A new way to selectively remove Si islands from polycrystalline silicon seed layers made by aluminum-induced crystallization Dries Van Gestel ⁎ , Ivan Gordon, Agnes Verbist, Lodewijk Carnel, Guy Beaucarne, Jef Poortmans IMEC, Kapeldreef 75, B-3001 Leuven, Belgium Available online 23 December 2007 Abstract Polycrystalline silicon (grain size ~ 0.1–100 μm) solar cells on foreign substrates are a promising approach for the next generation silicon solar cells. Aluminum-induced crystallization AIC in combination with epitaxy is a possible way to obtain such absorber layers. It is believed that Si islands present on the surface of AIC seed layers have a negative effect on the epitaxy. The removal of these islands could therefore lead to an increased absorber layer quality and solar cell performance. In this paper, we present a selective island removal procedure based on the Al layer already present after AIC annealing. By selecting an etchant which removes Si at least as fast as Al (in this paper plasma etching using SF 6 ), the Al layer acts as a perfectly aligned etching mask for the fully developed islands. © 2007 Elsevier B.V. All rights reserved. Keywords: Aluminum-induced crystallization; Si island; Polycrystalline silicon; Thin-film solar cell; Selective etching 1. Introduction The large growth of the photovoltaic (PV) energy market during the last ten years has resulted in a scarcity of high-quality silicon material and an increase of its price. Thin-film poly- crystalline silicon (pc-Si) solar cells on foreign substrates might become a low-cost alternative for wafer-based silicon solar cells if sufficiently high efficiencies are obtained [1]. Direct depo- sition of Si on foreign substrates typically results in absorber layers with small grain sizes (0.2–1 μm) and a relatively high density of defects due to grain boundaries. To improve the electrical quality, a possible approach is the formation of a large-grained seed layer (1–100 μm) in combination with epi- taxial growth. At IMEC an increase in absolute efficiency of ~ 1.5% per year up to 8% at the moment was achieved for pc-Si solar cells made by aluminum-induced crystallization (AIC) of amorphous silicon and thermal CVD [2]. The AIC seed layer itself can not be used as absorber layer due to its high p-type Al doping level of ~ 3 × 10 18 cm - 3 and due to the difficulty to make AIC layers with a thickness above 0.5 μm. To obtain highly efficient pc-Si solar cells however, the material quality has to be further optimized and different cell processes have to be developed. On top of a typical AIC seed layer, secondary silicon crystallites (secondary crystallites with vertical side walls and a smooth upper surface are also called Si islands) are present after crystallization [3]. In the past we showed that an increase of the grain size (and removal of these secondary crystallites) of the AIC seed layer before epitaxy, does not necessarily lead to an increase in solar cell efficiency [4]. Recently, we showed that this was due to the presence of a very high density of electrically active intragrain defects [5]. These intragrain defects are a major limiting factor for the cell performance and therefore need to be reduced in number. From defect etching and electron backscattered diffraction (EBSD) measurement we concluded that most of these defects originate in the seed layer or at the seed layer/epi layer interface [5,6]. Despite the fact that removal of the secondary crystallites does not always lead to a better layer quality [4,5], we believe that their presence on the surface of the AIC seed layers has a negative effect on epitaxy, and/or will limit a further improve- ment of the absorber layer quality. Most Si islands have a Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 6907 – 6911 www.elsevier.com/locate/tsf ⁎ Corresponding author. IMEC, Solar Cell Technology Group, Kapeldreef 75, B-3001 Leuven, Belgium. Tel.: +32 16 288683; fax: +32 16 281501. E-mail address: Dries.VanGestel@imec.be (D. Van Gestel). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.12.122