Microstructure Analysis of Agglomerated Epitaxial Columns of Si/PtSi/Si(111) Double Heterostructure
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1. INTRODUCTION In the last few years, many successful attempts have been made to grow Si/CoSi2/Si double heteroepitaxial structures on the Si(l 11) surface, which were aimed at fabrication of the permeable base and metal base transistors[I-2]. Recently, unique epitaxial structures were demonstrated in the CoSi2/Si( 11)[3] and PtSi/Si( 11)[4] systems, which consist of epitaxial columns of CoSi2 or PtSi surrounded by epitaxial Si. These structures were obtained by
coevaporation of Co and Pt and Si on a heated Si substrate in a highly Si rich condition in an ultra high vacuum (UHV). This growth was named "columnar epitaxy." Since PtSi has been widely used in Si devices and ICs, formation method[5] and thermal
stability[6] have been studied extensively. However, single crystal films on Si-substrate have not been demonstrated, though partial epitaxial growth of PtSi on Si(1 11) has been reported[7]. This is mainly due to the crystal structure of PtSi that is an orthorhombic structure with a--0.593nm, b=0.559nm and c=0.360nm[8], while CoSi2 has a similar crystal structure (cubic
fluorite) to that of Si, with lattice mismatch of about 1.2%. There is no report on epitaxial growth of PtSi layers on Si substrates by coevaporation of Pt and Si except our previous preliminary work on Si(001) substrates[9]. The original purpose of this work was to demonstrate the formation of Si/PtSi/Si(1 11) double heterostructure using a popular silicide, such as PtSi. However, it was found that the planar to columnar transformation of PtSi occurred in the process of Si deposition on PtSi layer when the substrate temperature was high during Si over-capping deposition[IO]. In this paper we show microstructure analysis on the interface of the agglomerated columns of PtSi and the Si substrate.
2. EXPERIMENTAL PROCEDURE N-type CZ Si(1 11) substrates (0.16-0.24 ncm) with a nominal misorientation angle of less than 0.50 were used. The substrates were chemically cleaned and a protective oxide layer was formed following the procedure of ISHIZAKA and SHIRAKI method[ 11] except the HNO3 boiling step. Then the substrate was loaded into the growth chamber through a load lock and 403 Mat. Res. Soc. Symp. Proc. Vol. 320. ©1994 Materials Research Society
heated to a temperature of 800 °C to remove oxide layer, which yielded a well-developed 7x7 reflection high energy electron diffraction (RHEED) pattern. In addition, the substrate was heated up to 1000 OC for 20 minutes to clean the surface thoroughly. The growth was carried out in a MBE system equipped with separate electron gun evaporation sources for Pt and Si. The base pressure of the MBE system was about 2x10- 8 Pa. The deposition rate of Pt and Si were controlled by an electron impact emission spectroscopy (EIES) sensor. After a cleaning process on the surface of Si substrate, the substrate temperature was cooled down to 500 oC, then Pt and Si were coevaporated to form PtSi layer on Si substrate. The deposition rates for Pt and Si were 0.03 nm/s and 0.03x1.33 nm/s respectively, for a atomic rat
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