Control of Domain Size and Doping of Barium Disilicide Epitaxial Thin Films for Earth-abundant High-efficiency Solar Cells

IMR KINKEN Research Highlights 2012 Orthorhombic barium disilicide (BaSi2) has been proposed as a promising, novel candidate for high-efficiency thin-film solar cells owing to its large absorption coefficient of 105 cm-1 at 1.5 eV [1] and bandgap that is tunable to 1.4 eV by substituting half of the Ba sites with isoelectronic Sr atoms [2]. A base layer only 1 μm thick is considered sufficient to absorb all the photons with energies higher than the bandgap. Importantly, all the constituent atoms are earth-abundant and suitable for the global deployment of photovoltaic technology. To fabricate a solar cell based on BaSi2, it is of crucial importance to realize high-quality thin film crystals with controlled doping. So far, epitaxial BaSi2 thin films with (100) orientations have been grown on Si(111) substrates. However, the epitaxial relationship allows formation of three epitaxial variants that are equivalent in terms of 60° in-plane rotations. As a consequence, many crystal domains and lattice defects exist in the films. A possible route toward realization of high-quality BaSi2 with large crystal domains is to reduce the symmetry of the surface to selectively grow one of the epitaxial variants. For this purpose, we investigated the impact of a miscut given to the Si(111) substrate on the microstructures of epitaxially grown BaSi2 thin films [3]. BaSi2 epitaxial films were prepared by reactive deposition epitaxy followed by molecular beam epitaxy. First, Ba atoms were deposited on the Si(111) substrate at a deposition rate of 0.9–1.0 nm/ min for 5 min, thereby producing a thin template BaSi2 layer. An a-axis-oriented BaSi2 epitaxial film with a thickness of 80–90 nm was formed subsequently by depositing Ba and Si atoms on the template layer. On-axis and vicinal (with a 2° miscut towards the [112] direction) Si(111) substrates were used. Fig. 1 compares the electron backscatter diffraction (EBSD) orientation maps of the BaSi2 films on (a) on-axis and (b) vicinal Si(111) substrates. In Fig. 1(a), it is seen that three epitaxial variants randomly appear and that the domain shape is complicated. The size of most domains is less than several micrometers. On the other hand, one of the three epitaxial variants was dominant when the BaSi2 film was grown on the 2°-inclined substrate, as shown in Fig. 1(b). In the dominant variant, the b axis was found to be parallel to the miscut direction. Atomic force microscopy clarified that, on the 2°-inclined substrate, barium silicide platelets are found next to the step-bunched regions and the proportion of one epitaxial variant is increased. This result suggests that lattice matching with the step edge is an important factor for orientation selection. Control of Domain Size and Doping of Barium Disilicide Epitaxial Thin Films for Earth-abundant High-efficiency Solar Cells Barium disilicide (BaSi2) is regarded as a promising candidate for a novel earth-abundant material for highefficiency thin-film solar cells owing to its large absorption coefficient and ideal bandgap. We attempted to realize high-quality BaSi2 epitaxial thin film crystals with large crystal domains and controlled doping. Intentional introduction of a miscut to the Si(111) substrate selectively enlarged one of the three equivalent epitaxial variants by modifying the nucleation sites; this led to extraordinarily large crystal domains. Ion-implantation of BF2 and subsequent annealing were found to be useful to obtain p-type BaSi2 with a carrier concentration of around 1018 cm-3.