Smith Purcell effect in GaAs/AlGaAs heterostructures

We report a new experimental technique to study the form of the hot electron distribution function in GaAs/AlGaAs heterostructures. A weak periodic surface potential induces Smith-Purcell-FIR-radiation of the electric field driven hot electrons in the 2-dimensional electron gas directly reflecting their velocity distribution. The FIR- radiation is detected by a magnetic field tuned InSb-detector. In samples with very low electron concentration and high mobility the emission spectra show a significant shift to higher energies and develop a steep high energy slope with increasing electric field when we use the geometry with grating vector q directed parallel to the electric field (q ∥ E). In the geometry q ⊥ E smooth decays are observed at lower energies. Comparison of the results with theory gives experimental evidence of a non-equilibrium shape of the distribution caused by the onset of LO-Phonon emission. In addition, the hot electron mean free path of the heated distribution is derived by investigating the experimental emission spectra as a function of the grating period length. The influence of a limited hot electron mean free path on the spectral width is described in terms of a Fourier-analysis of the interaction potential. In drift direction a mean free path of λ = 200 nm is obtained, whereas the mean free path is smaller in the direction perpendicular to the drift direction.     

Effect of roughness of two-dimensional heterostructures on weak localization

The effect that a longitudinal magnetic field exerts on the transverse negative magnetoresistance by suppressing the interference quantum correction is studied in GaAs/In_xGa_l?xAs/GaAs structures with a single quantum well. It is shown that the variation in the shape of the transverse magnetoresistance curve caused by a longitudinal magnetic field depends strongly on the relation between the mean free path, the phase-breaking length, and the correlation length characterizing the roughness of the two-dimensional layer. It is shown that the experimental results allow one to estimate the parameters of large-scale and small-scale roughness of the two-dimensional layer in the structures under study. The results obtained are in good agreement with the data of probe microscopy.     

Nonmonotonic bias voltage dependence of the magnetocurrent in GaAs-based magnetic tunnel transistors

Magnetic tunnel transistors are used to study spin-dependent hot electron transport in thin CoFe films and across CoFe/GaAs interfaces. The magnetocurrent observed when the orientation of a CoFe base layer moment is reversed relative to that of a CoFe emitter, is found to exhibit a pronounced nonmonotonic variation with electron energy. A model based on spin-dependent inelastic scattering in the CoFe base layer and strong electron scattering at the CoFe/GaAs interface, resulting in a broad electron angular distribution, can well account for the variation of the magnetocurrent in magnetic tunnel transistors with GaAs(001) and GaAs(111) collectors.     

2D-1D coupling in cleaved edge overgrowth

We study the scattering properties of an interface between a one-dimensional (1D) wire and a two-dimensional (2D) electron gas. Experiments were conducted in the highly controlled geometry provided by molecular bean epitaxy overgrowth onto the cleaved edge of a high quality GaAs /AlGaAs quantum well. Such structures allow for the creation of variable length 1D-2D coupling sections. We find ballistic 1D electron transport through these interaction regions with a mean free path as long as 6 &mgr;m. Our results explain the origin of the puzzling nonuniversal conductance quantization observed previously in such 1D wires.     

Nonlinear magnetotransport in a two-dimensional electron system in a square array of antidots in GaAs/AlAs heterostructures

Nonlinear magnetotransport of two-dimensional electrons in square antidot lattices prepared on the basis of selectively doped GaAs/AlAs heterostructures with the period that is much less than the electron mean free path in the initial GaAs quantum wells but is much larger than their Fermi wavelength has been studied. It has been shown that the character of the nonlinear transport of the two-dimensional electrons in the lateral lattices under study changes from classical to quantum with the decrease in the antidot radius. It has been found that the quantum lifetime increases in the magnetic field corresponding to the condition of equality of the cyclotron diameter of two-dimensional electrons and the antidot lattice period.     

Investigating the surface morphology and magnetic contrast of epitaxial ferromagnetic structures

The surface morphology of nickel thin films is investigated via atomic force microscopy. The multistage film growth mechanism is found to be dependent on substrate temperature and film thickness. It is shown that conduction electron scattering from the irregularities of the outer and inner surfaces of structures are influenced by the surface morphology and determined by an integrated contribution of the surface’s fluctuation density spectrum. The morphology influence can be decreased under certain growth conditions so that the residual mean free path of conduction electrons can reach 1000 nm, exceeding the film thickness. Epitaxial nanostructures with high electron mobility have been fabricated. Investigation of their magnetic structure has shown that their magnetic domain dimensions are less than the residual mean free path of electrons determined by the surface morphology.     

Discrepancy between photoluminescence and photoresponse intensities in n-InAs/GaAs and InAs/n-GaAs quantum-dot heterostructures

We report a discrepancy between near-infrared photoluminescence (PL) and far-infrared photoresponse (PR) efficiencies in self-assembled InAs/GaAs quantum-dot (QD) heterostructures with silicon doping in either InAs QDs or GaAs barriers. The structure with n-GaAs barriers reveals a much higher PR intensity in spite of a weaker PL intensity in comparison with n-InAs QD structure. This discrepancy is explained by differences in the electron occupation of QD sublevel associated with the Fermi-level position and in the mean free path of photogenerated carriers in GaAs barriers due to impurity scattering.     

Low temperature electron mobility in Ga0.5In0.5P/GaAs quantum well structures

We analyse the low temperature subband electron mobility in a Ga_0.5In_0.5P/GaAs quantum well structure where the side barriers are delta-doped with layers of Si. The electrons are transferred from both the sides into the well forming two dimensional electron gas (2DEG). We consider the interface roughness scattering in addition to ionised impurity scattering. The effect of screening of the scattering potentials by 2DEG on the electron mobility is analysed by changing well width. Although the ionized impurity scattering is a dominant mechanism, for small well width the interface roughness scattering happens to be appreciable. Our analysis can be utilized for low temperature device applications.      

Hot phonon effects in heterolayers

The strength of the hot-phonon effect generated by hot carriers in a two-dimensional heterolayer is estimated, specifically for electrons in GaAs. Both acoustic-mode phonons, at low temperature, and LO phonons are considered. For the former the phonon mean free path is taken to be large compared to the heterolayer thickness. The LO phonons are, however, assumed to decay locally. In both cases the estimates indicate that substantial hot phonon effects are to be expected in realized conditions.     

Probing the interface fluctuations in semiconductor superlattices using a magneto-transport technique

We have analysed the vertical transport properties of a GaAs/AlAs short period superlattice as the crossed magnetic field is rotated in the plane of the layers. The rotation of the magnetic field has been used to probe the interface roughness along the different in-plane crystallographic directions. Two maxima separated by 180° in the current intensity have been observed when the magnetic field is rotated through 360°. We interpret our results in terms of anisotropic scattering from elliptical interface defects of which the great axis is along [11̄0] and the minor one along [110].     

Anomalous magnetotransport in chemically doped carbon nanotubes

We report on anomalous magnetotransport features in chemically doped, weakly disordered carbon nanotubes. Under the application of a magnetic field parallel to the tube axis, hole conduction is shown to be strongly affected by impurity scattering with short mean free path and negative magnetoresistance, strongly different from electron conduction with much longer mean free path and positive magnetoresistance behavior.     

PHOTOLUMINESCENCE STUDY OF GaAs/AlGaAs QUANTUM WELL HETEROSTRUCTURE INTERFACES

Photoluminescence (PL) ia used to study the interface properties of GaAs/AlGaAs quan-tum well (QW) heterostructures prepared by molecular beam epitaxy with growth interrup-tion (GI). The discrete luminescence lines observed for the sample with GI are assigned to the splitting of the heavy-hole exciton associated with heterointerface islands with the lateral size greater than excjton diameter and mean height less than one monolayer, and the spectra have the Gaussian lineshapes. The results strongly support the microroughness model. We also study the temperature dependence of the exciton energies and find that excitons are localized at the interface roughness at low temperature even in the sample with GI, The lateral size of the microroughness of the GI sample is estimated to be less than 5nm from the exciton localization energy.     

The oscillatory galvanomagnetic size-effect in multilayered structures

Within the framework of the modified semi-classical Fuchs-Sondheimer model, we investigated theoretically the electrical resistivity of multilayered structures (MLS) consisting of alternating metallic layers (of different purity and different thicknesses) in a transverse magnetic field as functions of the ratio of the adjacent layer thicknesses and the magnetic field value. We have derived both a general formula (valid at arbitrary values of layer thicknesses) and asymptotic expressions that are valid when metallic layers are thick or thin compared with the electron mean free path. We found a non-monotonic behavior in the resistivity vs. the value of an applied magnetic field. As we demonstrated, this behavior is sensitive to the characteristics of the electron scattering in the interlayer interfaces in low magnetic fields. Moreover, the MLS resistivity oscillates in high magnetic fields with the field value (or with the layer thicknesses). The oscillation includes the harmonics that correspond both to the each layer thicknesses and the total thickness. The intensity of the oscillation is determined by the diffusive electron scattering in the interfaces, and the oscillation amplitude is proportional to the coefficient of the electron transmission through the interlayer interfaces. We have calculated numerically the resistivity in a wide range of fields and layer thicknesses at various values of the parameters of the interface and bulk electron scattering.      

Effect of interface roughness on the density of states of finite barrier height quantum wells

We calculate the density of states of a 2D electron gas in finite barrier height quantum wells with the explicit inclusion of the interface roughness effect. By using Feynman path-integral method, the analytic expression is derived. The results show that the 2D density of states is dependent on the RMS of the fluctuation potential. The interface roughness causes localized states below the subband edge. We also apply the theory to model the finite barrier height quantum wells in Al_xGa_1?xAs/GaAs.     

Role of specular reflection of electrons in the radio frequency surface magneto impedance of metals

The effect of a parallel magnetic field on the 10-mHz surface resistance of gallium single crystals is reported. We suggest that the observed structure can be interpreted in terms of an angular dependence of the probability of specular reflection of the electron at the crystal surface. This model allows a quantitative determination from the data of the temperature-dependent electron mean free path for electron wave vectors with a narrow range that is determined by the magnetic field and crystal orientation.     

Effect of Interface Roughness on Perpendicular Magnetic Anisotropy of Fe/Tb Multilayers

Fe/Tb multilayers have been prepared which exhibit significant perpendicular magnetic anisotropy (PMA) even at RT. The effect of systematic variation in the interface roughness on PMA has been studied in these multilayers which were deposited simultaneously on a set of float glass substrates prepared with varying root mean square surface roughness. The amount of intermixing at the interfaces and uncorrelated part of the interface roughness of different multilayers remain similar, thus allowing one to separate out the effect of the correlated interface roughness only. X-ray reflectivity and conversion electron Mössbauer spectroscopy and SQUID magnetometry was used to characterise the systems. PMA was found to depend weakly on the correlated interface roughness.     

Experimental determination of the mean free path of screened edge magnetoplasmons in a two-dimensional electron gas

Magnetic oscillations of the photovoltage in a two-dimensional electron system with the back gate, exposed to microwave radiation, are studied. The oscillations result from the interference of screened edge magnetoplasmons (EMPs). The mean free path of the EMPs is quantitatively determined by analyzing the dependence of the oscillation amplitude on the electron density. The dependences of the mean free path of the EMPs on the two-dimensional electron density, microwave frequency, electron relaxation time, and the magnetic field are studied. It is found that the dependences agree qualitatively with the known theoretical calculations.     

A study of the interface roughness effect in Si nanowires using a full 3D NEGF approach

The effect of interface roughness in ballistic Si nanowires is investigated using a full 3D non-equilibrium Green's Functions formalism. The current density, the electron density and the transmission function are calculated for nanowires with different interface roughness configurations. Interface roughness is randomly generated using an exponential autocorrelation function. The interface roughness profile in nanowires with 2 nm diameter and 6 nm length is reflected in the current density landscape showing macroscopic 3D patterns. These macroscopic patterns affect the transmission probability causing resonances coming from the constrictions in the channel. The shape of the electron density in cross-sections along the wire follows the distortion of electron transversal wave function.     

Interface Polaron Within Parallel Electric and Magnetic Fields

A combinative method of variational wavefunction and harmonic oscillator operator algebra is used to treat the interface polaron in a semi-infinite polar crystal within parallel electric and magnetic fields perpendicular to the interface. Both the bulk longitudinal optical phonon and the interface optical phonon together with the anisotropic mass of the electron are included. The energy level correction up to the second-order perturbation, cyclotron-resonance frequency and cyclotron mass are expressed as functions of the electric and magnetic fields and a parameter characterizing the mean distance of the polaron from the interface. This theory is used to calculate numerically the single heterostructure AlAs/GaAs, when the electron is at the X high-symmetry point of the conduction band of AlAs. The results show that the magnetic field greatly enhances the polaronic correction of the electron energy levels while the electric field only increases the correction of their surface optical phonon part but obviously decreases that of their bulk optical mode part and thus the total energy correction decreases as the electric field increases. The change of red shift due to the electron-phonon interaction with electric and magnetic fields is also obf ained.     

Magnetic field effects on THz quantum cascade laser: A comparative analysis of three and four quantum well based active region design

We consider the influence of additional carrier confinement, achieved by application of strong perpendicular magnetic field, on inter Landau levels electron relaxation rates and the optical gain, of two different GaAs quantum cascade laser structures operating in the terahertz spectral range. Breaking of the in-plane energy dispersion and the formation of discrete energy levels is an efficient mechanism for eventual quenching of optical phonon emission and obtaining very long electronic lifetime in the relevant laser state. We employ our detailed model for calculating the electron relaxation rates (due to interface roughness and electron–longitudinal optical phonon scattering), and solve a full set of rate equations to evaluate the carrier distribution over Landau levels. The numerical simulations are performed for three- and four-well (per period) based structures that operate at 3.9 THz and 1.9 THz, respectively, both implemented in GaAs/Al_0.15Ga_0.85As. Numerical results are presented for magnetic field values from 1.5 T up to 20 T, while the band nonparabolicity is accounted for.