Abstract In this work, the influence of the c-Si surface finishing (hydrophobic/hydrophilic) prior to the deposition of the Al2O3 passivation layer on the passivation quality is investigated. The samples are characterized by a combination of Quasi-Steady-State-PhotoConductance (QSSPC) Capacity-Conductance (CV), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transformed InfraRed (FTIR) measurements. Furthermore, FTIR measurements are used to determine the thickness of interfacial SiOx layer

Goverde, H.

Horzel, J.

John, J.

Meneghesso, G.

Morato, A.

Poortmans, J.

Vermang, B.

English

In this work, the influence of the c-Si surface finishing (hydrophobic/hydrophilic) prior to the deposition of the Al2O3 passivation layer on the passivation quality is investigated. The samples are characterized by a combination of Quasi-Steady-State-PhotoConductance (QSSPC) Capacity-Conductance (CV), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transformed InfraRed (FTIR) measurements. Furthermore, FTIR measurements are used to determine the thickness of interfacial SiOx layer

H. Goverde

B. Vermang

A. Morato

J. John

J. Horzel

J. Poortmans

MENEGHESSO, GAUDENZIO

Archivio istituzionale della ricerca - Università di Padova

Al2O3 Surface Passivation Characterized on Hydrophobic and Hydrophilic c-Si by a Combination of QSSPC, CV, XPS and FTIR

Open Access Repository

 Energy Procedia  27 ( 2012 )  355 – 360 1876-6102 © 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientifi c committee of the SiliconPV 2012 conference.doi: 10.1016/j.egypro.2012.07.076 SiliconPV: April 03-05, 2012, Leuven, Belgium Al2O3 surface passivation characterized on hydrophobic and hydrophilic c-Si by a combination of QSSPC, CV, XPS and FTIR H. Goverdea,b, B. Vermanga,c, A. Moratoa,d, J. Johna, J. Horzela, G. Meneghessod, J. Poortmansa,c aIMEC, Kapeldreef 75, 3001 Heverlee, Belgium  bEindhoven University of Technology (TU/e), P.O. Box 513, 5600 MB Eindhoven, The Netherlands cKatholieke Universiteit Leuven (ESAT), Kasteelpark Arenberg 10, 3001 Heverlee, Belgium dDept. of Information Engineering University of Padova, Via Gradenigo 6B, 35131 Padova, Italy  Abstract In this work, the influence of the c-Si surface finishing (hydrophobic/hydrophilic) prior to the deposition of the Al2O3 passivation layer on the passivation quality is investigated. The samples are characterized by a combination of Quasi-Steady-State-PhotoConductance (QSSPC) Capacity-Conductance (CV), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transformed InfraRed (FTIR) measurements. Furthermore, FTIR measurements are used to determine the thickness of interfacial SiOx layer.    © 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific committee of the SiliconPV 2012 conference  Keywords: Si; Surface Passivavation; Al2O3; FTIR; CV; XPS; QSSPC 1. Introduction  Thin (<30 nm) Al2O3 layers has been proven to provide an excellent level of surface passivation of n- and p-type c-Si due to its unique property of a high negative fixed charge density (Qf) and a low interface defect density (Dit) [1]. In the field-effective passivation mechanism, interfacial SiOx layers between the bulk Al2O3 and the c-Si play an important role [2-4]. The thickness and the quality of the SiOx interfacial layer is influenced by the c-Si surface state prior to deposition and the post-deposition anneal [1-6].  In this work, Al2O3 films were deposited on c-Si with oxidized or H-terminated surfaces. Prior and after post-deposition anneal, effective minority carrier lifetimes were measured using QSSPC. For Al2O3 layers on Czochralski (CZ) grown silicon, the fixed charge density and the interface defect density were determined using Capacitance-Voltage (C-V) measurements. Al2O3/SiOx structures layers were characterized using non-destructive Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Vailable online at www.sciencedirect.com 12 Published by Elsevier Ltd. Sel ction and peer-revi w under r sponsibility of the sci ntifi c committee of the SiliconPV 2012 conference. Open access under CC BY-NC-ND license.Open access under CC BY-NC-ND license.356   H. Goverde et al. /  Energy Procedia  27 ( 2012 )  355 – 360 Photoelectron Spectroscopy (XPS). Furthermore, it will be shown that the thickness of the interfacial SiOx layer can be calculated using non-destructive FTIR measurements.  2. Experimental Al2O3 layers (30 nm) were deposited using thermal Atomic Layer Deposition (ALD) (Al(CH3)3 + H2O) at a temperature (Tdep) of 200 ˚C, on both sides of 4” mirror polished p-type Float-Zone (FZ) c-Si with a resistivity of 2 Ω·cm and a thickness of 200 μm. Table 1 shows the cleaning process prior to Al2O3 deposition by ALD.  Table 1. Schematic overview of c-Si cleaning process to achieve the hydrophobic (left) or hydrophilic (right) c-Si surface state Hydrophobic   Hydrophilic   Solution Time   Solution Time 1 H2O + HF (2%) + HCl (5%) 5 min.  1. H2O + HF (2%) + HCl (5%) 5 min.     2. NH4OH + H2O2 + H2O (1:1:5) 10 min.  The samples received a post-deposition thermal anneal in N2 environment for 20 min. at a temperature of 300, 500 or 800 ˚C. FTIR spectra were constructed from 400 to 4000 cm-1 by a Brucker TENSOR FT-IR spectrometer. Furthermore, the effective carrier lifetimes in the c-Si sample (τeff) were measured by QSSPC measurement using a Sinton instrument lifetime tester in the generalized mode at an injection level of Δn = 1×1014 (low) or at Δn = 1×1015 (high) cm-3. Prior and after post-deposition anneal (20 min. 300 ˚C), the thickness of the interfacial SiOx layer between a 3 nm thick Al2O3 layer and the c-Si surface was measured using a Thermo Fish Theta 300 XPS spectrometer at an analyzing angle of 28˚.  Furthermore, Al2O3 films with a thickness of 5, 10, 15 or 20 nm were synthesized by an equivalent deposition process as described above, on both sides of 4” Saw Damage Removed (SDR) p-type Czochralski (CZ) c-Si with a resistivity of 1.5 Ω·cm and a thickness of 150 ± 10 μm with hydrophilic or hydrophobic surfaces. The Al2O3 layers were removed from the rear by HF vapor, prior to the deposition of 2 μm thick Al layer by e-beam evaporation. Pt dots electrodes with an area of (2.0 ± 0.5) × 10-3 cm2 were sputtered on the front, after the activation of the passivation layer by a thermal anneal as described above. Capacity-Voltage and Conductance-Voltage measurements were recorded using HP 4156 precision LCR-meter at low and high (1 – 100 kHz) frequency. All measurements were carried out at room temperature. 30 nm thick Al2O3 films were grown using the thermal ALD process as above, on both sides of  4” mirror polished p-type FZ c-Si with a resistivity of 2 Ω·cm and a thickness of 200 μm capped by a thermal grown SiOx layer with varying layer thickness. The SiOx layer thickness is varied by etching 100 nm thick, high quality, thermal grown (1050 ˚C) SiOx layers using a buffer HF solution. The thickness was determined by Spectroscopic Ellipsometry (SE) (SENTECH SE400adv-PV) prior to Al2O3 deposition. FTIR spectra from 400 to 4000 cm-1 were constructed by a Brucker TENSOR FT-IR spectrometer. 3. Result/Discussion 3.1. c-Si surface finishing Effective minority carrier lifetimes (Δn = 1×1015 cm-3) in H-terminated and oxidized c-Si samples passivated by Al2O3 are shown in Fig. 1(a) as a function of the post-deposition annealing temperature. Furthermore, the ratio between τeff at low (Δn = 1×1014 cm-3) injection level and high (Δn = 1×1015 cm-3) injection level is calculated, see Fig. 1(b).  H. Goverde et al. /  Energy Procedia  27 ( 2012 )  355 – 360 357Fig. 2(a) displays the fixed charge density and the interface defect density of c-Si samples passivated by an Al2O3 layer with a thickness of 10 nm as a function of the annealing temperature. Fig. 2(b) shows the thickness of the Al2O3 and interfacial SiOx layers prior and after a thermal anneal at 300 ˚C.    Fig. 1. Passivation properties of ALD Al2O3 deposited on c-Si with H-terminated or oxidized surface after an anneal at a temperature displayed on the x-axis (N2, 20 min.) (a) Effective carrier lifetime at Δn = 1×1015 cm-3 (b) ratio between effective carrier lifetime at low (Δn = 1×1014 cm-3) injection level and high (Δn = 1×1015 cm-3) injection level. The black line is a guide for the eye The Al2O3 layer and the Si/Al2O3 interface is furthermore characterized using FTIR. The absorption and differential spectrum of as-deposited Al2O3 layers is shown in Fig. 3(a) and Fig. 3(b) displays the absorption and differential spectrum of Al2O3 after a post-deposition anneal at 800 °C.  3.2. As-deposited For as-deposited layers, lower surface recombination velocities are observed for Al2O3 deposited on c-Si with H-terminated surface. This difference is caused by a higher interface defect density for Al2O3 deposited on c-Si with an oxidized surface, as shown in Fig. 2(a). Fig. 2(b) shows the presence of 1.0 nm thick interfacial SiOx layer, grown by the wet-chemical oxidation step. The low-quality oxide layer on the c-Si surfaces provides a poor chemical passivation, resulting in surface recombination velocity of 270 ± 20 cm·s-1. A surface recombination velocity of 11 ± 2 cm·s-1 has been observed in c-Si samples with hydrophobic surface prior Al2O3 deposition due to the low Dit.  A positive fixed charge density (Qf = (8 ± 2) ×1011 cm-2) is reported for as-deposited layers. The presence of positive charges is also indicated by the low τlow/τhigh ratio. Martin et al. hypothesized that an inversion layer in the subsurface region, present due to the absence of negative fixed charges, is responsible for the reduction of lifetime at low injection level [7].  Absorption peaks related to Al-O and O-H bonds are present in the FTIR spectrum. In contrast with Al2O3 layers synthesized by plasma-assisted ALD using Al(CH3)3 and an O2 plasma, no C-H absorption peak is observed in the noise signal [1,8]. The H2O precursor is less volatile than O radicals from the oxygen plasma; therefore, less C contamination is incorporated. The differential spectrum demonstrates by an absorption peak (1060 cm-1) which originates from the TO mode of thermally grown amorphous SiOx the difference between both Al2O3/Si interfaces [9]. The absorption peak indicates the presence of a interfacial SiOx layer between the Al2O3 layer and the c-Si with hydrophilic surface finishing. No significant difference is visible for the absorption peak associated with Si-H bonds. 358   H. Goverde et al. /  Energy Procedia  27 ( 2012 )  355 – 360   Fig. 2. (a) Qf and Dit of 10 nm as-deposited, low (≤ 500 ˚C) or high (> 500 ˚C) temperature annealed Al2O3 deposited on the hydrophilic or hydrophobic c-Si surface states. (b) Al2O3 and SiOx thickness of as-deposited and annealed (N2, 20 min. at 300 ˚C) c-Si samples with hydrophilic or hydrophobic surface 3.3. Post-deposition anneal An excellent level of passivation is reported for both surface finishing after a post-deposition anneal at Tann = 300 ˚C; Seff decreases to 7.5 ± 0.5 cm·s-1 and 22 ± 2 cm·s-1 for Al2O3 deposited on c-Si in the hydrophobic or hydrophilic surface state respectively. Fixed negative charges are formed in the passivation layer (Qf = - (1.0 ± 0.7) ×1012 cm-2 for Tann = 300 ˚C) and the interface defect density of c-Si samples with a hydrophilic surface state decreases while the Dit of passivated c-Si samples with hydrophobic surface finishing prior to Al2O3 deposition remained low (Dit = (5 ±1) ×1010 and (6 ± 1) ×1010 cm-2 for hydrophobic and hydrophilic surface finishing respectively after a thermal anneal at Tann = 300 ˚C). Also the ratio τlow/τhigh of both surface finishes indicates that no subsurface inversion layer is present after anneal due to the fixed negative charges in the passivation layer.  Fig 2(b) shows the formation of an interfacial SiOx layer between the Al2O3 and the hydrophobic c-Si surfaces after a thermal anneal at 300 ˚C while the thickness of the SiOx interfacial layer between Al2O3 and the hydrophilic c-Si surfaces remains constant. The formation of a high quality (confirmed by a low Dit) interfacial SiOx layer for both c-Si surface finishes is also depicted by an absorption peak at 1060 cm-1 in the FTIR spectrum.  For Tann = 500 ˚C, the fixed charge density (Qf = - (2.4 ± 0.4) ×1012 cm-2) increases compared with layers annealed at 300 ˚C, the interface defect density increases slightly (Dit = (10 ±1) ×1010 and (7 ± 1) ×1010 cm-2 for hydrophobic and hydrophilic surface finishing respectively, leading to an increase of the effective surface recombination velocity. No difference in surface passivation is observed between both surface finishes. By inspecting the differential spectrum, an interfacial SiOx layer of equivalent thickness is confirmed by the absence of an absorption peak at 1060 cm-1.  At Tann > 500 ˚C, surface recombination velocities increase due to blistering formation and an increase of the interface defect density (see Fig. 2(a)) [10]. Si-H bonds break at higher annealing temperature and H is effused from the c-Si interface resulting in dangling bonds, and therefore, an increase of the Dit [11]. The use of forming gas (H2) during the annealing would probably prevent the increase of Dit. Furthermore, fixed charges are removed from the c-Si surface due to blistering of the passivation layer. The negative fixed charge density is not affected by the high (> 500 ˚C) annealing temperature, leading to the conclusion that the level of passivation is reduced due to blistering formation and an increase of the interface defect density.   H. Goverde et al. /  Energy Procedia  27 ( 2012 )  355 – 360 359 Fig.  3. Infrared absorption spectra of 30 nm Al2O3 deposited on both sides of c-Si with hydrophobic or hydrophilic surfaces. The differential absorption spectrum is displayed to indicate the difference between the passivation layers. (a) The spectrum of as-deposited Al2O3 layers and (b) after a post-deposition anneal (N2, 20 min. at 500 ˚C). The spectra are off-set for clarity  3.4. Interfacial SiOx thickness As described in Section 3.1 above, the interfacial SiOx layer causes an absorption peak at 1060 cm-1 in the FTIR spectrum. For non-destructive determination of the thickness of the interfacial SiOx layer after annealing, FTIR spectra of Al2O3/SiOx/c-Si structures with 30 nm thick Al2O3 and varying SiOx thickness are constructed. Fig. 4 shows the absorbance at 1060 cm-1 as a function of the SiOx thickness.  Fig. 4. Absorbance at 1060 cm-1 of the Al2O3/SiOx/c-Si samples a function of the SiOx thickness. The SiOx thickness has been measured by SE prior to the deposition of the 30 nm thick Al2O3 layer by ALD  As displayed in Fig. 4, the absorbance at 1060 cm-1 depends linearly on the thickness of the SiOx layer. This can be understood, since, FTIR spectra apply to Lambert-Beer’s law:  -ln[I/I0]=αl  (1)  With I the transmittance intensity of the sample, I0 the absorbance of the reference sample (bare c-Si), the α absorbance coefficient and l the transmittance length. Eq. (1) shows that the logarithm of the absorption divided by the absorption of the reference sample depends linearly on the transmittance length. Therefore, Fig. 4 can be used to determine the thickness of the SiOx layer after a thermal treatment, under the assumption that the quality of the SiOx interlayer is equal to the quality of the thermally grown SiOx layer. The linear fit shows that α = (6.5 ± 0.1) × 10-3 nm-1, leading to a SiOx thickness of 2.0 ± 0.2 and 1.7 360   H. Goverde et al. /  Energy Procedia  27 ( 2012 )  355 – 360 ± 0.2 nm for the hydrophilic and hydrophobic c-Si finishing respectively after a post-deposition anneal at 500 ˚C, thus, no difference interfacial SiOx layer is observed between both c-Si surface finishes after a thermal anneal at 500 ˚C.  4. Conclusion The influence of the c-Si surface finishing on the characteristics of Al2O3 passivation layers has been investigated by a combination of FTIR, CV, XPS and QSSPC measurements. It has been proven that FTIR is powerful, non-destructive technique to characterize the thickness of the interfacial SiOx layer. The interface defect density is the dominating parameter for the surface passivation quality. The hydrophobic c-Si surface finishing is preferred because of the low Dit for as-deposited layers; however, an equivalent level of surface passivation has been observed after a thermal treatment. For both thicknesses, a high quality SiOx interfacial layer is formed during a thermal treatment.  Acknowledgements The authors greatly acknowledge the support of the IMEC Industrial Affiliated Partner (IIAP-PV) program and the IMEC PV support team. T. Bearda and S. Granada are kindly acknowledged for the FTIR measurements.  References [1] Hoex B, Heil SBS, Langereis E, van de Sanden MCM, Kessels WMM, Ultralow surface recombination of c-Si substrates passivated by plasma-assisted atomic layer deposition Al2O3, Appl Phys Lett, 2006;89, 04112 . [2] Agostinelli G, Delabie A, Vitanov P, Alexieva Z, Dekkers HFW, De Wolf S, Beaucarne G, Very low surface recombination velocities on p-type silicon wafers passivated with a dielectric with fixed negative charge, Sol Energ Mater Sol C, 2006; 90, 3438-3443  [3] Werner F, Veith B, Zielke D, Kuhnemund L, Tegenkamp C, Electronic and chemical properties of the c-Si/Al2O3 interface,  J Appl Phys, 2011; 109, 113701 [4] Benick J, Richter A, Li TTA, Grant NE, Mcintosh KR, Ren Y, Weber KJ, Hermle M, Glunz SW, Effect of a post-deposition anneal on Al2O3 properties, 35th IEEE PVSC, Honolulu, 2010; 2, 891-896  [5] Verlaan V. van den Elzen LRJG, Dingemans G, van de Sanden MCM, Kessels WMM, Composition and bonding structure of plasma-assisted ALD Al2O3 films, Phys Status Solidi C 7, 2010; 3-4 976-979  [6] Dingemans G, Terlinden NM, Verheijen MA, van de Sanden MCM, Kessels WMM, Controlling the fixed charges and passivation properties of Si(100)/Al2O3 interfaces using ultrathin SiO2 interlayers sythesized by atomic deposition,  J Appl Phys, 2011; 110, 093715 [7] Martin I, Hoex B, van de Sanden MCM, Alcubilla R, Kessels WMM, The origin of emitter-like recombination for inverted c-Si surfaces, Proc 23rd European PVSEC, Valencia, 2008; 1388  [8] Langereis E, Keijmel J, van de Sanden MCM, Kessels WMM, Surface chemistry of plasma-assisted atomic layer deposition of Al2O3 studied by infrared spectroscopy, Appl Phys Lett, 2008; 92, 231904 [9] Kirk CT, Quantitative analysis of the effect of disorder-induced mode coupling on infrared absorption in silica, Phys Rev B, 1988; 38, 1255-1273 [10] Vermang B, Goverde H, Uruena A, Lorenz A, Cornagliotti E,  Rothschild A, John J, Poortmans J, Mertens R, Blistering in ALD Al2O3 passsivation layers as rear contacting for local Al BSF Si solar cells, Sol Energ Mat Sol C, in press [11] Dingemans G, Engelhart P, Seguin R, Einsele F, Hoex B, van de Sanden MCM, Kessels WMM, Stability of Al2O3 and Al2O3/a-SiNx:H stacks for surface passivation of crytalline silicon, J Appl Phys, 2009; 106, 114907  

G. Meneghesso

Crossref

Energy Procedia

AbstractIn this work, the influence of the c-Si surface finishing (hydrophobic/hydrophilic) prior to the deposition of the Al2O3 passivation layer on the passivation quality is investigated. The samples are characterized by a combination of Quasi-Steady-State-PhotoConductance (QSSPC) Capacity-Conductance (CV), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transformed InfraRed (FTIR) measurements. Furthermore, FTIR measurements are used to determine the thickness of interfacial SiOx layer

Elsevier - Publisher Connector 

 Energy Procedia  27 ( 2012 )  355 – 360 
1876-6102 © 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientifi c committee of the 
SiliconPV 2012 conference.
doi: 10.1016/j.egypro.2012.07.076 
SiliconPV: April 03-05, 2012, Leuven, Belgium 
Al2O3 surface passivation characterized on hydrophobic and 
hydrophilic c-Si by a combination of QSSPC, CV, XPS 
and FTIR 
H. Goverdea,b, B. Vermanga,c, A. Moratoa,d, J. Johna, J. Horzela, G. Meneghessod, 
J. Poortmansa,c 
aIMEC, Kapeldreef 75, 3001 Heverlee, Belgium  
bEindhoven University of Technology (TU/e), P.O. Box 513, 5600 MB Eindhoven, The Netherlands 
cKatholieke Universiteit Leuven (ESAT), Kasteelpark Arenberg 10, 3001 Heverlee, Belgium 
dDept. of Information Engineering University of Padova, Via Gradenigo 6B, 35131 Padova, Italy 
 
Abstract 
In this work, the influence of the c-Si surface finishing (hydrophobic/hydrophilic) prior to the deposition of the Al2O3 
passivation layer on the passivation quality is investigated. The samples are characterized by a combination of Quasi-
Steady-State-PhotoConductance (QSSPC) Capacity-Conductance (CV), X-ray Photoelectron Spectroscopy (XPS) 
and Fourier Transformed InfraRed (FTIR) measurements. Furthermore, FTIR measurements are used to determine 
the thickness of interfacial SiOx layer.   
 
© 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific 
committee of the SiliconPV 2012 conference 
 
Keywords: Si; Surface Passivavation; Al2O3; FTIR; CV; XPS; QSSPC 
1. Introduction  
Thin (<30 nm) Al2O3 layers has been proven to provide an excellent level of surface passivation of n- 
and p-type c-Si due to its unique property of a high negative fixed charge density (Qf) and a low interface 
defect density (Dit) [1]. In the field-effective passivation mechanism, interfacial SiOx layers between the 
bulk Al2O3 and the c-Si play an important role [2-4]. The thickness and the quality of the SiOx interfacial 
layer is influenced by the c-Si surface state prior to deposition and the post-deposition anneal [1-6].  
In this work, Al2O3 films were deposited on c-Si with oxidized or H-terminated surfaces. Prior and 
after post-deposition anneal, effective minority carrier lifetimes were measured using QSSPC. For Al2O3 
layers on Czochralski (CZ) grown silicon, the fixed charge density and the interface defect density were 
determined using Capacitance-Voltage (C-V) measurements. Al2O3/SiOx structures layers were 
characterized using non-destructive Fourier Transform Infrared Spectroscopy (FTIR) and X-ray 
Vailable online at www.sciencedirect.com
 12 Published by Elsevier Ltd. Sel ction and peer-revi w under r sponsibility of the sci ntifi c committee of 
the SiliconPV 2012 conference. Open access under CC BY-NC-ND license.
Open access under CC BY-NC-ND license.
356   H. Goverde et al. /  Energy Procedia  27 ( 2012 )  355 – 360 
Photoelectron Spectroscopy (XPS). Furthermore, it will be shown that the thickness of the interfacial 
SiOx layer can be calculated using non-destructive FTIR measurements.  
2. Experimental 
Al2O3 layers (30 nm) were deposited using thermal Atomic Layer Deposition (ALD) (Al(CH3)3 + 
H2O) at a temperature (Tdep) of 200 ˚C, on both sides of 4” mirror polished p-type Float-Zone (FZ) c-Si 
with a resistivity of 2 Ω·cm and a thickness of 200 μm. Table 1 shows the cleaning process prior to Al2O3 
deposition by ALD.  
Table 1. Schematic overview of c-Si cleaning process to achieve the hydrophobic (left) or hydrophilic (right) c-Si surface state 
Hydrophobic   Hydrophilic  
 Solution Time   Solution Time 
1 H2O + HF (2%) + HCl (5%) 5 min.  1. H2O + HF (2%) + HCl (5%) 5 min. 
    2. NH4OH + H2O2 + H2O (1:1:5) 10 min. 
 
The samples received a post-deposition thermal anneal in N2 environment for 20 min. at a temperature 
of 300, 500 or 800 ˚C. FTIR spectra were constructed from 400 to 4000 cm-1 by a Brucker TENSOR FT-
IR spectrometer. Furthermore, the effective carrier lifetimes in the c-Si sample (τeff) were measured by 
QSSPC measurement using a Sinton instrument lifetime tester in the generalized mode at an injection 
level of Δn = 1×1014 (low) or at Δn = 1×1015 (high) cm-3. Prior and after post-deposition anneal (20 min. 
300 ˚C), the thickness of the interfacial SiOx layer between a 3 nm thick Al2O3 layer and the c-Si surface 
was measured using a Thermo Fish Theta 300 XPS spectrometer at an analyzing angle of 28˚.  
Furthermore, Al2O3 films with a thickness of 5, 10, 15 or 20 nm were synthesized by an equivalent 
deposition process as described above, on both sides of 4” Saw Damage Removed (SDR) p-type 
Czochralski (CZ) c-Si with a resistivity of 1.5 Ω·cm and a thickness of 150 ± 10 μm with hydrophilic or 
hydrophobic surfaces. The Al2O3 layers were removed from the rear by HF vapor, prior to the deposition 
of 2 μm thick Al layer by e-beam evaporation. Pt dots electrodes with an area of (2.0 ± 0.5) × 10-3 cm2 
were sputtered on the front, after the activation of the passivation layer by a thermal anneal as described 
above. Capacity-Voltage and Conductance-Voltage measurements were recorded using HP 4156 
precision LCR-meter at low and high (1 – 100 kHz) frequency. All measurements were carried out at 
room temperature. 
30 nm thick Al2O3 films were grown using the thermal ALD process as above, on both sides of  4” 
mirror polished p-type FZ c-Si with a resistivity of 2 Ω·cm and a thickness of 200 μm capped by a 
thermal grown SiOx layer with varying layer thickness. The SiOx layer thickness is varied by etching 100 
nm thick, high quality, thermal grown (1050 ˚C) SiOx layers using a buffer HF solution. The thickness 
was determined by Spectroscopic Ellipsometry (SE) (SENTECH SE400adv-PV) prior to Al2O3 
deposition. FTIR spectra from 400 to 4000 cm-1 were constructed by a Brucker TENSOR FT-IR 
spectrometer. 
3. Result/Discussion 
3.1. c-Si surface finishing 
Effective minority carrier lifetimes (Δn = 1×1015 cm-3) in H-terminated and oxidized c-Si samples 
passivated by Al2O3 are shown in Fig. 1(a) as a function of the post-deposition annealing temperature. 
Furthermore, the ratio between τeff at low (Δn = 1×1014 cm-3) injection level and high (Δn = 1×1015 cm-3) 
injection level is calculated, see Fig. 1(b). 
 H. Goverde et al. /  Energy Procedia  27 ( 2012 )  355 – 360 357
Fig. 2(a) displays the fixed charge density and the interface defect density of c-Si samples passivated 
by an Al2O3 layer with a thickness of 10 nm as a function of the annealing temperature. Fig. 2(b) shows 
the thickness of the Al2O3 and interfacial SiOx layers prior and after a thermal anneal at 300 ˚C.  
  
Fig. 1. Passivation properties of ALD Al2O3 deposited on c-Si with H-terminated or oxidized surface after an anneal at a 
temperature displayed on the x-axis (N2, 20 min.) (a) Effective carrier lifetime at Δn = 1×1015 cm-3 (b) ratio between effective carrier 
lifetime at low (Δn = 1×1014 cm-3) injection level and high (Δn = 1×1015 cm-3) injection level. The black line is a guide for the eye 
The Al2O3 layer and the Si/Al2O3 interface is furthermore characterized using FTIR. The absorption 
and differential spectrum of as-deposited Al2O3 layers is shown in Fig. 3(a) and Fig. 3(b) displays the 
absorption and differential spectrum of Al2O3 after a post-deposition anneal at 800 °C.  
3.2. As-deposited 
For as-deposited layers, lower surface recombination velocities are observed for Al2O3 deposited on c-
Si with H-terminated surface. This difference is caused by a higher interface defect density for Al2O3 
deposited on c-Si with an oxidized surface, as shown in Fig. 2(a). Fig. 2(b) shows the presence of 1.0 nm 
thick interfacial SiOx layer, grown by the wet-chemical oxidation step. The low-quality oxide layer on the 
c-Si surfaces provides a poor chemical passivation, resulting in surface recombination velocity of 270 ± 
20 cm·s-1. A surface recombination velocity of 11 ± 2 cm·s-1 has been observed in c-Si samples with 
hydrophobic surface prior Al2O3 deposition due to the low Dit.  
A positive fixed charge density (Qf = (8 ± 2) ×1011 cm-2) is reported for as-deposited layers. The 
presence of positive charges is also indicated by the low τlow/τhigh ratio. Martin et al. hypothesized that an 
inversion layer in the subsurface region, present due to the absence of negative fixed charges, is 
responsible for the reduction of lifetime at low injection level [7].  
Absorption peaks related to Al-O and O-H bonds are present in the FTIR spectrum. In contrast with 
Al2O3 layers synthesized by plasma-assisted ALD using Al(CH3)3 and an O2 plasma, no C-H absorption 
peak is observed in the noise signal [1,8]. The H2O precursor is less volatile than O radicals from the 
oxygen plasma; therefore, less C contamination is incorporated. The differential spectrum demonstrates 
by an absorption peak (1060 cm-1) which originates from the TO mode of thermally grown amorphous 
SiOx the difference between both Al2O3/Si interfaces [9]. The absorption peak indicates the presence of a 
interfacial SiOx layer between the Al2O3 layer and the c-Si with hydrophilic surface finishing. No 
significant difference is visible for the absorption peak associated with Si-H bonds. 
358   H. Goverde et al. /  Energy Procedia  27 ( 2012 )  355 – 360 
  
Fig. 2. (a) Qf and Dit of 10 nm as-deposited, low (≤ 500 ˚C) or high (> 500 ˚C) temperature annealed Al2O3 deposited on the 
hydrophilic or hydrophobic c-Si surface states. (b) Al2O3 and SiOx thickness of as-deposited and annealed (N2, 20 min. at 300 ˚C) c-
Si samples with hydrophilic or hydrophobic surface 
3.3. Post-deposition anneal 
An excellent level of passivation is reported for both surface finishing after a post-deposition anneal at 
Tann = 300 ˚C; Seff decreases to 7.5 ± 0.5 cm·s-1 and 22 ± 2 cm·s-1 for Al2O3 deposited on c-Si in the 
hydrophobic or hydrophilic surface state respectively. Fixed negative charges are formed in the 
passivation layer (Qf = - (1.0 ± 0.7) ×1012 cm-2 for Tann = 300 ˚C) and the interface defect density of c-Si 
samples with a hydrophilic surface state decreases while the Dit of passivated c-Si samples with 
hydrophobic surface finishing prior to Al2O3 deposition remained low (Dit = (5 ±1) ×1010 and (6 ± 1) 
×1010 cm-2 for hydrophobic and hydrophilic surface finishing respectively after a thermal anneal at Tann = 
300 ˚C). Also the ratio τlow/τhigh of both surface finishes indicates that no subsurface inversion layer is 
present after anneal due to the fixed negative charges in the passivation layer.  
Fig 2(b) shows the formation of an interfacial SiOx layer between the Al2O3 and the hydrophobic c-Si 
surfaces after a thermal anneal at 300 ˚C while the thickness of the SiOx interfacial layer between Al2O3 
and the hydrophilic c-Si surfaces remains constant. The formation of a high quality (confirmed by a low 
Dit) interfacial SiOx layer for both c-Si surface finishes is also depicted by an absorption peak at 1060 cm-
1 in the FTIR spectrum.  
For Tann = 500 ˚C, the fixed charge density (Qf = - (2.4 ± 0.4) ×1012 cm-2) increases compared with 
layers annealed at 300 ˚C, the interface defect density increases slightly (Dit = (10 ±1) ×1010 and (7 ± 1) 
×1010 cm-2 for hydrophobic and hydrophilic surface finishing respectively, leading to an increase of the 
effective surface recombination velocity. No difference in surface passivation is observed between both 
surface finishes. By inspecting the differential spectrum, an interfacial SiOx layer of equivalent thickness 
is confirmed by the absence of an absorption peak at 1060 cm-1.  
At Tann > 500 ˚C, surface recombination velocities increase due to blistering formation and an increase 
of the interface defect density (see Fig. 2(a)) [10]. Si-H bonds break at higher annealing temperature and 
H is effused from the c-Si interface resulting in dangling bonds, and therefore, an increase of the Dit [11]. 
The use of forming gas (H2) during the annealing would probably prevent the increase of Dit. 
Furthermore, fixed charges are removed from the c-Si surface due to blistering of the passivation layer. 
The negative fixed charge density is not affected by the high (> 500 ˚C) annealing temperature, leading to 
the conclusion that the level of passivation is reduced due to blistering formation and an increase of the 
interface defect density.  
 H. Goverde et al. /  Energy Procedia  27 ( 2012 )  355 – 360 359
 
Fig.  3. Infrared absorption spectra of 30 nm Al2O3 deposited on both sides of c-Si with hydrophobic or hydrophilic surfaces. The 
differential absorption spectrum is displayed to indicate the difference between the passivation layers. (a) The spectrum of as-
deposited Al2O3 layers and (b) after a post-deposition anneal (N2, 20 min. at 500 ˚C). The spectra are off-set for clarity  
3.4. Interfacial SiOx thickness 
As described in Section 3.1 above, the interfacial SiOx layer causes an absorption peak at 1060 cm-1 in 
the FTIR spectrum. For non-destructive determination of the thickness of the interfacial SiOx layer after 
annealing, FTIR spectra of Al2O3/SiOx/c-Si structures with 30 nm thick Al2O3 and varying SiOx thickness 
are constructed. Fig. 4 shows the absorbance at 1060 cm-1 as a function of the SiOx thickness. 
 
Fig. 4. Absorbance at 1060 cm-1 of the Al2O3/SiOx/c-Si samples a function of the SiOx thickness. The SiOx thickness has been 
measured by SE prior to the deposition of the 30 nm thick Al2O3 layer by ALD  
As displayed in Fig. 4, the absorbance at 1060 cm-1 depends linearly on the thickness of the SiOx layer. 
This can be understood, since, FTIR spectra apply to Lambert-Beer’s law: 
 
-ln[I/I0]=αl  (1) 
 
With I the transmittance intensity of the sample, I0 the absorbance of the reference sample (bare c-Si), 
the α absorbance coefficient and l the transmittance length. Eq. (1) shows that the logarithm of the 
absorption divided by the absorption of the reference sample depends linearly on the transmittance length. 
Therefore, Fig. 4 can be used to determine the thickness of the SiOx layer after a thermal treatment, under 
the assumption that the quality of the SiOx interlayer is equal to the quality of the thermally grown SiOx 
layer. The linear fit shows that α = (6.5 ± 0.1) × 10-3 nm-1, leading to a SiOx thickness of 2.0 ± 0.2 and 1.7 
360   H. Goverde et al. /  Energy Procedia  27 ( 2012 )  355 – 360 
± 0.2 nm for the hydrophilic and hydrophobic c-Si finishing respectively after a post-deposition anneal at 
500 ˚C, thus, no difference interfacial SiOx layer is observed between both c-Si surface finishes after a 
thermal anneal at 500 ˚C.  
4. Conclusion 
The influence of the c-Si surface finishing on the characteristics of Al2O3 passivation layers has been 
investigated by a combination of FTIR, CV, XPS and QSSPC measurements. It has been proven that 
FTIR is powerful, non-destructive technique to characterize the thickness of the interfacial SiOx layer. 
The interface defect density is the dominating parameter for the surface passivation quality. The 
hydrophobic c-Si surface finishing is preferred because of the low Dit for as-deposited layers; however, an 
equivalent level of surface passivation has been observed after a thermal treatment. For both thicknesses, 
a high quality SiOx interfacial layer is formed during a thermal treatment.  
Acknowledgements 
The authors greatly acknowledge the support of the IMEC Industrial Affiliated Partner (IIAP-PV) 
program and the IMEC PV support team. T. Bearda and S. Granada are kindly acknowledged for the 
FTIR measurements.  
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Al2O3 Surface Passivation Characterized on Hydrophobic and Hydrophilic c-Si by a Combination of QSSPC, CV, XPS and FTIR 

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Al2O3 Surface Passivation Characterized on Hydrophobic and Hydrophilic c-Si by a Combination of QSSPC, CV, XPS and FTIR

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