GaAs AlGaAs Research Articles

Perturbative approach to space-charge field dynamics in photorefractive semiconductors

A general perturbative approach to solving photorefractive material equations for a two-wave mixing geometry is developed. The method allows the study of both stationary and transient states of the space-charge field. As an example, solutions were given for GaAs–AlGaAs photorefractive multiple quantum well structures working in the Franz–Keldysh geometry. Approximate explicit expressions describing the recording and erasure dynamics of photorefractive gratings have been derived. The results are compared with numerical solutions of a complete set of material equations.

Trion formation in GaAs–AlGaAs quantum dots by tunneling

Based on time-resolved microscopic photoluminescence ( μ PL ) from single self-assembled GaAs/Al x Ga 1− x As quantum dots, we identify one of the emission lines as arising from positive trions (two holes and one electron) in these nominally undoped quantum dots. The trion is formed via tunneling of one electron out of the dot after optical excitation of a biexciton. The rise time of trion emission line matches the decay of the biexciton due to electron tunneling.

Scalable Monolithically Grown AlGaAs–GaAs Planar Nanowire High-Electron-Mobility Transistor

Monolithically grown planar nanowire (NW) high-electron-mobility transistors (NW-HEMTs) are demonstrated using self-aligned 〈110〉 GaAs NWs capped with Si-doped AlxGa1-xAs shell as the channel on semi-insulating (100) GaAs substrates. The planar Al0.35Ga0.65As-GaAs NW-HEMT with ~ 1-μm-long gate exhibits excellent dc characteristics, with extrinsic Gm of ~ 80 mS/mm and estimated intrinsic Gm of ~ 260 mS/mm, where the device width is defined as the entire periphery of the NWs. The ION/IOFF ratio is ~104 and the threshold is -1.5 V operating in depletion mode. The output current increases linearly with the number of NWs in the channel, while the threshold voltage does not change at all. This indicates excellent uniformity and scalability of the bottom-up-grown NW devices. Compared to field-effect transistors with doped NWs as channels, the structure reported here circumferences the inherent doping nonuniformity issues in NWs grown by the vapor-liquid-solid mechanism, and self-aligned lateral epitaxy nature of our NW structure makes scaling up to NW array-based transistors from the bottom up feasible.

Defect-Free GaAs/AlGaAs Core–Shell Nanowires on Si Substrates

We report straight and vertically aligned defect-free GaAs nanowires grown on Si(111) substrates by metal–organic chemical vapor deposition. By deposition of thin GaAs buffer layers on Si substrates, these nanowires could be grown on the buffer layers with much less stringent conditions as otherwise imposed by epitaxy of III–V compounds on Si. Also, crystal-defect-free GaAs nanowires were grown by using either a two-temperature growth mode consisting of a short initial nucleation step under higher temperature followed by subsequent growth under lower temperature or a rapid growth rate mode with high source flow rate. These two growth modes not only eliminated planar crystallographic defects but also significantly reduced tapering. Core–shell GaAs–AlGaAs nanowires grown by the two-temperature growth mode showed improved optical properties with strong photoluminescence and long carrier life times.

Noise, Gain, and Responsivity in Low-Strain Quantum Dot Infrared Photodetectors With up to 80 Dot-in-a-Well Periods

We present a systematic study of noise, gain, responsivity, and specific detectivity D* , in a series of low-strain dot-in-a-well (DWELL) quantum dot infrared photodetectors (QDIPs). The lattice-matched GaAs quantum wells and AlGaAs barriers in these devices prevent the accumulation of excessive strain and allow the growth of up to 80 DWELL periods. We show that the photoconductive gain in these QDIPs is inversely proportional to the number of periods, while the total quantum efficiency is proportional to the number of periods, meaning that the responsivity remains constant at a given mean electric field as the number of periods is varied. The dark current in each QDIP was also found to be constant at a given mean electric field.

GaAs–AlGaAs heterostructure thyristors with completely optical transfer of emitter current

Several variants of thyristors based on GaAs-AlGaAs heterostructures with optical transfer of the emitter current are considered. The possibility that thyristors with fully optical transfer of the emitter current can be, in principle, created is demonstrated by means of the results of a study of n-p-n and p-n-p optoelectronic transistors in which the emitter current is converted into light, this light, in turn, being converted into a collector current. Structures of optoelectronic switches of this kind are presented. A switch comprising three constituent transistors has been suggested and fabricated taking into account specific features of the technique for growth of undoped GaAs layers and fabrication of high-voltage p0-n0 junctions with back-ground impurities from these layers, which makes the turn-on delay cardinally shorter and raises the working frequency.

High extinction ratio metal–insulator–semiconductor waveguide surface plasmon polariton polarizer

The attenuation characteristics of a multilayer metal clad GaAs–AlGaAs optical waveguide polarizer are theoretically investigated. The dispersion relations and field distribution of the multilayer structures are calculated for different geometrical parameters and material properties. The polarizer studied consists of a single mode finite/infinite metal clad GaAs–AlGaAs waveguide with a dielectric (SiO2/Si3N4) buffer layer inserted between the metal and the waveguide.Conventionally, the TM polarized waves are found to exhibit an absorption peak at a particular buffer thickness (called critical buffer thickness).We shall show that the maximum TM absorption can be improved by a multiple factor up to 7 by choosing a buffer layer thicker than its critical value. This corresponds to an extinction ratio of 1470dB for a polarizer length of 1mm. Further, thicker buffers reduce the insertion losses and values as low as 0.1dB can be obtained. The strong TM absorption in these structures is interpreted as resonant coupling of the guided mode to the lossy surface plasmon polariton supported by the thin metal film. Thicker buffer also reduces the TE losses (insertion losses) and hence increases the extinction ratio (ratio of TM to TE losses).This can be achieved by optimizing the buffer and the metal thicknesses. Another equally efficient polarizer can be designed by positioning a dielectric (same as buffer) layer (superstrate layer) above the metal film and then optimizing the buffer, metal and the superstrate thicknesses. We also show that the proposed polarizer with the superstrate layer is highly stable even when exposed to the extreme atmospheric changes.

Preparation characterization of MnSb–GaAs spin LED

Spin light-emitting-diodes (spin-LEDs) composed of an epitaxial MnSb layer having in-plane magnetization and GaAs–AlGaAs double hetero-structures were prepared using a two-chamber MBE system. Electroluminescence extracted through the cleaved side walls showed elliptically polarized light emission with circular polarization of 2.1% at 30K without an auxiliary external magnetic field.

Carrier Lifetime and Electron Spin Relaxation Time in (110)-Oriented GaAs–AlGaAs Quantum-Well Micro-Posts

Carrier lifetime and electron spin relaxation time in (110)-oriented GaAs-AlGaAs quantum-well (QW) micro-posts are investigated by polarization- and time-resolved photoluminescence measurements. The long electron spin relaxation time in QWs on GaAs (110) is found to be preserved even when the sidewall boundaries with fast surface recombination are introduced and the carrier lifetime is drastically shortened. Rate equation analysis shows that spin-controlled vertical-cavity surface-emitting lasers with such (110)-QW micro-posts will exhibit faster switching of lasing circular polarizations than that we previously demonstrated. In particular, 20-GHz switching is expected with 0.5-μm posts.

Low temperature photoluminescence and Raman phonon modes of Au-catalyzed MBE-grown GaAs–AlGaAs core–shell nanowires grown on a pre-patterned Si (1 1 1) substrate

We present the optical properties of MBE-grown GaAs–AlGaAs core–shell nanowires (NWs) grown on anodized-aluminum-oxide (AAO) patterned-Si (1 1 1) substrate using photoluminescence and Raman scattering spectroscopy. The GaAs NWs were grown via the vapor–liquid–solid method with Au-nanoparticles as catalysts. Enhancement in emission of at least an order of magnitude was observed from the GaAs–AlGaAs core–shell NWs as compared to the bare GaAs NWs grown under similar conditions, which is an indication of improved radiative efficiency. The improvement in radiative efficiency is due to the passivating effect of the AlGaAs shell. Variation in bandgap emission energy as a function of temperature was analyzed using the semi-empirical Bose–Einstein model. Results show that the free exciton energy of the GaAs core–shell agrees well with the known emission energy of zinc blende (ZB) bulk GaAs. Further analysis on the linear slope of the temperature dependence curve of photoluminescence emission energy at low temperatures shows that there is no difference between core–shell nanowires and bulk GaAs, strongly indicating that the grown NWs are indeed predominantly ZB in structure. The Raman modes show downshift and asymmetrical broadening, which are characteristic features of NWs. The downshift is attributed to lattice defects rather than the confinement or shape effect.

Slow relaxation of magnetoresistance in AlGaAs–GaAs quantum well structures quenchedin a magnetic field

We observed a slow relaxation of the magnetoresistance in response to an applied magneticfield in selectively doped p-GaAs–AlGaAs structures with a partially filled upper Hubbardband. We have paid special attention to excluding the effects related to temperaturefluctuations. Although these effects are important, we have found that the general featuresof slow relaxation persist. This behavior is interpreted as related to the properties of theCoulomb glass formed by charged centers with account taken of spin correlations, which aresensitive to an external magnetic field. Variation of the magnetic field changesthe numbers of the impurity complexes of different types. As a result, it affectsthe shape and depth of the polaron gap formed at the states belonging to thepercolation cluster responsible for the conductance. The suggested model explainsboth the qualitative behavior and the order of magnitude of the slowly relaxingmagnetoresistance.

Slow and fast light in photorefractive GaAs–AlGaAs multiple quantum wells in transverse geometry

We show theoretically that, based on the dispersive phase coupling effect during the wave mixing process, both slow and fast light can be achieved in GaAs–AlGaAs photorefractive multiple quantum wells (PRMQWs) films applied with a transverse direct-current electric field. The general formula for the group velocity of the diffracted beams in the Raman–Nath regime during the wave mixing process in a nonlinear thin film is derived and is then applied to the case of the PRMQWs films in the transverse geometry. The simulation results in the transverse-geometry PRMQWs films show that the group velocity and bandwidth of slow light can be on the order of centimeter per second and 100 kHz, respectively. The extremely low group velocity and the relatively broad bandwidth are mainly originated from the strong quadratic electro-optic effect and the fast response rate of the PRMQWs films, respectively. Our results show that the delay-bandwidth product of slow light can be significantly improved in PRMQWs films as compared to the reported results in other photorefractive materials.

Submicron Etched Beam Splitters Based on Total Internal Reflection in GaAs–AlGaAs Waveguides

Submicron etched beam splitters are designed, fabricated and characterized in Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> As-GaAs waveguides. Beam splitter transmission and reflection characteristics show clear dependence on gap dimension and angle of incidence. It is possible to obtain 8 to 30% power transmission by adjusting the gap dimension and angle of incidence. The experimental results agree well with three-dimensional (3-D) finite difference time domain (FDTD) simulations. The effect of imperfections, mainly the slope of etched sidewalls and variations in etch depth are investigated using 3-D FDTD. Design guidelines for low loss etched beam splitters are also given.

Wavelength Conversion of 10-Gb/s RZ-OOK Using Filtered XPM in a Passive GaAs–AlGaAs Waveguide

Wavelength conversion of a 10-Gb/s return-to-zero on-off-keyed (RZ-OOK) signal has been achieved using transient cross-phase modulation in a passive GaAs-AlGaAs bulk-waveguide, followed by a detuned filter. The converted RZ-OOK signal showed a power penalty of <1 dB at a bit-error-rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> relative to the baseline RZ-OOK signal.

Electrons in one dimension.

In this article, we present a summary of the current status of the study of the transport of electrons confined to one dimension in very low disorder GaAs–AlGaAs heterostructures. By means of suitably located gates and application of a voltage to ‘electrostatically squeeze’ the electronic wave functions, it is possible to produce a controllable size quantization and a transition from two-dimensional transport. If the length of the electron channel is sufficiently short, then transport is ballistic and the quantized subbands each have a conductance equal to the fundamental quantum value 2e2/h, where the factor of 2 arises from the spin degeneracy. This mode of conduction is discussed, and it is shown that a number of many-body effects can be observed. These effects are discussed as in the spin-incoherent regime, which is entered when the separation of the electrons is increased and the exchange energy is less than kT. Finally, results are presented in the regime where the confinement potential is decreased and the electron configuration relaxes to minimize the electron–electron repulsion to move towards a two-dimensional array. It is shown that the ground state is no longer a line determined by the size quantization alone, but becomes two distinct rows arising from minimization of the electrostatic energy and is the precursor of a two-dimensional Wigner lattice.

High contrast ratio, high uniformity multiple quantum well spatial light modulators

Our latest research results on GaAs–AlGaAs multiple quantum well spatial light modulators are presented. The thickness uniformity of the epitaxial layers across the 3-inch wafer grown by our molecular beam epitaxy is better than 0.1% and the variation of cavity resonance wavelength within the wafer is only 0.9 nm. A contrast ratio (CR) of 102 by varying bias voltage from 0 to 6.7 V is achieved after fine tuning the cavity by etching an adjust layer. Both theoretical and experimental results demonstrate that incorporating an adjust layer is an effective tuning method for obtaining high CR.

Intense THz laser effects on off-axis donor impurities in GaAs–AlGaAs coaxial quantum well wires

The binding energy of an off-axis shallow-donor impurity in a GaAs coaxial cylindrical quantum well-wire under the action of an intense, high-frequency laser field is calculated using a variational procedure within the effective-mass approximation. We take into account the laser dressing effects on both the impurity Coulomb potential and the confinement potential. Numerical calculations of the ground state subband energy levels based on a finite element method are performed for different barrier thicknesses and laser field parameters. Our model indicates a possible tuning of the well potential shape along the laser polarization direction. Strong distortions of the electron probability density under intense laser field conditions are also predicted. The study proves that the presence of the laser field partially breaks down the degeneracy of the states for donors symmetrically positioned within the structure. The results obtained show that the impurity energy levels in coaxial quantum wires can be significantly modified and controlled by intense laser fields.

The challenge of unity wall plug efficiency: The effects of internal heating on the efficiency of light emitting diodes

We develop a self-consistent model to describe the internal heating of high power light emitting diodes (LEDs) and use this model to simulate the operation of GaAs–AlGaAs double heterostructure LEDs. We account for the heating by nonradiative recombination processes in the simulations and solve self-consistently the steady state junction temperature. Based on the simulation results, we discuss the plausibility of unity conversion efficiency in LEDs and also the mechanisms underlying the efficiency droop. We show that the rise in the junction temperature limits the light output available from LEDs and further degrades the efficiency of operation at high operating currents. In addition to high power applications we study the optimal operating point and discuss the methods to increase the efficiency of LEDs toward the thermodynamical limits.

Structural, Compositional, and Optical Characterizations of Vertically Aligned AlAs/GaAs/GaP Heterostructure Nanowires Epitaxially Grown on Si Substrate

We structurally, compositionally, and optically characterize vertically aligned AlAs/GaAs/GaP heterostructured nanowires (NWs) grown on a Si substrate used for the integration of an optically active material into Si-based technology and its band-gap engineering. The NWs were grown using Au colloidal nanoparticles as catalysts via the vapor–liquid–solid mode. By alternately changing the source material between Ga and Al, we grew GaAs/AlAs/GaAs/AlAs/GaAs NWs with a well-controlled periodic structure and composition on a GaP segment, which was epitaxially grown on a Si substrate. No dislocations induced by the lattice mismatch were found in the GaAs segment of the NWs grown on the GaP segment despite a lattice mismatch of as large as 4%. This is because the NWs have a particular columnar structure with nanoscale diameters and can therefore relax laterally and accommodate a high strain. Stacking faults exist in zinc-blende-structured GaP and GaAs segments, while the AlAs segment has a pure wurtzite crystal structure without any stacking faults. It is found that the stacking fault in III–V NWs is significantly dependent on the stacking fault energy and ionicity. With increasing ionicity, stacking faults can be more easily introduced, and these NWs tend to have a wurtzite crystal structure. In addition, owing to the high surface nonradiative recombination rate resulting from the surface states on the GaAs NW surface, the excitonic emission of photoluminescence from the bare GaAs NW segment has a decay time of as short as 30 ps. With the growth of an AlGaAs capping layer and GaAs outer shell layer, the decay time of the excitonic emission increased 46-fold, indicating an excellent passivation effect on the GaAs segment surface.

Development of Highly Reliable Point Source Infrared Light-Emitting Diodes and Analysis Using a New Parameter of Dark Area Ratio

Surface-emitting light-emitting diodes with a small opening, 150 µm in diameter, having an InGaAs–AlGaAs multi-quantum-well active layer were grown by metal-organic chemical vapor deposition. Their emission wavelength was approximately 850 nm. The manner of their degradation was investigated by operating the diodes for 10,000 h at room temperature, and high reliability was realized. By measuring the dark area and comparing it with output power, we show that the degradation mechanism differs from that of light-emitting diodes with a single GaAs layer. In light-emitting diodes with InGaAs–AlGaAs multi-quantum wells, the p–n junction functions well, despite initial degradation, and further degradation is suppressed.