Short Notes K 3 1

phys. stat . sol. (a ) - 11, K 3 1 (1972)

Subject classification: 1 4 . 1 ; 1.5; 21

Solid State Physics Laboratory, Delhi

Effect of Artificial Ageing on the Hall Effect in Palladium Films 1 )

BY RAMESH CHANDER

The electrical and galvanomagnetic propertins of thin films prepared by vacuum

evaporation show irreversible changes with t ime and on thermal treatment (artificial

ageing) (1 to 3) . In a recent paper Romanowsky and Potoczna-Petru (2) have ob- - '(

served that N i and Co f i lms , when heated in vacuum ( 3 10 T o r r ) , showed a re-

sistance minimum at v 120 C. The properties of the palladium metal a r e s imi la r

to those of nickel and when the f resh ly deposited palladium films (40 to 700 8 thick)

a r e heated in vacuum (= 10

at temperatures in the range 120 to 160 C (4 ) . These changes in the resistance also

influence the Hall effect and the results of such measurements a r e reported in this

note.

0

-6 T o r r ) , these films a l so show a resist'mce minimum

0

Palladium films were prepared by vacuum evaporation of 99.94% pure palladium

foil (kindly supplied by D r . R . E . Howard of N.B.S. Washington D.C.) on vacuum

baked (at = 150 C) polished glass (gold sea l micro slides) substrates held at room

temperature. All the evaporations were made at a pressure of e 10

methods for the film thickness, resistivity, and Hall effect measurements have been

described ea r l i e r ( 4 , 5).

0

-6 Tor r . The

The Hall voltage in the as-deposited <and the annealed films (after heating the 0 -6

film upto 200 C at a pressure of = 10

pendent on the applied electric 'and magnetic fields. The sign of the Hall coefficient

is negative implying an electron dominant mechanism for the electrical conduction.

Curves 1 and 2 in Fig. 1 show the variation of the Hall coefficient

film thickness for the as-deposited and the annealed fi lms, respectively. It i s seen

from these plots that R

nesses it increases very rapidly with decrease in film thickness. It can be further

Torr and cooling to RT) was linearly de-

versus the 5-l

does not change much above 300 2 while at lower thick- H

1) The work was done by the author when h e was at t h e National Physical Laboratory, New Delhi-12, India.

K32 physica s ta tus solidi (3) 14

Fig. 1. T h e var ia t ion of Hal l coefficient R of the as-deposited ‘and annealed Pd f i rms as a function of th ickness (curves 1 and 2 respect ively) . C u r v e s 3 and 4

30 i show t h e var ia t ion of Hall mobility p -7 with th ickness f o r the as-deposi ted *c and annealed f i l m s , respect ively. Curve 5 F shows t h e var ia t ion of p with p /d

--u H H

pHiH:di:d(cm L’.‘s-‘j - C I 2

H

-_ 20 at for annealed f i lms

seen f r o m these c u r v e s that the annealed

f i lms with th ickness > 250 8, show a de-

H c r e a s e in R when compared with the R

of the as-deposi ted f i lms w h e r e a s f i l m s

< 250 2 show an i n c r e a s e in the value of

:0

H -*

0 200 LOO . 500 thickness /A1 - - -

the Hall coefficient. Such changes in the Hall effect can b e explained on the basis

of the modification of the s t r u c t u r e of the f i lms on t h e r m a l t rea tment . Vand (6) was

the f i r s t to explain the decay of imperfec t ions on ageing in the freshly deposi ted

f i lms a s well a s in the f i l m s subjected to t h e r m a l t rea tment , F r o m t h e e lec t ron

diffraction s t u d i e s , Fuj ime (7) showed

27

250

et 31. (8) showed that t h e res i s t iv i ty of the palladium f i lms deposi ted and measured 0 0 0

at 78 K and m e a s u r e d a t 78 K .

This shows that the f i l m s deposited at 300 K w e r e s t ruc tura l ly m o r e perfect a s

compared to those deposited at 78 K . M u r r (9) h a s recent ly shown f r o m electron

microscopic s tud ies that the g r a i n s i z e of the palladium f i l m s i n c r e a s e s with in-

c r e a s e in the s u b s t r a t e t e m p e r a t u r e . T h e r e f o r e the effect of heating the f i lm a f t e r

formation and of depositing i t on a s u b s t r a t e held a t t h e s a m e annealing t e m p e r a t u r e

h a s on analogous effect on the s t r u c t u r e . T h e g n i n growth may b e l a r g e r in t h e

la t te r c a s e . It i s t h e r e f o r e poss ib le that the d e c r e a s e in the res i s t iv i ty observed

a f t e r annealing may be due to ( i ) the removal of thermal ly unstable imper tec t ions

.and ( i i ) the i n c r e a s e in the gra in s i z e and order ing in the f i l m s . T h i s will resu l t

in the smoothening of the film. Koppe and Bryan (10) have shown that t h e i m p e r -

fect ions ‘affect the resis t ivi ty much m o r e than the Hall coefficient. T h e d e c r e a s e

that the palladium f i lms deposi ted at 0 K had an a m o r p h o u s , s t r u c t u r e which turned polycrystal l ine on warming above 0 K . Grain growth therefore a l s o o c c u r s in the p r o c e s s of annealing. blikolaichuk

K was l a r g e r than that of the f i lms deposi ted at 300 0

0

Short Notes K33

in the Hall coefficient above 250 8 indicates the increase in the number of electrons

taking part in conduction. This is understandable because with the smoothening of

the film, the specular reflection coefficient also increases (4). Mikolaichuk et al. (8)

have also shown that the structural behaviour of nickel and palladium films is

similar. Swanson et al. (11) have shown that nickel films below 200 w possess a

discontinuous structure. Therefore when the palladium films 250 ware heated

in vacuum, there may be chemisorption of the residual gases along the grain

boundaries. The chemisorption process removes the electrons otherwise taking part

in conduction by forming a covalent bond (12) between the metal and gas atoms. This

will result in the increase in the value of the Hall ceefficient. The films become

less prone to chemisorption when these become more and more continuous a s the

film thickness increases.

Curve 3 and 4 in the figure show the variation of the Hall mobility with H thickness for the unannealed and annealed films, respectively. The values of the

Hall mobility for the annealed films a re larger than those for the as-deposited films

in the entire thickness range. The larger mobility values a re indicative of the fact

that the annealing decreased the resistivity to a larger extent than the corresponding

changes of the Hall coefficient, The variation of the Hall mobility with thickness (12)

can be utilised for determining the conduction parameter, Accordingly by using the

size effect theory the Hall mobility p has been plotted as a function of p /d in

curve 5. For ideal and reproducible films, the points should have been plotted on a

straight line because the Fuchs-Sondheimer (13) theory has been formulated for

idealized plane-parallel films. Therefore the straight line represented by curve 5

gives a kind of an average. From the slope of this plot we can get the value of the

mean free path 1. It comes out to be 260 2. This value i s smaller than the value

480 2 obtained from the resistivity data (4) and supports the view that the carr ier

density changes with the thickness,

H

The author is grateful to Prof. S. C. Jain for his valuable guidance and per-

mission to publish this note.

References

(1) G. HASS (Ed,), Physics of Thin Films, Vol. 2, Academic Press , New York 1963.

(2) W. ROMANOWSKY and D. POTOCZNA-PETRU, Thin Solid Films S , 35 (1971).

3 physica (a)

K 34 physica status solidi (a) 14

(3) S.C. JAIN and RAMESH CHANDER, Thin Solid F i l m s 4, R11 (1969). (4) RAMESH CHANDER, Ph.D. Thes i s , Delhi University, 1970; unpublished.

(5) RAMESH CHANDER, R.E. HOWARD, and S .C . JAIN, J. appl. Phys. 38,

4092 (1967).

(6) V. VAND, Proc. Phys. SOC. 55, 222 (1943). (7) S. FUJIME, Japan.J. appl. Phys. 1% 305 (1967).

(8) N.A. MIKOLAICHUK, R.S. PANCHISHIN, and Z.V. STASYUK, Ukr. fiz.

Zh. 14, 747 (1969). (9) L.E. MURR, Thin Solid Films 1, 101 (1971). (10) H. KOPPE and J.M. BRYAN, Canad. J. Phys. 29- 274 (1951).

(11) J.G. SWANSON, D.S. CAMPBELL, and J.C. ANDERSON, Thin Solid F i lms I , 325 (1967/68).

(12) R. SUHRMANN, G. WEDLER, and S. SCHUMICKI, S t ruc ture and P rope r t i e s

of Thin F i lms , Wi ley , New York 1959 (p. 268).

(13) E.H. SONDHEIMER, Adv. Phys. 1, l(1952).

(Received August 10, 1972)