On the mechanism of recombination at oxide precipitates in silicon (original) (raw)

Recombination at Oxide Precipitates in Silicon

Solid State Phenomena, 2011

Transient and quasi-steady-state photoconductance methods were used to measure minority carrier lifetime in p-type Czochralski silicon processed in very clean conditions to contain oxide precipitates. Precipitation treatments were varied to produce a matrix of samples, which were then characterised by chemical etching and transmission electron microscopy to determine the density and morphology of the precipitates. The lifetime component associated with the precipitates was isolated by preventing or factoring out the effects of other known recombination mechanisms. The lifetime component due to unstrained precipitates could be extremely high (up to ~4.5ms). Recombination at unstrained precipitates was found to be weak, with a capture coefficient of ~8 x 10 -8 cm 3 s -1 at an injection level equal to half the doping level. Strained precipitates and defects associated with them (dislocations and stacking faults) act as much stronger recombination centres with a capture coefficient of ~3 x 10 -6 cm 3 s -1 at the same level of injection. The lifetime associated with strained precipitates increases with temperature with a ~0.18eV activation energy over the room temperature to 140°C range. The shape of the injection level dependence of lifetime was similar for all the specimens studied, with the magnitude of the lifetime being dependent on the precipitate density, strain state and temperature, but independent of precipitate size.

Recombination via point defects and their complexes in solar silicon

physica status solidi (a), 2012

Electronic grade Czochralski and float zone silicon in the as grown state have a very low concentration of recombination generation centers (typically <10 10 cm À3). Consequently, in integrated circuit technologies using such material, electrically active inadvertent impurities and structural defects are rarely detectable. The quest for cheap photovoltaic cells has led to the use of less pure silicon, multi-crystalline material, and low cost processing for solar applications. Cells made in this way have significant extrinsic recombination mechanisms. In this paper we review recombination involving defects and impurities in single crystal and in multi-crystalline solar silicon. Our main techniques for this work are recombination lifetime mapping measurements using microwave detected photoconductivity decay and variants of deep level transient spectroscopy (DLTS). In particular, we use Laplace DLTS to distinguish between isolated point defects, small precipitate complexes and decorated extended defects. We compare the behavior of some common metallic contaminants in solar silicon in relation to their effect on carrier lifetime and cell efficiency. Finally, we consider the role of hydrogen passivation in relation to transition metal contaminants, grain boundaries and dislocations. We conclude that recombination via point defects can be significant but in most multi-crystalline material the dominant recombination path is via decorated dislocation clusters within grains with little contribution to the overall recombination from grain boundaries.

On the recombination activity of oxygen precipitation related lattice defects in silicon

The recombination activity of oxygen precipitation related lattice defects in p-and n-type silicon is studied with photoluminescence (PL) and microwave absorption (MWA) techniques. A direct correlation is observed between the amount of precipitated oxygen and the extended defect density on one hand and the minority carrier lifetime and PL activity on the other hand. The PL analyses show as dominant features in the spectra the D l and D2 lines. The relative amplitude of the D-lines in the different samples is investigated as a function of the oxygen content, defect density and excitation level. The results are correlated with those of complementary techniques and are interrelated on the basis of Shockley-Read-Hall (SRH) theory.

The effect of oxide precipitates on minority carrier lifetime in n-type silicon

Journal of Applied Physics, 2015

Supersaturated levels of interstitial oxygen in Czochralski silicon can lead to the formation of oxide precipitates. Although beneficial from an internal gettering perspective, oxygen-related extended defects give rise to recombination which reduces minority carrier lifetime. The highest efficiency silicon solar cells are made from n-type substrates in which oxide precipitates can have a detrimental impact on cell efficiency. In order to quantify and to understand the mechanism of recombination in such materials, we correlate injection level-dependent minority carrier lifetime data measured with silicon nitride surface passivation with interstitial oxygen loss and precipitate concentration measurements in samples processed under substantially different conditions. We account for surface recombination, doping level, and precipitate morphology to present a generalised parameterisation of lifetime. The lifetime data are analysed in terms of recombination activity which is dependent on precipitate density or on the surface area of different morphologies of precipitates. Correlation of the lifetime data with interstitial oxygen loss data shows that the recombination activity is likely to be dependent on the precipitate surface area. We generalise our findings to estimate the impact of oxide precipitates with a given surface area on lifetime in both n-type and p-type silicon. V

Minority carrier lifetime in silicon photovoltaics: The effect of

2014

Single-crystal Czochralski silicon used for photovoltaics is typically supersaturated with interstitial oxygen at temperatures just below the melting point. Oxide precipitates therefore can form during ingot cooling and cell processing, and nucleation sites are typically vacancy-rich regions. Oxygen precipitation gives rise to recombination centres, which can reduce cell efficiencies by as much as 4% (absolute). We have studied the recombination behaviour in p-type and n-type monocrystalline silicon with a range of doping levels intentionally processed to contain oxide precipitates with a range of densities, sizes and morphologies. We analyse injection-dependent minority carrier lifetime measurements to give a full parameterisation of the recombination activity in terms of Shockley-Read-Hall statistics. We intentionally contaminate specimens with iron, and show recombination activity arises from iron segregated to oxide precipitates and surrounding defects. We find that phosphorus diffusion gettering reduces the recombination activity of the precipitates to some extent. We also find that bulk iron is preferentially gettered to the phosphorus diffused layer rather than to oxide precipitates.

Fast-forming boron-oxygen-related recombination center in crystalline silicon

Applied Physics Letters, 2005

The mechanism of a fast carrier lifetime degradation effect proceeding within seconds in boron-doped Czochralski silicon is investigated. The decrease in the carrier lifetime is attributed to the formation of a deep boron-oxygen-related recombination center with a strongly asymmetric electron-to-hole capture cross section ratio of 100. The center is activated in a two-step process. In a first step, the electron quasi-Fermi level has to be shifted above an energy level at E V + 0.635 eV before, in the second step, the actual activation occurs via a thermally activated process with an energy barrier of 0.23 eV. A defect model is proposed to explain the observed activation behavior.

Performance Degradation of Silicon Solar Cells Triggered by Carrier Recombination

The formation process of the metastable boron-oxygen-related defect complex is analysed by investigating the degradation of the open-circuit voltage of Cz-Si solar cells in the dark as a function of an applied voltage at temperatures ranging from 298 to 373 K. We provide clear experimental evidence of the defect formation not only occurring as a consequence of illumination or the injection of minority carriers via a pn-junction but also under apparent equilibrium conditions at elevated temperatures in the dark. By applying a reverse voltage a partial suppression of the defect formation can be achieved. All results can consistently be explained by a recombination-enhanced defect formation mechanism correlated with the total recombination rate. Combining the experimental results with calculations of the minority carrier densities corresponding to the applied voltages, we are able to show that 50% of the maximum defect concentration is already being formed at minority carrier concentrations as low as 5×10 9 cm -3 .

Performance-Limiting Oxygen-Related Defects in Silicon Solar Cells

ECS Transactions, 2006

The energy conversion efficiency of solar cells fabricated from oxygen-containing crystalline silicon wafers with boron as p-type dopant is ultimately limited by boron-oxygen-related recombination centers which form under illumination or forward-biasinduced electron injection into the p-type base of the cell. This paper reviews the recent progress in understanding the physics of this degradation effect. It is shown that two different types of boronoxygen centers are simultaneously formed at very different formation rates. Electronic defect properties, formation mechanisms and the impact on device properties are discussed.

Recombination centers in electron-irradiated Czochralski silicon solar cells

Journal of Applied Physics, 1994

The defect responsible for the minority-carrier lifetime in p-type Czochralski silicon introduced by electron irradiation has been detected and characterized by deep-level transient spectroscopy and spin-dependent recombination. From the isotropic g value (2.00.55), the defect is tentatively identified as a Si dangling bond originating from a vacancy cluster. Its energetic location in the gap is at 630 meV below the conduction band. The electron and hole cross sections and their variation with temperature have been determined, and found to account for the tninority-carrier lifetime of the material.