Step-bunched Surface Research Articles

Nature effect of the gas during high temperature treatments of 4H-SiC substrates

4H-SiC seeds have been treated at high temperatures (1650–1900 °C) under Ar or N 2 in a sublimation like graphite crucible with SiC powder at the hotter part. It was found that the surface morphology is significantly altered by the nature of the atmosphere. Surfaces without step bunching under 1 bar of N 2 appear for low temperature range (≤1700 °C) whereas at higher temperature or lower pressure a step bunch morphology appears. Ar always gives step bunched surfaces but with more regular and parallel steps. Thermodynamical calculations performed on the SiCN (Ar) system show that N 2 plays an important role on the gas phase chemistry of decomposition of SiC by forming gaseous species of nitrides. The theoretical results correlate well with the observations of surface morphology and graphitisation of the SiC powder.

Growth of spatially ordered InAs quantum dots on step-bunched vicinal GaAs (1 0 0) substrates

Spatially ordered growth of InAs quantum dots (QD) was demonstrated via metalorganic vapour phase epitaxy on step-bunched vicinal GaAs substrates. Regular terraces of step-bunched surfaces were achieved by growing on GaAs (1 0 0) substrates off-oriented 2°, 4° and 6° towards the 〈1 1 0〉 and 〈1 1 1〉 directions, thus serving as in situ templates for the growth of the QD layer. Multilayer stacks of strain-aligned QDs were successfully grown to improve the size homogeneity and therefore optoelectronic properties. Furthermore, the possibility of fabricating laterally ordered quantum wires (QWR) in the same manner was shown.

Effect of V/III ratio and PH 3 annealing on InAs dots grown by MOVPE on InP(0 0 1) step-bunched surfaces

The self-organisation of InAs nanostructures grown by MOVPE on 0.2° misoriented (0 0 1)InP substrates has been studied as a function of growth parameters. The surface morphology of thin layers has been observed by atomic force microscopy for nominal thicknesses of about 3 and 6 ML. The InAs material was deposited at 500°C with V/III flux ratios ranging from 50 to 240. We observed that InAs dots were self-aligned along the bunched steps of the surface forming strips of several micrometers length, regularly spaced every 3–4 terraces. The surface distribution of the strips could be varied with the growth conditions, i.e. V/III flux ratio. The influence of the cooling atmosphere was also studied for samples cooled down from 500°C to 350°C during 6 min under either arsine or phosphine flux. This step has been found to largely determine the surface morphology of the InAs nanostructures. Under phosphine atmosphere, we will take into account the strong As→P exchange which takes place.

Correlation between surface step structure and phase separation in epitaxial GaInAsSb

A strong correlation between the surface step structure and phase separation in metastable GaInAsSb epitaxial layers grown by organometallic vapor-phase epitaxy is reported. The full width at half maximum (FWHM) of the 4 K photoluminescence (PL) peak energy is used as a semiquantitative measure of the degree of phase separation: FWHM values increase with phase separation. The step structure of GaInAsSb grown at 525 °C is vicinal, while it is step bunched for layers grown at 575 °C. The corresponding 4 K PL FWHM data indicate smaller FWHM values for layers grown at the lower temperature, and suggest a lower degree of phase separation. Extreme PL broadening is associated with an aperiodic surface structure. Furthermore, the PL FWHM values decrease when the growth rate increases from 1.2 to 5 μm/h. It is proposed that longer adatom lifetimes, which are associated with longer terrace lengths of a step-bunched surface compared to a vicinal one, allow more time for the adatoms to reach equilibrium and thus phase separate.

Properties of spin-valve structures deposited on step-bunched vicinal surfaces

We have investigated the properties of Co/Cu/FeNi spin-valve structures deposited on step-bunched vicinal Si(1 1 1) substrates. We first discuss the in-plane uniaxial magnetic anisotropy induced in each magnetic layer by the initial surface corrugation. A ferromagnetic coupling of the two magnetizations is found, which is analyzed as a function of the spacer-layer thickness. Giant magnetoresistance (GMR) of the spin-valve structures is then investigated as a function of the in-plane current direction. Because of the initial corrugation, we observe current perpendicular to the plane (CPP) contributions to the GMR of the spin-valve structures. These CPP contributions can lead to an increase of the absolute GMR by up to 70% at room temperature, when the current is applied perpendicular to the steps direction.

Material properties of InAlAs layers grown by MBE on vicinal (1 1 1)B InP substrates

InAlAs layers and InGaAs/InAlAs quantum well heterostructures, grown by molecular beam epitaxy on vicinal (1 1 1)B InP substrates, exhibited extensive surface step bunching, compositional inhomogeneities and degraded crystalline quality, with variations depending on the exact misfit strain and growth conditions. Silicon and beryllium doping of (1 1 1) InAlAs behaved similarly to the (1 0 0) case but a different predominant deep level was detected in n-type (1 1 1) material.

Properties of In0.52Al0.48As and In0.53Ga0.47As/In0.52Al0.48As Quantum Well Structures Grown on (111)B InP Substrates by Molecular Beam Epitaxy

In0.52Al0.48As and In0.53Ga0.47As/In0.52Al0.48As quantum well (QW) structures were grown on (111)B InP substrates by molecular beam epitaxy, and their optical and electrical properties were studied. The photoluminescence (PL) spectrum and electron mobility of the InAlAs layers grown on the (111)B InP substrates were found to have a remarkable dependence on the V/III ratio and the growth temperature. It was found that the PL peak energy of the InGaAs/InAlAs QWs grown under optimized conditions on the (111)B InP substrates shows a remarkable red shift compared to that grown on the (100) InP substrates. An atomic force microscopy observation revealed that the observed red shift for the (111)B QW is induced by a large surface step-bunching.

Effects of Step-bunching on Ordered Domain Structure in GaInP.

The effects of step bunching on a CuPt-B ordered structure in Ga0.5In0.5P grown on (11n) B and (11n) A (n=8, 13) GaAs substrates by metalorganic vapor phase epitaxy (MOVPE) are described. In the GaInP layers, a CuPt-B ordered structure in the [111] B and [111] B directions are formed. The epitaxial growth surfaces are often undulated due to the atomic-step bunching. Experimental results for a low V/III ratio of 55 shows that a disordered region of the CuPt-B or-dered structure is formed at the step-bunched surface. Also, anti-phase domain boundaries of the CuPt-B ordered structure are formed on terraced surfaces for V/III ratio of 1500. The ordered structure domain formation is thought to depend on the growth on surface facets which are formed through P-dimer distribution there.

Correlation Between GaInAsSb Surface Step Structure and Phase Separation

AbstractA strong correlation between the surface step structure and phase separation in metastable GaInAsSb epitaxial layers grown by organometallic vapor phase epitaxy has been identified. The full width at half maximum (FWHM) of 4-K photoluminescence (PL) peak energy is used as a semi-quantitative measure of the degree of phase separation. The step structure of GaInAsSb grown at 525 °C is vicinal, while it is step-bunched for layers grown at 575 °C. The corresponding 4-K PL FWHM data indicate that the degree of phase separation is minimized when the layers aregrown at the lower growth temperature. It is proposed that the longer terrace lengths of a step-bunched surface are associated with a longer adatom lifetime compared to a vicinal surface, and thus the adatoms have more time to cluster and phase separate, which is the preferred equilibrium state. Increasing the growth rate, which reduces the adatom lifetime, also reduces the PL FWHM, and thus, the degree of phase separation.

Effects of step bunching on CuPt-B ordered structures in Ga 0.5In 0.5P grown by MOVPE

The effects of step bunching on a CuPt-B ordered structure in Ga 0.5In 0.5P grown on (1̄18)B and (1̄ 1 13)B substrates by metalorganic vapor phase epitaxy (MOVPE) were studied. The growth was carried out at 660°C with V/III ratios of 55 and 1500. In the GaInP layers, a CuPt-B ordered structure in the [1̄11]B direction was dominantly formed. The epitaxial growth surfaces were undulated caused by the atomic-step bunching. When GaInP was grown at a V/III ratio of 55, a disordered region of CuPt-B ordered structure was formed at the step-bunched surface. At a V/III ratio of 1500, anti-phase domain boundaries of a CuPt-B ordered structure were formed on terraced surfaces, resulting in stretched terrace surfaces. The ordered structure is expected to be caused by the epitaxial growth on the undulated surface which has a distribution of P-dimer coverage. From a transmission electron microscopy lattice image, CuPt-B ordered-structure formation at the lower V/III ratio was weaker. The difference in the ordered-structure formation between two V/III ratios indicates that P-dimer coverage of surface reconstructions is a factor that affects the ordering formation intensity.

Temperature effect on surface flatness of molecular beam epitaxy homoepitaxial layers grown on nominal and vicinal (111)B GaAs substrates

For the first time a comparison is made between the surface characteristics of layers simultaneously grown on nominal and on vicinal (111)B substrates under the 1×1 reconstruction regime which provides flat surfaces for both orientations. The influence of growth temperatures, 600 and 700 °C, on surface characteristics is thoroughly reviewed based on an atomic force microscopy study. This study yields additional insight into results already reported on growth mechanisms occurring along these two orientations for the 19×19 growth regime. For the nominal (111)B layers, it will be shown that monoatomic steps at the surfaces define large atomically flat plateaus at both temperatures. At 600 °C, growth will be shown to proceed mainly through the development of two-dimensional nuclei, which are limited in size to a critical value and can coalesce by a proximity effect. These nuclei will be shown to be much smaller at 700 °C, thereby turning the step flow into the main mechanism occurring at that temperature. For the vicinal orientation, monoatomic-stepped and step-bunched surfaces grown at 600 and 700 °C, respectively, will be obtained under our 1×1 growth conditions, the same as in the 19×19 regime.

Surface step bunching and crystal defects in InAlAs films grown by molecular beam epitaxy on (111)B InP substrates

The surface morphology and crystal structure of InAlAs films grown by molecular beam epitaxy on (111)B InP substrates misoriented 1° toward [2̄11] have been investigated. Combined plane view transmission electron microscopy and atomic force microscopy observations have revealed spectacular terracelike topographies, induced by surface step bunching during the growth. Furthermore, cross section transmission electron microscopy analysis has shown the presence of threading dislocations, related to the giant steps, as well as strain inhomogeneities attributed to composition modulation. We have also demonstrated the potential use of the giant steps for local deposition of InAs.