Boron-doped Czochralski-grown Silicon Research Articles

Carrier Lifetime Stability of Boron-Doped Czochralski-Grown Silicon Materials for Years After Regeneration in an Industrial Belt Furnace

We examine the long-term stability of the carrier lifetime in boron-doped Czochralski-grown silicon materials with different boron and oxygen concentrations, which were regenerated in an industrial belt furnace. After firing and subsequent regeneration in an industrial conveyor-belt furnace, the silicon samples are exposed to long-term illumination at an intensity of 0.1 suns and a sample temperature of about 30 °C for more than two years. After regeneration, we observe a minor re-degradation (30–72% reduced compared to the degradation observed without regeneration step). We attribute this re-degradation to a non-completed regeneration within the belt furnace due to the short regeneration period. Our results show that the industrial process consisting of firing with subsequent regeneration in the same unit is very effective for industrially relevant silicon materials. Typical industrial silicon wafers with a resistivity of (1.75 ± 0.03) Ωcm and an interstitial oxygen concentration of (6.9 ± 0.3) × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">–3</sup> show lifetimes larger than 2 ms after regeneration and two years of light exposure.

Impact of hydrogen on the boron-oxygen-related lifetime degradation and regeneration kinetics in crystalline silicon

We examine the impact of hydrogen on the boron-oxygen-related lifetime degradation and regeneration kinetics in boron-doped p-type Czochralski-grown silicon wafers. We introduce the hydrogen into the silicon bulk by rapid thermal annealing. The hydrogen source are hydrogen-rich silicon nitride (SiNx:H) layers. Aluminum oxide (Al2O3) layers of varying thickness are placed in-between the silicon wafer surfaces and the SiNx:H layers. By varying the Al2O3 thickness, which acts as an effective hydrogen diffusion barrier, the hydrogen bulk content is varied over more than one order of magnitude. The hydrogen content is determined from measured wafer resistivity changes. In order to examine the impact of hydrogen on the degradation kinetics, all samples are illuminated at a light intensity of 0.1 suns near room temperature. We observe no impact of the in-diffused hydrogen content on the degradation rate constant, confirming that hydrogen is not involved in the boron-oxygen degradation mechanism. The regeneration experiments at 160°C and 1 sun, however, show a clear dependence on the hydrogen content with a linear increase of the regeneration rate constant with increasing bulk hydrogen concentration. However, extrapolation of our measurements toward a zero in-diffused hydrogen content shows that the regeneration is still working even without any in-diffused hydrogen. Hence, our measurements demonstrate that there are two distinct regeneration processes taking place. This is in good agreement with a recently proposed defect reaction model and is also in agreement with the finding that the permanent boron-oxygen deactivation also works on non-fired solar cells, though at a lower rate.

Fluorine Passivation of Defects and Interfaces in Crystalline Silicon.

Defects and impurities in silicon limit carrier lifetimes and the performance of solar cells. This work explores the use of fluorine to passivate defects in silicon for solar cell applications. We present a simple method to incorporate fluorine atoms into the silicon bulk and interfaces by annealing samples coated with thin thermally evaporated fluoride overlayers. It is found that fluorine incorporation does not only improve interfaces but can also passivate bulk defects in silicon. The effect of fluorination is observed to be comparable to hydrogenation, in passivating grain boundaries in multicrystalline silicon, improving the surface passivation quality of phosphorus-doped poly-Si-based passivating contact structures, and recovering boron-oxygen-related light-induced degradation in boron-doped Czochralski-grown silicon. Our results highlight the possibility to passivate defects in silicon without using hydrogen and to combine fluorination and hydrogenation to further improve the overall passivation effect, providing new opportunities to improve solar cell performance.

On the Correlation between Light-Induced Degradation and Minority Carrier Traps in Boron-Doped Czochralski Silicon.

Boron-doped Czochralski-grown silicon wafers dominate the photovoltaic market. Light-induced degradation of these wafers is one of the most significant roadblocks for high-efficiency solar cells. Despite a very large number of publications on this topic, only a few studies have directly investigated the precursor of the defect responsible for this degradation. In this study, using the photoconductance decay measurement method, we identify the precursor of the defect responsible for light-induced degradation. By comparing the photoconductance decay of samples in the different states, we observe the presence of a minority carrier trap in the annealed state, which is not present after degradation. Trap annihilation shows a clear anticorrelation with the generation of the recombination-active boron-oxygen defect, as determined from minority carrier lifetime measurements. Furthermore, it is concluded that a model based on a single-level trap cannot explain the doping-dependent measurements, meaning that the detected trap has two or more energy levels.

Light-induced lifetime degradation effects at elevated temperature in Czochralski-grown silicon beyond boron-oxygen-related degradation

The effect of ‘Light and elevated Temperature Induced Degradation’ (LeTID) of the carrier lifetime is well known from multicrystalline silicon (mc-Si) wafers and solar cells. In this contribution, we perform a series of carrier lifetime measurements to examine, whether the same effect may also be observable in boron-doped Czochralski-grown silicon (Cz-Si). The Cz-Si samples of our study are illuminated (i) at room temperature, (ii) under standard regeneration conditions eliminating the boron-oxygen (BO) related defect (i.e. at 185 °C) and (iii) at a temperature of 80 °C, typical for the examination of the LeTID effect in mc-Si. We observe the typical decay of the carrier lifetime due to the activation of the BO-related defect. Beyond the BO degradation, applying standard solar cell processes, there is no indication for the activation of a second defect. On samples, whose surfaces are passivated by fired hydrogen-rich silicon nitride layers, an additional bulk lifetime degradation effect on a long timescale is observed in the Cz-Si material. However, defect generation rate and injection dependence of the lifetime suggest another defect type than the mc-Si-specific LeTID defect. We conclude that by applying processing steps that trigger LeTID in mc-Si, the same defect does not occur in the Cz-Si samples examined in this study. On a long timescale, however, a hitherto unknown type of defect is activated, which is different from the mc-Si-specific LeTID defect. A careful differentiation between the various kinds of recombination centres which may form during illumination at elevated temperatures is hence of utmost importance.

Excessive light-induced degradation in boron-doped Cz silicon PERC triggered by dark annealing

This work investigates the impact of annealing at elevated temperatures on the light-induced degradation (LID) of passivated emitter and rear cells (PERC) processed on boron-doped Czochralski-grown silicon substrates. The boron-oxygen (BO) defect has been stabilised prior to annealing and subsequent LID treatment. Excessive LID of up to 19.1 %rel. is observed upon illumination after extended dark annealing at 150 °C for 552 h, which is well above the BO defect-related LID of 5.6 %rel. measured upon illumination after cell processing if BO is not stabilised. Light and elevated Temperature Induced Degradation (LeTID), iron-boron pairs and surface recombination are excluded as root causes for the observed increased LID, which shows a similar behaviour as the BO defect but which cannot be explained by the well-established three-state model of the BO defect with the assumption of an empty regenerated state prior to BO stabilisation. Two speculative hypothesis for an explanation are (i) that further BO defect precursors are formed, which could be described via a reservoir or (ii) that a high percentage of the in-principle available BO defects are already in the stabilised state even without dedicated BO defect stabilisation. This increased LID does not occur when at least a small level of excess carrier concentration is induced during extended annealing and, hence, is expected not to occur during field operation. However, the observed behaviour is highly relevant for accelerated aging testing such as, e.g., damp heat testing during IEC and UL certification.

In-situ characterization of electron-assisted regeneration of Cz-Si solar cells

We examine the regeneration kinetics of passivated emitter and rear solar cells (PERCs) fabricated on boron-doped p-type Czochralski-grown silicon wafers in darkness by electron injection via application of a forward bias voltage at elevated temperature (140 °C) in order to discriminate between electronic and photonic effects. Based on these dark regeneration experiments, we address the existing inconsistency regarding the measured linear dependence of the regeneration rate constant on the excess carrier density. Using the method of dark regeneration by current injection into the solar cell, we are able to measure the total recombination current of the solar cell at the actual regeneration temperature under applied voltage, i.e., at the physically relevant regeneration conditions. The direct comparison of the regeneration rate constant as a function of electronically injected carrier concentration in the dark and the regeneration rate constant during illumination clearly shows that the regeneration is a purely electronically stimulated effect and that photons are not directly involved.

Light and elevated temperature induced degradation in p-type and n-type cast-grown multicrystalline and mono-like silicon

We compare light induced degradation behaviours in lifetime samples and fully fabricated solar cells made from p-type boron-doped high performance multicrystalline silicon, p-type boron-doped mono-like silicon, n-type phosphorus-doped high performance multicrystalline silicon and p-type boron-doped Czochralski-grown silicon. Our results confirm that the degradation in multicrystalline silicon is triggered by the rapid cooling after the firing process. All cast-grown silicon samples subjected to fast cooling show lifetime reduction after light soaking. Interestingly, the degradation rate in n-type multicrystalline silicon is found to be orders of magnitude slower than in p-type multicrystalline silicon, suggesting that the defect formation mechanism could be affected by the positions of the quasi fermi levels.

Mass production of p-type Cz silicon solar cells approaching average stable conversion efficiencies of 22 %

Within this work, both the performance and reliability of industrial p-type monocrystalline solar cells with dielectrically passivated rear side and corresponding modules are investigated. Results of the mass production of Q.ANTUM solar cells at Hanwha Q CELLS on boron-doped p-type Czochralski-grown silicon (Cz-Si) substrates are presented, exceeding 21.5 % average conversion efficiency. Without power-enhancing measures such as the use of half cells, multi-wire approaches or light-capturing ribbons, essentially all currently (as of March 2017) produced Cz-Si Q.ANTUM solar modules exhibit output powers of > 300 Wp with 60 full 4-busbar cells. In terms of reliability, light-induced degradation (LID) is investigated in detail, with conditions relevant for the activation of, both, the boron-oxygen (BO) defect, and, so-called “Light and Elevated Temperature Induced Degradation” (LeTID). While the formation of the BO defect has been considered the most prominent LID mechanism in boron-doped p-type Cz-Si, LeTID has so far been discussed mainly as a potential issue for passivated emitter and rear cells (PERC) on multicrystalline silicon (mc-Si) substrates. This work shows that, if not adequately suppressed, LeTID can also occur in p-type Cz-Si PERC with a degradation in output power of up to > 6 %, which cannot be suppressed in a straightforward manner by conventional processing steps to permanently deactivate the BO defect. In contrast to conventional PERC, Hanwha Q CELLS’ Q.ANTUM technology is shown to reliably suppress, both, LID due to BO defect formation, and, LeTID in modules manufactured from, both, p-type mc-Si and Cz-Si substrates.

On the equilibrium concentration of boron-oxygen defects in crystalline silicon

We determine the equilibrium concentration of the BO defect in boron-doped Czochralski-grown silicon after prolonged (up to 150h) annealing at relatively low temperatures between 200 and 300°C. We show that after sample processing, the BO concentration has not necessarily reached the equilibrium state. The actually reached state depends on the detailed temperature profile of the last temperature treatment before the light-induced degradation (LID) is performed. For the investigated Cz-Si materials with base resistivities ranging between 0.5 and 2.5Ωcm, we observe that an annealing step at 200°C for 50h establishes the equilibrium, independent of the base resistivity. Experiments performed at different temperatures reveal that the equilibrium defect concentration decreases with increasing annealing temperature. This observation can be understood, assuming a mobile species which is distributed between at least two different sinks. A possible defect model is discussed.

Kinetics of the permanent deactivation of the boron-oxygen complex in crystalline silicon as a function of illumination intensity

Based on contactless carrier lifetime measurements performed on p-type boron-doped Czochralski-grown silicon (Cz-Si) wafers, we examine the rate constant Rde of the permanent deactivation process of the boron-oxygen-related defect center as a function of the illumination intensity I at 170°C. While at low illumination intensities, a linear increase of Rde on I is measured, at high illumination intensities, Rde seems to saturate. We are able to explain the saturation by assuming that Rde increases proportionally with the excess carrier concentration Δn and take the fact into account that at sufficiently high illumination intensities, the carrier lifetime decreases with increasing Δn and hence the slope of Δn(I) decreases, leading to an apparent saturation. Importantly, on low-lifetime Cz-Si samples no saturation of the deactivation rate constant is observed for the same illumination intensities, proving that the deactivation is stimulated by the presence of excess electrons and not directly by the photons.

Light-induced Recovery of Effective Carrier Lifetime in Boron-doped Czochralski Silicon at Room Temperature

We report on the enhancements of effective carrier lifetime by light-induced “recovery” instead of “degradation” in oxygen-rich boron-doped Czochralski-grown silicon (Cz-Si) wafers at room temperature. The highest measured lifetime of τ=550μs was obtained under light illumination at room temperature. We found that the increasing rate of the lifetimes depends on the firing condition and the illumination intensity. These dependencies are very similar to those of permanent recovery caused by hydrogenation of B-O defects or so called regeneration induced by illumination at higher temperatures. Therefore, it can be presumed that the significant increase of minority carrier lifetime is caused by hydrogenation of recombination active defects. However, the defect-inactive state is not stable and the lifetime was found to decrease slowly after the illuminated recovery step. The thermal activation energy of this defect reactivation reaction was obtained to be 0.36eV from the Arrhenius plot, which is much smaller than that of the thermal dissociation of the hydrogenated B-O complex of 1.25eV. This result indicates that the measured defect transitions are markedly different from those of B-O related defects. Possible mechanisms to control defect deactivation are discussed based on a given fractional concentration of charged hydrogen. By combining this deactivation process and a regeneration process, a very high lifetime value could be achieved in Cz-Si for solar cells.

Photocarrier Radiometry Investigation of Light-Induced Degradation of Boron-Doped Czochralski-Grown Silicon Without Surface Passivation

Light-induced degradation (LID) effects of boron-doped Cz silicon wafers without surface passivation are investigated in details by photocarrier radiometry (PCR). The resistivity of all samples is in the range of 0.006 Omega.cm to 38 Omega.cm. It is found that light-induced changes in surface state occupation have a great effect on LID under illumination. With the increasing contribution of light-induced changes in surface state occupation, the generation rate of the defect decreases. The light-induced changes in surface state occupation and light-induced degradation dominate the temporal behaviors of the excess carrier density of high- and low-resistivity Si wafers, respectively. Moreover, the temporal behaviors of PCR signals of these samples under laser illumination with different powers, energy of photons, and multiple illuminations were also analyzed to understand the light-induced change of material properties. Based on the nonlinear dependence of PCR signal on the excitation power, a theoretical model taking into account both light-induced changes in surface state occupation and LID processes was proposed to explain those temporal behaviors.

Lifetimes exceeding 1 ms in 1-Ω cm boron-doped Cz-silicon

We perform carrier lifetime investigations on oxygen-rich boron-doped Czochralski-grown silicon (Cz-Si) wafers. As a characteristic feature of oxygen-rich boron-doped silicon materials, their lifetime is generally limited by boron–oxygen-related defects, intensifying their recombination-active properties under illumination or, more generally speaking, minority-carrier injection. In this study, we examine the following characteristic lifetime values of boron-doped Cz-Si: τ0 after annealing in darkness (i.e. complete boron–oxygen defect deactivation), τd after illumination at room-temperature (i.e. in the completely degraded state) and τ0p after illumination at elevated temperature (i.e. after ‘permanent recovery’). We show that the permanent recovery process can be strongly influenced by a rapid thermal annealing (RTA) step performed in a conventional belt-firing furnace in advance of the permanent recovery process. We show that all measured lifetimes, i.e. τ0, τd as well as τ0p, are strongly influenced by the RTA process. We observe a strong increase of the lifetime after permanent recovery, depending critically on the RTA process parameters. On 1-Ωcm Cz-Si material after permanent recovery we measure lifetimes of τ0p(Δn=1.5×1015cm−3)=210µs without applying the RTA process and up to τ0p(Δn=1.5×1015cm−3)=2020µs using optimized RTA conditions. Apart from the very high lifetimes achieved, the applied RTA process step also strongly influences the kinetics of the permanent recovery process. The recovery process is accelerated by almost two orders of magnitude, compared to a non-treated sample, which proves the industrial relevance of the process. We discuss the results within a recently proposed defect model which ascribes the observed dependence of the kinetics of the recovery process to the presence of boron nano-precipitates and their interaction with free interstitial boron atoms.

Inductively coupled plasma chemical vapour deposited AlOx/SiNy layer stacks for applications in high-efficiency industrial-type silicon solar cells

Passivated emitter and rear cells (PERC) are considered to be the next generation of industrial-type screen-printed silicon solar cells. Deposition methods for rear passivation layers have to meet both the high-throughput and low-cost requirements of the PV industry in combination with high-quality surface passivation properties. In this paper, we evaluate and optimise a novel deposition technique for AlOx passivation layers by applying an inductively coupled plasma (ICP) plasma-enhanced chemical vapour deposition (PECVD) process. The ICP AlOx deposition process enables high deposition rates up to 5nm/s as well as excellent surface recombination velocities below 10cm/s after firing. A fixed negative charge of −4×1012cm−2 is measured for ICP AlOx single layers with an interface state density of 11.0×1011eV−1cm−2 at midgap position. When applied to PERC solar cells the ICP AlOx layer is capped with a PECVD SiNy layer. We achieve independently confirmed conversion efficiencies of up to 20.1% for large-area (15.6×15.6cm2) PERC solar cells with screen-printed metal contacts and ICP AlOx/SiNy rear side passivation on standard boron-doped Czochralski-grown silicon wafers. The internal quantum efficiency reveals an effective rear surface recombination velocity Srear of (90±30)cm/s and an internal rear reflectance Rb of (91±1)% which demonstrates the excellent rear surface passivation of the ICP AlOx/SiNy layer stack.

Room-temperature method for minimizing light-induced degradation in crystalline silicon

Although 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 interstitial copper 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.

Interaction of copper impurity with radiation defects in silicon doped with boron

The spectrum of deep levels formed in boron-doped Czochralski-grown silicon single crystals as a result of interaction of radiation defects with copper impurity is studied. It is shown that, irrespective of the order of introduction of defects (both in the case of low-temperature copper diffusion into crystals preliminarily irradiated with electrons and in the case of irradiation of the samples contaminated with copper), the same set of deep levels appears. In addition to conventional radiation defects, three types of levels have been detected in the band gap of copper-containing crystals. These levels include the level E v + 0.49 eV (already mentioned in available publications), the level E v + 0.51 eV (previously not related to copper), and a level close to the donor level of a vacancy. Based on the analysis of concentration profiles, the interstitial carbonoxygen pair is excluded from possible precursors of the copper-containing center with level E v + 0.49 eV.

Impact of oxygen on the permanent deactivation of boron–oxygen-related recombination centers in crystalline silicon

The carrier lifetime in boron-doped Czochralski-grown silicon is ultimately limited by light-induced boron–oxygen-related recombination centers. These centers can be permanently deactivated by illumination at elevated temperature (70–220 °C). However, the detailed defect reactions leading to permanent deactivation are still unresolved. In this work, we study the impact of oxygen on the deactivation process. We examine the dependence of the deactivation rate on the interstitial oxygen concentration as well as the impact of long-term annealing at 450 °C, leading to the generation of oxygen clusters acting as donors (thermal donors). We find a decrease in the deactivation rate with both increasing interstitial oxygen concentration and increasing thermal donor concentration, suggesting that oxygen is involved in the deactivation process.

Changes in Concentrations of Copper and Nickel on Boron-Doped Czochralski-Grown Silicon Surface at Room Temperature

In the diffusion of metal impurities in single-crystal silicon (c-Si), it is well known that Cu and Ni atoms diffuse very quickly within the crystal matrix. From various studies on diffusivity in bulk Si, it is clear that such elements can easily move through the Si matrix. In this paper, the time-dependent change in the concentrations of Cu and Ni metals on the Si surface at room temperature is reported. In this study, this change is evaluated by wet chemical analysis, and it is shown that this change results from the high diffusivity of Cu and Ni in Si at room temperature. Furthermore, it is shown that the Ni atoms move independently of the B concentration in Si, and it is possible to predict the diffusion behavior of Cu and Ni in p-type Si at room temperature on the basis of using the conventional thermal diffusion theory.

Solar cells on low-resistivity boron-doped Czochralski-grown silicon with stabilized efficiencies of 20%

Recently, it was shown that the boron-oxygen complex responsible for the light-induced lifetime degradation in oxygen-rich boron-doped silicon can be permanently deactivated by illumination at elevated temperatures. Since the degradation is particularly harmful in low-resistivity Czochralski silicon (Cz-Si), we apply the deactivation procedure to a high-efficiency rear interdigitated single evaporation emitter wrap-through solar cell made on 1.4Ωcm B-doped Cz-Si. The energy conversion efficiency is thereby increased by more than 1% absolute compared to the degraded state to 20.3% on a designated area of 92cm2 and is furthermore shown to be stable under illumination at room temperature.