Surface passivation for Si solar cells: a combination of advanced surface cleaning and thermal atomic layer deposition of Al2O3 Bart Vermang1,2,a, Aude Rothschild1,b, K. Kenis1, K. Wostyn1, T. Bearda1, 3 1 1 1 1,2 1,2 A. Racz , X. Loozen , J. John , P. Mertens , J. Poortmans , R. Mertens 1
imec, Kapeldreef 75, 3001 Heverlee, Belgium Katholieke Universiteit Leuven (ESAT), Kasteelpark Arenberg 10, 3001 Heverlee, Belgium 3 University of Debrecen, Pf. 2., Bem tér 18/b, 4010 Debrecen, Hungary a b
[email protected],
[email protected]
2
Keywords: Si, atomic layer deposition, ALD, Al 2O3, surface passivation, cleaning
Thermal atomic layer deposition (ALD) of Al 2O3 provides an adequate level of surface passivation for both p-type and n-type Si solar cells. To obtain the most qualitative and uniform surface passivation advanced cleaning development is required. The studied pre deposition treatments include an HF (Si-H) or oxidizing (Si-OH) last step and finish with simple hot-air drying or more sophisticated Marangoni drying. To examine the quality and uniformity of surface passivation - after cleaning and Al2O3 deposition - carrier density imaging (CDI) and quasi-steady-state photo-conductance (QSSPC) are applied. A hydrophilic surface clean that leads to improved surface passivation level is found. Si-H starting surfaces lead to equivalent passivation quality but worse passivation uniformity. The hydrophilic surface clean is preferred because it is thermodynamically stable, enables higher and more uniform ALD growth and cons equently exhibits better surface passivation uniformity. Introduction The present photovoltaic (PV) market is dominated by crystalline Si solar cells. Its cumulative level of capacity announced in 2009 was 24 GW, of which 78 % c-Si modules and 22 % thin film modules (including Si-based thin film). The European PV Industry Association (EPIA) expects these announced capacities to grow by about 30 % in 2010 after which the compound annual growth rate will level off to about 20 % during later years to exceed 65 GW in 2014 [1]. As can be seen, it is generally expected that the Si dominance in the PV market will continue for at least the next decade. In solar cell technology, given that ever thinner wafers imply an increased surface-to-volume ratio, sufficient surface passivation gains importance. The cost of Si constitutes about 1/3 of the module cost [2]. Therefore, in order to be less dependent on price fluctuations of poly-silicon feedstock and wafers, and to eventually realize cost targets significantly below € 1.00/Wp, an evolution towards a reduction of “grams of pure Si/Wp” is taking place. As solar cell efficiency cannot be sacrificed for the trend to ever thinner wafers, this requires quite drastic and cost-effective improvements in crystalline Si solar cell technology. A next generation material for surface passivation is Al2O3. It has been shown that both thermal and plasma-assisted (PA) atomic layer deposition (ALD) Al2O3 provide an adequate level of surface passivation for both p-type and n-type Si substrates. The underlying mechanism is based on chemical and field-effect passivation [3,4,5,6,7]. Lab-scale high-efficiency [8, 9,10,11,12] and large-area industrial type [13, 14,15] Si solar cells with ALD Al2O3 passivation layers have previously been demonstrated, using passivated emitter and rear cell (PERC) as well as passivated emitter and rear locally diffused (PERL) cell concepts. To obtain the most qualitative and uniform level of surface passivation advanced cleaning development is required. This cleaning can leave an oxidized Si-OH surface (hydroxylated SiO 2) or an HF last Si-H surface. H-terminated Si surfaces are well-known to exhibit substrate-inhibited or island
growth due to a lack of nucleation, whereas oxidized surfaces cause a more rapidly and more uniform growth mechanism [16,17]. In addition, Si-H surfaces are identified as unstable (due to radicals and hydrocarbons); an interfacial native oxide is formed rather rapidly in air ambient [18]. Description of the work In this work, advanced treatments derived from semiconductor industry are compared to conventional cleanings used in PV. The studied pre-deposition treatments have an HF-last or oxidizing last step and finish with a simple hot-air drying or more sophisticated Marangoni drying. To examine the quality and uniformity of surface passivation after cleaning and Al2O3 deposition, carrier density imaging (CDI) and quasi-steady-state photo-conductance (QSSPC) are applied. By using the 200 mm clean-room apparatus at imec, large area wafers can be cleaned in a controlled manner. Hence, surface passivation characterization is performed on 200 mm wafers with a typical solar cell p-type base resistivity: polished p-type CZ Si samples of 1.75 Ω.cm resistivity and 710 μm thickness. As listed in table I, advanced treatments derived from semiconductor industry (a,b and c) and conventional cleanings used in PV (d) are applied, leaving Si-OH (a,b) or Si-H (c,d) terminated surfaces. Wafer drying is done by using a Marangoni dryer (a, b and c) or a simple hot-air dryer (d). The Marangoni effect is introduced by a surface-tension gradient in the wetting film (IPA:H 2O) on the substrate, causing the water film quickly drain backwards into the rinse bath. As a result, a completely dry substrate emerges from the bath [19]. Table I: Cleaning sequences, leaving Si-H (HF last) or Si-OH (chemical oxidation last) terminated surfaces. a b c d SPM* HF* DIW:O3:HCl APM** Marangoni dryer Hot air dryer Marangoni dryer Si-OH Si-OH Si-H Si-H * SPM = H2O2:H2SO4 and ** APM = NH4OH:H2O2:H2O Thermal ALD Al2O3 films with a thickness of 30 nm are grown in a commercial (Cambridge Nanotech, Savannah S200) ALD reactor at a substrate temperature of 200 °C. The used precursors are trimethylaluminium (TMA) and de-ionized (DI) water. QSSPC is used to quantify the surface passivation by measuring the effective lifetime (τ eff). It is measured by a lifetime tester (Sinton WCT-100) in the generalized approach [20]. In first approximation τ eff of a symmetrically passivated c-Si wafer can be written as in Eq. 1.
1
eff
1
bulk
2S eff W
(1)
With τ bulk the bulk lifetime, W the c-Si wafer thickness and S eff the effective surface recombination velocity [21]. An upper limit of the S eff is estimated by setting τ bulk as infinity. On the other hand, CDI is used to qualify the passivation uniformity. It has been introduced by Isenberg, Riepe and co-workers [22,23] and is a further development of infrared lifetime mapping (ILM). In this study, CDI is used in absorption mode. Discussion of results
Wafers cleaned as listed in table I are covered on both sides by 30 nm of thermal ALD Al2O3 (a0, b0, c0 and d0) and subsequently given a 30 min forming gas (FG) anneal at 350 °C (a1, b1, c1 and d1) or the anneal leading to the best passivation, at a temperature between 300 and 550 °C in N 2 or FG environment for 15 or 30 minutes (a2, b2, c2 and d2). CDI pictures for samples a1 to d1 and QSSPC measurements of a0 to d0 and a2 to d2 can be found in respectively Fig. 1 and Fig. 2. It is clear that (i) in Fig. 1, the surface passivation uniformity is best for samples a1 and b1: there are almost no traces of contamination (defects or particles) or drying marks and (ii) in Fig. 2, the lowest S eff is obtained on samples b2 and c2. Consequently, the best surface passivation uniformity and quality is obtained by making use of chemical treatment b.
Fig. 1: Carrier density images of double side polished p-type CZ Si samples of 1.75 Ω.cm resistivity and 710 μm thickness, passivated at both sides by 30 nm Al2O3 and annealed in forming gas at 350 °C for 30 minutes. The sample size is ¼ of a 200 mm wafer and the scale is in arbitrary units: white and black resemble respectively the lowest and highest carrier densities. The large circle of lower carrier density found in the middle of each image is an artifact caused by reflection of the mirror polished wafers. a0
b0
c0
b2
c2
d2
1000
Seff (cm/s)
Seff (cm/s)
1000
10 1E+13
a2
d0
1E+14 1E+15 1E+16 Excess carrier density (cm-3)
1E+17
10 1E+13
1E+14 1E+15 1E+16 Excess carrier density (cm-3)
1E+17
Fig. 2: Effective surface recombination velocities of double side polished p-type CZ Si samples of 1.75 Ω.cm resistivity and 710 μm thickness. Samples a0 to d0 and a2 to d2 are passivated at both sides by 30 nm Al2O3 and respectively as-deposited or annealed in N 2 or forming gas at an optimal temperature. In Fig. 2, it is also apparent that the annealing effect is different for Si-H or Si-OH samples. After Al2O3 deposition, the chemically oxidized samples show higher S eff compared to HF-last samples. However, after the most advantageous anneal in N 2 or FG equivalent S eff can be obtained. Nevertheless, this is not valid for all chemical oxidations as can be seen by comparing samples a2 to b2. Even after optimal annealing, sample a2 never arrives at equivalent low S eff as samples b2 and c2. If a Si-OH sample dried in hot air is used, CDI images look like samples c1 or d1 in Fig. 1. Hence, it becomes obvious that almost no contamination or drying marks are added to the surface during Marangoni drying, which is not the case for basic hot air drying.
Furthermore, the surface of cleaned wafers a, b and c is scanned by a KLA/Tencor SP1 DLS demonstrating that samples c contained up to 8 times more light point defects (LPD’s = point defects detected by scattering light) compared to samples a and b. Conclusions It is concluded that advanced cleaning is key to obtain optimal surface passivation using a low temperature deposition method as thermal ALD Al2O3. As well HF-last (Si-H) as oxidized (Si-OH) treatments can lead to optimal surface passivation quality. Nevertheless, Si-OH surfaces are preferred because they are thermodynamically stable and enable higher and more uniform ALD growth. Consequently, Si-OH terminated surfaces lead to better surface passivation uniformity. The best chemical oxidation tested is the ammonia peroxide mixture (APM), which is expected to be too expensive to be industrially applicable. Therefore, also cheaper alternatives are screened and recent results show that chemical treatment a can be optimized to obtain similar results as in the case of cleaning b. Also the drying method is essential: the better developed Marangoni drying clearly adds almost no contamination or drying marks, contrarily to hot air drying generally used in PV. Due to cost consideration, also drying in a controlled N 2 environment is being investigated: in certain circumstances it leads to comparable CDI results as Marangoni drying. Supplementary research is ongoing. Chemical oxidized wafers typically have an oxide layer of barely 0.5 to 1.5 nm thick and therefore are difficult to characterize. However, more characterization as for example X-ray photo-electron or infrared spectroscopy (XPS or IRS) is required. In addition, the better stability in air of Si-OH surfaces as compared to Si-H surfaces has to be quantified. Finally, it has to be verified that the enhanced surface passivation is evident at solar cell level. References [1] EPIA Global market outlook for photovoltaics until 2014 , www.epia.org (2010) [2] W. C. Sinke, C. del Cañizo, and G. del Coso, Proc. 23rd EU PVSEC, p. 3700, Valencia, Spain (2008) [3] G. Agostinelli, A. Delabie, P. Vitanov, Z. Alexieva, H. F. W. Dekkers, S. De Wolf, and G. Beaucarne, Sol. Energ. Mat. Sol. C. 90, 3438-3443 (2006) [4] B. Hoex, J. Schmidt, R. Bock, P. P. Altermatt, M. C. M. van de Sanden, and W. M. M. Kessels, Appl. Phys Lett. 91, 112107 (2007) [5] B. Hoex, J. Schmidt, P. Pohl, M. C. M. van de Sanden, and W. M. M. Kessels, J. Appl. Phys. 104, 044903 (2008) [6] J. J. H. Gielis, B. Hoex, M. C. M. van de Sanden, and W. M. M. Kessels, J. Appl. Phys. 104, 073701 (2008) [7] N. M. Terlinden, G. Dingemans, M. C. M. van de Sanden, and W. M. M. Kessels, Appl. Phys. Lett. 96, 112101 (2010) [8] J. Schmidt, A. Merkle, R. Brendel, B. Hoex, M. C. M. van de Sanden and W. M. M. Kessels, Prog. Photovolt. 16, 461-466 (2008) [9] J. Benick, B. Hoex, M. C. M. van de Sanden, W. M. M. Kessels, O. Schultz and S. W. Glunz, Appl. Phys Lett. 92, 253504 (2008) [10] J. Benick, B. Hoex, O. Schultz, and S. W. Glunz, Proc. 33rd IEEE PVSC, p. 1 (2008) [11] J. Benick, B. Hoex, G. Dingemans, W. M. M. Kessels, A. Richter, M. Hermle, and S. W. Glunz, Proc. 24th EU PVSEC, p. 863, Hamburg, Germany (2009)
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