Magnetophonon resonances of the two-dimensional electron gas in GaAs/AlGaAs heterostructures

We have studied magnetophonon resonances of the two-dimensional (2D) electron gas at the GaAs/AlGaAs interface in a single interface heterostructure and a superlattice. In magnetic fields up to 30T, one set of oscillations is observed, corresponding to the coupling of 2D electrons with LO phonons of GaAs. The effective mass obtained directly from the magnetic field position of the fundamental resonance, where the Landau splitting equals the bulk phonon energy, and of the next two harmonics is $\text{m}{}^1\text{= (0.071 ± 0.0015)m}{}_0$. A comparison with cyclotron resonance measurements on the same system and with bulk GaAs data gives an upper limit of about 2% for the mass corrections due to polaron effects and due to the confinement of the electron gas.     

Measurements of the density-dependent many-body electron mass in two dimensional GaAs/AlGaAs heterostructures

We determine the density-dependent electron mass m(*) in two-dimensional electron systems of GaAs/AlGaAs heterostructures by performing detailed low-temperature Shubnikov-de Haas measurements. Using very high-quality transistors with tunable electron densities we measure m(*) in single, high mobility specimens over a wide range of r(s) (6 to 0.8). Toward low densities we observe a rapid increase of m(*) by as much as 40%. For 2>r(s)>0.8 the mass values fall approximately 10% below the band mass of GaAs. Numerical calculations are in qualitative agreement with our data but differ considerably in detail.     

Optically detected electron spin polarization and Hanle effect in AlGaAs/GaAs heterostructures

The spin polarization of optically created conduction electrons in p-type AlGaAs/GaAs heterostructures was observed via the degree of circular polarization of the photoluminescence. Application of a magnetic field perpendicular to the propagation of the light allows one to determine the spin relaxation time T₁ and the electron lifetime τ in the conduction band. By tilting the magnetic field with respect to an estimate of the effective nuclear field acting on the electrons can be obtained.     

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.     

Time-resolved photoluminescence of delta-doped AlGaAs/GaAs heterostructures

The photoluminescence (PL) at low temperature of three delta-doped AlGaAs/GaAs heterostructures is investigated under continuous and pulsed excitations. The PL under continuous excitations allows the identification of the trapping centres and probes the carrier's transfer to the GaAs channel. The time-resolved photoluminescence (TRPL) inquires about carrier dynamics and gives radiative lifetimes of different levels. The DX level shows two time constants of the intensity decay relating the splitting of the valence band under impurity strains which reduce the crystal symmetry. The two time constants evolve with temperature and exhibit an increase near T=50 K.     

Origin of current instability in GaAs/AlGaAs heterostructures

We propose a mechanism to explain the electric instability often observed in modulation-doped heterostructures GaAs/AlGaAs when current is passed along the heterostructure layers. The instability is caused by hot electron transport in AlGaAs layer that is not only heavily doped, but also strongly compensated due to the presence of DX-centers. This layer contains a large-scale random potential of significant magnitude, which strongly affects electron transport. The heating of electrons in the percolation cluster net and electron transfer from the cluster into the random potential wells result in the appearance of latent negative differential conductivity causing the current instability. When the instability gives rise to the formation of a high electric field domain, one of the domain walls blocks the current flow through the two-dimensional electron gas. Experimental results supporting this mechanism are given.     

p-type ohmic contacts to AlGaAs/GaAs heterostructures

Ohmic contacts to p-AlGaAs/GaAs heterostructures have larger resistivities compared to n-type structures because holes are heavier than electrons and the heterointerface barrier for high mobility structures is thicker. This work shows that diffusion of Zn during an alloy process of a Au/Zn/Au metallization establishes high acceptor concentrations throughout the heterostructure resulting in a thin metal-semiconductor barrier and probably a degradation of the heterointerface barrier by disordering, while the heterostructure outside the contact region remains unchanged. Contact resistivities down to 3.5ωmm at 25K could thus be achieved.     

Microwave waveguide method for the measurement of electron mobility and conductivity in GaAs/AlGaAs heterostructures

The microwave waveguide method for contactless determination of the electron mobility and conductivity of thin active layers is reported. The method is based on relative measurements of the magnetic field dependences of the derivative of the reflection coefficient with respect to the magnetic field from a semiconductor wafer bridging the waveguide.Experiments are performed on GaAs/AlGaAs heterostructures at microwave frequency = 36.4 GHz and liquid nitrogen temperature. For the analysis of the experimental data the theoretical basis for arbitrary frequencies is developed. The main advantage of the proposed method is that this method enables one to determine material parameters - mobility and conductivity - without careful calibration of the microwave system and does not require the accurate measurements of the absolute values of the reflection coefficient and phase of the reflected wave.     

Quantized conductance in extended electron waveguides fabricated on shallow etched modulation-doped GaAs/AlGaAs heterostructures

By high-resolution electron beam lithography and wet chemical etching extended electron waveguides with lengths up to 20 μm were fabricated on modulation-doped GaAs/AlGaAs heterostructures showing conductance quantization. For short electron waveguides 15 well-resolved steps in the conductance trace are observable. Conductance quantization is observed up to wire lengths comparable to the transport mean free path of electrons in the unconstrained two-dimensional electron gas, indicating that scattering due to fabrication induced defects is negligible in the present structures. By an analysis of temperature-dependent measurements the one-dimensional subband spacings are determined to be greater than 10 meV.     

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.     

Raman scattering and photoluminescence studies of two-dimensional electron systems in Ge/GaAs heterostructures

Resonant Raman scattering experiments on n-Ge/n-GaAs (100) heterostructures reveal transitions involving quasi-two-dimensional electron states in Ge-accumulation layers. The experiments were performed in the range of the E₁-gap of Ge. The electronic scattering transforms into E₁-luminescence as the laser energy is tuned above the resonance. Spectra obtained at higher excitation energies show luminescence bands associated with the E₁ and E₁ + Δ₁-gap of Ge. The latter results are compared with recent data for bulk heavily-doped Ge.     

Suppressed intermixing in InAlGaAs/ AlGaAs/GaAs and AlGaAs/GaAs quantum well heterostructures irradiated with a KrF excimer laser

The influence of gallium arsenide surface modification induced by irradiation with a KrF excimer laser on the magnitude of the quantum well (QW) intermixing effect has been investigated in InAlGaAs/AlGaAs/GaAs QW heterostructures. The irradiation in an air environment with laser pulses of fluences between 60 and 100 mJ/cm² has resulted in the formation of a gallium oxide-rich film at the surface. Following the annealing at 900 °C, up to 35 nm suppression of the band gap blue shift was observed in all the laser irradiated samples when compared to the non-irradiated samples. The origin of suppression has been discussed in terms of stress controlled diffusion. PACS 78.55.Et; 66.30.Lw; 73.21.Fg     

Ultrasmall nanoscale devices fabricated from compensating-layer GaAs/AlGaAs heterostructures

Nanoscale devices are fabricated from modulation-doped GaAs/AlGaAs heterostructures, where the two-dimensional electron system is initially depleted. Upon removing the p-type capping layer that compensates for the n-type supply layer, the electron system is induced. Arbitrarily shaped areal, line, and dot elements, i.e. the nanostructures and 2D leads, are simultaneously fabricated by patterning a thin resist layer with an atomic force microscope and subsequent selective wet etching. In this way a single-electron transistor (SET) with a 60 nm diameter island, a 60 nm wide electron waveguide (EWG), and an Aharonov–Bohm (AB) loop of 110 nm average diameter are prepared. Measurements at T=1.5 K reveal Coulomb-blockade, quantized conductance and AB-oscillations for the SET, EWG, and AB loop, respectively. Finally, an EWG is demonstrated in split-gate geometry where the compensating layer is used as split gate.     

Shake-up intersubband transitions observed in GaAs/AlGaAs quantum wells

Shake-up transitions involving QW hole subbands have been observed as satellites in selective photoluminescence spectra of undoped GaAs/AlGaAs QWs. These shake-up transitions are explained in terms of an interaction between localized exciton and valence-band hole states attached to the QW subbands, in which holes are shaken up from the n=1 heavy hole subband to higher subbands, either the n=1 light hole subband or the n=2 heavy hole subband. The required localization is due to the interface roughness; thus these new transitions are of intrinsic origin. From the observation of the intersubband shake-up processes we derive direct information about the hole inter-subband energies. Furthermore, the satellite intensity is strikingly enhanced in the presence of a magnetic field due to an increasing exciton localization related to the compression of its wave function in the field. The exciton wave function compression continues until its radius in the plane of the well is comparable with the radius of the "flat island" characterized by constant QW width. Accordingly, from the magnetic field dependence of the shake-up satellite intensity we can roughly estimate the size of the "flat islands" and consequently probe the interface roughness.     

Novel excitonic transitions in n-type GaAs/AlGaAs quantum wells

We report on a novel peak, the F-line, observed in photoluminescence spectra of GaAs/AlGaAs quantum wells (QWs) with various donor layer positions and concentrations. The F-line is well-defined and red shifted by approximately 1.3 meV (dependent on the experimental conditions) relatively the free exciton (FE) in a 200 Å wide QW. The F-line exhibits a strong magnetic field dependence. The enhanced intensity with increasing field is due to an increasing wave function overlap caused by the enhanced localization of the involved charge carriers. In accordance, the derived thermal activation energy for the F-line is magnetic field dependent. The F-line exhibits a diamagnetic shift as expected for an excitonic transition and splits into four components with increasing magnetic field. Another associated higher energy peak, the E-line, is observed preferably in the presence of a magnetic field, between the heavy hole- and light hole-FE in PL excitation spectra. The E-line also exhibits a striking magnetic field and temperature dependence. The observed properties of the F-line with a striking dependence on the excitation intensity, magnetic field and temperature are consistent with the observation of an exciton bound at the negatively charged D^- donor state or a negatively charged X^- exciton.     

NS junctions using high-mobility GaAs:AlGaAs heterostructures

The fabrication, analysis and measurement of superconductor/semiconductor junctions with GaAs:AlGaAs heterostructure channels is described. Microscopic analysis was used together with electrical measurement to optimize the characteristics of the annealed Sn and In alloy contacts, leading to the observation of ballistic electron mediated supercurrents. These contacts also had a high critical magnetic field, allowing the study of Andreev reflection at high fields. Epitaxially grown and unannealed single-crystal Al contacts showed high transparency.     

Monolithic two-dimensional surface emitting arrays of GaAs/AlGaAs lasers

Surface emitting lasers are of interest for various applications such as monolithic two-dimensional arrays and optical interconnects for integrated optics. Moreover, surface emitting lasers offer the advantage of wafer processing and testing. Several approaches for achieving surface emission are described. In addition, TRW s fabrication of a large monolithic two dimensional array of GaAs/AlGaAs surface emitting lasers which contains a total of 100 lasers is reported.