Electrical Transport in Ultrathin Films of CoSi 2 on Si(111)
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ELECTRICAL TRANSPORT IN ULTRATHIN FILMS OF CoSi 2 ON Si(1 11) J. C. HENSEL, J. M. PHILLIPS, J. L. BATSTONE, W. M. AUGUSTYNIAK, AND F. C. UNTERWALD AT&T Bell Laboratories, Murray Hill, NJ 07974 ABSTRACT We have performed electrical transport measurements on ultrathin films of epitaxial CoSi 2 on Si(l 11) with film thickness ranging down to - 10A. The resistivities exhibit temperature dependences characteristic of a metal and a thickness dependence which rises steeply with decreasing thickness suggestive of a quantum size effect. At the lowest temperatures (R 10K) the resistivities of the thinner films increase logarithmically with inverse temperature characteristic of transport in the weak localization regime as has been confirmed by magnetoresistance measurements. Hall effect measurements establish that carrier densities (holes) in the ultrathin films are essentially identical to those in bulk CoSi 2, i.e. 2.6 x 1022 cm-3 . INTRODUCTION The good electrical properties of certain metallic silicides are a key factor in their finding successful utilization in present-day integrated circuit technology as well as allowing one to envision more exotic device applications such as metal-base and permeable-base transistors [1,21. In previous work [31 it has been shown that CoSi 2 grown on Si(lll) is not only an interesting material from a technological standpoint but also represents an excellent model system for the investigation of transport phenomena in thin films. Specifically, amongst its noteworthy properties are a high conductivity (long electronic mean free path) and a favorable morphology, i.e. single-crystal epitaxy and nearly perfect interfaces are achievable for CoSi2 on Si owing to their compatible structures (fluorite and diamond, respectively) and moderately small lattice mismatch (1.2%). Most earlier work referred to above has been essentially limited to "thick" films of - 100 A or greater. In a new development, it has been found possible to grow thinner films of excellent quality, down to thicknesses as low as - 10 A. TEM shows these films to be essentially continuous and, provided the film thickness does not exceed approximately 30A, tc be pseudomorphic with the Si substrate. As far as transport is concerned this is a major advance; for besides the unique feature of a high degree of crystalline perfection in the pseudomorphic films, possibly even more intriguing is the prospect of exploring transport in a metal as one approaches the quantum limit. Such conditions are to be expected as the film thickness becomes comparable to a deBroglie wavelength (-7 A for CoSi 2) whereupon motion transverse to the plane becomes essentially quantized as one or at most a few transverse subbands are occupied. We are approaching this regime, and the ramifications in transport may prove to be quite interesting. EXPERIMENTAL A description of the growth of the films and their structural characterization is presented elsewhere in these proceedings [4]. The films used in the electrical transport experiment we will discuss here are t
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