Preferred orientation in erbium thin films observed ... - IUCr Journals

770

J. Appl. Cryst. (1992). 25, 770-777

Preferred Orientation in Erbium Thin Films Observed Using Synchrotron Radiation BY M. A. PLAYER, G. V. MARR, E. Gu, H. SAVALONIAND N. ONCAN

Department of Engineering, University of Aberdeen, Aberdeen AB9 2UE, Scotland AND I. H. MUNRO

SERC Daresbury Laboratory, Warrington WA4 4AD, England (Received 13 March 1992; accepted ll June 1992)

We have used the energy-dispersive powder diffraction camera on Station 9.7 at the Daresbury SRS This paper describes the use of energy-dispersive (Clark, 1989) to obtain pole figures for erbium films of diffraction using synchrotron radiation to obtain prenominal thickness 6000 A on molybdenum substrates liminary measurements of pole figures for a 6000 A of surface roughness Rq -- 1500 A. These films were erbium film deposited by UHV evaporation on deposited by electron-beam evaporation of 99.99% molybdenum substrates. A low glancing angle is used purity erbium metal targets under UHV conditions, and the pole-figure polar distance is scanned by rotausing a Balzers UMS 500 system. Pressure during ting the sample about the direction of the incident evaporation was 10-5-10 -6 Pa and the deposition beam. Correction formulae are derived for pole-figure rate was 25 A s- 1. A full account of the experimental intensity and position in this geometry. Results conditions and of the microstructures observed in confirm strong 002 orientation of films deposited at a these films is given elsewhere (Savaloni, Player, Gu & 673 K substrate temperature (near the middle of zone Marr, 1992; Gu, Savaloni, Player & Marr, 1992). In II for erbium), show that at an ambient substrate this paper we give results for the textures of three temperature (zone I) there is a mixture of 002 and 101 samples, deposited at substrate temperatures T~ of orientations, and demonstrate strong dependence of 673, 573 and 303 K. The first two temperatures lie the 002 orientation direction (for zone II tempera- respectively near the centre and in the lower part of tures) on the angle of vapour incidence during deposi- zone II of the Movchan & Demchishin or Thornton tion. structure-zone models (which broadly describe the microstructure of such films) while the 303 K film lies in zone I (Savaloni, Player, Gu & Marr, 1992; Movchan & Demchishin, 1969; Thornton, 1975). The samIntroduction ple for T~ = 573 K was deposited using an angle of The advantages of synchrotron radiation and energy- incidence of approximately 45 ° for the erbium evapodispersive diffraction for structural studies of thin rant, while the other two samples were deposited at films have been appreciated for some time (Cernik, near normal incidence, with angle of incidence apClark & Pattison, 1989). To enhance the relative proximately 8.5 °. It is known from comparison of the relative inintensity of diffraction from a thin film, it is necessary to use a low glancing angle (Iyengar, Santana, Wind- tensities of the 002, 101 and 100 diffraction peaks, ischmann & Engler, 1986; Larsen, McNulty, Goehner using energy-dispersive diffraction with a fixed sample & Crystal, 1988), so the strong collimation as well as orientation, that the zone-II films show strong 002 the high intensity of synchrotron radiation is very preferred orientation peaking at around Ts = 673 K, desirable. Departure from Bragg-Brentano geometry while on this basis the zone-I films show no clear requires intensity and position correction of pole evidence of preferred orientation (Savaloni, Player, figures, but this may be done with satisfactory accur- Gu & Marr, 1992). Our aims in making the measureacy (Heizmann, Vadon, Schlatter & Bessirres, 1988) ments presented in this paper were to establish the and collimated radiation avoids the defocusing effects viability of our method, to confirm and examine in usually associated with this departure. Exploitation of more detail the (002) preferred orientation of the the broad available bandwidth, by using energy-dis- zone-II films, to elucidate the texture of lower-tempersive detection, further increases the speed of data perature films and to examine the effect on preferred orientation of oblique vapour incidence. collection. Abstract

0021-8898/92/060770-08506.00

© 1992 International Union of Crystallography

M. A. PLAYER, G. V. MARR, E. GU, H. SAVALONI, N. O N C A N AND I. H. M U N R O

Pole-figure corrections

771

and proceeding similarly to give

Our experimental arrangement differs from that described by Heizmann, Vadon, Schatter & Bessi+res (1988) in that (apart from the use of synchrotron radiation) the scan of polar distance ~ is accomplished by rolling the sample about the axis of the incident beam, rather than an axis in the plane of the sample. This means that, for our geometry, the incident glancing angle 0~ remains constant as ~ is scanned. Scanning of azimuth ~o is accomplished as usual by rotating the sample about its normal. The diffraction angle 20 remains fixed, as is usual for energydispersive measurement.

sin A~o = (cos a sin 0 -- sin a cos 0 cos @')/]sin ~ I. (9) For intensity correction, considered in the next subsection, the angles between the sample normal and the incident and diffracted beams will be required (Fig. 2). They are given respectively by cos fl = n. z = sin ~

(10)

and cos ~, = n. d' = cos ~ sin 20 cos ~,' - sin ~ cos 20, (11)

Position correction The relationship between the true pole-figure position (~,, ~o) and the applied roll angle ~' and azimuth tp' may be determined by reference to Fig. 1. If b' denotes the scattering vector rotated through the roll angle q / f r o m its reference position at ~' = 0 represented by b, then inspection of Fig. 1 shows that we can write for the true polar distance ~ and the azimuth deviation Aq~ = ~o - q~' cos ~

--

b"n,

n cos A~p = [(b' x n) x y]/Ib' x nl = [(b' x n) x y]/Isin ~1.

(1)

Detector All~

'

t//?~ -.... ~ \+l \ ..... I~~--~T~,Y/

A~

"PAC ",, \ lY .{-o . z I

\\ ~

\ I/

Incident beam

%_,.--~