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Effects of low-temperature annealing on polycrystalline silicon for solar cells
Solar Energy Materials & Solar Cells 95 (2011) 559–563
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Effects of low-temperature annealing on polycrystalline silicon for solar cells Robert Slunjski a, Ivana Capan a, Branko Pivac a,n, Alessia Le Donne b, Simona Binetti b a b
Rudjer Boskovic Institute, Bijenicka 54, HR-10000 Zagreb, Croatia CNISM and Milano-Bicocca Solar Energy Research Center (MIB-SOLAR), Department of Material Science, University of Milano-Bicocca, via Cozzi 53, 20125 Milano, Italy
a r t i c l e in fo
abstract
Article history: Received 14 June 2010 Received in revised form 14 September 2010 Accepted 16 September 2010 Available online 8 October 2010
The polycrystalline silicon material grown by the edge-defined film-fed growth technique, and often used in solar cell production, is known to be carbon and dislocation rich. Aim of this work was to explore the effect of low-temperature annealing in vacuum on properties of these structural defects, often present in different solar-grade materials. Electrical measurements by deep level transient spectroscopy revealed the presence of the defects typically found in dislocated silicon. Detailed analysis further suggested that they are also carbon related, exhibiting quite unexpected behavior at such lowtemperature annealing. Moreover, photoluminescence results showed electron–hole droplet condensation at dislocations after such low-temperature annealing. This further supports the hypothesis that point defects are incorporated at dislocation cores rather than in a cloud at its proximity. & 2010 Elsevier B.V. All rights reserved.
Keywords: Silicon Defects Dislocations DLTS Photoluminescence Solar cells
1. Introduction Edge-defined film-fed growth (EFG) polycrystalline Si has been extensively investigated as a low-cost solar-cell material since the early 1970s [1,2]. This material offers an advantage for low-cost solar cells production due to the capillary action shaping growth method. Such a technique eliminates wafering step in the production by growing films 200–500 mm thick, several meters long and in the form of cylinder [3]. The specific puller design and introduction of non-standard gasses (CO or CO2) close to the meniscus during growth cause many effects on impurity distribution in the bulk [1,4]. Among the others, in situ gettering effect occurs [5], cleaning therefore the growing material in the course of production from detrimental metallic impurities. However, at the same time, ambient gasses and graphite crucibles and dies introduce a huge amount of carbon that is homogeneously distributed over the bulk on the microscopic scale [6] and remains in high supersaturation up to very high annealing temperatures [7]. Both carbon and oxygen impurities, intentionally or unintentionally introduced into the bulk, interact with extended structural defects such as dislocations and/or grain boundaries affecting its electrical features [8]. Nevertheless, the EFG production and processing related advantages and their continuous upgrading led to its successful implementation in solar cell production as described in Refs. [3,9]. Furthermore, one of the latest upgrading provided solar cells with very high
n
Corresponding author. E-mail address: [email protected] (B. Pivac).
0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.09.016
efficiency ( 18%) [10], demonstrating therefore the importance of defect engineering for this and similar class of materials. In order to further optimize the solar cell production steps it is important to understand the properties of dislocation-related deep levels as well as possible interaction effects of the other impurities with dislocations. In spite of many years of investigations, little is known about the influence of impurities on the electrical activity of dislocations. Nevertheless, it is apparent that dislocations have no simple and regular influence on the material electrical properties or on the device ones. Their electrical activity arises from intrinsic structural disorder at dislocation cores [11], shallow defect bands due to long-range strain fields [12] and extrinsic impurity decoration [13]. Indeed, there is a strong evidence that the presence of decorating impurities have a large effect on extended defects in Si [14]. In oxygen-rich Si crystals (i.e. Czochralski grown), it is widely demonstrated that oxygen aggregation at dislocations drastically increases their influence on carriers diffusion length and lifetime [15]. Besides oxygen, carbon is often present in the bulk, particularly in solar cell materials. In the present case, carbon is highly supersaturated, as typically occurs for this kind of material. Nevertheless, to our best knowledge, there are few reports on possible carbon influence on dislocations electrical activity [8]. Therefore, the first aim of this work is to study the properties of dislocations in highly carbon doped polycrystalline Si material, this topic being highly relevant for solar-grade Si-based solar cell production. Several approaches were used to improve the quality of solargrade multicrystalline Si. One of them, i.e. annealing at high temperatures ( 41170 1C), allows dislocation movement, leading
560
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to dislocation annihilation within minutes [16]. Moreover, it has been also recently shown that low-temperature annealing processes leads to an increase in Ld and the efficiency of solar cells [17]. Therefore, as a second aim of this work, we explored the effect of low-temperature annealing on the EFG material properties. Such material being highly inhomogeneous makes difficult to track changes introduced on microscopic scale by thermal treatments. To avoid this problem, we followed the effects of annealing on the same Schottky diode, by I–V and DLTS measurements, up to the point of its destruction and compared it with photoluminescence (PL) analysis. PL spectroscopy was in fact shown to be a valuable tool for the determination of the material quality and for the identification of defects affecting the lifetime of minority carriers [18].
2. Experiment EFG silicon sheets, cut from octagon cylinders, were grown at ASE Americas Inc. The samples were boron doped with resistivity 2–4 O cm, with high carbon and very low oxygen concentrations of 8 1017 and 3 1016 atoms/cm3, respectively. Their thickness was about 200 mm. A special care was taken during the polycrystalline tube growth, i.e. samples were grown from an electronic grade feedstock material and very clean dies were applied rendering therefore polycrystalline samples with metallic impurities below the detection limit of atomic absorbance spectroscopy. Before the diode formation, approximately 20 mm of the layer close to the surface was removed by planar etching to avoid the effects of subsurface defects [7,8]. The Schottky barriers were subsequently formed by thermal evaporation of aluminum, while ohmic contacts were formed by thermal evaporation of gold. The diodes were characterized by dark I–V plots at room temperature to evaluate the diode integrity upon thermal treatments. C–V measurements were carried out under dark conditions and 1 MHz test signal at room temperature to obtain an effective free carrier concentration. All I–V, C–V plots and DLTS spectra were taken with a SULA Technologies spectrometer. The DLTS measurements were performed at temperatures between 77 and 300 K. Eight different rate windows were used in order to determine the defect DLTS signature. The sample was analyzed by photoluminescence (PL) spectroscopy both before any thermal treatment and after annealing at temperatures higher that 200 1C. PL measurements at 14 K were performed with different spectral resolutions (6.6, 3 or 2 nm), using a standard lock-in technique in conjunction with a single grating monochromator and an InGaAs detector. For the excitation, a quantum well laser (lexc ¼805 nm) with a power density around 20 W/cm2 was used. After the electrical and PL characterization carried out before any thermal treatment, samples with electrical contacts were annealed in vacuum higher than 10 6 Pa in 50 1C steps from RT to 250 1C. After each step the integrity of diodes was checked by I–V and C–V measurements.
3. Results and discussion 3.1. DLTS analysis Fig. 1 shows the typical I–V curve taken on the as-received diode and on the same diode upon thermal treatments at different temperatures in vacuum. Fig. 1 clearly shows that the diode integrity was preserved up to the highest temperature here explored, i.e. even for the highest annealing temperature and
Fig. 1. Typical I–V curves of as-received (squares) dislocated carbon-rich polycrystalline EFG silicon sample, and the same sample annealed at 150 1C (triangles) and 225 1C (circles) in vacuum for 1 h. The inset shows C–V curve from as-received EFG sample.
Fig. 2. DLTS spectra of dislocated carbon-rich polycrystalline EFG silicon samples. The inset shows the Arrhenius plot for hole traps from EFG samples. Reverse bias Ur ¼ 3 V; filling pulse height Up ¼2.5 V; filling pulse width tp ¼ 10 ms; emission rate eh ¼ 12.5 s 1.
reverse biases up to 5 V, the leakage current was acceptable, assuring therefore reliable DLTS measurements. At the same time, annealing improved the series resistance, which was high at the beginning, as the contacts, particularly ohmic, were deposited under non-optimized conditions to allow the study of material properties under low thermal treatment conditions. The inset in the figure shows a C–V curve measured on the as-received sample, from which the doping concentration was calculated. Fig. 2 shows a typical DLTS spectrum taken on as-received polycrystalline EFG sample. The deep level of the hole trap shown in the figure is found at about 0.46 eV above the valence-band edge (Ev). The activation energy was determined from the Arrhenius plot of the hole emission rates, as shown in the inset of Fig. 2. This level, which is attributed to dislocations (as it will be discussed further on), is found in the range between 0.3 and 0.5 eV in different EFG samples and dislocated single crystal Si [8,19]. As amply discussed in Ref. [8], it shows all attributes of dislocation-related defect, i.e. having the full-width at halfmaximum more than 0.1 Tmax, as expected for point defects, and exhibiting a shift to lower energies and a decrease in peak height with the rate window. Finally, the behavior of carrier capture kinetics as a function of fill-pulse duration time exhibits a logarithmical dependence [8,20,21]. Using the approximations
R. Slunjski et al. / Solar Energy Materials & Solar Cells 95 (2011) 559–563
561
described in Refs. [8,20], for tp 4 t the density of filled traps is nT ðtp Þ ¼ s/uth SntNT expðqf0 =kTÞlogðtp =tÞ
ð1Þ
where
t ¼ ðkT=qf0 ÞðnT0 =NT Þð1=s/un SnÞ
ð2Þ
and f0 and nT0 are the equilibrium values of the barrier height and occupation density. Fig. 3 shows nT as a function of filling pulse duration, varied from 0.05 ms to 50 ms. The observed trapping behavior is rather complicated. The nT increases continuously with the filling pulse, then apparently saturates until 5 10 4 s and then again increases until 0.05 s. A least-squares fit of the DLTS data for the first and second part of the curve in Fig. 3 indicates that t is 17.1 and 3.1, respectively. Although, a clear trap saturation is not observed in Fig. 3, we have estimated nT for tp 50 ms at around 5 1014 cm 3. Combining the obtained values of t and the saturation value trap density, an effective capture cross-section can be determined. Therefore, for the second part seff ¼2.1 10 18 cm2. For the first part, the condition tp 4 t was not satisfied and therefore this cross-section cannot be determined. The effective cross-section of the second part is very close to that of ‘‘clean’’ dislocations in FZ material, while the other part is quite different. Since the main aim of this work was to investigate the effect of annealing on the properties of poly-Si material, we annealed samples with diode structures in vacuum higher than 10 6 Pa for 1 h to perform measurements after each step exactly on the same spot. After each annealing step the diodes integrity was checked with I–V and C–V measurements. Fig. 4 shows the results of DLTS signal variation with the annealing temperature. After thermal treatment at 100 1C for 1 h the signal increases more than seven times and is still high after annealing at 150 1C. Conversely, for higher temperatures (at about 200 1C) it completely vanishes. Such a large increase in DLTS
Fig. 5. Typical PL spectrum of EFG as-received sample (T¼ 14 K, PD ¼20 W/cm2, Dl ¼ 6.6 nm).
signal after annealing at so low temperatures was not recorded before in EFG poly-Si samples. As discussed in detail in [8] this DLTS signal is due to carbonrelated defects formed in the proximity to larger structural defects such as dislocations. Since annealing at low temperatures, such as 100–150 1C, is not able to produce significant mass transport of defects across the sample, we believe that this signal is due to local defect reconfiguration and activation. To further explore the defects responsible for enhanced signal after such annealing around 100 1C, we checked the effect of filling pulse duration on the signal. Namely, it has been shown by ¨ Schroter and Cerva [22] that influence of filling pulse duration on the peak position of DLTS signal allows to distinguish between levels (localized states) and band-like states. In the case of levels there is a huge range of filling pulses, for which the position of the line maximum stays almost constant. On the other hand for bandlike states, a range of filling pulses for which the position of the line maximum shifts toward lower temperature with an increase in tp exists. Using this criterion we found that for the broad range of filling pulses, namely from 0.1 to 100 ms, the line maximum stays at the same position suggesting that the effect of annealing created active point defects/impurities. Moreover, following the arguments given by Knobloch et al. [23], a rather narrow line of our DLTS spectrum suggests that we have the case of point defects/impurities accommodated at dislocation cores rather than in the cloud.
3.2. PL analysis
Fig. 3. Concentration of filled deep levels nT as a function of the filling pulse time tp for FZ and EFG samples.
Fig. 4. DLTS signal variation as a function of annealing temperature.
Fig. 5 shows the PL spectra collected in different positions of EFG as-received sample. These spectra show the presence of the free exciton (FE) emission around 1.1 eV, a broad band around 0.8 eV and the typical dislocation-related signals: D3 line around 0.94 eV and D4 line around 0.99 eV. The narrow dislocationrelated bands labeled D1 (0.807 eV) and D2 (0.847 eV) as observed by Sauer et al. [24] are not resolvable due to the overlap with a broad impurity related band (D1p) around 0.8 eV, which is about 60 meV wide and stays clearly visible up to room temperature [25]. Koshka et al. [25] have shown that the defect band is observed both in as-grown material and after processing including final solar cell stage. Moreover, they also claimed that the FE emission increases significantly, up to two orders of magnitude during cell processing, which they attributed to the gettering process for heavy metals and the hydrogenation effect. After annealing at temperature higher than 200 1C, the sample PL response is well represented by the spectrum shown in Fig. 6.
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Fig. 6. PL spectrum of EFG sample after annealing at 200 1C (T¼14 K, PD¼ 20 W/cm2, Dl ¼ 6.6 nm).
Fig. 8. High-resolution PL spectra of EFG sample after annealing at 200 1C (T ¼14 K, Dl ¼ 2 nm) with different laser power densities (black solid line PD ¼20 W/cm2, grey solid line PD ¼ 2 W/cm2).
4. Conclusions
Fig. 7. High-resolution PL spectrum of EFG sample after annealing at 200 1C (T¼14 K, PD ¼ 20 W/cm2, Dl ¼2 nm).
Fig. 6 shows that the intensity of both band around 0.8 eV and dislocation-related signals strongly decreased, while a line around 1.084 eV is present beside the FE band. As reported in the literature [26,27], electron hole droplets (EHD), or bound multiple-exciton complexes, in defect-free Si give rise to luminescence around 1.086 eV. In a Si crystal containing a significant number of dislocations, the processes of EHD formation and migration display different features from those for a dislocation-free crystal. Low-temperature PL study of Si with dislocations [27] showed that dislocations are centers of EHD condensation. Therefore, the radiative spectrum of EHD in Si with dislocations contains both the line for dislocation-free Si with a maximum around 1.086 eV and an additional line with a maximum around 1.078 eV, which is due to the appearance of specific EHD, extended or spread along the dislocation. This spreading is associated with the fields of elastic stresses around dislocations, producing the additional possibility for cylindrical EHD nucleation near the dislocation core [27]. The intensities of the EHD bands decrease sharply with increase in competitive channels of exciton capture and recombination and, therefore, with increase in density of both dislocations and defects or impurities [27]. In the case of the present EFG sample, the band around 1.086 eV observed in Fig. 6 can be assigned to EHD recombinations. In fact, as can be observed in the high-resolution spectrum shown in Fig. 7, the peak at 1.086 eV clearly shows a shoulder at 1.078 eV. The presence of the EHD signal related to dislocations is well pointed out in the high-resolution spectra carried out with different laser power densities and shown in Fig. 8.
The behavior of carbon-rich EFG polycrystalline silicon after low-temperature thermal treatments was explored. Electrical measurements by DLTS revealed the presence of defects that were attributed to carbon-related point-like defects in proximity to dislocations. The activity of such defects is significantly enhanced by the thermal treatment at about 100 1C, while is completely suppressed by further annealing at about 200 1C. Such low-temperature treatments cannot involve significant mass transport of defects across the sample, but rather a local structural reconfiguration. The analysis of DTLS spectra provides two possible explanations of the effect of annealing at 100 1C. The former is that the annealing related local reconfiguration creates a cloud of active point defects/impurities in proximity to dislocations. The latter is that point defects/impurities are accommodated at dislocation cores rather than in a cloud in proximity to dislocations. Further local reconfiguration should occur after annealing at about 200 1C giving rise to deactivation of these carbon-related point-like defects. PL results seem to confirm the hypothesis that point defects/ impurities are accommodated at dislocation cores rather than in a cloud in proximity to dislocations. In fact, while untreated sample shows mainly dislocation PL fingerprints (i.e. D bands), after annealing at 200 1C, typical PL signals related to EHD in silicon with dislocations were observed beside D lines. In particular, the EHD line around 1.078 eV is due to cylindrical EHD nucleation near the dislocation core. The presence of this PL line suggests that the local reconfiguration due to the annealing at 200 1C reduces the competitive channels of exciton capture and recombination related to point-like defects near the dislocation cores. References [1] J.P. Kalejs, Impurity redistribution in EFG, J Cryst. Growth 44 (1978) 329–344; J.P. Kalejs, L.-Y. Chin, Modeling of ambient-meniscus melt interactions associated with carbon and oxygen transport in EFG of silicon ribbon, J Electrochem. Soc. 129 (1982) 1356–1361. [2] B. Pivac, U.V. Desnica, Oxygen- and carbon-related defects in edge-defined film-fed growth silicon ribbon, J. Appl. Phys. 64 (1988) 2208–2210. [3] F.V. Wald, EFG crystal growth technology for low cost terrestrial photovoltaics: review and outlook, Sol. Energy Mater. 23 (1991) 175–182. [4] J.P. Kalejs, L.-Y. Chin, F.M. Carlson, Interface shape studies for silicon ribbon growth by the EFG technique I. Transport phenomena modeling, J Cryst. Growth 61 (1983) 473–484; B. Pivac, A. Borghesi, A. Sassella, L. Ottolini, J. Kalejs, Interaction of ambient gas and meniscus surface during growth of egde-defined film-fed growth polycrystalline silicon samples, J Appl. Phys. 70 (1991) 2963–2967;
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Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Effects of low-temperature annealing on polycrystalline silicon for solar cells Robert Slunjski a, Ivana Capan a, Branko Pivac a,n, Alessia Le Donne b, Simona Binetti b a b
Rudjer Boskovic Institute, Bijenicka 54, HR-10000 Zagreb, Croatia CNISM and Milano-Bicocca Solar Energy Research Center (MIB-SOLAR), Department of Material Science, University of Milano-Bicocca, via Cozzi 53, 20125 Milano, Italy
a r t i c l e in fo
abstract
Article history: Received 14 June 2010 Received in revised form 14 September 2010 Accepted 16 September 2010 Available online 8 October 2010
The polycrystalline silicon material grown by the edge-defined film-fed growth technique, and often used in solar cell production, is known to be carbon and dislocation rich. Aim of this work was to explore the effect of low-temperature annealing in vacuum on properties of these structural defects, often present in different solar-grade materials. Electrical measurements by deep level transient spectroscopy revealed the presence of the defects typically found in dislocated silicon. Detailed analysis further suggested that they are also carbon related, exhibiting quite unexpected behavior at such lowtemperature annealing. Moreover, photoluminescence results showed electron–hole droplet condensation at dislocations after such low-temperature annealing. This further supports the hypothesis that point defects are incorporated at dislocation cores rather than in a cloud at its proximity. & 2010 Elsevier B.V. All rights reserved.
Keywords: Silicon Defects Dislocations DLTS Photoluminescence Solar cells
1. Introduction Edge-defined film-fed growth (EFG) polycrystalline Si has been extensively investigated as a low-cost solar-cell material since the early 1970s [1,2]. This material offers an advantage for low-cost solar cells production due to the capillary action shaping growth method. Such a technique eliminates wafering step in the production by growing films 200–500 mm thick, several meters long and in the form of cylinder [3]. The specific puller design and introduction of non-standard gasses (CO or CO2) close to the meniscus during growth cause many effects on impurity distribution in the bulk [1,4]. Among the others, in situ gettering effect occurs [5], cleaning therefore the growing material in the course of production from detrimental metallic impurities. However, at the same time, ambient gasses and graphite crucibles and dies introduce a huge amount of carbon that is homogeneously distributed over the bulk on the microscopic scale [6] and remains in high supersaturation up to very high annealing temperatures [7]. Both carbon and oxygen impurities, intentionally or unintentionally introduced into the bulk, interact with extended structural defects such as dislocations and/or grain boundaries affecting its electrical features [8]. Nevertheless, the EFG production and processing related advantages and their continuous upgrading led to its successful implementation in solar cell production as described in Refs. [3,9]. Furthermore, one of the latest upgrading provided solar cells with very high
n
Corresponding author. E-mail address: [email protected] (B. Pivac).
0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.09.016
efficiency ( 18%) [10], demonstrating therefore the importance of defect engineering for this and similar class of materials. In order to further optimize the solar cell production steps it is important to understand the properties of dislocation-related deep levels as well as possible interaction effects of the other impurities with dislocations. In spite of many years of investigations, little is known about the influence of impurities on the electrical activity of dislocations. Nevertheless, it is apparent that dislocations have no simple and regular influence on the material electrical properties or on the device ones. Their electrical activity arises from intrinsic structural disorder at dislocation cores [11], shallow defect bands due to long-range strain fields [12] and extrinsic impurity decoration [13]. Indeed, there is a strong evidence that the presence of decorating impurities have a large effect on extended defects in Si [14]. In oxygen-rich Si crystals (i.e. Czochralski grown), it is widely demonstrated that oxygen aggregation at dislocations drastically increases their influence on carriers diffusion length and lifetime [15]. Besides oxygen, carbon is often present in the bulk, particularly in solar cell materials. In the present case, carbon is highly supersaturated, as typically occurs for this kind of material. Nevertheless, to our best knowledge, there are few reports on possible carbon influence on dislocations electrical activity [8]. Therefore, the first aim of this work is to study the properties of dislocations in highly carbon doped polycrystalline Si material, this topic being highly relevant for solar-grade Si-based solar cell production. Several approaches were used to improve the quality of solargrade multicrystalline Si. One of them, i.e. annealing at high temperatures ( 41170 1C), allows dislocation movement, leading
560
R. Slunjski et al. / Solar Energy Materials & Solar Cells 95 (2011) 559–563
to dislocation annihilation within minutes [16]. Moreover, it has been also recently shown that low-temperature annealing processes leads to an increase in Ld and the efficiency of solar cells [17]. Therefore, as a second aim of this work, we explored the effect of low-temperature annealing on the EFG material properties. Such material being highly inhomogeneous makes difficult to track changes introduced on microscopic scale by thermal treatments. To avoid this problem, we followed the effects of annealing on the same Schottky diode, by I–V and DLTS measurements, up to the point of its destruction and compared it with photoluminescence (PL) analysis. PL spectroscopy was in fact shown to be a valuable tool for the determination of the material quality and for the identification of defects affecting the lifetime of minority carriers [18].
2. Experiment EFG silicon sheets, cut from octagon cylinders, were grown at ASE Americas Inc. The samples were boron doped with resistivity 2–4 O cm, with high carbon and very low oxygen concentrations of 8 1017 and 3 1016 atoms/cm3, respectively. Their thickness was about 200 mm. A special care was taken during the polycrystalline tube growth, i.e. samples were grown from an electronic grade feedstock material and very clean dies were applied rendering therefore polycrystalline samples with metallic impurities below the detection limit of atomic absorbance spectroscopy. Before the diode formation, approximately 20 mm of the layer close to the surface was removed by planar etching to avoid the effects of subsurface defects [7,8]. The Schottky barriers were subsequently formed by thermal evaporation of aluminum, while ohmic contacts were formed by thermal evaporation of gold. The diodes were characterized by dark I–V plots at room temperature to evaluate the diode integrity upon thermal treatments. C–V measurements were carried out under dark conditions and 1 MHz test signal at room temperature to obtain an effective free carrier concentration. All I–V, C–V plots and DLTS spectra were taken with a SULA Technologies spectrometer. The DLTS measurements were performed at temperatures between 77 and 300 K. Eight different rate windows were used in order to determine the defect DLTS signature. The sample was analyzed by photoluminescence (PL) spectroscopy both before any thermal treatment and after annealing at temperatures higher that 200 1C. PL measurements at 14 K were performed with different spectral resolutions (6.6, 3 or 2 nm), using a standard lock-in technique in conjunction with a single grating monochromator and an InGaAs detector. For the excitation, a quantum well laser (lexc ¼805 nm) with a power density around 20 W/cm2 was used. After the electrical and PL characterization carried out before any thermal treatment, samples with electrical contacts were annealed in vacuum higher than 10 6 Pa in 50 1C steps from RT to 250 1C. After each step the integrity of diodes was checked by I–V and C–V measurements.
3. Results and discussion 3.1. DLTS analysis Fig. 1 shows the typical I–V curve taken on the as-received diode and on the same diode upon thermal treatments at different temperatures in vacuum. Fig. 1 clearly shows that the diode integrity was preserved up to the highest temperature here explored, i.e. even for the highest annealing temperature and
Fig. 1. Typical I–V curves of as-received (squares) dislocated carbon-rich polycrystalline EFG silicon sample, and the same sample annealed at 150 1C (triangles) and 225 1C (circles) in vacuum for 1 h. The inset shows C–V curve from as-received EFG sample.
Fig. 2. DLTS spectra of dislocated carbon-rich polycrystalline EFG silicon samples. The inset shows the Arrhenius plot for hole traps from EFG samples. Reverse bias Ur ¼ 3 V; filling pulse height Up ¼2.5 V; filling pulse width tp ¼ 10 ms; emission rate eh ¼ 12.5 s 1.
reverse biases up to 5 V, the leakage current was acceptable, assuring therefore reliable DLTS measurements. At the same time, annealing improved the series resistance, which was high at the beginning, as the contacts, particularly ohmic, were deposited under non-optimized conditions to allow the study of material properties under low thermal treatment conditions. The inset in the figure shows a C–V curve measured on the as-received sample, from which the doping concentration was calculated. Fig. 2 shows a typical DLTS spectrum taken on as-received polycrystalline EFG sample. The deep level of the hole trap shown in the figure is found at about 0.46 eV above the valence-band edge (Ev). The activation energy was determined from the Arrhenius plot of the hole emission rates, as shown in the inset of Fig. 2. This level, which is attributed to dislocations (as it will be discussed further on), is found in the range between 0.3 and 0.5 eV in different EFG samples and dislocated single crystal Si [8,19]. As amply discussed in Ref. [8], it shows all attributes of dislocation-related defect, i.e. having the full-width at halfmaximum more than 0.1 Tmax, as expected for point defects, and exhibiting a shift to lower energies and a decrease in peak height with the rate window. Finally, the behavior of carrier capture kinetics as a function of fill-pulse duration time exhibits a logarithmical dependence [8,20,21]. Using the approximations
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described in Refs. [8,20], for tp 4 t the density of filled traps is nT ðtp Þ ¼ s/uth SntNT expðqf0 =kTÞlogðtp =tÞ
ð1Þ
where
t ¼ ðkT=qf0 ÞðnT0 =NT Þð1=s/un SnÞ
ð2Þ
and f0 and nT0 are the equilibrium values of the barrier height and occupation density. Fig. 3 shows nT as a function of filling pulse duration, varied from 0.05 ms to 50 ms. The observed trapping behavior is rather complicated. The nT increases continuously with the filling pulse, then apparently saturates until 5 10 4 s and then again increases until 0.05 s. A least-squares fit of the DLTS data for the first and second part of the curve in Fig. 3 indicates that t is 17.1 and 3.1, respectively. Although, a clear trap saturation is not observed in Fig. 3, we have estimated nT for tp 50 ms at around 5 1014 cm 3. Combining the obtained values of t and the saturation value trap density, an effective capture cross-section can be determined. Therefore, for the second part seff ¼2.1 10 18 cm2. For the first part, the condition tp 4 t was not satisfied and therefore this cross-section cannot be determined. The effective cross-section of the second part is very close to that of ‘‘clean’’ dislocations in FZ material, while the other part is quite different. Since the main aim of this work was to investigate the effect of annealing on the properties of poly-Si material, we annealed samples with diode structures in vacuum higher than 10 6 Pa for 1 h to perform measurements after each step exactly on the same spot. After each annealing step the diodes integrity was checked with I–V and C–V measurements. Fig. 4 shows the results of DLTS signal variation with the annealing temperature. After thermal treatment at 100 1C for 1 h the signal increases more than seven times and is still high after annealing at 150 1C. Conversely, for higher temperatures (at about 200 1C) it completely vanishes. Such a large increase in DLTS
Fig. 5. Typical PL spectrum of EFG as-received sample (T¼ 14 K, PD ¼20 W/cm2, Dl ¼ 6.6 nm).
signal after annealing at so low temperatures was not recorded before in EFG poly-Si samples. As discussed in detail in [8] this DLTS signal is due to carbonrelated defects formed in the proximity to larger structural defects such as dislocations. Since annealing at low temperatures, such as 100–150 1C, is not able to produce significant mass transport of defects across the sample, we believe that this signal is due to local defect reconfiguration and activation. To further explore the defects responsible for enhanced signal after such annealing around 100 1C, we checked the effect of filling pulse duration on the signal. Namely, it has been shown by ¨ Schroter and Cerva [22] that influence of filling pulse duration on the peak position of DLTS signal allows to distinguish between levels (localized states) and band-like states. In the case of levels there is a huge range of filling pulses, for which the position of the line maximum stays almost constant. On the other hand for bandlike states, a range of filling pulses for which the position of the line maximum shifts toward lower temperature with an increase in tp exists. Using this criterion we found that for the broad range of filling pulses, namely from 0.1 to 100 ms, the line maximum stays at the same position suggesting that the effect of annealing created active point defects/impurities. Moreover, following the arguments given by Knobloch et al. [23], a rather narrow line of our DLTS spectrum suggests that we have the case of point defects/impurities accommodated at dislocation cores rather than in the cloud.
3.2. PL analysis
Fig. 3. Concentration of filled deep levels nT as a function of the filling pulse time tp for FZ and EFG samples.
Fig. 4. DLTS signal variation as a function of annealing temperature.
Fig. 5 shows the PL spectra collected in different positions of EFG as-received sample. These spectra show the presence of the free exciton (FE) emission around 1.1 eV, a broad band around 0.8 eV and the typical dislocation-related signals: D3 line around 0.94 eV and D4 line around 0.99 eV. The narrow dislocationrelated bands labeled D1 (0.807 eV) and D2 (0.847 eV) as observed by Sauer et al. [24] are not resolvable due to the overlap with a broad impurity related band (D1p) around 0.8 eV, which is about 60 meV wide and stays clearly visible up to room temperature [25]. Koshka et al. [25] have shown that the defect band is observed both in as-grown material and after processing including final solar cell stage. Moreover, they also claimed that the FE emission increases significantly, up to two orders of magnitude during cell processing, which they attributed to the gettering process for heavy metals and the hydrogenation effect. After annealing at temperature higher than 200 1C, the sample PL response is well represented by the spectrum shown in Fig. 6.
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Fig. 6. PL spectrum of EFG sample after annealing at 200 1C (T¼14 K, PD¼ 20 W/cm2, Dl ¼ 6.6 nm).
Fig. 8. High-resolution PL spectra of EFG sample after annealing at 200 1C (T ¼14 K, Dl ¼ 2 nm) with different laser power densities (black solid line PD ¼20 W/cm2, grey solid line PD ¼ 2 W/cm2).
4. Conclusions
Fig. 7. High-resolution PL spectrum of EFG sample after annealing at 200 1C (T¼14 K, PD ¼ 20 W/cm2, Dl ¼2 nm).
Fig. 6 shows that the intensity of both band around 0.8 eV and dislocation-related signals strongly decreased, while a line around 1.084 eV is present beside the FE band. As reported in the literature [26,27], electron hole droplets (EHD), or bound multiple-exciton complexes, in defect-free Si give rise to luminescence around 1.086 eV. In a Si crystal containing a significant number of dislocations, the processes of EHD formation and migration display different features from those for a dislocation-free crystal. Low-temperature PL study of Si with dislocations [27] showed that dislocations are centers of EHD condensation. Therefore, the radiative spectrum of EHD in Si with dislocations contains both the line for dislocation-free Si with a maximum around 1.086 eV and an additional line with a maximum around 1.078 eV, which is due to the appearance of specific EHD, extended or spread along the dislocation. This spreading is associated with the fields of elastic stresses around dislocations, producing the additional possibility for cylindrical EHD nucleation near the dislocation core [27]. The intensities of the EHD bands decrease sharply with increase in competitive channels of exciton capture and recombination and, therefore, with increase in density of both dislocations and defects or impurities [27]. In the case of the present EFG sample, the band around 1.086 eV observed in Fig. 6 can be assigned to EHD recombinations. In fact, as can be observed in the high-resolution spectrum shown in Fig. 7, the peak at 1.086 eV clearly shows a shoulder at 1.078 eV. The presence of the EHD signal related to dislocations is well pointed out in the high-resolution spectra carried out with different laser power densities and shown in Fig. 8.
The behavior of carbon-rich EFG polycrystalline silicon after low-temperature thermal treatments was explored. Electrical measurements by DLTS revealed the presence of defects that were attributed to carbon-related point-like defects in proximity to dislocations. The activity of such defects is significantly enhanced by the thermal treatment at about 100 1C, while is completely suppressed by further annealing at about 200 1C. Such low-temperature treatments cannot involve significant mass transport of defects across the sample, but rather a local structural reconfiguration. The analysis of DTLS spectra provides two possible explanations of the effect of annealing at 100 1C. The former is that the annealing related local reconfiguration creates a cloud of active point defects/impurities in proximity to dislocations. The latter is that point defects/impurities are accommodated at dislocation cores rather than in a cloud in proximity to dislocations. Further local reconfiguration should occur after annealing at about 200 1C giving rise to deactivation of these carbon-related point-like defects. PL results seem to confirm the hypothesis that point defects/ impurities are accommodated at dislocation cores rather than in a cloud in proximity to dislocations. In fact, while untreated sample shows mainly dislocation PL fingerprints (i.e. D bands), after annealing at 200 1C, typical PL signals related to EHD in silicon with dislocations were observed beside D lines. In particular, the EHD line around 1.078 eV is due to cylindrical EHD nucleation near the dislocation core. The presence of this PL line suggests that the local reconfiguration due to the annealing at 200 1C reduces the competitive channels of exciton capture and recombination related to point-like defects near the dislocation cores. References [1] J.P. Kalejs, Impurity redistribution in EFG, J Cryst. Growth 44 (1978) 329–344; J.P. Kalejs, L.-Y. Chin, Modeling of ambient-meniscus melt interactions associated with carbon and oxygen transport in EFG of silicon ribbon, J Electrochem. Soc. 129 (1982) 1356–1361. [2] B. Pivac, U.V. Desnica, Oxygen- and carbon-related defects in edge-defined film-fed growth silicon ribbon, J. Appl. Phys. 64 (1988) 2208–2210. [3] F.V. Wald, EFG crystal growth technology for low cost terrestrial photovoltaics: review and outlook, Sol. Energy Mater. 23 (1991) 175–182. [4] J.P. Kalejs, L.-Y. Chin, F.M. Carlson, Interface shape studies for silicon ribbon growth by the EFG technique I. Transport phenomena modeling, J Cryst. Growth 61 (1983) 473–484; B. Pivac, A. Borghesi, A. Sassella, L. Ottolini, J. Kalejs, Interaction of ambient gas and meniscus surface during growth of egde-defined film-fed growth polycrystalline silicon samples, J Appl. Phys. 70 (1991) 2963–2967;
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