Low-energy Array Research Articles

On the origin of threading dislocations in GaN films

A series of GaN films were grown by metalorganic vapor phase epitaxy on nitrided sapphire using an initial annealed low-temperature nucleation layer (LT-NL), without employing any conventional threading dislocation (TD) reduction methods. Film thicknesses ranging from the LT-NL to 500 nm were used. The island network morphology was investigated at each growth stage using atomic force microscopy. Data from cathodoluminescence studies showed initially uniform luminescence, followed by the gradual development of bright (low TD) regions which had lateral sizes different from the island sizes at all times and which continued to increase in size after coalescence. The formation of low-energy arrays of a-type TDs also continued after island coalescence. X-ray diffraction, transmission electron microscopy (TEM) and AFM data indicated that the highest (a+c)-type TD densities were found in the LT-NL, but subsequently decreased due to TD loop formation (promoted by island facets) and reaction to produce a-type TDs. a-type TD densities were also high in the LT-NL but subsequently increased slightly, due to the reaction of (a+c)-type TDs. A very sharp dynamical ‘correlation’ peak was also observed in XRD of the LT-NL, related to TDs with an a-component. Furthermore, defect formation was observed within the LT-NL using high-resolution TEM. These data are consistent with TD formation predominantly in the LT-NL, followed by TD movement at elevated growth temperatures. Initially, coalesced films had a high TD density with a spatially random TD arrangement, but progressively altered into a lower TD density, spatially clustered arrangement during growth. This type of microstructure may mistakenly be interpreted as arising from island coalescence.

Low energy dislocation structures produced by cyclic deformation

Cyclic deformation in push-pull straining provides ideal conditions for achieving low energy dislocation structures because large cumulative strains give rise to high dislocation densities and the to-and-fro dislocation motions enhance entrapment probabilities. At low strain amplitudes the structures are dominated by clusters of prismatic loops; evidence for the dislocation content and interaction of the clusters is reviewed and is shown to be consistent with the establishment of low energy arrays. At higher amplitudes the dislocation structures are dominated by dislocation walls which are strongly dipolar. The deformation which occurs in these structures at low numbers of cycles does not seem to be consistent with low energy arrays, or at least only with those of moderately low energy. However, with sufficient cycling, lower energy arrays seem to become established and to promote homogeneity of the strain. At still higher amplitudes, complex cellular structures are observed and the complications of these structures can be interpreted convincingly in terms of low energy dislocation structures.

Energetics of dislocation relaxation associated with cleavage in cadmium-doped zinc crystals as probed by synchrotron topography

An almost tenfold increase in the cleavage energy of zinc crystals doped with 0.1 at.% Cd has been previously reported in the literature and attributed to an increase in the intrinsic surface energy. The energetics of the cleavage process as a function of dopant concentration over the range 0.01, 0.03, 0.06 and 0.13 at.% Cd has been systematically measured and correlated with the observed dislocation microstructure induced by crack initiation and growth. The dislocation relaxation zones were determined by non-destructive observations using synchrotron radiation topography. Since the primary cleavage and slip systems are coplanar, previous workers have ignored plastic relaxation and assumed that fracture had to proceed in a purely brittle fashion. Synchrotron topography results indicated extensive plastic relaxation at the crack tip and also downstream during crack propagation. While plastic relaxation accompanied fracture for both pure zinc and cadmium-doped zinc crystals, the topographs indicated a conversion of the plastic relaxation mode from a low energy array akin to a Taylor lattice to a high energy dislocation microstructure which caused severe lattice distortion of the entire cleavage surface. On the basis of these observations a modified Griffith analysis could be used to determine the relative contributions to the cleavage energy due to the intrinsic surface and dislocation relaxation processes. It was concluded that the latter process is the dominant factor in controlling the fracture toughness of these alloys.

The role of residual dislocation arrays in slip induced cavitation, migration and dynamic recrystallization at grain boundaries

Experiments with bicrystals of nickel were designed to study whether different reactions between dislocations at grain boundaries can lead to different effects. The experiments were carried out at 573 K in low cycle fatigue. The large majority of the bicrystals were Σ9, and a few were Σ21. The results can be broadly classified into two groups. In one the dislocations activated in the adjacent crystals were symmetrical and in the other they were nonsymmetrical. The symmetrical case consisted of two types, screw-screw (S-S) where one the dislocations was of opposite sign, and symmetrical-mixed (M-M) where both had an edge component similarly tilted with respect to the boundary plane. The S-S case left no residuals in the boundary plane and produced no effects at the boundary. The M-M case gave rise to a low energy array of edge residuals in the boundary, which led to onset of dynamic recrystallization, but did not produce cavitation nor boundary migration. In the nonsymmetrical group three cases were examined, screw-mixed (S-M), edge sessile-rigid (ES-R, where edge dislocations with their Burgers vector normal to the boundary plane were activated in only one crystal) and edge glissile-rigid (EG-R, Burgers vector parallel to the boundary plane). The S-M reaction led to heavy cavitation and migration (they were inversely related), while the EG-R reaction produced only migration. The ES-R case gave evidence of all three effects to some degree. In summary nonsymmetrical reactions appeared to promote migration and cavitation, while the symmetrical reactions produced predominantly dynamic recrystallization.

The role of structural defects in the growth of nickel oxide films

A transmission electron microscopy investigation was made of the structural defects in nickel oxide films formed at 500° and 600°C on metallographically polished polycrystalline nickel. These films within the thickness range 500–3000 Å were composed of small crystallites. For a given film orientation, the crystallites increased in size with increasing oxidation time. This increase in crystallite size may be considered as a recrystallization reaction occurring during film growth. At later stages of film growth, the preferential migration of some boundaries led to the formation of large oxide grains. These latter films contained dislocations whose distribution varied from a random arrangement to well-defined subboundaries containing dislocations in low energy arrays. All observations were consistent with the conclusion that incoherent boundaries and dislocations in the oxide lying normal to the metal surface acted as short-circuit paths for selective diffusion of nickel. An oxide film model is advanced for correlation of the reaction kinetics to the observed structural defects.