Properties of purified direct steam grown silicon thermal oxides Sebastian Mack a,n , Andreas Wolf a , Alexandra Walczak a , Benjamin Thaidigsmann a , Edgar Allan Wotke a , Jeffrey J. Spiegelman b , Ralf Preu a , Daniel Biro a a Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstrasse 2, D-79110 Freiburg, Germany b Rasirc Inc., San Diego, CA 92121, USA article info Article history: Received 8 September 2010 Received in revised form 25 February 2011 Accepted 1 March 2011 Available online 29 April 2011 Keywords: Thermal oxidation Passivation SiO 2 Steam Solar cell PERC abstract Thermal silicon oxides are known to very effectively passivate silicon surfaces. Choosing a water vapor ambient instead of a dry oxygen atmosphere increases the oxidation rate by about one order of magnitude and considerably reduces process time and costs. State of the art pyrox systems produce steam by pyrolysis of hydrogen and oxygen gas. A new approach is the purification of vaporized deionized (DI) water. In this work we present a direct comparison of both steam generation systems, which are connected to the same quartz tube of an industrial high quality tube furnace. The higher steam saturation of the direct steam process enhances the growth rate by about 20% compared to a pyrolytic steam based process. On low-resistivity p-type substrates, excellent surface recombination velocities of around 25 cm/s are found for both systems after a forming gas anneal. Detailed characterization shows similar physical properties of the oxide layers grown by either steam from pyrolysis or purified steam. Moreover, thermal oxide rear surface passivated silicon solar cells show similar high efficiencies exceeding 18.0% irrespective of the applied steam generation technology. & 2011 Elsevier B.V. All rights reserved. 1. Introduction The passivation of silicon surfaces by thermal oxides has been a subject of intensive research for many years. Silicon thermal oxide films are regularly grown in an oxygen atmosphere (dry process), which gives excellent interface properties with interface trap densities D it below 4  10 9 cm 2 eV 1 [1] and surface recombination velocities below 15 cm/s on 1 O cm floatzone silicon [2]. Recently, we presented several approaches for the implemen- tation of thermal oxide processes for rear surface passivation into industrial cell structures with local rear contacts. Two approaches use a  10 nm thin oxide layer, upon which we deposit other dielectric layers by means of plasma-enhanced chemical vapor deposition to enhance the optical and electronical properties of the rear surface [3–5]. The use of only a thin oxide layer on the rear surface between the silicon wafer and the aluminum rear contact would lead to a reduced short-circuit current density due to a lower rear surface reflection as was shown by Green [6] and Kray et al. [7]. In addition, the use of capping layers on top of the thin thermal oxide layer can improve the electronic properties of the silicon–insulator interface [8–11]. Another approach uses a thick oxide layer [12,13], as investigated in this paper. The oxide layer thickness on the rear surface decreases during processing to a final thickness of around 100 nm in the solar cell device [13], which already allows for an average internal rear surface reflec- tion of 94% in combination with an evaporated aluminum layer [7]. Nevertheless, with increasing oxide film thickness, the oxide growth rate decreases, which prolongs the duration of dry oxidation processes to several hours, when growing oxide layers thicker than 100 nm. Using water vapor (wet oxidation) instead of oxygen as an oxidant increases the oxide growth rate by up to one magnitude [14,15] due to a higher solid solubility of steam in the silicon oxide, which makes wet oxidation interesting for pro- cesses, where thicker oxide layers are required. In addition, the thermal stress to the wafers is reduced, compared to the use of a dry oxygen ambient [16], due to the incorporation of hydroxyl groups. However, wet oxidation does not fully reach the surface passivation quality of dry processes [16,17]. State of the art systems generate steam in a pyrolytic torch, using high purity hydrogen and oxygen gas. An alternative approach uses vaporized and consecutively purified deionized water [18], which reduces the cost [19] and improves the safety due to the elimination of hydrogen and oxygen gas from the facility. Benick et al. [20] showed that for high efficiency solar cells, both purified steam and pyrolysis yield similar high open circuit voltages. However, the two steam generation systems were con- nected to different oxidation tubes and no detailed information on the properties of the oxide layers and the interface were presented. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.03.002 n Corresponding author. Tel.: þ49 761 4588 5596; fax: þ49 761 4588 9250. E-mail address: sebastian.mack@ise.fraunhofer.de (S. Mack). Solar Energy Materials & Solar Cells 95 (2011) 2570–2575