Aplicación del procedimiento de diagnóstico para la Dirección Estatal de Comercio.

This study relies on re-passivation at each processing stage using a temporary chemical solution and annealing without surface passivation. Therefore, bulk lifetime values in Chapter 5 are probably not influenced by hydrogenation or changes in surface passivation quality. Previous low-temperature annealing studies tend to report interstitial iron concentration [36, 50], cell efficiency [50, 51], or lifetimes after high-temperature processing [48, 49]. Whilst these all suggest beneficial effects in certain circumstances, this study focuses, for the first time, on the thermal effect of low-temperature annealing on lifetime in as-grown mc-Si. The key findings are discussed in the context of the height position in the ingot.

5.5.1.1

Bottom wafers

The average lifetime in bottom wafers was increased by annealing at all temperatures investigated, as shown in Figure 5.2 and Figure 5.3 and Table 5.1. The largest improvement is at 400 °C when 35 h of cumulative annealing increases the lifetime to ~7 times the as-received value. Annealing at 300 °C and 500 °C gives ultimate improvements of ~ 6 and ~ 4 times.

Figure 5.4 shows an ultimate reduction in the average interstitial iron concentration in bottom wafers. At 300 °C, the iron decays approximately exponentially. At 400 °C, there is a slight initial increase in bulk iron concentration followed by a decay, and at 500 °C, the bulk iron concentration increases for a period before it starts to decay. The annealing time dependence of the interstitial iron concentration is discussed in Section 5.5.3. Figure 5.13 shows the correlation between normalised changes in recombination rate (reciprocal lifetime) and normalised change in interstitial iron concentration, according to Equation 5.1. At all temperatures investigated, there is a good correlation for wafers from the bottom of the ingot. This means that the lifetime change in bottom wafers can be explained mainly by the removal of interstitial iron from the bulk.

grown samples, the lifetime is initially low in the bulks of the grains and very low at certain grain boundaries and in the vicinity of dislocation clusters. With increasing cumulative annealing time the lifetime improves, particularly in the bulks of the grains. Figure 5.9 also shows substantial changes in the interstitial iron distribution which occur during annealing of the same samples. Initially, the interstitial iron concentration is >1012

cm-3throughout the wafer, whereas after 35 h of cumulative annealing it is reduced to be

of order 1010 _cm-3 _{in many regions, with a few regions remaining with higher}

concentration. The evolution of the spatial distribution of interstitial iron is broadly

similar to the observations of Liu and Macdonald [49]. The absence of any external

gettering layer means it is likely that interstitial iron is gettered to internal features, such as iron-containing precipitates, grain boundaries and dislocations. The kinetics do not exclude the possibility of interstitial iron gettering to the samples’ surfaces, although previous work in single crystal silicon suggests this is unlikely [128]. Figure 5.14 shows that an increase in interstitial iron concentration at regions with high dislocation density (> 104cm-2) upon annealing at 400 °C for 35 h. In the bulk regions with low dislocation

density (≤ 104 _cm-2_{), the interstitial iron concentration decreases upon annealing}

compared to the as-received states. This indicates gettering of interstitial iron to the region with high dislocation density upon low-temperature annealing.

5.5.1.2

Middle wafers

Data for the bottom middle (MB) and top middle (MT) samples are discussed together as similar effects are observed in both sample types. In most cases, annealing samples from the middle of the block initially result in a lifetime reduction (Figure 5.2 and 5.3). There is also an initial increase in the interstitial iron concentration (Figure 5.4). With further annealing the lifetime recovers and the interstitial iron concentration reduces. Given sufficient time there are small ultimate lifetime improvements at 300 °C and 400 °C. Annealing at 500 °C results in a very abrupt initial reduction in lifetime. There is a lot of scatter in the data, and it is unclear whether substantial stable improvements are ever realised at 500 °C. In middle wafers, the impact of low-temperature annealing (without bulk hydrogenation effects) at best results in marginal improvements; at worst the lifetime is reduced substantially if the samples are not annealed for sufficient time.

The evolution of the spatial distribution of lifetime in the bottom middle and top middle samples at all three temperatures are shown in Figure 5.6 and Figure 5.7,

respectively. These Figures also show the spatial distribution of dislocation density (presented in Chapter 4). The initial annealing reduces lifetime throughout the wafer. Lifetime then begins to recover in regions where the dislocation density is low. The average lifetime after the last annealing step is lower than the as-grown lifetime. It is notable that the low lifetime regions within the high dislocation-containing areas are much more diffuse (less sharp) after annealing than before. The evolution of interstitial iron concentration distribution in the bottom middle and top middle samples at all three temperatures are shown in Figure 5.10 and Figure 5.11, respectively. The annealing appears to have caused higher interstitial iron concentrations in regions with high dislocation densities than in the as-grown state. The relationship in Figure 5.14 also shows the change in interstitial iron concentration is high in regions with high dislocation density (> 104cm-2). Annealing increases recombination associated with certain grain boundary types. Some (but not all) boundary types which exhibited no, or very little, detectable recombination contrast in the as-grown state exhibit stronger recombination contrast. This is presumably because they have gettered impurities from the bulk. This observation is consistent with electron beam induced current (EBIC) studies of Chenet al., which found increased EBIC contrast in samples subjected to slow cooling [65], which will have had similar effects to low-temperature annealing.

For middle samples, lifetime changes which occur upon low-temperature annealing do not appear to correlate in a simple way with the change in interstitial iron concentration (Figure 5.13). The correlation is particularly poor at 500 °C, which was also the case for top samples. Although annealing at 500 °C does affect the interstitial iron concentration (Figure 5.4), it can be concluded that annealing at this temperature must also affect the concentration of another defect or other defects. The same is probably also true at 300 °C and 400 °C, although the change due to defects other than interstitial iron is smaller at these temperatures.

5.5.1.3

Top wafers

Top wafers respond differently to low-temperature annealing compared to bottom wafers. Although lifetime is ultimately increased for all temperatures used, the magnitude of the improvement and also the final lifetime is lower than in annealed bottom wafers (Figure 5.2 and Figure 5.3 and Table 5.1). The largest improvement in lifetime is a factor

The annealing time dependence of lifetime is more complicated in the top wafers compared the bottom wafers. Annealing at 500 °C, lifetime falls substantially from ~ 12

s to ~ 2s within 15 minutes. This is accompanied by a substantial initial increase in interstitial iron concentration to a peak of ~ 3  1012 cm-3. The initial impact of the annealing appears to be to release interstitial iron into the bulk from somewhere else in the material, as discussed in Section 5.5.2.

The spatial distribution of lifetime and interstitial iron concentration for the top samples annealed at 300 °, 400 °C and 500 °C are presented in Figure 5.8 and Figure 5.12, respectively. After 1 h of annealing, the lifetime generally decreases slightly, but it recovers and improves with further annealing. The initial decrease in lifetime appears to be due to a lowering of lifetime in the bulk of the grains. This does not correlate with the interstitial iron map which shows the interstitial iron concentration to fall in most parts of the material. This suggests that reconfiguration of a defect other than interstitial iron is responsible for the initial lifetime reduction. With the exception of this initial decrease, for top samples annealed at 300 °C and 400 °C, there is however a quite good correlation between lifetime change and interstitial iron change (Figure 5.13). Lifetime improvements brought on by low-temperature annealing at these temperatures are therefore most likely due to reconfiguration of interstitial iron into a state or complex in which it is less recombination active. The correlation between the change in interstitial iron and dislocation density (Figure 5.14) suggests that interstitial iron is likely to be gettered at regions with high dislocation density (> 104cm-2). The lifetime changes in top samples at 500 °C do not, in general, correlate with the interstitial iron changes in a simple way. This could mean that another impurity is involved at this temperature, or that iron is present in a different state.

It is instructive to compare the lifetime maps with the top dislocation density maps in Figure 5.8. Parts of the sample with relatively low dislocation densities undergo a much larger lifetime improvement upon annealing than those with higher densities. In fact, the regions with very high dislocation densities are found to get slightly worse after annealing. This shows that impurities, including interstitial iron, are internally gettered to regions with high dislocation densities. This is to be expected as dislocations provide low energy sites for the segregation of impurities such as iron [192] and similar results are found after phosphorus diffusion gettering at higher temperatures [35]. The different

response between the top and bottom wafers is therefore probably due to their different microstructures (Figures 5.5 and 5.8). The top wafers have a higher dislocation density on average, and so impurities can segregate to or precipitate at relatively more sites. This gives rise to more recombination sites than in the bottom, in which there are fewer dislocations and hence fewer sites for iron decoration or precipitation. Figure 5.14 shows that the interstitial iron concentration is relatively high in regions with high dislocations (> 104_cm-2_{). As suggested by Boulfradet al.[48], it could also be the case that the higher}

oxygen concentration in the bottom wafers due to the segregation coefficient being greater than unity causes oxide precipitates to form upon low-temperature annealing and act as trapping sites for iron [47, 116]. This explanation is speculative, however.