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MgF2 cermet thin films
$30
Thin Solid Films, 24 (1974) $30 - $32 © Elsevier Sequoia S.A., Lausanne -- Printed in Switzerland
Short Communication
Resistivity and composition of Mn/MgF 2 cermet thin films J. BEYNON and L. OLUMEKOR
Physics Department, Brunel University, Kingston Lane, Hillingdon, Middlesex (Gt. Britain) (Received September 13, 1974; accepted September 20, 1974)
The variation of the resistivity of Mn/MgF2 cermets with thickness (50 - 1500 A) and composition (0 - 50% MgF2, by weight) has been investigated. The cermets were prepared by single-boat evaporation in a vacuum of about 10 - 5 torr (= 1.33 mPa). The starting material and the deposited thin films were analysed using, respectively, X-ray diffraction and electron microscopy. The starting material consisted of Mn powder, 99.9% pure (Griffin & George Ltd,), and MgF2 4-40 mesh (B.D.H. Ltd.). The MgF2 was first ground into a fine powder before the Mn was added in the correct proportion by weight. Both materials were again ground thoroughly to ensure complete mixing. The cermet mixture was evaporated from a tantalum boat-shaped filament in a vacuum of about 1 X 10 - 5 tort; the residual pressure was n o t allowed to rise above 2 × 10 - 5 tort. The substrate (Coming 7059 glass), cleaned by conventional techniques 1, was maintained at 100 ° + 2 °C throughout the deposition. The deposition rate was fixed at about 8 A s- 1 . Resistor tracks, 3 m m × 1.2 cm in size, were fabricated using a Mn/Cu alloy mechanical mask, and the contact lands were of manganin. The resistance was measured using a four-terminal technique 2. The variation of the resistivity p with film thickness, shown in Fig. 1, appears to consist of three regions, labelled A, B and C for convenience. Region A refers to films approximately 100 A thick or less. This region is generally characterized by a sharp rise in p for a small fall in thickness. In our case p rises from about 7 × 10 - 4 fi cm for a 100 A film to about 2.5 f~ cm for a 50 A film with a composition of 80% Mn. No electron micrographs were taken for the 50 A films but those for the 250 A films showed a discontinuous structure, consisting of discrete metallic islands separated by dielectric. Hence films in region A must also be characterized by a similar structural discontinuity. The electrical conduction in films in region A depends on the applied electric field, the potential between conducting islands, the island work function, image forces, the island separation and the dielectric constant of the insulator. Two models, electron tunnelling3" 5 and thermionic emission s. v, are frequently used to explain the conduction process in thin discontinuous films, although the electron tunnelling model appears to be more satisfactory.
$31
L^~
°~I- '
8
~
' ~-~.__~I .~(1~
41110
GoO
~-
~ IrO0
)OI~D
,o.~.~ 1200
I~.I~D
I/'O0
Fig. 1. Variation of the resistivity of Mn/MgF 2 cermets with thickness.
In region B (~ 100 - 800 A) the conducting islands have grown larger in size and the fall in p m a y be attributed to a fall in the dielectric spacing between islands. However, the conduction process appears to be the same as in region A. In region C (~ 800 A) p is lower than in regions A and B and is nearly constant for a given composition. The lower value of p may be accounted for by the considerable increase in the island size, due to agglomeration, with a corresponding fall in the activation energy for tunnelling. It is only in this region that the product of resistance and thickness is constant. Also, it is found that experimental results are more reproducible here than in either of the other regions. It may be noticed that p for 100% Mn at 1500 A is a b o u t three times larger than the bulk lVln value. The starting material was analysed using X-ray diffraction (Phillips spectrometer, model 1008) and found to consist of a-Mn, ~-Mn and MgF 2 sellalte. The cermet thin films were analysed using transmission electron diffraction (JEM 7 electron microscope). The films were prepared on cellulose acetate, also maintained at 100 °C during evaporation, and were 250 A thick. The results for three compositions, 100, 80 and 60% Mn, are given in Table I. It was observed that an increase in the MgF 2 content of the starting material produced a decrease in the Mn island size and an increase in the island separation. The activation energy E was estimated using Neugebauer and Webb's conduction model s and was found to rise as the MgF 2 content rose (Table II).
$32 TABLE I Composition of cermet thin films Composition (wt.% Mn)
Identification
100 80 60
Manganese(I) oxide, gamma 2U Mn Manganese(II) oxide, fl-Mn, ~-MgF 2 MnO 2 (pyrolusite), a-Mn, fl-Mn, ~-MgF 2
TABLE II Variation of island radius, separation and activation energy with composition for cermet films 250 • thick Composition (wt.% Mn)
Island radius (10 - 7 cm)
Island separation (10 --6 cm)
Activation energy (10 - 2 eV)
100 80 60
8.13 + 2.11 3.73 + 0.90 2.14 +- 0.40
3.33 + 0.85 4.28 + 1.39 7.14 + 1.90
2.46 + 0.60 5.90 + 1.64 10.80 + 2.15
These values are a factor of 10 larger than those obtained experimentally for Mn/SiO cermets 1. We are in the process of obtaining experimental values of the activation energy for Mn/MgF2 cermets, which will be made the subject of a future communication. The values for the island radius and separation quoted in Table II were estimated from a sample of 50 separate measurements. Critical metal composition s was n o t observed in these cermet films. The crystalline structures of MgF2 found by Beckerman and Thun 9 in Cr/MgF2 cermets containing less than 82% Cr were not observed either, but the blue-black coloration which these investigators found for Au/MgF 2 cermets of 60% Au was observed in our cermets at thicknesses greater than 1500 A and compositions in the range 50 - 90% Mn. However, unlike Beckerman and Thun, no electrical discontinuities were observed in the present study. One of us (L. O.) wishes to thank the Nigerian Government for the award of a post graduate maintenance grant. I J. Beynon and N. R. P. Milway, Thin Solid Films, 14 (1972) 387. 2 L. I. Maissel and R. Glang, Handbook o f Thin Film Technology, McGraw-Hill, New York, 1970. 3 C. A. Neugebauer and M. B. Webb, J. Appl. Phys., 33 (1962) 74. 4 T. E. Hartman, J. Appl. Phys., 34 (1963) 943. 5 D. S. Herman and T. N. Rhodin, J. Appl. Phys., 37 (1966) 1594. 6 K. van Steensel, Phillips Res. Rep., 22 (1967) 246. 7 Sen-Sik Minn, J. Rech. Centre Nat. Sci. Lab. (Paris), 5 (1960) 131. 8 A. A. Milgram and C. S. Lu, J. Appl. Phys., 39 (1968) 4219. 9 M. Beckerman and R. E. Thun, Trans. 8th Amer. Vacuum Soc., 1962, p. 903.
Thin Solid Films, 24 (1974) $30 - $32 © Elsevier Sequoia S.A., Lausanne -- Printed in Switzerland
Short Communication
Resistivity and composition of Mn/MgF 2 cermet thin films J. BEYNON and L. OLUMEKOR
Physics Department, Brunel University, Kingston Lane, Hillingdon, Middlesex (Gt. Britain) (Received September 13, 1974; accepted September 20, 1974)
The variation of the resistivity of Mn/MgF2 cermets with thickness (50 - 1500 A) and composition (0 - 50% MgF2, by weight) has been investigated. The cermets were prepared by single-boat evaporation in a vacuum of about 10 - 5 torr (= 1.33 mPa). The starting material and the deposited thin films were analysed using, respectively, X-ray diffraction and electron microscopy. The starting material consisted of Mn powder, 99.9% pure (Griffin & George Ltd,), and MgF2 4-40 mesh (B.D.H. Ltd.). The MgF2 was first ground into a fine powder before the Mn was added in the correct proportion by weight. Both materials were again ground thoroughly to ensure complete mixing. The cermet mixture was evaporated from a tantalum boat-shaped filament in a vacuum of about 1 X 10 - 5 tort; the residual pressure was n o t allowed to rise above 2 × 10 - 5 tort. The substrate (Coming 7059 glass), cleaned by conventional techniques 1, was maintained at 100 ° + 2 °C throughout the deposition. The deposition rate was fixed at about 8 A s- 1 . Resistor tracks, 3 m m × 1.2 cm in size, were fabricated using a Mn/Cu alloy mechanical mask, and the contact lands were of manganin. The resistance was measured using a four-terminal technique 2. The variation of the resistivity p with film thickness, shown in Fig. 1, appears to consist of three regions, labelled A, B and C for convenience. Region A refers to films approximately 100 A thick or less. This region is generally characterized by a sharp rise in p for a small fall in thickness. In our case p rises from about 7 × 10 - 4 fi cm for a 100 A film to about 2.5 f~ cm for a 50 A film with a composition of 80% Mn. No electron micrographs were taken for the 50 A films but those for the 250 A films showed a discontinuous structure, consisting of discrete metallic islands separated by dielectric. Hence films in region A must also be characterized by a similar structural discontinuity. The electrical conduction in films in region A depends on the applied electric field, the potential between conducting islands, the island work function, image forces, the island separation and the dielectric constant of the insulator. Two models, electron tunnelling3" 5 and thermionic emission s. v, are frequently used to explain the conduction process in thin discontinuous films, although the electron tunnelling model appears to be more satisfactory.
$31
L^~
°~I- '
8
~
' ~-~.__~I .~(1~
41110
GoO
~-
~ IrO0
)OI~D
,o.~.~ 1200
I~.I~D
I/'O0
Fig. 1. Variation of the resistivity of Mn/MgF 2 cermets with thickness.
In region B (~ 100 - 800 A) the conducting islands have grown larger in size and the fall in p m a y be attributed to a fall in the dielectric spacing between islands. However, the conduction process appears to be the same as in region A. In region C (~ 800 A) p is lower than in regions A and B and is nearly constant for a given composition. The lower value of p may be accounted for by the considerable increase in the island size, due to agglomeration, with a corresponding fall in the activation energy for tunnelling. It is only in this region that the product of resistance and thickness is constant. Also, it is found that experimental results are more reproducible here than in either of the other regions. It may be noticed that p for 100% Mn at 1500 A is a b o u t three times larger than the bulk lVln value. The starting material was analysed using X-ray diffraction (Phillips spectrometer, model 1008) and found to consist of a-Mn, ~-Mn and MgF 2 sellalte. The cermet thin films were analysed using transmission electron diffraction (JEM 7 electron microscope). The films were prepared on cellulose acetate, also maintained at 100 °C during evaporation, and were 250 A thick. The results for three compositions, 100, 80 and 60% Mn, are given in Table I. It was observed that an increase in the MgF 2 content of the starting material produced a decrease in the Mn island size and an increase in the island separation. The activation energy E was estimated using Neugebauer and Webb's conduction model s and was found to rise as the MgF 2 content rose (Table II).
$32 TABLE I Composition of cermet thin films Composition (wt.% Mn)
Identification
100 80 60
Manganese(I) oxide, gamma 2U Mn Manganese(II) oxide, fl-Mn, ~-MgF 2 MnO 2 (pyrolusite), a-Mn, fl-Mn, ~-MgF 2
TABLE II Variation of island radius, separation and activation energy with composition for cermet films 250 • thick Composition (wt.% Mn)
Island radius (10 - 7 cm)
Island separation (10 --6 cm)
Activation energy (10 - 2 eV)
100 80 60
8.13 + 2.11 3.73 + 0.90 2.14 +- 0.40
3.33 + 0.85 4.28 + 1.39 7.14 + 1.90
2.46 + 0.60 5.90 + 1.64 10.80 + 2.15
These values are a factor of 10 larger than those obtained experimentally for Mn/SiO cermets 1. We are in the process of obtaining experimental values of the activation energy for Mn/MgF2 cermets, which will be made the subject of a future communication. The values for the island radius and separation quoted in Table II were estimated from a sample of 50 separate measurements. Critical metal composition s was n o t observed in these cermet films. The crystalline structures of MgF2 found by Beckerman and Thun 9 in Cr/MgF2 cermets containing less than 82% Cr were not observed either, but the blue-black coloration which these investigators found for Au/MgF 2 cermets of 60% Au was observed in our cermets at thicknesses greater than 1500 A and compositions in the range 50 - 90% Mn. However, unlike Beckerman and Thun, no electrical discontinuities were observed in the present study. One of us (L. O.) wishes to thank the Nigerian Government for the award of a post graduate maintenance grant. I J. Beynon and N. R. P. Milway, Thin Solid Films, 14 (1972) 387. 2 L. I. Maissel and R. Glang, Handbook o f Thin Film Technology, McGraw-Hill, New York, 1970. 3 C. A. Neugebauer and M. B. Webb, J. Appl. Phys., 33 (1962) 74. 4 T. E. Hartman, J. Appl. Phys., 34 (1963) 943. 5 D. S. Herman and T. N. Rhodin, J. Appl. Phys., 37 (1966) 1594. 6 K. van Steensel, Phillips Res. Rep., 22 (1967) 246. 7 Sen-Sik Minn, J. Rech. Centre Nat. Sci. Lab. (Paris), 5 (1960) 131. 8 A. A. Milgram and C. S. Lu, J. Appl. Phys., 39 (1968) 4219. 9 M. Beckerman and R. E. Thun, Trans. 8th Amer. Vacuum Soc., 1962, p. 903.