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Large scale inhomogeneities in granular metals
Physica A 157 (1989) 220-222 North-Holland, Amsterdam
LARGE SCALE INHOMOGENEITIES
R.S. NEWROCK, Physics Department,
IN GRANULAR
M.G. DiSTEFANO*
METALS
and H.-K. SIN**
University of Cincinnati, Cincinnati, OH 45221, USA
S.A. DODDS Physics Department,
Rice University, Houston,
TX 772-51, USA
We present direct evidence for large-scale inhomogeneities in granular metal films. Using Rutherford backscattering data and measurements of the superconducting critical fields, we show that granular materials (Al:Al>Te, and AI:AI,O,) are deposited in layers. Each film contains 3-5 layers approximately 4OOA thick. The layer thicknesses are significantly less than the film thicknesses and are of the order of the coherence length. This layering has significant consequences when using such films to investigate dimensionally dependent theories.
Thin films are of great importance; the limited geometry allows exploration of critical regions inaccessible in bulk materials. Granular metals, composed of “islands” of metal in an insulating matrix, with high resistivities and considerable disorder, are useful for studying localization, percolation, superconductivity, and the intersections of these areas [l]. These films are assumed homogeneous, at least on long length scales, lengths significantly greater than the average grain size. Recently, indirect evidence [2,3] showed that in Al:Al,O,, the archetypal granular metal, inhomogeneities may exist. Data from critical field and fluctuation conductivity measurements indicated that supposedly homogeneous threedimensional films are composed of layers much thinner than the nominal thickness. The effective layer thickness was estimated to be about 300 A [3]. The existence of such inhomogeneities in films presumed to be homogeneous is important and needs verification. This note presents direct evidence for the layers and demonstrates that granular metals are not simple systems and care must be taken when evaluating experiments which depend crucially on sample homogeneity or on dimensionality. * Current address: Science Division, Northeast Missouri State University, Kirksville, MO 63501, USA. ** Current address: Department of Physics, Ajou University, 5 Wonchun-dong, Suwon, Korea.
0378-4371/89/$03.50 (North-Hnllnnd
0 Elsevier Science Publishers B.V.
Phwirr
Pllhlichinn
l-Grririnn\
R.S. Newrock et al. I Inhomogeneities
in granular metals
221
To determine if the layering in Al:Al,O, appears in other granular systems, and to obtain direct evidence for the layering, we investigated a different This system and the oxide have similar aluminum-based system, Al:Al,Te,. properties. We used Rutherford backscattering (RBS) to examine a large number of films and did critical field measurements. We investigated aluminum and tellurium films co-deposited on glass substrates. Sample preparation information will be presented elsewhere. Fig. 1 shows a typical example of the RBS data obtained; these data are for a metallic sample, 74~01% metal. The channel number is roughly proportional to the mass of the scatterer which determines the location of the peaks. The width of each peak is due to slowing of the alpha particles as they penetrate the specimen. The number of counts is proportional to the intensity and thus to the concentration of the scattering element at the depth in the film represented by the channel number. The width, shape and area of the peaks yield the thickness of the specimen and the relative concentration of each element as a function of depth in the film. The figure shows clear evidence for regions that are aluminum (or tellurium) rich and poor; the constituents are not deposited uniformly but segregate at, or soon after, deposition into reasonably distinct layers. The data do not indicate the existence of pure aluminum or tellurium strata-only that there exist well-defined strata that are rich or poor in a particular element. (However, the depth of the valleys is limited by instrumental resolution and random scattering, and the relative concentration differences are greater than these data indicate.) In an aluminum-rich layer all the tellurium will be in the telluride and the excess aluminum will (apparently) form a continuous layer of conduct-
3200-
Tellurium 2400-
:: q: J . .?! : I: . j:
if 5
1600-
’ .
Aluminum
z
l’Y.-J-h_ 0 300
I 500
;_i:.
r
d I 700
CHANNEL
‘$.
I 900
NUMBER
Fig. 1. Rutherford backscattering data for an Al:Al,Te, specimen. The channel number is proportional to the mass of the scattering element and the loss due to multiple scattering. The intensity is proportional to the amount of the scattering element present.
222
R.S. Newrock et al.
I Inhomogeneities
in granular metals
ing grains. We observe this layering over the entire metallic concentration range and well into the insulating regime - down to concentrations at least as low as 36% aluminum. We estimate each aluminum layer to be about 400 A thick; this correlates well with the effective thickness deduced from superconducting the properties [3]. The measured film thickness is greater than the coherence length in granular aluminum ( t( 1.5 K) = 300-800 A), but the conducting layer thickness is less than, or equal to, the coherence length. If these layers are not tightly coupled, and they do not appear to be, these presumed homogeneous threedimensional (5 < d) specimens are actually highly inhomogeneous, two-dimensional (5 > d) systems. Similar results were obtained for films over a wide concentration range, well into the insulating regime. We have measured the critical field and fluctuation conductivity for many of these specimens (the data will be published elsewhere); the apparent twodimensional nature is reflected in the superconducting properties. In summary, we observe macroscopic inhomogeneities in at least one family of granular films. The films segregate into distinct conducting layers and nominally three-dimensional films are actually two-dimensional. One must therefore be quite cautious in using granular films to investigate dimensionally dependent theories.
References [l] The papers on these topics are too numerous to list. A good starting point is the proceedings of the NATO Advanced Studies Conference Proceedings, Percolation, Localization and Superconductivity, S.A. Wolf and A.M. Goldman, NATO ASI Series B, vol. 109 (Plenum Press, New York, 1984). [2] G. Deutscher and S.A. Dodds, Phys. Rev. B 16 (1977) 3936. [3] S.A. Dodds, S.N. Harrington, R.S. Newrock and K. Loeffler, Phys. Rev. B 33 (1986) 3115.
LARGE SCALE INHOMOGENEITIES
R.S. NEWROCK, Physics Department,
IN GRANULAR
M.G. DiSTEFANO*
METALS
and H.-K. SIN**
University of Cincinnati, Cincinnati, OH 45221, USA
S.A. DODDS Physics Department,
Rice University, Houston,
TX 772-51, USA
We present direct evidence for large-scale inhomogeneities in granular metal films. Using Rutherford backscattering data and measurements of the superconducting critical fields, we show that granular materials (Al:Al>Te, and AI:AI,O,) are deposited in layers. Each film contains 3-5 layers approximately 4OOA thick. The layer thicknesses are significantly less than the film thicknesses and are of the order of the coherence length. This layering has significant consequences when using such films to investigate dimensionally dependent theories.
Thin films are of great importance; the limited geometry allows exploration of critical regions inaccessible in bulk materials. Granular metals, composed of “islands” of metal in an insulating matrix, with high resistivities and considerable disorder, are useful for studying localization, percolation, superconductivity, and the intersections of these areas [l]. These films are assumed homogeneous, at least on long length scales, lengths significantly greater than the average grain size. Recently, indirect evidence [2,3] showed that in Al:Al,O,, the archetypal granular metal, inhomogeneities may exist. Data from critical field and fluctuation conductivity measurements indicated that supposedly homogeneous threedimensional films are composed of layers much thinner than the nominal thickness. The effective layer thickness was estimated to be about 300 A [3]. The existence of such inhomogeneities in films presumed to be homogeneous is important and needs verification. This note presents direct evidence for the layers and demonstrates that granular metals are not simple systems and care must be taken when evaluating experiments which depend crucially on sample homogeneity or on dimensionality. * Current address: Science Division, Northeast Missouri State University, Kirksville, MO 63501, USA. ** Current address: Department of Physics, Ajou University, 5 Wonchun-dong, Suwon, Korea.
0378-4371/89/$03.50 (North-Hnllnnd
0 Elsevier Science Publishers B.V.
Phwirr
Pllhlichinn
l-Grririnn\
R.S. Newrock et al. I Inhomogeneities
in granular metals
221
To determine if the layering in Al:Al,O, appears in other granular systems, and to obtain direct evidence for the layering, we investigated a different This system and the oxide have similar aluminum-based system, Al:Al,Te,. properties. We used Rutherford backscattering (RBS) to examine a large number of films and did critical field measurements. We investigated aluminum and tellurium films co-deposited on glass substrates. Sample preparation information will be presented elsewhere. Fig. 1 shows a typical example of the RBS data obtained; these data are for a metallic sample, 74~01% metal. The channel number is roughly proportional to the mass of the scatterer which determines the location of the peaks. The width of each peak is due to slowing of the alpha particles as they penetrate the specimen. The number of counts is proportional to the intensity and thus to the concentration of the scattering element at the depth in the film represented by the channel number. The width, shape and area of the peaks yield the thickness of the specimen and the relative concentration of each element as a function of depth in the film. The figure shows clear evidence for regions that are aluminum (or tellurium) rich and poor; the constituents are not deposited uniformly but segregate at, or soon after, deposition into reasonably distinct layers. The data do not indicate the existence of pure aluminum or tellurium strata-only that there exist well-defined strata that are rich or poor in a particular element. (However, the depth of the valleys is limited by instrumental resolution and random scattering, and the relative concentration differences are greater than these data indicate.) In an aluminum-rich layer all the tellurium will be in the telluride and the excess aluminum will (apparently) form a continuous layer of conduct-
3200-
Tellurium 2400-
:: q: J . .?! : I: . j:
if 5
1600-
’ .
Aluminum
z
l’Y.-J-h_ 0 300
I 500
;_i:.
r
d I 700
CHANNEL
‘$.
I 900
NUMBER
Fig. 1. Rutherford backscattering data for an Al:Al,Te, specimen. The channel number is proportional to the mass of the scattering element and the loss due to multiple scattering. The intensity is proportional to the amount of the scattering element present.
222
R.S. Newrock et al.
I Inhomogeneities
in granular metals
ing grains. We observe this layering over the entire metallic concentration range and well into the insulating regime - down to concentrations at least as low as 36% aluminum. We estimate each aluminum layer to be about 400 A thick; this correlates well with the effective thickness deduced from superconducting the properties [3]. The measured film thickness is greater than the coherence length in granular aluminum ( t( 1.5 K) = 300-800 A), but the conducting layer thickness is less than, or equal to, the coherence length. If these layers are not tightly coupled, and they do not appear to be, these presumed homogeneous threedimensional (5 < d) specimens are actually highly inhomogeneous, two-dimensional (5 > d) systems. Similar results were obtained for films over a wide concentration range, well into the insulating regime. We have measured the critical field and fluctuation conductivity for many of these specimens (the data will be published elsewhere); the apparent twodimensional nature is reflected in the superconducting properties. In summary, we observe macroscopic inhomogeneities in at least one family of granular films. The films segregate into distinct conducting layers and nominally three-dimensional films are actually two-dimensional. One must therefore be quite cautious in using granular films to investigate dimensionally dependent theories.
References [l] The papers on these topics are too numerous to list. A good starting point is the proceedings of the NATO Advanced Studies Conference Proceedings, Percolation, Localization and Superconductivity, S.A. Wolf and A.M. Goldman, NATO ASI Series B, vol. 109 (Plenum Press, New York, 1984). [2] G. Deutscher and S.A. Dodds, Phys. Rev. B 16 (1977) 3936. [3] S.A. Dodds, S.N. Harrington, R.S. Newrock and K. Loeffler, Phys. Rev. B 33 (1986) 3115.