Chemical bulk properties of multicrystalline silicon ingots for solar cells cast in silicon nitride crucibles C. Modanese a,n , M. Di Sabatino a , M. Syvertsen b , L. Arnberg a a NTNU—Department of Materials Science and Engineering, N-7491 Trondheim, Norway b SINTEF Materials and Chemistry, N-7465 Trondheim, Norway article info Article history: Received 26 February 2012 Received in revised form 24 May 2012 Accepted 25 May 2012 Communicated by P. Rudolph Available online 2 June 2012 Keywords: A1. Diffusion A1. Directional solidification A1. Doping A1. Impurities B1. Silicon nitride B2. Semiconducting silicon abstract Silicon nitride is an alternative material to the widely used silica crucibles for directional solidification of mc-Si ingots, its main advantages being the reusability in successive castings and elimination for a source for oxygen contamination of the ingot. In this work, several ingots were cast in these crucibles and compared to reference ingots cast in silica crucibles. The thermal properties of the Si 3 N 4 crucible differ from those of the SiO 2 crucible and lead to a different thermal history during melting and casting. The oxygen contamination of the ingot was observed to depend mainly on the melting and holding temperature, rather than on the crucible material. The lowest oxygen concentration was observed in the ingots with the lowest melting temperature. However, the thermal properties of the Si 3 N 4 crucible influence the oxygen profile along ingot height, with a faster decrease in the concentration with increasing ingot height. This is believed to be due to a different mechanism for oxygen transport compared to that of the silica crucibles. The concentration of dopants in the ingots showed that contamination from the Si 3 N 4 crucible occurred, probably due to diffusion of B- and P-oxides into the Si melt. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Multicrystalline (mc) silicon is the most widely used material in the solar cell industry, accounting for approximately 50% of the global solar cell market [1]. Cell manufacturers must decrease manufacturing costs, and improve solar cell performances in order to be competitive. Silicon nitride (Si 3 N 4 ) crucibles for directional solidification are a promising alternative to silica (SiO 2 ) crucibles. The use of Si 3 N 4 crucibles has two main advantages: (i) the reusability of the crucible for several castings and (ii) potentially very low oxygen content in the solidified ingots. Si 3 N 4 is a well-known material for high temperature applica- tions, due to its tolerance for thermal shocks and its chemical stability [2]. Hence its application as a crucible material for mc-Si ingots casting can overcome the single-run limitation related to the use of SiO 2 crucibles. Silica crucibles undergo phase transfor- mation from quartz (higher density) to cristobalite (lower den- sity) during the melting of the Si charge. During the post- solidification cooling, the crucibles recrystallize into quartz and the resulting stresses lead to crucible fracture. Therefore the cost of mc-Si ingots can be appreciably decreased by replacing a single-use silica crucible with a reusable silicon nitride crucible. Moreover, diffusion of oxygen into the melt from the silica crucible walls and the silicon nitride coating layer has been reported as a relevant source of oxygen contamination in the solidified ingot [3–5]. Hence, using a silicon nitride based crucible for casting of mc-Si ingots allows eliminating one source of contamination for the silicon melt, and thus it offers the potential to solidify mc-Si ingots with very low oxygen concentrations. A novel technique to produce crucibles of silicon nitride has been developed [6]. Numerical and experimental investigations of such crucibles have been reported by Bellmann et al. [7] for the solidification furnace used in this work. They suggested increas- ing the furnace temperature by 20 K during melting, in order to counterbalance the higher heat loss through the bottom due to higher heat conductivity of the silicon nitride crucibles compared to silica crucibles. In the present work, Si 3 N 4 crucibles have been used to directionally solidify mc-Si ingots for solar cells. The effect on ingot properties of different combinations of (i) purity of the Si 3 N 4 crucible, (ii) firing procedures for applying the Si 3 N 4 coating layer, and (iii) holding temperatures during silicon melting, are pre- sented and compared to those of reference ingots cast in SiO 2 crucibles, under standard solidification parameters. Focus is set on dissolved oxygen concentration and on contamination of dopant elements and their distribution along the ingot height. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.05.033 n Corresponding author. E-mail addresses: chiara.modanese@material.ntnu.no, chiara@material.ntnu.no (C. Modanese). Journal of Crystal Growth 354 (2012) 27–33