Influence of Γ–X mixing on carrier transport and photoluminescence in GaAs/AlAs type-I superlattices

We report on the effects of field-induced Γ–X resonances on carrier transport and optical properties of GaAs/AlAs type-I short-period superlattices (SLs). We have observed an anomalously delayed photocurrent and Γ2-hh1 photoluminescence originating from X1–Γ2 mixing. These observations clearly suggest that electrical conductivity and optical properties even in type-I SLs are seriously affected by X states in the applied electric field.     

Excited state resonant tunneling in GaAsAlxGa1−xAs double barrier heterostructures

Resonant tunneling through the ground and first excited state of single quantum well, double barrier GaAsAl_xGa_1−xAs heterostructures is reported. Negative differential resistance from both quantum well states is observable up to room temperature in one of these structures. The observed positions of the quantum well states agree well with theory, though there exists an asymmetry in the current-voltage characteristics about the origin.     

Effect of Γ–X Intervalley Scattering on the Electron Transport in Double-Barrier GaAs/AlAs(001) Heterostructures

Volt-ampere characteristics, transmission spectra of electrons, and tunneling times are calculated for a three-valley model by the multiband method used for studying the quantum transport in GaAs/AlAs(001) semiconductor heterostructures. The effect of –X intervalley scattering of electrons on the heteroboundaries of the structures is examined.     

Enhanced electron tunneling and Γ–X interlayer intervalley electron transfer in a triple-barrier heterostructure

The enhanced electron tunneling effect and the electron Γ–X intervalley interlayer transfer in the AlAs/GaAs (001) triple-barrier heterostructure have been investigated both experimentally and theoretically. The effects of the external bias on the electronic structure, Γ–X state mixing and higher lying excited energy states are studied. The experimental observations show good agreement with the theoretical predictions based on the scattering theoretical approach of Green's function theory, which can handle electron interlayer intervalley propagation through the layered aperiodic heterostructure under the external bias.     

Resonant tunneling in InGaAsInP double-barrier structures and superlattices

We have studied the perpendicular transport in the double-barrier structures of the InGaAs system, with particular focus on reducing the large leakage current caused by conduction at the perimeter of the device. Results obtained on a sample which had low doping in the electrode layers, such that the Fermi level of the emitter electrode was below the first subband level at low bias, are presented. These results demonstrate unambiguously that the process of selectively etching the device has been successful in suppressing the leakage current. A peak-to-valley ratio of 4.3 was obtained for this sample at 4.2K. We have also carried out the first investigation of the perpendicular transport in InGaAs superlattice structures. These structures exhibit a large series of periodic negative differential resistance whose number and period are controlled by the number of the width of the quantum wells respectively.     

Resonant tunneling and enhanced Goos–Hänchen shift in a graphene double velocity barrier structure

Resonant transmission and Goos–Hänchen (GH) shift for Dirac fermion beams tunneling through graphene double velocity barrier structures (DVBs) are investigated theoretically. Analytical and numerical results demonstrate that strong resonant tunneling effect occurs in this structure and is highly dependent on the incident angle and the structure of velocity barriers. The resonant tunneling in graphene DVBs belongs to the Fabry–Pérot resonance and leads to oscillated conduction at wide energy range. It is also found that GH shifts in this structure can be enhanced by the resonant tunneling and multi-GH shift peaks with giant magnitudes can occur at these resonant energy positions. These special properties of GH shifts in graphene DVBs may have good application in lateral manipulation of electron beams and valley or spin beam splitter.     

Photoluminescence response of a quantum well to a change in the magnetic field of the Mn δ Layer in InGaAs/GaAs heterostructures

Ferromagnetic ordering of two types (depending on the sample geometry) is found to occur in a thin Ga_{1 − x} Mn _x As alloy layer (Mn δ layer) in heterostructures containing an InGaAs/GaAs quantum well. Singular samples in which the δ Mn layer is parallel to the (001) GaAs plane exhibit the “3/2” Bloch temperature dependence of magnetization, and vicinal samples in which the δ Mn layer deviates from the (001) GaAs plane exhibit a “percolation” ferromagnetic transition. The photoluminescence polarization of the quantum well is shown to follow changes in the magnetization of the Mn δ layer as a function of temperature according to the Bloch law in the singular samples and to a percolation law in the vicinal samples.     

Photoluminescence spectroscopy on excitonic states of narrow GaAs–AlGaAs single quantum wells in high magnetic field

Magneto-photoluminescence (PL) experiments of very thin GaAs/Al_0.3Ga_0.7As quantum wells were performed in a magnetic field (B) of up to 20 T. It has been observed that the diamagnetic shift changes abruptly from βB²to αBaround 5 T asBincreases, and both α and β become larger as the well-width increases. ThisB-dependent transition is indicative of the change of electron–hole (e–h) coupling as the cyclotron radius becomes comparable with that of the exciton. The change of PL linewidth byBalso supports the concept that thee–hrecombination is sensitive to the dimensionality.     

Dipolar magnetic field and fermi contact field at the μ+ site in α-Fe at very low temperatures

In a longitudinal ⁺SR experiment on a high-purity-Fe single crystal sphere magnetically saturated in a 111 direction damped oscillations (wiggles) were observed in a temperature range 30 mK to 600 mK and in a certain regime of applied magnetic fieldsB _appl. Meassurements of the wiggle frequency as a function ofB _appl give us directly the Fermi fieldB _Fermi=(–1.13±0.02)T and the dipolar magnetic field ¦B _dip ¦=(0.66±0.03)T.B _dip was used to determine the prefactor in the Arrhenius law obeyed by the ⁺ hopping rate between 100 K and 1000 K. A comparision with the corresponding values for protons and deuterons suggests diffusion via the adiabatic mechanism.     

Fundamental studies and device application of δ-doping in GaAs Layers and in AlxGa1−xAs/GaAs heterostructures

In addition to the realization of atomically abrupt interfaces in III–V semiconductors by molecular beam epitaxy, the confinement of donor and acceptor impurities to an atomic plane normal to the crystal growth direction, called-doping, is important for the fabrication of artifically layered semiconductor structures. The implementation of-function-like doping profiles by using Si donors and Be acceptors generates V-shaped potential wells in GaAs and Al_xGa_1–xAs with a quasi-two-dimensional (2D) electron (or hole) gas. In this review we define three areas of fundamental and device aspects associated with-doping. (i) The prototype structure of-doping formed by a single atomic plane of Si donors in GaAs allows to study the 2D electron gas by magnetotransport and tunneling experiments, to study the metal-insulator transition, and to study central-cell and multivalley effects. In addition, non-alloyed ohmic contacts to GaAs and GaAs field-effect transistors (-FETs) with a buried 2D channel of high carrier density can be fabricated from-doped material. (ii) GaAs sawtooth doping superlattices, consisting of a periodic sequence of alternating n- and p-type-doping layers equally spaced by undoped regions, emit light of high intensity at wavelengths of 0.9 < <1.2 [m], which is attractive for application in photonic devices. The observed carrier transport normal to the layers due to tunneling indicates the feasibility of this superlattice as effective-mass filter. (iii) The confinement of donors (or acceptors) to an atomic (001) plane in selectively doped Al_xGa_1–xAs/GaAs heterostructures leads to very high mobilities, to high 2D carrier densities, and to a reduction of the undesired persistent photo-conductivity. These-doped heterostructures are thus important for application in transistors with improved current driving capabilities.Extended version of a paper presented at the18th Int. Symp. GaAs Related Compounds (Heraklion, Crete, 1987)     

Dependence of “Reststrahlen” bands in far-infrared reflectivity on configuration of GaAs/AlAs multiple quantum well heterostructures

Far infrared reflectivity measurements are performed on a series of GaAs/AlAs multiple quantum well (MQW) heterostructures with systematically varied thicknesses of the constituent layers. In addition to the artificial anisotropy we observe two distinct bulk-like Reststrahlen regions. The widths of the GaAs-like and the AlAs-like Reststrahlen bands strongly depend on the relative thicknesses of the constituent layers of the MQW heterostructures, in excellent agreement with the predictions of the effective-medium theory.Prof. Aldo Cingolani passed away just before the publication of this article. We would like to dedicate this paper to his memory     

Cauchy magnetic field component and magnitude distribution studied by the zero‐field muon spin relaxation technique

Zero‐field muon spin relaxation (ZF‐μSR) data for dilute spin magnetic systems have been widely interpreted with what is called a Kubo–Toyabe form based on a Lorentzian distribution of local field components. We derive here the proper magnetic field magnitude distribution using independent and uncorrelated component distributions. Our result is then compared to the previously accepted formula for ZF‐μSR. We discuss the origins of the magnetic field component and magnitude distributions. Further, we found that after rescaling the magnetic field, the differences that are amenable to experimental examination are quite small, although the interpretations behind them are quite different. This revised version was published online in August 2006 with corrections to the Cover Date.     

Finite confining effect on electron–phonon interaction in GaAs/Ga1−xAlxAs single heterostructures

The form factors of the electron–phonon interaction for GaAs/Ga_1−xAl_xAs single heterostructures have been evaluated using a finite height barrier. The calculations are performed within the extreme quantum limit approximation, assuming for the envelope electronic wavefunction a modified Fang–Howard wavefunction. Both types of long-wave phonons, longitudinal optical and interface phonons, are considered. It is found that the effect of the finite height is to reduce the strength of the electron–phonon interaction.     

Resonant DX centers in highly doped Sn-Ga1−xAlxAs under hydrostatic pressure in magnetic field

We report experimental evidence of deep impurity states producing a localized resonance in the Γ_1c band continuum in highly doped Sn-Ga_1−xAl_xAs. At low temperatures and increasing pressure, electrons were transferred from the Γ_1c band to the deep Sn donor state; this transfer of carriers was directly related to the pressure dependence of Shubnikov de Haas oscillations. Persistent photoconductivity, due to the DX nature of this deep level, was observed through the increase in the Γ carrier density resulting from illumination with a light emitting diode. This is interpreted as a photoionization of the DX centers up to the Γ_1c valley.     

Lowering of magnetic field intensity at the surfaces of α-Fe2O3 and FeBO3 single crystals

Depth-selective conversion electron Mössbauer spectroscopy was used to study magnetic properties of the thin surface layers of the α-Fe₂O₃ and FeBO₃ single crystals. An analysis of the experimental spectra indicates that the magnetic properties of the layers at a depth of more than ～100 nm from the surface are similar to the properties of crystal bulk, and the corresponding spectra consist of narrow lines. The lines gradually broaden as the crystal surface is approached. The spectra of the ～10-nm-thick surface layers consist of broad lines, indicating a wide distribution δ=2.1 T of the effective magnetic field about its mean value of 32.2(4) T. The experimental spectra were used to determine the effective magnetic fields (H _eff) for the iron ions situated in the surface layers of thickness ～100 nm. The effective fields in these layers were found to gradually decrease at room temperature (291 K) as the crystal surface was approached. The H _eff values in the 2.4(9)-nm-thick surface layer of the α-Fe₂O₃ crystal and 4.9(9)-nm layer of FeBO₃ are 0.7(2) and 1.2(3)%, respectively, smaller than for the nuclei of the ions in the bulk of these crystals.     

Magnetic field effects and k?-filtering in BEEM on GaAsAlGaAs resonant tunneling structures

In this work, GaAsAlGaAs double barrier resonant tunneling diodes (RTDs) are investigated by ballistic electron emission microscopy (BEEM). RTDs grown directly below the sample surface exhibit characteristic steplike features in the BEEM spectrum, whereas for buried RTDs, a linear spectrum is observed. Moreover, the BEEM spectra of sub-surface RTDs show Shubnikov-de Haas-like oscillations in magnetic fields. To investigate the origin of these effects, the BEEM spectra were calculated using a scattering formalism within the framework of a semi-empirical tight binding method. As a main result we found that, independent of the applied bias, only electrons within a narrow k_ distribution are transferred resonantly through the RTD. Hence, a k_ filter is established for ballistic electrons close to k_=0. The calculated filter width is consistent with the magnetic field data.     

Zero,positive and negative quantum Goos–Hänchen shifts in graphene barrier with vertical magnetic field

We have explored the zero, positive and negative quantum Goos–Hänchen (GH) shifts of the transmitted Dirac carriers in graphene through a potential barrier with vertical magnetic field. Numerical results show that only one energy position at the zero GH shift exists and is highly dependent on the y-directional wave vector, the energy gap, the magnetic field and the potential. The positive and negative GH shifts happen when the incident energy is more and less than the energy position at the zero GH shift, respectively. In addition, we found that there are two values of potential at the zero GH shifts, where a potential window can always keep the positive GH shifts. These results may be useful in designing a graphene-based valley or spin splitter as well as manipulating the electrons and holes in graphene nanostructure.     

Influence of delta〈Mn〉 doping parameters of the GaAs barrier on circularly polarized luminescence of GaAs/InGaAs heterostructures

The circular polarization of low-temperature electroluminescence of diodes based on heterostructures with an undoped quantum well InGaAs/GaAs and a delta〈Mn〉 layer in the GaAs barrier has been investigated. The possibility of changing the degree of circular polarization of the electroluminescence by varying the main structural parameters of diodes (spacer layer thickness, i.e., the distance between the delta〈Mn〉 layer and the quantum well, atomic concentration in the delta〈Mn〉 layer, and introduction of an additional acceptor delta layer) has been analyzed. It has been revealed that the variation in the spacer layer thickness is the most effective method for controlling the degree of circular polarization of the electroluminescence.     

Large scale magnetic field influence on trap recharging waves in InP:Fe and GaAs:Cr

An impressive linear influence of a magnetic field on optically generated trap-recharging waves (TRW) has been observed in InP:Fe and GaAs:Cr. The phenomenon appears for the particular orientation of parallel to the samples’ surface and orthogonal to the direction of the electric field and wave vector of the TRW . The results are qualitatively explained taking into account the Lorentz force and a pronounced inhomogeneity of the charge transport and of the TRW parameters.