Punched-out Dislocations Research Articles

Enhanced diffusion of boron by oxygen precipitation in heavily boron-doped silicon

The enhanced diffusion of boron has been investigated by analyzing out-diffusion profiles in the vicinity of the interface between a lightly boron-doped silicon epitaxial layer and a heavily boron-doped silicon substrate with a resistivity of 8.2 mΩ cm and an oxide precipitate (O.P.) density of 108–1010 cm−3. It is found that the boron diffusion during annealing at 850–1000 °C is enhanced with the increase of the oxide precipitate density. On the basis of a model for boron diffusion mediated by silicon self-interstitials, we reveal that the enhanced diffusion is attributed to self-interstitials supersaturated as a result of the emission from oxide precipitates and the absorption by punched-out dislocations. In addition, the temperature dependence of the fraction of the self-interstitial emission obtained analyzing the diffusion enhancement well explains the morphology changes of oxide precipitates reported in literature.

Transition from a punched-out dislocation to a slip dislocation revealed by electron tomography

Punched-out dislocations emitted from an octahedral oxide precipitate in single-crystal silicon were investigated using high-voltage electron microscopy and tomography (HVEM-tomography) to understand the mechanism of softening caused by the oxide precipitates. In the present paper, direct evidence of the transition of a punched-out prismatic dislocation loop to a slip dislocation is presented. The punched-out dislocation grows into a large matrix dislocation loop by absorption of interstitial atoms, which were produced during oxide precipitation.

Interaction of metal impurities with extended defects in crystalline silicon and its implications for gettering techniques used in photovoltaics

Multicrystalline silicon materials for photovoltaic applications inherently contain extended defects like grain boundaries, dislocations, microdefects and in some cases also second phase precipitates due to high concentrations of light elements (carbon, nitrogen or oxygen) and transition metal impurities. The latter are known to reduce the minority carrier lifetime and hence should be removed by gettering during solar cell processing. This paper discusses the influence of extended defects on the spatial distribution of copper- and nickel-related silicide precipitates for a model system containing a small angle grain boundary and in one part silicon oxide pecipitates partly associated with punched-out dislocations. Phosphorus-diffusion gettering under conditions of mostly precipitated metal impurities is discussed in terms of quantitative simulations. It is shown that two regimes can be distinguished where gettering kinetics are either limited by precipitate dissolution or phosphorus in-diffusion. Finally, binding of metal impurities to dislocations is considered and its effect on gettering kinetics is illustrated in terms of gettering simulations.

Oxidation-induced stacking faults in nitrogen-doped czochralski silicon investigated by transmission electron microscope

The extended-defects generated during oxidation in both of nitrogen-doped Czochralski (NCZ) silicon and common Czochralski (CZ) silicon have been investigated by Transmission Electron Microscopy(TEM). It is found that the size of the oxidation-induced stacking faults (OSFs) decreases with the increase of oxidation time in NCZ silicon, and punched-out dislocations could also be observed. While in CZ silicon, there are many polyhedral oxygen precipitates generated, and the size of OSFs increases with increasing oxidation time.

Mechanical properties of 300 mm wafers

A simulator to calculate slip length during thermal processes on the basis of dislocation kinetics was developed. Using the simulator, slip length during the thermal process of 300 mm wafers was calculated. From the results of the calculations it was clarified that: (1) control of the ramping rate above 1000°C is important to avoid slip generation; (2) when the wafer spacing is 9.5 or 6.75 mm, slip dislocation will generate during the process at a ramping rate greater than 1.8 or 1.3°C/min, respectively; and (3) slip length decreased using a ring-like support instead of a conventional four-point support. The generation of punched-out dislocations and slip in a heavily boron-doped p+ epitaxial substrate was also examined by indentation tests and thermal stress tests. It was found that the generation of dislocations could be reduced compared with that of p/p− wafers by using p+ epitaxial substrates.

Pinning Effect of Punched-Out Dislocations in Carbon-, Nitrogen- or Boron-Doped Silicon Wafers

The pinning effect of punched-out dislocations in carbon-, nitrogen- or boron-doped Czochralski-grown silicon wafers was investigated using an indentation method. The size of a rosette pattern which corresponds to the distance of dislocation movement was measured after heat-treatment without any thermal stress. It was found that the rosette size decreased by carbon, nitrogen and boron doping with a concentration of 2.0×1016–1.4×1017, 5.3×1013–5.3×1014 and 2.0×1018–2.0×1019 atoms/cm3, respectively. The rosette size was approximately proportional to the power -1/3 of carbon, -1/10 of nitrogen and -1/10 of boron concentration, which differed from the reported power of -2/3 of oxygen concentration.

Effect of heavy boron doping on the lattice strain around platelet oxide precipitates in Czochralski silicon wafers

The effect of heavy boron doping on local lattice strain around platelet oxide precipitates in Czochralski silicon wafers was investigated quantitatively by convergent beam electron diffraction (CBED). Lightly boron doped (p−) polished wafers, including platelet precipitates with density of about 5×109/cm3 and with an edge length of about 500 nm, were prepared with an isothermal annealing at 800 °C for 700 h. Heavy boron doped (p/p+) epitaxial wafers, including an almost equal precipitate density and length to p− wafers, were also prepared with an isothermal annealing at 800 °C for 200 h. It was found by strain analysis from high-order Laue zone patterns in the CBED disk that (i) the type of lattice strain was coincident in p− and p/p+ wafers: the strain along the normal direction of the platelet precipitate was compressive, while the strain along the parallel direction of the platelet was tensile; (ii) the strain in p/p+ wafers was smaller than that in p− wafers; and (iii) the punched-out dislocations were generated by platelets only in p− wafers. The results of (ii) and (iii) indicate that the strain around the platelets is relaxed due to the heavy boron doping.

Gettering of copper by bulk stacking faults and punched-out dislocations in Czochralski-grown silicon

The characteristics of Cu precipitation on various types of defects associated with oxygen precipitation in Czochralski-grown silicon are investigated by transmission electron microscopy and the electron-beam-induced-current technique. Specimens containing dominantly either punched-out dislocations or bulk stacking faults were intentionally contaminated with Cu at various temperatures and cooled at three different rates. Colonies of Cu precipitates developed irrespective of cooling rate, apparently originating from punched-out dislocations developed around oxygen precipitates. In heavily contaminated specimens cooled fast from the contamination temperature, Cu also precipitates on Frank partial dislocations bounding stacking faults. During slow cooling, precipitation of Cu takes place on Frank partials only in lightly contaminated specimens but never in heavily contaminated specimens. Cu precipitates in colonies are thermally more stable than those formed on Frank partials. It is concluded that punched-out dislocations are more favorable precipitation sites for Cu than Frank partials.

Modelling and numerical computation of transient internal damping due to thermal expansion mismatch between matrix and particles in metal matrix composites

A new model of the transient internal damping (ID) associated with the emission and movements of dislocations around particles in metal matrix composites (MMCs) is developed. These movements on which the proposed model is based are mainly induced during thermal cycles by the internal stress field around particles, which results from the thermal expansion mismatch between particles and matrix. First, from this thermally induced internal stress field, calculated by the Eshelby method, and the critical shear stress opposing the motion of dislocations in their glide plane in the matrix, the number and positions of punched-out dislocations are determined as a function of temperature. Second, the actual positions due to the superposition on the thermal stress field of the alternating shear stress associated with the pendulum oscillations are calculated by a perturbation method. Then the internal damping is derived from the contribution of the dislocation movements to the inelastic strain over a period of oscillation. The role of the experimental parameters is investigated. This simulated ID is compared with experimental results obtained in the case of aluminium-based MMCs. A good agreement between simulated and experimental IDs is found.

Strengthening of a particulate metal matrix composite by quenching

The strengthening of a particulate metal matrix composite due to quenching was studied both experimentally and theoretically. The strengthening was attributed to two mechanisms: punched-out dislocations due to CTE mismatch strain and back stress. Both mechanisms were analyzed by theoretical modeling leading to a good agreement between the experimentally observed strengthening and the prediction by the models. A parametric study revealed that the volume fraction of particulate, quenching temperature and its temperature differential and particulate size are the major variables influencing the increase in composite flow stress.

Formation of clean stacking faults and oxide microdefects in Czochralski silicon

The effect of intrinsic gettering action by microdefects in Czochralski silicon containing substantial amounts of oxygen on the properties of stacking faults (SF’s) has been studied. It has been found that high-temperature oxidation of swirl-free wafers results in the generation of SF’s decorated with probably heavy metallic impurity precipitates around the Frank partial dislocations, while high-temperature oxidation of swirled wafers results in the generation of clean SF’s. The occurrence of clean SF’s is correlated with the presence of microdefects in the bulk of wafers. Transmission electron microscopy (TEM) observations revealed that the microdefects were the precipitates (believed to be some form of silicon oxide) with generator dislocations or punched-out dislocations and that these dislocations captured impurity precipitates. These observations suggest that the microdefect dislocations are more attractive to metallic impurities in silicon than the Frank partial dislocations of the SF’s.