Surface Lattice Constant Research Articles

Effect of surface tension on the growth mode of highly strained InGaAs on GaAs(100)

We have investigated the molecular beam epitaxy growth of highly strained InGaAs on GaAs(100) as a function of the anion to cation flux ratio. Using reflection high energy electron diffraction the evolution of the film morphology is monitored and the surface lattice constant is measured. It is found that the cation to anion flux ratio dramatically affects the growth mode. Under arsenic-rich conditions, growth is characterized by a two-dimensional (2D) to three-dimensional (3D) morphological transformation. However, for cation-stabilized conditions, 3D islanding is completely suppressed, and 2D planar growth is observed. We associate these differences in the growth mode with corresponding changes in the surface tension of the overlayer. A high surface tension stabilizes 2D growth. An analysis which relates surface tension to a critical thickness for the onset of coherent island formation supports this view.

In Situ Analysis of magnetic and Structural Properties of Epitaxial and Polycrystalline Ni80Fe20 Thin Films

ABSTRACTWe have investigated structural and magnetic properties of epitaxial Ni80Fe20 films grown on relaxed epitaxial Cu/Si (001) films. The crystallographic texture of these films was analyzed in situ by reflection high energy electron diffraction (RHEED), and ex situ by x-ray diffraction and cross-sectional transmission electron Microscopy (XTEM). In particular, RHEED intensities were recorded during epitaxial growth, and intensity profiles across Bragg rods were used to calculate the surface lattice constant, and hence, find the critical epitaxial thickness for which Ni80Fe20 grows pseudomorphically on Cu (100). XTEM analysis indicated that the epitaxial films had atomically-abrupt interfaces which was not the case for polycrystalline Cu and Ni80Fe20 film interfaces. The Magnetic properties of these epitaxial films were Measured in situ using Magneto-optic Kerr effect magnetometry and were compared with those of polycrystalline films grown on SiO2/Si. Large Hc (∼ 35 Oe) was observed for epitaxial Ni80Fe20 films less than 3.0 nm thick whereas for increasing thickness, Hc decreased approximately monotonically to a few Oersteds. Correlations were made between magnetic properties of these epitaxial films, the strain in the film and the interface roughness obtained from XTEM analysis.

Structural effects on the Pb(110) surface between 160 and 580 K observed by low-energy ion scattering.

Low-energy ion scattering and surface channeling is used to study structural changes of the Pb(110) surface. There is experimental evidence for point defects (vacancies) formed at 160 K, step and/or adatom formation at 270 K, and liquidlike surface phase at 580 K. In the neutral impact collision ion scattering spectrometry mode a thermal expansion of the [11\ifmmode\bar\else\textasciimacron\fi{}0] surface lattice constant of 10% at 400 K is measured.

Epitaxial growth of coronene and hexa-peri-benzocoronene on MoS 2(0001) and graphite (0001): a LEED study of molecular size effects

The molecular order in thin films of the aromatic hydrocarbon coronene, C 24H 12, and its larger homologue, hexa-peri-benzocoronene (“HBC”), C 42H 18, obtained by vacuum-deposition onto the (0001) cleavage plane of graphite and molybdenite, MoS 2, has been investigated by low-intensity low-energy electron diffraction (LEED). Under well-defined preparation conditions (low evaporation rates and moderate substrate temperatures), epitaxial films could be obtained; their epitaxial relations could be characterized by well-resolved LEED-patterns. For coverages in the monolayer region, commensurate graphite (0001)-p(√21 × √21 R ± 10.9°)-coronene and MoS 2(0001)-p(√13 × √13 R ± 13.9°)-coronene superstructures are formed, while hexa-peri-benzoncoronen forms commensurate graphite (0001)-p(√31 × √31 R ± 9.0°)-HBC and MoS 2(0001)-p(√21 × √21 R ± 10.9°)-HBC superstructures. From the dependence of the observed superstructure lattice constants on the size and shape of the adsorbed molecules relative to the respective substrate surface lattice constants a parallel orientation of the molecular planes with respect to the substrate surface can be inferred. It is shown that under the influence of the substrate the distances and azimuthal orientations of the molecules are determined by the closest packing permitted by their in-plane van der Waals radii. These structures are different from the respective bulk crystal structures. For the system coronene on MoS 2 a model of film formation is proposed.

The molecular beam epitaxy growth of InGaAs on GaAs(100) studied by i n s i t u scanning tunneling microscopy and reflection high-energy electron diffraction

I n situ real space images and reflection high energy electron diffraction (RHEED) data for the initial stages of molecular beam epitaxy growth of highly strained InxGa1−xAs (0.3≤x≤0.6) on GaAs(100) are reported. RHEED patterns, specular intensity oscillations, and surface lattice constant data are recorded simultaneously during growth. From the RHEED analysis we divide the film evolution into two stages, corresponding to a largely unrelaxed and relaxed surface lattice constant. Scanning tunneling microscopy images of the surface of the film as it appeared during growth reveal that the initial phase has a rippled morphology. With further growth the film evolves to a distinctive three-dimensional island microstructure. Both of these characteristic morphologies are interpreted as manifestations of strain in the growing film.

Multiple states in surface lattice constant behavior during the molecular beam epitaxial growth of ZnTe-ZnS strained layer superlattices

ZnTe-ZnS short period (∼30 Å) strained layer superlattices were grown by molecular beam epitaxy and the growth process was observed in situ by using reflection high energy electron diffraction (RHEED). Surface lattice constant (a∥) behavior shows a transition from one state in which a∥ is equal to that of the substrate to the other in which a∥ depends on the depositing constituents. RHEED patterns show the same streaks for both states except the difference in streak separation. Transmission electron micrograph reveals that the resultant multilayer structures of the two states have the same quality. The experimental data may suggest a possible assumption that there exists multiple states for two-dimensional growth of a strained system with substantial lattice mismatch.

Low energy ion scattering from the Ir(110) surface: A quest for the (1 × 2) structure

pA study of an Ir(110) surface by ion scattering spectrometry (ISS) and low energy electron diffraction (LEED) gives evidence for a (1 × 3) reconstruction of this surface. Two modes of ISS have been used, i.e. (i) variation of the azimuthal angle with fixed primary energy (2 keV Na +), fixed angle of incidence (5°) and fixed scattering angle (40°), and (ii) variation of the angle of incidence with fixed primary energy (2 keV Na +), fixed scattering angle (115°) at different crystallographic directions. Technique (i) allows the definition of the periodicity of the surface, technique (ii) leads to the estimate of surface lattice constants.

Growth of highly strained InGaAs on GaAs

The early stages of the molecular beam epitaxial growth of InxGa1−xAs on GaAs have been studied with reflection high-energy electron diffraction. Measurement of the in-plane surface lattice constant as a function of film thickness clearly showed a pseudomorphic to incoherent transition. Changes in the diffraction streaks indicated a corresponding two- to three-dimensional growth transition. The results are compared with various models of dislocation nucleation and good support is found for heterogeneous misfit accommodation by 60° dislocations.

Role of strain and growth conditions on the growth front profile of InxGa1−xAs on GaAs during the pseudomorphic growth regime

Theoretical and experimental studies are presented to understand the initial stages of growth of InGaAs on GaAs. Thermodynamic considerations show that, as strain increases, the free-energy minimum surface of the epilayer is not atomically flat, but three-dimensional in form. Since by altering growth conditions the strained epilayer can be grown near equilibrium or far from equilibrium, the effect of strain on growth modes can be studied. In situ reflection high-energy electron diffraction studies are carried out to study the growth modes and surface lattice spacing before the onset of dislocations. The surface lattice constant does not change abruptly from that of the substrate to that of the epilayer at the critical thickness, but changes monotonically. These observations are consistent with the simple thermodynamic considerations presented.

Sensitivity of helium diffraction to surface geometry.

In order to examine the sensitivity of He diffraction to surface structure, we develop explicit expressions for the principal features of the He scattering potential in terms of simple atomic and lattice parameters. A measure of structural sensitivity evolves naturally out of these relationships. In particular, the surface lattice constant emerges as the most crucial factor in determining when He diffraction will provide useful structural information. For surface lattice constants less than approx.5--6 A, the potential is often well approximated by a simple universal form, and has little sensitivity to the precise structure. The conclusions are illustrated with recent experimental results for selected metal, semiconductor, and adsorbate-covered surfaces.

EPITAXIAL GROWTH AND SOME PROPERTIES OF SAMARIUM CRYSTALS ON TUNGSTEN

Samarium epitaxial crystalline layers have been grown by vapor deposition onto either tungsten field-electron emitters or a single macro-cyrstal, (011)-oriented tungsten low-energy-electron diffraction specimen. Optimum growth occurred for substrate temperatures in the range of 650 - 750 K. The epitaxial relationship most commonly observed was (0001)Sm // (011)W with [1120]Sm // [001]W. The surface lattice constant of Sm(0001) appears to be a few per cent larger than the bulk value.

The characterization of some vanadium (100) surface using LEED and AES

Low energy electron diffraction (LEED) supplemented by Auger electron spectroscopy (AES) was used to investigate the properties of the (100) crystallographic face of vanadium metal. A (1×1) surface structure is characteristics of the clean metal surface. Sulfur segregates to the surface upon heating, forming first a C(2×2) structure and then a (√2×2√5) R27° structure. A kinematical analysis of the diffracted beam intensity vs voltage curves proved appropriate for the V(100)−(1×1) structure and yields a value of −9.9 V for the inner potential and a value of 3.02 Å for the surface lattice constant. Room temperature gas adsorption studies on the V(100)−(1×1) surface indicate that oxygen diffuses into the lattice and produces a disordered surface, while carbon monoxide either does not absorb or is desorbed under the influence of the electron beam.

A new method to determine surface lattice constants by a low energy electron diffraction system

The method makes efficient use of critical reflection of electron beams along a crystal surface and allowed sources of error such as improper alignment of the apparatus and warping of its components to be more easily minimized. Possible errors in this method were analysed and expressions for these errors were derived. The method was applied to measurements on the (001) surface of single crystal alpha iron. The distance d110 between atomic rows in the (110) direction was measured and the result d110=2.0263 AA was obtained with a total error of less than 0.18%.

A low energy electron diffraction technique for accurate absolute determination of surface lattice constants

Commercially available LEED equipment permits a relative accuracy of the order of a few per cent, in the absolute measurement of the lattice identity period parallel to the surface. A technique is described by which the accuracy is increased to 0.5 per cent. Precautions necessary for control of the residual fields and the geometry of the diffraction system are listed. The manipulation of the specimen is optically monitored and a built-in reference ensures the accuracy of the diffractogram evaluation on the polaroid film. The surface lattice constants are calculated by linear regression analysis. The technique has been applied to investigate the (111)-surface of silicon. The measured distance between atomic rows in the (111) plane amounted to 3.366± 0.012 Å which deviates significantly from the bulk value of 3.326 Å.