Ga Migration Research Articles

Relationship between silicon incorporation and electrical characteristics for lateral p - n junctions grown on patterned (110) and (100) GaAs substrates

We have undertaken a systematic study of silicon incorporation on planar and patterned GaAs substrates. In this paper we link the observed electrical characteristics of lateral p - n junctions formed on patterned (100) and (110) substrates to silicon incorporation on the chosen surfaces and subsequent defect formation. Lateral p - n junctions were fabricated on patterned (100) and (110) substrates. These were located at the upper and lower boundaries between the (100) - (311)A and (110) - (100) flat - facet combinations. Electrical measurements revealed two main current mechanisms, dominant over separate sections of the voltage characteristic. Under low bias, a tunnel current was observed in all samples and this was ascribed to indirect electron tunnelling into band-tail states, followed by recombination via phonon emission. The magnitude of the excess current in various junction combinations was observed to be related to the degree of compensation in the junction regions, determined by the growth conditions and Ga migration effects; the highest levels were measured in the most compensated material. Recombination and diffusion currents were dominant under high bias conditions. The value of the ideality factor was found to be dependent on the relative magnitudes of the majority carrier densities on either side of the depletion region.

Ga migration process in Au film on (100) GaAs under temperature treatment in vacuum

The investigated structures look like patterns with separated circular windows, opened in SiO 2 covering GaAs surface with thin metal film (Au) deposited above (honey-comb structures). The method of precise chemical analysis developed and used in this work allowed to determine the total quantity of Ga in Au layer rubbed off the substrate. It was found that at the initial stage of heating (up to 400°C) Ga migration from windows (Au on GaAs) into Au on SiO 2 had not been observed but physical and chemical reactions with forming of Au-Ga eutectic compound were found in Au layer on GaAs. The melting temperatures and specific melting heat were calculated from experimental data ( T 0 = 340°C; Q 0 = 3.5kcal/mol). After temperature increasing Ga migration in Au on SiO 2 was noticed and identified as a result of diffusion process with Au-GaAs windows as ‘sources’ and Au on SiO 2 as ‘drains’. The process of Ga migration manifests itself for 500A˚Au and 0.2–0.3 μm SiO 2 thickness as the grain boundary diffusion with activation energy ∼ 7.3 kcal/mol.

Growth of (411)A Faceted GaAs/AlGaAs Ridges by Growth-interrupted Chemical Beam Epitaxy

Faceted (411) and (100) ridges of GaAs/AlGaAs have been successfully grown on mesa-etched GaAs(100) substrates by chemical beam epitaxy employing growth-interruption technique. The growth-interruption method was found to be very efficient in evolving and forming (411) faceted ridge structures. The formation of (411) ridge structures were attributed to the Ga migration enhanced by growth-interruption and also to the increased desorption of Ga on the (411) face compared to (100) face. We also observed the growth of triangular-shaped GaAs structure with (411)-(100) planes which are surrounded by AlGaAs barrier layer. This results shows that the formation of quantum wires could be possible using this method.

MBE growth of submicron carrier confinement structures on patterned GaAs(111)A substrates using only silicon dopant

Abstract Silicon-doped GaAs layers have been grown on patterned GaAs(111)A substrates with equilateral triangles by molecular beam epitaxy. Lateral carrier confinement structures are confirmed by cathodoluminescence (CL) spectra: a (111)A surface is p -type and (311)A sidewalls are n -type. The confined (111)A region is reduced to submicron scale by reducing the size of the photoresist pattern. Furthermore, strong emission from the confined region and lateral modulation of peak wavelength are observed in the CL spectra of Al 0.3 Ga 0.7 As GaAs single quantum well grown on the patterned (111)A substrate. This modulation is caused by a decrease in GaAs layer thickness due to Ga migration from the (111)A surface to the (311)A surface. These results indicate that MBE growth on the patterned (111)A substrate is effective for fabricating submicron carrier confinement or optical confinement structures.

First-principles calculations of surface adsorption and migration on GaAs surfaces

This paper investigates surface adsorption and migration on GaAs surfaces theoretically based on the ab initio calculations. First, we calculate the Ga migration potential on GaAs(001)-(2 × 4) surfaces by the ab initio calculations. The calculated migration potential sensitively depends on the Ga coverage. At lower Ga coverages, the most favorable sites are located in the As-dimer region. Whereas, as the Ga coverage increases, favorable lattice sites move from the As-dimer region to the missing-dimer region. Next, we clarify the mechanism of the coverage dependence of migration potential by considering the surface atomic structure and the energy band structure at each Ga coverage. The calculated results show that surface distortion mainly determines the most favorable site at lower Ga coverages. However, after the surface distortions are relaxed at higher Ga coverages, favorable sites are determined by the electron counting model. In addition, to investigate the dynamical behavior of cation adatoms at finite temperatures, we perform Monte Carlo simulations based on the ab initio results. The simulations show that the dynamical behavior of cation adatoms also strongly depends on the Ga coverage. At the initial stage of growth, randomly impinged cation adatoms predominantly occupy the lattice sites in the As-dimer region. While, as the Ga coverage increases, favorable lattice sites change from the lattice sites in the As-dimer region to those in the missing-dimer region, reflecting the coverage dependence of migration potential obtained by the ab initio calculations.

Transmission electron microscopy observation of GaAs/Al0.3Ga0.7As T-shaped quantum well structure fabricated by glancing angle molecular beam epitaxy on GaAs(100) reverse-mesa etched substrates

GaAs/Al0.3Ga0.7As multi-layer structures were grown on GaAs (100) reverse-mesa etched substrates by glancing angle molecular beam epitaxy (GA-MBE). A(111)B facet was formed as a side-facet. Surface migration of Ga and Al atoms from the (100) flat region to the (111)B side-facet region has been investigated to fabricate T-shaped GaAs/AlGaAs quantum wells (QWs) under the condition that Ga and Al atoms impinge only on the (100) flat region and do not impinge on the (111)B side-facet. Observation of T-shaped GaAs/AlGaAs quantum wires (QWRs) by cross-sectional transmission electron microscopy (TEM) revealed that there is no migration of Al atoms from the (100) to the (111)B facet region at a substrate temperature (Ts) as high as 630°C, under a VIII ratio of 28 (in pressure ratio). On the other hand, very thin GaAs epitaxial layers grown on the (111)B side-facet region owing to the Ga migration were observed for substrate temperatures of 600 and 630°C. It was found that the mass flow of Ga atoms from the (100) region to the (111)B side-facet region increases, with the thermal activation energy of 2.0 eV, as the substrate temperature increases from 570 to 630°C. The GA-MBE growth on a reverse-mesa etched GaAs substrate at a low temperature 570°C or lower is desirable to fabricate a nm-scale GaAs/AlGaAs QWR structure with nm-scale precision.

Transmission electron microscopy observation of GaAs/Al 0.3Ga 0.7As T-shaped quantum well structure fabricated by glancing angle molecular beam epitaxy on GaAs(100) reverse-mesa etched substrates

GaAs/Al 0.3Ga 0.7As multi-layer structures were grown on GaAs (100) reverse-mesa etched substrates by glancing angle molecular beam epitaxy (GA-MBE). A(111)B facet was formed as a side-facet. Surface migration of Ga and Al atoms from the (100) flat region to the (111)B side-facet region has been investigated to fabricate T-shaped GaAs/AlGaAs quantum wells (QWs) under the condition that Ga and Al atoms impinge only on the (100) flat region and do not impinge on the (111)B side-facet. Observation of T-shaped GaAs/AlGaAs quantum wires (QWRs) by cross-sectional transmission electron microscopy (TEM) revealed that there is no migration of Al atoms from the (100) to the (111)B facet region at a substrate temperature ( T s ) as high as 630°C, under a V III ratio of 28 (in pressure ratio). On the other hand, very thin GaAs epitaxial layers grown on the (111)B side-facet region owing to the Ga migration were observed for substrate temperatures of 600 and 630°C. It was found that the mass flow of Ga atoms from the (100) region to the (111)B side-facet region increases, with the thermal activation energy of 2.0 eV, as the substrate temperature increases from 570 to 630°C. The GA-MBE growth on a reverse-mesa etched GaAs substrate at a low temperature 570°C or lower is desirable to fabricate a nm-scale GaAs/AlGaAs QWR structure with nm-scale precision.

Scanning tunneling microscopy of the GaAs(001) surface morphology prepared by migration enhanced epitaxy

Employing field ion-scanning tunneling microscopy (FI-STM), we have studied the GaAs(001) surface prepared by the migration enhanced epitaxy (MEE) technique in a wide range of growth temperatures. The STM images clearly show the advantageous effect of enhanced Ga migration of MEE on the surface morphology of the GaAs(001) surface.

Computer simulation of Ga migration in molecular-beam epitaxial growth of GaAs

Some quantitative features of surface Ga atom migration (migration length, migration time, hopping frequency), which have been obtained via Monte Carlo simulation of molecular-beam epitaxial growth of GaAs on flat and stepped (001) surfaces under different growth conditions are presented in this paper. The evolution of these features during growth time and the relations between migration, surface roughness and bulk defect concentration are demonstrated.

GaAs/AlGaAs Quantum Wells Grown by Low-Temperature Molecular Beam Epitaxy with Atomic Hydrogen Irradiation

Optical properties of GaAs/AlGaAs quantum wells grown by molecular beam epitaxy with atomic hydrogen irradiation have been investigated by photoluminescence (PL) method. Narrow PL linewidths have been obtained for samples grown at a low-temperature of 330° C with atomic hydrogen and without any growth interruptions. This is due to enhanced Ga migration on GaAs (100) surface under the presence of atomic hydrogen thereby resulting in atomically smooth interfaces. It has also been shown that the PL intensities can be improved as the non-radiative recombination centers could be effectively passivated by the hydrogen atoms.

The Role of Vertical Exchange in the Growth of GaAs/AlAs Lateral and Vertical Superlattices

AbstractWe report on the formation of lateral superlattices in short period vertical GaAs/AlAs superlattices. To explain the observed self-organized phase separation, we propose a model of vertical intermixing, driven by the exchange of Ga on the surface with impinging Al atoms. The model correctly describes the formation of lateral superlattices for both integer and fractional monolayer deposition. It also predicts a far-reaching intermixing at GaAs-AlAs interfaces. Insitu RHEED studies of the initial growth stage of both GaAs-AlAs and AlAs-GaAs interfaces support the assumption of an asymmetric exchange at the growing surface and confirm the longrange Ga migration predicted by the model.

Nucleation and growth of gallium arsenide on silicon

We have studied the early stages of growth of GaAs on Si (001), misoriented by 4° towards [110], by migration-enhanced epitaxy (MEE) in a molecular beam epitaxy (MBE) system. We present results using in situ and ex situ analyses, reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD) and X-ray double crystal diffractometry (DCD) performed at each stage of growth. We show that the nucleation by MEE induces a surface roughness decreasing as the layer becomes thicker. XPD experiments at the onset of the growth show a stoichiometric GaAs without antiphase domains. The relaxation of stress for the layers deposited at low temperature (300°C) occurs via partial dislocation migration developing between them, stacking faults and microtwins. The post annealing of films with thicknesses less than 60 nm drives the formation of three-dimensional islands on the silicon surface. These islands develop (114) facets in the [ 1 1 ¯ 0 ] direction where the Ga migration is greater. The height over the base ratio of these islands is uniform and from Bauer's relation, we calculate the interface energy which can be correlated to the strain energy due to the dislocations near the interface. There are two reasons for the restruction of the surface. First, from a thermodynamical approach including surfaces and interface energies of the GaAs/Si system, we can demonstrate that the growth of GaAs/Si is a three-dimensional Volmer-Weber growth. Second, the bulk energy due to the compressive strain of a continuous pseudomorphic GaAs film is higher than that of an islanding GaAs where relaxation from the free surfaces of the islands occurs. The post annealing of films with thickness higher than 60 nm has a smoothing effect and can give a perfect two-dimensional (001) surface. Growth at higher temperatures (580°C) suppresses plane defects, creates a Lomer dislocation network at the interface and induces the 60° dislocations in the bulk. We perform X-ray DCD experiments on 90 nm thick layers. We observe, before annealing a compressive biaxial stress of about −1.9×10 8Pa which is the driving force for the elimination of the plane defects.

Growth modes and reduction of hillock density in migration enhanced epitaxy of GaAs(111)B

Abstract The growth mode in migration enhanced epitaxy (MEE) of GaAs(111)B on just (111) oriented substrate was revealed by 0.5 monolayer Ga irradiation per cycle. The RHEED specular beam intensity oscillation observed indicates that the step flow mode tends to occur under growth conditions with enhanced Ga migration length. The realization of step flow mode is effective in reducing the hillock density. The hillock density was reduced by 4 orders of magnitude using MEE with the period where Ga and As 4 beams were cut.

The application of RHEED intensity effects to interrupted growth and interface formation during MBE growth of GaAs/(Al, Ga)As structures

There is a diffraction-induced phase effect in the RHEED intensity oscillation technique used in MBE, whereby intensity maxima only correspond to monolayer completion for very restricted conditions. In particular, the angle of incidence of the primary beam is extremely critical. The effect occurs because the total intensity at the measured position of the specular beam is always derived from at least two different diffraction processes, which do not have the same phase relation to monolayer formation. It can be accomodated either by a systematic series of measurements to establish an empirical relationship between incidence angle and phase, or by Fourier transform techniques. Unless full account is taken of this purely diffraction-induced effect, very misleading results can be obtained for the time constants of the recovery period following cessation of growth and this is illustrated for GaAs. The effect also has important implications for the growth-interrupt technique. In addition, it is shown that for heterojunction formation in the GaAs/(Al, Ga)As system, adatom (Ga and Al) migration lengths are of greater importance than the position in the monolayer at which the composition is changed, and that RHEED can provide only limited information on the interface structure.

Magnetic properties of laser annealed garnet films as determined by FMR and bubble statics

The effects of high power laser irradiation upon the magnetic properties of Ga-substituted garnet films were studied by ferromagnetic resonance (FMR) and bubble statics. FMR measurements showed that for a certain range of laser power, annealing changes the magnetic properties in a layer located at the free surface, the thickness of which increases with increasing laser power P. For a sufficiently high P the whole film is transformed, and for even higher P the film is melted in a thin surface layer. The change of collapse field and stripe width as a function of P is fairly well explained by this model. The modification of the magnetic properties can be well explained by a migration of about 3% of Ga <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> from the tetra- to the octahedrical site, but surprisingly the amount of Ga migration is independent of the laser power.

GaAs/Ta2O5 and GaAs/Al2O3 interface structures

Tantalum and aluminum oxides were grown on n-type GaAs by thermal oxidation of Ta and by anodic oxidation of vacuum deposited Ta and Al films. The insulating properties of these films are better than those of the native oxide films on GaAs. The interface properties of the GaAs/Al2O3 interface are better than those of GaAs/Ta2O5 structures obtained so far; this is due to the fact that the interaction between GaAs and Al and/or Al2O3 is less than that occurring during oxidation of Ta on GaAs. Even a relatively small film–substrate interaction can deteriorate the properties of the oxide film and the interface. However, some controlled interaction is desired to obtain good interface properties but it is difficult to achieve. Preliminary C–V measurements at 1 MHz indicate that the properties of the GaAs/Al2O3 interface depend on the original GaAs/Al interface. If a thin native oxide is present, then the accumulation capacitance is lower than the insulator capacitance; this effect is probably due to accumulation of arsenic at the interface. From the capatitance minimum it was also found that the effective donor density at the GaAs surface decreased by about one order of magnitude; this is probably due to Ga migration during the oxidation. However, an important observation was that an accumulation capacitance coinciding with the insulator capacitance could be obtained if a preoxidation heat treatment of the GaAs/Al interface was employed. This allows the incroporation of the room ambient oxide film on GaAs into Al2O3 film; i.e., a new interface is established which has promising properties, but the essential features of the Al2O3 films are maintained.

Metallurgical Structure of Be‐Au and Si‐Au Ohmic Contacts to GaP

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