Room Temperature Epitaxy of AL (100) on SI (111)
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Marek Sosnowski*, Samuel Ramac*, Walter L. Brown**, and Young 0. Kim*** * Department of Electrical and Computer Engineering, New Jersey Institute of Technology, University Heights, Newark, New Jersey 07102 **AT&T Bell Laboratories, Murray Hill, New Jersey, 07974 ***AT&T Bell Laboratories, Holmdel, New Jersey, 07733
ABSTRACT There are a number of published reports on the epitaxial growth of Al(1 11) on Si(1 11) surfaces usually following a high temperature treatment of the Si surface in UHV. In contrast to these results, we have for the first time observed dominant epitaxial growth of Al(100) films on Si(1 11) surfaces that have been carefully cleaned and hydrogen terminated and not heated prior to effusion cell deposition of Al at room temperature in UHV. X-ray diffraction shows sharp and intense Al (200) diffraction, enhanced by post deposition annealing. Crystal quality and the dominance of Al (100) structure depend strongly on the substrate treatment and the off-cut angle, both of which control the steps on the Si(1 11) surface. The steps were found responsible for the epitaxial alignment of the film and the substrate lattices. Details of this alignment were observed in TEM cross-sectional images of the interface.
1. INTRODUCTION Aluminum films are known to grow on substrates such as Si and SiO2 in a polycrystalline form with preferential Al (111) orientation. There have been a number of reports, however, that single crystals of Al (11) can be grown on a clean Si (111) surface, despite a large difference between the lattice constants of the film and the substrate [1 - 7]. High quality Al (111) single crystals were reported to be grown on atomically clean Si (111) surface in ultra high vacuum (UHV) under keV ion bombardment [1] and by MBE [2,3]. Similar results were obtained in conventional vacuum by thermal evaporation [4,5], and by deposition from partially ionized vapor whose ions were accelerated to a keV [6], but achieving high quality crystals required in these cases post deposition annealing. Al(1 I 1)/Si(1 11) epitaxy was also obtained by CVD at a substrate temperature of 4000 C [7]. Such results may seem surprising as the lattice mismatch between the film and the substrate exceeds 25 %. Straight forward geometrical considerations require that the two crystals in an epitaxial arrangement should have almost equal lattice constants, so that they can match at the interface without a major disruption of their periodicity. Heteroepitaxy, however, can be achieved with materials of different atomic spacing or even different lattice types, when the lattices are commensurate [8]. Such is the case of Al (111) films grown on Si (111). As the ratio of the two lattice constants is very close to 4:3, every fourth Al plane matches every third Si plane with only a small residual misfit of 0.56 %. Such an arrangement was observed in high resolution TEM images of the Al(1 I1)/Si(l 11) interface [35,9]. The interface was found to be very smooth, especially after annealing which improves 213 Mat. Res. Soc. Symp. Proc. Vol.
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