scanning electron diffraction and its application to

Corrosion Science. 1976. Vol. 16. pp. 819 to 836. Pergamon Press. Printed in Great Britain

SCANNING ELECTRON DIFFRACTION AND ITS APPLICATION TO THE OXIDATION OF IRON* C. J. R.

SHEPPARDt

and H.

AHMED

Engineering Department, Cambridge University, Cambridge, England Abstract-The merits of scanning electron diffraction as a method of studying thin films and solid surfaces are discussed. An ultra-high vacuum scanning electron diffraction system designed for the study of surfaces and surface reactions is described. Results of experiments on the oxidation of the (110) surface of iron single crystals using this apparatus are presented. Epitaxial growths of oxide phases were observed, some of these phases exhibiting orientations different from those normally reported. The proportions of the various phases present were found to change as the oxidation proceeded. INTRODUCTION

electron diffraction is a method whereby high-energy (10-100 keV) electron diffraction patterns may be measured directly. By this technique diffraction intensities may be determined quickly and accurately. Electrons which have been scattered inelastically and therefore do not contribute to the diffraction pattern may be filtered out. This improves the contrast between the peaks and the background and allows comparison with theoretical predictions. Diffraction patterns may be compared to show the effects of different methods of processing, and growth sequences may be followed. SCANNING

THE FORMATION OF ELECTRON DIFFRACTION PATTERNS

Electron diffraction may be used to study the crystal structure of thin films or solid surfaces. How a diffraction pattern is formed by specimens of these two types is now explained. Consider a single row ofn atoms, with a spacing of 'a' between each atom (Fig. 1). If they are irradiated with an intensity 10 at an angle 91 to the atom row, the intensity of the diffracted beam at a particular angle 92 to the atom row is given by

=

I ,

where 0 =

2

Iof 2 sin no, sin2 0

a;: {cos 9

1 -

cos({3 - 9J}

,

and is a function of both 91 and {3, the scattering angle, and where f is the atomic scattering factor, which describes the scattering of a single atom. The intensity is thus proportional to the product of 12, which is a slowly varying function of {3, and the interference function sin2 no/sin2o. *Manuscript received 13 October 1975. tNow at the Department of Engineering Science, University of Oxford. 819

820

C. J. R.

SHEPPARD

and H.

AHMED

FIG. 1. The Laue diffraction condition for a single row of atoms.

If the way the interference function varies with 0 is now examined, it can be seen that it has large maxima where 0 = N1t, with N an integer (Fig. 2). It also has small maxima which are only important for very short atom rows. The condition 0 = N1t is known as the Laue condition. The breadth of the peaks is inversely proportional to the number of atoms in the row. For an infinite number of atoms, the peaks become delta functions. For a few atoms only, the peaks spread out, and this is described as a relaxation of the Laue condition. For a 3-dimensional solid, there are three such Laue conditions, and a maximum in the diffracted intensity will occur when all three Laue conditions are satisfied simultaneously. As the orientation of a crystall'elative to the electron beam has only two angular degrees of freedom, maxima will occur only rarely. However, in practice one or more of the conditions are often relaxed.

a J/n

o

7T

8 FIG. 2. The interference function sin 2nB/sin 28 as a function of B. The figure is drawn for n = 6. \,

Electron diffraction from a specimen consisting of a thin single-crystal film of cubic crystal structure can now be consiqered. The electron beam is incident perpendicular to the thin film. Figure 3 shows the lines in the diffraction pattern on which the Laue conditions are satisfied. The two perpendicular lines of atoms in the plane of the film result in Laue conditions which are satisfied on perpendicular arrays of parallel lines, whereas the line of atoms parallel to the incident beam, result in a Laue condition which is satisfied on a series of concentric circles, called Laue zones. The number of atoms in this latter row is small and hence the Laue condition resulting from that direction is relaxed. The diffraction pattern will thus consist of a square array of spots, at the intersection of the two sets of straight lines.

Scanning electron diffraction and its application to the oxidation of iron

Loue zones (Cubic,

0=

821

16A)

FIo. 3. Laue conditions for a thin single-crystal film of cubic crystal structure. Lattice dimensions, a = 16;'. The concentric circles are the Laue Zones.

If an electron beam is incident at a small grazing angle of incidence to the surface of a bulk single crystal, the Laue conditions are similarly satisfied on two arrays of straight lines and a series of circles. Although the range of the electrons in the solid is of the order of 10 nm, the penetration into the solid may be only two or three atomic layers because of the small angle of incidence. The Laue condition resulting from atom rows perpendicular to the surface, that is, one of the sets of parallel lines, is relaxed. If the crystal consists of a mosaic of crystallites arranged so that their orientations are nearly parallel, the Laue zones will also be relaxed. The pattern will then consist of a shadow edge and a series of streaks perpendicular to it. This is true if the surface of the specimen is flat on an atomic scale. If the beam is incident on a rough surface, as is produced by etching a crystal, the electrons can penetrate through the asperities, and if the specimen is a single crystal a diffraction pattern of spots will be formed (Fig. 4). So we distinguish between true reflection, and transmission through surface asperities. For a thin polycrystalline film, or transmission through surface asperities of a polycrystalline solid, the pattern will be made up of many spot patterns at different rotations about the main beam, and a series of concentric rings will be formed.

(0)

(b)

FIo. 4. Modes of reflection diffraction: (a) True reflection from a flat crystal surface. (b) Diffraction by transmission through sutface asperities.

C. J. R.

822

FIG. S.

SHEPPARD

and H. AIiMED

The Bragg condition for diffraction by parallel atom planes of spacing d.

The three Laue conditions for a 3-dimensional solid (Fig. 5) are together equivalent to the Bragg condition for diffraction from planes of atoms 2d sin M2 = mA, where ~ is the angle of diffraction, d is the interplanar spacing, and m is an integer. The interplanar spacing is dependent on the Miller indices of the planes, and hence a diffraction peak may be characterized by a set of Miller indices. If m is equal to unity these Miller indices are the integers in the corresponding Laue conditions. THE MODE OF OPERATION OF SCANNING ELECTRON DIFFRACTION

The mode of operation of scanning electron diffraction is shown in Fig. 6. A beam of high-energy electrons is produced by the electron gun. This is focused by a magnetic lens on to a phosphor screen. The beam is scattered by the specimen, producing a diffraction pattern on the phosphor screen. In conventional high-energy electron diffraction, this pattern is photographed, or the pattern is formed directly on a photographic plate. In scanning electron diffraction, the whole pattern is scanned by an electromagnetic field produced by the scan coils. A small aperture is situated in the phosphor screen and the electron current which penetrates this apei:tureis measured using a scintillator and a photomultiplier tube. The electron current is plotted against the current through the scan coils on either an oscUIoscope or an d.e.amplifier

x-y plotter

Scan coils

Lens Electron gun

=:=il

~

u n

~

luJ\luJ

Aperture Filter Photomultiplier tube

Scintillator

I~--~~r-B-ia-s------------------------J FIG. 6. The mode of operation of scanning electron diffraction.

Scanning electron diffraction and its application to the oxidation of iron

823

X-y plotter. For a polycrystalline ring pattern, a single radial scan is used. For single crystal patterns, two sets of scan coils are provided and the pattern is scanned in a series of lines similar to those of a television picture. The sum of the intensity and the y-scan current is then plotted against the x-scan current. This gives a series of sections through the intensity surface plotted in 3-dimensional space. The energy filter shown in Fig. 6 is a retarding-potential filter and is capable of filtering out electrons that have lost more than ca. 4 eV oftheir original50keV in their interaction with the specimen. SOME EXAMPLES OF THE USE OF SCANNING ELECTRON DIFFRACTION

Scanning electron diffraction may be used to study single crystal or polycrystalline specimens in either transmission or reflection. Figure 7 shows a radial scan through the ring pattern produced by a thin film of polycrystalline iron on a carbon substrate. 2 The large peaks are characteristic of ~-Fe and are labelled with their corresponding Miller indices. The smaller peaks are produced by impurities. The upper trace is magnified by a factor of 10, showing clearly the impurity peaks. These would be difficult to see on a photographically recorded diffraction pattern. The way in which the scale can be altered at will is a further advantage of scanning electron diffraction. The angular scale can also be varied. A scan like this could be displayed on an oscilloscope in a time of ca. 50 ms.

v Fe & IMPURITY

....

t

222

FIG. 7. A radial scan through the ring pattern produced by a thin film of polycrystalline iron on a carbon substrate. 2

Figure 8 shows a series of sectIons through a pattern of rings formed by polycrystalline iron. 2 The sum of the intensity and the y-scan current is plotted against the x-scan current. The intensity of the rings varies markedly around their circumference, showing that the grains have a preferential orientation. Figure 9 shows a series of sections through a spot pattern produced by reflection diffraction from a single crystal of ~-brass. 3 The pattern is produced by a beam of electrons at a small glancing angle to the surface of the etched crystal. The pattern is thus produced by transmission through asperities on the surface. This pattern is

824

c. J. R. SHEPPARD and H. ARMED

FIG. 8. A series of sections through a pattern of rings formed by polycrystalIine iron.-

Vm (431)

(321)

+300 +100 +30

+5--------'1

~[221]

(000)

[110]

FIG. 9. A series of sections through a spot pattern produced by reflection diffraction from an etched single crystal of ~.brass. The effect of altering the filter electrode potential is also shown. 8

unfiltered. Figure 10 shows a filtered pattern from the same specimen. The pattern shows an improved contrast and sharper peaks. Figure 11 shows the growth of a thin film of silver. The film was grown inside the diffraction instrument. Successive traces show the structure formed by the addition of monolayer thicknesses of material. The film is originally amdrphous and gradually

Scanning electron diffraction and its application to the oxidation of iron

825

~[2211

J

[110]

FIG. 10. A filtered pattern from the same specimen as Fig. 9, showing improved contrast and sharper peaks.

311

The Growth of Silver on Preheate d carbjm

220

0-3eV

12-7A/Min

2-5A

Increments

222

FIG.l1. Growth of a silver thin film. a

develops the usual polycrystalline structure.l' In this way the crystallite size has been monitored, and it has been shown that changes in lattice constant occur in the initial growth of some thin films.'

826

C. J. R.

SHEPPARD

and H. AHMED

HIGH-ENERGY ELECTRON DIFFRACTION AS A METHOD OF STUDYING SURFACES AND SURFACE REACTIONS

High-energy electron diffraction in the true reflection mode can be used to study the surface of a bulk specimen. Adsorption structures have been detected on various materials using this technique. 5,6 High-energy diffraction is also sensitive to the presence of nuclei on a surface, as are often formed during the process of oxidation. In comparison, low energy electron diffraction (LEED), because the electrons penetrate only a short distance, is a surface phenomenon and is usually insensitive to the presence of nuclei. Oxide nuclei which are large enough to be seen by electron microscopy have failed to be detected using LEED.7 Nuclei with well-developed facets parallel to the substrate will produce LEED patterns, but with inclined facets the pattern will be weaker.s For nuclei that are non-crystallographic in shape the sensitivity will be much reduced. If there is a non-zero misfit between the substrate and the nuclei the pattern will also be much weaker. For randomly-oriented nuclei, the LEED pattern only exhibits an increase in general background, but high-energy electrons will produce a polycrystalline ring pattern. High-energy electron diffraction has been shown to be capable of detecting a film consisting of nuclei of average thickness only 40 pm. 9 Owing to the greater penetration of high-energy electrons it is possible by increasing the angle of incidence to study the substrate through a few monolayers of adsorbed gas 5 or a thin film of oxide. Changing the angle of incidence can also give information about different depths in a thick surface film. It would thus be possible in principle to measure the variations in non-stoichiometry across an oxide film. A further advantage of high-energy electron diffraction is that because of the much smaller beam size, 1l;lore accurate measurements of the lattice dimensions are possible. However, the specimen must be viewed in more than one azimuth to build up the crystal structure if only information from the zero-order Laue Zone is used. Specimens are more difficult to prepare for high-energy electron diffraction. The method is very sensitive to surface non-flatness as the beam strikes the specimen in grazing incidence. This makes the calculation of diffraction intensities more difficult but has led to valuable information concerning the surface topography.lo The calculation of intensities in high-energy diffraction is made easier because the diffraction is thought to be less dynamic in nature than in the low~energy case. l1 Atomic scattering factors are also better known for high energies. A SCANNING ELECTRON DIFFRACTION SYSTEM FOR STUDYING SURFACE REACTIONS

A scanning electron diffraction system has been specifically developed for the study of surface reactions (Fig. 12). The complete instrument is of ultra-high vacuum and bakeable,construction in order to avoid any possibility of contamination. '. To be able to work with a pressure'of 10-3 Torr of oxygen in the specimen region while retaining a good vacuum in the electron gun and energy filter, the apparatus is divided into three regions; the specimen chamber, the electron gun section and the energy filter section. These regions are separated from each other by small apertures and pumped individually, the specimen region by a 100 lis ion pump, and the electron gun and energy filter regions each by a 15 1/s ion pump.

Scanning electron diffraction and its application to the oxidation of iron

827

AIiQnment coils I

. Main aperture

m....."