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The measurement of infrared absorption of excess carriers is a successful technique by which images of the excess free carrier density and recombination lifetime in silicon can be generated. Carrier density imaging (CDI) has recently been developed as a powerful tool by which one obtains such images. This article analyzes both the effect due to absorption and emission of infrared radiation by excess carriers in silicon. The former effect is the basis of the existing CDI technique whereas this article describes a lifetime measurement technique based on the latter effect. Both methods allow one to obtain lifetime images with high spatial resolution on the order of seconds and measure actual lifetimes. These techniques are contactless and nondestructive. The emission CDI mode facilitates the measurement of low-lifetime wafers on the order of seconds.

In this paper, we present new insight in the degradation and subsequent recovery of charge carrier lifetime upon light soaking at 75 °C observed in float-zone silicon wafers. Variations of doping type, dielectric passivation schemes and thermal treatments after layer deposition were performed. The degradation was only observed for p-type float-zone silicon wafers passivated with passivation schemes involving silicon nitride layers. An influence of thermal treatments after deposition was found. N-type wafers did not degrade independent of their passivation scheme. Room temperature re-passivation experiments showed the degradation to affect the wafer bulk, and photoluminescence studies demonstrated fine lateral striations of effective lifetime. We conclude that the degradation is caused by bulk defects that might be related to hydrogen complexes.

Identification of the lifetime limiting defects in silicon plays a key role in systematically optimizing the efficiency potential of material for solar cells. We present a technique based on temperature and injection dependent photoluminescence imaging to determine the energy levels and capture cross section ratios of Shockley-Read-Hall defects. This allows us to identify homogeneously and inhomogeneously distributed defects limiting the charge carrier lifetime in any silicon wafer. The technique is demonstrated on an n-type wafer grown with the non-contact crucible (NOC) method and an industrial Czochralski (Cz) wafer prone to defect formation during high temperature processing. We find that the energy levels for the circular distributed defects in the Cz wafer are in good agreement with literature data for homogeneously grown oxide precipitates. In contrast, the circular distributed defects found in NOC Si have significantly deeper trap levels, despite their similar appearance.

Light-induced degradation due to BO defects in silicon consists of a fast initial decay within a few seconds followed by a slower decay within hours to days. Determination of injection dependent charge carrier lifetime curves during the initial decay is challenging due to this short timeframe. We have developed a suitable measurement technique based on in situ photoluminescence measurements and present results of our studies of the fast degradation component. The temporal evolution of the recombination activity is studied and assessed by means of a two-level Shockley-Read-Hall statistics. A quadratic dependence of the fast defect activation on the hole concentration during illumination is demonstrated. We suggest a new parameterization of the recombination activity introduced by fast-formed BO defects featuring energy levels 0.34 eV below the conduction band and 0.31 eV above the valence band. The capture asymmetry ratio determined for the donor level of 18.1 is significantly smaller than previous paramet...

A study of the activation of the light-induced degradation in compensated n-type Czochralski grown silicon is presented. A kinetic model is established that verifies the existence of both the fast and the slow components known from p-type and proves the quadratic dependence of the defect generation rates of both defects on the hole concentration. The model allows for the description of lifetime degradation kinetics in compensated n-type silicon under various intensities and is in accordance with the findings for p-type silicon. We found that the final concentrations of the slow defect component in compensated n-type silicon only depend on the interstitial oxygen concentration and on neither the boron concentration nor the equilibrium electron concentration n0. The final concentrations of the fast defect component slightly increase with increasing boron concentration. The results on n-type silicon give new insight to the origin of the BO defect and question the existing models for the defect composition.

Measuring the excess charge carrier density is a widespread approach to accessing the charge carrier lifetime in silicon using an equation of continuity. In this paper, we investigate the scenario of a spatially non-uniform pulsed or modulated optical excitation and a measurement of the emitted photoluminescence intensity. In order to ascertain how to obtain the charge carrier lifetime in this scenario, a rigorous theoretical analysis of the induced charge carrier dynamics is elaborated. Emanating from a photoluminescence intensity-weighted average of charge carrier density, we obtain a macroscopic equation of continuity which accounts for the spatial non-uniformity of charge carrier density. An iterative solution to this equation is given. A numerical simulation reveals distortions to the measured charge carrier lifetime, which occur if the commonly used arithmetic average of the equation of continuity is used instead. We show that the transient measurement of low lifetimes below 10 μs regime is strongly affected and, in particular, that our approach renders necessary for an accurate lifetime determination if using focused excitation as for micro-photoluminescence spectroscopy.

Knowledge of the carrier mobility in silicon is of utmost importance for photovoltaic applications, as it directly influences the diffusion length and thereby the cell efficiency. Moreover, its value is needed for a correct quantitative evaluation of a variety of lifetime measurements. However, models that describe the carrier mobility in silicon are based on theoretical calculations or fits to experimental data in monocrystalline silicon. Multicrystalline (mc) silicon features crystal defects such as dislocations and grain boundaries, with the latter possibly leading to potential barriers through the trapping of charge carriers and thereby influencing the mobility, as shown, for example, by Maruska [Appl. Phys. Lett. 36, 381 (1980)]. To quantify the mobilities in multicrystalline silicon, we performed Hall measurements in p-type mc-Si samples of various resistivities and different crystal structures and compared the data to majority carrier Hall mobilities in p-type mo nocrystalline floatzone (FZ) silicon. For lack of a model that provides reliable values of the Hall mobility in silicon, an empirical fit similar to existing models for conductivity mobilities is proposed based on Hall measurements of monocrystalline p-type FZ silicon. By comparing the measured Hall mobilities obtained from mc silicon with the corresponding Hall mobilities in monocrystalline silicon of the same resistivity, we found that the mobility reduction due to the presence of crystal defects in mc-Si ranges between 0 and 5 only. Mobility decreases of up to 30 as reported by Peter [Proceedings of the 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain, 1-5 September 2008], or even of a factor of 2 to 3 as detected by Palais [Mater. Sci. Eng. B 102, 184 (2003)], in multicrystalline silicon were not observed.

Extensive investigations on industrial multicrystalline silicon solar cells have shown that, for standard 1 Omega cm material, acid-etched texturization, and in absence of strong ohmic shunts, there are three different types of breakdown appearing in different reverse bias ranges. Between -4 and -9 V there is early breakdown (type 1), which is due to Al contamination of the surface. Between -9 and -13 V defect-induced breakdown (type 2) dominates, which is due to metal-containing precipitates lying within recombination-active grain boundaries. Beyond -13 V we may find in addition avalanche breakdown (type 3) at etch pits, which is characterized by a steep slope of the I-V characteristic, avalanche carrier multiplication by impact ionization, and a negative temperature coefficient of the reverse current. If instead of acid-etching alkaline-etching is used, all these breakdown classes also appear, but their onset voltage is enlarged by several volts. Also for cells made from upgraded metallurgical grade material these classes can be distinguished. However, due to the higher net doping concentration of this material, their onset voltage is considerably reduced here. (C) 2011 American Institute of Physics. [doi:10.1063/1.3562200]

We studied the behavior of iron in multicrystalline silicon during phosphorus diffusion by spatially resolved measurements and physical modeling. We present improvements to the previously used models for internal gettering in multicrystalline silicon and phosphorus diffusion gettering. 2-dimensional simulations are used for optimization of the phosphorus diffusion processes for intentionally contaminated wafers regarding the iron distribution, without changing the emitter characteristics. Simulations and experimental results show a reduced interstitial iron concentration after an additional low temperature step at the end of the phosphorus diffusion. The concentration of iron precipitates was reduced by a short annealing at 900°C before the phosphorus diffusion, leading to a carrier lifetime three times higher than compared to the standard process.

Metals corrupt the performance of silicon solar cells severely. In this paper we investigate the recombination activity of metal precipitates and present a strong positive correlation between their recombination activity and the stress around them, independent of the type of metal forming the precipitate. This fundamental observation suggests that stress, together with the size of the precipitate, has a dominant effect on the recombination activity of metallic precipitates. We explain the recombination enhancing effect of stress near precipitates by the strong piezoresistance of silicon.

In this work a method for the spatially resolved detection of the Cr point defect distribution in p-type silicon based on photoluminescence imaging was developed and successfully applied on monocrystalline and multicrystalline wafers. Chromium was identified by its metastable defect configuration and the association time during the formation of chromium-boron pairs. Further defects which could influence the Cr concentration determination have been studied and their influence was suppressed. For a quantitative evaluation of the chromium concentration, the knowledge of exact values of defect parameters such as energy level and capture cross sections for electrons and holes for both Cr states is essential. From the doping dependence of the lifetime, published defect parameters have been analyzed and adjusted slightly in order to explain experimental results. With these parameters reasonable averaged Cr point defect concentrations have been determined for different monocrystalline and multicrystalline samples. Furthermore, space-resolved images of the Cr point defect concentration could be acquired, permitting study of the distribution within a multicrystalline sample with respect to grains and grain boundaries or dislocation clusters.

In the last fifteen years the measurement of the spatially resolved carrier lifetime has emerged as a valuable tool for the characterization of silicon wafers and solar cells. In most of the available measurement methods, the spatial resolution is constrained to the order of several 10 to 100 μm by the diffusion length of the charge carriers. In this paper we introduce a contactless quantitative technique to determine the Shockley–Read–Hall lifetime with a spatial resolution of 1 μm. This technique is based on high injection microphotoluminescence spectroscopy and allows a quantitative analysis of microscopic defects such as grain boundaries and metal precipitates by virtue of the high spatial resolution.

Solar cells made of upgraded metallurgical grade silicon (UMG-Si) feedstock material generally show an increased reverse current at a relatively low reverse bias, which is thought to be disadvantageous for the use in solar modules. In the solar cells used in this study, the reverse current flows through many soft breakdown sites at recombination active defects. The onset voltage of the soft breakdown is decreased primarily due to the increased net doping concentration in the wafer base of UMG-Si wafers compared to standard multicrystalline silicon. We attribute this behavior to the enhancement of the electric field around metal precipitates which form Schottky junctions with the surrounding semiconductor.

Carrier density is a frequently examined parameter for silicon material characterization especially to determine carrier lifetime. Typically integrated values are obtained which ignore nonhomogeneities in the depth dependent carrier profile and result in mean values for lifetime and injection density. Due to the averaging together with nonlinear recombination processes, these values may be subjected to substantial systematic errors. This work demonstrates the effect on iron concentration measurements which are based on the comparison of carrier lifetime measurements before and after light induced splitting of FeB pairs. Although this technique has been proven to be very sensitive, simple, and thus attractive, systematic quantitative errors are revealed in this work which are inherent to this method. It is demonstrated that the errors, while small for certain conditions, may be substantial in many practical cases. Simulations based on the calculation of carrier profiles and virtual measurements provide a tool to estimate the influence of material and setup parameters to the total systematic error. A correction method is developed from this analysis.

We present spatially resolved and injection dependent excess carrier lifetime measurements on silicon. At low level injection conditions an anomalous increase often interferes in such measurements. The origin of the anomalous increase is discussed. Assuming trapping as the origin, highly resolved images of trap parameters together with low level injection recombination lifetimes with strongly reduced trapping effects have been obtained. A theoretical model for infrared lifetime imaging based on the Hornbeck and Haynes model [J. A. Hornbeck and J. R. Haynes, Phys. Rev. 97, 311 (1955)] is presented which describes the trapping effect on the measured lifetime. This model is fitted to experimental spatially resolved data to extract trapping parameters, particularly trap density and the trap-escape ratio, i.e., the ratio between escape rate of minority carriers from the trap level into the minority band (detrapping) and trap rate from the minority band into the trap level (trapping). An upper bound for the low level injection recombination lifetimes is determined. Lifetime and trapping parameters are compared with dislocation density maps. A strong correlation is found between the total trap density and the crystal defect density.

A complex crystal defect in tungsten has been examined in detail by means of the field ion microscope. Field evaporation has been employed to remove the metal surface, making visible the three‐dimensional structure of the defect. The defect has been identified as a grain boundary appearing in(116) of one grain and(116) of the other, so that the grains are misoriented by an angle of 26.5° rotation about the direction [110], which corresponds to the axis of the wire sample. A screw dislocation intersects (110), which is shared by both grains. The intersection of the screw dislocation with the surface produces a spiral on (110) which can be made to rotate under field evaporation. The direction of the Burgers vector of the dislocation has been found to be [110], and the Burgers vector magnitude is an integral multiple of the (110) plane spacing. The Burgers vector magnitude of the dislocation increases as the sample is removed, producing multiple spirals on (110).