Lateral Oxidation Research Articles

Lateral oxidation kinetics of AlAsSb and related alloys lattice matched to InP

The lateral oxidation kinetics of AlAs0.56Sb0.44 on InP substrates have been investigated to understand the antimony segregation process during oxidation. Oxidation layers were grown between GaAsSb buffer and cap layers on InP substrates by molecular beam epitaxy. Oxidation temperatures between 325 and 500 °C were investigated for AlAsSb layer thicknesses between 100 and 2000 Å. At low oxidation temperatures (Tox⩽400 °C), the process is reaction limited with a linear dependence of oxidation depth on time. At intermediate oxidation temperatures (400<Tox<450 °C), the oxidation process becomes diffusion limited. At high oxidation temperatures, the oxidation process is termed self-limiting since at 500 °C the process stops entirely after oxidation times on the order of 5 min and distances of 40 μm. It is shown that the antimony float layer lags the oxidation front by a temperature-dependent distance, which suggests that the antimony may change the structure of the oxide at the front and cause self-limiting behavior. The oxidation kinetics of AlxGa1−xAsSb and AlxIn1−xAsSb have also been investigated. Antimony segregation is not suppressed during oxidation of Ga-containing layers and AlInAsSb quaternary alloys do not oxidize laterally at measurable rates in the range 400–525 °C. SiNx cap layers deposited after growth and before oxidation do not affect the Sb segregation or oxidation rate, but do smooth the cap surface by preventing uneven Sb metal segregation to the cap/oxide interface.

Microstructure and wet oxidation of low-temperature-grown amorphous (Al/Ga,As)

Amorphous and polycrystalline compounds of (Ga,As) and (Al,As) grown at very low temperatures by molecular-beam epitaxy are characterized. The ultimate microstructure and the amount of excess arsenic incorporated in the (Ga,As) or (Al,As) layers are found to depend on the arsenic overpressure during the low-temperature growth. With lower arsenic overpressure, a polycrystalline structure prevails and less excess arsenic is observed inside the layer. In contrast, a high incorporation of excess arsenic achieved by high-arsenic overpressures leads to the formation of amorphous films. Upon wet oxidation, the lateral oxidation rate of (Al,As) is found to depend on the crystallinity of the (Al,As) layer and the amount of excess arsenic. During the same process, recrystallization proceeds in the (Ga,As) layer.

Strain relaxation and defect reduction in InxGa1−xAs/GaAs by lateral oxidation of an underlying AlGaAs layer

The strain relaxation in In0.25Ga0.75As and In0.4Ga0.6As grown on GaAs substrates at low temperature has been studied before and after laterally oxidizing an underlying Al0.98Ga0.02As layer. The relaxation as a function of layer thickness has been measured by cross-sectional transmission electron microscopy and x-ray analysis. It is found that oxidation of the Al0.98Ga0.02As layer improves the relaxation of the strained InxGa1−xAs layer. Moreover, the interfacial misfit dislocations have been removed, and the threading dislocation density has decreased approximately by one order of magnitude after oxidation.

Increased lateral oxidation rates of AllnAs on InP using short-period superlattices

We present greatly increased lateral oxidation rates for AlInAs grown as a short-period superlattice of InAs and AlAs compared to the analog alloy. The tensile strain in the AlAs layers is balanced by the compressive strain in the InAs layers, creating a strain-compensated alloy lattice-matched to InP. Oxidation layers with superlattice periods up to 40 A and cladded by lattice-matched InGaAs layers were grown on InP substrates and laterally oxidized at temperatures ranging from 450°C to 525°C. The oxidation depth for a given time and temperature was seen to increase with superlattice period, allowing increased oxidation depths or reduced oxidation temperatures compared to the analog alloy. Oxidized layers were examined with transmission electron microscopy and were found to retain some of the superlattice structure.

Reactive removal of misfit dislocations from InGaAs on GaAs by lateral oxidation

Layers of hypercritical thickness InGaAs/AlAs/GaAs have been shown to strain relax when the underlying AlAs layer is laterally oxidized. InxGa1−xAs layers of composition 0.2⩽x⩽0.4 have been investigated at thicknesses in the range 5–20 times the Matthews–Blakeslee critical thickness hc. The amount of strain relieved does not depend on the InGaAs-layer thickness, the initial strain state, or the composition of the material, but it does strongly depend on the oxidation temperature. The structure of the strain-relaxed InGaAs layer has been investigated using plan-view transmission electron microscopy. It is shown that the misfit dislocation density in In0.3Ga0.7As grown to 15 times the critical thickness (660 Å) has been reduced by two orders of magnitude after lateral oxidation. It is proposed that this is due to interfacial oxidation, which consumes the misfit dislocation cores.

Improvement of the interface quality during thermal oxidation of Al0.98Ga0.02As layers due to the presence of low-temperature-grown GaAs

The role of a low-temperature-grown GaAs (LT GaAs) layer on the lateral oxidation of an Al0.98Ga0.02As/GaAs layer structure has been studied by transmission electron microscopy. Results show that structures incorporating LT GaAs develop better quality oxide/GaAs interfaces compared to reference samples without LT GaAs. While the latter have As accumulation in the vicinity of these interfaces, the structures with LT layers display sharper oxide–GaAs interfaces with a reduced concentration of As. These results are explained in terms of the high Ga vacancy concentration in the LT GaAs and the possible influence of those vacancies in enhancing As diffusion away from the oxide–semiconductor interface.

Strain relaxation of InGaAs by lateral oxidation of AlAs

Strained InGaAs layers grown on AlAs/GaAs have been shown to relax when the AlAs is laterally oxidized. A detailed microscopy study is reported of the InGaAs structure before and after oxidation. Plan-view transmission electron microscopy (TEM) reveals that the misfit dislocation density in the InGaAs/AlAs interface is reduced by 30 times after lateral oxidation. The mechanism proposed for this reduction is the oxidation of the InGaAs interface, including the core regions of the misfit dislocations. Threading dislocation densities in the InGaAs are not measurable by TEM either before or after oxidation for optimized In0.20Ga0.80As and In0.30Ga0.70As layers. Two possible strain relaxation mechanisms are examined: (i) the compressive strain in the InGaAs is relaxed by a tensile stress developing during oxidation in the cap layer; and (ii) the stress is relaxed by the threading dislocation motion in the cap layer during oxidation. The removal of misfit dislocations reduces the likelihood of threading dislocation blocking, which prevents threading dislocations from moving across misfit dislocations.

Strain relaxation in InGaAs lattice engineered substrates

Strain relaxation of hypercritical thickness InxGa1−xAs layers has been observed during lateral oxidation of underlying AlAs layers. Strain relaxation of InxGa1−xAs layers was studied as a function of indium composition and the AlAs oxidation temperature. It is proposed that the enhanced strain relaxation is due to two factors. The first is enhanced motion of threading dislocations due to stresses generated during the lateral oxidation process. The second is the porous nature of the InxGa1−xAs/Al2O3 interface that minimizes the interaction of threading dislocations with existing misfit dislocation segments. The extent of strain relaxation increases with increasing oxidation temperature, whereas the efficiency of strain relaxation was found to decrease with increasing indium composition. The efficiency of strain relaxation upon oxidation can be improved by reducing the misfit dislocation density at the InxGa1−xAs/AlAs interface prior to oxidation and by changing the nature of the InxGa1−xAs/Al2O3 interface.

A study on initial oxidation of Si(100)-2×1 surfaces by coaxial impact collision ion scattering spectroscopy

By using coaxial impact collision ion scattering spectroscopy (CAICISS), we have studied the structural changes extending over the first several layers from oxidized Si(100)-2×1 surfaces. A simulation of ion scattering was used for the analysis of CAICISS spectra. At an oxide thickness of 0.87 monolayer (ML), a peak in the CAICISS spectrum, which originates from the first-layer Si atoms, disappears. This finding indicates that the first-layer Si atoms move due to the structural relaxation accompanying the adsorption of oxygen atoms onto SiSi dimer sites and back-bond sites. In the CAICISS spectrum for 1.1-ML-thick oxide, some peaks appear due to the oxygen adsorption onto SiSi bonds between the second- and the third-layer Si atoms. From this result, we can deduce that the inward oxidation occurs before the lateral oxidation finishes.

Effect of Sb composition on lateral oxidation rates in AlAs1−xSbx

We demonstrate the effect of antimony (Sb) composition on the oxidation mechanism of AlAs1−xSbx (x<0.21) layers on GaAs substrates. It has been demonstrated that addition of a group-III element like Ga to AlAs slows the rate of oxidation. In contrast, addition of a group-V element like Sb to AlAs changes the oxidation mechanism in more than one way. The oxidation rate increases with Sb addition, and the oxidation reaction changes from a diffusion-limited mechanism to a reaction-rate-limited mechanism at higher oxidation temperatures. This is attributed to the increase in the permeability of the oxide. Nonuniform segregation of Sb is observed upon oxidation. The activation energy of the oxidation reaction-rate constant initially decreases with the Sb composition upto 10%, further Sb addition increases the activation energy.

Optimisation of λ= 850 nm hybrid-mirror vertical-cavity surface-emitting laser with 37 μA threshold current

A vertical-cavity surface-emitting laser (VCSEL) structure is optimised for low threshold current. The cavity comprises a GaAlAs bottom DBR with a dielectric SiO/sub 2/-Si/sub 3/N/sub 4/ output mirror on top. The asymmetrical active region consists of GaAs-QWs, emitting at 850 nm. Selective lateral oxidation is applied to the p-side AlAs layer for current confinement. With the realised structure, a record low threshold current for selectively oxidised, hybrid Ga(Al)As lasers of I/sub th/=37 /spl mu/A at an oxide-aperture diameter of O/sub ap/=4.9 /spl mu/m is achieved. The observed multimode emission spectrum is analysed using a simplified theoretical model for the transverse-mode guiding, and first experimental results of a redesigned VCSEL structure with single-mode operation at similar aperture diameters are presented.

Lateral Wet Oxidation of AlAS Layer Grown on High Index GaAs Substrate by MBE

Lateral Wet Oxidation of AlAS Layer Grown on High Index GaAs Substrate by MBE

Influence of Low Temperature-Grown GaAs on Lateral Thermal Oxidation of Al0.98Ga0.02As

AbstractThe lateral thermal oxidation process of Al0.98Ga0.02As layers has been studied by transmission electron microscopy. Growing a low-temperature GaAs layer below the Al0.98Ga0.02As has been shown to result in better quality of the oxide/GaAs interfaces compared to reference samples. While the later have As precipitation above and below the oxide layer and roughness and voids at the oxide/GaAs interface, the structures with low-temperature have less As precipitation and develop interfaces without voids. These results are explained in terms of the diffusion of the As toward the low temperature layer. The effect of the addition of a Si02 cap layer is also discussed.

Strain relaxation of InxGa1−xAs during lateral oxidation of underlying AlAs layers

Strain relaxation of hypercritical thickness InxGa1−xAs layers has been observed during lateral oxidation of underlying AlAs layers. Strain relaxation of InxGa1−xAs layers was studied as a function of indium composition and the AlAs oxidation temperature. It is proposed that the enhanced strain relaxation is due to two factors. The first is enhanced motion of threading dislocations due to stresses generated during the lateral oxidation process. The second is the porous nature of the InxGa1−xAs/Al2O3 interface that minimizes the interaction of threading dislocations with existing misfit dislocation segments. The extent of strain relaxation increases with increasing oxidation temperature, whereas the efficiency of strain relaxation was found to decrease with increasing indium composition.

Room-temperature pulsed operation of triple-quantum-well GaInNAs lasers grown on misoriented GaAs substrates by MOCVD

The lasing operation of three-quantum-well GaInNAs stripe geometry lasers grown by MOCVD on 0/spl deg/ and 6/spl deg/ misoriented [100] GaAs substrates, respectively, have been demonstrated and their performance is compared for the first time. Both devices achieved room temperature, pulsed lasing operation at an emission wavelength of 1.17 μm, with a threshold current density of 667 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for lasers grown on 6/spl deg/ misoriented substrates, and 1 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for lasers grown on 0/spl deg/ misoriented substrates. The threshold for the lasers grown on 6/spl deg/ misoriented substrates compares favorably with the best results for GaInNAs lasers. Lasers with narrower stripe width and a planar geometry have also been demonstrated by the use of lateral selective wet oxidation for current confinement, with a threshold current density of 800 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for 25-μm-wide devices.

Fabrication of a substrate-independent aluminum oxide-GaAs distributed Bragg reflector

We propose a method for forming a top distributed Bragg reflector (DBR) during very-low temperature (VLT) molecular-beam epitaxy (MBE) growth that is independent of the substrate being used. By varying the arsenic overpressure during VLT MBE, it was determined by Auger electron spectroscopy and cross-section transmission electron microscopy that alternating layers of polycrystalline GaAs and amorphous (Al,As) can be deposited. Because these layers are not single crystal, they can be grown on any host lattice. After lateral wet oxidation, the polycrystalline GaAs does not undergo any significant changes; whereas the amorphous (Al,As) becomes an amorphous aluminum oxide. An index step of Δn=1.88 is realized between these two layers which makes it possible to fabricate a high efficiency DBR with very few polycrystal-GaAs/amorphous-Al-oxide pairs on GaAs-, GaP-, or InP-based materials. Using reflectivity measurements, we demonstrate a five pair GaAs/AlAs-based DBR grown on an InP substrate that reflects wavelengths between 1.4 and 2.3 μm up to 95%.

Intermixing-induced resonance shift in GaAs/AlxOy distributed Bragg resonators

The effect of ion implantation-induced interdiffusion on the resonant wavelength of GaAs/AlxOy distributed Bragg reflectors is investigated. As interdiffusion becomes stronger, the resonant wavelength is seen to redshift. Shifts of more than 60 nm could be achieved for center wavelengths around 800 nm. A model is proposed to explain this behavior. This model agrees well with previous lateral oxidation studies.

Lattice mismatched molecular beam epitaxy on compliant GaAs/Al xO y/GaAs substrates produced by lateral wet oxidation

Large-area 16 nm GaAs–Al x O y –GaAs substrates were formed by lateral wet oxidation of patterned GaAs/Al 0.97Ga 0.03As/GaAs heterostructures. Wet digital etching followed by in situ dry iodine etching was used to prepare thin GaAs layers intended to serve as compliant substrates. Relaxed layers of In 0.15Ga 0.85As were deposited on the compliant substrates and on bulk GaAs substrates using In 0.15Ga 0.85As and In 0.15Al 0.85As nucleation layers. The In 0.15Ga 0.85As epitaxial layers grown on the compliant substrates using an In 0.15Al 0.85As nucleation layer showed no crosshatch pattern by Nomarski microscopy and had X-ray rocking curve linewidths that were narrower than those grown on bulk GaAs or on samples with In 0.15Ga 0.85As nucleation layers.

AlAs oxidation process in GaAs/AlGaAs/AlAs heterostructures grown by molecular beam epitaxy on GaAs (n11)A substrates

The lateral oxidation of AlAs layers grown on GaAs (100), (110) and (n11)A-oriented substrates (n=1, 2, 3, 4) was studied. The temperature dependence of the oxidation rate was measured between 390°C and 450°C. The oxidation rate is highly anisotropic and the anisotropy is related to the symmetry of the crystal structure. The oxidation process has an activation energy that depends on substrate orientation. The oxidation front line becomes irregular for temperatures higher than 450°C and the surface of the samples was degraded when the temperature exceeded 540°C. The time dependence of the oxidation rate was found to be similar to the Si oxidation process.

Lateral wet oxidation of AlAs layer in GaAs/AlAs heterostructures grown by MBE on GaAs (n 1 1)A substrates

Lateral sidewall oxidation of AlAs layers grown on GaAs (1 0 0) and ( n 1 1)A-oriented substrates ( n=1, 2, 3, or 4) within a heterostructure was studied. The oxidation rate for each type of substrate was measured between 390°C and 450°C. The oxidation data concerning the time dependence of the oxidation rate was very well fitted by a theoretical formula based on a Si oxidation model. The time dependence of oxidation rate followed a linear law for the initial process and successively a parabolic law. The oxidation rate is highly anisotropic which is related to the symmetry of the crystal structure. An oxidation model was proposed from the results of anisotropic activation energy of oxidation. Al x O y /GaAs DBRs (distributed Bragg reflectors) were fabricated and their reflection characteristics were measured. It is shown that the center wavelength of DBRs can be adjusted during the oxidation process.