. These estimates are supported by cathodoluminescence data.

Far-infrared cyclotron resonance of wide-gap semiconductors using pulsed high magnetic fields

A cyclotron resonance experiment for n-ZnO and the ‘camel's back’ n-GaP was carried out in pulsed magnetic fields using an optically pumped far-infrared laser as a radiation source. The cyclotron resonance transitions of n-GaP for 0 + → 1 −, 0 − → 2 + and 0 + → 2 − were observed for the first time as well as the usual cyclotron transitions associated with the transverse effective mass m t. The longitudinal effective mass m 1 was determined to be ∼ 10 m o by the angular dependence of cyclotron resonance at 433 μm. The electron effective mass of n-ZnO was estimated to be approximately 0.3 m 0 at 394 μm for B ⊥ ( 10 1 ¯ 0 ) surface.

Tuneable confinement energies in a quasi-one-dimensional electron gas regrown on a patterned GaAs backgate

A quasi-one dimensional (Q1D) electron gas has been fabricated from a high electron mobility transistor regrown by molecular beam epitaxy onto facets of a patterned backgate consisting of alternate layers of p- and n-GaAs. By applying biases independently to the p- and n-GaAs layers, both the carrier density and the confinement width were varied. The effects of changes in these quantities were detected by measuring far-infrared cyclotron resonance spectra of the structure, which were used to determine the Q1D confinement energy. Tuneability of the confinement energy has been demonstrated for different p n - GaAs bias combinations, and the effects of the backgate on the Q1D gas may be explained by use of the local plasmon model.