GaAs AlGaAs Research Articles

Effect of cavity length on stimulated emission from Fabry-Pérot Gunn lasers

Emission characteristics from GaAlAs–GaAs–GaAlAs Gunn devices of different lengths placed in Fabry-Pérot cavities have been investigated. The emission from the device of length of 194μm is stimulated in nature, whereas for the other two devices of lengths of 100 and 50μm, the emission is spontaneous. Lasing action in such devices, when biased above the threshold of negative differential resistance, arises from the band to band recombination of impact-ionized nonequilibrium electron hole pairs created within the propagating high field Gunn domains. Quantitative results evaluated using nonequilibrium semiconductor-statistics show that reduction in the length of the device leads to (a) a shift of the gain peak towards higher energies and (b) an increase in the average nonequilibrium carrier temperature. As a result, lasing in very short devices is inhibited and only ultra-bright-spontaneous emission is observed.

MBE-grown ultra-large aperture single-mode vertical-cavity surface-emitting laser with all-epitaxial filter section

A novel GaAs–AlGaAs vertical-cavity surface-emitting laser (VCSEL) design based on the coupled cavities and all-epitaxial non-transparent filter section (F-VCSEL) was proposed. The key point of this approach is that the absorption filter selectively provides the high losses for off-resonance optical modes and relatively narrow transparency window (∼1.3 nm). The broad area (100 μm×500 μm) F-VCSEL devices with a metal contact grid on the top demonstrate the 0.85 μm range narrow line emission and the Gaussian-like far-field patterns, which confirm the possibility of the achievement of the surface-emitting lasing by using the all-epitaxial wavelength-selective filter concept. Single-mode operation in wide pump current range in F-VCSEL was achieved at the larger oxide current aperture (3 μm) as compared with that for the conventional oxide-confined VCSEL (1 μm).

Post-Growth Fabrication and Characterization of Integrated Chirped Bragg Gratings on GaAs–AlGaAs

A fully post-growth integrated chirped Bragg grating (CBG) in III-V material is presented based on a tapered waveguide geometry. The deeply etched structure provides the high coupling coefficient required in CBG devices without recourse to overgrowth fabrication techniques and the taper method allows for arbitrary chirp functions to be implemented. A 5-nm stopband with 0.0335 ps/nm dispersion is demonstrated and DFB lasers based on the CBG structure are presented

Characteristics of high responsivity 8.5 μm InGaAs/InP QWIPs grown by metalorganic vapour phase epitaxy

Higher responsivity of quantum well infrared photodetectors based on In0.53Ga0.47As–InP material system compared to the well established GaAs–AlGaAs material system is analyzed. It is shown that the higher responsivity of the former results mainly from its smaller capture probability, pc, than that of the latter. Both transport as well as L valley occupancy appear important in determining the pc.

Special Issue: Frontiers in Semiconductor Nanoscience (Part A)

AbstractThe present special issue of physica status solidi is dedicated to Professor Gerhard Abstreiter on the occasion of his 60th birthday. Guest edited by Dominique Bougeard, Jonathan Finley, Matthew Grayson, and Marc Tornow, it contains 24 papers covering different aspects of semiconductor physics and nanoscience. Additional contributions can be found in Part B published in phys. stat. sol. (b) 243, No. 14 (2006). Together the twin volumes of pss(a) and pss(b) cover many advanced topics, including quantum transport, optical properties and coherent phenomena, nano‐biological systems and novel materials and nanostructures.The cover picture shows a schematic image of a coupled quantum dot nanostructure formed by two‐fold cleaved edge overgrowth of a GaAs–AlGaAs heterostructure [1]. Such advanced nanostructure fabrication technologies were pioneered in Gerhard Abstreiter's group at the Walter Schottky Institut almost ten years ago. The molecular like electronic structure in such an artificial molecule remains subject to intense study even today in the context of solid‐state implementations of quantum information technologies. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Terahertz Emission in Asymmetric Quantum Wells by Frequency Mixing of Midinfrared Waves

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Generation of terahertz (THz), and sub-THz, coherent waves has been demonstrated at room temperature in GaAs–AlGaAs asymmetric quantum wells by mixing two &lt;formula formulatype="inline"&gt;&lt;tex&gt;${\hbox {CO}}_{2}$&lt;/tex&gt; &lt;/formula&gt; laser beams. Two overlapping regimes were studied: the double resonance regime where three states are involved and the optical rectification which relies on two states only. The measured conversion efficiency of &lt;formula formulatype="inline"&gt;&lt;tex&gt;$\sim {\hbox {10}}^{-7}\ {\hbox {W}}^{-1}$&lt;/tex&gt; &lt;/formula&gt; is in reasonable agreement with theoretical predictions. In addition, this technique of THz generation provides a model system to study the emission profile from an ensemble of radiating dipoles. For instance, backward emission and strong diffraction effects have been observed. </para>

Temperature dependence of photoluminescence from single core-shell GaAs–AlGaAs nanowires

Temperature-dependent polarized microphotoluminescence measurements of single GaAs∕AlGaAs core-shell nanowires are used to probe their electronic states. The low-temperature emission from these wires is strongly enhanced compared with that observed in bare GaAs nanowires and is strongly polarized, reflecting the dielectric mismatch between the nanowire and the surrounding air. The temperature-dependent band gap of the nanowires is seen to be somewhat different from that observed in bulk GaAs, and the PL rapidly quenches above 120K, with an activation energy of 17meV reflecting the presence of nonradiative defects.

Electrical Control of Ballistic Spin-Dependent Conductance Through the 2D-Electron Gas of GaAs Heterostructure

We investigated the effect of magnetic–electric barriers on electron transport in the two-dimensional electron gas (2DEG) of a AlGaAs–GaAs heterostructure. It is found that electric potential can be controlled to achieve both spin and charge conductance modulation in this material system, with the requisite condition being the presence of magnetic barriers to break the time reversal symmetry of the system. The use of electrical potential rather than applied magnetic fields as an external control parameter to achieve spin/charge conductance modulation promises a simpler device design for practical realization.

Bandwidth enhancement phenomenon of a high-speed GaAs–AlGaAs based unitraveling carrier photodiode with an optimally designed absorption layer at an 830nm wavelength

In this letter, the authors introduce a GaAs∕AlGaAs based unitraveling carrier photodiode (UTC-PD) for a wavelength of around 830nm. There is significant bias- and output-current-dependent bandwidth enhancement phenomena observed with this device. According to their microwave and optical-to-electrical measurement results, such distinct phenomena can occur under a much lower current density (0.3mA∕μm2 vs 0.05mA∕μm2) than previously reported for InP–InGaAs UTC-PDs. This can be attributed to the self-induced field in the absorption region, made possible due to the optimized p-type doping profile.

Novel reconfigurable semiconductor photonic crystal-MEMS device

We present a novel device concept for a reconfigurable optoelectronic integrated circuit (OEIC) that can be reprogrammed for many different functionalities. We report on our preliminary design, modeling, and fabrication of a semiconductor based photonic crystal (PhC) nano-membrane device with MEMS activated waveguide which is the basic building block of the reconfigurable OEIC. We have developed a technique to fabricate such a three-dimensional device on GaAs–AlGaAs epitaxial layers grown on a GaAs substrate. The device has a top PhC membrane layer structure composed of hexagonal holes in a triangular lattice. Below that, a separate suspended bridge layer or cantilever layer can insert a line of posts into the PhC holes to create a defect line that forms or alters an optical waveguide. This MEMS feature can generate/cancel a section of the waveguide in the PhC platform, or (by partial removal) it can change the dispersion of the waveguide. Therefore, the same structure can be actively reconfigured as different types of waveguide devices.

Closely spaced, independently contacted electron–hole bilayers in GaAs–AlGaAs heterostructures

We describe a technique of fabricating closely spaced electron–hole bilayers in GaAs–AlGaAs heterostructures. Our device incorporates a novel method of making shallow ohmic contacts, to a low density ( < 10 11 cm - 2 ) 2-DEG, using MBE grown doped InAs. These contacts do not require annealing and are completely free from “spiking”. Independent four terminal measurements on both layers (25 nm apart) are possible. Hole mobilities μ h > 10 5 cm 2 V - 1 s - 1 and electron mobilities μ e > 10 6 cm 2 V - 1 s - 1 are simultaneously attained at 1.5 K. Preliminary Coulomb drag measurements made down to T = 300 mK are consistent with the Onsager reciprocity relation for four terminal conductance and indicate an enhancement of Coulomb interaction over the values obtained from a “Thomas–Fermi” calculation. The technique lends itself fully to conventional uniform MBE growth and does not require shadow masking or ion implantation. In the simplest mode of operation a single external voltage, slightly higher than the bandgap of GaAs, is required for simultaneous accumulation of electrons and holes separated by the 25 nm Al 0.3 Ga 0.7 As barrier. Both carrier densities increase with the applied interlayer bias and the measured densities are reproducible over several cooldowns. A backgate can be added to the structure to get independent control of the carrier densities. Our process is independent of the precise depth of the 2DEG/2DHG or the width of the barrier and will be useful in studying novel phases resulting from a possible excitonic pairing between electrons and holes.

Single-quantum-well grating-gated terahertz plasmon detectors

A grating-gated field-effect transistor fabricated from a single-quantum well in a high-mobility GaAs–AlGaAs heterostructure is shown to function as a continuously electrically tunable photodetector of terahertz radiation via excitation of resonant plasmon modes in the well. Different harmonics of the plasmon wave vector are mapped, showing different branches of the dispersion relation. As a function of temperature, the resonant response magnitude peaks at around 30K. Both photovoltaic and photoconductive responses have been observed under different incident power and bias conditions.

Spin-injection efficiency and magnetoresistance in a hybrid ferromagnetic–semiconductor trilayer with interfacial barriers

We present a self-consistent model of spin transport in a ferromagnetic (FM)–semiconductor (SC)–FM trilayer structure with interfacial barriers at the FM–SC boundaries. The SC layer consists of a highly doped n 2+ AlGaAs–GaAs 2DEG while the interfacial resistance is modeled as delta potential ( δ ) barriers. The self-consistent scheme combines a ballistic model of spin-dependent transmission across the δ -barriers, and a drift-diffusion model within the bulk of the trilayer. The interfacial resistance ( R I ) values of the two junctions were found to be asymmetric despite the symmetry of the trilayer structure. Transport characteristics such as the asymmetry in R I , spin-injection efficiency and magnetoresistance (MR) are calculated as a function of bulk conductivity σ s and spin-diffusion length (SDL) within the SC layer. In general a large σ s tends to improve all three characteristics, while a long SDL improves the MR ratio but reduces the spin-injection efficiency. These trends may be explained in terms of conductivity mismatch and spin accumulation either at the interfacial zones or within the bulk of the SC layer.

Intersubband infrared absorption in stepped quantum wells under intense irradiation

The effect of an intense THz irradiation on the relative intersubband absorption ofelectrons in stepped quantum wells of GaAs–GaAlAs is theoretically studied. Analyticalexpressions for the induced current are obtained by means of the adiabatic and resonantapproximations within the matrix density formalism. This method allows one to predict thepresence of a marked fine structure on the absorption, together with a shift and broadeningof the absorption peaks, when the pump intensity is around the megawatts level.

Negative charged excitons in double barrier diodes

We have studied the effects of excitonic complexes formation, such as excitons and trions, on the optical and on transport properties of GaAs–GaAlAs n–i–n double barrier diodes, by measuring the current–voltage characteristics and the photoluminescence emission, as function of bias. The observation of a pre-resonance shoulder in the I(V) curves, under high laser intensities, and a negative charged excitons in the photoluminescence spectra, under the same bias conditions, were associated to the dissociation of these complexes either by thermal excitation or by scattering with ‘free’ carriers in the quantum well layer. A simple rate equation model allows us to explain the kinetics of the excitonic complexes in double barrier devices.

Nanometre spaced electrodes on a cleaved AlGaAs surface

We present a novel technique for fabricating nanometre spaced metal electrodes on asmooth crystal cleavage plane with precisely predetermined spacing. Our method does notrequire any high-resolution nanolithography tools, all lateral patterning being based onconventional optical lithography. Using molecular beam epitaxy we embedded athin gallium arsenide (GaAs) layer in between two aluminium gallium arsenide(AlGaAs) layers with monolayer precision. By cleaving the substrate an atomicallyflat surface is obtained exposing the AlGaAs–GaAs sandwich structure. Afterselectively etching the GaAs layer, the remaining AlGaAs layers are used as asupport for deposited thin film metal electrodes. We characterized these coplanarelectrodes by atomic force microscopy and scanning electron microscopy; thisrevealed clean, symmetric and macroscopically flat surfaces with a maximumcorrugation of less than 1.2 nm. In the case of a device with a 20 nm thick GaAslayer the measured electrode distance was 22.5 nm with a maximum deviation ofless than 2.1 nm. To demonstrate the electrical functionality of our device wepositioned single colloidal gold nanoparticles between the electrodes by an alternatingvoltage trapping method; this resulted in a drop of electrical resistance from∼11 G Ω to∼1.5 k Ω at 4.2 K. The device structure has large potential for the manipulation of nanosized objectslike molecules or more complex aggregates on flat surfaces and the investigation of theirelectrical properties in a freely suspended configuration.

Cross-sectional ballistic electron emission microscopy for Schottky barrier height profiling on heterostructures

In this work, cross-sectional ballistic electron emission microscopy is introduced to determine a Schottky barrier height profile of a GaAs–AlGaAs multiheterostructure in cross-sectional geometry. Ballistic electron spectra measured across the heterostructure with nanometer resolution indicate that the measured Schottky barrier height profile is smeared out compared to the conduction band profile calculated from the sample growth parameters. We attribute this behavior to lateral band bending effects along the heterojunction. In addition, we have evidence that the barrier height profile is influenced by single impurities in the AlGaAs layers.

Highly polarized electrons from GaAs–GaAsP and InGaAs–AlGaAs strained-layer superlattice photocathodes

GaAs–GaAsP and InGaAs–AlGaAs strained-layer superlattice photocathodes are presented as emission sources for highly polarized electron beams. The GaAs–GaAsP cathode achieved a maximum polarization of 92(±6)% with a quantum efficiency of 0.5%, while the InGaAs–AlGaAs cathode provides a higher quantum efficiency (0.7%) but a lower polarization [77(±5)%]. Criteria for achieving high polarization using superlattice photocathodes are discussed based on experimental spin-resolved quantum efficiency spectra.

Light Sensitivity of Current DLTS and Its Implications on the Physics of DC-to-RF Dispersion in AlGaAs–GaAs HFETs

The light sensitivity of current deep-level transient spectroscopy (I-DLTS) is analyzed with the aim of gaining insight about the physics of surface-trap related dc-to-RF dispersion effects in AlGaAs-GaAs heterostructure field-effect transistors. I-DLTS experiments under dark reveals three surface-trap levels with activation energies 0.44 eV (h1), 0.59 eV (h2), and 0.85 eV (h3), as well as a bulk trap with activation energy 0.45 eV (e1). While the I-DLTS signal peaks associated with the two shallower surface traps h1 and h2 are suppressed by optical illumination with energy larger than the AlGaAs bandgap, that which is associated with the deepest surface trap h3 is nearly unaffected by light up to the highest intensity adopted. Two-dimensional device simulations assuming that surface traps behave as hole traps provide an interpretation for the observed different light sensitivity of surface traps, explaining it as the result of the temperature dependence of surface hole concentration and negative trap-charge density, making trap-charge modulation at increasing temperature less and less sensitive to excess carriers generated by light.

Pressure and magnetic field effects on the binding energy of excitonic states in single and coupled GaAs–AlGaAs quantum wells

We have calculated the binding energy for the ground state and the 2P_like state of a light-hole exciton in GaAs–Al0.3Ga0.7As single and double quantum wells, under the action of a hydrostatic pressure and a magnetic field applied in the growth direction. We have used a variational scheme and hydrogenic-like trial wave function for the ground exciton state. We have considered that the well and the barrier material have the same dielectric constant and the carriers have different effective mass in these regions. We have found that the effects of the magnetic field on the binding energy are more evident than those due to the hydrostatic pressure. Also, we have found a good agreement not also with previous theoretical results on the binding energy pressure dependence in quantum wells, but with recent experimental reports.