Thin SolidFilms, 53 (1978) L9-LI 1 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands L9

LeHer

On the resistivity-temperature variation of manganese and M n - M g F 2 thin films

L. OLUMEKOR* AND J. BEYNON

Physics Department, Brunel University, Hillingdon, Middlesex ( Gt. Britain)

(Received June 13, 1978; accepted June 30, 1978)

Manganese and M n - M g F 2 thin films were deposited by vacuum evaporation onto Corning 7059 glass substrates held at 22 °C at a residual gas pressure of about 10 -5 Torr (1.33 mPa) or less; the deposition rate was approximately 0.4 nm s-1. After deposition the films were annealed at various temperatures and the resistance was measured between 110 K and the annealing temperature using a four-terminal technique with aluminium contact pads.

Figure I shows the p-T variations for various film thicknesses, annealing temperatures and annealing times. The numbers on the left-hand side of the curves refer to thickness in nanometres and those on the right-hand side refer first to the annealing temperature in kelvins and second to the annealing time in hours.

After annealing for I h at 620 K the resistivity of all the manganese films studied (thickness />25 nm) increased slowly as the temperature increased; the resistivity variation is practically linear for the 100 nm film and the resistivity increases by about 15%, which agrees reasonably well with the value t of approximately 13% for high purity annealed bulk manganese. The 25 nm and 50 nm films are interesting because p is reasonably constant between 300 and 450 K. It is thought that the plateau is a consequence of incomplete annealing as a similar result was not obtained when the annealing time was extended to 3 h or when the temperature was increased to 720 K.

The results for the 25 nm and 50 nm cermets of 39 vol.% manganese composition also exhibit a plateau region which disappears under conditions similar to those for the manganese films. The predominant conduction mechanism is an activated one in contrast with a non-activated process in the 100 nm cermets. Thus it is expected that there will be a thickness for which the TCR is zero over a wide temperature range; this thickness lies between 50 and 100 nm for 39 vol.% manganese cermets. For comparison, this change in the sign of the TCR has been observed 2 in gold films at a thickness of approximately 16-17 nm.

In order to ascertain whether the slow variation in the resistivity of the annealed M n - M g F 2 cermets between 110 and 620 K was caused primarily by the presence of manganese or MgF2, it was decided to carry out a subsidiary experiment with Mn-S iO and C u - M g F 2 cermets. It was found that cermets with MgF 2 as the insulator had a higher resistivity than those containing SiO, with M n - M g F 2 having the highest resistivity. The percentage change in p across the temperature range was

* Present address : Department of Physics, University of Benin, Benin City, Nigeria.

L 10 LETTERS

7O0

£ 5 0 0

¢ 0 o

ioo I

o

i o ~ ~ "I 2.o, t ~$ " . 6 2 o , I 25

30 ~ r ~ . O j t Ioo

--TE.MP. T/IO~" K •

Fig. 1. Schematic representation ofthe variation of resistivity with temperature for manganese ( - - ) and 39 vol.~o Mn-MgF 2 ( - - - ) thin films.

- 14~o for M n - M g F / a n d - 35~o for C u - M g F z c o m p a r e d with - 20~o for M n - S i O and - 5 0 ~ o for C u - S i O . Thus the presence of manganese tends to p roduce a prac t ica l ly flat p - T curve agreeing with observa t ions on bu lk manganese , whilst the presence of M g F 2 tends to p roduce higher resistivit ies than SiO. This la t ter result is con t r a ry to tha t found 3 with A u - M g F 2 and A u - S i O .

Unannea l ed manganese and M n - M g F 2 cermets depos i ted at 295 K and cycled between 110 and 295 K gave l inear Ar rhen ius - type graphs. The ac t iva t ion energies defined convent iona l ly by

P = Po e x p ( E a / 2 k T )

are l isted in Table I.

TABLE I EXPERIMENTALLY AND THEORETICALLY DERIVED ACTIVATION ENERGIES FOR UNANNEALED MANGANESE AND

Mn-MgF 2 THIN FILMS

Vol.% Mn Film thickness (nm) E a (meV)

Experimental Calculated 4

100 25 31 25±6 100 50 8

63 25 79

39 25 150 39 50 83 39 100 21

59 + 16

108 + 22

The increase in E a with increase in insu la tor content and decrease in thickness is as expected. Po was found to increase by a factor of a b o u t two between 25 and 100 nm for the 39 voLVo manganese cermets. This increase agrees in pr inciple with Kazmersk i and Racine 5. It will be not iced tha t the exper imenta l ly ob ta ined ac t iva t ion energies agree fairly well with the ca lcula ted values ob ta ined from elect ron mic rographs if the upper l imit is taken.

LETTERS L 11

Arrhenius-type graphs for the annealed 25 nm 39 vol.% manganese cermets are decidedly non-linear above about 250 K (E~ is approximately 2 meV below about 250 K and approximately 40 meV at 500 K). However, the 50 nm cermets are reasonably linear over the whole temperature range studied, with E a ~ 2 meV. T - 1/2 and T - 1/4 laws have the effect of linearizing the results but it is not possible to distinguish between them at present; clearly resistivity measurements below about 110 K are necessary.

Electron micrographs of annealed 39 vol.~o manganese cermets indicate the presence of individual islands and a metallic maze structure. The conduction mechanism must be a combination of an activated and a metallic process acting in parallel. In the terminology of Abeles et al. 6, such cermets lie in the transitional region. Thus the sign of the TCR will depend critically on the Composition and thickness. As the manganese composition increases and the thickness increases the percolation path density increases and the contribution of the parallel activated process diminishes. This trend is indicated in Table I.

1 G.T. Meaden, Electrical Properties o f Metals, Heywood Books, 1965. 2 L.A. Weitzenkamp and N. M. Boshara, Trans. Metall. Soc. AIME, 236 (1966) 351. 3 M. Beckerman and R. E. Thun, 8th Annual American Vacuum Society Meeting, 2 (1961) 905. 4 J. Beynon and L. Olumekor, Thin Solid Films, 24 (1974) $30. 5 L.L. Kazmerski and D. M. Racine, J. Appl. Phys., 46 (1975) 791. 6 B. Abeles, P. Sheng, M. D. Coutts and Y. Arie, Adv. Phys., 24 (1975) 407.