Boron-doped Czochralski Silicon Research Articles

New properties of boron-oxygen dimer defect in boron-doped Czochralski silicon

Silicon doped with boron is the most widely used material in modern microelectronic devices based on p-Si. Therefore, it is important to thoroughly understand boron’s role in the processes of defect-impurity interaction in Si both on growing the material and during operation of devices. In this work, interactions of boron with oxygen in Si are investigated by studying boron absorption intracenter transitions, which are known to be highly sensitive to the local environment. In boron-doped Si, two lines with maxima at 228 and 261.3 cm−1 were detected. The linear dependence of lines intensity on boron concentration and the quadratic on oxygen content testifies that the defect responsible for the lines can be identified as BsO2i. The observed absorption lines correspond to the transitions from the ground to the excited states of boron, which are shifted toward lower frequencies relative to the main transitions due to a deformation perturbation from neighboring oxygen atoms. The activation energy of annealing and ionization energy of defect are determined. The properties of the registered ВsO2i defect differ from the known ВsO2 associated with the light-induced degradation of solar cells by local configuration. The data obtained testify that the ВsO2i defects with different properties can be formed in Si and must be taken into account when developing Si:B-based devices because they can play an important role in charge carrier transfer and affect the electrical and optical parameters of the material.

In Situ LID and Regeneration of PERC Solar Cells from Different Positions of a B-Doped Cz-Si Ingot

In order to investigate the light-induced-degradation (LID) and regeneration of industrial PERC solar cells made from different positions of silicon wafers in a silicon ingot, five groups of silicon wafers were cut from a commercial solar-grade boron-doped Czochralski silicon (Cz-Si) ingot from top to bottom with a certain distance and made into PERC solar cells by using the standard industrial process after measuring lifetimes of minority carriers and concentrations of boron, oxygen, carbon, and transition metal impurities. Then, the changes of their I - V characteristic parameters (efficiency η , open-circuit voltage V oc , short-circuit current I sc , and fill factor FF ) with time were in situ measured by using a solar cell I - V tester during the 1st LID (45°C, 1 sun, 12 h), regeneration (100°C, 1 sun, 24 h), and 2nd LID (45°C, 1 sun, 12 h). The results show that the LID and regeneration of the PERC solar cells are caused by the transition of B-O defects playing a dominant role together with the dissociation of Fe-B pairs playing a secondary role. The decay of η during the 1st LID is caused by the degradation of V oc , I sc , and FF , while the increase of η during the regeneration is mainly contributed by V oc and FF , and the decay of η during the 2nd LID is mainly induced by the degradation of I sc . After regeneration, the decay rate of η reduces from 4.43%–5.56% (relative) during the 1st LID to 0.33%–1.75% (relative) during the 2nd LID.

New insights on LeTID/BO-LID in p-type mono-crystalline silicon

In this work, light-induced degradation (LID) experiments were performed on p-type boron-doped Czochralski silicon (Cz-Si) wafers under different conditions, aiming to separately characterize boron-oxygen related LID (BO-LID) and “light and elevated temperature-induced degradation” (LeTID). We found that the regeneration rate of BO-LID, as well as the extent and kinetics of LeTID, were strongly influenced by peak firing temperature. These dependences can be explained by the variation of hydrogen concentration, which was modulated by peak firing temperature. Furthermore, we found that extending the duration of dark annealing pre-treatment decelerates the subsequent regeneration of BO defects. This result revealed that, under dark annealing, the evolution of LeTID was accompanied by a notable decrease in the mobile hydrogen concentration. Supported by this phenomenon, a hydrogen-related model based on reversible reaction is proposed to describe the LeTID behavior in crystalline silicon.

Characterization of grown-in defects in Si wafers by gas decoration

In this paper, a vapor gas etching method is developed to systematically characterize grown-in defects such as crystal originated particles (COPs), oxygen precipitates (OPs) and dislocations in 300 mm boron-doped (001) Czochralski (CZ) silicon wafers. For a given type of defect, an unique morphology reveals after selective etching by hydrogen chloride (HCl) at 1000 °C. The etching mechanism is discussed and the characteristic morphology of bulk defects is investigated by localized light scattering (LLS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). Analyzing the data obtained by these techniques, it can be found that the defect types could be differentiated by light scattering signal, i.e., latex sphere equivalent (LSE) size, and defect regions could be divided by the etch pit density. Moreover, due to the effect of HCl etching, the OP as small as several nanometers can be measured directly by SEM and AFM. This study provides Si industry with a fast and comprehensive defect delineation method.

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.

Atomic structure of light-induced efficiency-degrading defects in boron-doped Czochralski silicon solar cells

Using electron paramagnetic resonance, we show that under light exposure, nearly all the 1016 boron doping sites in Si degrade to form shallow traps. Of these 1016 traps, only 1012–1013 cm−3 are spin-active and responsible for light-induced degradation.

Study on the Electrical Injection Regeneration of Industrialized B-Doped Czochralski Silicon PERC Solar Cells

In this paper, 156 mm×156 mm boron-doped Czochralski silicon (Cz-Si) wafers were fabricated into PERC solar cells by using the industrial standard process; then, the as-prepared PERC solar cells were treated by the regeneration process using electrical injection and heating and the effects of different regeneration processes (temperature, time, and injection current) on the anti-light-induced degradation (anti-LID) performance of the PERC solar cells were investigated. The results show that under the condition of 10 A injection current and 30 min processing time, the optimal processing temperature is about 180°C for PERC solar cells to obtain the best anti-LID performance. Under the conditions of a temperature of 180°C, an injection current of 10 A, and a processing time of 0-30 min, the anti-LID performance of the PERC solar cells is enhanced with the increase in the processing time. When the processing time is 20 and 30 min, the efficiency, the short-circuit current, and the open-circuit voltage of the processed PERC solar cells are slightly higher than the initial values before the regeneration and remain stable in the subsequent 12-hour light degradation process at 45°C and 1-sun illumination. At a temperature of 180°C and a processing time of 30 min, the injection current of 6 A is enough to obtain a good regeneration effect, but the optimal injection current is around 10 A.

Degradation of Surface Passivation and Bulk in p-Type Monocrystalline Silicon Wafers at Elevated Temperature

The surface passivation quality and the bulk lifetime of boron-doped p-type Czochralski silicon wafers were studied in response to dark annealing at 175 °C, using in situ effective lifetime measurements. We investigated non-fired and fired silicon nitride (SiN x ), aluminum oxide (AlO x ) capped with SiN x , and thermally-grown silicon oxide (SiO2). Modulation in surface passivation quality and bulk lifetime was detected only in cases where hydrogen is assumed to be released into the silicon wafer from the dielectric (AlO x /SiN x stack and fired SiN x layer). Interestingly, the degradation of both the surface and the bulk were followed by a recovery. It is also interesting to note that the changes in the surface and the bulk seem to be related, as the surface degradation starts when the initial bulk degradation ends. This study indicates a possible involvement of hydrogen in both the degradation and the recovery processes. The evolution of the effective lifetime as a function of time is similar to that reported for carrier-induced degradation in multicrystalline wafers; however, occurring on a different time scale. Hence, these findings may also be valuable for investigation of other degradation mechanisms in different silicon materials.

Thermal stress during RTP processes and its possible effect on the light induced degradation in Cz-Si wafers

In this study, the carrier lifetime variation of p-type boron-doped Czochralski silicon (Cz-Si) wafers was investigated after a direct rapid thermal processing (RTP). Two wafers were passivated by silicon nitride (SiNx:H) layers, deposited by a PECVD system on both surfaces. Then the wafers were subjected to an RTP cycle at a peak temperature of 620 °C. The first wafer was protected (PW) from the direct radiative heating of the RTP furnace by placing the wafer between two as-cut Cz-Si shield wafers during the heat processing. The second wafer was not protected (NPW) and followed the same RTP cycle procedure. The carrier lifetime τeff was measured using the QSSPC technique before and after illumination for 5 h duration at 0.5 suns. The immediate results of the measured lifetime (τRTP) after the RTP process have shown a regeneration in the lifetime of the two wafers with the PW wafer exhibiting an important enhancement in τRTP as compared to the NPW wafer. The QSSPC measurements have indicated a good stable lifetime (τd) and a weak degradation effect was observed in the case of the PW wafer as compared to their initial lifetime value. Interferometry technique analyses have shown an enhancement in the surface roughness for the NPW wafer as compared to the protected one. Additionally, to improve the correlation between the RTP heat radiation stress and the carrier lifetime behavior, a simulation of the thermal stress and temperature profile using the finite element method on the wafers surface at RTP peak temperature of 620 °C was performed. The results confirm the reduction of the thermal stress with less heat losses for the PW wafer. Finally, the proposed method can lead to improving the lifetime of wafers by an RTP process at minimum energy costs.

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.

Impact of germanium co-doping on oxygen precipitation in heavily boron-doped Czochralski silicon

We have investigated the impact of germanium (Ge) co-doping on oxygen precipitation (OP) in heavily boron (B)-doped Czochralski (CZ) silicon subjected to low-high two-step anneal without or with the prior high temperature rapid thermal process (RTP). Herein, the Ge concentration is one order of magnitude higher than the B concentration. It is found that the Ge co-doping exhibits the effect of suppression or enhancement on OP in the heavily B-doped CZ silicon without or with the prior RTP. In the case without the prior RTP, the compressive stress introduced by the Ge co-doping compensates the tensile stress arising from the B-doping, which is not beneficial for the growth of oxide precipitates. While, in the case with the prior RTP, the Ge co-doping increases the amount of vacancies introduced by the RTP and, moreover, may enable to generate more heterogeneous nucleation centers of oxide precipitates, thus leading to the enhanced OP in the heavily B-doped CZ silicon.

Low temperature iron gettering by grown-in defects in p-type Czochralski silicon

Low temperature iron gettering in as-grown boron doped Czochralski silicon (Cz-Si) at temperatures between 220 and 500 °C is studied using microwave-photoconductive decay based minority carrier lifetime measurements. Scanning infrared microscopy technique is used to study the defect density/size distribution in the samples before and after anneal. It is found that the decrease of interstitial iron (Fei) concentration shows a double exponential dependence on annealing time at all temperatures. This suggests the existence of two sinks for Fei. Meanwhile, the observed bulk defect densities and sizes in contaminated and as-grown samples are nearly the same, implying that the grown-in defects could be the gettering centers in this process. The results are important for understanding and controlling low temperature Fei gettering during processing of Cz-Si based devices.

Grouping by bulk resistivity of production-line mono-crystalline silicon wafers and its influence on the manufacturing of solar cells modules

Weak light performance of crystalline silicon solar module presents a clear dependence on the type of cell used, mainly wafer resistivity and shunt resistance. This paper shows that a proper wafer and cell classification can provide a further optimization opportunity. This means a well-controlled product fabrication and production line yield improvement without an additional cost. For these reasons a resistivity boron-doped Czochralski silicon (Cz–Si) wafer classification has been implemented as the first stage of a photovoltaic monocrystalline silicon solar cell production line which allows to process solar device batches with similar raw material properties. This new production stage leads to a narrower solar cell efficiency distribution and a tailored power photovoltaic module fabrication. After that, solar cell devices have been sorted by shunt resistance criterion. Their behaviors under weak light conditions have been carefully studied at cell and module levels. Finally, one and two diode models have been used to justify the obtained results.

Effect of Thermal Annealing on Light-Induced Minority Carrier Lifetime Enhancement in Boron-Doped Czochralski Silicon**Supported by the National Natural Science Foundation of China under Grant No 61076056.

The effect of thermal annealing on the light-induced effective minority carrier lifetime enhancement (LIE) phenomenon is investigated on the p-type Czochralski silicon (Cz-Si) wafer passivated by a phosphorus-doped silicon nitride (P-doped SiNx) thin film. The experimental results show that low temperature annealing (below 300°C) can not only increase the effective minority carrier lifetime of P-doped SiNx passivated boron-doped Cz-Si, but also improve the LIE phenomenon. The optimum annealing temperature is 180°C, and its corresponding effective minority carrier lifetime can be increased from initial 7.5 μs to maximum 57.7 μs by light soaking within 15 min after annealing. The analysis results of high-frequency dark capacitance-voltage characteristics reveal that the mechanism of the increase of effective minority carrier lifetime after low temperature annealing is due to the sharp enhancement of field effect passivation induced by the negative fixed charge density, while the mechanism of the LIE phenomenon after low temperature annealing is attributed to the enhancement of both field effect passivation and chemical passivation.

Light-induced enhancement of the minority carrier lifetime in boron-doped Czochralski silicon passivated by doped silicon nitride

This study reports a doubling of the effective minority carrier lifetime under light soaking conditions, observed in a boron-doped p-type Czochralski grown silicon wafer passivated by a phosphorus-doped silicon nitride thin film. The analysis of capacitance–voltage curves revealed that the fixed charge in this phosphorus-doped silicon nitride film was negative, which was unlike the well-known positive fixed charges observed in traditional undoped silicon nitride. The analysis results revealed that the enhancement phenomenon of minority carrier lifetime was caused by the abrupt increase in the density of negative fixed charge (from 7.2×1011 to 1.2×1012cm−2) after light soaking.

Strain relief via silicon self-interstitial emission in highly boron-doped silicon: A diffuse X-ray scattering study of oxygen precipitation

Oxygen precipitation in highly boron-doped Czochralski silicon (5 mΩ cm resistivity) was studied, and a strain-relieving mechanism involving the emission of silicon interstitial atoms (ISi) was identified. Strain-sensitive X-ray diffraction was employed, and through a comparison with complementary electron microscopy measurements a linear misfit strain ϵ=0.01 was determined. Different behavior concerning strain relaxation was measured for wafer-type samples (760 μm thickness) and thick samples (2 mm thickness), which was explained with out-diffusion of ISi in the thinner samples. Based on the experimental findings, a model was developed which invokes an increase in the solubility of interstitial silicon atoms upon boron doping. The model successfully accounts for different effects of boron doping on oxygen precipitation in silicon, which were reported on in literature such as preferential octahedral morphology of precipitates, enhanced nucleation, and relaxed accommodation of the precipitates with respect...

Hydrogen passivation of defect-rich n-type Czochralski silicon and oxygen precipitates

High efficiency commercial silicon solar cell technologies are often fabricated on n-type wafers to avoid light-induced degradation and other recombination centres that are observed in boron-doped Czochralski silicon. The advanced cell structures often feature the p–n junction on the rear of the device and require high bulk lifetimes in order to achieve high efficiencies. However n-type Czochralski silicon is still subject to a range of impurity related defects which can substantially reduce the lifetime of such material. In this work, large improvements in bulk lifetimes were demonstrated on standard commercial grade n-type Czochralski silicon from values in the range of 90–190μs to values of 3.4–4.9ms through the use of a short, low temperature hydrogen passivation process only using atomic hydrogen released from layers of silicon oxynitride grown by plasma enhanced chemical vapour deposition. The large improvements suggest that wafers with inherent low lifetimes, often deemed unsuitable to achieve high efficiencies, could in fact be compatible with the use of advanced cell architectures, to realise cell efficiencies over 24%. Oxygen precipitates are one type of impurity related defect which can cause substantial lifetime and efficiency limitations in both n- and p-type Czochralski silicon solar cells. An advanced hydrogenation process incorporating minority carrier injection was observed to lead to the complete electrical deactivation of the recombination activity of the defects. Furthermore, the passivation was retained throughout the remainder of the fabrication sequence and hence, no recombination activity from oxygen precipitates was evident on finished devices.

Evidence for the role of hydrogen in the stabilization of minority carrier lifetime in boron-doped Czochralski silicon

This study demonstrates that the presence of a hydrogen source during fast-firing is critical to the regeneration of B-O defects and that is it not a pure thermally based mechanism or due to plasma exposure. Boron-doped p-type wafers were fired with and without hydrogen-rich silicon nitride (SiNx:H) films present during the fast-firing process. After an initial light-induced degradation step, only wafers fired with the SiNx:H films present were found to undergo permanent and complete recovery of lifetime during subsequent illuminated annealing. In comparison, wafers fired bare, i.e., without SiNx:H films present during firing, were found to demonstrate no permanent recovery in lifetime. Further, prior exposure to hydrogen-rich plasma processing was found to have no impact on permanent lifetime recovery in bare-fired wafers. This lends weight to a hydrogen-based model for B-O defect passivation and casts doubt on the role of non-hydrogen species in the permanent passivation of B-O defects in commercial-grade p-type Czochralski silicon wafers.

Light-induced degradation and metastable-state recovery with reaction kinetics modeling in boron-doped Czochralski silicon solar cells

Solar cells fabricated from boron-doped p-type Czochralski silicon suffer from light-induced degradation that can lower the conversion efficiency by up to 10% relative. When solar cells are exposed to temperatures between 100 °C and 200 °C under illumination, regeneration, in which the minority carrier lifetime is gradually recovered, occurs after the initial light-induced degradation. We studied the light-induced degradation and regeneration process using carrier injection within a design chamber and observed open-circuit voltage trends at various sample temperatures. We proposed a cyclic reaction kinetics model to more precisely analyze the degradation and recovery phenomenon. Our model incorporated the reaction paths that were not counted in the original model between the three states (annealed, degradation, and regeneration). We calculated a rate constant for each reaction path based on the proposed model, extracted an activation energy for each reaction using these rate constants at various temperatures, and calculated activation energies of redegradation and the stabilization reaction.

Effective surface passivation of p-type crystalline silicon with silicon oxides formed by light-induced anodisation

Electronic surface passivation of p-type crystalline silicon by anodic silicon dioxide (SiO2) was investigated. The anodic SiO2 was grown by light-induced anodisation (LIA) in diluted sulphuric acid at room temperature, a process that is significantly less-expensive than thermal oxidation which is widely-used in silicon solar cell fabrication. After annealing in oxygen and then forming gas at 400°C for 30min, the effective minority carrier lifetime of 3–5Ωcm, boron-doped Czochralski silicon wafers with a phosphorus-doped 80Ω/□ emitter and a LIA anodic SiO2 formed on the p-type surface was increased by two orders of magnitude to 150μs. Capacitance–voltage measurements demonstrated a very low positive charge density of 3.4×1011cm−2 and a moderate density of interface states of 6×1011eV−1cm−2. This corresponded to a silicon surface recombination velocity of 62cms−1, which is comparable with values reported for other anodic SiO2 films, which required higher temperatures and longer growth times, and significantly lower than oxides grown by chemical vapour deposition techniques. Additionally, a very low leakage current density of 3.5×10−10 and 1.6×10−9Acm−2 at 1 and −1V, respectively, was measured for LIA SiO2 suggesting its potential application as insulation layer in IBC solar cells and a barrier for potential induced degradation.