INVESTIGATING THE INFLUENCE OF INTERSTITIAL ...

33rd European Photovoltaic Solar Energy Conference and Exhibition

INVESTIGATING THE INFLUENCE OF INTERSTITIAL IRON ON THE STUDY OF BORON-OXYGEN DEFECTS

Moonyong Kim, Daniel Chen, Malcolm Abbott, Stuart Wenham, and Brett Hallam School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, AUSTRALIA ABSTRACT: In this paper, we explore the influence of interstitial iron on studying boron-oxygen related degradation in p-type Czochralski silicon. We show that the presence of interstitial iron can potentially explain several key experimental results used to derive theories on the fundamental properties of the boron-oxygen defect. A theoretical analysis of the influence of interstitial iron and the boron-oxygen defect concentrations on the apparent capture cross section ratio indicates that at a concentration of 4 × 1011 /cm3, interstitial iron can result in an apparent capture cross section ratio of >50 for a typical normalised boron-oxygen defect density after fast degradation. With the same interstitial iron concentration, after full degradation, the apparent capture cross section converged to the lifetime ratio with only boron-oxygen defects. We then compare the literature data of first annihilation rate of B-O defects after rapid degradation, to the kinetics of iron-boron pair association. The similar reaction rates suggest that with no iron, after either fast- or slow- degradation, only a single stage of recovery occurs. Another key potential influence of iron occurs when modulating the applied voltage used to induce B-O defect formation. Reducing the applied voltage can result in the association of Fe-B pairs and a subsequent recovery of VOC. We conclude that careful attention must be paid to interstitial iron when studying boron-oxygen defects and that the current models related to boron-oxygen defects may need to be revised. Keywords: Carrier-induced degradation, interstitial iron, boron-oxygen

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INTRODUCTION

the key results was a two-stage recovery of lifetime after rapid degradation [7]. This was compared to a single-stage recover of lifetime after full. The paper showed that the first stage of lifetime recovery could partially occur at close to room temperature. In another study by Niewelt et al., a revised singledefect parameterisation was presented with a k of 18 (kB-O,b) for the donor level that was also able to describe both fast- and slow- degradation on multiple samples [10]. It should be noted that the difference in the donor-level k from earlier work (kFRC,a) was due to the fact that Bothe et al. assumed a single-level defect, while in the more recent studies by Hallam et al. and Niewelt et al., the two-level properties of B-O defects were included [6], [11]–[13], as well as the different assumptions used in modelling [14]. Furthermore, we have demonstrated the method to modulate the fraction of fast and slow degradation [15]. This observation strongly suggested that the defect that there are two recombination inactive precursors, A1 and A2 where A1 is the precursor of A2. The degradation occurs when A2 transits to B under an illumination. In our previous work where we determined identical recombination properties for rapid and slow B-O related degradation, we eluded to the possibility of an influence of iron [16]. Iron is a common impurity in p-type Cz silicon that is influenced by carrier-injection. In the interstitial form, iron can associate with substitutional boron to form Fe-B pairs [17]. However, these pairs can be readily dissociated with carrier injection. Under illumination, the dissociation of Fe-B pairs usually occurs within 10 seconds, in contrast to fast and slow B-O related degradation, which can take up to 300 s and 48 hours, respectively [7], [16]. A peculiar behaviour of interstitial iron is when compared to the resultant lifetime in the Fe-B state, the lifetime above the cross-over point increases, and lifetime below the cross-over point decreases where the cross-over point depends on the background doping density [18]. In contrast, as B-O defects form, the lifetime in all injection levels decrease, although the effect is more pronounced at low-injection levels due to the increasing dominance of dark-saturation current density related recombination with

Carrier-induced boron-oxygen (B-O) related degradation in Czochralski silicon wafers has been widely studied over decades [1], [2]. However there is still significant debate about the structure and properties of the B-O defect [3]–[6]. One particular discrepancy in the literature is related to the capture cross-section ratio of the B-O defect. In early studies, the fast-forming recombination center (FRC) and slow-forming recombination center (SRC) were proposed to be two distinctively different defects of B-O due to the different time-constants of lifetime degradation and different capture cross section ratios (kFRC,a and kSRC) of 100 and 10 respectively [7], [8]. The k of FRC was later revised to be 65 (kFRC,b) [9]. It was also shown that an applied voltage of 0.4 V could suppress the fast degradation while still enabling the slow degradation. Furthermore, when the applied voltage is shifted from 0.8 V to 0.4 V, partial recovery can be observed, which then continues to degrade by B-O formation with the same degradation rate as the one that had applied voltage of 0.4 V only. Based on the observation, it was suggested that at higher applied voltage of 0.8 V, both fast and slow degradations occur while lower applied voltage of 0.4 V can only introduce the slow degradation. However, the observation is not sufficient to conclude the existence of two independent defects. The two-defect theory cannot certainly explain how improvement of VOC can be observed when applied voltage is changed from 0.8 V to 0.4 V. If the defects were independently formed, the VOC should have continued to decrease regardless of whether the fast degradation, as suggested in the paper, was suppressed with lower VOC instead of converging to the other condition with constant applied voltage. However, recently it was suggested that a single defect could be responsible for B-O degradation due to the measurement of identical recombination properties for fast and slow degradation with capture cross section ratio of close to 19 [6]. Two other key experimental results could also suggest the involvement of separate defects. One of

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increasing carrier concentration. Therefore, during lightsoaking/dark annealing, in samples containing iron, the association/dissociation of Fe-B pairs could potentially lead to a misinterpretation of the B-O related properties. This paper will provide a theoretical evidence, which demonstrates how the behaviour of interstitial iron can adequately provide an alternative explanation to explain the previous difference between the fast and slow degradation. The results will be compared to the key results published in the literature that has been used for the development of theories for the fundamental properties of the boron-oxygen defect, in the context of interstitial iron.

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whereas increasing the NDD for a given [Fei] decreases kapp. This conceptual simulation significantly highlights the possible explanation of the difference in k with different NDD with a fixed amount of interstitial iron, where it would be expected to have a higher initial kapp throughout the early stages of B-O degradation. The theoretical result offers a possible explanation for the higher capture cross-section ratio attributed to the ‘FRC’ in early studies and is in agreement with suggestions of a single defect causing both rapid and slow B-O related degradation [6], [15]. 2.2 Influence of iron to the annihilation of B-O defects The association kinetics of Fe-B pairs can have a significant impact on B-O studies during dark annealing. During dark annealing, the iron association can occur, which depends on the temperature and NA. Therefore, if BO defects and iron are both present in p-type silicon, you can expect to have the simultaneous reaction of Fe-B association and B-O annihilation. The Fe-B association rate [22], κFe,a with NA = 1.5 × 1016 /cm3 was compared to the known FRC annihilation rate [7], κFRC,a1 in Figure 2. In the temperature range of 295 K and 330 K, κFe,a is within the errors of the reported fast recovery rate, κFRC,a1. The measurement was taken with the intensity of 10 W/cm2 or 0.1 Sun, which can expect to be the injection point below the cross-over point. Below the cross-over point, Fe-B association can appear to increase the VOC measured. It should be noted that there was a discrepancy between experimental data from Bothe et al. at temperatures above 330 K and the Fe-B association rate [7]. However, there were only limited data points were used between the time frame of 1 to 100 seconds to calculate the rate in the paper[7] that it is unclear whether the number of measurements is sufficient to fit the rate of the reaction. Furthermore, the difference in kinetics of the second stage of fast defect annihilation (1.36 eV) and the annihilation rate for slow-degradation (1.32 eV) by Bothe et al. was very similar. Lim et al. also suggested the two reactions are due to same defect annihilation [23]. Therefore, the previous observation of first stage recovery of fast may have been due to the combination of Fe-B association and a single rate for B-O defect annihilation.

RESULTS AND DISCUSSION

2.1 Influence of interstitial iron and B-O defects in monocrystalline silicon To investigate the influence of interstitial iron in B-O defect studies, we performed a theoretical simulation to determine an apparent capture cross-section ratio (kapp) of Shockley-Read-Hall (SRH) recombination in the presence of both defects. We used the known recombination properties of the interstitial iron, kFei ≈ 286 [16], and B-O defects, kB-O,c ≈ 9.5 [5], [19], where a parameterisation method [20] was used for SRH lifetime approximation with normalised defect density of B-O defects, NDDB-O in Figure 1.

Figure 1 line plots of the theoretical apparent capture cross section ratio (kapp) as a function of normalised defect density of B-O defects (NDDB-O) with different concentration of Fe. Red dash line indicates kapp with no B-O kapp,Fei and black dash line indicates the kapp with no iron kapp,B-O. NDDfast and NDDfull indicate the typical NDD after fast- and full- degradation, respectively. For simplicity, interstitial iron in the Cz wafer was assumed fully dissociated from boron. The typical value of NDD after fast degradation and after full degradation was statistically estimated using the multiple measurements of identical structures with lifetime range of 60 to 90 µs. For the B-O defect, the relationship between the normalised defect density (NDD) and B-O defect concentration was estimated using Murphy’s method [21]. The kapp was estimated by taking the ratio of the SRH lifetime at two injection levels, 1016 /cm3 and 1012 /cm3[16]. The simulated impact of interstitial iron concentration and normalised B-O defect density on the kapp is shown in Fehler! Verweisquelle konnte nicht gefunden werden.. Increasing the [Fei] increases kapp for a given NDD,

Figure 2 Inverse temperature vs rate constant, κ of B-O annihilation [7], and iron association rate [22] with NA = 1.5 × 1016 /cm3. Highlighted in the yellow region represents the temperature range where iron association rate is within the range of B-O defect annihilation rate (κFe,a = κFRC,a1).

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room temperature is simulated in Figure 4. Condition 1 curve is plotted based on the fitted time constant of the results. κA2B of the Condition 2 is 20 times lower than κA2B of Condition 1. Condition 2 effectively shows a single exponential formation rate, despite that there are two reactions are occurring. This can explain the previous observation of a single formation rate, which was interpreted as the suppression of fast degradation [7]. Furthermore, the recovery of VOC, when the applied voltage is changed from 0.8 V to 0.4 V is likely that the dissociation rate of Fe-B had reduced due to the lower applied voltage, which can effectively improve the VOC. The applied voltage of 0.4 V may have been insufficient to dissociate Fe-B pairs such that no reduction was observed in the early stage of degradation by the constant applied voltage. The converging curves in the study by Bothe et al. [7] would, therefore, be adequately explained by the condition when the Fe-B dissociation rate is reduced while B-O defects continue to form at the same rate with the applied voltage.

2.3 Influence of iron on the formation of B-O defects The influence of iron dissociation to the B-O defect formation study was theoretically investigated. It was known that an applied voltage can form B-O defects as well as dissociate Fe-B pairs [24], [25]. The Fe-B dissociation rate is proportional to the light intensity [26], which strongly indicates that the dissociation rate is excess carrier concentration dependent. The VOC under 0.1 Sun with a different fraction of interstitial iron to the total iron concentration of 3×1011 /cm3 is shown in Figure 3. At this iron concentration, this trend was opposite to that previously obtained by Schmidt et al. [27], where for a total iron concentration below 1×1012 /cm3 an increase in the VOC with Fe-B pair dissociation would decrease in the VOC. However, as the VOC was estimated at 0.1 Sun instead of at 1 Sun, it is expected to operate below the cross-over point [16], where dissociation of Fe-B pair would lower the effective lifetime [28], hence lower VOC. The figure shows that dissociation, which can occur with the application of a bias voltage, can reduce the VOC. Furthermore, if Fe-B association occurs, for example, with slower dissociation rate, VOC can be increased.

Figure 4 Fractional defect density (FDD) vs light soaking time. Condition 1 is the fitted curve. Condition 2 is the modified condition of where κA2B of the Condition 2 is 20 times lower than κA2B of Condition 1 Figure 3 percentage of iron in interstitial form vs VOC at 0.1 Sun with total [Fe] = 3×1011 /cm3. CONCLUSION When the transition rate of A2 to B is significantly reduced, typically with lower excess or hole concentrations, it can appear as a single exponential degradation with two subsequent precursor stages, A1 and A2. In order to demonstrate the apparent single exponential degradation, the two degradation conditions were used, where Condition 2 had a lower transition rate of A2 to B than that of Condition 1. The normalised concentrations in each of the states Ni(t), where i = A1, A2, B, are governed by a system of linear differential equations given by Equations 1-3, and t is the light soaking time. Here, κij is the rate constant from state i to state j. 𝑑𝑁𝐴1 = −𝜅𝐴1𝐴2 𝑁𝐴1 𝑑𝑡 𝑑𝑁𝐴2 = 𝜅𝐴1𝐴2 𝑁𝐴1 − 𝜅𝐴2𝐵 𝑁𝐴2 𝑑𝑡 𝑑𝑁𝐵 = 𝜅𝐴2𝐵 𝑁𝐴2 𝑑𝑡

This paper highlights the significant influence that interstitial iron can have when studying the B-O defect system. Theoretical simulation results in this paper indicate that the presence of iron can greatly influence the apparent behaviour and property of B-O defects., which could offer an alternative explanation of previous observations. The higher k of the B-O defects that is formed by the fast degradation, or namely FRC may be the combination of Fei and B-O defect. The known behaviour of iron and boron association can adequately explain a two-stage recovery of carrier lifetime. We theoretically demonstrate that the dissociation of Fe-B decreases VOC at 0.1 Sun, which can adequately explain previous observation where the recovery of VOC is observed when the applied voltage is reduced. Reducing the applied VOC can reduce the dissociated stage of iron. The previous observation of the VOC improvement with the reduced applied voltage before the degradation is observed can be explained by the new equilibrium of Fei with different reaction rate. The apparent single exponential degradation, which was previously interpreted as a suppression of fast

(1) (2) (3)

Using the experimental results of fractional defect density over light soaking time with pre-dark annealing condition of 200 °C at 10 mins from Kim et al. [15], the formation rate of fast and slow B-O related degradation at

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degradation can be adequately explained by the slower transition rate of A2 and B, or κA2B. The theoretical results that are demonstrated in this study suggest that current defect models should be revised and the potential influence of iron should be considered as even in very small concentrations, can have significant impacts on studies of the B-O defect. The two independent defects that previous studies have suggested would be due to the pre-assumptions of the existence of two defects and likely the misperception of the influence of interstitial iron. Therefore, we conclude that the B-O defect is a single defect.

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ACKNOWLEDGEMENTS The authors would like to acknowledge Kyung Kim, Ly Mai, Nino Borojevic, Hongzhao Li, and the MAiA processing team who assisted with wafer processing. This Program has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA 1-A060) and the Australian Center for Advanced Photovoltaics (ARENA 1-SRI001). Brett Hallam would like to thank the Australian Research Council (ARC) for financial support through a Discovery Early Career Researcher Award (DE170100620) and the Australian Academy of Science through the J.G. Russell Prize. The views expressed herein are not necessarily the views of the Australian Government, and the Australian Government does not accept responsibility for any information or advice contained herein. The authors would like to thank the commercial partners of the ARENA 1A060 project, and the UK Institution of Engineering and Technology (IET) for their funding support for this work through the A.F. Harvey Engineering Prize.

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