Hang Cheong Sio

Carrier de-smearing of photoluminescence images on silicon wafers using the continuity equation

Photoluminescence images of silicon wafers with non-uniform lifetime distribution are often smeared by lateral carrier diffusion. We propose a simple method to de-smear the photoluminescence images by applying the two-dimensional continuity equation. We demonstrate the method on simulated silicon wafers and measured photoluminescence-based lifetime image of multicrystalline silicon wafer. The de-smearing is very effective in recovering the actual lifetime for wafers with gradual changes in lifetime but is less effective around localised recombination centres with high contrast such as grain boundaries and dislocations. The method is sensitive to measurement noise; therefore, the implementation of suitable noise filtering is often critical.

Quantifying carrier recombination at grain boundaries in multicrystalline silicon wafers through photoluminescence imaging

We present a method based on steady state photoluminescence (PL) imaging and modelling of the PL intensity profile across a grain boundary (GB) using 2D finite element analysis, to quantify the recombination strength of a GB in terms of the effective surface recombination velocity (Seff). This quantity is a more meaningful and absolute measure of the recombination activity of a GB compared to the commonly used signal contrast, which can strongly depend on other sample parameters, such as the intra-grain bulk lifetime. The method also allows the injection dependence of the Seff of a given GB to be explicitly determined. The method is particularly useful for studying the responses of GBs to different cell processing steps, such as phosphorus gettering and hydrogenation. The method is demonstrated on double-side passivated multicrystalline wafers, both before and after gettering, and single-side passivated wafers with a strongly non-uniform carrier density profile depth-wise. Good agreement is found between ...

Imaging crystal orientations in multicrystalline silicon wafers via photoluminescence

We present a method for monitoring crystal orientations in chemically polished and unpassivated multicrystalline silicon wafers based on band-to-band photoluminescence imaging. The photoluminescence intensity from such wafers is dominated by surface recombination, which is crystal orientation dependent. We demonstrate that a strong correlation exists between the surface energy of different grain orientations, which are modelled based on first principles, and their corresponding photoluminescence intensity. This method may be useful in monitoring mixes of crystal orientations in multicrystalline or so-called “cast monocrystalline” wafers.

Impact of Phosphorous Gettering and Hydrogenation on the Surface Recombination Velocity of Grain Boundaries in p-Type Multicrystalline Silicon

We compare the recombination properties of a large number of grain boundaries in multicrystalline silicon wafers with different contamination levels and investigate their response to phosphorous gettering and hydrogenation. The recombination activity of a grain boundary is quantified in terms of the effective surface recombination velocity SGB based on photoluminescence imaging and 2-D modeling of the emitted photoluminescence signal. Our results show that varying impurity levels along the ingot significantly impact the grain boundary behavior. Grain boundaries from the middle of the ingot become more recombination active after either gettering or hydrogenation alone, whereas grain boundaries from the top and bottom of the ingot have a more varied response. Hydrogenation, in general, is much more effective on gettered grain boundaries compared with as-grown grain boundaries. A close inspection of their injection dependence reveals that while some grain boundaries exhibit little injection dependence before gettering, others show a relatively large injection dependence, with their SGB increasing as the injection level decreases. The former type tend not to be recombination active after both gettering and hydrogenation and are less likely to impact the final cell performance, in comparison with grain boundaries of the latter type.

The influence of crystal orientation on surface passivation in multi-crystalline silicon

We present an approach to study the variation of the surface recombination velocity in multi-crystalline silicon wafers through photoluminescence imaging for thin, passivated and mirror polished wafers. The influence of crystal orientation on surface passivation is investigated for various passivating films, including silicon nitride and aluminum oxide. Our results show that the influence of surface orientation is negligible in well passivated multi-crystalline silicon wafers due to the detrimental effects of crystal defects. Our study on hydrogenated samples suggests that aluminum oxide passivation exhibits a similar surface dependence as native oxide passivation. A slight and different surface dependence is observed in one of the silicon nitride films used in the study.

Recombination sources in p-type high performance multicrystalline silicon

This paper presents a comprehensive assessment of the electronic properties of an industrially grown p-type high performance multicrystalline silicon ingot. Wafers from different positions of the ingot are analysed in terms of their material quality before and after phosphorus diffusion and hydrogenation, as well as their final cell performance. In addition to lifetime measurements, we apply a recently developed technique for imaging the recombination velocity of structural defects. Our results show that phosphorus gettering benefits the intra-grain regions but also activates the grain boundaries, resulting in a reduction in the average lifetimes. Hydrogenation can significantly improve the overall lifetimes, predominantly due to its ability to passivate grain boundaries. Dislocation clusters remain strongly recombination active after all processes. It is found that the final cell efficiency coincides with the varying material quality along the ingot. Wafers toward the ingot top are more influenced by carrier recombination at dislocation clusters, whereas wafers near the bottom are more affected by a combination of their lower intra-grain lifetimes and a greater density of recombination active grain boundaries.

Characterizing the Influence of Crystal Orientation on Surface Recombination in Silicon Wafers

We present two approaches for evaluating the influence of crystal orientation on surface passivation of silicon wafers using photoluminescence imaging. The methods allow a variety of orientations that are not limited to (1 0 0) and (1 1 1) planes to be studied. The first approach is based on imaging carrier lifetimes in silicon strips containing different surface orientations that have been created from a single monocrystalline silicon wafer via laser cutting. The second approach is based on imaging carrier lifetimes among different grains in multicrystalline silicon wafers, which make use of their random distribution of crystal orientations. Both approaches are demonstrated with silicon-oxide-passivated samples. The results from both methods are consistent with each other, showing that the studied silicon oxide films provide a better passivation on surfaces with higher surface energy, such as (1 0 0) or (1 0 6) surfaces, compared with those with lower surface energy, such as (2 3 5) or (1 1 1) surfaces. The advantages and limitations of both approaches are also discussed and compared.

Activation Kinetics of the Boron–oxygen Defect in Compensated n- and p-type Silicon Studied by High-Injection Micro-Photoluminescence

<italic>In</italic> <italic>situ</italic> measurement of the activation kinetics of the slowly forming recombination center (SRC) of the boron–oxygen defect in compensated n- and p-type silicon (n-Si and p-Si) under high-injection conditions is realized through micro-photoluminescence measurements. The high-injection conditions significantly accelerate the defect activation. Another advantage of this method is that the injection level can be kept almost constant during the defect activation and in differently doped samples, as the high-injection lifetime is dominated by Auger recombination. Courtesy of this, the activation time constant remains steady during the activation of the defects, and the activation time constant and defect concentration in differently doped samples can be compared more directly. The results confirm that the defect activation rate constant is the same at high-injection levels in both n- and p-type samples, and that it only depends on the hole concentration <italic>p</italic>, but not on [O <italic>_i</italic>] or [B]. The effective saturated defect concentration normalized with [O<italic>_i</italic>]² is independent of the doping in n-Si, and increases with the net doping in p-Si. The latent form reconfiguration model for the defect, instead of the B<italic>_s</italic>–O₂ <italic>_i</italic> model, is considered to be more compatible with these findings.

N-type high-performance multicrystalline and mono-like silicon wafers with lifetimes above 2 ms

Combined with advanced crystal growth technology and reduced dislocation densities, the higher tolerance to metal contamination of n-type silicon makes n-type cast-grown silicon a potential option for low cost high quality substrates for solar cells. Using a combination of photoconductance based lifetime testing and photoluminescence imaging, we have investigated the carrier lifetime in wafers from the bottom, middle, and top parts of a n-type high-performance multicrystalline (HPM) silicon ingot, and wafers from n-type mono-like silicon ingots after each high temperature solar cell processes, including after boron diffusion, phosphorus diffusion, and hydrogenation. Although boron diffusion leads to a degradation of the sample lifetime, phosphorus diffusion and hydrogenation is effective at recovering the lifetime in the intra-grain region and at the grain boundaries respectively. Quasi-steady-state photoconductance (QSSPC) measurements show that the arithmetic average lifetime of HPM silicon wafers and mono-like silicon wafers can reach up to 1.8 and 3.3 ms respectively for a process sequence including a boron diffusion, with corresponding implied open circuit voltage of about 720 mV. If the boron diffusion can be avoided, average lifetimes up to 3.0 and 6.6 ms can be achieved respectively, highlighting the excellent potential of n-type cast-grown materials.