AlGaAs Heterostructures Research Articles

Optimization of layer structure for InGaAs channel pseudomorphic HEMTs

Electron scattering in the pseudomorphic InGaAs channel of an InGaAs–AlGaAs heterostructure grown by molecular beam epitaxy has been studied at low temperatures to optimize the layer structure for use at 300 K in high-frequency transistors (p-HEMTs).

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.

On electron-electron scattering mechanisms in 2D degenerated systems

The detailed quantitative theory of electron-electron scattering processes in 2D electron degenerated systems in GaAs(AlGaAs) heterostructures has been developed on the basis of analytical treatment and numerical calculations. We have found the conditions and intervals of values of the characteristic parameters in which specific properties of 2D relaxation predicted previously on the theoretical level manifest themselves. New effects, i.e. a secondary beam of electrons scattered back and a very narrow beam of holes moving in the direction of injection, have been found.

Spatially-Controllable Quantum Well Intermixing with Stripe-Size Dependence in AlGaAs Heterostructures

Quantum well intermixing based on impurity-free vacancy disordering in Al0.07Ga0.93As /AlxGa1-xAs graded-index (x=0.23–0.5) heterostructures has been performed on a single wafer with various mesa structures by a one-step annealing. From photoluminescence and laser characteristics, it has been demonstrated that the extent of quantum well intermixing can be spatially controlled by simply varying the mesa widths. It has been found that the bandgap shift induced by the intermixing increases as the mesa width decreases.

Quantized current in one-dimensional channel induced by surface acoustic waves

We report an observation of the quantized acoustoelectric current induced by surface acoustic wave with frequency f≈3 GHz through a 1D channel defined in a GaAs–AlGaAs heterostructure by a split gate. When the split gate is pinched off the acoustoelectric current can be observed only for sufficiently high RF power levels, consistent with electrons being forced through the channel by the surface acoustic wave. As gate voltage is made less negative the current shows quantized steps in units I= e· f where e is the electron charge. The current on the first plateau coincides with the calculated value I= e· f with the accuracy limited by the error (0.3%) of our measuring instrument. The current value at the plateau was found to be stable over an interval of source-drain voltages and temperatures up to 4 K. The observed quantization is due to trapping of electrons in the moving potential minima, with the number of electrons in each minimum controlled by electron–electron repulsion. The accuracy of the current quantization observed in our experiments suggests that SAW devices may open a new route towards the creation of a current standard. Different possible designs of SAW based current standard are discussed.

Theory of potential modulation in lateral surface superlattices. II. Piezoelectric effect

Abstract We have calculated the piezoelectric coupling between a two-dimensional electron gas and the stress field due to a lateral surface superlattice, a periodic striped gate. Stress is assumed to arise from differential contraction between the metal gate and semiconductor. The piezoelectric potential is several times larger than the deformation potential and generally gives the dominant coupling. It depends on the orientation of the device and vanishes on a (100) surface if current flows parallel to a crystallographic axis. Most devices, however, are fabricated parallel to {011} cleavage planes in which case the piezoelectric potential is at a maximum. There are several sources of screening, including the partly-occupied donors in a typical GaAs–AlGaAs heterostructure. We also consider different elastic models for the gate. The best agreement with experiment is obtained if the force is distributed over the interface between the gate and semiconductor, rather than being concentrated at its ends.

In-plane birefringence of asymmetric (001) quantum wells

It is widely believed that [001]-grown semiconductor superlattices have an isotropic linear optical response in the (001) plane. In this letter, we use simple symmetry arguments to show that introduction of asymmetry in the [001] direction can induce anisotropy in the (001) plane. The latter is calculated for various AlGaAs heterostructures; it is found that the in-plane birefringence can be comparable to the birefringence of KH2PO4.

Nanolithography with an atomic force microscope

A lithographic technique, employing the vibrating tip of an atomic force microscope to mechanically pattern various materials such as photoresist, metals or semiconductors in the nanometre regime has been developed. We use this technique for the fabrication of etch masks as well as for the patterning of evaporation shadow masks.The tip quality has been found to be a crucial factor in the lithographic resolution. We therefore use ultra hard, amorphous carbon tips, which are prepared by electron beam deposition in an electron microscope. With these tips, additionally sharpened in an oxygen plasma, we now succeed in fabricating hole arrays with periods in the 10 nm regime. These hole arrays are transferred to the electron system of a GaAs–AlGaAs heterostructure, and the magneto resistance of such fabricated antidot arrays is discussed.

Single-particle and transport relaxation times in back-gated undoped AlGaAs/GaAs

The magnetotransport properties of a two-dimensional electron gas were studied at low temperatures in an inverted GaAs - AlGaAs heterostructure whose electron density could be changed between and by the application of a substrate bias. The epitaxial layers were grown undoped to reduce the influence of remote ionized impurity scattering and so a biased back-gate was needed to create the two-dimensional electron gas. The conductive back-gate required the use of a shallow Pd/AuGe/Ag/Au contact recipe. We present the magnetoresistance response for a range of back-gate biases (electron densities) and compare the mobilities, the transport scattering and the single-particle relaxation times with theory.

Quantum-well AlGaAs heterostructures grown by low-temperature liquid-phase epitaxy

An investigation of the low-temperature liquid-phase epitaxy (LTLPE) process at 400–650 °C for crystallization of AlGaAs heterostructures with ultrathin (2–100 nm) layers has been carried out. In particular, low-threshold quantum-well heterolasers (threshold current density jth as low as 120 A cm−2) and high-efficiency solar cells (efficiency as high as 24.0–24.7% under AMO conditions at concentration ratio 20–100 ×) have been fabricated by LT LPE.

Interplay of Landau band width and band separation in a square lateral superlattice

A laterally modulated gate electrode was evaporated on high-mobility two-dimensional electron systems embedded in GaAs AlGaAs heterostructures. At small lateral potential modulation of square geometry, as induced by a suitable gate voltage commensurability oscillations dominate the low-field magnetoresistance, indicating the oscillating width of the Landau bands. For increasing potential modulation the width of the Landau bands increases, the band contributions to the conductivity are suppressed, and scattering conductivity dominates. This reflects progress in the resolution of the internal structure of a Landau band.

Quantum transport in lateral superlattices

Lateral superlattices with very small lattice periods approaching the Fermi wavelength in size are fabricated on AlGaAs heterostructures by electron beam lithography. The potential modulation is induced by a gate voltage applied to a laterally modulated gate electrode. In our samples the two-dimensional electron gas is located very close to the sample surface, thus allowing us to induce very small lattice periods with a relatively large amplitude of the potential modulation. For weak potential modulation, which can be tuned by the gate voltage, we observe a modulation of the amplitude of the Shubnikov-de Haas oscillations arising in the magneto-resistance at quantizing magnetic fields. For more negative gate voltages, i.e. larger amplitudes of the potential modulation, certain maxima of the Shubnikov-de Haas oscillations split up reflecting the energy spectrum that arises when a given number of flux quanta penetrates a unit cell of the lattice. We compare our experimental results with theoretical predictions for the so-called Hofstadter butterfly energy spectrum and we conclude that we can experimentally detect the major energy bands and gaps.

Modeling effects of focused ion beams

A zero-order model is used to estimate the effects associated with focused ion beams. We assume that the energetic ions introduce electronic states at a fixed distance from the surface and at a single energy. Lateral and vertical straggle are ignored. Effects of implant depth and implant dose are investigated and results for carrier depletion at the edge of an implanted sheet and for antidot structures are presented. Some results are for GaAs AlGaAs heterostructures and some are for SiGe heterostructures.

Magneto transport on antidot arrays, fabricated by an atomic force microscope

A recently introduced lithographic tool employing an atomic force microscope is improved and utilized to fabricate nanometer-scale antidot arrays. The AFM periodically creates holes in a thin photoresist, covering a conventional mesa-etched GaAsAlGaAs heterostructure. To minimize the period of produced holes in the resist we now use carbon AFM supertips, prepared in a scanning electron microscope. To create holes or lines in the resist, a mechanical force of typically 1 μ N is exerted at ambient conditions. We thus succeed in fabricating hole arrays with a periodicity down to 15 nm and a hole diameter of only few nanometers. The resist mask is transferred onto the two-dimensional electron gas either by a shallow wet chemical etch step or by an ion beam irradiation technique, respectively. In magneto-resistance studies at T = 4.2 K on antidot-devices with periods down to 85 nm, we clearly observe commensurability oscillations of the longitudinal resistance R xx , demonstrating the successful pattern transfer to the electron system. Resistance fluctuations measured at T = 25 mK indicate a clear dependence on the period of the antidots.

Indications of a Luttinger liquid in the fractional quantum Hall regime

We have studied the transmission through a point contact in a gated GaAsAlGaAs heterostructure. At the v = 1 3 quantum Hall plateau measurements of the tunneling conductance exhibit non-Fermi liquid like resonance properties. The temperature dependence of the off-resonance conductances and resonance line shapes agrees with recent predictions of Luttinger liquid theory.

Sharpened electron beam deposited tips for high resolution atomic force microscope lithography and imaging

We employ the vibrating tip of an atomic force microscope as a lithographic tool to mechanically pattern a thin photoresist layer covering a GaAs–AlGaAs heterostructure. High aspect ratio electron beam deposited tips, additionally sharpened in an oxygen plasma, are used to minimize the dimensions of the fabricated quantum electronic devices. The fabrication parameters of the tips and the sharpening process are investigated. With these ultrasharp tips we are able to produce lines and holes with periods down to 9 nm in photoresist. In addition, the very sharp tips yield substantial improvements in the imaging mode.

Granular superconducting contacts to GaAs:AlGaAs semiconductor heterostructures

The differential resistance characteristics of junctions formed from annealed granular superconducting contacts and GaAs:AlGaAs heterostructures are reported. At low bias voltage there is a smooth resistance dip due to Andreev reflection at the interfaces and diffusive electron transport in the semiconductor. At higher bias there is a peak in the resistance corresponding to the onset of phonon-mediated processes, and further structure with an unusual temperature dependence. The width of the Andreev dip and its apparent insensitivity to temperature are due to a combination of the temperature dependence of the superconductor gap, the thermal carrier distribution in the normal regions, and the geometry of the contacts. Disorder near the interfaces is believed to be responsible for the high degree of Andreev reflection, and this increased probability persists in an applied magnetic field, as has been predicted.

Hot-electron multiquantum well microwave detector operating at room temperature

A new multiquantum well hot-electron microwave detector is demonstrated. The GaAs–AlGaAs heterostructure was grown by molecular beam epitaxy and tested in the X and K bands. It showed room-temperature operation with responsivity of several 103 V/W. The operation of the device is based on enhanced thermionic current due to carrier heating by the incident in-plane polarized microwave electric field. This principle yields a broad frequency range of operation extending up to the millimeter band without significant degradation of the responsivity.

EPITAXIAL DESIGN AND PERFORMANCE OF AlGaInP RED (650–690 nm) VCSELs

Progress in addressing critical epitaxial design issues associated with AlGaInP -based visible (red) vertical-cavity surface-emitting laser (VCSEL) diodes is reviewed, with emphasis on those issues that differentiate red VCSELs from conventional AlGaAs -based near-IR VCSELs. Key issues include epitaxial growth techniques for red VCSELs, the unique properties of AlGaInP alloys and heterostructures, design and growth of visible distributed Bragg reflectors, and integration of AlGaInP and AlGaAs hetero-structures into the hybrid VCSEL device structure. The performance characteristics of AlGaInP/AlGaAs red VCSEL diodes are overviewed in the context of the epitaxial design.

Buried heterostructure AlGaAs photonic devices fabricated by low temperature mesa melt etching and regrowth

Low temperature melt-etching and liquid phase epitaxial regrowth was used to fabricate buried heterostructure laser diode arrays, optical amplifiers and active waveguide branches. 12 element laser diode arrays emitted 1 W total optical power at 1.2 A (room temperature, continuous wave) at 980 nm. 6 dB fiber to fiber optical gain was obtained in buried heterostructure optical amplifiers operating at 830 nm. Active branching waveguides are shown to allow the compensation of 3 db branching losses.