MBE Growth of an Epitaxial Insulator-Metal-Semiconductor Structure: CaF 2 /CoSi 2 /Si(111)
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ABSTRACT
We report the first successful growth of an epitaxial insulator-metal-semiconductor structure. A layer of metallic CoSi 2, followed by a layer of insulating CaF 2 have been grown by molecular beam epitaxy on Si(1 11). The epitaxial quality of the CaF 2 layer improves upon rapid thermal annealing, while the already excellent crystallinity of the CoSi 2 layer is unaffected. The lattice of the CoSi 2 is rotated 180* with respect to the Si lattice while the CaF 2 lattice is aligned with the Si lattice.
The epitaxial growth of metals and insulators on semiconductors has been studied intensively for some time, 1' 2 spurred in part by the possibilities these materials offer for three-dimensional integration. While research into the growth' and properties of epitaxial metals and insulators individually on semiconductors has revealed much about the structural and other characteristics of these layers, no work has been reported in which both materials have been grown sequentially on the same substrate. We have grown an epitaxial insulator-metal-semiconductor structure using CaF 2 as the insulator, CoSi 2 as the metal, and Si as the semiconductor. Both CaF 2 and CoSi2 are closely lattice matched to Si. At room temperature the CaF 2 lattice is 0.6% larger than that of Si, while the CoSi 2 lattice is 1.2% smaller than the Si lattice. Rutherford backscattering/channeling has been used to determine the epitaxial quality of the layers as well as the influence of the substrate temperature during growth on the epitaxial quality of the CaF 2. We have also studied the effect of rapid thermal annealing (RTA) on the heterostructures. RTA can improve the crystallinity of the CaF 2 layer without affecting the CoSi2 layer. Finally, we have determined the epitaxial relations between the Si substrate and the CoSi 2 and CaF 2 overlayers. The CoSi 2 and CaF 2 films were grown in a VG V80H molecular beam epitaxy system. 3" Si(1 11) substrates were chemically cleaned prior to insertion in the system, as described previously. 3 The final volatile oxide was desorbed in situ by heating to 830'C. After this cleaning step the substrate was cooled to < 100'C, at which temperature 3-28 nm of Co was deposited by electron beam evaporation. Following the Co deposition, the temperature of the substrate was raised to 650 C to allow the Co and Si to react to form 11-100 nm of CoSi 2. Finally, the temperature of the substrate was adjusted to a value between 500° and 700*C, at
Mat. Res. Soc. Symp. Proc. Vol. 54. '1986 Materials Research Society
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which temperature 500-700 nm of CaF 2 were deposited from an effusion cell. Portions of all wafers were rapidly thermal annealed for 20 sec in a commercial AG 210 flash lamp annealing system using an Ar gas ambient. The peak temperature in these anneals was 1100I'C. Figure 1 shows the Xmin (defined as the ratio of the backscattered high energy ion yield in the normal < 111 > channeling direction to a random direction) as a function of the substrate temperature during the CaF 2 film growth. This
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