Materials Science and Engineering B73 (2000) 212–217Large-grained polycrystalline Si films obtained by selectivenucleation and solid phase epitaxyR.A. Puglisi1, H. Tanabe, C.M. Chen *, Harry A. AtwaterThomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA91125, USAAbstractWe investigated the formation of large-grain polycrystalline silicon films on glass substrates for application in low-cost thin filmcrystalline silicon solar cells. Since the use of glass substrates constrains process temperatures, our chosen approach to formlarge-grain polycrystalline silicon templates is selective nucleation and solid phase epitaxy (SNSPE). In this process, selectivecrystallization of an initially amorphous silicon film, at lithographically predetermined sites, enables grain sizes larger than thoseobserved via random crystallization. Selective heterogeneous nucleation centers were created for both P-doped, B-doped andundoped, 100 nm thick amorphous silicon films, by masked implantation of In or Ni islands, followed by annealing attemperatures below 600°C. Seeded crystallization begins at the metal islands and continues via lateral solid phase epitaxy (SPE),thus obtaining crystallized regions of several tens of square microns. The maximum achievable grain size depends on the productof the SPE rate and the incubation time for the spontaneous nucleation. We have studied the dependence of the SPE rate and theincubation time on the type of metal (In and Ni) inducing the nucleation and on the electronic dopant (e.g. P and B)concentration in the 1019–1021cm−3range. © 2000 Elsevier Science S.A. All rights reserved.Keywords:Solar cells; Si; Selective nucleation; Ni, Inwww.elsevier.com/locate/mseb1. IntroductionPolycrystalline silicon (poly-Si) thin films are cur-rently attracting a great deal of attention for use insolar cells. Poly-Si has superior carrier mobilities andstability under illumination, relative to amorphous sili-con (a-Si). Since grain boundaries act as minority car-rier recombination centers, poly-Si films withsufficiently large grain sizes are necessary in photo-voltaic applications [1]. In particular, if the grain size isin the range of 10–100 mm, AM1.5 cell efficiency canreach 17%, compared to an efficiency of approximately10% for a-Si [2]. In order to use low-cost glass sub-strates, which have softening temperatures about600°C, it is also necessary to be able to grow poly-Sifilms at low thermal budgets. High quality poly-Si filmscan be obtained by either surface energy driven sec-ondary grain growth of as-deposited poly-Si films, bypulsed laser-induced crystallization or by selective nu-cleation and subsequent solid phase epitaxy (SNSPE) ofthe a-Si. For secondary grain growth of as-depositedpoly-Si, the temperature necessary to achieve high qual-ity, i.e. large defect-free grains, poly-Si is generally inthe 1000°C range [3]. Pulsed laser-induced crystalliza-tion of a-Si, on the other hand, can produce poly-Siwith the required high quality. However, laser crystal-lization is likely to result in a low throughput photo-voltaic manufacturing process due to the small areaprocessed per pulse. The SNSPE has advantages overother poly-Si preparation processes, like smoother sur-faces, better uniformity, and, when compared to lasercrystallized poly-Si, a higher throughput. This processis based on metal induced nucleation of a-Si layers.Metal rich regions formed by ion implantation througha mask cause selective heterogeneous nucleation tooccur at a much earlier time than random nucleation.The enhanced nucleation rate has been attributed to theinteraction of the free electrons of the metal with thecovalent Si bonds near the growing interface [4]. Thuscrystals which are selectively nucleated can grow to* Corresponding author. Tel.: + 1-626-395-3826; fax: + 1-626-449-5678.E-mail addresses:[email protected] (R.A. Puglisi),[email protected] (C.M. Chen)1Present address: Universita` di Catania and INFM, Corso Italia57, 95100 Catania. Italy.0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.PII: S0921-5107(99)00466-3R.A. Puglisi et al./Materials Science and Engineering B73 (2000) 212 – 217213very large sizes via lateral solid phase epitaxy beforetheir growth is impeded by impingement with randomlynucleated crystal silicon grains. Furthermore, B and Pdoping has been shown to enhance SPE rate in Si,enabling even larger grains to be obtained in B andP-doped films.The formation of poly-Si in In-implanted a-Si wasfirst reported by Nygren et al. [5], who observed rapidcrystallization of a-Si implanted with 2% of In, andannealed at 550°C for 15 min, suggesting a crystalliza-tion mechanism via liquid In droplet migration. Theysuggested that the crystallization process is initiatedfrom molten In-rich precipitates that form within a-Siwhen it contains high concentrations of In atoms. An-other system that takes advantage of silicide formationin the transport of Si from the amorphous to crystallinephase, is that of Ni on Si. Low temperature crystalliza-tion of a-Si has been reported [6] following Ni deposi-tion on hydrogenated a-Si and was believed to resultfrom heterogeneous nucleation of c-Si at the interfacebetween NiSi2and a-Si.All the above findings refer to metal islands uni-formly deposited or implanted in a-Si. In this paper wepropose a new method to selectively induce the nucle-ation of c-Si in a-Si, by implanting indium or nickelislands into a-Si thin films. We observed crystallizationof the a-Si after annealing at temperatures of 550°C.The crystallized regions sizes were of the order ofseveral microns. Moreover, the crystallization processinduced by nickel was much faster than the one inducedby indium. In particular we measured a crystallizationvelocity of 2× 10−3mms−1at 620°C for indium-in-duced crystallization, while 8 ×10−2mms−1at 610°Cfor nickel-induced crystallization.2. Experimental procedures and resultsTo study the kinetics of the SNSPE, two differenttypes of samples were made. The first type was anamorphous Si thin film, 100 nm thick, deposited byultra-high vacuum electron beam evaporation onto 100nm thick thermally grown SiO2films on (100) Si. Agroup of these samples was doped with boron, im-planted at an energy of 13 keV with doses of 1 ×1012,3× 1012,1×1013, and 5 ×1013cm−2. We will refer tothis group as boron-doped samples. A second group ofsamples was doped with phosphorus