Edge-type Dislocations Research Articles

X-ray scattering from GaN epitaxial layers - an example of highlyanisotropic coherence

GaN(0001) epitaxial layers were grown by molecular beam epitaxy on afew-nanometres thick low-temperature GaN nucleation layers on c-planesapphire. Despite extremely high densities of extended defects, the layersshow a narrow (002) x-ray diffraction peak, superimposed by broad diffusescattering. Triple-axis transverse and radial scans were measured for (00l)reflections of different orders and for various GaN layer thicknesses. Theresults can be described by an interfacial displacement-difference correlationfunction. Its microscopic origin is assigned to either inversion domainboundaries or edge-type threading dislocations in the GaN layers, in agreementwith findings of transmission electron microscopy. These defects areassociated with an only weak rotational disorder perpendicular to the growthplane as proven by the x-ray scattering characteristics.

Electronic structure of molecular crystals containing edge dislocations

An attempt to model the electronic structure of molecular crystals containing an edge dislocation at the ab initio Hartree–Fock level is performed. The experimentally determined configurations for edge-type dislocations with the Burgers vector [001] in crystalline cyclotrimethylene trinitramine (RDX) and pentaetythritol tetranitrate (PETN) are theoretically simulated. It is shown that a shear stress, induced by the dislocations, produces local electronic states in the fundamental band gap of the crystal. These states are mainly formed by molecular orbitals of critical bonds (which are the N–NO2 group in RDX and the O–NO2 group in PETN) responsible for the stability of the materials. Optical absorption attributed to these electronic states is predicted and compared to the available experimental data. Properties of the defective solids are compared with those of the perfect crystals. Correlation of the electronic structure and sensitivity of the materials to initiation of a chemical reaction as well as some practical applications of the obtained results are discussed.

Characterization of high-quality epitaxial AlN films grown by MOVPE

AbstractThis paper presents the correlation between overall crystal mosaicities and dislocation behaviors of high-quality AlN epitaxial films grown on a C-plane sapphire substrate using a low-pressure metal organic vapor phase epitaxy (LP-MOVPE) method. Typical full width at half maximum (FWHM) values of x-ray rocking curves (XRC) for (0002) and (10-10) of the AlN are 80arcsec and 1800arcsec, respectively. From transmission electron microscopy (TEM) observations, edge-type dislocations thread the AlN layer, however, almost all screw-type dislocations disappear at an early AlN growth stage. The dislocation density of the AlN films in its surface region is as low as approximately 1x1010 cm-2. The distribution of the dislocations is considered to be caused by a large twisted mosaicity and a very small tilted mosaicity.

New buffer layer technique using underlying epitaxial AlN films for high-quality GaN growth

AbstractWe demonstrate high-quality epitaxial AlN films on C-plane sapphire as a new buffer layer technique for the growth of high-quality GaN. The obtained GaN films were atomically flat and the full width at half maximum (FWHM) values of the X-ray rocking curve (XRC) were 120arcsec and 510arcsec for (0004) and (10-10), respectively. XRC-FWHM values of the underlying AlN film were 90arcsec and 1800arcsec for (0004) and (10-10), respectively. The XRC results of the GaN indicated that tilted mosaicity of the GaN was much smaller than that of conventional GaN and that small tilted mosaicity coincided with small twisted mosaicity. It was characteristic that the twisted mosaicity drastically reduced from the AlN to the GaN. According to TEM results, it was clarified that the high-quality AlN layer had an effect in reducing the amount of screw-type threading dislocations and that the GaN/AlN interface played an important role in reducing the amount of edge-type dislocations.

Dislocation arrangement and density in deformed Al–Pd–Mn single-quasicrystals

We present results of microstructural investigations on plastically deformed Al–Pd–Mn single-quasicrystals. Deformations were carried out in uniaxial geometry at different temperatures and to different values of plastic strain and in bending geometry. In the uniaxial geometry, various glide systems are activated. The dependence of the total dislocation density on temperature and on plastic strain is studied. Additional heat treatments on deformed samples show that recovery processes occurring at the high deformation temperatures significantly influence the deformation process. The experimental data are interpreted using a kinetic equation describing the evolution of the dislocation density during deformation. In the bending geometry a defined arrangement of geometrically necessary dislocations of edge type is observed. We find straight dislocation lines with line directions in the plane spanned by the direction of the applied force and the bending axis. The Burgers vectors have twofold direction corresponding to glide planes with the highest shear stress.

Mass Transport of GaN and Reduction of Threading Dislocations

Mass transport of patterned GaN at around 1100°C in nitrogen with an ammonia atmosphere has been discovered for the first time. The mass transport process is found to be affected by the anisotropy of surface energy of GaN. Behaviors of threading dislocations which are predominantly of the mixed type and the pure edge type are affected by the anisotropy during mass transport. Mixed type dislocations are bent keeping the geometrical relationship normal to the surfaces, while pure edge type dislocations are bent horizontally. This new-found process, the so-called "mass transport epitaxy," is one of the best methods for achieving low dislocation density GaN on the highly lattice-mismatched substrate.

Edge-type misfit dislocations produced by thermal processing of pre-relaxed InxGa1−xAs/GaAs heterostructures

Dislocation structures are presented for GaAs/InxGa1−xAs/GaAs heterostructures before and after thermal processing. Cathodoluminescence has allowed nondestructive examination of bulk specimens, while transmission electron microscopy has been used to establish the details of the dislocation configurations. In each as-grown specimen the thickness of the InxGa1−xAs layer was above its critical value, so 60° misfit dislocations were already present. It is shown that new pure edge, i.e., 90°, dislocations are formed at the interfaces by thermal processing at 1040 K. Their Burgers vectors are a/2〈101〉 perpendicular to their 〈010〉 directions. Although individual 90° misfit dislocations are more effective relievers of strain than 60° ones, the self-energy for an array of such dislocations is higher and hence 60° misfit dislocations form first. A model of the formation of 90° edge misfit dislocations is proposed in which the climb of vacancy-producing jogs on pre-existing 60° dislocations leaves a trailing dislocation dipole.

Strain relaxation of low-temperature deposited epitaxial titanium-carbide films

The lattice misfit strain and relaxation during growth of 60–950 Å epitaxial TiC carbide films deposited by co-evaporation of C 60 and Ti on MgO(0 0 1) have been studied by reciprocal space mapping (RSM) and transmission electron microscopy (TEM). All the films exhibited a strained layer growth behavior with respect to the substrate. The strain ε, ranged from 2.1% for the 60 Å film to 0.8% for the 950 Å film. Initial misfit strain relaxation was by slip on {1 1 0}〈 1 0 1 ̄ 〉 and {1 1 1}〈 1 0 1 ̄ 〉. After dislocation rearrangement the films predominantly exhibited well-developed misfit dislocations of edge type with line direction 〈1 0 0〉 along the interface plane and Burgers vectors 1/2 1 0 1 ̄ inclined to the interface with MgO.

MASS TRANSPORT OF GaN AND REDUCTION OF THREADING DISLOCATIONS

Mass transport of patterned GaN at around 1100°C in nitrogen with an ammonia atmosphere has been discovered for the first time. The mass transport process is found to be affected by the anisotropy of surface energy of GaN. Behaviors of threading dislocations which are predominantly of the mixed type and the pure edge type are affected by the anisotropy during mass transport. Mixed type dislocations are bent keeping the geometrical relationship normal to the surfaces, while pure edge type dislocations are bent horizontally. This new-found process, the so-called "mass transport epitaxy," is one of the best methods for achieving low dislocation density GaN on the highly lattice-mismatched substrate.

Growth and microstructure of In xGa 1− xN films grown on SiC substrates via low pressure metalorganic vapor phase epitaxy

The growth and microstructures of In x Ga 1− x N films ( x≤0.23) grown on α(6H)–SiC(0001) wafer/AIN buffer layer/GaN heterostructures by low pressure metalorganic vapor phase epitaxy have been investigated. The system deposition pressure limited the InN content in these films. The maximum InN contents achievable at the deposition pressures of 45 and 90 torr were ∼13 and ∼23%, respectively. Kinetic phenomena based on the rates of adsorption and desorption of the In growth species off the growth surface are presented to explain the film composition dependence on the system pressure. The surface morphologies and microstructures of the In x Ga 1− x N films were analyzed using several techniques, and the formation of pinhole defects in the films was investigated. Most of the pinhole defects were associated with threading dislocations with a c-component Burgers vector. Edge-type dislocations were never observed to terminate in pinholes in the samples observed here. Indium segregation to areas around the defect areas was observed, as was an In compositional gradient in the growth direction. Based on experimental observations, the strain field around dislocations with a c-component Burgers vector could result in the increase of In atoms at the dislocation sites in the film, which result in a change to the local growth mode of the film and causes the pinhole defects to form.

Microstructures of GaN films grown by low pressure metal-organic vapor phase epitaxy on [formula omitted] sapphire substrates

Microstructures of GaN films grown by low pressure metalorganic vapor-phase epitaxy on (0 1 1 ̄ 2) sapphire substrates were investigated using transmission electron microscopy. The crystallographic structure of the GaN buffer layer grown at 550°C was uniquely hexagonal. Grain boundaries and stacking faults in the as-grown buffer layer are much less than those in the buffer layer grown on (0 0 0 1) sapphire substrates. Defects in the as-grown epitaxial layers are predominantly edge type dislocations with Burgers vector b= 1 3 〈1 1 ̄ 0 0〉 . No screw dislocations or domain boundaries in the epilayer were observed in the as-grown samples.

Plastic versus elastic misfit relaxation in III-nitrides grown by molecular beam epitaxy

With a view to understand the misfit relaxation mechanisms for binary III-nitride semiconductors, a combined RHEED and TEM study of strain accommodation for AlN/GaN, GaN/AlN and InN/GaN MBE heteroepitaxial combinations is presented. It is shown that AlN grows on GaN (2.7% tensile strain) in a two-dimensional way in accordance with a Frank-van der Merwe mode with plastic relaxation occurring through the formation of non-interacting edge-type dislocations. On the other hand, GaN epitaxy on AlN (2.7% compressive strain) proceeds in different modes according to the growth temperature: in particular, a strained Stranski–Krastanow mode can be evidenced at intermediate temperatures. For the case of InN on GaN (10% compressive strain), strain relaxation occurs through the formation of dislocated islands on the surface, i.e. through a relaxed Stranski–Krastanow mode. The implications of these different growth processes to the development of nitride-based quantum dots heterostructures are presented.

High resolution transmission electron microscopic study of some low‐dimensional nanostructures

High-resolution transmission electron microscopy (HRTEM) study of some low-dimensional nanostructures, such as carbon nanotubules and silicon-based blue-light emitting β-SiC nanoparticles will be reported. We have shown that nested hollow tubules formed by successive cylindrical graphite sheets are found, frequently, to be polyhedral or elliptical in cross-section, which are perpendicular to the tube-axis. Varied spacing between adjacent tube sheets is observed and edge-type dislocations can be distinguished in some tubules. These abnormal structural features are related to accommodations of various strains taking place simultaneously in tube sheets. Blue-light emitting porous β-SiC formed by C+ implantation of a single crystal silicon wafer have also been investigated. Buried layer structures with different β-SiC concentration were formed in the implanted silicon substrate. These β-SiC nano-particles are 2–8 nm in size and formed epitaxially from the upper and lower damaged layers, respectively, but randomly in the middle layer. The structural characteristics of the layered structure may be responsible for the blue-light emitting effect of the porous β-SiC material. Microsc. Res. Tech. 40:163–176, 1998. © 1998 Wiley-Liss, Inc.

High resolution transmission electron microscopic study of some low-dimensional nanostructures.

High-resolution transmission electron microscopy (HRTEM) study of some low-dimensional nanostructures, such as carbon nanotubules and silicon-based blue-light emitting beta-SiC nanoparticles will be reported. We have shown that nested hollow tubules formed by successive cylindrical graphite sheets are found, frequently, to be polyhedral or elliptical in cross-section, which are perpendicular to the tube-axis. Varied spacing between adjacent tube sheets is observed and edge-type dislocations can be distinguished in some tubules. These abnormal structural features are related to accommodations of various strains taking place simultaneously in tube sheets. Blue-light emitting porous beta-SiC formed by C+ implantation of a single crystal silicon wafer have also been investigated. Buried layer structures with different beta-SiC concentration were formed in the implanted silicon substrate. These beta-SiC nano-particles are 2-8 nm in size and formed epitaxially from the upper and lower damaged layers, respectively, but randomly in the middle layer. The structural characteristics of the layered structure may be responsible for the blue-light emitting effect of the porous beta-SiC material.

The hrem observation of cross-sectional structure of carbon nanotubes

Cross-sectional structural features of carbon nanotubes have been investigated by high resolution electron microscopy (HREM). Carbon nanotube-bundles with 0.2–0.6 mm in diameter and 5–10 mm in length were produced by means of an arc-discharge method, which is suitable for preparing TEM cross-sectional samples of carbon nanotubes. Our HREM observations have shown that most of carbon nanotubes have a polyhedral and elliptical shape in a cross section, and there are many defects in the microstructure of carbon nanotubes, including edge-type dislocations and variable spacing between adjacent tube sheets, revealing that the seamless cylindric and scroll-shaped graphene sheets co-exist within the same nanotube. The abnormal structure features are closely related to the non-equilibrium growth conditions of carbon nanotubes in the arc-discharge process, and also to the accommodations of various strains taking place simultaneously in tube sheets.

Characterization of the Induced Plastic Zone in a Single Crystal TiN(001) Film by Nanoindentation and Transmission Electron Microscopy

The slip system of TiN at room temperature has been determined to be {110} 〈110〉 by Burgers vector analysis using transmission electron microscopy and slip trace analysis of indents made on a TiN(001) film deposited on a MgO(001) substrate. Both small indents (0.4 mN maximum load) and large indents (40 mN maximum load) were used to study the dislocation structure in TiN. The nucleation of dislocations was investigated using small indents. Further development of the plastic zone was studied using large indents and microhardness indents (1.6 N). The critical resolved shear stress evaluated at the load when pop-in occurs was estimated to be 3.7 GPa, assuming a Hertzian elastic contact. Indents made with a 0.4 mN maximum load show a complex dislocation pattern with loops and straight segments that belong to the same slip system. Dislocations of mixed screw and edge type are dominant. The cascade of dislocations generated during pop-in is likely to nucleate from loops. For larger indents, the plastic zone extends more than three times the diameter of the imprint. The straight dislocations outside the large imprint are arranged in arrays along the 〈100〉 and 〈110〉 directions. A scanning force microscopy study of the surface outside a microhardness indent revealed a raised surface along 〈110〉 and formation of troughs along 〈100〉.

Formation of stacking faults in ZnS thin films epitaxially grown on GaAs(001)

Abstract The microstructure of single-crystal ZnS thin films epitaxially grown on GaAs(100) has been investigated by high resolution transmission electron microscopy (HRTEM). The dominant defect structures are {111} stacking faults and interface mismatch dislocations. The distribution of stacking faults is limited to the first 70 nm thickness range of the grown film, and almost no defects are observed in the thickness range beyond 100 nm. This could have important consequences in optoelectronic applications. The stacking faults of {111} layers are generated at the substrate-film interface in the following two ways. Firstly a small interface angle between the substrate and the film results in edge-type dislocations due to additional (002) ZnS layers (i.e. a low-angle boundary structure). Part of the stress created by these interface dislocations is relaxed by generating stacking faults. Secondly mismatch dislocations with additional {111} layers, due to the lattice mismatch between GaAs and ZnS (4.4%), als...

Energy analysis of dislocation arrays near bimaterial interfaces

Expressions for the elastic strain energies due to dislocation arrays near bimaterial interface or near the free surface of a semi-infinite body are derived. Arrays with periodic straight dislocations of edge, screw and combined type are considered. Expressions for the complete stress distribution and dislocation forces are also presented.

Microstructure and growth morphology as related to electro-optical properties of heteroepitaxial wurtzite GaN on sapphire (0001) substrates

We investigate the microstructural development during growth and the related electro-optical properties of gallium nitride (GaN) films deposited on (0001) sapphire substrates by gas source molecular beam epitaxy. We use transmission electron microscopy, atomic force microscopy, scanning tunnelling microscopy, spectrally resolved cathodoluminescence (CL) mapping and photoluminescence (PL). We report on specimens exhibiting UV luminescence (band-to-band transition at 358 nm/3.46 eV) in CL/PL and an additional strong yellow luminescence (Gaussian shaped CL peaks at around 528 nm/2.35 eV). The specimens show a surface topology with facetted hexagonal islands with a width of 1–2 μm at a height of 50 nm. A correlation with spectrally resolved CL images shows: the yellow luminescence is homogeneously distributed over the whole of the specimens as are the pure screw dislocations with b → = 〈 0001 〉 , while the UV luminescence is confined to troughs between adjacent hexagonal islands where edge type dislocations with b → = 1 / 3 〈 2 ¯ 110 〉 ) or b → = 1 / 3 〈 2 1 ¯ 1 ¯ 3 〉 ) form small angle boundaries. These dislocations do favour or at least do not impair UV luminescence. Formation mechanisms for the different defect types and possibilities for their reduction are briefly considered.

Comparative Study of Typical Defects in III-Nitride Thin Films and Their Alloys

AbstractThe microstructure and typical defects in GaN and GaN/GaAlN thin film heterostructures grown on (0001)α-A12O3 were investigated using different transmission electron microscopy (TEM) techniques including diffraction contrast analysis, multiple dark field imaging, and highresolution TEM. The films were grown by metal-organic chemical vapor deposition (MOCVD) technique. All of the films exhibited good electrical/optical properties. Yet, films were found to be of two distinctive types in terms of the microstructure. Films of the first type (A) were found to contain high, up to the 109 cm2, density of inversion domains (IDs) as well as pure edge (b=l/3[11 20]), screw and mixed type dislocations with the average density of 109 - 1010 cm−2. Smoother surface of the film, absence of IDs, and low (down to 107 cm−2 in the device quality layers) density of screw and mixed type dislocations were found to be characteristic for the second type (B) microstructure. The majority of defects present in these B-type GaN and GaN/GaA1N thin films were found to be threading pure edge dislocations associated with low angle tilt sub-grain boundaries. Despite the 1010 cm−2 density of the edge dislocations, the films displayed the devicequality electrical characteristics. Type A and type B microstructure can be obtained by the variation of growth conditions. The correlation between the optical and structural properties are discussed.