Preventing light-induced degradation in multicrystalline silicon

Multicrystalline silicon (mc-Si) is currently dominating the silicon solar cell market due to low ingot costs, but its efficiency is limited by transition metals, extended defects, and light-induced degradation (LID). LID is traditionally associated with a boron-oxygen complex, but the origin of the degradation in the top of the commercial mc-Si brick is revealed to be interstitial copper. We demonstrate that both a large negative corona charge and an aluminum oxide thin film with a built-in negative charge decrease the interstitial copper concentration in the bulk, preventing LID in mc-Si.Peer reviewe

Modeling phosphorus diffusion gettering of iron in single crystal silicon

We propose a quantitative model for phosphorus diffusion gettering (PDG) of iron in silicon, which is based on a special fitting procedure to experimental data. We discuss the possibilities of the underlying physics of the segregation coefficient. Finally, we show that the proposed PDG model allows quantitative analysis of gettering efficiency of iron at various processing conditions.Peer reviewe

Experimental and theoretical study of heterogeneous iron precipitation in silicon

Heterogeneous iron precipitation in silicon was studied experimentally by measuring the gettering efficiency of oxide precipitate density of 1×10exp10cm−3. The wafers were contaminated with varying iron concentrations, and the gettering efficiency was studied using isothermal annealing in the temperature range from 300 to 780°C. It was found that iron precipitation obeys the so called s-curve behavior: if iron precipitation occurs, nearly all iron is gettered. For example, after 30 min annealing at 700°C, the highest initial iron concentration of 8×10exp13cm−3 drops to 3×10exp12cm−3, where as two lower initial iron concentrations of 5×10exp12 and 2×10exp13cm−3 remain nearly constant. This means that the level of supersaturation plays a significant role in the final gettering efficiency, and a rather high level of supersaturation is required before iron precipitation occurs at all. In addition, a model is presented for the growth and dissolution of iron precipitates at oxygen-related defects in silicon during thermal processing. The heterogeneous nucleation of iron is taken into account by special growth and dissolution rates, which are inserted into the Fokker-Planck equation. Comparison of simulated results to experimental ones proves that this model can be used to estimate internal gettering efficiency of iron under a variety of processing conditions.Peer reviewe

Modeling boron diffusion gettering of iron in silicon solar cells

In this paper, a model is presented for boron diffusion gettering of iron in silicon during thermal processing. In the model, both the segregation of iron due to high boron doping concentration and heterogeneous precipitation of iron to the surface of the wafer are taken into account. It is shown, by comparing simulated results with experimental ones, that this model can be used to estimate boron diffusion gettering efficiency of iron under a variety of processing conditions. Finally, the application of the model to phosphorus diffusion gettering is discussed.Peer reviewe

Quantitative copper measurement in oxidized p-type silicon wafers using microwave photoconductivity decay

We propose a method to measure trace copper contamination in p-type silicon using the microwave photoconductivity decay (μ-PCD) technique. The method is based on the precipitation of interstitial copper, activated by high-intensity light, which results in enhanced minority carrier recombination activity. We show that there is a quantitative correlation between the enhanced recombination rate and the Cu concentration by comparing μ-PCD measurements with transient ion drift and total reflection x-ray fluorescence measurements. The results indicate that the method is capable of measuring Cu concentrations down to 10exp10cm−3. There are no limitations to wafer storage time if corona charge is used on the oxidized wafer surfaces as the charge prevents copper outdiffusion. We briefly discuss the role of oxide precipitates both in the copperprecipitation and in the charge carrier recombination processes.Peer reviewe

(oral talk) Effect of Oxygen in Low Temperature Boron and Phosphorus Diffusion Gettering of Iron in Czochralski-Grown Silicon

Peer reviewe

Chemical Excitation of Silicon Photoconductors by Metal-Assisted Chemical Etching

The chemical transformations taking place during many of the reactions of the Si surface have been well documented, but the in situ dynamics of the reactions remain largely unexplored even for widely used electrochemical processes such as metal-assisted chemical etching (MACE). In this work, we design both n- and p-type Si photoconductors covered with silver nanoparticles to demonstrate photoconductors’ sensitivity to the MACE process and their ability to provide in situ information about the dynamics of the reactions occurring during MACE. The experimental results show that both n- and p-type photoconductors exhibit a response to MACE that is much stronger than their response to light with an intensity of 2 mW/cm2. The observations are further explained by a thermodynamic analysis of the relevant energy levels of the system, showing how both electron and hole injection into the conduction and valence bands by the Si/etching solution interface contribute to the photoconductors’ response and excite Si. These results clearly demonstrate a new chemical operating mode for photoconductors, showing that in addition to being sensitive to excitation by light, a photoconductor can also be excited by chemical reactions providing means to monitor the dynamics of the reactions in situ and thus also for chemical sensing.Peer reviewe