Results

Fig. 5.4 shows both positions under investigation. The left column refers to position C and the right column to position D. The top images represent the reference gettered samples and the bottom row depicts the samples after 900^◦C + reference. No significant difference between these process schemes is observed. The mean values of the 900^◦C + reference diffusion are slightly lower. Both samples are positions of the same 15.6×15.6 cm²wafer located at a slightly higher ingot height than the wafer of the upper samples in Fig. 5.4. It is possible that grain structure and/or impurity content between upper and lower samples slightly vary. A typical standard deviation of±37% of two sister samples processed in the same POCl3diffusion is given in Chapter 3. The small difference between the two process schemes constitutes within this experimental error. Therefore, it can be stated that the 900^◦C + reference diffusion does not result in a more effective PDG for the investigated material. This might indicate the above described situation that the driving force to form and grow precipitates is not sufficiently strong due to the diminished supersaturation of the rather clean mc-Si under investigation. In such a material an additional high temperature peak before the POCl3 diffusion, aiming the dissolution of small and medium sized precipitates, is found not to be beneficial for the lifetime. This is in agreement with the findings of Schön et al. regarding the cleaner mc-Si as compared in Tab. 5.2[70].

It is interesting to note that position C and, in particular, D have considerably lower as-grown life-times than Schön et al.’s cleaner mc-Si. The absolute final lifetime yielded by PDG, however, is higher by a factor of two. This means a considerably stronger relative lifetime improvement∆τ in case of ma-terial IV which is comparable to the one of the contaminated mc-Si. This indicates a stronger influence of relaxation gettering than for Schön et al.’s cleaner mc-Si. Hence, it is surprising that material IV does

5.1 Pre-diffusion Gettering 89

(reference) (process)

(900°C Peak + reference)

(reference) (process)

(900°C Peak + reference)

Position C Position D

Figure 5.4: Comparison of a reference gettered sample (top) and its sister sample after 900^◦C + reference diffusion (bottom).

Left:Position C of material IV in bottom ingot height.Right:Position D of material IV in bottom ingot height. Arithmetical mean lifetimes with the respective process type in brackets are given on the left-hand side of each column.

not show a positive effect due to the high temperature peak at 900^◦C even though it seems to involve a stronger driving force for precipitation (higher∆τ). However, both positions under investigation contain a high amount of extended defects strongly hindering PDG and remaining with a strong recombination activity as described in Chapter3. It might be possible that these structural defects have a dominating impact so that the yielded precipitate dissolution by the high temperature peak is not beneficial. In ad-dition, the stoichiometry of the precipitates in the bottom ingot material is likely to incorporate oxygen since the concentration of interstitial oxygen[Oi] =5×10¹⁷cm⁻³is exceptionally high compared with higher ingot heights³.

Sattler et al. observed a lowered gettering efficacy during cooling at an oxygen concentration of [Oi] =7.8×10¹⁷cm⁻³[173]. They identified this effect with back diffusing cobalt atoms into the sample and their binding to oxygen related centers eventually degrading the material quality. They call this effect inverse gettering. Such mechanisms might compensate the optimized gettering of the 900^◦C + reference process. It might be also possible that the high interstitial oxygen concentration of the bottom material leads to a facilitated nucleation of precipitates as pointed out in [174,175]. This would change the situa-tion that small and medium sized precipitates are successfully reduced by the 900^◦C + reference process.

A facilitated nucleation might eventually result in the same amount of small and medium sized precipi-tates as in the case of the reference diffusion provided that the density in the reference gettered sample

3The oxygen concentration has been measured using fourier transform infrared (FTIR) spectroscopy by Dr. W. Seifert from BTU Cottbus within the framework of SolarWinS (see acknowledgement).

Table 5.2: Comparison of measured lifetimes of this work with data taken from Schön et al. [70]. The relative change∆τin brackets gives the relative lifetime improvement yielded by each PDG process compared with the as-grown lifetime before processing. Data of material IV refers to its bottom ingot height.

Material Lifetimeτ (µs) [rel. change∆τ (%)]

as-grown reference 900^◦C + reference Contaminated mc-Si ( [70]) 1.3 7 [438] 22 [1592]

Cleaner mc-Si ( [70]) 23 32 [39] 34 [48]

Material IV (position C) 16 80 [400] 74 [363]

Material IV (position D) 6 72 [1100] 64 [966]

is already high enough so that no new precipitates nucleate. These assumptions cannot be conclusively clarified without further investigations, in particular, without the determination of the chemical nature of the precipitates. Since the oxygen content in the upper part of the ingot is lower, a differing PDG response of the mc-Si material originating from central ingot height is expected. This and the influence of a PECVD SiNx:H step on the material quality will be addressed in the following section.

Influence of the PECVD SiNx:H Step

In bottom ingot height of material IV, both positions C and D degrade due to the PECVD SiNx:H step compared with their lifetime directly after PDG. This is observed for both POCl3 diffusion processes which is supplemented in Appendix B. It might indicate a further material degradation induced by back diffusing impurities into the sample forming deleterious oxygen related precipitates as described by Sattler et al. [173].

In the following, it is focused on position C of material IV in central ingot height since it exhibits an interesting effect after the PECVD SiN_x:H step⁴. Position C degrades due to the hydrogenation step af-ter the 900^◦C + reference process down to∅τ=55 µs but is improved after the complete reference + H process up to∅τ=101 µs. Fig. 5.5shows the results of both diffusions with and without the hydrogena-tion step in comparison with the as-grown state. Note that the as-grown sample is shown twice in the top row of the figure.

Tab.5.3 summarizes the relative lifetime changes∆τ yielded by the hydrogenation step after each POCl₃ diffusion. The lifetime of the reference + H process is even doubled. The 900^◦C + ref. + H process, however, exhibits a material degradation by 39% (marked in red). The different behavior of the differently gettered sister samples under the same SiNx:H step emphasizes the influence of the thermal pretreatment of each sample. Hence, the different modes of operation of the different POCl₃ diffusions shall be addressed first.

In agreement with the expectation mentioned in the previous section, the sample from central in-got height shows a different PDG response than the samples originating from the inin-got bottom. The 900^◦C + reference diffusion results in a higher material quality than the reference gettered sample. The obtained mean lifetime is 90 µs compared with only 51 µs of the reference diffusion. The latter even degraded compared with the as-grown lifetime of 72 µs possibly due to the lack of supersaturated im-purities. Hence, relaxation gettering cannot efficiently work during the reference diffusion within the

4As already mentioned, the process with an additional SiNx:H step to the 900^◦C + reference diffusion is called 900^◦C + ref. + H process.

5.1 Pre-diffusion Gettering 91

(as-grown)

(reference)

(reference + H) (process)

(as-grown)

(900°C + reference)

(900°C + ref. + H) (process)

101 72.2

50.5 90.4

72.2

55.4

Figure 5.5:Left:Comparison of the as-grown state (top) with the reference gettered sample (center) and its sister sample after the reference + H process (bottom). Right:Same for the 900^◦C + reference diffusion. All sister samples originate from po-sition C of material IV in central ingot height. Only a section of the 5×5 cm²C samples is shown since some are partly broken.

cleaner material originating from central ingot height. It rather seems to induce the dissolution of pre-cipitates without being capable of subsequently removing the dissolved impurities. This would explain the observation of structural defects as well as intra-grain regions showing a lower material quality than in the as-grown state.

The better performance of the 900^◦C + reference diffusion is particularly visible in intra-grain re-gions. In addition, grain boundaries also appear with a smaller lifetime contrast although their surround-ings are on a higher lifetime level than the ones in the reference gettered sample. It indicates a successful gettering even at structural defects wherein PDG is typically hindered (compare with findings of chap-ter3). This is also pointed out by Schön et al. [70]. Overall, the pre-diffusion gettering is successful in central ingot height of position C but not in its bottom ingot height. This strengthens the assumed impact of the interstitial oxygen concentration in bottom ingot height eventually leading to an inverse gettering effect during the hydrogenation step as suggested by Sattler et al. [173]. No statement can be made for position D in central ingot height due to sample breakage.

As was simulated by Schön et al., the high temperature peak halves the number of small and medium sized precipitates compared with the reference gettered material [70]. Buonassisi et al. showed that a high number of small precipitates is more detrimental to the lifetime than a few large precipitates [9]. In agreement with that, the pre-diffusion high temperature peak, leading to more large but significantly less small and medium sized precipitates, is shown to be beneficial for the lifetime of material IV originating from central ingot height. The question arises, however, why the previously more efficient PDG process (900^◦C + reference process) eventually results in a material degradation after the additional PECVD SiNx:H step by 39% whereas the reference gettered sample is improved by 100% (see Tab.5.3). It is

Table 5.3: Relative lifetime change of each diffusion due to hydrogenation compared with its lifetime before the PECVD SiNx:H step. The material degradation of the 900^◦C + ref. + H is depicted in red. Data of position C refers to the bottom ingot height of material IV.

Lifetime change∆τ(%)

Process Position C

reference + H 100

900^◦C + ref. + H -39.4

supposed that the emitter underneath the SiNx:H layers acts as a contamination source during further thermal processing since large amounts of impurities are segregated towards the external gettering layer.

Still, this does not explain why the reference emitter does not contaminate the material to the same way as the 900^◦C + reference emitter does. Hence, it is suggested that renewed small precipitates nucleate particularly within the material that does not offer sufficient precipitates in the close vicinity of dissolved impurities⁵. This is the case for the 900^◦C + reference gettered material which is assumed to contain considerably less small and medium sized precipitates. Such a scenario describing the nucleation of dispersed nanoprecipitates has been previously observed by Buonassisi et al [162].

As described in the introduction of the present experiment, the thermal processing of the SiN_x:H layer involves fast cooling ramps that may be critical and thus may lead to the formation of these dele-terious nanoprecipitates. It is suggested that the time during cooling is not sufficient for gettering the impurities dissolved by the preceding thermal processing. Hence, they may remain dissolved or may form nanoprecipitates dispersed over the whole sample explaining its material degradation. It seems plausible that the formation of nanoprecipitates mainly occurs in the 900^◦C + reference sample due to its reduced density of small and medium sized precipitates as already stated above. Note that it is always possible that impurities will remain dissolved if impurities are not supersaturated. During the sample cooling down to room temperature, which always occurs after thermal processing, impurities are highly likely supersaturated leading to the formation of such nanoprecipitates. Despite of the observed material degradation, most of the GBs vanish indicating their successful hydrogen passivation. The same is found for the reference + H gettered sample but on a higher lifetime level.