Formation Of Antiphase Boundaries Research Articles

Antiphase boundaries in GaAs layers on Si and Ge

A systematic study of the formation of antiphase boundaries and their properties in GaAs layers grown by metalorganic chemical vapor deposition (MOCVD) on Si 1 − x Ge x /Si (001) and Ge (001) substrates has been performed by chemical etching, scanning and transmission electron microscopy. The main attention is paid to the peculiarities of antiphase boundary propagation through the epitaxial layer and the interaction of boundaries with dislocations. Three various antiphase boundary configurations are determined. These experimentall observed configurations are in good agreement with those deduced from formal analysis of heterostructure crystal lattice. A connection of the boundary geometric configurations with the vapor phase composition oscillations in the crystallization zone is proposed. The stable [As]/[Ga] ratio in the vapor phase leads to the annihilation of inclined antiphase boundaries, whereas the concentration oscillations cause the formation of zigzag-like boundaries which propagate through the epitaxial layer.

Antiphase boundary formation energies in L10 and L11 superstructures

We described an approach suitable for obtaining an analytical expression for the energy of an antiphase boundary in an ordered alloy in the hard sphere model using pair interatomic interaction potentials. Depending on the orientation of the antiphase boundary, the crystal is subdivided into two-dimensional monatomic packings parallel to the defect, and its energy is determined by the sum of the energies of the two-dimensional packings. The results of the calculations for antiphase boundaries of different orientations are given in the form of matrix expressions for each of the considered superstructures.

FI-STM investigation of the Si(111) 2 × 1 cleaved surface

The (111) surfaces of Si obtained by cleaving in ultrahigh vacuum were investigated by a scanning tunneling microscope attached to a field ion microscope for in situ tip preparation. The (111) surfaces were invariably reconstructed to (2 x 1) with the [1 1 0] row direction in most cases normal to [11 2 ], the employed direction of cleavage. Other remarkable features were cleavage facets close to {779} besides the ordinary {111}, very different morphology of terrace edges dependent on the orientation with respect to the reconstruction row, and anti-phase boundaries of the 〈110〉 reconstructed rows exhibiting high chemical activity. Dislocations in Si were for the first time successfully imaged in STM as topological defects from which monatomic steps originate. The implications of the experimental findings were discussed in terms of atomistic models for the Si(111)2 × 1 superstructure and a dislocation mechanism for formation of anti-phase boundaries.

Energy of formation of anti-phase boundaries in a superstructure L12

A phenomenological model is developed which facilitates an analytical description of the energy of formation of anti-phase boundaries (APB) of arbitrary type and orientation for ordered alloys with L12 superstructure. In approximations, including interatomic bonds in seven coordination spheres, expressions are derived for the energy of formation of “glide” APB as a function of the ordering energy. To describe “thermal” type APB, an additional energy parameter is introduced Δ i =ϕ AA i −ϕ BB i (ϕ kL i is the binding energy of a pair K-L atoms in the i-th sphere). It is shown that the “complimentary” APBs of orientation {hkl}, in comparison to the stoichiometric composition of the alloy, have a constant energy gap except for an orientational factor. We propose new types of dislocation transformation reactions, including “glide” APB-“thermal” APB in an arbitrary plane {hkl}.

Defect Reduction in GaAs Grown on Si by Using Saw-Tooth-Patterned Substrates

ABSTRACTEpitaxial GaAs layers have been grown on saw-tooth-patterned (STP) Si substrates by metal-organic chemical vapor deposition and analyzed by transmission electron microscopy. The utilization of this special interface feature is effective in suppressing the formation of antiphase boundaries and reducing the threading dislocation density. The growth of GaAs has been studied with the epilayer thicknesses ranging from several hundred angstroms to several microns. Very flat growth front on (100) plane above the STP region is observed. The dislocation density decreases very rapidly in the area farther away from the interface. The dislocation configuration at this STP interface is very different from that at the extensively studied two-dimensional planar interface.

The role of elasticity in determining antiphase boundary anisotropy in ordered intermetallics

The sources of elastic stresses at an antiphase boundary (APB) are discussed as well as the contribution of these elastic effects to APB energy. Attention is focused on TEM contrast recorded from anisotropic B2 APBs in rapidly solidified Fe-Al-Mo alloys, suggesting that the elastic stresses are minimized when APBs are oriented parallel to (100) planes and that the APB anisotropy increases with increase in elastic stress at APBs. It is pointed out, however, that only in the presence of extra factors such as the formation of APBs in an Fe-Al-Mo matrix supersaturated with Mo the elastic effects become important. 10 refs.

A band theoretical study on the electronic specific heat in the phase transition CuAuI to CuAu II

The linear muffin-tin orbitals (LMTO) method is used to calculate the electronic density of states at the Fermi energy, characteristic d-band energies and charge transfers of CuAu I and CuAu II. It has previously been shown that the large difference observed in low-temperature specific heat between CuAu I and CuAu II is explained quite well by band structure calculations. It is shown that the dominant factor in the change of the electronic specific heat in the phase transition CuAu I to CuAu II is the formation of anti-phase boundaries in CuAu II, the effect of changes in lattice parameters, atomic positions and atomic potentials being less important. The electronic specific heat as a function of the length of the anti-phase domain is also studied.

A SISF formation mechanism in Ni3Al

In the ordered Ll2 structure, the superlattice intrinsic stacking fault (SISF) has often been observed to be formed through two steps, the formation of antiphase boundary (APB) by dissociation of superlattice dislocation at first and then the conversion of the APB to the SISF(1,2). The present paper studied the SISF in NijAl by TEM and found that the SISF can also be formed by superlattice dislocation dipoles.The tensile deformation of the directionally solidified Ni3Al (of composition in wt% of 82.88Ni, 8.5A1, 0.8Zr, 7.8Cr, and 0.02B) specimens was performed at room temperature. Thin foils were made normal to the specimen axis (parallel to [001]) and studied at 200 kv in H-800.As shown in Fig.l, a SISF is formed at the end of a superlattice dislocation dipole, according to the SISF-type dissociation [10] =1/3 [11]+1/3[2l] on (111) plane. In Fig.2, a superlattic dislocation dipole is moving away from a SISF.

The determination of the rigid-body translation across anti-phase boundaries in GaAs

The growth of a polar material on a non-polar substrate leads to the possibility of anti-phase boundary (APB) formation. Across the boundary there will be bonding between like atoms. The stoichiometry of the material may, or may not, be affected, depending on the relative proportion of A-A and B-B bonds in the boundary. This will depend on the crystallographic orientation of the boundary plane. Because of the different nature of the bonding at an APB, it has been assumed that the physical properties, particularly the electrical characteristics, of the polar layer will be adversely affected by the presence of APBs. Much effort has therefore gone into the development of growth conditions which minimize the likelihood of APB formation, particularly in the GaAs on Si system.Little work, however, has been done on the fine-structure of the APB itself. In this paper the results of a quantitative comparison of the α fringes produced in 2-beam images of APBs, with calculated images will be presented. The theoretical images were calculated using the COMIS program, which uses a manybeam formalism based on the Howie-Whelan equation. The samples were prepared from GaAs epilayers deposited on a (100) Ge substrate. The Ge substrate was removed by mechanical polishing prior to ion milling of the epilayer in a liquid nitrogen cooled specimen stage. The epilayer was examined at a number of low index zone axes, using a number of different 2-beam conditions. A representative selection is shown in figure 1. Images A, B, and C, were obtained close to the [001] zone axis, image D was obtained near the [101] zone axis and shows the horizontal APB nearly flat-on, but out of contrast. Near the [101] zone axis this APB appears as a line defect, and therefore must lie on the (101) plane.

Twinning in Pr 24O 44: An investigation by high-resolution electron microscopy

In the oxidation of Pr 7O 12 to Pr 24O 44, the diminished number of defects within the crystal are structurally redistributed. Both structures have a direct relationship to the fluorite lattice for the arrangement of the metal and the oxygen atoms. The defect cluster in these structures consists of two vacancies within the oxygen sublattice that are paired along the [111] direction of the fluorite cell. The transition from one phase to the other is very often accompanied by twinning and also by the formation of antiphase boundaries. It is shown here that twin domains can be related to the loss of symmetry during transformation from the fluorite-related rhombohedral Pr 7O 12 to the triclinic Pr 24O 44. The family tree for the rare-earth oxygen-deficient fluorite-related structures is used to explain the twinning observed in Pr 24O 44.

Morphology of Voids in Quenched β-Brass

The shape of voids in quenched β-brass has been studied by means of transmission electron microscopy. A variety of void shape is found, while the void image calculated by using the two-beam dynamical theory can verify that the fundamental shape is a rhombic dodecahedron with the {110} faces. The variety is arisen from two factors. One is truncation of the corners of voids and the other is deformation from the rhombic dodecahedron. From the truncation, the anisotropy of surface energy of β-brass can be estimated to be γ 100 /γ 110 ≧1.24. The deformation is due to void growth associated with the formation of antiphase boundaries.

On the use of convergent-beam electron diffraction for identification of antiphase boundaries in GaAs grown on Si

Dynamical coupling effects between the (200) reflection and weak odd-index reflections in the Bragg position in convergent-beam diffraction patterns has been used to detect antiphase boundaries (APBs) in GaAs grown on (001) Si. It was shown that APBs lie in most cases on energetically favorable {110} planes and effectively stop the propagation of dislocations, twins and stacking faults. The formation of APBs is promoted by Si surface contamination and irregularities.

Adsorption of H2S on Ni(100): Kinetics and structure

The kinetics of H 2 S adsorption on Ni(100) is studied at various temperatures, using X-ray photoelectron spectroscopy and low energy electron diffraction. The variation of the sticking coefficient with sulphur coverage is explained by computer simulation. The simulation is based on a precursor picture of the adsorption process and further includes the formation of antiphase boundaries in the sulphur overlayer. A change in the long range order of the overlayer is observed to take place at 600 K.

Structure of heteroepitaxial GaAs on Si

The current interest in GaAs grown on nonpolar substrates such as Si has been stimulated by the potential technological advantages of this system. Although the two major obstacles impeding the progress of heteroepitaxial growth of GaAs on Si (100) substrates, the large lattice mismatch and the formation of antiphase boundaries, have recently been overcome, the understanding of the microstructural growth process is still not satisfactory. We are presenting new x-ray scattering results which indicate that thin GaAs films are compressed in the film plane at room temperature, while thicker films are under tensil stress, the cross-over region being at about 1000Å. In addition, we show that the GaAs lattice is translationally incommensurate with the Si substrate and that the in-plane [001] axes are misaligned by 3–5°. Thermal expansion measurements of the out-of-plane lattice parameters of the film and substrate indicate that the GaAs in-plane thermal expansion follows from the anharmonicity of the substrate.

Atomic configuration of antiphase boundaries in alloys

Computer simulation methods in the approximation of pairwise interatomic Morse potentials are used to calculate the atomic configurations and the energy of formation of 1/2 {111} antiphase boundaries in ordered Cu3Au and Ni3Fe alloys. Atomic displacements result in considerable smearing of the first five planes closest to an antiphase boundary and a slip of atomic planes. These effects substantially reduce the energy of formation of antiphase boundaries. The influence of the size factor on the atomic configuration on the antiphase boundaries is discussed.

Plastic deformation mechanisms in tungsten carbide

Transmission electron microscopy studies of dislocations formed by plastic deformation in tungsten carbide have confirmed a slip deformation mechanism involving the {1 0 ¯1 0} 〈0 0 0 1〉 system. Direct visual evidence also confirmed the existence of extended dislocations formed by a suggested dissociation 1/3〈1 1 ¯2 3〉=1/6〈2 0 ¯2 3〉+1/6〈0 2 ¯2 3〉 which can further interact with other extended dislocations on pairs of intersecting {1 1 ¯2 2} pyramidal planes. The proposed interaction is suggested as a means of reducing both dislocation strain energy and atomic misfit strains due to antiphase boundary formation and leads to a form of sessile dislocation arrangement similar to the Lomer Cottrell lock.

Exsolution and inversion in pigeonite from the Whin Sill, Northern England

Grains of pigeonite, a calcium-poor silicate mineral of the pyroxene group, from the Whin Sill dolerite have been ion-thinned and examined by TEM. The pigeonite is strongly zoned chemically from the composition Wo8En64FS28 in the core to Wo13En34FS53 at the rim. Two phase transformations have occurred during the cooling of this pigeonite:- exsolution of augite, a more calcic pyroxene, and inversion of the pigeonite from the high- temperature C face-centred form to the low-temperature primitive form, with the formation of antiphase boundaries (APB's). Different sequences of these exsolution and inversion reactions, together with different nucleation mechanisms of the augite, have created three distinct microstructures depending on the position in the grain.In the core of the grains small platelets of augite about 0.02μm thick have farmed parallel to the (001) plane (Fig. 1). These are thought to have exsolved by homogeneous nucleation. Subsequently the inversion of the pigeonite has led to the creation of APB's.

Deformation behavior of long-range ordered AgMg

The deformation behavior of long-range ordered AgMg single crystals was investigated in tension and compression for several orientations and temperatures. The study revealed that AgMg exhibits a complex and asymmetric deformation behavior corresponding to the operation of two different primary slip vectors, and , whose activation depends on the crystallographic orientation and sense of the applied uniaxial stress and on temperature. Crystals of various orientations deformed between 20° and 298°K exhibited six different operative slip systems, {112} , {123} , {110} , {120} , {100} , and {110} . It is shown that the high strain-hardening rate exhibited by polycrystalline AgMg does not arise from the Vidoz-Brown5 mechanism of formation of antiphase boundary (APB) tubes on jogged superdislocations, but is associated with the operation of the {hko} slip systems.

Electron microscopic observations of dislocations in nickel and NiMo alloys

Dislocations induced by deformation of disordered f.c.c. 10 and 20 at. % MoNi alloys are closely restricted to the slip planes and pile-up readily at barriers. It is believed that Mo has lowered the stacking fault energy to cause sufficient separation of partial dislocation pairs so that cross-slip is difficult; however, the stacking faults were not resolvable, indicating that the separation was less than about 10 Å. The microstructure of the deformed ordered Ni 4Mo was characterized by numerous long ribbons which were assumed to be contrast effects caused by the formation of antiphase boundaries as dislocations passed through the domains. Dislocations were restricted to single slip planes, and appeared nearly always in pairs, which were concluded to be superdislocations.