Dislocation Clusters Research Articles

Impact of the firing temperature profile on light induced degradation of multicrystalline silicon

Light‐ and elevated temperature‐induced degradation in multicrystalline silicon can reduce the efficiency of solar cells significantly. In this work, the influence of the firing process and its temperature profile on the degradation behaviour of neighbouring mc‐Si wafers is analysed. Five profiles with measured high peak temperatures ≥800 °C and varying heating and cooling ramps are examined. With spatially resolved and lifetime calibrated photoluminescence images, normalized defect concentrations N*t are calculated to determine the degradation intensity. Wafers that underwent a fast firing process typical for industrial solar cell production show a significantly stronger degradation than samples that were subjected to the same peak temperature but with slower heating and cooling rates. A spatially resolved analysis of the carrier lifetime in the whole wafer shows that the degradation begins in low lifetime areas around dislocation clusters, spreading into good grains after several hours. By the use of optimized ramp‐up and/or ramp‐down rates during the firing even at very high peak temperatures, light and elevated temperature induced degradation can be suppressed. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

The relationship of dislocation and vacancy cluster with yield strength in magnetic annealed UFG 1050 aluminum alloy

The evolutions of tensile properties and microstructures of ultrafine grained (UFG) 1050 aluminum alloy after annealing at 90–210°C for 4h without and with 12T high magnetic field were investigated by tensile test, electron back scattering diffraction pattern (EBSD), transmission electron microscopy (TEM) and positron annihilation lifetime spectroscopy (PALS). When annealing temperature increases from 90°C to 150°C, the yield strength (YS) of UFG 1050 aluminum alloy increases, it is because that the increase in the density of vacancy clusters due to the activated monovacancies and the high angle boundaries (HABs) having more stable structures, both of them can act as effective barriers to dislocation motion during tensile deformation. When annealing at 210°C, the YS of UFG 1050 aluminum alloy deceases, it is because that the decrease in the vacancy clusters density due to the thermally activated the vacancy clusters annihilating at sinks and the dislocation density decreases. The YS of magnetic annealed samples are lower at 90°C and 150°C due to the lower density of dislocations and vacancy clusters. The difference of YS between samples annealed without and with magnetic field disappears at 210°C due to the sharply reduced strain hardening stage.

Impact of the nanostructuration on the corrosion resistance and hardness of irradiated 316 austenitic stainless steels

The influence of grain size and irradiation defects on the mechanical behavior and the corrosion resistance of a 316 stainless steel have been investigated. Nanostructured samples were obtained by severe plastic deformation using high pressure torsion. Both coarse grain and nanostructured samples were irradiated with 10MeV 56Fe5+ ions. Microstructures were characterized using transmission electron microscopy and atom probe tomography. Surface mechanical properties were evaluated thanks to hardness measurements and the corrosion resistance was studied in chloride environment. Nanostructuration by high pressure torsion followed by annealing leads to enrichment in chromium at grain boundaries. However, irradiation of nanostructured samples implies a chromium depletion of the same order than depicted in coarse grain specimens but without metallurgical damage like segregated dislocation loops or clusters. Potentiodynamic polarization tests highlight a definitive deterioration of the corrosion resistance of coarse grain steel with irradiation. Downsizing the grain to a few hundred of nanometers enhances the corrosion resistance of irradiated samples, despite the fact that the hardness of nanocrystalline austenitic steel is only weakly affected by irradiation. These new experimental results are discussed in the basis of couplings between mechanical and electrical properties of the passivated layer thanks to impedance spectroscopy measurements, hardness properties of the surfaces and local microstructure evolutions.

The dislocation structure of diamond crystals grown on seeds in the Mg-C system

The dislocation structure of diamond crystals grown in the Mg-C system at a pressure of 7GPa and temperatures of 1800–1900°C is studied by X-ray topography and selective etching. According to the selective etching data, diamond crystals have two types of dislocations whose outputs on the {100} faces of crystal are associated with two types of etch pits. It has been demonstrated that the etch pits with the side wall inclination angle of about 7° are formed at the outcrop points of full edge dislocations, while the etch pits with the inclination angle of about 4° are associated with 45° mixed dislocations. It has been found that the dislocation structure of diamonds grown at 1900°C is completely determined by the seed crystals structure and the dislocation density is 105cm−2. The dislocation density in the diamond crystals grown at 1800°C increases by two or three orders of magnitude due to nucleation of dislocations at the seed-overgrown layer interface and in the overgrown layer. The high dislocation density leads to the mosaic structure of crystals and misorientation of single blocks, up to 1°. Local ring clusters of edge dislocations were found to be the dominant source of growth layers on the {100} faces of diamond.

Different nucleation approaches for production of high-performance multi-crystalline silicon ingots and solar cells

Two different process approaches for growing high-performance multi-crystalline silicon (HPM-Si) material for photovoltaic applications are presented. Assisted nucleation was achieved by seeding on fine polycrystalline silicon chunks and on a rough silica crucible bottom, respectively. For each of the two methods industrially sized ingots of 650kg were directionally solidified. Wafers of these ingots and those from conventionally solidified multi-crystalline (mc) silicon ingots were used to produce solar cells on industrial line. Solar cell parameters were analyzed and compared.Both assisted nucleation methods are suitable for industrial growing of HPM-Si ingots. The grain size is more homogeneously distributed resulting in improved material quality. Less dislocation clustering results in higher average conversion efficiency of solar cells compared to those made of conventionally solidified mc-Si material. Moreover, it is also experimentally proven that assisted nucleation procedure is effective for both Siemens polysilicon feedstock and cheaper solar grade silicon. Further improvements of nucleation procedures, which can be used to achieve higher conversion solar cell efficiency and lower product cost, are proposed.

Effect of the fused quartz particle density on nucleation and grain control of high-performance multicrystalline silicon ingots

The nucleation process of high-performance multicrystalline silicon (HP mc-Si) growth seeded by fused quartz particles (FQP) through directional solidification is crucial for the ingot quality. To determine the optimal density of FQP and obtain a better nucleation process and the grain growth, we cast ingots using four different densities of FQP fixed on the bottom of the four quartz crucibles and covered them with a certain thickness of Si3N4 coating. FQP sizes of 30–50 mesh were used, and the influence of the fused quartz particle density on the nucleation mechanism, initial grain uniformity, grain size, density of dislocation clusters, and cell efficiency were analyzed. Compared with the ingots seeded with other three densities of FQP, the 220 particles/cm2 of FQP seeded ingot showed better uniformity of nucleation and initial grains. A large number of small uniform Si grains with lower density of dislocation clusters in the bottom of the ingot were observed. The average conversion efficiency of p-type solar cells manufactured with the 220 particles/cm2 seeded ingot (18.28%) was 0.19% higher than that manufactured with the 120 particles/cm2 seeded ingot (18.09%).

Plastic Deformation as an Origin of Dislocations in Cast Mono

The formation of dislocation clusters in cast mono ingots is investigated using etch pit density measurement and X-ray diffraction. The majority of detrimental dislocation clusters in the ingot originate in the seeding region.Beside the well-described dislocation source at the seed joints, a second origin of dislocation clusters is described in this work. Initial dislocation clusters appear in the seed by a rearrangement of dislocation cells to dislocation domains. It is demonstrated that a cell size below ∼ 50μm is required for this process. The dislocation density in these domains is about one order of magnitudes higher than in the cells. The high local dislocation density gains a very high shear stress. In consequence, the domain is able to penetrate the phase boundary between the remaining seed and the seeded material. Thus, the cluster is spreading unhindered into the ingot under the influence of thermo-mechanical stress during crystallization. X-ray diffraction measurements prove that the plastic deformation of the seeds leads in particular to such small cell sizes.

Spatially Resolved Analysis of Light Induced Degradation of Multicrystalline PERC Solar Cells

Light induced degradation at elevated temperatures in mc-Si PERC solar cells can be responsible for major efficiency losses. In this work, a spatially resolved analysis of the degradation behavior of a standard mc-Si PERC solar cell is performed based on carrier lifetime-calibrated photoluminescence (PL) images. An effective defect concentration is calculated from lifetime data at a fixed injection which allows for defect analysis taking into account any other defects. The time-dependent defect evolution is fitted to obtain the spatially resolved maximum effective defect concentration and the degradation time constant. No differences in the behavior between grains and dislocation clusters could be observed. Lower effective defect concentrations have been found around grain boundaries which could be an effect of denuded zones. In the case of our sample, slight local process and material parameter variations, namely the local firing temperature, have a much stronger impact than the crystal structure.

Secondary particles precipitates in Be-Fe alloys

Mossbauer spectra of monocrystalline Be-Fe alloy (0.85 % Fe) were obtained with the use of resonant detector after isothermal annealing at 600 °C for total duration of 2659 hours, and Mossbauer spectra of coarse-grained Be-Fe alloys (0,09-0,80 % Fe) samples were obtained after annealing at 500-600 °C for different durations. The alloys were prepared from the beryllium of different purity. Spectra of phases were fitted by a convolution equation of the three Lorentz lines. The coherent analysis of the solid solution decomposition process by means of the kinetic law classification and the secondary particles precipitate growth processes based on the diffusion models has been implemented. Nucleation on the numerous dislocation clusters and diffusion growth of the FeBe 11 nano-particles are the dominant processes in the analyzed materials. The phase distribution, the incubation period and the diffusion path were obtained. The dependence between the impurity concentration and Mossbauer parameters of the phases is discussed.

Local Strain Relaxation by A-type Dislocation Clusters in InxGal-xN/GaN Film with Indium Compositions of x = 0.07 and 0.12

Local Strain Relaxation by A-type Dislocation Clusters in InxGal-xN/GaN Film with Indium Compositions of x = 0.07 and 0.12

Numerical simulation of stresses and dislocations in quasi-mono silicon

The Alexander–Haasen model is applied for the analysis of dislocation dynamics in quasi-mono crystalline silicon. Model constants are re-calibrated using stress–strain measurements on small silicon samples under uniaxial compression. It is observed that the activation energy may decrease at low temperatures and the hardening parameter generally increases due to the presence of grown-in dislocation clusters. The calibrated model is applied to an idealized cooling process which allows for a discussion of the basic physical mechanisms leading to residual stresses in quasi-mono ingots. Residual stresses can be reduced by minimizing thermal stresses during the elastic–plastic transition, which was observed approximately between 1100°C and 750°C in the present case.

High-Performance and Traditional Multicrystalline Silicon: Comparing Gettering Responses and Lifetime-Limiting Defects

In recent years, high-performance multicrystalline silicon (HPMC-Si) has emerged as an attractive alternative to traditional ingot-based multicrystalline silicon (mc-Si), with a similar cost structure but improved cell performance. Herein, we evaluate the gettering response of traditional mc-Si and HPMC-Si. Microanalytical techniques demonstrate that HPMC-Si and mc-Si share similar lifetime-limiting defect types but have different relative concentrations and distributions. HPMC-Si shows a substantial lifetime improvement after P-gettering compared with mc-Si, chiefly because of lower area fraction of dislocation-rich clusters. In both materials, the dislocation clusters and grain boundaries were associated with relatively higher interstitial iron point-defect concentrations after diffusion, which is suggestive of dissolving metal-impurity precipitates. The relatively fewer dislocation clusters in HPMC-Si are shown to exhibit similar characteristics to those found in mc-Si. Given similar governing principles, a proxy to determine relative recombination activity of dislocation clusters developed for mc-Si is successfully transferred to HPMC-Si. The lifetime in the remainder of HPMC-Si material is found to be limited by grain-boundary recombination. To reduce the recombination activity of grain boundaries in HPMC-Si, coordinated impurity control during growth, gettering, and passivation must be developed.

Acoustic emission during the formation and breakdown of a dislocation cluster

We construct a model for the formation of a dislocation cluster and its evolution after the detachment of the head dislocation. The results obtained with the help of this model are used for calculating the acoustic emission signal accompanying the stages of cluster formation and breakdown. The elastic stresses in the signal under investigation are estimated. The data on relaxation of elastic stresses in the sample containing the cluster are reported.

TEM analysis of recrystallized double forged tungsten after exposure in JUDITH 1 and JUDITH 2

Five samples of recrystallized pure tungsten were exposed to transient heat loads using the electron beam of the JUDITH 1 and JUDITH 2 installations of Forschungszentrum Jülich. The heat flux and base temperature were the same for all samples; only the number of pulses and exposure device differed. Transmission electron microscopy was applied to determine the first defects that are introduced during exposure and to compare the effects of the two machines. With increasing number of pulses, first dislocations are formed near the grain boundaries, and then line dislocations and clusters of dislocations appear within the grains. Upon prolonged exposure, the dislocations migrate and cluster in dislocation pile-ups. Comparing exposure in JUDITH 1 to JUDITH 2, the amount of defects is much higher in the samples exposed in JUDITH 1.

Improved multicrystalline silicon ingot quality using single layer silicon beads coated with silicon nitride as seed layer

We propose to utilize single layer silicon beads (SLSB) coated with silicon nitride as cost-effective seed layer to grow high-quality multicrystalline silicon (mc-Si) ingot. The texture structure of silicon nitride provides a large number of nucleation sites for the fine grain formation at the bottom of the crucible. No special care is needed to prevent seed melting, which would lead to decrease of red zone owing to decrease of feedstock melting time. As we expected, mc-Si ingot seeded with SLSB was found to consist of small, different grain orientations, more uniform grain distribution, high percentage of random grain boundaries, less twin boundaries, and low density of dislocation clusters compared with conventional mc-Si ingot grown under identical growth conditions. These results show that the SLSB seeded mc-Si ingot has enhanced ingot quality. The correlation between grain boundary structure and defect structure as well as the reason responsible for dislocation clusters reduction in SLSB seeded mc-Si wafer are also discussed.

Characterization of electrically-active defects in ultraviolet light-emitting diodes with laser-based failure analysis techniques

Laser-based failure analysis techniques demonstrate the ability to quickly and non-intrusively screen deep ultraviolet light-emitting diodes (LEDs) for electrically-active defects. In particular, two laser-based techniques, light-induced voltage alteration and thermally-induced voltage alteration, generate applied voltage maps (AVMs) that provide information on electrically-active defect behavior including turn-on bias, density, and spatial location. Here, multiple commercial LEDs were examined and found to have dark defect signals in the AVM indicating a site of reduced resistance or leakage through the diode. The existence of the dark defect signals in the AVM correlates strongly with an increased forward-bias leakage current. This increased leakage is not present in devices without AVM signals. Transmission electron microscopy analysis of a dark defect signal site revealed a dislocation cluster through the pn junction. The cluster included an open core dislocation. Even though LEDs with few dark AVM defect signals did not correlate strongly with power loss, direct association between increased open core dislocation densities and reduced LED device performance has been presented elsewhere [M. W. Moseley et al., J. Appl. Phys. 117, 095301 (2015)].

The Nature and Multiscale Techniques for Characterizatoin of Mechanical Properties: from Nanostructured Materials to Single Macromolecules Part III. Nanoindentation and Atomic Mechanisms of Local Plastic Deformation

Up-to-date techniques for characterization of mechanical properties of materials (including biological ones) at all scale levels: macro-, micro-, nano- and atomic-molecular ones are considered. Particular attention is paid to atomic mechanisms of transition from elastic to plastic deformation. It is shown that a decrease in the size of crystalline object or its structural elements leads to a decrease in the activation volume, which characterizes the sizes of plastic deformation carrier, up to a volume comparable with atomic/ionic volume. It means that carriers of plastic deformation in nanoobjects are predominantly individual atoms and/or their small-atomic-clusters. As the size of the object or the size of plastic deformation zone increase, dislocation mechanisms start to operate. In macrobodies the formation of large clusters of dislocations, twins, grain boundaries migration, pore formation, micro-cracks become dominant.

Indentation-introduced dislocation rosettes and their effects on the carrier transport properties of CdZnTe crystal

Herein, we study the crystallographic configuration of rosette-like dislocation clusters introduced by indentation in CdZnTe (CZT) crystal and the effects of the dislocations on the carrier transport process. The Vickers hardness test was used to generate dislocations artificially on the () Te face of the CZT crystal at room temperature. Etch pit density (EPD) revealed by Everson etchant showed that the dislocation density increased from 4.9 × 104 cm−2 to a concentration too high to be evaluated after the indentation. A model was proposed to explain the rosette-like dislocation clusters. It was verified that the tetrahedron with small size and high EPD belonged to Te(g) dislocations on the () Te face. The hexagon-like distribution of pile-ups formed during the indentation was determined by the zinc-blende structure of CZT, rather than the direction of the indenter. The dislocation pile-up during propagation was observed and analyzed based on the rule proposed by Lomer and Cottrell. The ion beam induced charge (IBIC) measurements showed low carrier collection efficiency (CCE) in the dislocation generation areas, which confirms that the dislocations and related defects can significantly degrade the CCE of a CZT detector. Measurements of low temperature photo-luminescence (PL) showed that the emission peak located at 1.485 eV highly increased in the indentation area and was responsible for the performance degradation of the detectors.

Recent Progress of Crystal Growth Technology for Multi-Crystalline Silicon Solar Ingot

In recent years, silicon solar cells continue to remain the main stream in photovoltaic (PV) industry, particularly of made from multi-crystalline silicon (mc-Si). The progress of crystal growth technology for mc-Si ingot using directional solidification (DS) is particularly significant. With the breakthrough of the so-called high-performance (HP) mc-Si technology in 2011, the mc-Si solar cell efficiency had increased from 16.6% in 2011 to 18 % or beyond in 2013. Nowadays, HP mc-Si, solar cells from a normal screen-printing aluminum back surface field (Al-BSF) production line could easily reach 18.3%. With the passivated emitter and rear cell (PERC) structure using PECVDalumina passivation, an average efficiency of over 19.2% could also be obtained. The emerging of HP mc-Si almost blocked the development of mono-like technology in 2012, and pushed p-type mono-Si cells to higher efficiency by using advanced technology. Unlike the conventional way of having large grains and electrically-inactive twin boundaries, the growth of HP mc-Si is from small and uniform grains having more random GBs. The grains developed from such grain structures significantly relaxes the thermal stress and suppresses the massive generation and propagation of dislocation clusters. Currently, most of commercial mc-Si ingots are grown by this concept, which could be implemented by seeded with small silicon particles or using nucleation agent coatings. The seeded growth has been well adopted in industry. However, the melting control of the seed layer and the thick red zone induced remain key issues in mass production. Several methods have been considered to resolve these issues with some success. The use of nucleation agent layers is a simpler approach, but the control of initial grain structures remains challenging.

Xe diffusion and bubble nucleation around edge dislocations in UO2

Recently it has been suggested that dislocations, generated by radiation damage, may increase the rate of fission gas diffusion from the fuel grains, an affect which is at present not incorporated into fuel performance codes. Therefore, we perform molecular dynamics simulations employing empirical potentials to investigate the diffusion of Xe atoms around edge dislocations in UO2 to establish the importance of this pathway for fission gas release. The results suggest that for isolated atoms near the dislocation the activation energy for Xe diffusion is dramatically reduced relative to the bulk. However, Xe atoms diffusing along the dislocation cluster together to form small bubbles, these bubbles incorporate all of the isolated mobile Xe atoms thereby inhibiting fast diffusion of Xe along the dislocation core.