Electrical Transport in Thin Silicide Films
- PDF / 1,517,152 Bytes
- 12 Pages / 417.6 x 639 pts Page_size
- 70 Downloads / 232 Views
Mat. Res. Soc. Symp. Proc. Vol. 54. '1986 Materials Research Society
500
K
K hT Fig. 1. Intersection of the 7th, 8th, and 9th hole sheets of the CoSi 2 Fermi surface with the (11) plane of the BZ (calculations by L. F. Mattheiss).
K
K
represents a system of relative simplicity as far as metals go for which the analysis of transport should not prove intractable. It has been emphasized by Chabal, et al. [10] that CoSi 2 and NiSi 2 can be regarded as metallic, simple-cubic phases of Si, the structure being stabilized by a single metal atom centered in every other cube. The covalent nature of the sp 3 tetrahedral bonding to the Si atoms implies a tightly bound structure, rather "rigid" from the standpoint of lattice dynamics. Finally, CoSi 2 undergoes a superconducting transition at =1.45K (for the best specimens), but NiSi 2 remains normal down to at least 0.9K. ELECTRICAL TRANSPORT IN NiSi 2 AND CoSi2 Any characterization of the electrical resistivity of a metal is incomplete without the temperature dependence. Usually the resistivity can be expressed in the following fashion [13], Po + PL(T)
p(T)
(1)
as the sum of two terms (Matthiessen's rule): the "ideal" resistivity PL(T), an intrinsic contribution due to scattering from lattice vibrations, and a residual resistivity P, due to temperature independent scattering from point defects. The former can be described by the Bloch-Gruneisen formula, PL(T) - const C2.
]Jo @,/
e- 'd
2
(2)
which at very low temperatures (T OD) exhibits the standard "equipartition" behavior, a linear dependence T/0D. In (2) OD is the Debye temperature and C is a deformation potential constant. The residual resistivity can be written in terms of an elastic scattering length epas follows: mvF o ne 2
'•
where vF is the Fermi velocity and n is the carrier density. Resistivity data [3,4] for UHV grown films (-1000A thick) of CoSi 2 and NiSi 2 in Fig. 2 show the behavior typical of a metal. The most striking feature in Fig. 2 is the almost orderof-magnitude disparity in p0 between NiSi 2 and CoSi2 . This high residual resistivity, po >-_20 Atflcm, for NiSi 2 is characteristic of all samples we have looked at. More later on this point. Otherwise the two sets of data are quite similar; in particular the slopes of the linear temperature dependence are nearly identical. A fit of the Bloch-Gruneisen expression (2) to resistivity data [71 as shown in Fig. 3 is good except at low temperatures where it appears that a power of less than T5 is required, probably something between T3 and T4. A discrepancy from the canonical T5 law for metals is not uncommon and has frequently been attributed to deviations from the ideal Debye phonon spectrum at low w's. The rough values of 0D provided by the fit are fairly large for metals, consistent with a picture of strong covalent bonding in these materials.
501
30
15 30
40-*!
10
0.
eOD-450K
NiSi2/si 1111)UHV 25 30-
3-. 20
5 COS 2 /SI(1111UHV 1982L 2.62
0
0
100
200
300o
400
"TWK)
Fig. 2. Temperature dependence of resistivity for CoSi2 an
Data Loading...
