S. Senkader

Oxygen-dislocation interactions in silicon at temperatures below 700 °C: Dislocation locking and oxygen diffusion

The locking of dislocations by oxygen atoms in Czochralski–silicon at temperatures between 350 and 700 °C has been studied. Both experimental and theoretical investigations were carried out for different oxygen concentrations, different annealing times (from 10 to 3×107 s), and different point defect concentrations. It was found that the unlocking stress of dislocations at low temperatures follows similar trends to those previously observed at higher temperatures and is determined by annealing temperature, time, and oxygen concentration. However, in the present temperature range, experimental results indicate an enhanced transport of oxygen to dislocations. Numerical simulations solving the diffusion equation for oxygen transport to the dislocations show that the effective diffusivity of oxygen at lower temperatures diverges from “normal” diffusivity of oxygen. We have shown that oxygen transport can be as much as three orders of magnitude higher than that which would be assumed by extrapolation of the “n...

On the locking of dislocations by oxygen in silicon

Abstract Locking of dislocations by oxygen atoms in Czochralski silicon has been investigated both experimentally and theoretically. Experiments were performed at annealing temperatures between 700 and 850°C for different annealing times and different oxygen concentrations. These showed five distinct regimes for the unlocking stress as a function of annealing time. First the unlocking stress increases almost linearly with time and then saturates. The saturation stress, the time needed to reach saturation and the duration of saturation depend on the annealing conditions and oxygen content. Following the saturation a rapid increase and a second saturation of the unlocking stress with increasing annealing time were observed. Finally after long anneals the locking effect is much reduced. From the temperature dependence of the first saturation stress the interaction energy between an oxygen atom and a dislocation has been deduced and it is shown that the change in entropy when an oxygen atom is trapped at a dislocation is significant. The transport of oxygen to dislocations has also been investigated by solving the diffusion equation numerically. For these calculations both trapping and emission of oxygen atoms from the dislocation core have been considered.

Onset of slip in silicon containing oxide precipitates

We report a study of the behavior of dislocations at oxide precipitates in (001) Czochralski silicon wafers for different oxide-precipitate sizes (100–600 nm), densities (108−1011 cm−3), and background oxygen concentrations (7.7×1017−10.35×1017 cm−3) using a bending technique with annular knife edges causing a biaxial stress distribution in the samples. The main advantage of the method we use is the possibility of detecting single slip events that may be caused by precipitates with special properties. We found that the stress level at which dislocation movement can be detected around precipitates depends mainly on the mean-precipitate diameter. The stress threshold at which dislocations begin to move can be increased by a thermal treatment prior to application of an external stress. This effect is due to the diffusion of oxygen to the dislocations causing a locking effect and shows that the dislocations are associated with the oxide precipitates prior to any external stress being applied. It has been show...

Oxygen and Nitrogen Transport in Silicon Investigated by Dislocation Locking Experiments

The behavior of oxygen and nitrogen impurities in silicon has been investigated using a novel dislocation locking technique. The locking effect of oxygen in Czochralski silicon (CZ-Si) was investigated in the 350-850°C temperature range and was found to display five well-defined regimes as a function of annealing time. Results indicate that enhanced transport of oxygen to dislocations takes place for temperatures below ∼700°C . Numerical simulations of the enhanced oxygen transport indicate that the effective diffusivity becomes dependent on oxygen concentration with an activation energy of approximately 1.5eV . The same technique has been used to investigate nitrogen transport in nitrogen-doped float-zone silicon in the 550-830°C temperature range and shows nitrogen to have a comparable locking effect to oxygen in CZ-Si, despite being present in a concentration that is 2 orders of magnitude lower. The stress required to unlock dislocations at 550°C which have previously been immobilized by nitrogen during an annealing step, initially increases approximately linearly with the duration of the anneal before saturating to a steady-state value of approximately 50MPa for all anneal temperatures investigated. An expression for the transport of nitrogen to the dislocations was deduced, which has an activation energy of 1.45eV

Enhanced oxygen diffusion in highly doped p-type Czochralski silicon

The locking of dislocations by oxygen has been investigated experimentally in Czochralski silicon (Cz-Si) with different concentrations of shallow dopants. Specimens containing well-defined arrays of dislocation half-loops were subjected to isothermal anneals in the 350–550°C temperature range, and the stress required to bring about dislocation motion at 550°C was then measured. This dislocation unlocking stress was found to increase with annealing time due to oxygen diffusion to the dislocation core. The dislocation unlocking stress was measured in n-type Cz-Si with a high antimony doping level (∼3.4×1018cm−3) and p-type Cz-Si with a low boron doping level (∼1.3×1015cm−3). An analysis of the data taking the different oxygen concentrations into account showed that the rate of increase in dislocation unlocking stress was unaffected by the high level of antimony doping. This indicates that a high antimony doping level has no significant effect on oxygen transport for the conditions used in this experiment. ...

The influence of nitrogen on dislocation locking in float-zone silicon

Dislocation locking by nitrogen impurities has been investigated in float-zone silicon with nitrogen concentrations of 2.2 x 1015cm-3 and 3 x 1014cm-3. The stress required to unlock dislocations pinned by nitrogen impurities was measured as a function of annealing time (0 to 2500 hours) and temperature (550 to 830°C). For all conditions investigated the locking effect was found to increase linearly with annealing time before saturating. It is assumed that the rate of increase of unlocking stress with annealing time is a measure of transport of nitrogen to the dislocation core. This rate of increase was found to depend linearly on nitrogen concentration, which is consistent with transport by a dimeric species, whose activation energy for diffusion is approximately 1.4eV. The saturation unlocking stress has been found to be dependent on the nitrogen concentration. Additionally, the temperature dependence of the stress required to move dislocations immobilised by nitrogen impurities has been studied. By assuming a value for the binding energy of the nitrogen to the dislocation, the density of the locking species at the dislocation core has been calculated.

Generation of dislocation glide loops in Czochralski silicon

Critical stresses necessary to generate dislocation glide loops in Czochralski silicon containing oxide precipitates have been investigated. Using three-point bending and etching techniques, it was possible to determine the minimum shear stress required to generate dislocation glide loops from controlled distribution of precipitates under constant-stress conditions. The generation of glide dislocations was investigated in samples with different oxide precipitate sizes and different numbers of dislocations initially attached to precipitates. It has been found that the value of the critical resolved shear stress for generating dislocation glide loops depends on the duration of the applied stress. A qualitative model involving punched-out prismatic loops was considered for the explanation of the experimental data. It was found that glide dislocations must be generated from pre-existing large loops probably associated with particular oxide precipitates or other complex defects.

The use of numerical simulation to predict the unlocking stress of dislocations in Cz-silicon wafers

Under certain conditions, interstitial oxygen atoms in Czochralski-grown silicon (Cz-Si) are known to hinder or completely stop dislocation motion. As a result, oxygen impurities can remarkably improve the mechanical strength of silicon wafers as they are transported and bound to dislocations. The amount of oxygen bound to dislocations--and with it the wafers resistance to plastic deformation--is oxygen concentration, time, temperature and, importantly, thermal history dependent. It is also reversible. A numerical model has been developed to predict the shear stress necessary to move glide dislocations in Cz-Si wafers during the course (time evolution) of different heat treatments and sequences of heat treatments typical of integrated circuit fabrication. This model accurately accounts for the experimentally observed behaviour of isolated straight dislocations over a wide range of controlled conditions. Modifications to heat treatments can be predicted by using this numerical simulation so that wafer warpage can be minimised during device processing.

A study of oxygen dislocation interactions in CZ-Si

The interaction between dislocations and oxygen atoms in silicon has been studied. The effect of immobilization of dislocations by the segregation of oxygen to the dislocation core (dislocation locking) has been investigated for different oxygen concentrations and annealing conditions. It has been revealed that oxygen locking of dislocations shows three different regimes of behaviour. A numerical model for the dislocation locking process of oxygen atoms has been presented and used to interpret the experimental results.

Controlled field effect surface passivation of crystalline n-type silicon and its application to back-contact silicon solar cells

Surface passivation continues to be a significant requirement in achieving high solar-cell efficiency. Single layers of SiO2 and double layers of SiO2/SiN surface passivation have been widely used to reduce surface carrier recombination in silicon solar cells. Passivation films reduce surface recombination by a combination of chemical and electric field effect components. Dielectric films used for this purpose, however, must also accomplish optical functions at the cell surface. In this paper, field effect passivation is seen as a potential method to enhance the passivation properties of a dielectric film while preserving its optical characteristics. It is observed that the field effect can make a large reduction in surface recombination by using corona charged ions deposited on the surface of a dielectric film. The effect is studied for both SiO2 and SiO2/SiN layers, and surface recombination velocities of less than 9 cm/s and 16 cm/s are inferred, respectively, on n-type, 5 Ωcm, Cz-Si. This improvement in passivation was stabilized for period of over a year by chemically treating the films to prevent water absorption. Intense ultraviolet radiation was seen to diminish the surface recombination velocity to its initial value in a time period of up to 7 days. Additionally, external deposition of charge on to the SiO2/SiN passivated front surface of back-contact n-type silicon solar cells provides a 2.5 % relative improvement in conversion efficiency due to enhanced and controlled field effect passivation.