ThinSolidFilms, 113(1984) 151-156

PREPARATION AND CHARACTERIZATION 151

KIKUCHI RINGS FROM POLYCRYSTALLINE PLATINUM FILMS WITH A (111) FIBER AXIS

PANKAJ DIXIT AND R. W. VOOK

Department of Chemical Engineering and Materials Science, Syracuse University. Syracuse, NY 13210

(U.S.A.)

(Received August 3,1983; accepted November 23,1983)

Platinum films were grown on mica substrates in ultrahigh vacuum. These films have a strong (111) fiber axis orientation as found by low energy electron diffraction, reflection high energy electron diffraction and transmission electron diffraction studies. A new form of Kikuchi pattern, namely Kikuchi rings, was found in the diffraction pattern of these oriented polycrystalline films. This is the first time that a Kikuchi pattern has been reported from a polycrystalline film with a fiber axis.

1. INTRODUCTION

Electron diffraction patterns produced in a transmission electron microscope can be of various types depending on the microstructure of the specimen. They can be either a ring pattern corresponding to a polycrystalline specimen or a spot pattern corresponding to a single-crystal sample. Sometimes in relatively thick single-crystal specimens the spot pattern is associated with a Kikuchi line pattern, Kikuchi patterns are very important in electron microscopy because the positions of Kikuchi lines in the diffraction pattern are a more sensitive measure of the crystal orientation relative1 to theIelectronbeam than,the spot’positions’. In the present work a new form of Kikuchi pattern is reported from platinum films with a strong (111) fiber axis, namely Kikuchi rings.

2. EXPERIMENTAL TECHNIQUE

Platinum films have been formed in vacuum systems on glass’ as well as on mica substrates3v4. Films grown on mica at high substrate temperatures (about 1000 K) are (11 l)-oriented single crystals 3*4. In the present work platinum films were formed in a stainless steel ultrahigh vacuum system equipped with low energy -electron diffraction (LEED) apparatus, a single-pass cylindrical mirror analyzer for Auger electron spectroscopy (AES) and a quadrupole mass spectrometer. The evaporation source was a modified tungsten filament source’, and the rate of evaporation was maintained at 1 A mm-‘. The mica substrate was clamped on a molybdenum strip heater. The base pressure was about 8 x lo-* Pa (6 x lo- lo Torr) while the pressure during evaporation was about 4 x lo-’ Pa (3 x 10e9 Torr). The

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152 P. DIXIT, R. W. VDDK

substrate temperature was maintained at 723 K during deposition. The film thickness ranged from 1000 to 12OOA as measured in situ by a quartz crystal oscillator microbalance.

The films so formed were studied by LEED and AES to determine their crystal orientation and surface elemental composition. They were subsequently taken out of the ultrahigh vacuum system, removed from the mica substrate by floating in very dilute nitric acid solution and further studied by transmission electron microscopy. Film samples were also cut into small pieces and studied by reflection high energy electron diffraction (RHEED) techniques.

3. RESULTS AND DISCUSSION

Figure 1 shows a typical LEED pattern from a platinum film made as described above. The 11 ring pattern is due to the (111) fiber texture of the film. The fiber texture always consists of a rotational symmetry about the fiber axis. Since the ring is symmetrical about the electron beam direction and the electron beam strikes the film perpendicularly, the fiber axis is perpendicular to the film surface. The ring also has twelve intensity maxima on it which indicate a weak double-positioned (111) texture. Figure 2 is a RHEED pattern from the same film. The pattern remains the same irrespective of the azimuthal direction of the electron beam. The spots in Fig. 3 represent the calculated RHEED pattern for a platinum film having a (111) fiber axis perpendicular to the surface. The semicircles give the calculated pattern from a randomly oriented polycrystalline film. The calculated fiber axis pattern agrees well with the pattern obtained experimentally. The streaking in Fig. 2 is due to the smoothness of the surface of the film.

A transmission electron diffraction (TED) pattern (Fig. 4) from this film shows a sharp ring which is indicative of the polycrystalline nature of the film. The lattice parameter measurement shows that this ring corresponds to 220 reflections. We also

Fig. 1. LEED pattern of an evaporated platinum film 1200 A thick (primary voltage, 220 V).

KIKUCHI RINGS FROM FQLYCRYSTALLINE Pt FILMS 153

Fig. 2. RHEED pattern from the platinum film of Fig. 1 ( :accelerating voltage, 100 kV).

531 400 222 440

Fig. 3. Calculated RHEED pattern for a platinum film having a (111) fiber axis perpendicular to the film showing various circles and spots which satisfy the diffraction conditions. The spots give rise to the streaks, which correspond to the streaks in Fig. 2 and indicate a very smooth surface.

know that from the equation

hu+ku+lw = 0 (11

for the allowed reflections from a film having a fiber axis parallel to the electron beam, where (uuw) is the fiber axis and hki are the Miller indices of the reflecting planes, the only allowed reflection is 220 amongst the first few allowed reflections for an f.c.c. lattice. The ring does not have a uniform intensity. Instead, it appears to have

P. DIXIT, R. W. VOOK

Fig. 4. Diffraction pattern of a platinum film with a fiber axis showing a strong Kikuchi ring.

Ki kuchi Lines

Fig. 5. Calculated Kikuchi diffraction pattern for a monocrystalline film (-) and a polycrystalline film (- - -) having a (111) fiber axis. The usual 220 Debye ring is also shown.

twelve weak intensity maxima. This structure can again be explained by the double- positioned (111) texture. The most striking features in the diffraction pattern are the broad circular bands, one band between the 220 ring and the transmitted beam and another much weaker band located just outside the 220 ring and slightly overlapping it. This uneven character of the background intensity can be explained on the basis of a Kikuchi pattern6 whose origin involves both elastic and inelastic scattering of electrons. A comprehensive treatment requires the extension of the dynamical theory to include effects due to diffuse inelastic scattering processes’. The geometry of this Kikuchi ring can be explained onthe basis of Fig. 5, which shows the Kikuchi lines for a (111) orientation in a single crystal. Bright regions appear at the intersection of Kikuchi lines. These bright regions are somewhat broad and diffuse in nature because Kikuchi lines tend to broaden at their intersections. For an early thin film example see the work on epitaxial tin films*. Such bright regions in a single crystal give rise to a broad ring in a fiber-textured film because of the rotational symmetry of the film. One important characteristic of a Kikuchi pattern is

KIKUCHI RINGS FROM POLYCRYSTALLINE Pt FILMS 155

that it moves, when the specimen is tilted or rotated, as though it were rigidly attached to the specimen. It is evident from Fig. 6, which was obtained by tilting the film, that the Kikuchi ring pattern is no longer centered about the transmitted beam spot. Thus the diffuse ring in Fig. 4 does not represent, for example, the presence of an amorphous phase.

The microstructure of this film is shown in Fig. 7. The average grain size is about 0.3 pm. This micrograph was taken under the tilted condition of Fig. 6. A micrograph showing grain contrast could not be recorded under normal incidence when the Kikuchi ring was centered around the transmitted beam because under

Fig. 6. TED pattern of a platinum film with a (111) fiber axis tilted approximately 1.2” with respect to the incident electron beam.

Fig. 7. Bright field transmission electron micrograph of a platinum film with a (111) fiber axis under tilted conditions.

156 P. DIXIT, R. W. VOOK

such conditions all the grains are dark. The result makes sense if essentially all the grains are (111) oriented (perfect fiber axis) and diffract only into the 220 Debye ring.

4. CONCLUSION

This is the first reported observation of a Kikuchi ring pattern from a polycrystalline film with a fiber axis. The presence of a Kikuchi ring is further evidence therefore of an essentially perfect fiber axis in a polycrystalline material.

ACKNOWLEDGMENT

The authors are indebted to the U.S. Department of Energy, Basic Energy Sciences Division, for financial support under Contract DE AC02-83ER-45034.

REFERENCES

1 J. W. Edington, Practical Electron Microscopy in Materials Science, van Nostrand Reinhold, New York, 1976.

2 D. Brennan, D. 0. Hayward and B. M. W. Trapnell, Proc. R. Sot. London, Ser. A, 256 (1960) 81. 3 J. N. Smith, Jr., and R. L. Palmer, J. Chem. Phys., 56 (1972) 13. 4 R. L. Palmer and J. N. Smith, Jr., J. C/tern. Phys., 60 (1974) 1453. 5 P.DixitandR. W.Vook, ThinSolidFihns, 110(1983)Ll33. 6 S. Kikuchi, Jpn. J. Phys., 5 (1928) 83. 7 K. Artmann, 2. Phys., 125 (1944) 229. 8 R. W. Vook, J. Appl. Phys., 33 (1962) 2498.