Two Step Growth and Characterization of BaF 2 /Si(III) Heterostructures
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A.BELENCHUK*, A.FEDOROV**, V.LUCASH*, A.VASILYEV*, V.ZENCHENCO* *Institute of Applied Physics, Kishinev 277028, Moldova "**Institute of Device Technology, Kharkov, Ukraina
ABSTRACT The influence of growth conditions on structural perfections and epitaxial relation in BaF 2 films on Si(1 11) are considered. It is shown that rotational-twin-free BaF2 films (Atype orientation) can be grown on Si(1 11) by two step growth method. It is also shown that a mixed (A+B)-type orientation, which usually observed in BaF 2 films grown by conventional one step growth method, can be converted into A-type orientation by postgrowth annealing. The correlation between type of epitaxial orientation in BaF 2 films and structures of interface is discussed.
INTRODUCTION
The epitaxial growth of Ila group's fluorides on silicon substrates has attracted much attention because of potential applications of such heterostructures for the construction of
micro- and optoelectronic devices. In the case of Si( 11) -oriented substrates there is a problem of rotational twins generations in fluoride films [1]. The film has an A-type epitaxial relation when crystallographic axes of the film and substrate fully coincide. In the case of B-type epitaxial relation, the crystallographic axes of the film are 1800 rotated about the surface normal [III] axis with respect to substrate axes. The CaF 2 films prefer to have B-type epitaxial relation on the silicon (111) substrates whereas in the SrF2 and BaF2 films on the Si(1 11) a mixture of A- and B-type domains are observed [1]. The polydomain state in the epitaxial films is undesirable because of a structural defects connected with domain boundaries. B-type epitaxial relation also leads to structural defects localized at the steps on the substrate surface [2]. It was expected that A-type epitaxial relation gave a maximum structural perfection in fluoride Ila group films on Si(1 11) substrates. Recently it was shown that a two-step growth method made it possible to obtain uniform type-A CaF2 films with a good crystallinity on Si substrates [3,4]. The use of two step method for the growth of heterostructure with a large lattice mismatch has an additional advantage. This method allows to organize layer growth at the early stage of epitaxy and to localize structural defects, connected with the lattice mismatch, within a buffer layer thickness. In some cases such defect localization has an advantage compared with its uniform distribution, through the film thickness. 337
Mat. Res. Soc. Symp. Proc. Vol. 356 01995 Materials Research Society
Taking into consideration these arguments we have used two step method for the growth of BaiF2 /Si(1 11) heterostructures [5]. This method permitted us to obtain the BaLF2 surface films directly on Si(lll1) substrates with good crystallinity and good morphology. In this paper we investigate the effects of growth conditions on crystalline quality and epitaxial relations in BaF2 films on Si(1 11). It is shown that BaF2 films with A-type orientation can be grown directly on Si(
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