A. I. Toropov

Raman study of self-assembled InAs quantum dots embedded in AlAs: influence of growth temperature

Abstract Phonon spectra of self-assembled InAs quantum dots (QDs) in an AlAs matrix were studied by Raman spectroscopy. A series of InAs QDs was grown by molecular beam epitaxy at substrate temperatures varied in the range of 420–550°C. The observed asymmetric line shape of LO phonons in InAs QDs and its low-frequency shift with increasing excitation energy are explained by QD size distribution and phonon confinement in small-size dots. Phonons of the InAs wetting layer are also observed in the Raman spectra of QD structures. Two bands of interface phonons in the AlAs frequency region are attributed to phonons associated with two types of interfaces: the planar interface wetting layer/AlAs matrix and the three-dimensional QD/matrix interface. A comparison of the position and the line shape of phonon features of InAs QDs grown at different temperatures reveals that dots grown at low temperatures (∼420°C) have the smallest average size. Increasing the temperature leads to the formation of larger InAs islands. At temperatures higher than 520°C partial re-evaporation of InAs occurs.

Giant magnetoresistance oscillations induced by microwave radiation and a zero-resistance state in a 2D electron system with a moderate mobility

The effect of a microwave field in the frequency range from 54 to 140 GHz on the magnetotransport in a GaAs quantum well with AlAs/GaAs superlattice barriers and with an electron mobility no higher than 106 cm2/V s is investigated. In the given two-dimensional system under the effect of microwave radiation, giant resistance oscillations are observed with their positions in the magnetic field being determined by the ratio of the radiation frequency to the cyclotron frequency. Earlier, such oscillations had only been observed in GaAs/AlGaAs heterostructures with much higher mobilities. When the samples under study are irradiated with a 140-GHz microwave field, the resistance corresponding to the main oscillation minimum, which occurs near the cyclotron resonance, appears to be close to zero. The results of the study suggest that a mobility value lower than 106 cm2/V s does not prevent the formation of zero-resistance states in a magnetic field in a two-dimensional system under the effect of microwave radiation.

Millisecond photoluminescence kinetics in a system of direct-bandgap InAs quantum dots in an AlAs matrix

Anomalously long millisecond kinetics of photoluminescence (PL) is observed at low temperatures (4.2–50 K) in direct-bandgap InAs quantum dots formed in an AlAs matrix. An increase in temperature leads to a decrease in the duration of PL decay down to several nanoseconds at 300 K, whereas the integral PL intensity remains constant up to 210 K. In order to explain the experimental results, a model is proposed that takes into account the singlet-triplet splitting of exciton levels in small quantum dots.

Interface phonons in semiconductor nanostructures with quantum dots

The vibrational spectra of structures with InAs quantum dots in an AlGaAs matrix and AlAs quantum dots in an InAs matrix are investigated experimentally and theoretically. The Raman spectra exhibit features that correspond to transverse-optical (TO), longitudinal-optical (LO), and interface phonons. The frequencies of interface phonons in InAs and AlAs quantum dots and in an AlGaAs matrix with various concentrations of aluminum are calculated with the use of experimental values of transverse-and longitudinal-optical phonons in the approximation of a dielectric continuum. It is shown that the model of a dielectric continuum adequately describes the behavior of interface phonons in structures with quantum dots under the assumption that the quantum dots are spheroidal.

Photoluminescence of high-quality AlGaAs layers grown by molecular-beam epitaxy

We report a photoluminescence study of high-purity AlxGa1−xAs layers grown by molecular-beam epitaxy over the 0⩽x⩽0.295 composition range. The intense excitonic line dominates in the photoluminescence spectra of the layers. The full width at half maximum of the excitonic line is in excellent agreement with values calculated by Lee and Bajaj [J. Appl. Phys. 73, 1788 (1993)] for perfectly random alloys, and in the spectra of the layers with AlAs fractions of x=0.15 and x=0.209 it equals to 1.24 and 1.48 meV, respectively. A linear dependence of the exciton line intensity on excitation power evidences negligible concentration of nonradiative recombination centers in the layers.

Nanotechnologies in semiconductor electronics

The various technologies for fabrication micro- and nanosized systems included semiconductor, metal and insulator structures are reviewed on the base of the data obtained in Novosibirsk Institute of Semiconductor Physics. Main attention is devoted to development of method of molecular beam epitaxy, silicon-on-insulator fabrication process, electron beam lithography and scanning probe nanolithography.

Resonant Raman scattering in nanostructures with InGaAs/AlAs quantum dots

Raman scattering by optical phonons in InxGa1 − xAs/AlAs nanostructures with quantum dots has been studied experimentally for compositions corresponding to x = 0.3−1 under out-resonance conditions. Features due to scattering by GaAs-and InAs-like optical phonons in quantum dots have been detected, and the phonon frequencies have been determined as a function of the dot composition. With increasing excitation energy, a red shift is observed in the frequency of the GaAs-like phonon in quantum dots, which testifies to Raman scattering selective by the size of quantum dots. Under resonant conditions, multiphonon light scattering by optical and interface phonons is observed up to the third order, including overtones of the first-order phonons of InGaAs and AlAs materials and their combinations.

Aharonov-Bohm Oscillation Amplitude in Small Ballistic Interferometers

Small-radius (110 nm) ring interferometers were fabricated by the local anodic oxidation of AlGaAs/GaAs heterostructures containing 2D electron gas. Measurements and modeling show that a small ring asymmetry, which is detected by an atomic force microscope, leads to a small amplitude of Aharonov-Bohm oscillations, while a stronger asymmetry completely suppresses these oscillations.

Vibrational spectroscopy of InAlAs epitaxial layers

Raman and IR spectroscopies were applied for the investigation of optical phonons in relaxed InxAl1−xAs alloys grown by molecular-beam epitaxy on GaAs. From the analysis of Raman and IR data, the frequencies of longitudinal and transverse InAs- and AlAs-like optical phonons were derived in the whole composition range of the alloy (x=0–1). Analytical expressions are obtained for the frequency variation with In composition for the optical phonon modes, as well as the variations in their intensity and linewidth. Multiple-phonon Raman scattering up to the fourth order is observed under resonant conditions.

Magnetotransport properties of a ballistic ring interferometer on the basis of a GaAs quantum well with a high concentration of 2D electron gas

Magnetotransport properties of ballistic ring interferometers made on the basis of 2D electron gas in a GaAs quantum well with AlAs/GaAs superlattice barriers are studied. An asymmetry of magnetoresistance and a phase reversal in h/e oscillations are observed when the bias voltage across the ring exceeds kT/e.