Chaotic dynamics dependence on doping density in weakly coupled GaAs/AlAs superlattices

A discrete sequential tunneling model is used for studying the influence of the doping density on the dynamical behaviors in weakly coupled GaAs/AlAs superlattices.Driven by the DC bias,the system exhi...     

Negative differential conductance in GaAs/AlAs superlattices

Negative differential conductance has been characterised on a series of n-doped GaAs/AlAs superlattices due to Esaki's negative effective mass mechanism. High electron peak velocities (up to 10/sup 7/ cm/s) were obtained together with high current densities (up to 150 kA/cm/sup 2/). These preliminary results are promising for microwave applications.<>     

Oscillating electric field domains in GaAs/AlAs superlattices

We report on the observation of self-sustained oscillations associated with electric-field domains in GaAs/AlAs superlattices. The samples are n-doped superlattices embedded between n⁺-contacts. The observed oscillation frequencies between 700 kHz and 100 MHz depend strongly on the coupling strength between adjacent wells. Monitoring the oscillations with time-resolved photoluminescence spectroscopy indicates that the current oscillations are due to the movement of a charge accumulation layer at the domain boundary.     

Donor-acceptor recombination in short-period silicon-doped GaAs/AlAs superlattices

A low-frequency band is observed along with an exciton band in photoluminescence spectra of short-period GaAs/AlAs superlattices doped with Si in the barriers or in the barriers and wells. This band is ascribed to donor-acceptor recombination on the basis of the dependence of its frequency on the excitation intensity under cw excitation and on the time delay under pulsed excitation. Mainly type-II superlattices are investigated. The estimate E _A+E _D≈120 meV can be obtained from the peak energy of the donor-acceptor band with a very weak excitation intensity. The estimates E _A≈23 meV and E _D≈90 meV are obtained from the temperature dependence of the band intensity. It is suggested that the deep donor level is associated with a DX center in the AlAs layers. Fiz. Tekh. Poluprovodn. 32, 839–842 (July 1998)     

Negative differential perpendicular velocity in GaAs/AlAs superlattices

We investigate the doping level, hydrostatic pressure, temperature and superlattice parameter dependence of the perpendicular negative differential velocity (NDV) of GaAs/AlAs superlattices. The results show that NDV is a single Γ miniband effect. A qualitative agreement is found with Esaki's negative effective mass model, although a few limitations of this model are pointed out and some improvement is suggested.     

Vertical transport of hot electrons in GaAs/AlAs superlattices

Ballistic transport of hot photoexcited electrons injected from a superlattice into an enlarged quantum well is studied using the polarized hot photoluminescence technique. It is established that most photoexcited electrons thermalize before they are captured by the enlarged QW; however, a minor fraction of them reach the enlarged quantum well ballistically, keeping their momentum-distribution anisotropy or spin orientation arising due to the absorption of linearly or circularly polarized light in the superlattice.     

Exciton recombination in δ-doped type-II GaAs/AlAs superlattices

Radiative recombination of excitons in δ-doped type-II GaAs/AlAs superlattices (SLs) is studied experimentally. With an increase in the impurity density in δ-layers from 2×10¹⁰ to 7.5×10¹¹ cm^?2, the integrated intensity of SL photoluminescence (PL) decreases by a factor of 4–6; the intensity of excitonic PL drops considerably (up to 70–80 times), which is accompanied by an increase in the exciton radiative decay rate. Uniform doping of the SL does not result in the exciton PL quenching. Analysis of the temperature dependence and the kinetics of the PL indicate that impurity quenching of the excitonic PL in δ-doped structures is not related to a reduction in the exciton localization energy and cannot be explained by an increase in the density of nonradiative recombination centers. We conclude that the PL quenching is mainly caused by the appearance of built-in electric fields originating from ionized impurities, which hinders the formation of the excitons.     

Optical properties of GaAs/AlAs superlattices grown on (311)A GaAs surfaces

We present a temperature-dependence photoluminescence of (GaAs)₅/(AlAs)₅ superlattice grown on (311)A-oriented semi-insulating substrate by molecular beam epitaxy. The temperature dependence reveals an anomalous decrease of the PL width, which is explained in terms of phonon-assisted thermal activation of localized excitons.     

Transverse optical phonon splitting in GaAs/AlAs superlattices grown on the GaAs(311) surface studied by the method of Raman light scattering

GaAs_n/AlAs_m superlattices grown on the GaAs (311)A and (311)B surfaces by molecular-beam epitaxy were studied by Raman light scattering. The form of the Raman scattering tensor allowed the TO _y and TO _x modes to be separately observed using various scattering geometries (the y and x axes correspond to atomic displacements along and across facets formed on the (311)A surface, respectively). The TO1_y and TO1_x modes exhibited splitting in superlattices grown on a faceted GaAs(311)A surface. The degree of splitting increased for superlattices with an average GaAs layer thickness of 6 monoatomic layers and less. No splitting was observed for superlattices grown under the same conditions on the (311)B surface, which indicates that the splitting effect is probably due to the formation of GaAs quantum wires on the faceted (311)A surface.     

Far infrared measurements of bulk and surface phonons in GaAs/AlAs superlattices

We have used far-infrared oblique-incidence reflection spectroscopy to study bulk phonon polaritons, and attenuated total reflection (ATR) spectroscopy to study surface phonon polaritons, in long-period GaAs/Al_xGa_1?xAs and short-period GaAs/AlAs superlattices. Results on the former are in good agreement with an effective-medium bulk-slab model of the dielectric tensor of the superlattice; results on the latter are analysed in terms of a model that contains dielectric-tensor contributions from the confined optic phonons.     

Miniband spectra of (AlAs)M(GaAs)N(111) superlattices

Semiconductors - Electron states in the conduction band of (111)-oriented (AlAs)M(GaAs)N superlattices (SLs) with M≥N and N&;lt;10 are considered. The properties of such SLs are mainly...     

Multi-day dynamic storage of holes at the AlAs/GaAs interface

We have used a deep-depletion capacitance-voltage (CV) technique to measure the storage time of holes in AlAs/n-GaAs MIS capacitors in the dark. We measure a storage time of 75 s at 175 K, increasing to 500 min at 140 K. At 77 K, the storage time is greater than 220 h. These values are far in excess of what is needed to make a one-transistor dynamic RAM for the MODFET technology.     

Characteristics of planar diodes based on heavily doped GaAs/AlAs superlattices in the terahertz frequency region

Characteristics of ohmic InGaAs contacts in planar diodes based on semiconductor superlattices with a small-area active region (1–10 μm²) are studied. The diodes were formed on the basis of short (18 or 30 periods) heavily doped (10¹⁸ cm⁻³) GaAs/AlAs superlattices with a miniband width of 24.4 meV. The reduced resistance of the ohmic contact was equal to 2×10⁻⁷ Ω cm² at room temperature. It is shown that the properties of fabricated planar diodes make it possible to use these diodes later on in semiconductor devices that operate in the terahertz frequency region in a wide temperature range (4–300 K). __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 9, 2004, pp. 1141–1146. Original Russian Text Copyright ? 2004 by Pavel’ev, Demarina, Koshurinov, Vasil’ev, Semenova, Zhukov, Ustinov.     

Forbidden Resonant Raman Scattering in GaAs/AlAs Superlattices: Experiment and Calculations

The Raman spectra of GaAs/AlAs(100) superlattices are calculated and studied experimentally for various wave-vector directions. The experiments are performed when applying a confocal optical microscope combined with a micro-Raman spectrometer for various scattering geometries both for phonons with a wave vector directed along the normal to a superlattice and in the in-plane geometry. The frequencies and eigenvectors of phonons are calculated in the extended Born model approximation taking into account Coulomb interaction in the rigid-ion approximation. The Raman spectra are calculated in the scope of the deformation- potential mechanism; herewith, it turns out that additional peaks, which are not described in the scope of this approach, appear in the experimental spectra. It seems likely that these peaks appear due to the manifestation of Raman scattering forbidden by selection rules under resonance conditions. An attempt is made to explain the appearance of these peaks in the experimental spectra within the scope of inelastic phonon scattering at bound charges (phonons with a large dipole moment).     

Experimental study of frequency multipliers based on a GaAs/AlAs semiconductor superlattices in the terahertz frequency range

Frequency multipliers based on a GaAs/AlAs semiconductor quantum superlattice have been experimentally studied. The power spectrum of the harmonics in the output signal from a multiplier with an input-signal frequency of 140–160 GHz has been measured. Planar diodes with a small active region (an area of 1–2 μm²) have been used in this study. For fabrication of the diodes, structures of heavily doped superlattices with the miniband width 24 meV have been used, these structures were grown by the molecular-beam epitaxy method. Measurements have been conducted using a BOMEM DA3.36 Fourier spectrometer equipped with a detector based on a bolometer cooled to the temperature of liquid helium. The results of the measurements have been used to plot the dependences of the power of the harmonics on the frequency in the range from 0.4 to 8.1 THz. It has been found that the character of the microwave-power distribution over the number of harmonics is close to the spectrum of a sequence of sign-alternating pulses which appear in the diode circuit when the applied voltage of the input signal exceeds the threshold of the diode. The minimal time of establishment of the pulse front and pulse duration are equal to 123 and 667 fs, respectively.     

DX centers in AlxGa1-xAs bulk alloy,AlAs/GaAs ordered,and disordered superlattices

Properties of theDX centers in Al_0.5Ga_0.5As bulk alloy (b-AL), (AlAs)₂ (GaSa)₂ ordered superlattice (o-SL) and (AlAs) _m (GaAs) _n disordered superlattice (d-SL) (m = 1, 2, 3,n = 1, 2, 3) with the same macroscopic composition were measured and compared. By deconvolution of deep level transient spectroscopy (DLTS) spectrum due to theDX center, we have found a decrease in the number of separate peaks in DLTS spectrum in an intentionally atomic ordered arrangement. Visiting Scholar of the Japan Society for the Promotion of Science. On leave from Department of Electrical Engineering, San Jose State University, San Jose, California 95192-0084, USA.     

Magnetic field effects in the luminescence spectra of type II GaAs/AlAs double layer structures

We report on low temperature luminescence studies of type II GaAs/AlAs heterostructures in magnetic fields up to 20 T. Application of magnetic fields perpendicular to the two-dimensional planes lead to a tremendous quenching of the indirect luminescence transitions. At higher excitation intensities, when the luminescence transitions are of a more Γ-Γ character within the GaAs layer, the application of magnetic field leads to an increase of the luminescence intensity. These effects are attributed to field induced lateral localization of photocreated carriers.     

Simulation of electron transport in GaAs/AlAs superlattices with a small number of periods for the THz frequency range

The electron transport in superlattices based on GaAs/AlAs heterostructures with a small number of periods (6 periods) is calculated by the Monte Carlo method. These superlattices are used in terahertz diodes for the frequency stabilization of quantum cascade lasers in the range up to 4.7 THz. The band structure of superlattices with different numbers of AlAs monolayers is considered and their current–voltage characteristics are calculated. The calculated current–voltage characteristics are compared with the experimental data. The possibility of the efficient application of these superlattices in the THz frequency range is established both theoretically and experimentally.     

Wet Oxidation in a Square Sandwich Composite of GaAs/AlAs/GaAs

Wet oxidation in a square sandwich composite, GaAs/AlAs/GaAs, with varying thickness of the AlAs layer was investigated in a temperature range of 400°C–480°C. At a given temperature and time, the oxidation depth increases with increasing thickness of the AlAs layer. A model based on the boundary layer diffusion in a sandwich composite is used to interpret the thickness effect, and the theoretical predictions are in good agreement with the measured oxidation data. The theory also predicts a value of 0.53 eV ± 0.03 eV to be the difference in activation energies of water vapor diffusion in the central layer AlAs and the outer layers GaAs in the temperature range studied. Such a difference remains to be verified experimentally.     

Mechanism of the effect of the electric field of a surface acoustic wave on the low-temperature photoluminescence kinetics in type-II GaAs/AlAs superlattices

Kinetics of the low-temperature photoluminescence (PL) of type-II GaAs/AlAs superlattices under the effect of the electric field of a surface acoustic wave is studied experimentally. It is found that application of electric-field pulses results in speeding up the PL kinetics of free and localized excitons and phonon replicas irrespective of the pulse duration. Analysis of the experimental data demonstrates that acceleration of the PL kinetics is related to the transport of excitons towards nonradiative recombination centers; this transport is promoted by the interaction of excitons with hot free charge carriers ejected from localized states.