Structure origins of diffraction pattern features of diamond-like films

Diamond and Related Materials, 2 (1993) 573-574

573

Structure origins of diffraction pattern features of diamond-like films E. A. Pilankiewicz Institute of Surface Chemistry, Prospekt Nauki 31,252-028 Kiev (Ukraine)

S. Mitura Institute of Materials Science and Engineering, Technical University of L6d~, ul. Stefanowskiego 1, 90-924 L6d~ (Poland)

Abstract Possible explanations for the appearance of a forbidden peak for diamond-like carbon films are analysed. The impossibility of a purely diffractional diagnosis of the structure in such a situation is shown.

1. Introduction The problem of forbidden peaks in the diffraction pattern of diamond-like lattices has been discussed for a long time [1]. Nevertheless, the only reasonable explanation for this phenomenon is based on multiple diffraction [2]. This explanation, however, does not seem to be universally applicable, especially for polycrystalline and faulted materials. So, another explanation, a twinning and stacking faults effect, is also often cited. This explanation is also supported by the characteristic streaking on the diffraction patterns.

to the appearance of new maxima. This phenomenon is well known for close-packed lattices. Let us analyse this possibility for diamond-like lattices. Any diamond-like lattice can be presented as a superposition of two close-packed lattices. These sublattices 1 are shifted by ~ space diagonally to the f.c.c, unit cell. They are identical for diamond, silicon and germanium, and consist of different kinds of atoms, as in the case of A"B s-" compounds (such as SiC, ZnS etc.). That is why the Laue function for the diamond-like lattice can be represented by the superposition of two such functions for close-packed lattices, i.e. F=

~

f, exp(isr,)

n - - -- c~3

2. Analysis

= ~ {fA exp(isr,) +fB exp Its(r, " + ~d111 1 )]} n

There are other possibilities worth mentioning before proceeding to the present investigation. Firstly, in the case of highly oriented graphite with a basic plane parallel to the substrate, all (00/) peaks disappear, and the apparent diffraction pattern can be identified as an f.c.c, lattice with the same constant as diamond. Nevertheless, all other properties of the film will be those of graphite. Secondly, in the case of very thin films with an odd number of layers these forbidden peaks can appear owing to a difference between the two sublattices from which the diamond lattice is formed. Direct calculation of the Laue function for this case shows that only in very thin films (up to approximately 200 A) can this be the case. Moreover, other additional peaks also appear. Despite the complicated form of the diffraction pattern, all the physical properties are those of diamond. The formation of (1ll) twinning and stacking faults in the case of a non-random distribution can also lead

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- _ (fA _ + j~, e x p ~isd111~ )~exp(isr,) -

where s is the diffraction vector in the reciprocal lattice coordinates, r is the radius vector of the nth atom, fA and fB are the atomic scattering factors for A and B close-packed lattices respectively, and d111 is the vector coinciding with space diagonal to the f.c.c, unit cell. The sum in the last expression, as is easily observed, is the Laue sum for the close-packed lattice with the same sequence of close-packed layer positions, i.e. with the same distribution of twinning and stacking faults. The final expression for the structure scattering factor F21 for the diamond-like lattice can be written as

Fd21=

(sd111~G

f 2 + fi] -I- 2fAfB C0S t ~ ) /

F~pl

fc2pl is the structure scattering factor of the closepacked lattice. where

© 1993 - Elsevier Sequoia. All rights reserved

E. A. Pilankiewicz, S. Mitura / Structure and diffraction patterns

574

Fc2pl J

pared. As was shown previously by one of the authors [3], no one-dimensional disordering can result in a peak in the position forbidden for a diamond-like structure such as (200), where cos(sdlll/4) = 0. This excludes twinnlng and stacking faults as possible explanations of the appearance of the (200) peak, although shifting and streaking of peaks are sound evidence of these structure imperfections. The same F21 expression shows that for non-identity of the close-packed sublattices the (200) peak appears in the case of corresponding layer stacking, as in cubic SiC. Such non-identity can also be caused by non-uniform distributions of vacancies and impurities (such as hydrogen and oxygen) which markedly change other physical properties of the material as well.

/

3. Conclusions

111

200

Fig. 1. Distribution of the diffracted intensity for f.c.c. (upper curve) and diamond (lower curve) lattices with one-dimensional random disordering. The stacking fault concentration is 0.2. Bragg positions for 111 and 200 peaks are marked with vertical lines.

Summing up, we can draw two explicit conclusions: (i) the problem of the appearance of the (200) peak in the diffraction pattern of a diamond-like structure cannot be treated as closed, and (ii) in the case of such an appearance the nature of the material under investigation has to be determined not only by diffraction but also by other methods such as measurements of the Raman shift and refractive index.

In the case of diamond F 2' = 2 f 2

Sdll?~ Fc2pl 1 -'l-cos ~ - )

and F]l = 0 for the (200) peak position, irrespective of twinning and stacking faults. This is clearly seen in Fig. 1, where F21 and F2pl for faulted crystals are com-

References 1 E. G. Spencer, P. H. Schmidt, D. C. Joy Phys. Lett., 29 (1976) 118-120. 2 K. N. Tu and A. Howie, Philos. Mag. B, 3 A. N. Pilankiewicz, E. A. Pilankiewicz, Chuistov, Sverkhtvordye Mater., 5 (1983)

and F. J. Sansalone, Appl.

37 (1978) 73-81. A. N. Ustinov and K. V. 23-25.