Basal Plane Dislocation Loops Research Articles

Formation mechanism of horizontal-half-loop arrays and stacking fault expansion behavior in thick SiC epitaxial layers

The formation mechanism of half-loop arrays (HLAs) that form parallel (horizontal) to the step-flow direction in 120 μm thick 4H-silicon carbide (SiC) epitaxial layers was investigated using ultraviolet photoluminescence (UVPL) imaging and x-ray topography (XRT). The horizontal-HLAs are generated by the multiplication and glide of basal plane dislocation (BPD) loops that are created within the epitaxial layer. The BPD loops were initiated after ∼40–50 μm of growth from a small BPD segment, which glides toward the surface as well as the substrate interface. BPD multiplication occurs and several loops are generated. Some of these loops are terminated by the growth front and create HLAs due to the 4° offcut of the wafer. XRT images show that successive BPD loops interact with previously generated HLA segments. Successive loops also interact with the moving growth front and create new HLAs that are spatially displaced from the previous HLA segments. These appear as a string of horizontal-HLAs in the UVPL images. The expansion of stacking faults (SFs) from these horizontal-HLAs was investigated, and we show that they all lie on the same basal plane. The complex defect structure is created in the epitaxial layer from a single BPD loop but extends over a large (∼5 × 0.5 cm2) region of the SiC wafer during epitaxial growth. The high density of HLAs and BPDs would generate several SFs upon device operation leading to severe device degradation.

Identification of fine structures at the surface of epi-ready GaN wafer observed by confocal differential interference contrast microscopy

Confocal differential interference contrast microscopy detected dots and line contrasts on the surface of epi-ready (0001) GaN wafers. Large and small dots consisted of pits, ∼1.4 μm in width and ∼7 nm in depth, and ∼0.5 μm in width and ∼2.7 nm in depth, respectively; the pits mainly formed on the outcrops of dislocations, with Burgers vectors ≥ c and b = a, respectively, via inadequate chemical mechanical polishing. Lines consisted of scratches induced via polishing. Deep scratches accompanied dense basal plane dislocation loops.

Defects and Polytype Instabilities

A model based on the generation and recombination of defect was developed to describe the stability of stacking faults and basal plane dislocation loops in crystals with layered polytype structures. The stability of the defects configuration was analysed for stacking faults surrounded by Shockley and Frank partial dislocation as well as Shockley dislocation dipoles with long range elastic fields. This approach allows the qualitative prediction of defect subsystems in polytype structure in external fields.

Morphology of single Shockley-type stacking faults generated by recombination enhanced dislocation glide in 4H–SiC

Morphology of single Shockley-type stacking faults (SFs) generated by recombination enhanced dislocation glide (REDG) in 4H–SiC are discussed and analysed. A complete set of the 12 different dissociated states of basal-plane dislocation loops is obtained using the crystallographic space group operations. From this set, six different double rhombic-shaped SFs are derived. These tables indicate the rules that connect shapes of SFs with the locations of partial dislocations having different core structures, the positions of slip planes in a unit cell, and the Burgers vectors of partial dislocations. We applied these tables for the analysis of SFs generated by the REDG effect reported in the past articles. Shapes, growing process of SFs and perfect dislocations for origins of SFs were well analysed systematically.

Effect of foil orientation on damage accumulation during irradiation in magnesium and annealing response of dislocation loops

The effect of foil orientation on damage accumulation behavior in commercial purity magnesium is investigated by in situ electron and ion irradiation. Transmission electron microscope has been used to study the dislocation loops formed by the agglomeration of point defects during irradiation. It has been observed that the ratio of prism plane to basal plane defects increases as the foil orientation is changed from basal to the prism foil. The ratio of vacancy to interstitial defects also increases in prism foils as compared to the basal foils. This point defect accumulation behavior is reversed when magnesium is irradiated with 1MeV Kr2+ ions and the formation of basal plane dislocation loops were only observed in prism foils and did not take place in the basal foils. Analysis showed that all the basal plane dislocation loops have Burgers vector of the type 16〈202¯3〉 and are interstitial in nature whereas prism plane dislocation loops have Burgers vector of the type 13〈112¯0〉 and are of mixed interstitial/vacancy in character.In situ annealing experiments at different temperatures performed on electron irradiated magnesium foils suggest that those dislocation loops that become thermodynamically unstable anneal out in a matter of few seconds whereas other stable dislocation loops continue to shrink by absorbing surrounding vacancy clusters. The activation energy for the shrinkage of the interstitial dislocation loops has been derived and the results show that the shrinkage of interstitial dislocation loops takes place by the mechanism of vacancy assisted self diffusion.

Irradiation damage in commercial purity magnesium

In-situ electron irradiation damage within the transmission electron microscope has been studied and compared with ion irradiation on commercial purity magnesium. Magnesium has the same crystal structure (hcp) and a similar c/a ratio to that of zirconium and is considered a suitable model system and useful analogue to zirconium (an important structural material for nuclear reactors). The results presented in this paper suggest that microstructure developed during irradiation is modified by the nature of the irradiating particles. The basal plane dislocation loops with Burgers vector 1 / 6 〈 2 0 2 ¯ 3 〉 are formed by electron irradiation and analysis showed that all of these dislocation loops are interstitial in nature. Vacancy type prism plane dislocation loops with Burgers vector 1 / 3 〈 1 1 2 ¯ 0 〉 also form but to a lesser degree than the basal plane dislocation loops. Ion irradiation for a dose up to 0.1 dpa produces a majority of interstitial type prism plane dislocation loops with Burgers vector 1 / 3 〈 1 1 2 ¯ 0 〉 . Void formation also takes place in ion irradiated samples subjected to a higher dose of 0.7 dpa.

Ostwald ripening of interstitial-type dislocation loops in 4H-silicon carbide

The annealing behavior of interstitial-type basal plane dislocation loops in Al ion implanted 4H-SiC is investigated. It is shown that the loops undergo a dynamical ripening process. For annealing below 1700°C the total area of dislocation loops increases, indicating that point defects are still available for accumulation, but for annealing times longer than 100min at this temperature the value of the total loop area saturates. For longer annealing times, or higher temperatures, the dislocation loops are subjected to a conservative coarsening process, also known as Ostwald ripening. In this process the mean loop radius increases with increasing annealing time and temperature while the number of loops decreases. Meanwhile the summarized area of the loops stays constant. The observed ripening is suggested to occur by a mechanism, which involves coarsening by direct loop coalescence. Through this mechanism, loops on the same basal plane move towards each other until they coalesce into one, but loops on neighboring basal planes can only move until their loop edges meet (in a basal plane projection) where they remain. Climb along the c axis is not favorable as shown by experimental results and is suggested to be caused by the atomic configuration of the loop. Upon continuous annealing, this results in a situation where the loops are confined in clusters.

Multiplication of Basal Plane Dislocations via Interaction with c-Axis Threading Dislocations in 4H-SiC

AbstractSilicon carbide (SiC) substrates with chemical vapor deposition (CVD) grown epilayers have been characterized by synchrotron white beam X-ray topography (SWBXT). Large numbers of circular basal plane dislocation loops (BPDs) were observed in the substrate which were anchored by threading screw dislocations (SDs). Threading edge dislocations (TEDs) are not observed to play an important role in the multiplication of BPDs. A SD-assisted “conservative climb” model is proposed to explain the multiplication of BPDs during growth and/or post-growth processes. BPDs are shown to multiply on adjacent parallel basal planes via single SD-assisted as well as opposite sign SD-pair-assisted “conservative climb”.

Amorphization processes in Au ion irradiated GaN at 150–300 K

Epitaxial single-crystal gallium nitride (GaN) films on sapphire were irradiated at temperatures between 150 and 300 K using 1.0 MeV Au 2+ ions over a range of fluences. The accumulation of disorder on the Ga sublattice has been investigated based on 2.0 MeV He + RBS along the 〈0 0 0 1〉-axial channeling direction. In general, the degree of disorder in the irradiated GaN increases at low doses and saturates at intermediate doses; at higher doses, a rapid amorphization process occurs as a result of the ingrowth of surface defects. Results from this study indicate that there may be a dynamic recovery stage on the Ga sublattice in GaN between 250 and 300 K. High-resolution TEM studies show that the microstructure in the disorder saturation stage contains a dense network of planar defects (basal-plane dislocation loops and stacking faults), while the more highly disordered regime includes amorphous domains and small crystalline zones that are randomly oriented.

Dislocation motion around loaded notches in ice single-crystals

Synchrotron X-ray topography has been used to study dislocation behavior around notches in single-crystals of ice during in-situ deformation at a constant strain-rate. During deformation at −8°C a dislocation-depleted zone (DDZ) formed above the notch while a vertical array of dislocations formed below the notch. Modeling the interaction between basal plane dislocation loops and the notch stress field suggested that this DDZ arose from dislocations gliding completely through the specimen. Dislocation activity around the notch at −8°C was much greater than that observed at −40°C and −60°C.

Dislocation motion around loaded notches in ice single crystals

ABSTRACTSynchrotron X-ray topography has been used to study dislocation behavior around a notch in single crystal ice during in-situ deformation at a constant strain-rate of 1 × 10−8s−1 and a temperature of -8 °C. During deformation a dislocation depleted zone (DDZ) formed above the notch. Modeling the interaction between basal plane dislocation loops and the notch stress field suggested that this DDZ arose from dislocations gliding completely through the specimen.

The formation of c-component defects in zirconium alloys during neutron irradiation

There is a correlation between the existence of c- component defects and accelerated irradiation growth of annealed Zr and Zircaloy-2 and -4. Analysis shows that these defects are vacancy, basal plane dislocation loops having Burgers vectors of b = 1 6 〈202̄3〉. Their formation is related to the solute content of the Zr (either as an impurity or as an alloying element) and, in many cases, appears to be dependent on the dissolution of intermetallic particles during irradiation. This could account for why irradiation growth of annealed Zircaloys increases at high temperatures and why there is an incubation period for the onset of accelerated or “Breakaway” growth, i.e. because fluence and temperature are factors governing the extent of solute dissolution.