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Phosphorous diffusion gettering of trapping centers in upgraded metallurgical-grade solar silicon

In this paper we show experimental evidence indicating the beneficial impact of a phosphorous diffusion gettering (PDG) in the reduction of trapping centers, as observed by means of inductively-coupled photoconductance decay and lifetime measurements performed on upgraded metallurgical grade silicon (UMG-Si) wafers. We have unambiguously identified the presence of trapping species dominating the long time range of the photoconductance decay of UMG-material (slow traps), which can be effectively removed after a PDG carried out at 780 Celsius. Furthermore, the beneficial effect of the PDG is studied as a function of processing time, showing minority carrier bulk lifetime improvements up to 18-fold, up to the range of 70 microseconds. This work paves the way for developing gettering strategies capable of successfully removing trap centers and thereby improving the bulk lifetime of unconventional Si material.

The photovoltaic industry is currently mostly based on polysilicon feedstock purified by the Siemens process, which is expensive in terms of both economic and environmental cost. To overcome this issue, alternative approaches, such as metallurgical purification routes, have been widely investigated.
Specifically, upgraded metallurgical-grade silicon (UMG-Si) has gained attention from the scientific community over the last years and has become an alternative feedstock source for different companies [1][2][3] in the Si photovoltaic industry. UMG-Si offers not only lower cost than conventional polysilicon (whereby the cost of the final PV module can be reduced by about 8%), but also a reduced energy payback time (by 50%) and environmental impact due to its reduced-emission purification process (quantified in as much as 70% cut of green-house-gas emissions, depending on the energy mix at the fabrication site) 3 . Despite the typically lower purity of the resulting material, containing a significantly high impurity concentration, it has been reported that multicrystalline solar cells based on UMG-Si show a negligible penalty in terms of efficiency (with values up to 20.76%) in Al-BSF and PERC architectures, as compared to standard polysilicon-based cells [3][4][5] and hence it can compete with solar-grade Si.
Lifetime measurements 6,7 are a common way of characterization of all types of Si wafers, and is of particular importance in our work. Carrier lifetime, defined as the characteristic time it takes the excess minority carrier to recombine, is mainly affected by three mechanisms: band-to-band recombination, negligible in Si; Auger recombination, and Shockley-Read-Hall recombination, the latter proportional to the amount of recombination centers present in the bulk of the material, and therefore, the most critical mechanism, particularly in UMG-Si.
Different characterization techniques to measure carrier lifetime have been proposed, being photoconductance (PC) decay in its various forms the most popular one. For materials with low minority carrier lifetime the method used is the quasi-steady-state photoconductance (QSSPC) 8 .
Working with this technique, an anomalously characteristic behavior is often observed, particularly at low injection levels 9 . The PC decay after a light pulse does not follow a single exponential function, which implies the existence of more than a single electronic process involved in the PC-decay, resulting in prolonged effective lifetimes that increase markedly as the excess carrier density decreases. These apparent lifetimes can reach values in the range of hundreds of microseconds or longer, a phenomenon typically associated to trapping effects and which has been observed in different types of Si (single crystal, polysilicon, thin film) 10 .
Most of the previous works agreed to treat traps as electronic centers whose main characteristic is a significant difference in the values of the capture cross-sections of minority and majority carriers, whereas in the case of very effective recombination centers those values are typically of the same order of magnitude. Nevertheless, it is firmly accepted that traps do increase the recombination.
Indeed, the Shockley-Read-Hall (SRH) formalism has been adapted to treat traps as recombination centers 11 , and it has been observed experimentally, when comparing with other characterization techniques, that they do actually increase the recombination rate at low injection levels 12 .
Along this line, in this work we report on the net reduction of bulk recombination in UMG Si wafers resulting from the removal of trapping centers upon a phosphorous diffusion gettering (PDG) step.
There have been just a few works that reported on a beneficial effect regarding the suppression and/or reduction of trapping effects in multicrystalline (mc-Si) after physico-chemical treatments. For instance, S. Jafari et al. 13 achieved a full removal of trapping effects in mc-Si by applying an infrarred laser treatment; however, the bulk lifetime values of treated wafers were also affected and considerably reduced due to a higher recombination rate. On the other hand, Macdonald et al. 14 showed that PDG treatment can have a two-fold effect: an increase of the minority carrier lifetime and a moderate reduction of the trap concentration. The latter approach is of great interest for UMG material, because of its low starting lifetime values. Therefore, with the aim of studying the removal of trap centers, we have performed PDG processes on wafers grown from UMG-Si feedstock. The fabrication details of the UMG-Si material can be found elsewhere 3 , although it has to be noted that the wafers used in this study correspond to a highlycontaminated batch, while the solar cell results in [3] were obtained from wafers coming from an optimized purification process. We have worked with sixteen UMG-Si bare wafers of 15x15 cm 2 size, 148 ± 2 μm thickness and 1.05 ± 0.07 ·cm average resistivity. The wafers were analyzed with the Sinton Instruments WCT-120 lifetime tester. The minority carrier lifetime is obtained from the values of the PC and represented as a function of the excess carrier density, as shown in Figure 1 (a). As a result of the high impurity concentration, the recombination rate is very high in this type of Si and hence its bulk lifetime is also very low, in the range of 1 s for the case of non-passivated surfaces.
As it can be observed, at low injection levels, trapping, revealed as a large increase in the apparent lifetime (with values above 10 s), is the dominant process. Depletion-region modulation (DRM) can be discarded as the main cause of the lifetime increase since there are no p-n junctions in the bare wafers 9,11 .
We have attempted to quantitatively analyze the trapping. The model postulated by Hornbeck and Baynes 15 and optimized by Macdonald 16 for the quantification of the trap parameters has been applied to the curves. However, the curves cannot be fitted succesfully to the model. An alternative strategy is to analyze the PC raw measurements whithout any of the assumptions needed to convert them into lifetime values (in particular those related to the carrier mobility). Figure   1 (b) shows the photovoltage signal, which is proportional to the photoconductance, of the analyzed wafers, together with the corresponding signal measured at the reference cell of the instrument (green dots) as a function of time. The flash mode used has a decay time constant of 1.8 ms, which is standard for stationary conditions (QSSPC). Under these conditions, the PC should follow the same exponential trend of the flash with a similar decay time. However, the blue square points clearly deviate from linearity (mind the log-scale). This deviation is well understood as the result of trapping effects, as explained elsewhere 13 . The PC measuraments could be satisfactorily fitted with a twoexponential function of the form y=A 1 ·exp(-x/t 1 )+A 2 ·exp(-x/t 2 ) with characteristic decay times t 1 and t 2 . The first exponential function represents physical processes that happen in the time lapse of the flash such as SRH-recombination (t 1~tflash ), while the second one represents events that last longer, such as the ones governed by slow traps. As an example, a PC measurement is fitted and their two different exponential functions are plotted in Figure 1   The samples were subsequently subjected to a PDG treatment. It is a well-known method to eliminate a high range of interstitial impurities in Si material, particularly effective with metallic species, and its bases are well stablished and understood 17,18 . After carefully cleaning the samples by RCA1 and CP4 (HNO 3 /HF) chemical treatments, the samples were introduced in a tubular furnace at 780 ºC under N 2 and O 2 gas fluxes for the subsequent PDG from a POCl 3 liquid source. The process time has been varied from 30 to 120 min in 4 batches of 4 samples each followed by a 10 min drive-in providing an n-type emitter atop the Si wafer surface. The emitter is subsequently removed with CP4 etching and the wafers are passivated with 0.1 M iodine-ethanol 19 to minimize surface recombination effects. Recombination lifetime is measured after each step of the process, namely: before PDG, with and without passivation (already shown in Figure 1 (a)), with an emitter in both sides of the wafer, and after emitter removal with and without passivation. Iodine-ethanol passivation avoids other derived consequences on trapping, such as those related to hydrogen incorporation or additional temperature effects, as it could happen with passivation layers based in SiN x :H.
For the sake of clarity, the passivated lifetime curves of the four wafers of the 90 min PDG batch are presented in Figure 2 (a). There is a clear difference in lifetime values before (green spheres) and after (red stars) the PDG. The gettering process successfully works by removing recombination impurities, thereby increasing the lifetime from 3.5-5 μs to 18-26 μs, as evaluated at 10 15 cm -3 injection level. Moreover, it is worth noting that the PDG does not completely suppress the trapping, since it is still present at low injection levels. Figure 2   From the lifetime curves of Figure 2 (a), changes in trapping effects due to the gettering process cannot easily observed. To do so, we have proceeded with the same procedure as in Figure 1 (b). We have analyzed the PC spectra of the samples after PDG and compared them with pre-gettering data.
Results are presented in Figure 3 (a) for an exemplary case of a PDG time of 30 min. Two major changes are observed after the gettering process. Firstly, the PC signal is increased at short times, which means lower overall recombination, in agreement with the lifetime improvement shown in Figure 2. Secondly, the PC curvature at longer times decreases after gettering. The latter is related with the decay time constant of the second exponential ascribed to the slow trap center. Figure 2 (b) shows this parameter (t 2 ) for all the samples before and after the PDG. As we concluded from Figure   1, all as-grown samples are dominated by the same slow trap center, whose decay time constant was estimated around 67 ms (blue spheres). However, after the PDG process, t 2 decreases to an average value of 14 ± 7 ms (still significantly larger than the 1.7 ms of the reference decay), thus revealing a change of the main trapping mechanism. In a few words, despite the PDG process clearly removes one type of slow trap, one additional trapping mechanism still remains active in the samples. Interestingly, Macdonald reported two types of traps in mc-Si, one related to dislocations, impervious to gettering; and a second one which was getterable 14 . This scenario seems to be also valid for contaminated UMG-Si, according to Figure 3.
Since the PDG process removes one type of slow trap and it is in general much more effective in the removal of defects associated to metallic impurities, it is likely that the presence of an interstitial metallic impurity acting with a dual behavior 21 , as a trap and as a recombination center, governs the electronic transport of the samples.
In summary, it has been shown that a phosphorous diffusion gettering process has a significant impact on the trapping of contaminated UMG-Si wafers. Traps controlling the long photoconductance decay time constant have been observed in as-grown samples. After PDG those trapping centers disappear, revealing the presence of fast traps associated with a shorter PC decay time constant that are less detrimental to the bulk recombination lifetime. The impact of the PDG process on trapping adds up to its well-known beneficial effect on the reduction of active recombination centers, resulting in an 18-fold increase of the lifetime values, and thereby reaching dozens of μs. The mechanisms unveiled in this work can help understand potential side effects of PDG on the reduction of recombination in mc-Si linked to the dual behavior of metallic impurities acting simultaneously as traps and recombination centers, that may have previously been overseen.