Heli Talvitie

Effective Passivation of Black Silicon Surfaces by Atomic Layer Deposition

The poor charge-carrier transport properties attributed to nanostructured surfaces have been so far more detrimental for final device operation than the gain obtained from the reduced reflectance. Here, we demonstrate results that simultaneously show a huge improvement in the light absorption and in the surface passivation by applying atomic layer coating on highly absorbing silicon nanostructures. The results advance the development of photovoltaic applications, including high-efficiency solar cells or any devices, that require high-sensitivity light response.

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.

Phosphorus and boron diffusion gettering of iron in monocrystalline silicon

We have studied experimentally the phosphorus diffusion gettering (PDG) of iron in monocrystalline silicon at the temperature range of 650–800 °C. Our results fill the lack of data at low temperatures so that we can obtain a reliable segregation coefficient for iron between a phosphorus diffused layer and bulk silicon. The improved segregation coefficient is verified by time dependent PDG simulations. Comparison of the PDG to boron diffusion gettering (BDG) in the same temperature range shows PDG to be only slightly more effective than BDG. In general, we found that BDG requires more carefully designed processing conditions than PDG to reach a high gettering efficiency.

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.

Detection of Nickel in Silicon by Recombination Lifetime Measurements

The impact of nickel on minority carrier recombination lifetime has been studied in ptype CZ silicon using SPV and μ-PCD techniques. The results show that small oxide precipitates can be used to improve drastically the detection limit of nickel. This is explained by the decoration of oxide precipitates by nickel, which results in the enhanced recombination activity. In the absence of oxide precipitates or other related bulk microdefects nickel precipitates preferably to wafer surfaces, which does not have such a high impact on the measured recombination lifetime, at least on a low concentration level. Low temperature anneal at 180°C or light illumination of the wafers after nickel in-diffusion did not reveal any further change in lifetime in any of the wafers, which may indicate that nickel precipitates efficiently during air-cooling from high temperature.

Analysis of Heterogeneous Iron Precipitation in Multicrystalline Silicon

Iron precipitation in multicrystalline silicon has been modeled aiming at the optimization of intrinsic gettering of iron in multicrystalline silicon. Iron precipitation during both crystal growth and following phosphorus diffusion gettering (PDG) are simulated and compared to experimental results as the iron precipitate density after these processes is essential in the modeling of intrinsic gettering in multicrystalline silicon solar cell processing. The PDG decreases the density of iron precipitates compared to the as-grown state and as expected the effect is larger at lower initial iron concentrations. Due to this effect the iron precipitation is significantly reduced almost throughout the whole ingot height and it can be concluded that intrinsic gettering has a beneficial effect only in the case of high initial iron concentration, in accordance with the experimental results. The simulated change in interstitial iron concentration as a function of intrinsic gettering temperature suggests the same optimum intrinsic gettering temperature as the experiments. With the given model it is however much easier to find optimal parameters compared to expensive and time consuming experiments.

Diffusion gettering of metal impurities in crystalline silicon

We present here our latest results of boron and phosphorus diffusion gettering of iron in crystalline silicon. The gettering efficiency is evaluated measuring both the minority carrier lifetime as well as the solar cell parameters. The results indicate that the optimal dopant and time-temperature profiles strongly depend on the thermal history as well as the initial iron level present in silicon.

Effect of transition metals on oxygen precipitation in silicon

Effects of iron and copper impurities on the amount of precipitated oxygen and the oxide precipitate and stacking fault densities in Czochralski-grown silicon have been studied under varying thermal anneals. Silicon wafers were intentionally contaminated with iron or copper and subsequently subjected to different two-step heat treatments to induce oxygen precipitation. The iron contamination level was 2 × 1013 cm-3 and copper contamination level 6 × 1013 cm-3. Experiments did not show that iron contamination would have any effect on the amount of precipitated oxygen or the defect densities. Copper contamination tests showed some indication of enhanced oxygen precipitation.