Average Dislocation Density Research Articles

Growth and characteristics of low dislocation density GaN grown on Si(111) from a single process

From one uninterrupted growth process, GaN films were deposited on maskless stripe-patterned Si(111) substrates using the facet-initiated growth technique. The epilayer with a flat surface has a thickness of ∼1.3μm. The influence of stress on the behavior of dislocations in the crystal during growth was observed by the transmission electron microscopy (TEM). Concentrated lines of dislocations were found along the coalescence boundaries by atomic force microscopy (AFM). Few dislocations were detected in the other area. The average threading dislocation density of the GaN layer was decreased to ∼1.7×108cm−2. These dislocations have pure or partial screw dislocation characteristics.

N-type doping of GaN/Si(1 1 1) using Al 0.2Ga 0.8N/ALN composite buffer layer and Al 0.2Ga 0.8N/GaN superlattice

The characteristics of Si-doped and undoped GaN/Si(1 1 1) heteroepitaxy with composite buffer layer (CBL) and superlattice are compared and discussed. While as-grown Si-doped GaN/Si(1 1 1) heteroepitaxy shows lower quality compared to undoped GaN, crack-free n-type and undoped GaN with the thickness of 1200 nm were obtained by metalorganic chemical vapor deposition (MOCVD). In order to achieve the crack-free GaN on Si(1 1 1), we have introduced the scheme of multiple buffer layers; composite buffer layer of Al 0.2Ga 0.8N/AlN and superlattice of Al 0.2Ga 0.8N/GaN on 2-in. Si(1 1 1) substrate, simultaneously. The FWHM values of the double-crystal X-ray diffractometry (DCXRD) rocking curves were 823 arcsec and 745 arcsec for n-GaN and undoped GaN/Si(1 1 1) heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85×10 9 and 1.32×10 9 cm −2 for n-GaN and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of edge emission peaks onto the 2 inch whole Si(1 1 1) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0 nm related with 3.377 and 3.378 eV, respectively, using 325 nm He-Cd laser as an excitation source under room temperature.

Diffraction whole-pattern modelling study of anti-phase domains in Cu 3Au

The theory of diffraction line broadening from anti-phase domains in L1 2 intermetallics is reviewed and incorporated in a whole powder pattern modelling (WPPM) approach. The new analytical procedure was tested on Cu 3Au filings before and after a thermal treatment to promote a disorder/order transformation with generation of anti-phase domains. The results confirm a prevalence in the ordered phase of conservative {1 0 0} boundaries, even though fractions of different boundaries are also likely to be present. The information provided by the WPPM is completed by the distribution of diffraction domain sizes and by values of average dislocation density and interaction parameters.

Lattice imperfections in intermetallic Ti–Al alloys: an X-ray diffraction study of the microstructure by the Rietveld method

Microstructure in intermetallic Ti–Al alloys with compositions (Al=45, 50 and 55%: all compositions are in at. %) in the homogenized bulk and cold-worked powder states has been extensively studied using Rietveld whole X-ray profile fitting technique, adopting the recently developed software, MAUD (Material Analysis Using Diffraction). The analysis also considers the quantitative estimation of different phases and lattice defect related features of the evoluted microstructures, namely crystallite sizes, microstrains, stacking fault probabilities and dislocation density values, etc. in both the states of the materials (homogenized bulk and cold-worked powder). The study revealed reverse α 2→γ phase transformations due to plastic deformation in all the considered alloy compositions. Upon cold-working, high densities of stacking faults were observed in the α 2-Ti 3Al microstructure, accompanied by moderate values (∼10 11 cm −2) of average dislocation densities in the γ-TiAl microstructure. In the homogenized bulk alloys, the respective Vickers microhardness values were found to be 464, 365 and 339 HV, decreasing with increase in Al concentration. The values of all the above defect parameters have been evaluated and compared for elucidating a better structure-property relationship.

Evolution of the statistical properties of dislocation ensembles

X-ray line profile analysis is an efficient non-destructive technique to determine some key statistical properties of dislocation structures developing during plastic deformation. In the first part of the paper recent developments describing the asymptotic behavior of the line profiles are outlined. A new evaluation method is presented to determine the average dislocation density and its spatial variation. The method is demonstrated on measurements obtained on compressed Cu single crystals. After this, it is shown that the relative dislocation density fluctuation exhibits a sharp maximum at the stages II–III transition. A nanoindentation tests indicate that the fluctuations of other quantities behave similarly. The results give new insight into the dislocation phenomena taking place in the stage III deformation regime.

Effect of cold rolling on the indentation deformation of AA6061 aluminum alloy

The indentation behavior of cold-rolled AA6061 Al alloy was investigated. Following the approach suggested by Tabor, indentation stress–indentation strain curves were constructed and analyzed. The indentation stress required to create the same indentation strain increases with an increase in the reduction of thickness, suggesting a strong effect of plastic deformation history on the deformation behavior of materials. Through the dislocation dynamics, the evolution of the dislocations underneath the indentation was correlated with the plastic deformation history and the indentation load. The plastic energy dissipated in indentation was then calculated and found to be proportional to the 3/2 power of the indentation load and the 3/4 power of the average dislocation density underneath the indentation. The ratio of the dissipated plastic energy to the total energy in the indentation was demonstrated to be a function of the deformation state in materials, independent of the indentation load.

Growth of crack‐free GaN on maskless Si(111) by MOVPE

A ∼1.5 µm thick crack-free GaN film is successfully grown on periodic grooved Si(111) substrate. Two low temperature (LT) interlayers are used in the epitaxial film to compensate tensile stress induced by thermal mismatch between GaN and Si. The width of parallel trenches etched in the substrate along the Si〈110〉 is 1 µm. In this case, crystallographic tilt in GaN epilayer is not detected by x-ray diffraction. The average dislocation density of the GaN crystal deposited over the trenches in the surface layer is ∼1 × 109 cm–2, which is much lower than that of GaN on the Si mesas. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of residual dislocations in VGF-grown Si-doped GaAs

The VGF-method was applied for the growth of Si-doped GaAs crystals of 3 and 4 inch diameter ( n>10 18 cm −3). When the average dislocation density in these crystals falls below 200 cm −2 the stress-induced 60°-dislocations disappear and other dislocation types dominate the crystal. These remaining dislocations are classified and the conditions for their occurence are analyzed. Close to the crystal axis, dislocation walls are found which extend along [1 1 0]- and [ 1 1 ¯ 0 ] -directions and grow through the whole crystal. These walls are formed by edge dislocations belonging to the (1 1 0) 1 2 [ 1 ¯ 1 0 ] - or the ( 1 ¯ 1 0 ) 1 2 [1 1 0]-glide system. Apart from the dislocation walls, a dislocation arrangement in the form of a cross is observed in Si-doped GaAs. The orientation of the Burgers vector relative to the dislocation line of this dislocation type was determined. They include an angle close to 45°. Dislocations of this type belong to the (0 1 0) 1 2 [1 0 1] or the (0 1 0) 1 2 [ 1 0 1 ¯ ] -glide system. The examination of the growth in the seed well shows that the dislocation structures found in the crystal are partly already formed in the seed well. The growth in the seed well leads to a reduction of the EPD although the solid–liquid interface is concave. A reduction of the EPD in the seed well is accomplished by annihilation of dislocations.

Fourier analysis of X-ray micro-diffraction profiles to characterize laser shock peened metals

X-ray micro-diffraction profiles using a synchrotron light source were analyzed via Fourier transformation for single crystal Aluminum and Copper samples subjected to micro-scale laser shock peening. Specifically, the asymmetric and broadened diffraction profiles registered across the shock peen region were observed and analyzed by classic Warren and Averbach (W–A) method [Warren, B.E., Averbach, B.L., 1950. The effect of cold-work distortion on X-ray patterns. Journal of Applied Physics 21, 595–599] and modified W–A method [Ungar, T., Borbely, A., 1996. The effect of dislocation contrast on X-ray line broadening: A new approach to line profile analysis. Applied Physics Letters 69, 3173–3175]. Average strain deviation, mosaic size and dislocation density were estimated for the first time with a spatial resolution of 5 μm. The results compare well with the simulation results obtained from FEM analysis and from electron backscatter diffraction (EBSD) measurements. Differences in response caused by different materials and crystalline orientations (1 1 0 and 0 0 1) were also studied.

Microindentation of aluminum

A simple model was developed to correlate the plastic energy dissipated in microindentation to the applied load and the average dislocation density underneath the indentation. The model predicted that the plastic energy dissipated in a loading-unloading cycle was proportional to the 3/2 power of the indentation load as well as to the 3/4 power of the average dislocation density underneath the indentation. The experimental results from the microindentation tests of the annealed Al and the equal channel angular extrusion (ECAE)-deformed Al supported the proposed model. A variation of the microhardness to the various sections of the ECAE-deformed Al was observed, suggesting that the Al rod was deformed to varying degrees of plastic deformation after only a single ECAE pass. The hardness of the most severely deformed sample showed the highest hardness, and the annealed sample showed the lowest hardness. Also, plastic recovery was observed in the indentation of the ECAE-deformed Al, which suggested a kind of Bauschinger effect took place.

Temperature rise in crystals subjected to ultrasonic influence

The nonuniform temperature distribution in the surface of Hg1−xCdxTe semiconductor crystals during ultrasonic excitation was detected. This phenomenon was associated with a sonic-stimulated temperature rise around dislocations and a macroscopic heating of nonperfect regions. The dislocation moving in an ultrasonic field was considered as a linear thermal source and the temperature distribution around the dislocation was calculated. The discrete distribution of thermal sources is realised for Hg1−xCdxTe solid solutions at the average dislocation density ∼1010m−2. The model of the sonic-stimulated activation of internal sources of the infrared radiation was proposed.

Deformation-induced ordering of dislocation structures

Abstract On the basis of the experimental data, the deformation-induced transformation of homogeneous dislocation structures into heterogeneous ones with the formation of a substructure are considered. Order parameters that describe these transformations are introduced. Critical values of average and local dislocation densities, at which the transformations and the formation of new substructures take place, can be obtained from the experimental data.

Dynamic recrystallization: plasticity enhancing structural development

Dislocation mobility upsurge usually causes significant improvement in the plasticity of crystalline solids, partly through its ability to deter crack formation. Dynamic recrystallization (DRX) that proceeds during deformation reduces the flow stress and markedly raises the ductility, characterizing the hot working range. While it facilitates straining, it’s nucleation rate and equilibrium grain size are defined by the deformation temperature and strain rate. The flow stress in steady state is not directly determined by the grain size but rather by the average dislocation density, which is tied to both dynamic recovery (DRV) and DRX in so far as the migration of grain boundaries reduces the dislocation density below that attained by DRV annihilation and polygonization of substructure. Dynamic recrystallization does not, in its operation, produce any strain. It is a restoration or softening process, which facilitates straining with respect to ease and extent. In contrast to its static counterpart, its progress and effects are intimately associated with the plastic deformation in real time.

X-ray imaging study of lattice defects related to diffusion of helium in quartz

Production of He isotopes in surface rocks enables one to study their origin. Quartz is a widespread mineral; understanding He mobility in it will be valuable to helium studies in many environments. Measurements of the He diffusivity in quartz demonstrated that a perfect crystalline quartz lattice was virtually impermeable for He. In this paper, we show that lattice dislocations serve as the high diffusivity paths for helium in quartz. Synthetic crystals were grown to contain a certain amount of helium, which was then degassed under heating. Synchrotron imaging was applied to characterize and map grown-in defects. Samples for desorption were prepared from the chosen growth zones where the average dislocation densities were evaluated. From the highly dislocated samples helium appeared to release more rapidly and to have a lower activation energy. The gain in diffusivity was interpreted due to a smaller amount of energy required to permit the passage of a He atom through an increased `doorway' provided by a dislocation pipe.

Crucible Selection in AlN Bulk Crystal Growth

ABSTRACTGrowth of AlN bulk single crystals by sublimation of AlN powder was carried out in a resistively heated reactor at temperatures up to 2300°C. A variety of crucible materials, such as BN, W, Ta, Re, ZrO2, TaN, and TaC, were evaluated. Our studies have shown that the morphology of crystals grown by spontaneous nucleation strongly depends on the growth temperature and contamination in the reactor. Crucible selection had a profound effect on contamination in the crystal growth environment, thus influencing nucleation, coalescence, and crystal morphology. Spontaneously grown single crystals up to 15 mm in size were characterized by x-ray diffraction (XRD), x-ray topography (XRT), glow discharge mass spectrometry (GDMS), and secondary ion mass spectrometry (SIMS). Average dislocation densities were on the order of 103 cm−3, with extended areas virtually dislocation-free, while high-resolution XRD showed rocking curves as narrow as 7 arcsec.

On the stability with temperature of the dislocation structure in 6061Al-15vol%SiC composites as studied by hardness and differential scanning calorimetry

The relative difference between the average dislocation density of 6061Al-15vol%SiC composites investigated by two independent methods (namely hardness, H, and differential scanning calorimetry, DSC), is discussed. They are interpreted in terms of the “actual” temperature at which the measurements are made with each technique. The present analysis accounts for the high microstructural stability of these composites at elevated temperatures.

TSSG of RbTiOPO 4 single crystals from phosphate flux and their characterization

Large-size inclusion-free RbTiOPO 4 (RTP) single crystals have successfully been grown from Rb 6P 4O 13 flux with the addition of a small amount of MoO 3 using TSSG technique. The solubility of RTP in the flux composition Rb 6P 4O 13 has been determined. Seed crystals were obtained by spontaneous nucleation process. The seed–liquid interface was maintained by the surface tension forces, and the criteria of avoiding the solution inclusion in the initial stages of the growth are discussed. Crystals grown from Rh-incorporated platinum crucibles shift the UV cut-off towards lower wavelength. The effect of coloration and impurities on optical properties were characterized by optical absorption spectra. Single-crystal X-ray diffraction has been carried out to evaluate the lattice parameters of the grown crystals. Average dislocation density was in the range 400–5000/mm 2. Second harmonic generation (SHG) elements have been fabricated.

A simple model for dislocation behavior, strain and strain rate hardening evolution in deforming aluminum alloys

In this work, modeling of the stress–strain behavior is carried out using a simple dislocation model. This model uses three variables to characterize the dislocation population: The average forest and mobile dislocation densities, ρf and ρm, and the average dislocation mean free path L. However, it is shown that within reasonable assumptions, only two of these variables are independent. The mathematical form derived from this dislocation-based model was applied to experimental stress–strain data determined at room temperature for pure aluminum, 3003-O, 2008-T4, 6022-T4, 5182-O and 5032-T4 aluminum alloy sheets. The evolution of the state variables was calculated for these materials from a single stress–strain curve. The average dislocation mean free paths at a strain of 0.5 were compared with TEM observations of dislocation cell sizes or inter-dislocation spacing for specimens deformed equal biaxially with the hydraulic bulge test. A very good agreement was obtained between predictions and experiments.

Characterization of self-similar dislocation structures by X-ray diffraction

The development of the statistical parameters of the dislocation structure during plastic deformation of copper single crystals (average dislocation density, and average dislocation density fluctuation) was investigated by the method of X-ray diffraction peak profile analysis. It is found that during the deformation process, while the dislocation density increases monotonically, the average fluctuation exhibits a maximum corresponding to the transition from stage II to stage III of work hardening. The fractal dimension of the dislocation structure was also investigated. A strong correlation was found between the fractal dimension and the relative dislocation density fluctuation.

Growth of InSb epitaxial layers on GaAs (0 0 1) substrates by LPE and their characterizations

The growth of epitaxial InSb layers on highly lattice mismatched GaAs substrates has been successfully achieved via the traditional liquid phase epitaxy. Details about nucleation, growth and optimization have been dealt with. High-resolution X-ray diffraction studies on the epilayers reveal all reflections with distinct layer and substrate peaks even up to a highest scattering angle of 153°. However, the layer peaks are considerably broader, indicating extensive dislocations. The average dislocation density was estimated from the Full-width at half-maximum of symmetric reflection. The grown films were n-type and the typical value of the sheet carrier density at 80 K obtained for these samples was 6.12×10 15/cm 2. The Hall mobility at 80 K was 7.058×10 3 cm 2/ Vs . The room-temperature band gap was determined to be 0.198 eV. Raman scattering measurements have also been performed to characterize the grown epilayers.