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Impacts of phosphorus and aluminum gettering with porous silicon damage for p-type Czochralski silicon used in solar cells technology
Thin Solid Films 511 – 512 (2006) 377 – 380 www.elsevier.com/locate/tsf
Impacts of phosphorus and aluminum gettering with porous silicon damage for p-type Czochralski silicon used in solar cells technology A. Ben Jaballah a,*, M. Hassen a, H. Rahmouni b, M. Hajji a, A. Selmi b, H. Ezzaouia a b
a Institut National de Recherche Scientifique et Technique, Laboratoire de Photovoltaı¨que et des Semiconducteurs, PB 95 2050 Hammam-Lif, Tunisia Laboratoire de Physique des Semiconducteurs et des Composants Electroniques Faculte´ des Sciences de Monastir, Rue de Kairouan 5000 Monastir, Tunisia
Available online 18 January 2006
Abstract In this paper, porous silicon damage (PSD) is introduced like a simple sequence for efficient extrinsic gettering schemes. The technique consists to create a sacrificial porous silicon layer on both sides of the silicon substrates with randomly hemispherical voids. Then, two main sample types are processed. In the first group, thin aluminium layers ( 1 Am) are thermally evaporated followed by photo-thermal annealing at 700 and 800 -C, under N2 atmosphere. In the second group, phosphorous is continually diffused during heating at one of several temperatures ranging between 750 and 950 - C for 1 h in a solid phase from POCl3 solution, in N2/O2 ambient. Hall Effect and Van Der Pauw methods prove the existence of an optimum temperature in the case of phosphorus gettering equal to 900 -C yielding a hall mobility of about 982 cm2 V 1 s 1. FTIR investigations show an increase of interstitial oxygen enhancing the precipitation phenomena and proving that extrinsic and intrinsic gettering are in work at the same time. However, in the case of aluminum gettering, there isn’t a gettering limit in the as mentioned temperature range. Moreover, silicon solar cells are processed to clarify this effect. D 2005 Elsevier B.V. All rights reserved. Keywords: Porous silicon; Al and P gettering; Hall mobility; Solar cells
1. Introduction It is well-known that B-doped Cz Si materials suffer lightinduced degradation due to boron-oxygen related defects [1] which is responsible of a reduction in bulk diffusion length and hence yields of electronic devices. In this context, Zhao et al [2] propose to use Float-zone silicon material gallium doped in order to overcome this problem. In addition, P and Al gettering [3,4] thermal treatments are explored as techniques to enhance Cz material qualities. External P and Al gettering with sacrificial porous silicon layer has been reported to substantially enhance the majority carrier mobility in monocrystalline silicon [5]. It is shown that Al and P gettering is able to recover mobility, reducing both Fingot growth-induced_ and Fprocessinduced_ impurities, and allowing also for a reduction of the defect concentration after the thermal step. In this paper, we summarize experimental results which outline the tentatives made to introduce PS damage as a
* Corresponding author. Tel.: +216 71 430 160; fax: +216 71 430 934. E-mail address: [email protected] (A. Ben Jaballah). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.11.101
preliminary step for extrinsic gettering in Czochralski silicon by rapid thermal diffusion of aluminum (Al) and phosphorus (P) using an infrared furnace. Hall Effect and Van Der Pauw techniques are used to determine the mobility of majority charge carriers. Moreover, dark current– voltage investigations of standard silicon solar cells are used to monitor the effects of these gettering treatments. Scanning electron microscope investigations are processed in the case of aluminum gettering to show the formation of the transition captured region. Based on the experimental results, we discuss the mechanisms involved by external gettering using a sacrificial porous silicon layer. Indeed, we aim to optimize the gettering temperatures and time in order to use the more convenient conditions for the conception of adequate Al – P co-gettering treatments. 2. Experimental In these experiments, we have used a single crystal Cz wafers, p type, boron doped, (100) oriented, and 330-Amthick with the resistivity ranging from 1 to 3 V cm. Porous silicon layer was formed on both sides of samples using
378
A. Ben Jaballah et al. / Thin Solid Films 511 – 512 (2006) 377 – 380
Table 1 The effect of porous silicon damage on majority charge mobility and carriers concentrations of silicon substrates treated under N2 ambient at 750 -C during 60 min Sample type Reference Without PS With PS
Resistivity (V .cm)
Mobility (cm2.V 1.s
1,53 1,43 1,27
89 358 582
1
)
Concentration (cm 3) 4,64 1016 1,21 1016 8,43 1015
stain-etching method which consisting of dipping wafers in aqueous (HF (40%)/HNO3 (65%)/H2O) solution with (1:3:5) volume composition. Each wafer was cut into 0.5 0.5 cm2 pieces to minimize the variation resulting from different wafers. These samples were divided on two groups. For the first group, a thick aluminum layer was evaporated onto the both sides of samples. Then, the wafers are annealed for 30 min in the temperature range of 600– 800 -C in dry N2 in an infrared furnace. For the second group phosphorus is continuously diffused from POCl3 sources in the solid phase. Then, the wafers are thermally treated for temperatures ranging between 800 -C and 950 -C for 1 h under N2/O2 ambient (gas vector). After annealing, the as formed Al –Si alloy and the phosphorus doped region are etched using a (HF (40%): HNO3 (65%)) acid solution with (3:1) volume composition. As a consequence, these approaches improve the efficiency of the Al and P gettering which are studied and compared using the Hall mobility measurements as well as the dark I –V characteristics of the solar cells. In the case of phosphorus, Fourier Transformation Infrared (FTIR) spectroscopy may help to evaluate qualitatively the interstitial oxygen concentrations. For silicon solar cells based on wafers treated with aluminium and phosphorus, a pieces cut into 2 2 cm2 are used. Phosphorus diffusion was carried out in a rapid thermal infrared tubular furnace. The metallic contacts are made by screen printing of Al/Ag pastes for the rear contacts and silver paste for the emitter contacts. The metallic contacts are separately annealed at temperatures of 650 -C and 700 during 10 min and 15 min
for rear and emitter contacts respectively, in an infrared furnace. 3. Results and discussions 3.1. Porous silicon damage The use of porous silicon as an intermediate layer to getter unwanted impurities has stimulated a great interest in epitaxial growth [6]. This has been found by the improvements of the transport parameters like the majority charge mobility (l d) and the carrier concentration determined by both Hall Effect measurements and resistivity calculated using Van Der Pauw method. Table 1 shows the enhancement of l d caused by means of porous silicon damage from 358 cm2 V 1 s 1 for silicon wafer without PS to 582 cm2 V 1 s 1 for sample with PS treated at 750 -C for 60 min under N2 atmosphere. This gettering effect is probably based on processes which induce segregation of impurities and clusters type defects at extended defects. This suggestion is mainly supported by the saturation of the gettering effect as the temperature and the treatment period increased [7]. These behaviours are explained by the reduction of vacancy concentration and dangling bonds due to the annealing of damages and chemical adsorption, respectively [8]. 3.2. Aluminum gettering Fig. 1 exhibits cross section views of silicon wafers treated by aluminum once and annealed at temperature equal to 900 -C for 30 min in a clean tubular furnace under N2 atmosphere. It shows the formation of a highly rear doped silicon layer which reinforce the effect of a back surface field. The top microstructure has been changed due to annealing. The rear aluminum is known to form a Shottky barrier to p-type silicon with a relatively low barrier height of 0.4 –0.6 eV, which allow the realization of a concentration gradient on the rear. Table 2 shows the enhancement of the Hall mobility of majority carriers as the temperature rise. However, the boron concen-
Fig. 1. Cross section views of Al/PS/Si structures: (a) as realized, (b) annealed at 900 -C for 30 min under N2 atmosphere, showing the change of the microstructure of porous silicon and the diffusion of aluminum to form a highly doped layer on the back of the sample.
A. Ben Jaballah et al. / Thin Solid Films 511 – 512 (2006) 377 – 380 Table 2 Enhancement of majority carrier mobility for silicon wafers treated with aluminum once and subsequently annealed under N2 atmosphere for 30 min Treatment
Resistivity (V .cm)
Mobility (cm2.V
Reference 700 -C 800 -C
1,53 1,28 1,32
89 512 573
1
.s
1
Untreated 800 -C 900 -C 950 -C
6,0x10
-3
4,0x10
-3
2,0x10
-3
-6
-4
-2
0 -2,0x10
-3
-4,0x10
-3
Mobility (cm2.V 1.s
1,53 1,24 1,31 1,34
89 644 982 514
Concentration (cm 3)
1
)
4,6 1016 7,8 1015 4,85 1015 9,07 1015
30 min in presence of a sacrificial PS layer. One can conclude from Tables 1 and 2 that these two treatments yield the same results but with less duration for the aluminum case under the assumption that no steady state gettering phenomenon may be occurred during treatments. 3.3. Phosphorus gettering In Table 3, the resistivity and the active boron concentration are presented in addition to the annealing temperatures, the treatment cycle time is kept equal to 60 min. It seems that the boron concentrations are more sensitive to treatment conditions than the resistivity. Moreover, the Hall mobility investigations prove the existence of an optimum treatment temperature. During the procedure, the two competing mechanisms of impurity gettering and thermal degradation are simultaneously at work. Another effect is the compensation phenomenon occurring between phosphorus in diffusion and boron atoms which dominates as the annealing temperature increased. The optimal temperature is about 900 -C, where the mobility is multiplied by ten. This optimum could be the result of competition between the release of impurities from the bulk and a capture of impurities in the gettering layer. Below the optimum temperature, gettering process is limited by the release or the diffusion of metallic impurities towards the gettering layer. In Fig. 3, infra-red spectra of treated samples are shown; the band relative to interstitial oxygen [9] appear at 1110 cm 1.
(b) 2,7
(a)
2,4
Voltage (V)
0,0 -8
Resistivity (V .cm)
The wafers are annealed under N2/O2 atmosphere during a cycle of 60 min.
Absorbance (a.u)
-3
Current (A)
8,0x10
Table 3 Phosphorus treatments induced improvements of majority charge mobility due reduction of defect centers
)
tration of majority charge carriers decreases as the temperature increase. These behaviors may indicate in a part the occurrence of gettering phenomena. In the present work the nature and the concentration of contaminants in the bulk are unknown, which seems to be a weak point to discuss the nature of undesirable impurities gettered away from the bulk. On the other hand, this can be viewed as a strong point in the sense that it corresponds to a real world device fabrication situation for which the starting silicon material contaminant nature and concentration are unknown. Owing to the low gettering temperature and short gettering time used, we believe that the gettered species are the fast moving interstitials metal atoms such as those of Fe, Ni, Cu, etc. Substitutionally dissolved metal atoms, like gold (Au) not have been significantly affected by the present gettering treatment. Indeed, the weak decrease of substitutional boron [Bs] concentration can stimulate a gettering effect because, the defect concentration centers in Cz silicon is founded [1] to be proportional to [Bs]. In Fig. 2, the dark I –V characteristics of silicon solar cells based on samples treated by the combination of porous silicon and aluminum are shown. It appears that the diode quality is improved: in the forward bias, the leakage current due to the escape of holes through deep recombination centers is less pronounced for sample treated at 900 -C in comparison of untreated one. This effect indicates the enhancement of the electronic properties of the silicon material subjected to the above gettering process. For instance, we try to give a comparison between two samples: the first treated only with PS layer and thermally annealed at 750 -C for 60 min and the second treated by aluminum and annealed at 800 -C for
379
2
2,1
1200
Fig. 2. Effect of aluminum gettering on leakage current densities of the dark I – V characteristics of silicon based solar cells: (a) untreated sample, (b) sample treated at 900 -C.
800°C 750°C 900°C 950°C untreated 1150
1100
1050
1000
Wave number (cm-1) Fig. 3. The evolution of the infrared absorption spectra of interstitial oxygen due to phosphorus treatments and subsequent annealing under N2/O2 atmosphere. All samples have been treated with a porous silicon layer and cleaned by CP4 chemical solution after infrared heating.
A. Ben Jaballah et al. / Thin Solid Films 511 – 512 (2006) 377 – 380 Current (A)
380
0,008
0,006
4. Conclusion (c) (d)
(e)
0,004
0,002
Voltage (V) 0,000 -10
-8
-6
-4
-2
0
2
-0,002
Fig. 4. The effect of phosphorus treatments on the saturation current density of three silicon solar cells for various annealing temperatures: (c) 900 -C, (d) 800 -C and (e) 950 -C; during 60 min.
One can notice that the band intensity tend to be lower for Si substrate treated at 900 -C. This appears to be sufficient to eliminate any previously formed nucleation centres that could enhance the precipitation of oxygen. Roughly, for the specimen annealed at temperature equal to 950 -C, an unexpected result for the FTIR investigation is obtained. The corresponding band of interstitial oxygen in this case shows that more oxygen precipitates are formed in the case samples when the interstitial oxygen concentration is higher. Oxygen is in fact the main impurities in Cz silicon induced light degradation of solar cells. Oxygen precipitation in combination with dislocations and dangling bands generated by PS, they could enhance the precipitation of metals at these extended defects [10]. Since a useful gettering experiment needs to include more than the majority charge carrier mobility measurements to provide a reasonable degree of confidence in the results, solar cells were processed on selected CZ samples to show the influence of the gettering step on the device properties. The I – V characteristics of the CZ solar cells measured under dark are represented in Fig. 4. It is clear from the curves that the leakage current density ( J L) and the diffusion potential (V d) are enhanced by P gettering at 850 and 900 -C; however, 950 -C gettering did not improve the electrical parameters of the cells. Fig. 4 displays the relationship between the wafers. Both V d and J L follow the Hall mobility trend in the cases of 850 and 900 -C P gettering, while no clear picture could be drawn from gettering the CZ wafers at 950 -C.
In conclusion, porous silicon damage may be used in conjunction with Al and P gettering procedures for silicon gettering by generation of dangling bands and vacancies. These observations are mainly supported by the improvements of the majority carrier mobility from 89 to 573 cm2 V 1 s 1 and 982 cm2 V 1 s 1 for aluminium and phosphorus, respectively, by using a sacrificial porous silicon layer. FTIR investigations prove that the precipitation of oxygen at extended defects may occur in some treatment conditions. This means that for P gettering with porous silicon damages intrinsic and extrinsic gettering may operate conjointly, this is a reason to clarify that phosphorus is more effective than aluminum. The reduction of substitutional boron and interstitials oxygen concentrations may indicate in part that the effect of light induced degradation in boron doped Cz silicon can be reduced. The main step in this work is the applications of these treatments in the field of silicon solar cells pointing out the reduction of the recombination centres in the gap band of the material. Acknowledgements This work was supported by the Ministe`re de la Recherche Scientifique et de la Technologie. The authors would like to thank Pr. B. Bessais for the technical support. References [1] Jan Schmidt, Karsten Bothe, Phys. Rev., B 69 (2004) 024107. [2] J. Zhao, A. Wang, M. Green, Sol. Energy Mater. Sol. Cells 65 (2001) 429. [3] Anzhong Lin, Xuejian Wong, Youxin Li, Liande Chou, Sol. Energy Mater. Sol. Cells 62 (2000) 149. [4] S. Martinuzzi, H. El Ghitani, D. Sarti, P. Torchio, Proceedings of the 20th IEEE Photovoltaic Specialists Conference, p. 1575. [5] S. Martinuzzi, I. Perichaud, Mater. Sci. Forum 144 (147) (1994) 1629. [6] N. Khedher, M. Hajji, M. Hassen, A. Ben Jaballah, B. Ouertani, H. Ezzaouia, B. Bessais, A. Selmi, R. Bennaceur, Sol. Energy Mater. Sol. Cells 87 (2005) 605. [7] R. Bilyalov, L. Stalmans, G. Beaucarne, R. Loo, M. Caymax, J. Poortmans, J. Nijs, Sol. Energy Mater. Sol. Cells 65 (2001) 477. [8] M. Hassen, A. Ben Jaballah, M. Hajji, N. Khedher, B. Bessais, H. Ezzaouia, H. Rahmouni, F. Rziga Ouaja, A. Selmi, Sol. Energy Mater. Sol. Cells 87 (2005) 493. [9] Stefano Pizzini, Phys. Status Solidi, A Appl. Res. 171 (1999) 123. [10] H.J. Mo¨ller, L. Long, M. Werner, D. Yang, Phys. Status Solidi, A Appl. Res. 171 (1999) 175.
Impacts of phosphorus and aluminum gettering with porous silicon damage for p-type Czochralski silicon used in solar cells technology A. Ben Jaballah a,*, M. Hassen a, H. Rahmouni b, M. Hajji a, A. Selmi b, H. Ezzaouia a b
a Institut National de Recherche Scientifique et Technique, Laboratoire de Photovoltaı¨que et des Semiconducteurs, PB 95 2050 Hammam-Lif, Tunisia Laboratoire de Physique des Semiconducteurs et des Composants Electroniques Faculte´ des Sciences de Monastir, Rue de Kairouan 5000 Monastir, Tunisia
Available online 18 January 2006
Abstract In this paper, porous silicon damage (PSD) is introduced like a simple sequence for efficient extrinsic gettering schemes. The technique consists to create a sacrificial porous silicon layer on both sides of the silicon substrates with randomly hemispherical voids. Then, two main sample types are processed. In the first group, thin aluminium layers ( 1 Am) are thermally evaporated followed by photo-thermal annealing at 700 and 800 -C, under N2 atmosphere. In the second group, phosphorous is continually diffused during heating at one of several temperatures ranging between 750 and 950 - C for 1 h in a solid phase from POCl3 solution, in N2/O2 ambient. Hall Effect and Van Der Pauw methods prove the existence of an optimum temperature in the case of phosphorus gettering equal to 900 -C yielding a hall mobility of about 982 cm2 V 1 s 1. FTIR investigations show an increase of interstitial oxygen enhancing the precipitation phenomena and proving that extrinsic and intrinsic gettering are in work at the same time. However, in the case of aluminum gettering, there isn’t a gettering limit in the as mentioned temperature range. Moreover, silicon solar cells are processed to clarify this effect. D 2005 Elsevier B.V. All rights reserved. Keywords: Porous silicon; Al and P gettering; Hall mobility; Solar cells
1. Introduction It is well-known that B-doped Cz Si materials suffer lightinduced degradation due to boron-oxygen related defects [1] which is responsible of a reduction in bulk diffusion length and hence yields of electronic devices. In this context, Zhao et al [2] propose to use Float-zone silicon material gallium doped in order to overcome this problem. In addition, P and Al gettering [3,4] thermal treatments are explored as techniques to enhance Cz material qualities. External P and Al gettering with sacrificial porous silicon layer has been reported to substantially enhance the majority carrier mobility in monocrystalline silicon [5]. It is shown that Al and P gettering is able to recover mobility, reducing both Fingot growth-induced_ and Fprocessinduced_ impurities, and allowing also for a reduction of the defect concentration after the thermal step. In this paper, we summarize experimental results which outline the tentatives made to introduce PS damage as a
* Corresponding author. Tel.: +216 71 430 160; fax: +216 71 430 934. E-mail address: [email protected] (A. Ben Jaballah). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.11.101
preliminary step for extrinsic gettering in Czochralski silicon by rapid thermal diffusion of aluminum (Al) and phosphorus (P) using an infrared furnace. Hall Effect and Van Der Pauw techniques are used to determine the mobility of majority charge carriers. Moreover, dark current– voltage investigations of standard silicon solar cells are used to monitor the effects of these gettering treatments. Scanning electron microscope investigations are processed in the case of aluminum gettering to show the formation of the transition captured region. Based on the experimental results, we discuss the mechanisms involved by external gettering using a sacrificial porous silicon layer. Indeed, we aim to optimize the gettering temperatures and time in order to use the more convenient conditions for the conception of adequate Al – P co-gettering treatments. 2. Experimental In these experiments, we have used a single crystal Cz wafers, p type, boron doped, (100) oriented, and 330-Amthick with the resistivity ranging from 1 to 3 V cm. Porous silicon layer was formed on both sides of samples using
378
A. Ben Jaballah et al. / Thin Solid Films 511 – 512 (2006) 377 – 380
Table 1 The effect of porous silicon damage on majority charge mobility and carriers concentrations of silicon substrates treated under N2 ambient at 750 -C during 60 min Sample type Reference Without PS With PS
Resistivity (V .cm)
Mobility (cm2.V 1.s
1,53 1,43 1,27
89 358 582
1
)
Concentration (cm 3) 4,64 1016 1,21 1016 8,43 1015
stain-etching method which consisting of dipping wafers in aqueous (HF (40%)/HNO3 (65%)/H2O) solution with (1:3:5) volume composition. Each wafer was cut into 0.5 0.5 cm2 pieces to minimize the variation resulting from different wafers. These samples were divided on two groups. For the first group, a thick aluminum layer was evaporated onto the both sides of samples. Then, the wafers are annealed for 30 min in the temperature range of 600– 800 -C in dry N2 in an infrared furnace. For the second group phosphorus is continuously diffused from POCl3 sources in the solid phase. Then, the wafers are thermally treated for temperatures ranging between 800 -C and 950 -C for 1 h under N2/O2 ambient (gas vector). After annealing, the as formed Al –Si alloy and the phosphorus doped region are etched using a (HF (40%): HNO3 (65%)) acid solution with (3:1) volume composition. As a consequence, these approaches improve the efficiency of the Al and P gettering which are studied and compared using the Hall mobility measurements as well as the dark I –V characteristics of the solar cells. In the case of phosphorus, Fourier Transformation Infrared (FTIR) spectroscopy may help to evaluate qualitatively the interstitial oxygen concentrations. For silicon solar cells based on wafers treated with aluminium and phosphorus, a pieces cut into 2 2 cm2 are used. Phosphorus diffusion was carried out in a rapid thermal infrared tubular furnace. The metallic contacts are made by screen printing of Al/Ag pastes for the rear contacts and silver paste for the emitter contacts. The metallic contacts are separately annealed at temperatures of 650 -C and 700 during 10 min and 15 min
for rear and emitter contacts respectively, in an infrared furnace. 3. Results and discussions 3.1. Porous silicon damage The use of porous silicon as an intermediate layer to getter unwanted impurities has stimulated a great interest in epitaxial growth [6]. This has been found by the improvements of the transport parameters like the majority charge mobility (l d) and the carrier concentration determined by both Hall Effect measurements and resistivity calculated using Van Der Pauw method. Table 1 shows the enhancement of l d caused by means of porous silicon damage from 358 cm2 V 1 s 1 for silicon wafer without PS to 582 cm2 V 1 s 1 for sample with PS treated at 750 -C for 60 min under N2 atmosphere. This gettering effect is probably based on processes which induce segregation of impurities and clusters type defects at extended defects. This suggestion is mainly supported by the saturation of the gettering effect as the temperature and the treatment period increased [7]. These behaviours are explained by the reduction of vacancy concentration and dangling bonds due to the annealing of damages and chemical adsorption, respectively [8]. 3.2. Aluminum gettering Fig. 1 exhibits cross section views of silicon wafers treated by aluminum once and annealed at temperature equal to 900 -C for 30 min in a clean tubular furnace under N2 atmosphere. It shows the formation of a highly rear doped silicon layer which reinforce the effect of a back surface field. The top microstructure has been changed due to annealing. The rear aluminum is known to form a Shottky barrier to p-type silicon with a relatively low barrier height of 0.4 –0.6 eV, which allow the realization of a concentration gradient on the rear. Table 2 shows the enhancement of the Hall mobility of majority carriers as the temperature rise. However, the boron concen-
Fig. 1. Cross section views of Al/PS/Si structures: (a) as realized, (b) annealed at 900 -C for 30 min under N2 atmosphere, showing the change of the microstructure of porous silicon and the diffusion of aluminum to form a highly doped layer on the back of the sample.
A. Ben Jaballah et al. / Thin Solid Films 511 – 512 (2006) 377 – 380 Table 2 Enhancement of majority carrier mobility for silicon wafers treated with aluminum once and subsequently annealed under N2 atmosphere for 30 min Treatment
Resistivity (V .cm)
Mobility (cm2.V
Reference 700 -C 800 -C
1,53 1,28 1,32
89 512 573
1
.s
1
Untreated 800 -C 900 -C 950 -C
6,0x10
-3
4,0x10
-3
2,0x10
-3
-6
-4
-2
0 -2,0x10
-3
-4,0x10
-3
Mobility (cm2.V 1.s
1,53 1,24 1,31 1,34
89 644 982 514
Concentration (cm 3)
1
)
4,6 1016 7,8 1015 4,85 1015 9,07 1015
30 min in presence of a sacrificial PS layer. One can conclude from Tables 1 and 2 that these two treatments yield the same results but with less duration for the aluminum case under the assumption that no steady state gettering phenomenon may be occurred during treatments. 3.3. Phosphorus gettering In Table 3, the resistivity and the active boron concentration are presented in addition to the annealing temperatures, the treatment cycle time is kept equal to 60 min. It seems that the boron concentrations are more sensitive to treatment conditions than the resistivity. Moreover, the Hall mobility investigations prove the existence of an optimum treatment temperature. During the procedure, the two competing mechanisms of impurity gettering and thermal degradation are simultaneously at work. Another effect is the compensation phenomenon occurring between phosphorus in diffusion and boron atoms which dominates as the annealing temperature increased. The optimal temperature is about 900 -C, where the mobility is multiplied by ten. This optimum could be the result of competition between the release of impurities from the bulk and a capture of impurities in the gettering layer. Below the optimum temperature, gettering process is limited by the release or the diffusion of metallic impurities towards the gettering layer. In Fig. 3, infra-red spectra of treated samples are shown; the band relative to interstitial oxygen [9] appear at 1110 cm 1.
(b) 2,7
(a)
2,4
Voltage (V)
0,0 -8
Resistivity (V .cm)
The wafers are annealed under N2/O2 atmosphere during a cycle of 60 min.
Absorbance (a.u)
-3
Current (A)
8,0x10
Table 3 Phosphorus treatments induced improvements of majority charge mobility due reduction of defect centers
)
tration of majority charge carriers decreases as the temperature increase. These behaviors may indicate in a part the occurrence of gettering phenomena. In the present work the nature and the concentration of contaminants in the bulk are unknown, which seems to be a weak point to discuss the nature of undesirable impurities gettered away from the bulk. On the other hand, this can be viewed as a strong point in the sense that it corresponds to a real world device fabrication situation for which the starting silicon material contaminant nature and concentration are unknown. Owing to the low gettering temperature and short gettering time used, we believe that the gettered species are the fast moving interstitials metal atoms such as those of Fe, Ni, Cu, etc. Substitutionally dissolved metal atoms, like gold (Au) not have been significantly affected by the present gettering treatment. Indeed, the weak decrease of substitutional boron [Bs] concentration can stimulate a gettering effect because, the defect concentration centers in Cz silicon is founded [1] to be proportional to [Bs]. In Fig. 2, the dark I –V characteristics of silicon solar cells based on samples treated by the combination of porous silicon and aluminum are shown. It appears that the diode quality is improved: in the forward bias, the leakage current due to the escape of holes through deep recombination centers is less pronounced for sample treated at 900 -C in comparison of untreated one. This effect indicates the enhancement of the electronic properties of the silicon material subjected to the above gettering process. For instance, we try to give a comparison between two samples: the first treated only with PS layer and thermally annealed at 750 -C for 60 min and the second treated by aluminum and annealed at 800 -C for
379
2
2,1
1200
Fig. 2. Effect of aluminum gettering on leakage current densities of the dark I – V characteristics of silicon based solar cells: (a) untreated sample, (b) sample treated at 900 -C.
800°C 750°C 900°C 950°C untreated 1150
1100
1050
1000
Wave number (cm-1) Fig. 3. The evolution of the infrared absorption spectra of interstitial oxygen due to phosphorus treatments and subsequent annealing under N2/O2 atmosphere. All samples have been treated with a porous silicon layer and cleaned by CP4 chemical solution after infrared heating.
A. Ben Jaballah et al. / Thin Solid Films 511 – 512 (2006) 377 – 380 Current (A)
380
0,008
0,006
4. Conclusion (c) (d)
(e)
0,004
0,002
Voltage (V) 0,000 -10
-8
-6
-4
-2
0
2
-0,002
Fig. 4. The effect of phosphorus treatments on the saturation current density of three silicon solar cells for various annealing temperatures: (c) 900 -C, (d) 800 -C and (e) 950 -C; during 60 min.
One can notice that the band intensity tend to be lower for Si substrate treated at 900 -C. This appears to be sufficient to eliminate any previously formed nucleation centres that could enhance the precipitation of oxygen. Roughly, for the specimen annealed at temperature equal to 950 -C, an unexpected result for the FTIR investigation is obtained. The corresponding band of interstitial oxygen in this case shows that more oxygen precipitates are formed in the case samples when the interstitial oxygen concentration is higher. Oxygen is in fact the main impurities in Cz silicon induced light degradation of solar cells. Oxygen precipitation in combination with dislocations and dangling bands generated by PS, they could enhance the precipitation of metals at these extended defects [10]. Since a useful gettering experiment needs to include more than the majority charge carrier mobility measurements to provide a reasonable degree of confidence in the results, solar cells were processed on selected CZ samples to show the influence of the gettering step on the device properties. The I – V characteristics of the CZ solar cells measured under dark are represented in Fig. 4. It is clear from the curves that the leakage current density ( J L) and the diffusion potential (V d) are enhanced by P gettering at 850 and 900 -C; however, 950 -C gettering did not improve the electrical parameters of the cells. Fig. 4 displays the relationship between the wafers. Both V d and J L follow the Hall mobility trend in the cases of 850 and 900 -C P gettering, while no clear picture could be drawn from gettering the CZ wafers at 950 -C.
In conclusion, porous silicon damage may be used in conjunction with Al and P gettering procedures for silicon gettering by generation of dangling bands and vacancies. These observations are mainly supported by the improvements of the majority carrier mobility from 89 to 573 cm2 V 1 s 1 and 982 cm2 V 1 s 1 for aluminium and phosphorus, respectively, by using a sacrificial porous silicon layer. FTIR investigations prove that the precipitation of oxygen at extended defects may occur in some treatment conditions. This means that for P gettering with porous silicon damages intrinsic and extrinsic gettering may operate conjointly, this is a reason to clarify that phosphorus is more effective than aluminum. The reduction of substitutional boron and interstitials oxygen concentrations may indicate in part that the effect of light induced degradation in boron doped Cz silicon can be reduced. The main step in this work is the applications of these treatments in the field of silicon solar cells pointing out the reduction of the recombination centres in the gap band of the material. Acknowledgements This work was supported by the Ministe`re de la Recherche Scientifique et de la Technologie. The authors would like to thank Pr. B. Bessais for the technical support. References [1] Jan Schmidt, Karsten Bothe, Phys. Rev., B 69 (2004) 024107. [2] J. Zhao, A. Wang, M. Green, Sol. Energy Mater. Sol. Cells 65 (2001) 429. [3] Anzhong Lin, Xuejian Wong, Youxin Li, Liande Chou, Sol. Energy Mater. Sol. Cells 62 (2000) 149. [4] S. Martinuzzi, H. El Ghitani, D. Sarti, P. Torchio, Proceedings of the 20th IEEE Photovoltaic Specialists Conference, p. 1575. [5] S. Martinuzzi, I. Perichaud, Mater. Sci. Forum 144 (147) (1994) 1629. [6] N. Khedher, M. Hajji, M. Hassen, A. Ben Jaballah, B. Ouertani, H. Ezzaouia, B. Bessais, A. Selmi, R. Bennaceur, Sol. Energy Mater. Sol. Cells 87 (2005) 605. [7] R. Bilyalov, L. Stalmans, G. Beaucarne, R. Loo, M. Caymax, J. Poortmans, J. Nijs, Sol. Energy Mater. Sol. Cells 65 (2001) 477. [8] M. Hassen, A. Ben Jaballah, M. Hajji, N. Khedher, B. Bessais, H. Ezzaouia, H. Rahmouni, F. Rziga Ouaja, A. Selmi, Sol. Energy Mater. Sol. Cells 87 (2005) 493. [9] Stefano Pizzini, Phys. Status Solidi, A Appl. Res. 171 (1999) 123. [10] H.J. Mo¨ller, L. Long, M. Werner, D. Yang, Phys. Status Solidi, A Appl. Res. 171 (1999) 175.