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Institute Silicon Photovoltaics

Wet-chemical Conditioning of Silicon surfaces

The influence of the most important wet-chemical cleaning and passivation procedures on the electronic properties of Si interfaces was investigated by combining two surface sensitive techniques, the spectroscopic ellipsometry (SE) and the surface photo voltage method (SPV).

 

enlarged view

AFM images and Dit,min as a function of the effective surface micro-roughness <dr>, obtained on n- and p-type Si(111) after etching in HF (48%) (0 s to 600 s) (curves1 and 2), and on initially atomically flat p-type Si(111) surfaces prepared by wet-chemical oxidation in (i) RCA II solution, (ii) in H2SO4/H2O2 and (iii) in DIW at 80◦C and subsequent oxide removal in NH4F solution (curve 3)

The band-bending and the energetic distribution of the interface states Dit(E) were determined by contactless SPV measurements. For sensing the surface roughness and the oxide coverage, SE measurements in the ultra-violet and visible (UV-VIS) region were applied.

In contrast to recently published attenuated total reflection (ATR) measurements, which require a special Si crystal geometry, here, for the first time, the hydrogen coverage of the Si surface was successfully detected by Fourier-transform infrared ellipsometry (FT IR SE) using a single light reflection only.

By simultaneous SPV and SE investigations of differently HF etched, H terminated and wet-chemically oxidized Si wafers during the wet-chemical process and storage in clean-room air, correlations were established between the preparation induced surface morphology and the density and energetic distributions of surface and interface states. It was shown that the electronic properties of wet-chemically treated surfaces and the stability of the surface passivation strongly depend on the resulting surface micro-roughness and remaining surface coverage.

The results of these investigations have been successfully used for the optimization of a H termination procedure yielding atomically smooth Si(111) surfaces without any native oxide coverage, which are characterised by an intrin¬sic surface state distribution and a very low surface state density Dit,min< 2x1010 cm-2eV-1. By using a newly developed hot water treatment on H terminated Si(111) and Si(100) surfaces, very thin passivating oxide layers (5 to 25 Å) were obtained with surface state densities of about  4x 1011 cm-2eV-1 that are significantly lower than those of conven¬tionally prepared chemical and thermal oxides.

These methods were successfully applied for surface conditioning of different smoothed and textured Si solar cell substrates, carried out before preparation of hydrogenated, amorphous Si (a-Si:H)/c-Si, Si carbide (a-SiC:H)/c-Si, Si nitride (a-SiNx:H)/c-Si hetero-junctions und various other passivation (AlOx) and contact systems.

dit

Dit(E) distributions as obtained by SPV measurements on polished Si surfaces after (1) thermal oxidation (dox∼=1000˚A), (2) H-termination by NH4F treatment (dr∼=1˚A), and on ultra-thin wet-chemical oxides (dox∼=10 . . . 20˚A) prepared in (3) ultra pure water 80◦C (dox∼=15˚A) or in concentrated solution of (4) HNO3130◦C, (5) H2SO4/H2O2, (6) NH4OH/H2O2/H2O: RCA I), and (7) HCl/H2O2/H2O:RCA II

Surface morphology and electronic properties of wet-chemically prepared Silizium (111)-surfaces

Stability of H-terminierted Si surfaces during storage on clean-room air

Stabilität

Duration of the initial phase of oxidation, tini, in clean-room air as a function of the initial Dit,min on H-terminated Si(111) prepared by (i) by hot water oxidation + NH4F (<dr> 1 Å), (ii) by RCA + NH4F (<dr> 3 Å), (iii) by HF 48 % treatments (<dr> 4,5 …12 Å). The time tini was obtained by UV-VIS SE measurements of the oxide-thickness reaching one monolayer

publication

  • Angermann, H.; Balamou, P.; Lu, W.; Korte, L.; Leendertz, C.; Stegemann, B.: Oxidation of Si surfaces: Effect of ambient air and water treatments on surface charge and interface state density. Solid State Phenomena 255 (2016), p. 331-337
  • Angermann, H.; Laades, A.; Kegel, J.; Klimm, C.; Stegemann, B.: Improvement of silicon solar cell substrates by wet-chemical oxidation studied by surface photovoltage measurements. Solid State Phenomena 219 (2015), p. 291-296
  • Balamou, P.; Angermann, H.; Stegemann, B.: Reduction of the Interface Defect Density on Crystalline Silicon Solar Cell Substrates by Wet-chemical Preparation of Ultrathin SiOx Passivation Layers. In: Photovoltaic Specialist Conference (PVSC) New Orleans, LA, 2015 IEEE 42nd. IEEE Journal of Photovoltaics 5, 2015. - ISBN 978-1-4799-7944-8, p. 1-5
  • Angermann, H.: Conditioning of Si-interfaces by wet-chemical oxidation: electronic interface properties study by surface photovoltage measurements. Applied Surface Science 312 (2014), p. 3-16
  • Lu, W.; Leendertz, C.; Korte, L.; Töfflinger, J.A.; Angermann, H.: Passivation properties of subnanometer thin interfacial silicon oxide films. Energy Procedia 55 (2014), p. 805-812 
  • Angermann, H.; Stürzebecher, U.; Kegel, J.; Gottschalk, C.; Wolke, K.; Laades, A.; Conrad, E.; Klimm, C.; Stegemann, B.: Wet-chemical conditioning of H-terminated silicon solar cell substrates investigated by surface photovoltage measurements. Solid State Phenomena 195 (2013), p. 301-304
  • Angermann, H.; Wolke, K.; Gottschalk, Ch.; Moldavan, A.; Roczen, M.; Fittkau, J.; Zimmer, M.; Rentsch, J.: Surface charge and interface state density on Silicon substrates after Ozone based wet-chemical oxidation and Hydrogen-termination. Solid State Phenomena 195 (2013), p. 314-317
  • Angermann, Heike; Wolke, Klaus; Gottschalk, Christiane; Moldovan, Ana; Roczen, Maurizio; Fittkau, Jens; Zimmer, Martin; Rentsch, Jochen: Electronic interface properties of Silicon substrates after Ozone based wet-chemical oxidation studied by SPV measurements. APPLIED SURFACE SCIENCE 258 (2012) p. 8387-8396
  • Varache, R.; Angermann, H.; Farret, M.-E.; Kleider, J.P.; Korte, L.: Controlled interfacial native oxide for amorphous silicon/crystalline silicon heterojunction solar cells. Proc. of the 27th EU PVSEC - European Photovoltaic Solar Energy Conference and Exhibition Frankfurt / Main, Germany, (2012) 1582 – 1585.
  • Wolke, K. ; Gottschalk, Ch. ; Rentsch, J. ; Angermann, H.; :Ozone based chemical oxide growth for chrystalline solar cell production. In: Proc. of 10th International Symposium on Ultra Clean Processing of Semiconductor Surfaces Thermae Palace, Ostend (Belgium)., 20.09.2010 - 22.09.2010 (2010) pp. 99-100
  • Angermann, H. ; Rappich, J. ; Klimm, C.: Wet-chemical treatment and electronic interface properties of silicon solar cell substrates. Central European Journal of Physics 7 (2009), p. 363-370
  • H. Angermann: Passivation of structured p-type silicon interfaces: Effect of surface morphology and wet-chemical pre-treatment,  Appl. Surf. Scie. 254 (2008), pp. 8067-8074
  • Angermann, J. Rappich, Surface states and recombination loss on wet-chemically passivated Si studied by Surface Photovoltage (SPV) and Photoluminescence (PL). Solid State Phenomina 134 (2008) pp. 41-44
  • Angermann, H.; Rappich, J.; Sieber, I.; Hübener, K.; Hauschild, J.: Smoothing and passivation of special Si(111) substrates: studied by SPV, PL, AFM and SEM measurements. Analytical and Bioanalytical Chemistry (ABC, Springer-Verlag) Analytical and Bioanalytical Chemistry, Anal Bioanal Chem 390 (2008) 1463-1470, DOI: 10.1007/s00216-007-1738-5
  • Angermann H.: Interface state densities and surface charge on wet-chemically prepared Si(100) surfaces. Solid State Phenomena 103-104 (2005), p. 23-26
  • H. Angermann, W. Henrion, M. Rebien, A. Röseler, Wet-chemical Preparation and Spectroscopic Characterization of Silicon Interfaces. Appl. Surf. Sci. 235 (2004), pp. 322-339.
  • H. Angermann, W. Henrion, M. Rebien, A. Röseler, Wet-chemical passivation and characterization of silicon interfaces for solar cell applications. Solar Energy Materials and Solar Cells 83 (2004) pp. 331-346.
  • H. Angermann, Characterisation of wet-chemically treated silicon interfaces by surface photovoltage measurements. Anal. Bioanal. Chem. 374 (2002) 676
  • W. Henrion, M. Rebien, H. Angermann, A. Röseler, Spectroscopic Investigations of Hydrogen Termination, Oxide Coverage, Roughness, and Surface State Density of Silicon During Native Oxidation in Air. Appl. Surf. Sci. 202 (2002) 199.
  • H. Angermann, W. Henrion, A. Röseler,  Wet-Chemical Conditioning of Silicon: Electronic Properties Correlated with the Surface Morphology. in Silicon-Based Materials and Devices, edited by H. S. Nalwa (Academic Press,  San Diego 2001) pp. 267-298
  • H. Angermann, W. Henrion, M. Rebien, D. Fischer, J.-T. Zettler, A. Röseler, H-terminated Silicon: Spectroscopic Ellipsometric Measurements correlated the surfaces electronic properties. Thin Solid Films 313-314 (1998), 552
  • H. Angermann, W. Henrion, M. Rebien, J.-T. Zettler, A. Röseler, Characterisation of chemically prepared Si-surfaces by UV-VIS and IR spectroscopic ellipsometry and SPV measurements. Surf. Sci. 388 (1997), 15
  • H. Angermann, K. Kliefoth,  H. Flietner, Preparation of H-Terminated Si-Surfaces and their Characterisation by Measuring the Surface State Density. Appl. Surf. Sci. 104 / 105 (1996), 107
  • H. Angermann, Th. Dittrich, H. Flietner, Investigation of Native Oxide Growth on HF-Treated Si(111) Surfaces Measuring the Surface State Distribution. Appl. Phys. A 59 (1994), 193