104455889
Impact of iron on the room temperature luminescence efficiency of oxygen-containing precipitates in silicon
Oxygen precipitation in silicon has been associated with a weak room temperature sub-bandgap luminescence emission at around 1600 nm. We show that the additional presence of iron impurities enhances this emission by an order of magnitude and results in a red shift of the peak luminescence by approximately 45 nm. We not only observe an increase in the luminescence emission with iron contamination level but also with the density and size of the oxide precipitates. Moreover, we provide evidence that the sub-bandgap luminescence emission increases proportionally with the concentration of iron segregated to oxide precipitates after high temperature (>700 °C) annealing and thus allows evaluation of the gettering efficiency of oxygen-containing precipitates. Annealing of iron-contaminated samples at low temperatures (550 °C) results in a considerable reduction in the interstitial iron concentration without changing the sub-bandgap luminescence, indicating that the sink to which iron diffuses depends upon temperature.
122032482
Mechanisms of transition-metal gettering in silicon
The atomic process, kinetics, and equilibrium thermodynamics underlying the gettering of transition-metal impurities in Si are reviewed. Methods for mathematical modeling of gettering are discussed and illustrated. Needs for further research are considered.
29344957
Mechanism of internal gettering of interstitial impurities in Czochralski-grown silicon.
Results of the low-temperature precipitation of interstitial Fe (\ensuremath{\approxeq}250 \ifmmode^\circ\else\textdegree\fi{}C) in Czochralski-grown Si after different annealing treatments for the precipitation of oxygen are presented. It is concluded that the Fe solubility is not affected by the oxygen-precipitation process. The Fe-precipitation kinetics are found to be strongly accelerated by oxygen-precipitation\char21{}induced defects. Based on the results the mechanism of internal gettering is derived. For the first time the contribution of oxygen precipitates and dislocations to the internal-gettering process have been determined quantitatively.
23404908
The effect of oxide precipitates on minority carrier lifetime in n-type silicon
Supersaturated levels of interstitial oxygen in Czochralski silicon can lead to the formation of oxide precipitates. Although beneficial from an internal gettering perspective, oxygen-related extended defects give rise to recombination which reduces minority carrier lifetime. The highest efficiency silicon solar cells are made from n-type substrates in which oxide precipitates can have a detrimental impact on cell efficiency. In order to quantify and to understand the mechanism of recombination in such materials, we correlate injection level-dependent minority carrier lifetime data measured with silicon nitride surface passivation with interstitial oxygen loss and precipitate concentration measurements in samples processed under substantially different conditions. We account for surface recombination, doping level, and precipitate morphology to present a generalised parameterisation of lifetime. The lifetime data are analysed in terms of recombination activity which is dependent on precipitate density or on the surface area of different morphologies of precipitates. Correlation of the lifetime data with interstitial oxygen loss data shows that the recombination activity is likely to be dependent on the precipitate surface area. We generalise our findings to estimate the impact of oxide precipitates with a given surface area on lifetime in both n-type and p-type silicon.
56317982
Parameterisation of injection-dependent lifetime measurements in semiconductors in terms of Shockley-Read-Hall statistics: An application to oxide precipitates in silicon
Injection-dependent minority carrier lifetime measurements are a valuable characterisation method for semiconductor materials, particularly those for photovoltaic applications. For a sample containing defects which obey Shockley-Read-Hall statistics, it is possible to use such measurements to determine (i) the location of energy levels within the band-gap and (ii) the ratios of the capture coefficients for electrons and holes. In this paper, we discuss a convenient methodology for determining these parameters from lifetime data. Minority carrier lifetime is expressed as a linear function of the ratio of the total electron concentration to the total hole concentration for p-type (or vice versa for n-type) material. When this is plotted on linear scales, a single-level Shockley-Read-Hall centre manifests itself as a straight line. The gradient and intercepts of such a plot can be used to determine recombination parameters. The formulation is particularly instructive when multiple states are recombination-ac...
56307184
Minority carrier lifetime in silicon photovoltaics: The effect of oxygen precipitation
Single-crystal Czochralski silicon used for photovoltaics is typically supersaturated with interstitial oxygen at temperatures just below the melting point. Oxide precipitates therefore can form during ingot cooling and cell processing, and nucleation sites are typically vacancy-rich regions. Oxygen precipitation gives rise to recombination centres, which can reduce cell efficiencies by as much as 4% (absolute). We have studied the recombination behaviour in p-type and n-type monocrystalline silicon with a range of doping levels intentionally processed to contain oxide precipitates with a range of densities, sizes and morphologies. We analyse injection-dependent minority carrier lifetime measurements to give a full parameterisation of the recombination activity in terms of Shockley–Read–Hall statistics. We intentionally contaminate specimens with iron, and show recombination activity arises from iron segregated to oxide precipitates and surrounding defects. We find that phosphorus diffusion gettering reduces the recombination activity of the precipitates to some extent. We also find that bulk iron is preferentially gettered to the phosphorus diffused layer rather than to oxide precipitates.
123528362
On the nature of striations in n-type silicon solar cells
In n-type Czochralski silicon (Cz-Si) wafer, swirl shaped regions with low lifetime (known as striations) can cause degradation up to 1% absolute or even more in homojunction industrial solar cells. Nevertheless, the nature of the defects responsible for the occurrence of these striations is still unclear. In this work, n-type Cz-Si solar cell precursors cut from industrial size ingots with different feedstock quality and oxygen content were analyzed by microwave photo-conductance decay and photoluminescence in order to investigate the nature of such defects. The results demonstrate that the defects responsible for the occurrence of striations are oxide nanoprecipitates formed during the high temperature steps for the solar cell realization, due to the presence of grown-in oxygen nuclei.
123434163
On the mechanism of recombination at oxide precipitates in silicon
Oxide precipitates are well known to degrade minority carrier lifetime in silicon, but the mechanism by which they act as recombination centres is not fully understood. We report minority carrier lifetime measurements on oxide precipitate-containing silicon which has been intentionally contaminated with iron. Analysis of the injection-dependence of lifetime demonstrates the same recombination centres exist in iron-contaminated and not intentionally contaminated samples, with the state density scaling with iron loss from the bulk. This shows that recombination activity arises from impurity atoms segregated to oxide precipitates and/or surrounding crystallographic defects.
56230077
Competitive gettering of iron in silicon photovoltaics: Oxide precipitates versus phosphorus diffusion
Experiments have been conducted to understand the behaviour of iron in silicon containing oxide precipitates and associated defects (dislocations and stacking faults), which is subjected to phosphorus diffusion gettering. Injection-dependent minority carrier lifetime measurements are analysed to provide quantitative information on the degree to which the precipitates and associated defects are decorated with iron impurities. These data are correlated with bulk iron measurements based on the photodissociation of FeB pairs. Iron in the vicinity of oxide precipitates in samples with relatively low levels of bulk iron contamination ( 1.2 × 1013 cm−3) result in irreversible behaviour, suggesting iron precipitation in the vicinity of oxide precipitates. Bulk iron is preferentially gettered to the phosphorus diffused layer opposed to the oxide precipitates and associated defects.
67800954
Room temperature sub-bandgap photoluminescence from silicon containing oxide precipitates
Room temperature photoluminescence was measured from p-type Czochralski silicon processed to contain oxide precipitates. No detectable luminescence was associated with unstrained oxide precipitates. Strained oxide precipitates gave rise to a broad luminescence peak centred at ∼1600 nm. The intensity of the peak increased with the density of strained precipitates, with band-to-band luminescence being reduced correspondingly. Dislocations and stacking faults around the strained precipitates were found to introduce competing non-radiative recombination centres which reduced the sub-bandgap photoluminescence. A mechanism is proposed for the sub-bandgap luminescence due to strained precipitates in terms of a transition between defect bands.
97310891
Evaluation of Oxygen Precipitated Silicon Crystals by Deep‐Level Photoluminescence at Room Temperature and Its Mapping
A deep-level photoluminescence (PL), which corresponds to the DI line (0.81 eV) at 4.2 K, is investigated at room temperature for oxygen precipitated Czochralski Si crystals in their growth directions. Analyses are made of the macroscopic and microscopic distributions of the room temperature D1 line intensity (I D ) the room temperature band-to-band PL intensity (I B ), carrier lifetime (γ), and precipitated oxygen concentration (Δ[O i ])
123437541
Optical properties of oxygen precipitates and dislocations in silicon
Photoluminescence (PL) and deep level transient spectroscopy (DLTS) measurements were used to study the origin of optical emissions in the 0.8–1.0 eV region of selected oxygen precipitated and dislocated silicon samples. It was shown that the D1 band, present in both types of samples, is the convolution of different sub-bands, narrowly spaced between 0.802 and 0.820 eV. The emission at 0.807 eV, assigned in the literature to dislocations, was found only in samples where dislocations were intentionally generated by plastic deformation or induced by the clustering of self-interstitials generated during the growth of oxide precipitates. A comparison of the results of PL, DLTS, and optical DLTS measurements allows the assignment of levels involved in the radiative recombination processes.
135525601
Radiative recombination processes of thermal donors in silicon
Abstract : There is a recent, renewed attention on the possible development of optical emitters compatible with silicon microelectronic technology and it has been recently shown that light emitting diodes could be manufactured on dislocated silicon, where dislocations were generated by plastic deformation or ion implantation. Among other potential sources of room temperature light emission, compatible with standard silicon-based ULSI technology, we have studied old thermal donors (OTD), as the origin of their luminescence is still matter of controversy and demands further investigation. In this work we discuss the results of a spectroscopical study of OTD using photoluminescence (PL) and Deep Level Transient Spectroscopy (DLTS) on standard Czochralsky (Cz) silicon samples and on carbon-doped samples. We were able to show that their main optical activity, which consists of a narrow band at 0.767 eV (P line), is correlated to a transition from a shallow donor level of OTD to a deep level at E(sub v)+0.37 eV which is tentatively associated to C-O complexes. As we have shown that the P line emission persists at room temperature, we discuss about its potentialities to silicon in optoelectronic applications.
96297218
Photoluminescence Analysis of Iron Contamination Effect in Multicrystalline Silicon Wafers for Solar Cells
We investigated the effect of Fe contamination on the electronic properties of dislocation clusters in relation to oxygen precipitation in multicrystalline silicon (mc-Si). Photoluminescence (PL) spectroscopy and mapping were performed at room and liquid-He temperatures on mc-Si wafers before and after Fe contamination. PL spectra consisted of the band-edge emission, the 0.78-eV emission associated with oxygen precipitates, and the dislocation-related D-lines. The Fe contamination increased the electrically active dislocation clusters. Part of these clusters acted as preferential oxygen precipitation sites, and their electronic properties were not further influenced by the Fe contamination.
99978417
Thresholds for effective internal gettering in silicon wafers
The gettering of transition metals by Internal Gettering (IG) is an important tool for the management of metal contamination in IC processing. But unlike most external gettering systems, IG systems must be set up in a silicon wafer via a controlled precipitation of dissolved interstitial oxygen in the wafer during heat treatments. This paper addresses the question of the threshold to effective IG as the oxygen in the system goes from an essentially fully dissolved state to varying degrees of precipitation. The central step in the transition to an efficient gettering system is a morphological transformation of a growing oxygen cluster from an unstrained state to a strained platelet. Experimental data of the transformation rates as a function of nuclei site density and oxygen concentration are given. The dependence of the transformation rate has been successfully modeled and the time to create efficient IG systems as a function of site density, temperature and oxygen concentration is calculated.
94609566
A fast, preparation‐free method to detect iron in silicon
Iron is one of the most important impurities in silicon integrated‐circuit technology. We present a fast (5 min), essentially preparation‐free large‐area (25 cm2 ) technique to determine the Fe concentration in boron‐doped silicon with a sensitivity of 2–5×1011 cm−3. The principle of the method is based on the fact that interstitially dissolved Fe undergoes a reversible pairing reaction with boron and that the minority‐carrier diffusion length—as measured by the surface photovoltage method—is modified by this reaction. The method has been calibrated by deep‐level transient spectroscopy and is also suitable to measure a surface Fe contamination in combination with a rapid thermal annealing diffusion step.
93403774
Electronic properties of interstitial iron and iron-boron pairs determined by means of advanced lifetime spectroscopy
As dissolved iron is one of the most common lifetime-killing contaminants in silicon, its coexisting defect configurations, interstitial iron (Fei) and iron-boron pairs (FeB), are investigated on an intentionally iron-contaminated silicon sample by means of temperature-dependent lifetime spectroscopy (TDLS) and injection-dependent lifetime spectroscopy (IDLS). In good agreement with the literature, the study identifies the known Fei donor level at Et−EV=(0.394±0.005)eV and determines its symmetry factor as k=σn∕σp=51±5, which is an order of magnitude lower than expected from the literature. Using the well-confirmed k factor, the poorly confirmed electron-capture cross section is redetermined as σn=(3.6±0.4)×10−15cm2 at 300K. In addition, the observed exponential σ(T) dependence identifies the multiphonon emission mechanism as the dominant capture mechanism for electrons with an activation energy E∞=0.024eV. Concerning the defect related to the iron-boron pair, the study unambiguously identifies the deep F...
93879608
Contamination of silicon by iron at temperatures below 800 °C
Iron-related defects are deleterious in silicon-based integrated circuits and photovoltaics, ruining devices and acting as strong recombination centres. Unless great care is taken, iron contamination will result from high temperature processing and so it is essential to understand the degree to which this can occur. Iron solubility data above ∼800 °C have been summarised by Istratov et al. (Appl. Phys. A 69, 13 (1999)), but many processes are performed at lower temperatures for which solubility data are scarce. We have studied iron contamination below ∼800 °C. Iron concentrations in intention- ally contaminated air-annealed Czochralski silicon samples were determined from the change in minority carrier lifetime due to photodissociation of FeB pairs measured by quasi-steady-state photoconductance. In the ∼600 to 800 °C temperature range the iron concentration was found to vary according to $ 1.3 \times 10^{21} \; {\rm exp} \;\left (- { {1.8\;{\rm eV} } \over {kT} } \right)\;{\rm cm}^{-3}. It is therefore the case that significantly more iron can dissolve in silicon at these temperatures than extrapolation of higher temperature data suggests, with the enhancement being by a factor of >20 at 600 °C. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
67842486
The relaxation behaviour of supersaturated iron in single-crystal silicon at 500 to 750 °C
Iron-related defects cause major problems in silicon for both microelectronic devices and photovoltaics. Iron contamination can occur during high temperature processing or, particularly in the case of low-cost photovoltaics, from the feedstock. In many situations, silicon is cooled too rapidly for the establishment of equilibrium, and so the bulk iron concentration exceeds the solubility value. We have investigated the relaxation of supersaturated bulk iron to the equilibrium solubility in single-crystal silicon. Bulk iron concentrations are measured by analysing the change in minority carrier lifetime that occurs when iron-boron pairs are dissociated. High-purity silicon is rubbed with iron and annealed at 750 °C for 24 h. This process creates an iron silicide phase on the rubbed surface and allows the equilibrium solubility of ∼2 × 1012 cm−3 to be established. Samples are then annealed at lower temperatures (500 to 700 °C) for a range of times. The rate of decay in iron concentration depends upon whethe...
94189080
Optimization and characterization of remote plasma-enhanced chemical vapor deposition silicon nitride for the passivation of p-type crystalline silicon surfaces
In a recent letter [Lauinger et al., Appl. Phys. Lett. 68, 1232 (1996)] we have shown that record low effective surface recombination velocities Seff of 4 cm/s have been obtained at ISFH on low-resistivity (1 Ω cm) p-type crystalline silicon using microwave-excited remote plasma-enhanced chemical vapor deposition (RPECVD) of silicon nitride at low temperature (300–400 °C). As an important application, this technique allows a simple fabrication of rear-passivated high-efficiency silicon solar cells with monofacial or bifacial sensitivity. In this work, we present details of the required optimization of the PECVD parameters and a characterization of the resulting silicon nitride films. All deposition parameters are shown to strongly affect Seff as well as the stability of the films against the ultraviolet (UV) photons of terrestrial sunlight. A clear correlation between Seff and the film stoichiometry is observed, allowing a simple control and even a rough optimization of the surface passivation quality by ...
13411639
Gettering of interstitial iron in silicon by plasma-enhanced chemical vapour deposited silicon nitride films
It is known that the interstitial iron concentration in silicon is reduced after annealing silicon wafers coated with plasma-enhanced chemical vapour deposited (PECVD) silicon nitride films. The underlying mechanism for the significant iron reduction has remained unclear and is investigated in this work. Secondary ion mass spectrometry (SIMS) depth profiling of iron is performed on annealed iron-contaminated single-crystalline silicon wafers passivated with PECVD silicon nitride films. SIMS measurements reveal a high concentration of iron uniformly distributed in the annealed silicon nitride films. This accumulation of iron in the silicon nitride film matches the interstitial iron loss in the silicon bulk. This finding conclusively shows that the interstitial iron is gettered by the silicon nitride films during annealing over a wide temperature range from 250 °C to 900 °C, via a segregation gettering effect. Further experimental evidence is presented to support this finding. Deep-level transient spectrosc...
43610796
Experimental setup for camera-based measurements of electrically and optically stimulated luminescence of silicon solar cells and wafers.
We report in detail on the luminescence imaging setup developed within the last years in our laboratory. In this setup, the luminescence emission of silicon solar cells or silicon wafers is analyzed quantitatively. Charge carriers are excited electrically (electroluminescence) using a power supply for carrier injection or optically (photoluminescence) using a laser as illumination source. The luminescence emission arising from the radiative recombination of the stimulated charge carriers is measured spatially resolved using a camera. We give details of the various components including cameras, optical filters for electro- and photo-luminescence, the semiconductor laser and the four-quadrant power supply. We compare a silicon charged-coupled device (CCD) camera with a back-illuminated silicon CCD camera comprising an electron multiplier gain and a complementary metal oxide semiconductor indium gallium arsenide camera. For the detection of the luminescence emission of silicon we analyze the dominant noise sources along with the signal-to-noise ratio of all three cameras at different operation conditions.
55433698
Contactless determination of current–voltage characteristics and minority‐carrier lifetimes in semiconductors from quasi‐steady‐state photoconductance data
A simple method for implementing the steady‐state photoconductance technique for determining the minority‐carrier lifetime of semiconductor materials is presented. Using a contactless instrument, the photoconductance is measured in a quasi‐steady‐state mode during a long, slow varying light pulse. This permits the use of simple electronics and light sources. Despite its simplicity, the technique is capable of determining very low minority carrier lifetimes and is applicable to a wide range of semiconductor materials. In addition, by analyzing this quasi‐steady‐state photoconductance as a function of incident light intensity, implicit current–voltage characteristic curves can be obtained for noncontacted silicon wafers and solar cell precursors in an expedient manner.
95114574
Photoconductance‐calibrated photoluminescence lifetime imaging of crystalline silicon
We use photoluminescence (PL) measurements by a silicon charge-coupled device camera to generate high-resolution lifetime images of multicrystalline silicon wafers. Absolute values of the excess carrier density are determined by calibrating the PL image by means of contactless photoconductance measurements. The photoconductance setup is integrated in the camera-based PL setup and therefore identical measurement conditions are realised. We demonstrate the validity of this method by comparison with microwave-detected photoconductance decay measurements. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
1394236
Electronic properties of iron-boron pairs in crystalline silicon by temperature- and injection-level-dependent lifetime measurements
Iron-boron pairs in crystalline silicon are studied by measuring the recombination lifetime as a function of injection density, doping concentration, and temperature. The characteristic crossover point of the injection-level-dependent carrier lifetime curves measured before and after optical dissociation of the iron-boron pairs is analyzed to determine the energy level as well as the electronand hole-capture cross sections of the acceptor level of iron-boron pairs, assuming known recombination parameters for interstitial iron. The doping concentration dependence of the crossover point gives an electron-capture cross section of s1.4± 0.2 d 3 10 ˛14 cm 2 , while the temperature dependence results in a hole-capture cross section in the range from 0.5 3 10 ˛15 to 2.5 3 10 ˛15 cm 2 and an energy level of s0.26± 0.02 d eV below the conduction-band edge. © 2005 American Institute of Physics . fDOI: 10.1063/1.1897489g
94551115
Temperature dependence of the radiative recombination coefficient in silicon
The temperature dependence of the intrinsic radiative recombination coefficient B in silicon was measured from 100 to 400 °K. In contrast to previous calculations, B decreases with temperature. For interpretation of the measurements, a unique theory of the indirect radiative band-to-band and free exciton recombination is given. The decrease of B with increasing T is due to the decrease to the exciton concentration and of the Coulomb-enhancement of the band-to-band recombination rate. The latter however is smaller than the exciton recombination part even at room temperature. The constants of the exciton and band-to-band recombination are determined.
98129516
A study of oxygen precipitation in silicon using high-resolution transmission electron microscopy, small-angle neutron scattering and infrared absorption
Abstract Czochralski silicon samples have been heated at 750°C for periods of 48,96 and 431 h under clean conditions appropriate to device fabrication. The loss of oxygen from solution was measured from the strength of the 9 pm absorption band with the samples at 4·2 K; detailed information about the SiO2 precipitates was obtained by measurements on many foils using transmission electron microscopy, and by small-angle neutron scattering. The three techniques were applied to common samples. The measurements show excellent overall self-consistency and some new features have been found in the transmission electron microscopy analysis. In general, platelets are observed with length increasing from 12 to 26 nm, and with thicknesses of about 2 nm. The number density of particles drops from 2 × 1013 to 7 × 1012 cm−3 as the heating time is increased. We deduce a value of the oxide density to be 2·6 g cm−3, which is somewhat higher than the normally quoted value of 2·3 g cm−3 for amorphous silica.
121624346
Internal gettering of iron in multicrystalline silicon at low temperature
The interstitial iron concentration in multicrystalline silicon wafers, determined from recombination lifetime measurements, is effectively reduced by annealing the wafers at very low temperature (300–500 °C). During annealing, the iron concentration decreases by more than one order of magnitude. The observed disappearance of interstitial iron is explained by internal gettering of the iron by crystallographic defects.
31977692
Precipitation of iron in multicrystalline silicon during annealing
In this paper, the precipitation kinetics of iron in multicrystalline silicon during moderate temperature annealing are systematically studied with respect to annealing time, temperature, iron super-saturation level, and different types and densities of precipitation sites. The quantitative analysis is based on examining the changes in the concentrations and distributions of interstitial iron in multicrystalline silicon wafers after annealing at 400–700 °C. This is achieved by using the photoluminescence imaging technique to produce high-resolution spatially resolved images of the interstitial iron concentrations. The concentrations of interstitial iron are found to decrease exponentially with the annealing time. Comparison of the precipitation time constants of wafers annealed at different temperatures and of different initial interstitial iron concentrations indicates that higher levels of iron super-saturation result in faster precipitation processes. The impact of iron super-saturation on the precipit...