Electrical Transport Properties of Cu 3 Ge thin films
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ABSTRACT We present results of electrical transport studies performed on thin films of E1-Cu 3 Ge in the temperature range 4.2 - 300 K. It is found that E1-Cu 3 Ge which has a long-range ordered monoclinic crystal structure, exhibits a remarkably low metallic resistivity of - 6 pfQ cm at room temperature. The density of charge carriers, which are predominantly holes, is - 8 x 1022/crn 3 and is independent of temperature and film thickness. The Hall mobility at 4.2 K is - 132 cm 2/V s, considerably higher than in pure copper. The elastic mean free path is found to be -- 1200A, which is surprisingly large for a metallic compound film. The results show that the residual resistivity is dominated by surface scattering rather than grain-boundary scattering. It is also found that by varying the Ge concentration from 0 to 40 at. % the resistivity exhibits anomalous behavior. This behavior is correlated with changes observed in the crystal structure of the thin-film alloys as the Ge concentration is increased. The resistivity remains close to that of the ':'-Cu 3 Ge phase over a range of Ge concentration which extends from 25 to 35 at. 0/
INTRODUCTION In recent years, there has been extensive interest in the study of electrical transport properties of metal silicide thin films [1] owing to their technological importance. Most silicides, with a few exceptions, are metals with good electrical conductivity at room temperature. Regarding their electronic properties, the transition metal silicide NiSi 2 , for example, can be viewed as a noble-metal compound with conduction resulting from the Si s and p electrons with the Ni d electrons remaining essentially localized [2]. Metal germanides, on the other hand, are in general less conductive than silicides. Recently, however, we have found [3] '-'-Cu3 Ge to be exceptional, with a room-temperature resistivity (-, 5.5 /K, cm) considerably lower than that of the epitaxial, single-crystal films of NiSi 2 (34 pn cm) and CoSi 2 (15 p/A cm) [4] and only a factor of three higher than that of pure Cu films (.- 2 PK2 cm) [3]. Here, we investigate the electrical transport in thin films of E1 -Cu3 Ge formed on Ge and SiO 2 -covered Si substrates as a function of temperature and film thickness. We also examine the effect of deviations from stoichiometry and more specifically of varying the Ge concentration on electrical transport in thin films of Cu-Ge alloys. 269 Mat. Res. Soc. Symp. Proc. Vol. 320. ©1994 Materials Research Society
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EXPERIMENTAL PROCEDURE Films of r.'Cu 3 Ge with thickness ranging from 200 to 2000A were formed by deposition of a Cu film on undoped Ge(111) wafers and by sequential deposition of Cu and Ge films on SiO 2-covered Si(100) wafers at room temperature followed by an anneal. The Cu and Ge films were deposited using e
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