AlAs Layer Thickness Research Articles

Lateral Oxidation of AlAs for Circular and Inverted Mesa Saturable Bragg Reflectors

A model of the thermal oxidation kinetics that is based on the continuity equation is adapted to the fabrication of circular saturable Bragg reflector (SBR) mesa structures, as well as inverted mesa array structures. A comparison between theoretical calculations and experimental measurements shows good agreement, making the model a useful tool for the controllability of the structure-dependent oxidation process. The model is applied to the wet oxidation of buried AlAs layers for the fabrication of ultra-broadband, high index contrast, III-V/ AlxOy SBRs. The rate of oxidation is found to be dependent not only on the oxidation temperature, but also on oxidation time, mesa geometry, and AlAs layer thickness. The two distinct large-area SBR structures, the circular mesa and the inverted mesa, are fabricated.

High-resolution X-ray diffraction analysis of InGaAs/AlAsSb coupled double quantum wells grown by molecular beam epitaxy

High-resolution X-ray diffraction was used to study the structural properties of InGaAs/AlAs/AlAsSb coupled double quantum wells (CDQWs) grown on an InP (0 0 1) substrate by molecular beam epitaxy. The InGaAs composition and the AlAs layer thickness were the considered parameters. High-resolution rocking curve measurements and two-dimensional reciprocal space mapping (RSM) were employed in both symmetrical and asymmetrical reflections. All samples showed extended satellite peaks in the rocking curves, although the sample with largest residual strain (03125) had broader satellite peaks. Symmetric RSMs indicated that none of the samples had a crystallographic tilt between the quantum well (QW) layers and the substrate. A clear strain gradient in the CDQW layers was observed in the RSM of the sample 03125, which was caused by a large residual strain.

The influence of the additional confining potentials on ferromagnetism in III–V digital ferromagnetic heterostructures

The effect of the additional confining potentials on ferromagnetism in III–V digital ferromagnetic heterostructures has been studied by ab initio calculations in combination with pseudopotential plane-wave method. The electronic and magnetic properties are shown as a function of the thickness of AlAs layers in the GaAs∕AlAs digital ferromagnetic heterostructures. It is found that all the structures show ferromagnetic alignment for the most favored configuration and their electronic structures are half metallicity. The exchange coupling constants N0β are estimated by using the spin splitting of the valence band. It is also shown that the charge density and the strong spin polarization are concentrated mostly on the magnetic layers for all structures. Furthermore, the hole distributions are analyzed in terms of their orbital projected density of states. The concentration of confined hole within the magnetic layer increases with increasing the additional potentials, which is responsible for the enhancement of ferromagnetic interaction in III–V digital ferromagnetic heterostructures.

Peculiar photocurrent response due to Γ–X coupling in a GaAs/AlAs heterostructure

Peculiar current jumps and hysteresis in current–voltage curves are reported in an illuminated heterostructure consisting basically of a thick AlAs layer and a narrow GaAs quantum well. These novel features come from the photon-assisted transfer of electron–hole pairs and the resultant charge polarization in the structure, mainly caused by the resonant Γ–X coupling at the heterointerfaces. Using the transfer-matrix method, the simulated current density–voltage curve reproduces the main features of the experimental observations in the case where the influence of resonant Γ–X coupling at the heterointerfaces is included, further confirming the physical mechanism involved. The structure presented here may be used as a new type of photonic memory cell and also as an optically controlled switch.

Novel semiconductor/superlattice distributed Bragg reflector: Experiment and simulation

We design a novel semiconductor/superlattice AlAs/(GaAs/AlAs) distributed Bragg reflector (DBR), and the reflection spectrum is calculated by employing transfer matrix method. In experiment, this kind of DBR is grown on GaAs (1 0 0) substrate. From the measured reflection spectrum, the central wavelength is about 810 nm with high reflectivity, and the reflection bandwidth is about 100 nm. By adjusting the thickness of AlAs layer and the period of superlattice GaAs/AlAs, the DBR with center wavelength at 1.3 μm is also investigated theoretically. The calculated results show that the central wavelength is about 1.3 μm, and the peak reflectivity is up to 99.5% with 30 pair.

Infrared reflectivity of (GaAs)m/(AlAs)n superlattices

We report an original analysis method to model the p-polarized infrared reflectivity (IR) spectra of (GaAs)m/(AlAs)n superlattices (m=36, 4<n<7) grown on semi-insulating GaAs (100). In our fitting procedure, the first derivative of the IR spectrum was calculated numerically and the respective dielectric functions of the AlAs barriers and GaAs quantum wells were described by two independent factorized models, with the thickness of the AlAs layers treated as a free parameter. The longitudinal and transverse optical phonon frequencies of the GaAs layers were found to be independent of the AlAs layer thickness, while those of the AlAs layer decreased in accordance with theory and their linewidths increased with increasing confinement.

Wet oxidation kinetics of AlAs at elevated temperatures

Wet oxidation in the AlAs layer sandwiched between two GaAs plates was investigated for the temperature range of 400 to 480 °C. The oxidation rate increased with increasing thickness of the AlAs layer. Theoretical analysis based on the boundary layer diffusion was performed to account for the thickness effect. The theory is in excellent agreement with the experimental measurement.

Temperature and thickness dependence of steam oxidation of AlAs in cylindrical mesa structures

The Deal-Grove model of thermal oxidation kinetics is adapted to cylindrically symmetric mesa structures and applied to study steam oxidation of AlAs. Oxidation process parameters are extracted from available experimental data as functions of temperature and the AlAs layer thickness. The oxidation rate is found to be very sensitive not only to temperature, but also to the oxidation front position inside the mesa. The oxidation rate slows down as the oxidation front moves into the mesa, reaches a minimum, and then accelerates at the final stages of the oxidation process. Complex nonmonotonic dependence of the oxidation process on layer thickness is also revealed.

The study of thermal stability during wet oxidation of AlAs

A structure in which a 1000 Å thick AlAs layer is sandwiched between 1000 Å thick GaAs cap layer and 2000 Å GaAs buffer layer was subsequently grown by molecular beam epitaxy on the GaAs substrate. The AlAs layer was laterally oxidized in N 2 bubbled H 2O vapor ambient at 400°C for 20, 30, 40, 60, and 120 min. It was found that the thermal stability of the mesas is very dependent on the removal rate of volatile products, such as As and As 2O 3. The removal of volatile products is dependent on the oxidation time and temperature. The Raman spectra presented here proved that the spectra were different from samples, oxidized for different time durations.

Fabrication of a depletion mode GaAs MOSFET using Al 2O 3 as a gate insulator through the selective wet oxidation of AlAs

A 1 μm thick undoped GaAs buffer layer, a 1500 Å thick n-type GaAs layer, an undoped 500 Å thick AlAs layer and a 50 Å thick GaAs cap layer were consecutively grown by molecular beam epitaxy (MBE) on a [100] oriented semi-insulating GaAs substrate. The AlAs layer was oxidized in a N 2 bubbled H 2O vapor ambient at 400°C for 3 h and fully converted to Al 2O 3 for use as a gate insulator. The I– V characteristics, having a maximum drain current of 10.6 mA, a current cut-off voltage of −4.5 V and a maximum transconductance value of 11.25 mS/mm, indicate that the selective wet thermal oxidation of AlAs/GaAs was successful in producing a depletion mode GaAs MOSFET.

Super-lattice AlAs/AlInAs for lateral-oxide current confinement in InP-based lasers

We propose a novel super-lattice AlAs/AlInAs lateral-oxide current confinement structure for InP-based semiconductor lasers, including surface emitting lasers and opto-electronic devices. We also demonstrate an oxide current confined AlGaInAs/InP multi-quantum well edge emitting laser at 1.55 μm. The lasers can be fabricated by a single step MOCVD growth followed by a lateral oxidation. A tensile-strained AlAs layer on InP substrate has a large hetero-barrier and works as an electron stopping layer in p-type region. The optimized super-lattice AlAs/AlInAs layer exhibited no defects in spite of thick AlAs layer growth beyond critical thickness. In the super-lattice, AlAs layers are carbon-δ-doped for low turn-on voltage, between which narrow gap AlGaInAs layers are inserted for reducing resistance. Broad area lasers with the super-lattice structure exhibited a threshold current density of 130 A/cm 2 per well and characteristic temperature of 85 K.

Dependence of lateral oxidation rate on thickness of AlAs layer of interest as a current aperture in vertical-cavity surface-emitting laser structures

The dependence of the wet oxidation process on the AlAs layer thickness used in selectively oxidized vertical-cavity surface-emitting-laser structures is studied in detail. A theoretical model based on a diffusion-reaction process is proposed. A rapid reduction in the oxidation rate is predicted with a reduction in the layer thickness of the ultrathin AlAs layer. The theoretical predictions are verified through experiments.

Type-II biexcitons in GaAs/AlAs short-period superlattices

We report on the photoluminescence properties of type-II biexcitons consisting of the X electrons of AlAs and Γ heavy holes of GaAs in (GaAs) m /(AlAs) n short-period superlattices with ( m, n)=(13, 13), (12, 12), (10, 10), (10, 14), and (10, 20) in units of monolayer. In all the samples, we have observed the biexciton formation with a superlinear dependence of the excitation power. From the line-shape analysis of the photoluminescence bands, the biexciton binding energies are estimated to be around 3 meV. We find that the biexciton binding energy decreases with the increase of the AlAs layer thickness in the (10, n) superlattices. This indicates that the spatial separation of electrons and holes of the type-II biexciton affects the binding energy. In addition, we discuss the transient processes of the type-II biexciton from time-resolved photoluminescence spectra.

Observation of composition in surface monolayers by X-ray scattering spectra caused by crystal truncation and interferences.

X-ray crystal-truncation-rod (CTR) scattering measurements using synchrotron radiation and an imaging plate can reveal the composition of a surface monolayer. Even when the composition is changed in only one atomic layer on the top surface, the X-ray CTR spectrum can change due to the differences in composition. X-ray CTR spectra are greatly enhanced by X-ray interference when a sample is designed properly. In this paper, it is shown by theoretical calculations and experiments for AlAs/GaAs samples grown by MBE that a 1 ML (monolayer)-thick AlAs layer embedded under 10 ML below the surface can enhance the modulation of an X-ray CTR spectrum.

Coupling coefficient, temperature stability and mass sensitivity of the Rayleigh-type mode on (001) [110] AlAs/GaAs

The use of AlAs/GaAs layered structures for SAW sensor applications is discussed with the aim of exploring the potential of the material system for the integration of SAW and electronic devices. Based on the acoustic wave spectrum on the (001) cut of GaAs the development of the Rayleigh-type mode in the [110] direction of the AlAs/GaAs structure is described. Using a transfer matrix algorithm the phase velocity and the coupling coefficient of the dispersive structure are calculated as a function of the relative layer thickness , with k being the wave number and the layer thickness. Results of SAW phase velocity measurements carried out by a thermoelastic laser excitation method and a time delay technique are presented. The coupling coefficient has a maximum at kh = 1.8 being twice as high as the coefficient of bare GaAs. The temperature stabilization with and Au layers is calculated and the relation between and is determined for a vanishing temperature coefficient of delay (TCD). The mass sensitivity is increased by the application of the temperature stabilizing layers. It reaches a maximum value at and . Some aspects of optimization procedures including the AlAs layer thickness are discussed.

Surface energy model for the thickness dependence of the lateral oxidation of AlAs

The lateral oxidation rate of AlAs layers decreases dramatically for layers thinner than about 500 Å, because the activation energies for the rate constant of the reaction at the oxidation front increases by an amount inversely proportional to the layer thickness. We derive a model for the thickness dependence of the lateral oxidation rate of AlAs based on the surface energy of the curvature observed at the oxide tip. From the model, we show that the linear oxidation rate has an exp(−θ0/θ) dependence on the AlAs layer thickness θ, and we can predict the slowing of oxidation when the AlAs layer is cladded with AlGaAs barriers. Also, we estimate the surface energy of the AlAs/oxide interface to be 50 eV/nm2.

Capture dynamics and far-infrared response in photovoltaic quantum well intersubband photodetectors

Recent progress in photovoltaic quantum well intersubband photodetectors (QWIP) makes these devices suitable for high-performance imaging and heterodyne detection. We report on investigations concerning the basic physics of the transport mechanism, the dynamical behavior, and the further optimization of these structures. GaAs/AlAs/AlGaAs double-barrier QWIPS designed for 3-5 μm wavelength operation provide an interesting model system to study the dynamical aspects of the photovoltaic response. We find strong evidence that carrier transfer across the AlAs barriers mainly occurs due to Γ-X intervalley scattering, even for AlAs layer thicknesses of 1-2 nm. We also discuss experimental results on photovoltaic QWIPs operating in the 8-12 μm regime. In these structures, photovoltaic operation is achieved using a combination of single-barrier quantum wells with built-in space-charge fields. We report on a photovoltaic QWIP with a cutoff energy of 118 meV and a zero-bias detectivity of 2.5×10 9 cm√Hz/W, which is only three times less than the detectivity of a photoconductive QWIP with the same cutoff energy.

Formation of GaAs/AlAs(001) interfaces studied by scanning tunneling microscopy.

We have investigated the interface formation of molecular-beam epitaxy (MBE) grown GaAs/AlAs(001) heterostructures by in situ scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). Starting from a smooth As-rich (2\ifmmode\times\else\texttimes\fi{}4) reconstructed GaAs surface the formation of the normal interface was studied at AlAs layer thicknesses ranging from 1 to 50 ML. With increasing AlAs thickness the reconstruction changes from (2\ifmmode\times\else\texttimes\fi{}4) to (2\ifmmode\times\else\texttimes\fi{}3) via a (1\ifmmode\times\else\texttimes\fi{}1) and a (1\ifmmode\times\else\texttimes\fi{}3) symmetry under the conditions used. For the inverted interface, the RHEED pattern changes immediately after the deposition of only 1 ML of GaAs on AlAs from a (2\ifmmode\times\else\texttimes\fi{}3) to a clear (2\ifmmode\times\else\texttimes\fi{}4) symmetry. The STM images of both interfaces show that the interface formation is accompanied by a disordering of the (2\ifmmode\times\else\texttimes\fi{}4) reconstruction by forming kinks in the missing dimer rows that can be related to intrinsic point defects. We attribute the different incorporation of these intrinsic point defects to the different electronic properties of the two interfaces. Large-scale STM images also reveal a different surface roughness for the normal and inverted interface. \textcopyright{} 1996 The American Physical Society.

Low-temperature photoluminescence of disordered thin-layer GaAs/AlAs superlattices: Experiment.

Low-temperature photoluminescence spectra for various disordered thin-layer (GaAs${)}_{\mathit{m}}$(AlAs${)}_{\mathit{n}}$ superlattices have been experimentally studied. The samples were grown by molecular-beam expitaxy (MBE) growth, and disorder was introduced by varying the thickness of GaAs and AlAs layers in the growth direction in various specific but randomly generated sequences. The photoluminescence from disordered superlattice samples shows a strong disorder sequence dependence. Disordered (GaAs${)}_{\mathit{m}}$(AlAs${)}_{\mathit{n}}$ superlattices with m,n distributed in a range of 1--6 shows mainly a sharp peak at the high-energy side and a broad peak at the low-energy side. According to the study of the temperature, excitation-intensity, and disorder-sequence dependence of the photoluminescence, the sharp peak is assigned to bound-exciton recombination and the broad peak to the recombination of electron-hole pairs localized by a disorder potential induced by layer thickness disorder. Samples with m,n larger than 6 exhibit multiple peaks of similar recombination mechanism. In addition, the results demonstrate that the luminescence intensity of disordered superlattices at T=10 K can be increased by up to two orders of magnitude over that of ordered short-period superlattices. \textcopyright{} 1996 The American Physical Society.

Photoluminescence properties of [formula omitted] disordered superlattices with fixed GaAs or AlAs layer thickness

We have shown that disordered superlattices (d-SLs) possess a remarkably enhanced photoluminescence (PL) capability. The d-SLs have a disorderly distributed layer thickness which is selected from 2, 4, and 6 molecular layers (MLs). We investigate the effects of a 6 ML GaAs layer, which is the widest well width, through d-SLs with fixed GaAs or AlAs layer thickness. PL thermal quenching properties are measured for the investigation. The enhanced PL capability is caused not by the existence of the 6 ML GaAs layer but by the number of disorderly distributed layer patterns. The effect of the number of disorderly distributed layer patterns is also investigated through the d-SL with fixed AlAs layer thickness.