Locally aluminum-doped p-type silicon regions are formed by in-line high-rate evaporation of aluminum. We deposit aluminum layers of 28 μm thickness at dynamic deposition rates of 20 μm×m/min on locally laser-ablated Al2O3 / SiNx passivation layers. Due to the high substrate temperature of up to 778°C during deposition an Al-doped p+ region is formed. Using the camera-based dynamic infrared lifetime mapping technique we determine a contact recombination velocity of 1000 ± 100 cm/s for local Al-p+ regions on p-type silicon wafers of 1.5 ωcm resistivity. The recombination velocity between the contacts is determined to 4.4 ± 0.5 cm/s after deposition.State of Lower Saxon

Bock, Robert

Brendel, Rolf

Mader, Christoph

Müller, Jens

Schmidt, Jan

AbstractLocally aluminum-doped p-type silicon regions are formed by in-line high-rate evaporation of aluminum. We deposit aluminum layers of 28μm thickness at dynamic deposition rates of 20μm×m/min on locally laser-ablated Al2O3 / SiNx passivation layers. Due to the high substrate temperature of up to 778°C during deposition an Al-doped p+ region is formed. Using the camera-based dynamic infrared lifetime mapping technique we determine a contact recombination velocity of 1000±100cm/s for local Al-p+ regions on p-type silicon wafers of 1.5Ωcm resistivity. The recombination velocity between the contacts is determined to 4.4±0.5cm/s after deposition

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SiliconPV: 17-20 April 2011, Freiburg, Germany 
Formation of locally aluminum-doped p-type silicon regions 
by in-line high-rate evaporation 
 
Christoph Madera,*, Robert Bocka, Jens Müllera, Jan Schmidta, Rolf Brendela,b
aInstitute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, 31860 Emmerthal, Germany 
bInstitute for Solid State Physics, Leibniz University of Hannover, Appelstrasse 2, 30167 Hannover, Germany 
 
Abstract 
Locally aluminum-doped p-type silicon regions are formed by in-line high-rate evaporation of aluminum. We deposit 
aluminum layers of 28 μm thickness at dynamic deposition rates of 20 μmum/min on locally laser-ablated  
Al2O3 / SiNx passivation layers. Due to the high substrate temperature of up to 778°C during deposition an Al-doped 
p+ region is formed. Using the camera-based dynamic infrared lifetime mapping technique we determine a contact 
recombination velocity of 1000 r 100 cm/s for local Al-p+ regions on p-type silicon wafers of 1.5 :cm resistivity. 
The recombination velocity between the contacts is determined to 4.4 r 0.5 cm/s after deposition. 
 
Keywords: Crystalline silicon solar cell; in-line evaporation; aluminum doped silicon; alloying; local contacts 
1. Introduction 
The rear side of a standard industrial-type silicon solar cell is contacted by screen-printing. This 
process has three steps: (i) An aluminum paste is printed onto the silicon wafer, (ii) the paste is dried at a 
temperature of a 200°C, and (iii) the Al paste is fired in a belt furnace at peak temperatures of a 850°C 
 
* Corresponding author. Tel.: +49-5151-999-642; fax: +49-5151-999-400. 
E-mail address: mader@isfh.de. 
1876–6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV 2011.
doi:10.1016/j.egypro.2011.06.176
Energy Procedia 8 (2011) 521–526
Open access under CC BY-NC-ND license.
© 2011 Published by Elsevier Ltd. 
Selection and/or peer-review under responsibility of SiliconPV 2011
Open access under CC BY-NC-ND license.
522  Christoph Mader et al. / Energy Procedia 8 (2011) 521–526
[1]. The firing provides an ohmic contact on the entire rear side and also forms an aluminum-doped  
p+ (Al-p+) region, which reduces the rear surface recombination. As an alternative metalization scheme the 
in-line high-rate evaporation of aluminum has recently been introduced [2,3]. Similar to the screen-
printing, drying and firing process an ohmic contact and an Al-p+ region is formed by single evaporation 
of aluminum. In Refs. 2 aluminum layers of up to 28 μm thickness were deposited at dynamic deposition 
rates of 20 μmum/min. The deposition leads to an increase in wafer temperature due to the enthalpy of 
deposition. The maximum substrate temperatures during the deposition are well above the eutectic 
temperature of the Al-Si system and thus are leading to the formation of highly aluminum-doped p-type 
silicon regions. The Al-p+ regions are formed on the complete rear side and emitter saturation current 
densities of (695 r65) fA/cm2 for the fully metalized Al-p+ regions are determined [2]. This corresponds 
to an implied solar cell open-circuit voltage of only (635 r 2) mV thus limiting the energy conversion 
efficiencies of solar cells. Higher open circuit voltages can be achieved by a passivated and only locally 
contacted rear side [4,5,6]. In this paper, we present the formation of locally aluminum-doped p-type 
silicon regions by a single in-line high rate evaporation process without any further temperature treatment. 
2. Sample preparation 
For the Seff measurements we fabricate asymmetric test structures where both sides are well passivated 
with a stack of atomic-layer-deposited (ALD) aluminum oxide Al2O3 and plasma-enhanced-chemical-
vapor-deposited (PECVD) silicon nitride SiNx. The p+ region is prepared on one side of the samples by 
local laser ablation of the dielectric stack and subsequent high-rate Al deposition. For the sample 
preparation we use 2.5 u 2.5 cm², single-crystalline, (100)-oriented and 270 μm thick p-type float-zone 
(FZ) silicon wafers of 1.5 ȍcm resistivity. After KOH etching and RCA cleaning we deposit on both 
wafer sides a 10 nm thick ALD-Al2O3 layer and a 90 nm thick PECVD-SiNx layer with a refraction index 
of n = 2.05 at a wavelength of O = 632 nm [7]. For the single sided contact openings laser pulses of 8 to 9 
picoseconds at a wavelength of 532 nm are applied to the samples using a Nd: YVO4 laser. The 
contacting scheme consists of parallel line contacts with a width of b = 85 μm and a period length of  
p = 2000 μm. Finally, we evaporate Al layers of 13, 20, and 28 μm thickness on the locally opened wafer 
side. The Al deposition process parameters are summarized in Table 1. 
Table 1: Dynamic deposition rate r, tray speed vtray and Al layer thickness d of the evaluated processes. The dynamic deposition rate 
of all processes is fixed at r = 20 μmum/min. The variation in Al layer thickness is achieved by different tray speeds. 
 
Process Dynamic deposition rate  r [μmum/min] 
Tray speed  
vtray [m/min] 
Al layer thickness  
d [μm] 
1 20 1.5 13 
2 20 1 20 
3 20 0.7 28 
All processes are carried out at dynamic deposition rates of r = 20 μmum/min using an in-line high-
rate metallization system (ATON 500 from Applied Materials). Figure 1 shows a sketch of the deposition 
process. The high deposition rate leads to an increase in the wafer temperature of up to 778°C 
(simulated). No active heating is applied. We calculate the substrate temperature using two-dimensional 
finite-element simulations [8,9]. The simulations are based on a simplified model of the heat flows j 
during evaporation. These heat flows into and out of the wafer are caused by the enthalpy of deposition of 
aluminum and heat radiation of the wafer. 
Christoph Mader et al. / Energy Procedia 8 (2011) 521–526 523
  
Fig. 1. Schematic sketch of the Al deposition process. An aluminum wire is steered onto heated boron nitride (BN) crucibles, 
resulting in a continuous generation of aluminum vapor. The evaporated aluminum deposits on the sample that moves on a carrier 
through the deposition chamber. Many BN crucibles are positioned next to each other in z-dimension to have a homogeneous 
deposition on a width of 500 mm. 
Please note that phase transitions are not considered in the simulation. The simulation has been 
verified with temperature measurements at deposition processes with a dynamic deposition rate of  
r = 5 μmum/min leading to maximum substrate temperatures of 400°C. Simulated and measured peak 
temperatures agree within 3% [8,9]. 
3. Experimental results 
Figure 2 shows the calculated substrate temperature for three deposition processes with layer 
thicknesses d of 13 μm, 20 μm, and 28 μm. 
 
Fig. 2. Substrate temperature - during deposition of Al layers of various thicknesses d (see Table 1) deduced from numerical 
simulations. The Si wafer thickness is fixed at W = 270 μm. The increase in temperature is caused by the deposition of aluminum. 
The deposition of Al layers of 20 μm and 28 μm on 270 μm-thick Si wafers leads to temperatures well 
above the eutectic temperature of 577°C of the Al-Si system. Thus, an Al-p+ region forms similar to the 
screen-printing process. Please note that the temperature during deposition depends on the ratio of wafer 
thickness W and Al layer thickness d. Using thinner Si wafers at fixed Al layer thickness leads to higher 
temperatures and thus the formation of an Al-p+ region is also possible by depositing Al layers < 10 μm. 
The integration of a heater in the deposition system could further decrease the Al layer thickness, which is 
524  Christoph Mader et al. / Energy Procedia 8 (2011) 521–526
required for the formation of an Al-p+ layer. The high temperature during evaporation improves the 
passivation quality of the Al2O3-SiNx stack [10,11]. On symmetric reference samples passivated with an 
Al2O3-SiNx stack we measure surface recombination velocities (SRV) of SAl2O3  25 cm/s before and of 
SAl2O3 d 5 cm/s after Al deposition. Effective SRV Seff are obtained from metalized samples using the 
camera-based dynamic infrared lifetime mapping technique (dyn ILM) [12,13]. We do not observe an 
injection-dependent lifetime in the range from ǻn = 1014  3u1015 and extract the effective surface 
recombination velocities Seff from the measured lifetime at an injection level ǻn = 2x1015 cm 3 on an area 
of 2 u 2 cm² using the equation [14] 
 
,     (1) 
 
where W is the wafer thickness. It is assumed that the excess carrier concentration ǻn is uniform 
throughout the base, which is not strictly valid for large surface recombination velocities. We assume that 
the bulk lifetime of the substrate is Auger-limited [15]. Figure 3 shows cross-sectional SEM images of the 
evaporated local Al-p+ samples of the three deposition processes 1 through 3. The images are obtained 
from an ultra-high resolution Hitachi S-4800 field emission SEM. The Al-doped p+-region appears 
brighter than the high-resistivity bulk of the silicon wafer due to the potential contrast [16]. Process 1 
from Table 1 results in a temperature of -max = 560°C, which is below the eutectic temperature of 577°C, 
thus an Al-p+ region has not formed [see Figure 3 (a)]. Process 2 results in wafer temperatures of -max = 
640°C, leading to the formation of an Al-p+ region. Nevertheless, Figure 3 (b) shows a discontinuous 
aluminum-doped region. The Process 3 from table 1, where a peak substrate temperature of -max = 778°C 
has been simulated, results in a completely closed Al-p+ region within the contact openings as shown in 
Figure 3 (c). From this SEM image we determine the Al-p+ depth to be (5 ± 3) μm. 
 
 
Fig. 3. SEM micrographs of cross sections of samples processed with processes 13. The samples feature an Al2O3-SiNx passivation 
stack with line openings of a width of 85 μm before Al deposition. The line width increases during the formation of the Al-p+ layer 
to 120 μm. (a) SEM micrograph of a sample processed with process 1. The process temperature below 577°C is not leading to a 
local Al-p+ region. (b) Process 2 results in a higher maximum wafer temperatures of -max = 640°C, leading to the formation of a 
discontinuous Al-p+ region represented by the dotted line. The emitter thickness is (150 ± 150) nm. The Al-p+ region is clearly 
visible as brighter contrast. (c) Process 3 (-max = 778°C) results in a completely closed Al-p+ region as represented by the dotted 
line. The emitter thickness is (5 ± 3) μm. 
W
S+
W
S
Ĳ
=
Ĳ
OAl
bulkeff
eff3211 
Christoph Mader et al. / Energy Procedia 8 (2011) 521–526 525
At temperatures above the eutectic temperature silicon dissolves into the aluminum layer. The 
passivation layer acts hereby as a diffusion barrier and the alloying process can only take place in the area 
of the local openings. This results in the surface morphology shown in Fig. 6 (b) and (c). Please note, that 
the width of the line contacts after the formation of the Al-p+ region increases to b = 120 μm. A detailed 
analysis of the contact formation of local Al alloyed Si contacts is given in Ref. [17]. Figure 4 shows the 
measured rear SRV Seff depending on Al layer thickness. Process 1 (see Table 1) with an Al layer 
thickness of 13 μm is not leading to the formation of an Al-p+ region. The corresponding recombination 
velocity is Seff = (114 ± 15) cm/s. Processes 2 and 3 (see Table 1) lead to the formation of an Al-p+ region 
and are thus reducing the effective SRV. Process 2 results in a SRV of (99 ± 10) cm/s and process 3 in a 
value of (50 ± 5) cm/s. Latter value is in the same range as published SRVs of screen-printed local Al-p+ 
regions [6]. 
 
Fig. 4. Measured effective surface recombination velocity as function of the Al layer thickness d of a 1.5 ȍcm FZ p-Si wafer. One 
side of the wafer is passivated with a stack of Al2O3 and SiNx. The other side is passivated with the same stack and features local 
laser openings of a width of 85 μm and a period length of 2000 μm. The local Al-p+ region forms at the interface of Al to Si in the 
area of the laser openings. 
We extract the contact recombination velocity Scont from the measured Seff values using equation [18] 
 
,    (2) 
where Rs is the series resistance of the majority carriers in the bulk, U is the base resistivity, W the 
sample thickness, D the diffusion coefficient and f the metallization fraction. Spass is the surface 
recombination velocity between and Scont the surface recombination velocity under the contacts. Using 
equation 2 we determine Scont = (7 r 2)u103 cm/s for process 1. Processes 2 and 3 are reducing the contact 
recombination velocity to values of (3 r 0.5)u103 cm/s and (1 r 0.1)u103 cm/s, respectively. 
 
f
S
fSD
WR=S
-1
1 pass
1
cont
S
eff ¸¸
¹
·
¨¨
©
§


U
U
526  Christoph Mader et al. / Energy Procedia 8 (2011) 521–526
[15] Kerr M, Cuevas A. General parameterization of Auger recombination in crystall. silicon. J. Appl. Phys. 2002; 91 : 2473. 
[16]  Sealy CP, Castell MR, Wilshaw PR. Mechanism for secondary electron dopant contrast in the SEM. Journal of Electron 
Microscopy 2000; 49: 311-321. 
[17] Müller J, Bothe K, Gatz S, Plagwitz H, Schubert G, Brendel R. Recombination at local aluminum alloyed silicon solar 
cell base contacts by dynamic infrared lifetime mapping. Energy Procedia, 1st SiliconPV 2011, Freiburg. 
[18] Fischer B. Loss analysis of crystalline silicon solar cells using photoconductance and quantum efficiency measurements. 
PhD thesis, Cuvillier, Göttingen 2003. 
4. Conclusion 
In conclusion, we have presented results on in-line high-rate evaporation of aluminum on silicon 
substrates. Using numerical simulations, we have shown that the deposition process leads to temperatures 
of -max = 778°C. These conditions lead to the formation of an Al-p+ region without any further 
temperature treatment. We determine a contact recombination velocity of 1000 r 100 cm/s for local Al-p+ 
regions on silicon wafers of 1.5 :cm resistivity resulting in an effective surface recombination velocity of 
only (50 r5) cm/s for a typical rear contacting scheme with a line width of b = 85 μm and a period length 
of p = 2000 μm. 
Acknowledgements 
This work is supported by the State of Lower Saxony. 
 
References 
[1] Ralph E. Recent advancement in low cost solar cell processing. Proceedings of the 11th IEEE Photovoltaic Specialists 
Conference 1975; Scottsdale: 315 - 316. 
[2] Mader C, Bock R, Schmidt J, Brendel R. Formation of highly aluminum-doped p-type silicon regions by in-line high-rate 
evaporation. Sol. Energy Mater. Sol. Cells 2011, doi:10.1016/j.solmat.2011.01.039. 
[3] Mader C, Bock R, ISFH, Patent pending. 
[4] Agostinelli G, Choulat P, Dekkers H, De Wolf S, Beaucarne G. Screen printed larg area crystalline silicon solar cells on 
thin substrates Proceedings of the 20th European Photovoltaic Solar Energy Conference (WIP, Barcelona, 2005). pp. 
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[5] Ramanathan S, Das A, Cooper I, Rohatgi A, Payne A, Koehler I. 20% efficient screen printed LBSF cell fabricated using 
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[6] Gatz S, Hannebauer H, Hesse R, Werner F, Schmidt A, Dullweber T, Schmidt J, Bothe K, Brendel R. 19.4%-efficient 
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[7] Lauinger T, Schmidt J, Aberle AG, Hezel R. Record low surface recombination velocities on 1 :cm p-silicon using 
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[9] Mader C, Eitner U, Kessler M, Brendel R. Temperature of silicon wafers during in-line high-rate evaporation of 
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[10] Hoex B, Heil S, Langereis E, van de Sanden M, Kessels W. Ultralow surface recombination of c-Si substrates passivated 
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Formation of locally aluminum-doped p-type silicon regions by in-line high-rate evaporation 

Institutionelles Repositorium der Leibniz Universität Hannover

Available online at www.sciencedirect.comSiliconPV: 17-20 April 2011, Freiburg, Germany Formation of locally aluminum-doped p-type silicon regions by in-line high-rate evaporation  Christoph Madera,*, Robert Bocka, Jens Müllera, Jan Schmidta, Rolf Brendela,baInstitute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, 31860 Emmerthal, Germany bInstitute for Solid State Physics, Leibniz University of Hannover, Appelstrasse 2, 30167 Hannover, Germany  Abstract Locally aluminum-doped p-type silicon regions are formed by in-line high-rate evaporation of aluminum. We deposit aluminum layers of 28 μm thickness at dynamic deposition rates of 20 μmum/min on locally laser-ablated  Al2O3 / SiNx passivation layers. Due to the high substrate temperature of up to 778°C during deposition an Al-doped p+ region is formed. Using the camera-based dynamic infrared lifetime mapping technique we determine a contact recombination velocity of 1000 r 100 cm/s for local Al-p+ regions on p-type silicon wafers of 1.5 :cm resistivity. The recombination velocity between the contacts is determined to 4.4 r 0.5 cm/s after deposition.  Keywords: Crystalline silicon solar cell; in-line evaporation; aluminum doped silicon; alloying; local contacts 1. Introduction The rear side of a standard industrial-type silicon solar cell is contacted by screen-printing. This process has three steps: (i) An aluminum paste is printed onto the silicon wafer, (ii) the paste is dried at a temperature of a 200°C, and (iii) the Al paste is fired in a belt furnace at peak temperatures of a 850°C  * Corresponding author. Tel.: +49-5151-999-642; fax: +49-5151-999-400. E-mail address: mader@isfh.de. 1876–6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV 2011.doi:10.1016/j.egypro.2011.06.176Energy Procedia 8 (2011) 521–526Open access under CC BY-NC-ND license.© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV 2011Open access under CC BY-NC-ND license.522  Christoph Mader et al. / Energy Procedia 8 (2011) 521–526[1]. The firing provides an ohmic contact on the entire rear side and also forms an aluminum-doped  p+ (Al-p+) region, which reduces the rear surface recombination. As an alternative metalization scheme the in-line high-rate evaporation of aluminum has recently been introduced [2,3]. Similar to the screen-printing, drying and firing process an ohmic contact and an Al-p+ region is formed by single evaporation of aluminum. In Refs. 2 aluminum layers of up to 28 μm thickness were deposited at dynamic deposition rates of 20 μmum/min. The deposition leads to an increase in wafer temperature due to the enthalpy of deposition. The maximum substrate temperatures during the deposition are well above the eutectic temperature of the Al-Si system and thus are leading to the formation of highly aluminum-doped p-type silicon regions. The Al-p+ regions are formed on the complete rear side and emitter saturation current densities of (695 r65) fA/cm2 for the fully metalized Al-p+ regions are determined [2]. This corresponds to an implied solar cell open-circuit voltage of only (635 r 2) mV thus limiting the energy conversion efficiencies of solar cells. Higher open circuit voltages can be achieved by a passivated and only locally contacted rear side [4,5,6]. In this paper, we present the formation of locally aluminum-doped p-type silicon regions by a single in-line high rate evaporation process without any further temperature treatment. 2. Sample preparation For the Seff measurements we fabricate asymmetric test structures where both sides are well passivated with a stack of atomic-layer-deposited (ALD) aluminum oxide Al2O3 and plasma-enhanced-chemical-vapor-deposited (PECVD) silicon nitride SiNx. The p+ region is prepared on one side of the samples by local laser ablation of the dielectric stack and subsequent high-rate Al deposition. For the sample preparation we use 2.5 u 2.5 cm², single-crystalline, (100)-oriented and 270 μm thick p-type float-zone (FZ) silicon wafers of 1.5 ȍcm resistivity. After KOH etching and RCA cleaning we deposit on both wafer sides a 10 nm thick ALD-Al2O3 layer and a 90 nm thick PECVD-SiNx layer with a refraction index of n = 2.05 at a wavelength of O = 632 nm [7]. For the single sided contact openings laser pulses of 8 to 9 picoseconds at a wavelength of 532 nm are applied to the samples using a Nd: YVO4 laser. The contacting scheme consists of parallel line contacts with a width of b = 85 μm and a period length of  p = 2000 μm. Finally, we evaporate Al layers of 13, 20, and 28 μm thickness on the locally opened wafer side. The Al deposition process parameters are summarized in Table 1. Table 1: Dynamic deposition rate r, tray speed vtray and Al layer thickness d of the evaluated processes. The dynamic deposition rate of all processes is fixed at r = 20 μmum/min. The variation in Al layer thickness is achieved by different tray speeds.  Process Dynamic deposition rate  r [μmum/min] Tray speed  vtray [m/min] Al layer thickness  d [μm] 1 20 1.5 13 2 20 1 20 3 20 0.7 28 All processes are carried out at dynamic deposition rates of r = 20 μmum/min using an in-line high-rate metallization system (ATON 500 from Applied Materials). Figure 1 shows a sketch of the deposition process. The high deposition rate leads to an increase in the wafer temperature of up to 778°C (simulated). No active heating is applied. We calculate the substrate temperature using two-dimensional finite-element simulations [8,9]. The simulations are based on a simplified model of the heat flows j during evaporation. These heat flows into and out of the wafer are caused by the enthalpy of deposition of aluminum and heat radiation of the wafer. Christoph Mader et al. / Energy Procedia 8 (2011) 521–526 523  Fig. 1. Schematic sketch of the Al deposition process. An aluminum wire is steered onto heated boron nitride (BN) crucibles, resulting in a continuous generation of aluminum vapor. The evaporated aluminum deposits on the sample that moves on a carrier through the deposition chamber. Many BN crucibles are positioned next to each other in z-dimension to have a homogeneous deposition on a width of 500 mm. Please note that phase transitions are not considered in the simulation. The simulation has been verified with temperature measurements at deposition processes with a dynamic deposition rate of  r = 5 μmum/min leading to maximum substrate temperatures of 400°C. Simulated and measured peak temperatures agree within 3% [8,9]. 3. Experimental results Figure 2 shows the calculated substrate temperature for three deposition processes with layer thicknesses d of 13 μm, 20 μm, and 28 μm.  Fig. 2. Substrate temperature - during deposition of Al layers of various thicknesses d (see Table 1) deduced from numerical simulations. The Si wafer thickness is fixed at W = 270 μm. The increase in temperature is caused by the deposition of aluminum. The deposition of Al layers of 20 μm and 28 μm on 270 μm-thick Si wafers leads to temperatures well above the eutectic temperature of 577°C of the Al-Si system. Thus, an Al-p+ region forms similar to the screen-printing process. Please note that the temperature during deposition depends on the ratio of wafer thickness W and Al layer thickness d. Using thinner Si wafers at fixed Al layer thickness leads to higher temperatures and thus the formation of an Al-p+ region is also possible by depositing Al layers < 10 μm. The integration of a heater in the deposition system could further decrease the Al layer thickness, which is 524  Christoph Mader et al. / Energy Procedia 8 (2011) 521–526required for the formation of an Al-p+ layer. The high temperature during evaporation improves the passivation quality of the Al2O3-SiNx stack [10,11]. On symmetric reference samples passivated with an Al2O3-SiNx stack we measure surface recombination velocities (SRV) of SAl2O3  25 cm/s before and of SAl2O3 d 5 cm/s after Al deposition. Effective SRV Seff are obtained from metalized samples using the camera-based dynamic infrared lifetime mapping technique (dyn ILM) [12,13]. We do not observe an injection-dependent lifetime in the range from ǻn = 1014  3u1015 and extract the effective surface recombination velocities Seff from the measured lifetime at an injection level ǻn = 2x1015 cm 3 on an area of 2 u 2 cm² using the equation [14]  ,     (1)  where W is the wafer thickness. It is assumed that the excess carrier concentration ǻn is uniform throughout the base, which is not strictly valid for large surface recombination velocities. We assume that the bulk lifetime of the substrate is Auger-limited [15]. Figure 3 shows cross-sectional SEM images of the evaporated local Al-p+ samples of the three deposition processes 1 through 3. The images are obtained from an ultra-high resolution Hitachi S-4800 field emission SEM. The Al-doped p+-region appears brighter than the high-resistivity bulk of the silicon wafer due to the potential contrast [16]. Process 1 from Table 1 results in a temperature of -max = 560°C, which is below the eutectic temperature of 577°C, thus an Al-p+ region has not formed [see Figure 3 (a)]. Process 2 results in wafer temperatures of -max = 640°C, leading to the formation of an Al-p+ region. Nevertheless, Figure 3 (b) shows a discontinuous aluminum-doped region. The Process 3 from table 1, where a peak substrate temperature of -max = 778°C has been simulated, results in a completely closed Al-p+ region within the contact openings as shown in Figure 3 (c). From this SEM image we determine the Al-p+ depth to be (5 ± 3) μm.   Fig. 3. SEM micrographs of cross sections of samples processed with processes 13. The samples feature an Al2O3-SiNx passivation stack with line openings of a width of 85 μm before Al deposition. The line width increases during the formation of the Al-p+ layer to 120 μm. (a) SEM micrograph of a sample processed with process 1. The process temperature below 577°C is not leading to a local Al-p+ region. (b) Process 2 results in a higher maximum wafer temperatures of -max = 640°C, leading to the formation of a discontinuous Al-p+ region represented by the dotted line. The emitter thickness is (150 ± 150) nm. The Al-p+ region is clearly visible as brighter contrast. (c) Process 3 (-max = 778°C) results in a completely closed Al-p+ region as represented by the dotted line. The emitter thickness is (5 ± 3) μm. WS+WSĲ=ĲOAlbulkeffeff3211 Christoph Mader et al. / Energy Procedia 8 (2011) 521–526 525At temperatures above the eutectic temperature silicon dissolves into the aluminum layer. The passivation layer acts hereby as a diffusion barrier and the alloying process can only take place in the area of the local openings. This results in the surface morphology shown in Fig. 6 (b) and (c). Please note, that the width of the line contacts after the formation of the Al-p+ region increases to b = 120 μm. A detailed analysis of the contact formation of local Al alloyed Si contacts is given in Ref. [17]. Figure 4 shows the measured rear SRV Seff depending on Al layer thickness. Process 1 (see Table 1) with an Al layer thickness of 13 μm is not leading to the formation of an Al-p+ region. The corresponding recombination velocity is Seff = (114 ± 15) cm/s. Processes 2 and 3 (see Table 1) lead to the formation of an Al-p+ region and are thus reducing the effective SRV. Process 2 results in a SRV of (99 ± 10) cm/s and process 3 in a value of (50 ± 5) cm/s. Latter value is in the same range as published SRVs of screen-printed local Al-p+ regions [6].  Fig. 4. Measured effective surface recombination velocity as function of the Al layer thickness d of a 1.5 ȍcm FZ p-Si wafer. One side of the wafer is passivated with a stack of Al2O3 and SiNx. The other side is passivated with the same stack and features local laser openings of a width of 85 μm and a period length of 2000 μm. The local Al-p+ region forms at the interface of Al to Si in the area of the laser openings. We extract the contact recombination velocity Scont from the measured Seff values using equation [18]  ,    (2) where Rs is the series resistance of the majority carriers in the bulk, U is the base resistivity, W the sample thickness, D the diffusion coefficient and f the metallization fraction. Spass is the surface recombination velocity between and Scont the surface recombination velocity under the contacts. Using equation 2 we determine Scont = (7 r 2)u103 cm/s for process 1. Processes 2 and 3 are reducing the contact recombination velocity to values of (3 r 0.5)u103 cm/s and (1 r 0.1)u103 cm/s, respectively.  fSfSDWR=S-11 pass1contSeff ¸¸¹·¨¨©§UU526  Christoph Mader et al. / Energy Procedia 8 (2011) 521–526[15] Kerr M, Cuevas A. General parameterization of Auger recombination in crystall. silicon. J. Appl. Phys. 2002; 91 : 2473. [16]  Sealy CP, Castell MR, Wilshaw PR. Mechanism for secondary electron dopant contrast in the SEM. Journal of Electron Microscopy 2000; 49: 311-321. [17] Müller J, Bothe K, Gatz S, Plagwitz H, Schubert G, Brendel R. Recombination at local aluminum alloyed silicon solar cell base contacts by dynamic infrared lifetime mapping. Energy Procedia, 1st SiliconPV 2011, Freiburg. [18] Fischer B. Loss analysis of crystalline silicon solar cells using photoconductance and quantum efficiency measurements. PhD thesis, Cuvillier, Göttingen 2003. 4. Conclusion In conclusion, we have presented results on in-line high-rate evaporation of aluminum on silicon substrates. Using numerical simulations, we have shown that the deposition process leads to temperatures of -max = 778°C. These conditions lead to the formation of an Al-p+ region without any further temperature treatment. We determine a contact recombination velocity of 1000 r 100 cm/s for local Al-p+ regions on silicon wafers of 1.5 :cm resistivity resulting in an effective surface recombination velocity of only (50 r5) cm/s for a typical rear contacting scheme with a line width of b = 85 μm and a period length of p = 2000 μm. Acknowledgements This work is supported by the State of Lower Saxony.  References [1] Ralph E. Recent advancement in low cost solar cell processing. Proceedings of the 11th IEEE Photovoltaic Specialists Conference 1975; Scottsdale: 315 - 316. [2] Mader C, Bock R, Schmidt J, Brendel R. Formation of highly aluminum-doped p-type silicon regions by in-line high-rate evaporation. Sol. Energy Mater. Sol. Cells 2011, doi:10.1016/j.solmat.2011.01.039. [3] Mader C, Bock R, ISFH, Patent pending. [4] Agostinelli G, Choulat P, Dekkers H, De Wolf S, Beaucarne G. Screen printed larg area crystalline silicon solar cells on thin substrates Proceedings of the 20th European Photovoltaic Solar Energy Conference (WIP, Barcelona, 2005). pp. 647-650. [5] Ramanathan S, Das A, Cooper I, Rohatgi A, Payne A, Koehler I. 20% efficient screen printed LBSF cell fabricated using UV laser for rear dielectric removal. Proceedings of the 35th IEEE Photovoltaic Specialists Conference (IEEE, Hawaii, 2010). pp. 678-682. [6] Gatz S, Hannebauer H, Hesse R, Werner F, Schmidt A, Dullweber T, Schmidt J, Bothe K, Brendel R. 19.4%-efficient large-area fully screen-printed silicon solar cells. Phys. Status Solidi RRL (2011) 5, No. 4, 147–149. [7] Lauinger T, Schmidt J, Aberle AG, Hezel R. Record low surface recombination velocities on 1 :cm p-silicon using remote plasma silicon nitride passivation. Appl. Phys. Lett. 1996; 68: 1233. [8] Mader C, Eitner U, Brendel R. Experimentelle Untersuchungen und Simulationsrechnungen zur Temperaturentwicklung bei Hochraten- Aufdampfprozessen. Workshop “Hochratenverdampfen für die Photovoltaik” 2009; Hameln. [9] Mader C, Eitner U, Kessler M, Brendel R. Temperature of silicon wafers during in-line high-rate evaporation of aluminum. Sol. Energy Mater. Sol. Cells, submitted. [10] Hoex B, Heil S, Langereis E, van de Sanden M, Kessels W. Ultralow surface recombination of c-Si substrates passivated by plasma-assisted atomic layer deposited Al2O3. J. Appl. Phys. 2006; 89: 042112. [11] Schmidt J, Merkle A, Brendel R, Hoex B, van de Sanden M, Kessels W. Surface passivation of high-efficiency silicon solar cells by atomic-layer-deposited Al2O3. Prog. Photovolt: Res. Appl. 2008; 16:461–466. [12] Ramspeck K, Reissenweber S, Schmidt J, Bothe K, Brendel R. Dynamic carrier lifetime imaging of silicon wafers using an infrared-camera-based approach. Appl. Phys. Lett. 2008; 93: 102104. [13] Müller J, Bothe K, Gatz S, Haase F, Mader C, Brendel C. Recombination at laser-processed local base contacts by dynamic infrared lifetime mapping. J. Appl. Phys. 2010:108; 124513. [14] Kane DE, Swanson RM. Measurement of the emitter saturation current by a contactless photoconductivity decay method. Proceedings of the 18th IEEE Photovoltaic Specialists Conference; Las Vegas: 578. 

Formation of locally aluminum-doped p-type silicon regions by in-line high-rate evaporation

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Formation of locally aluminum-doped p-type silicon regions by in-line high-rate evaporation

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