Browsing by Author "Lindroos, Jeanette"
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- Accelerated light-induced degradation for detecting copper contamination in p-type silicon
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2015) Inglese, Alessandro; Lindroos, Jeanette; Savin, HeleCopper is a harmful metal impurity that significantly impacts the performance of silicon-based devices if present in active regions. In this contribution, we propose a fast method consisting of simultaneous illumination and annealing for the detection of copper contamination in p-type silicon. Our results show that, within minutes, such method is capable of producing a significant reduction of the minority carrier lifetime. A spatial distribution map of copper contamination can then be obtained through the lifetime values measured before and after degradation. In order to separate the effect of the light-activated copper defects from the other metastable complexes in low resistivity Cz-silicon, we carried out a dark anneal at 200 C, which is known to fully recover the boron-oxygen defect. Similar to the boron-oxygen behavior, we show that the dark anneal also recovers the copper defects. However, the recovery is only partial and it can be used to identify the possible presence of copper contamination. - Copper-related light-induced degradation in crystalline silicon
School of Electrical Engineering | Doctoral dissertation (article-based)(2015) Lindroos, JeanetteUnintentional copper and nickel impurities are common in silicon-based devices due to the abundance of contamination sources in industrial silicon crystallization and wafer processing lines. High solubility and diffusivity result readily in significant impurity concentrations, which cause charge-carrier recombination and reduce the device response. This work confirms that nickel diffuses as fast as copper in silicon, emphasizing the importance of contamination control in silicon-based devices. Copper contamination is known to form recombination-active defects in silicon during illumination, which is observed as copper-related light-induced degradation (Cu-LID). In order to identify the extent of degradation in silicon-based devices, this work focuses on determining the properties of Cu-LID in gallium-doped Czochralski (Cz) silicon, boron-doped Cz-Si, and boron-doped multicrystalline silicon. Cu-LID is determined to be predominantly a bulk recombination effect, and the formed defects are found to be stable at 200°C. Slower Cu-LID is observed in Ga-Si compared to B-Si, suggesting that Cu-LID formation is limited by the effective copper diffusivity. Cu-LID is shown to completely disappear after negative sample surface charging and illumination. The negative surface charge is achieved by corona charging or aluminum oxide deposition. Cu-LID removal is observed to have no impact on classical boron-oxygen-related light-induced degradation (BO-LID), which has previously been shown to recover at 200°C. Unlike BO-LID, the activation energy of Cu-LID is found to depend on the silicon doping concentration. Hence, Cu-LID and BO-LID are concluded to be two different degradation effects, which can occur simultaneously in silicon-based devices. - Experimental evidence on removing copper and light-induced degradation from silicon by negative charge
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2014) Boulfrad, Yacine; Lindroos, Jeanette; Wagner, Matthias; Wolny, Franziska; Yli-Koski, Marko; Savin, HeleIn addition to boron and oxygen, copper is also known to cause light-induced degradation (LID) in silicon. We have demonstrated previously that LID can be prevented by depositing negative corona charge onto the wafer surfaces. Positively charged interstitial copper ions are proposed to diffuse to the negatively charged surface and consequently empty the bulk of copper. In this study, copper out-diffusion was confirmed by chemical analysis of the near surface region of negatively/positively charged silicon wafer. Furthermore, LID was permanently removed by etching the copper-rich surface layer after negative charge deposition. These results demonstrate that (i) copper can be effectively removed from the bulk by negative charge, (ii) under illumination copper forms a recombination active defect in the bulk of the wafer causing severe light induced degradation. - Formation kinetics of copper-related light-induced degradation in crystalline silicon
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2014) Lindroos, Jeanette; Savin, HeleLight-induced degradation (LID) is a deleterious effect in crystalline silicon, which is considered to originate from recombination-active boron-oxygen complexes and/or copper-related defects. Although LID in both cases appears as a fast initial decay followed by a second slower degradation, we show that the time constant of copper-related degradation increases with increasing boron concentration in contrast to boron-oxygen LID. Temperature-dependent analysis reveals that the defect formation is limited by copper diffusion. Finally, interface defect density measurements confirm that copper-related LID is dominated by recombination in the wafer bulk. - Iron gettering with aluminium and back-surface passivation of chrystalline silicon solar cells
School of Electrical Engineering | Master's thesis(2010) Lindroos, JeanetteThe purpose of this Master's thesis was to study aluminium gettering of iron impurities in single crystalline silicon. The intention was to determinate the iron segregation coefficient of aluminium segregation gettering at temperatures lower than 850°C. Hence, 20 nm of pure aluminium was sputtered onto the backside of 400 µm thick, 2.87 OMEGA -cm p-doped Czochralski silicon wafers, which had been intentionally iron contaminated to a level of (3.36 ± 0.14) x 1013cm-3. The iron concentration of the silicon wafer was measured through Surface Photovoltage (SPV) diffusion length measurements. Iron gettering with aluminium at temperatures 800 - 840°C reduced the initial iron concentration to the range 1010 - 1011 cm-3. Therefore, the iron segregation coefficient ranged from (5.28± 3.38) x 107 at 800°C to (5.23±2.48) x 105 at 840°C. The resulting high iron segregation coefficient also led to a large iron segregation enthalpy of 21.73 eV at 820 - 840°C. High temperature annealing of an aluminized crystalline silicon wafer also creates an aluminium back-surface field at the backside of the wafer. This back-surface field reduces the back-surface recombination and improves the solar cell efficiency together with the iron gettering. Hence, the back-surface recombination velocity of the aluminium back-surface field was also studied at 800, 850 and 900°C. In these experiments, 1 and 3.5 µm of pure aluminium was sputtered onto the back surface of clean 200 µm thick, 2.87 OMEGA -cm p-doped crystalline silicon wafers. The back-surface fields were formed by 30 min anneals and the diffusion lengths of the wafers were measured with Surface Photovoltage. The measurements resulted in back-surface recom bination velocities of (3.1+1x107-1.85) x 104 cm/s for 3.5 µm aluminium annealed at 850°C and (5.5+3.7-1.6) x 103 cm/s for 3.5 µm aluminium at 900°C. The 1 µm aluminium layer and the 3.5 µm layer annealed at 800°C did not reduce the back-surface recombination from its initial ohmic value 107 cm/s. The results confirmed the theoretical assumption that the recombination decreases as a function of increasing aluminium thickness and increasing formation temperature. - Light-induced degradation in copper-contaminated gallium-doped silicon
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2013) Lindroos, Jeanette; Yli-Koski, Marko; Haarahiltunen, Antti; Schubert, Martin C.; Savin, HeleTo date, gallium-doped Czochralski (Cz) silicon has constituted a solar cell bulk material free of light-induced degradation. However, we measure light-induced degradation in gallium-doped Cz silicon in the presence of copper impurities. The measured degradation depends on the copper concentration and the material resistivity. Gallium-doped Cz silicon is found to be less sensitive to copper impurities than borondoped Cz silicon, emphasizing the role of boron in the formation of copper-related light-induced degradation. - Light-induced degradation in multicrystalline silicon: the role of copper
A4 Artikkeli konferenssijulkaisussa(2016-09-25) Inglese, Alessandro; Focareta, Alessia; Schindler, Florian; Schön, Jonas; Lindroos, Jeanette; Schubert, Martin C.; Savin, HeleIn this contribution, we provide an insight into the light-induced degradation of multicrystalline (mc-) silicon caused by copper contamination. Particularly we analyze the degradation kinetics of intentionally contaminated B- and Ga-doped mc-Si through spatially resolved photoluminescence (PL) imaging. Our results show that even small copper concentrations are capable of causing a strong LID effect in both B- and Ga-doped samples. Furthermore, the light intensity, the dopant and the grain qualitywere found to strongly impact the degradation kinetics, since faster LID was observed with stronger illumination intensity, B-doping and in the grains featuring low initial lifetime. Interestingly after degradation we also observe the formation of bright denuded zones near the edges of the B-doped grains, which might indicate the possible accumulation of copper impurities at the grain boundaries. - Nickel: A very fast diffuser in silicon
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2013) Lindroos, Jeanette; Fenning, David P.; Backlund, Daniel J.; Verlage, Erik; Gorgulla, Angelika; Estreicher, Stefan K.; Savin, Hele; Buonassisi, TonioNickel is increasingly used in both IC and photovoltaic device fabrication, yet it has the potential to create highly recombination-active precipitates in silicon. For nearly three decades, the accepted nickel diffusivity in silicon has been DNi(T)=2.3×10exp−3 exp(−0.47 eV/kBT) cm2/s, a surprisingly low value given reports of rapid nickel diffusion in industrial applications. In this paper, we employ modern experimental methods to measure the higher nickel diffusivity DNi(T)=(1.69±0.74)×10exp−4 exp(−0.15±0.04 eV/kBT) cm2/s. The measured activation energy is close to that predicted by first-principles theory using the nudged-elastic-band method. Our measured diffusivity of nickel is higher than previously published values at temperatures below 1150 °C, and orders of magnitude higher when extrapolated to room temperature. - Preventing light-induced degradation in multicrystalline silicon
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2014) Lindroos, Jeanette; Boulfrad, Yacine; Yli-Koski, Marko; Savin, HeleMulticrystalline silicon (mc-Si) is currently dominating the silicon solar cell market due to low ingot costs, but its efficiency is limited by transition metals, extended defects, and light-induced degradation (LID). LID is traditionally associated with a boron-oxygen complex, but the origin of the degradation in the top of the commercial mc-Si brick is revealed to be interstitial copper. We demonstrate that both a large negative corona charge and an aluminum oxide thin film with a built-in negative charge decrease the interstitial copper concentration in the bulk, preventing LID in mc-Si. - Recombination activity of light-activated copper defects in p-type silicon studied by injection- and temperature-dependent lifetime spectroscopy
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-09-26) Inglese, Alessandro; Lindroos, Jeanette; Vahlman, Henri; Savin, HeleThe presence of copper contamination is known to cause strong light-induced degradation (Cu-LID) in silicon. In this paper, we parametrize the recombination activity of light-activated copper defects in terms of Shockley—Read—Hall recombination statistics through injection- and temperature dependent lifetime spectroscopy (TDLS) performed on deliberately contaminated float zone silicon wafers. We obtain an accurate fit of the experimental data via two non-interacting energy levels, i.e., a deep recombination center featuring an energy level at Ec−Et=0.48−0.62 eVEc−Et=0.48−0.62 eV with a moderate donor-like capture asymmetry (k=1.7−2.6) k=1.7−2.6) and an additional shallow energy state located at Ec−Et=0.1−0.2 eVEc−Et=0.1−0.2 eV, which mostly affects the carrier lifetime only at high-injection conditions. Besides confirming these defect parameters, TDLS measurements also indicate a power-law temperature dependence of the capture cross sections associated with the deep energy state. Eventually, we compare theseresults with the available literature data, and we find that the formation of copper precipitates is the probable root cause behind Cu-LID. - Reduction of Light-induced Degradation of Boron-doped Solar-grade Czochralski Silicon by Corona Charging
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2013) Boulfrad, Yacine; Lindroos, Jeanette; Inglese, Alessandro; Yli-Koski, Marko; Savin, HeleThis study aims at the reduction of light-induced degradation of boron-doped solar-grade Czochralski silicon wafers by corona charging. The method consists of deposition of negative charges on both surface sides of wafer and keeping the wafer in dark for 24 hours to allow the diffusion of positively-charged interstitial copper towards the surfaces. This method proves to be useful to reduce or eliminate light-induced degradation caused by copper. The degradation was significantly reduced in both intentionally (copper-contaminated of the negative charge was found to be proportional to the reduction strength - Room-temperature method for minimizing light-induced degradation in crystalline silicon
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2012) Lindroos, Jeanette; Yli-koski, Marko; Haarahiltunen, Antti; Savin, HeleAlthough light-induced degradation (LID) in crystalline silicon is attributed to the formation of boron-oxygen recombination centers, copper contamination of silicon has recently been observed to result in similar degradation. As positively charged interstitialcopper stays mobile at room temperature in silicon, we show that the bulk copper concentration can be reduced by depositing a large negative charge onto the wafer surface. Consequently, light-induced degradation is reduced significantly in both low- and high-resistivity boron-doped Czochralski-grown silicon. - Valon aiheuttama rappeutuminen monikiteisessä piissä
Sähkötekniikan korkeakoulu | Bachelor's thesis(2012-12-30) Hyvärinen, Sampo - Yksikiteisen piin valmistusmenetelmien vaikutus aurinkokennon toimintaan
Sähkötekniikan korkeakoulu | Bachelor's thesis(2013-12-31) Puokka, Ohto