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Observations of surface-channelling phenomena with a stem instrument
Ultramicroscopy 27 (1989) 319-328 North-Holland, Amsterdam
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OBSERVATIONS OF SURFACE-CHANNELLING PHENOMENA WITH A STEM INSTRUMENT J.M. C O W L E Y Department of Physics, Arizona State Unioersity, Tempe, Arizona 85287-1504, USA
Received 20 November 1988
The surface-resonance-channelingphenomena which have been explored in recent years by using transmission electron microscopes in the reflection electron microscopy(REM) mode may be observed also in scanning transmission electron microscopes used in the reflection (SREM) mode. When SREM images are formed using surface-channelled diffracted beams close to the "shadow-edge"of the diffraction pattern, high-intensity images are formed of down-steps on the surface which are almost invisible in SREM images formed with specularly reflected beams. In an alternative configuration the incident beam may be injected at the edge of a crystal face so that the electron beam in the crystal runs parallel to a flat surface. Bright, near-axial features appear in the diffraction pattern corresponding to electrons channelled along the surface layers. Also bright resonance parabolas are generated, as in RHEED patterns, but coming in this case from scattering of elastic, rather than inelastic, resonance-channelledelectrons. High resolution STEM images formed by detecting the axially channelled electrons show very bright lattice fringes at the surface. Low-loss EELS analysis of the axially channelled electrons and of the bright parabolas shows strong peaks due to surface excitations, with some dependence on channelling directions. Results are presented for MgO cleavage faces and MgO smoke crystals.
1. Introduction Interest in the surface-channelling phenomenon, long known to influence R H E E D patterns from near-perfect crystal faces, has revived in recent years. Apart from the fundamental interest of the phenomenon, which may now be investigated by use of more powerful techniques, it is of interest in relation to several possible areas of application. It has significance for the interpretation of R E M images of surface steps and other features [L2] and as a,pbssible l~asis for the analysis of surface structure and composition l~y use ~of electron energy-loss spectroscopy [3-5] and possibly by secondary ~electron emission and A u g e r electron analysis [6]. Recently developed methods for computing : t h e wave-fields of electrons in crystals f o r the R H E E D geometry have shown that under surface resonance c o n d i t i o n s t h e electron beam in the crystal may be confined to the top one or two layers of atoms for certain directions of the incident beam [7-10,3] although in other circumstances, under resonance conditions
the electron intensity is distributed over 3 or 4 layers of atoms [10,11]. Most observations of surface resonance phenomena have been made, in recent years, using transmission electron microscopes to produce the R H E E D patterns and corresponding R E M images. The incident beam is so oriented that a strong diffracted beam is generated running parallel to the surface or else directed slightly upwards towards the surface so that it is reflected back from the surface potential battier and trapped inside the crystal [12]. '1~1a¢ electibn wave field is then established in the top few layers of atoms in the surface and may propagat e along the surface for considerable distances. Scattering from this wave field may then greatly enhance ~the intensity o f the specular beam and other features of the diffraction pattern. A large proportion o f the R H E E D pattern is composed of inelastically scattered electrons. Electrons scattered inelastically through relatively large angles give rise to the Kikuchi lines. The strong continuous parabolas and circles in the Kikuchi line patterns correspond to the directions
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J.M. Cowley / Observations of surface-channelling phenomena with STEM instrument
for which the inelastically scattered electrons satisfy the surface resonance conditions [13]. When one of the parabolas passes through a Bragg spot, the conditions for the surface resonance are satisfied for the elastic Bragg scattering and its associated small-angle inelastic scattering. For the Bragg peak, so enhanced, the inelastic scattering component is usually strong, amounting to from about 50% to about 9096 of the total [4,10]. A scanning transmission electron microscopy (STEM) instrument m a y be used to form equivalent R H E E D patterns and scanning reflection electron microscopy (SREM) images. However the geometry is usually somewhat different in that the b e a m incident on the crystal surface is normally a convergent b e a m of convergence determined by the objective aperture. The diffraction pattern is a convergent-beam pattern and the strong diffraction features in the pattern are commonly those produced for the particular incident b e a m directions which satisfy the surface resonance conditions [14]. There are, however, other geometries which may be exploited with the STEM instru' ment whereby the surface resonance condition m a y be created, and the conditions of surface channeling may be investigated, using microdiffraction patterns and electron energy loss analysis in a manner which is not feasible with a T E M instrument. The incident b e a m m a y be injected through the edge of a crystal or a large step on the crystal face so that it creates the wavefield running parallel to the crystal surface directly rather than
through a diffraction process. For a cry finite dimensions in the b e a m direction, t fraction pattern, the high resolution image EELS spectrum may then be obtained froJ trons transmitted along the surface in th( nelling mode using either the forward-sc electrons or those scattered out of the surf~ the diffraction pattern. In this report, examples are given of such means whereby the surface-charmellil nomenon may be explored and, in some exploited.
2. SREM images with surface-channelled el In the R H E E D - R E M geometry, it he shown that surface-channelled electrons me the crystal if there is a large step or the tc tion of the crystal at an edge. The extr~ a p p e a r in the R H E E D pattern belt "shadow-edge" which, for a perfect, infinite face, terminates the usual R H E E D pattel resulting diffraction patterns have been ref( as transmission-RHEED or T R H E E D [15]. When a STEM instrument is used, SOl of the convergent incident b e a m usually surface resonance channelling. If there is step-down on the surface (as viewed fr( objective lens, i.e. from the incident bean tion) additional diffracted beams appear shadow edge. This is illustrated in fig. L
Fig. 1. Reflection electron diffraction patterns obtained from a (100) cleavage face of MgO using a beam of diameter ab incident close to a down-step on the surface. The (400) reflection is at the Bragg angle. The (200) and (220) reflections el( shadow edge appear only because surface-channelled electrons escape from the crystal at the step. White spots have been indicate the zero beam position.
J.M. Cowley/ Observations of surface-channellingphenomena with STEM instrument
shows diffraction patterns obtained from a (100) cleavage face of a large MgO crystal. The incident b e a m direction is close to [001] and the (400) reflection is strongly excited. The (200) reflection and the (220) and (220) reflections would not escape from the surface of the perfectly flat crystal, being propagated almost parallel to the crystal surface. When the incident beam of diameter about 10 A strikes the crystal a few hundred ,~ before a large down-step, however, these spots appear. The positions of these spots vary with the incident b e a m orientation and with the position of the incident b e a m relative to the surface step. Thus the diffraction patterns of fig. 1 do not usually show the square array of spots which might be expected from MgO kinematically and is almost achieved in fig. la. The 220 spot m a y be displaced perpendicular to the surface to almost the 320 position and may appear to be displaced parallel to the surface also, as is apparent in figs. l b and lc. Such variations of spot position may be attributed to the oscillatory nature of the surface resonance channelling [16]. In terms of the classical channelling picture, the channelled electron m a y execute "rosette" or " w e a v o n " motion, relative to the lines or planes of atoms in the surface and so m a y emerge from the crystal in any direction over a considerable range. If such spots are detected in order to form the S R E M image, as the incident beam is scanned over the surface, the image will show bright lines
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Fig. 2. Diagram showing ray paths for electrons diffracted from a crystal surface before and after a down-step and channelled along the surface to leave the crystal at the step. at the positions of the down-steps. Such steps do not give strong contrast in S R E M images obtained using the specular diffracted beam. The incident b e a m is diffracted with much the same intensity from the top surface, before the step, and the bottom surface, beyond the step, with no interruption (fig. 2). The step m a y appear as a dark line because, as the incident beam approaches the step, the limitation of the surface-channelling at the step may decrease the total reflected intensity. This is in contrast to the case of a step-up, for which a strong black line of twice the step height is produced as first the diffracted b e a m and then the incident beam is blocked by the step (fig. 2). Fig. 3a is an image of a MgO (100) cleavage face obtained using the 400 specular diffracted beam. The flat parts of the surface give high intensity. The up-steps appear as strong dark lines or bands. Fig. 3b is the image of the same area formed by detecting the surface-channelled 220 beam. 'The bright lines are large steps down, scarcel~ visible as weak dark lines in fig. 3a.
i
Fig. 3. SREM images of the (100) cleavage surface of a MgO crystal obtained with (a) the specular (400) reflection and (b) the
surface-channeUed (220) reflection which emerges at down-steps. Marker = 10 nm.
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Smaller bright lines and patches correspond to smaller down-steps, approaching single-atom height. A remnant of the contrast of fig. 3a is present in fig. 3b, presumably because the detector aperture also includes some inelastic or elastic diffuse scattering generated with the specular reflection. Images similar to fig. 3b are given by the weaker (200) spot. Also similar images of weaker contrast are obtained if, instead of being set to include the main 400 specular reflection, the collector aperture is set to collect only the uppermost part of the 400 reflection. This follows because the 400 reflection coming from the flat surface of the crystal is displaced appreciably towards the shadow-edge by the refraction effect, whereas a b e a m diffracted in the (400) direction after having left a step edge does not suffer this displacement.
3. Shadow images of edges When a dedicated STEM instrument is used with no objective aperture, or with a very large objective aperture, characteristic shadow images of crystals are formed by beams focussed to a cross-over close to a crystal. With coherent illumination the effects of both Fresnel and Fraunhofer diffraction are visible and are strongly affected by the spherical aberration of the objective lens. The effects of astigmatism may be ob-
served and corrected, with high precisio cognition of the perturbations of sym images thereby introduced [17]. For transmission through thin cry., orientation of the crystal relative to the a objective lens m a y be determined by ob of the K-lines (Kikuchi- or Kossel-ty readily visible. When a principal set of oriented nearly parallel to the lens axis, acteristic electron Ronchi fringes are obse These fringes depend on, and m a y be t measure of, the crystal lattice plane spe defocus and the spherical aberration con., When the thin straight edge of a crystal to the objective lens axis, the aberration the lens appears as in the "Knife-edge" light optics, with a well-defined "circle c magnification". If the lens axis is parallel face of the crystal, striking patterns ha, contrast appear and a combination is Ronchi fringes from the lattice planes p the surface and the strong Fresnel-lik from the aligned face of the crystal [20] 4a. The shadow image of a crystal, align fig. 4a, changes rapidly as the incident b, or the crystal, is moved sideways, in the perpendicular to the crystal edge. Within circle of infinite magnification, a bri~ moves across the field as the b e a m ax over the edge, see fig. 4b. This b a n d m
Fig. 4. Shadow images of the edge of a MgO smoke crystal with the incident beam axis aligned parallel to a crystal face, negative defocus. In (a) Ronchi fringes produced by interference of diffracted beams from the (200) planes interact with fringes at the edge. In (b) a bright fringe is seen crossing the axis within the circle of infinite magnification. In (c) bright K and resonance parabolas are seen in the vacuum part of the image.
J.M. Cowley/ Observationsof surface-channellingphenomena with STEM instrument
spond to electrons channelled along the surface planes of atoms (see below). It is especially bright for those orientations for which surface-resonance-channelling conditions are expected to apply, as judged from the K-line patterns seen by transmission through the bulk of the crystal. The brightness varies with crystal thickness and with defocus of the electron beam. At the same time bright Kikuchi lines and arcs are visible in the high-angle background in the vacuum region of the shadow image (fig, 4c). These correspond to portions of the patterns of Kikuchi lines and resonance parabolas observed in R H E E D patterns as a result of wide-angle inelastic scattering. As in the case of the R H E E D patterns, the bright parabolas are not the envelopes of the Kikuchi lines, but there is a clear gap between them and the Kikuchi lines. This gap has been attributed to the difference of effective inner potential values for the electrons channelled along the potentials wells of rows of atoms, as compared with electrons transmitted through the average potential of the three-dimensional crystal structure [13]. There is an important difference from the R H E E D case in that, under the conditions of the experiment in cases such as fig. 4c, it is the primary beam, rather than the beams inelastically scattered through wide angles, which is channelled along the lines or planes of atoms in the surface. There is an associated difference in intensity distribution. For a particular position of the incident beam and a particular defocus only a relatively small part of the resonance parabola, and associated Kikuchi line, is bright. The bright portion moves outward, away from the surface shadow edge as the axis of the electron beam moves further into the crystal.
4. S T E M images of crystal edges If a detector is placed at the center of the circle of infinite magnification of the shadow image, and hence on the objective lens axis, and the incident beam is scanned, a high resolution STEM image of the crystal is obtained, showing the lattice fringes with the periodicity of the planes of atoms. At the edge of a crystal with the incident beam
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Fig. 5. (a) STEM image obtained at the edge of a MgO smoke crystal aligned so that the incident beam axis is parallel to the crystal face. The bright fringes are attributed to surface channelling. (b) The intensity profile along a line through the middle of the image (a).
axis running parallel to the crystal face, strong contrast effects are seen at the edge of the crystal, as in the image of a M~O smoke crystal, fig. 5a. In this picture the 2.1 A periodicity of the (200) lattice planes parallel to the surface is clearly resolved through the coherent interference effects [21], although the incident beam diameter at the specimen level, and the STEM image resolution, as normally defined, are about 3 ,~. At the edge of the crystal, in such images, one or more bright fringes appear. The appearance of these bright fringes is strongly dependent on the crystal orientation, the crystal thickness and the defocus. In the case of fig. 5a the crystal is oriented so that the incident beam direction is a few degrees away from the [021] azimuth and the crystal thickness is of the order of 1000 ,~. The defocus is optimum for the production of the 2.1 ,A fringes, i.e. about - 2 5 0 0 ,A. Fig. 5b is an intensity profile along a
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J.M. Cowley/ Observations of surface-channellingphenomena with STEM instrument
line perpendicular to the crystal edge near the middle of fig. 5a. One fringe is seen to be particularly bright with a m a x i m u m intensity considerably higher than the intensity produced when the b e a m is well outside the crystal. The appearance of this fringe corresponds to the occurrence of the very bright band in the shadow image, as in fig. 4b. T h u s , for a particular position of the incident b e a m spot, centered within a few lattice plane spacings of the crystal edge, the electrons are transmitted along the lattice planes parallel to the surface with high efficiency, and also a focussing effect takes place so that the intensity detected by an axial detector is greater than that for transmission of the b e a m through a vacuum. Although the incident b e a m diameter is about 3 A, this heightened transmission takes place for a range of positions of the incident b e a m only 2 ,~ or less in extent, clearly indicating that the effect arises from coherent diffraction of electron waves incident on the crystal over a wide range of angles of incidence. As the defocus of the incident beam is changed from under-focus to over-focus, i.e. increasing the objective lens current, the position of the bright fringe, or group of two or three bright fringes, is seen to move from the interior of the crystal to the edge and beyond it. The m a x i m u m intensity of the fringes is greatest when they are within a few a
lattice planes of the edge. This direction of ment is opposite to that to be expected 1 bright Fresnel fringe at the edge of a crystal is outside the crystal for under-focus and for over-focus. The bright "channelled" frir fact, appear to have m a x i m u m intensity wh~ coincide with the bright Fresnel fringe. The variation of the bright fringe intensi thickness is illustrated in figs. 6a and 6b. TI magnification image, fig. 6a, was obtaine the incident beam in the [110] direction. "I little indication of any enhancement of the sity near the crystal edge. In the image tained with five times lower magnification, line is seen near the edge. In this case the cl ing is not very strong and the bright frinl less intense than the Fresnel fringe at the edge. Their intensity is seen to vary w: thickness of the wedge-shaped crystal (90 o The thickness varies from zero at the right .~ at the left in the picture. The bright frin varies in intensity with a periodicity of ab~ in thickness. This is in contrast to t proximate periodicity of about 300 .~ for tl~ field in the crystal, away from the edge, flected in the contrast variations of the fringe images for 100 keV electrons [22]. The minor indentation in the crystal e suited from removal of one or two layers ol from the surface by intense electron ra
Fig. 6. (a) High resolution image of the edge of a MgO crystal with the incident beam axis in the [110] direction for a thi~ about 500 .~. (b) Image with five times lower magnification and different defocus showing the variation of the bright fringe for thicknesses from zero (right-hand side) to 630 .~ (left-hand side). Marker = 10 nm.
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when the incident beam was held stationary at that position for focussing and stigmation of the objective lens.
5. Energy loss for surface-channelled electrons The low-loss portion (0-50 eV) of the EELS spectrum was obtained for electrons channelled along the surfaces of MgO crystals for each of the modes of observation used to demonstrate the channelling phenomena in the diffraction patterns or images. For diffraction patterns such as those of fig. 1 the surface-channelled spots close to the shadow edge were passed through the aperture to the EELS spectrometer. With the incident beam parallel to the (100) crystal face, spectra were obtained from both the high-angle bright parabolas and from the forward-scattered electrons on the axis. The peaks observed in the EELS spectra were much the same in all cases. The most clearly defined sets of peaks were obtained with an axial detector and the incident beam parallel to the (100) crystal face. Spectra obtained with the beam in the [001] and [011] azimuths are compared in fig. 7 with the spectrum obtained by transmission through a thin crystal. The transmission spectrum shows the prominent bulk plasmon peak at 22 eV with weak maxima at 8.2, 11.3, 14.5, 19.6 and 25 eV. The spectra, figs. 7b and 7c, were selected from series of spectra, obtained as the beam axis was moved in steps of a few ~, towards the crystal edge, as was done in earlier experiments [23]. Fig. 7b shows four spectra obtained in such a series for an MgO crystal in the [001] orientation. The exact positions Of the beam axis, relative to the edge, could not be determined with any accuracy because of the small amount of drift of the specimen position over the period taken to collect the series of spectra. It may be assumed, however, that the maximum intensity of the energy loss peaks corresponds to the beam position giving the maximum intensity in the image, i.e. for the strongest surface channelling Cbndition. For both figs. 7b and 7c, the bulk plasmon has negligible intensity. The surfacerelated peaks, weak in fig. 7a, are strong. For the
Fig. 7. EELS spectra in the low-loss region obtained from a MgO smoke crystal: (a) for transmission, showing the bulk plasmon peak at 22 eV; (b) for surface-channelled electrons with incident beam parallel to the (100) surface in the [001] direction, and (c) as for (b) but with the incident beam in the [011] direction.
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[001] orientation, fig. 7b, the peak positions are 8.2, 11.0, 14.0 and 19.2 eV. For the [011] orientation, fig. 7c, the peaks are at 8.2, 10.6, 13.6 and 19.0 eV. The differences between these values and the values for the minor peaks of fig. 7a are probably not significant, firstly because the spectra are rather noisy and, secondly, because the peak positions appear to vary with crystal orientation and thickness and with the position of the beam relative to the crystal edge. The zero-loss peaks, recorded with reduced intensity in fig. 7, indicate a peak width of 1.5 eV so that energy loss values may be determined to within approximately 0.5 eV. The peak appearing at about 8.2 eV for the [001] orientation varies from 8 to 9.3 eV for the [011] and [012] orientations. The peak at about 11 eV has a value averaging 10.5 eV for [001] and 11.5 for the [011] and [012] orientation. The peak at around 14 eV for the [011] orientation appears around 14.0 eV for [001] and 13.5 eV for [012]. The peak at 19-20 eV appears around 19.5 eV for [001] and [012], but for [011] appears over a wide range, from 18.0 to almost 22 eV, with a bi-modal distribution having preferred values of around 19.0 and 20.5 eV. This peak is commonly seen to be broader for the [011] orientation than for other orientations, as is evident from fig. 7c. N o clear evidence has been found for systematic differences in energy-loss values for EELS spectra obtained from the surface-resonance diffraction spots, the high-angle bright parabolas or the axial positions on the shadow image patterns. The correlation of the surface-related energy-loss peaks with the surface-channelling effects, giving bright fringes in the images such as fig. 5, is confirmed by obtaining similar images using electrons which have lost the corresponding amounts of energy. Fig. 8 shows intensity profiles across the crystal edge obtained in the same manner as for fig. 5b except that the EELS spectrometer is used as an energy filter with an energy window of approximately 3 eV. The zero-loss trace shows a group of stron~ fringes, with the lattice-plane periodicity of 2.1 A, at the crystal edge. For an energy loss of around 10 eV the fringes are relatively more prominent. The signal strength falls slowly in the region outside the crystal. This observation
Fig. 8. Intensity profiles across the edge of a Mgq crystal as for fig. 5b but with energy filtering, for enel of 0, 10 and 16 eV for (a), (b) and (c), respective is consistent with the evidence of fig. 6b ir the lower curves, obtained with the beam the crystal, show relatively strong excitati low energy losses [23,24]. For the energ 3 around 16 eV the intensity in the surface fr much higher than the intensity in the inside or outside the crystal.
6. Discussion and conclusions
It has been demonstrated that surfw nance-channelling effects may have signifi~ fluence on the observations of surfaces the STEM instrument in the imaging, difJ and EELS modes. In the SREM ima! surfaces, detection of the surface-channell, trons emerging from step edges allows hi,,
J.M. Cowley/ Observationsof surface-channellingphenomena with STEM instrument
trast to be obtained from the down-steps which appear with low contrast in images obtained with specular reflections from the crystal surface. Thus a comparison of SREM images obtained with the various diffracted beams allows a more complete analysis of the surface morphology. The imaging of the down-steps is not strongly dependent on the incident beam orientation because, with a convergent incident beam, the intense diffraction spots in the reflection diffraction pattern were produced by the surface-channelled electrons, and detection of these intense features ensures that the resonance conditions are satisfied. For the crystals viewed in transmission with one flat face parallel to the incident beam axis, the crystal orientation can readily be determined by observation of the shadow images. The pattern of K-lines produced by the convergent beam allows the crystal orientation to be adjusted. The appearance of the bright parabolas in the vacuum region of the image indicates that the surface resonance condition is established in the surface planes of atoms at the crystal edge. The high-intensity fringes appearing at the crystal edge in high resolution STEM images of a properly aligned crystal are attributed to surface channelling effects. The form and intensity distribution of these fringes are strongly dependent on the experimental conditions, including the crystal orientation and thickness, the defocus and aberrations of the objective lens and the detector position. It is also to be expected that the surfacechannelling effect will be dependent on the form and composition of the surface layers, since it has been shown that REM and REELS observations can be strongly affected by the presence of an absorbed monolayer on a MgO (100) surface [3]. The intense irradiation of the surface by the incident electron beam has an unknown effect on absorbed layers of gas and may, in some cases, remove surface layers of atoms from the MgO crystals itself. Hence it is not surprising that great difficulty was experienced in reproducing exactly the intensity distributions in the profiles such as fig. 5b. Although many similar profiles were obtitined, it was not possible to correlate the intensity distributions in the profiles with values for the various experimental parameters.
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Similar uncertainties exist in the correlation of the energy-loss values observed in the EELS spectra with the experimental parameters. There appear to be differences in the energy-loss peak positions or differences in the ranges of energy-loss values, for spectra obtained in different azimuthal orientations, close to th~ various principal axial directions, but no correlations with other experimental variables can be made. It may. be anticipated that differences in surface channelling conditions, corresponding to the channelling of electrons along different rows of atoms in the surface, might give large differences in the energy-loss values. In REELS observations from the (110) cleavage surfaces of GaAs, strong additional energy-loss peaks have been found at 6.3 eV [25] and, for a different orientation, at 4.5 eV [10]. These peaks occur over very narrow ranges (less than 10 -3 rad) of the incident beam orientation. They are attributed to transitions taking place between the possible lateral energy states of the electrons channelled along lines or planes of atoms. For MgO it has been shown by Kambe et al. [26] than for channelling along the [011] direction in the bulk crystal, bound channelling states differ for channelling along the rows of Mg atoms and the rows of O atoms. It is to be expected that for channelling along rows of alternate Mg and O atoms in the [001] direction, some bound states with intermediate energy values may exist. Hence, if the existence of these energy states is reflected in the EELS spectra for the surface layers, some clear differences should be observed for the spectra obtained in the [001] and [011] azimuths. The observed differences in the EELS peak positions could be explained in this way if the energies associated with channelling in the (011) direction along Mg and O atoms were about 20.5 and 9 eV and if the energy for channelling in the [001] direction was about 10.5 eV, but no basis exists for assuming that the energies could have these values.
Acknowledgements This work was supported by N H S F grants DMR-8510059 and DMR-8810238, and made use
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of the resources of the ASU Faculty for High Resolution Electron Microscopy, supported by NSF grant DMR-8611609.
References [1] T. Hsu and L.-M. Peng, Ultramicroscopy 22 (1987) 217. [2] L.-M. Peng, J.M. Cowley and T. Hsu, Ultramicroseopy 29 (1989), in press. [3] Z.L. Wang, P. Lu and J.M. Cowley, Ultramicroscopy 23 (1987) 205. [4] Z.L. Wang and J.M. Cowley, Surface Sci. 193 (1988) 501. [5] Z.L. Wang and J.M. Cowley, J. Microsc. Spectrosc. Electron. 13 (1988) 184. [6] A. Ichimiya and Y. Takeuchi, Surface Sei. 12 (1983) 343. [7] H. Marten and G. Meyer-Ehmsen, Surface Sci. 151 (1985) 570. [8] L.-M. Peng and J.M. Cowley, Acta Cryst. A42 (1986) 545. [9] L.-M. Peng and J.M. Cowley, Surface Sci. 199 (1988) 609. [10] Z.L. Wang, J. Liu and J.M. Cowley, Acta Cryst. A45 (1989), in press. [11] A.L. Bleloch, A. Howie, R.H. Milne and M.G. Walls, in press.
[12] L.-M. Peng and J.M. Cowley, J. Electron Micros (1987) 43. [13] L.-M. Peng, J.M. Cowley and Nan Yao, Ultram 26 (1988) 189. [14] G. Lehmpfuhl and W.C.T. Dowell, Acta Cryst. ? 569. [15] L.-M. Peng and J.M. Cowley, Surface Sci. 201 (1 [16] Z.L. Wang, J. Liu, Ping Lu and J.M. Cowley, croscopy 27 (1989) 101. [17] J.M. Cowley, Ultramicroscopy 4 (1979) 413. [18] J.M. Cowley, Ultramicroscopy 4 (1979) 435. [19] J.A. Lin and J.M. Cowley, Ultramicroseopy 19 (] [20] J.M. Cowley and M. Disko, Ultramicroscopy 469. [21] J.M. Cowley and J.C.H. Spence, Ultramicroscop: 433. [22] J.C.H. Spence and H. Kolar, Phil. Mag. A39 (l [23] J.M. Cowley, Surface Sci. 114 (1982) 587. [24] L.D. Marks, in: Electron Microscopy and Anal3 Inst. Phys. Conf. Ser. 61, Ed. M.J. Goringe (I~ London-Bristol, 1981) p. 259. [25] L.-M. Peng and J.M. Cowley, Surface Sci. 204 (1 [26] K. Kambe, G. Lehmpfuhl and F. Fujimoto, , forsch. 29a (1974) 1034.
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OBSERVATIONS OF SURFACE-CHANNELLING PHENOMENA WITH A STEM INSTRUMENT J.M. C O W L E Y Department of Physics, Arizona State Unioersity, Tempe, Arizona 85287-1504, USA
Received 20 November 1988
The surface-resonance-channelingphenomena which have been explored in recent years by using transmission electron microscopes in the reflection electron microscopy(REM) mode may be observed also in scanning transmission electron microscopes used in the reflection (SREM) mode. When SREM images are formed using surface-channelled diffracted beams close to the "shadow-edge"of the diffraction pattern, high-intensity images are formed of down-steps on the surface which are almost invisible in SREM images formed with specularly reflected beams. In an alternative configuration the incident beam may be injected at the edge of a crystal face so that the electron beam in the crystal runs parallel to a flat surface. Bright, near-axial features appear in the diffraction pattern corresponding to electrons channelled along the surface layers. Also bright resonance parabolas are generated, as in RHEED patterns, but coming in this case from scattering of elastic, rather than inelastic, resonance-channelledelectrons. High resolution STEM images formed by detecting the axially channelled electrons show very bright lattice fringes at the surface. Low-loss EELS analysis of the axially channelled electrons and of the bright parabolas shows strong peaks due to surface excitations, with some dependence on channelling directions. Results are presented for MgO cleavage faces and MgO smoke crystals.
1. Introduction Interest in the surface-channelling phenomenon, long known to influence R H E E D patterns from near-perfect crystal faces, has revived in recent years. Apart from the fundamental interest of the phenomenon, which may now be investigated by use of more powerful techniques, it is of interest in relation to several possible areas of application. It has significance for the interpretation of R E M images of surface steps and other features [L2] and as a,pbssible l~asis for the analysis of surface structure and composition l~y use ~of electron energy-loss spectroscopy [3-5] and possibly by secondary ~electron emission and A u g e r electron analysis [6]. Recently developed methods for computing : t h e wave-fields of electrons in crystals f o r the R H E E D geometry have shown that under surface resonance c o n d i t i o n s t h e electron beam in the crystal may be confined to the top one or two layers of atoms for certain directions of the incident beam [7-10,3] although in other circumstances, under resonance conditions
the electron intensity is distributed over 3 or 4 layers of atoms [10,11]. Most observations of surface resonance phenomena have been made, in recent years, using transmission electron microscopes to produce the R H E E D patterns and corresponding R E M images. The incident beam is so oriented that a strong diffracted beam is generated running parallel to the surface or else directed slightly upwards towards the surface so that it is reflected back from the surface potential battier and trapped inside the crystal [12]. '1~1a¢ electibn wave field is then established in the top few layers of atoms in the surface and may propagat e along the surface for considerable distances. Scattering from this wave field may then greatly enhance ~the intensity o f the specular beam and other features of the diffraction pattern. A large proportion o f the R H E E D pattern is composed of inelastically scattered electrons. Electrons scattered inelastically through relatively large angles give rise to the Kikuchi lines. The strong continuous parabolas and circles in the Kikuchi line patterns correspond to the directions
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for which the inelastically scattered electrons satisfy the surface resonance conditions [13]. When one of the parabolas passes through a Bragg spot, the conditions for the surface resonance are satisfied for the elastic Bragg scattering and its associated small-angle inelastic scattering. For the Bragg peak, so enhanced, the inelastic scattering component is usually strong, amounting to from about 50% to about 9096 of the total [4,10]. A scanning transmission electron microscopy (STEM) instrument m a y be used to form equivalent R H E E D patterns and scanning reflection electron microscopy (SREM) images. However the geometry is usually somewhat different in that the b e a m incident on the crystal surface is normally a convergent b e a m of convergence determined by the objective aperture. The diffraction pattern is a convergent-beam pattern and the strong diffraction features in the pattern are commonly those produced for the particular incident b e a m directions which satisfy the surface resonance conditions [14]. There are, however, other geometries which may be exploited with the STEM instru' ment whereby the surface resonance condition m a y be created, and the conditions of surface channeling may be investigated, using microdiffraction patterns and electron energy loss analysis in a manner which is not feasible with a T E M instrument. The incident b e a m m a y be injected through the edge of a crystal or a large step on the crystal face so that it creates the wavefield running parallel to the crystal surface directly rather than
through a diffraction process. For a cry finite dimensions in the b e a m direction, t fraction pattern, the high resolution image EELS spectrum may then be obtained froJ trons transmitted along the surface in th( nelling mode using either the forward-sc electrons or those scattered out of the surf~ the diffraction pattern. In this report, examples are given of such means whereby the surface-charmellil nomenon may be explored and, in some exploited.
2. SREM images with surface-channelled el In the R H E E D - R E M geometry, it he shown that surface-channelled electrons me the crystal if there is a large step or the tc tion of the crystal at an edge. The extr~ a p p e a r in the R H E E D pattern belt "shadow-edge" which, for a perfect, infinite face, terminates the usual R H E E D pattel resulting diffraction patterns have been ref( as transmission-RHEED or T R H E E D [15]. When a STEM instrument is used, SOl of the convergent incident b e a m usually surface resonance channelling. If there is step-down on the surface (as viewed fr( objective lens, i.e. from the incident bean tion) additional diffracted beams appear shadow edge. This is illustrated in fig. L
Fig. 1. Reflection electron diffraction patterns obtained from a (100) cleavage face of MgO using a beam of diameter ab incident close to a down-step on the surface. The (400) reflection is at the Bragg angle. The (200) and (220) reflections el( shadow edge appear only because surface-channelled electrons escape from the crystal at the step. White spots have been indicate the zero beam position.
J.M. Cowley/ Observations of surface-channellingphenomena with STEM instrument
shows diffraction patterns obtained from a (100) cleavage face of a large MgO crystal. The incident b e a m direction is close to [001] and the (400) reflection is strongly excited. The (200) reflection and the (220) and (220) reflections would not escape from the surface of the perfectly flat crystal, being propagated almost parallel to the crystal surface. When the incident beam of diameter about 10 A strikes the crystal a few hundred ,~ before a large down-step, however, these spots appear. The positions of these spots vary with the incident b e a m orientation and with the position of the incident b e a m relative to the surface step. Thus the diffraction patterns of fig. 1 do not usually show the square array of spots which might be expected from MgO kinematically and is almost achieved in fig. la. The 220 spot m a y be displaced perpendicular to the surface to almost the 320 position and may appear to be displaced parallel to the surface also, as is apparent in figs. l b and lc. Such variations of spot position may be attributed to the oscillatory nature of the surface resonance channelling [16]. In terms of the classical channelling picture, the channelled electron m a y execute "rosette" or " w e a v o n " motion, relative to the lines or planes of atoms in the surface and so m a y emerge from the crystal in any direction over a considerable range. If such spots are detected in order to form the S R E M image, as the incident beam is scanned over the surface, the image will show bright lines
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Fig. 2. Diagram showing ray paths for electrons diffracted from a crystal surface before and after a down-step and channelled along the surface to leave the crystal at the step. at the positions of the down-steps. Such steps do not give strong contrast in S R E M images obtained using the specular diffracted beam. The incident b e a m is diffracted with much the same intensity from the top surface, before the step, and the bottom surface, beyond the step, with no interruption (fig. 2). The step m a y appear as a dark line because, as the incident beam approaches the step, the limitation of the surface-channelling at the step may decrease the total reflected intensity. This is in contrast to the case of a step-up, for which a strong black line of twice the step height is produced as first the diffracted b e a m and then the incident beam is blocked by the step (fig. 2). Fig. 3a is an image of a MgO (100) cleavage face obtained using the 400 specular diffracted beam. The flat parts of the surface give high intensity. The up-steps appear as strong dark lines or bands. Fig. 3b is the image of the same area formed by detecting the surface-channelled 220 beam. 'The bright lines are large steps down, scarcel~ visible as weak dark lines in fig. 3a.
i
Fig. 3. SREM images of the (100) cleavage surface of a MgO crystal obtained with (a) the specular (400) reflection and (b) the
surface-channeUed (220) reflection which emerges at down-steps. Marker = 10 nm.
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Smaller bright lines and patches correspond to smaller down-steps, approaching single-atom height. A remnant of the contrast of fig. 3a is present in fig. 3b, presumably because the detector aperture also includes some inelastic or elastic diffuse scattering generated with the specular reflection. Images similar to fig. 3b are given by the weaker (200) spot. Also similar images of weaker contrast are obtained if, instead of being set to include the main 400 specular reflection, the collector aperture is set to collect only the uppermost part of the 400 reflection. This follows because the 400 reflection coming from the flat surface of the crystal is displaced appreciably towards the shadow-edge by the refraction effect, whereas a b e a m diffracted in the (400) direction after having left a step edge does not suffer this displacement.
3. Shadow images of edges When a dedicated STEM instrument is used with no objective aperture, or with a very large objective aperture, characteristic shadow images of crystals are formed by beams focussed to a cross-over close to a crystal. With coherent illumination the effects of both Fresnel and Fraunhofer diffraction are visible and are strongly affected by the spherical aberration of the objective lens. The effects of astigmatism may be ob-
served and corrected, with high precisio cognition of the perturbations of sym images thereby introduced [17]. For transmission through thin cry., orientation of the crystal relative to the a objective lens m a y be determined by ob of the K-lines (Kikuchi- or Kossel-ty readily visible. When a principal set of oriented nearly parallel to the lens axis, acteristic electron Ronchi fringes are obse These fringes depend on, and m a y be t measure of, the crystal lattice plane spe defocus and the spherical aberration con., When the thin straight edge of a crystal to the objective lens axis, the aberration the lens appears as in the "Knife-edge" light optics, with a well-defined "circle c magnification". If the lens axis is parallel face of the crystal, striking patterns ha, contrast appear and a combination is Ronchi fringes from the lattice planes p the surface and the strong Fresnel-lik from the aligned face of the crystal [20] 4a. The shadow image of a crystal, align fig. 4a, changes rapidly as the incident b, or the crystal, is moved sideways, in the perpendicular to the crystal edge. Within circle of infinite magnification, a bri~ moves across the field as the b e a m ax over the edge, see fig. 4b. This b a n d m
Fig. 4. Shadow images of the edge of a MgO smoke crystal with the incident beam axis aligned parallel to a crystal face, negative defocus. In (a) Ronchi fringes produced by interference of diffracted beams from the (200) planes interact with fringes at the edge. In (b) a bright fringe is seen crossing the axis within the circle of infinite magnification. In (c) bright K and resonance parabolas are seen in the vacuum part of the image.
J.M. Cowley/ Observationsof surface-channellingphenomena with STEM instrument
spond to electrons channelled along the surface planes of atoms (see below). It is especially bright for those orientations for which surface-resonance-channelling conditions are expected to apply, as judged from the K-line patterns seen by transmission through the bulk of the crystal. The brightness varies with crystal thickness and with defocus of the electron beam. At the same time bright Kikuchi lines and arcs are visible in the high-angle background in the vacuum region of the shadow image (fig, 4c). These correspond to portions of the patterns of Kikuchi lines and resonance parabolas observed in R H E E D patterns as a result of wide-angle inelastic scattering. As in the case of the R H E E D patterns, the bright parabolas are not the envelopes of the Kikuchi lines, but there is a clear gap between them and the Kikuchi lines. This gap has been attributed to the difference of effective inner potential values for the electrons channelled along the potentials wells of rows of atoms, as compared with electrons transmitted through the average potential of the three-dimensional crystal structure [13]. There is an important difference from the R H E E D case in that, under the conditions of the experiment in cases such as fig. 4c, it is the primary beam, rather than the beams inelastically scattered through wide angles, which is channelled along the lines or planes of atoms in the surface. There is an associated difference in intensity distribution. For a particular position of the incident beam and a particular defocus only a relatively small part of the resonance parabola, and associated Kikuchi line, is bright. The bright portion moves outward, away from the surface shadow edge as the axis of the electron beam moves further into the crystal.
4. S T E M images of crystal edges If a detector is placed at the center of the circle of infinite magnification of the shadow image, and hence on the objective lens axis, and the incident beam is scanned, a high resolution STEM image of the crystal is obtained, showing the lattice fringes with the periodicity of the planes of atoms. At the edge of a crystal with the incident beam
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Fig. 5. (a) STEM image obtained at the edge of a MgO smoke crystal aligned so that the incident beam axis is parallel to the crystal face. The bright fringes are attributed to surface channelling. (b) The intensity profile along a line through the middle of the image (a).
axis running parallel to the crystal face, strong contrast effects are seen at the edge of the crystal, as in the image of a M~O smoke crystal, fig. 5a. In this picture the 2.1 A periodicity of the (200) lattice planes parallel to the surface is clearly resolved through the coherent interference effects [21], although the incident beam diameter at the specimen level, and the STEM image resolution, as normally defined, are about 3 ,~. At the edge of the crystal, in such images, one or more bright fringes appear. The appearance of these bright fringes is strongly dependent on the crystal orientation, the crystal thickness and the defocus. In the case of fig. 5a the crystal is oriented so that the incident beam direction is a few degrees away from the [021] azimuth and the crystal thickness is of the order of 1000 ,~. The defocus is optimum for the production of the 2.1 ,A fringes, i.e. about - 2 5 0 0 ,A. Fig. 5b is an intensity profile along a
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line perpendicular to the crystal edge near the middle of fig. 5a. One fringe is seen to be particularly bright with a m a x i m u m intensity considerably higher than the intensity produced when the b e a m is well outside the crystal. The appearance of this fringe corresponds to the occurrence of the very bright band in the shadow image, as in fig. 4b. T h u s , for a particular position of the incident b e a m spot, centered within a few lattice plane spacings of the crystal edge, the electrons are transmitted along the lattice planes parallel to the surface with high efficiency, and also a focussing effect takes place so that the intensity detected by an axial detector is greater than that for transmission of the b e a m through a vacuum. Although the incident b e a m diameter is about 3 A, this heightened transmission takes place for a range of positions of the incident b e a m only 2 ,~ or less in extent, clearly indicating that the effect arises from coherent diffraction of electron waves incident on the crystal over a wide range of angles of incidence. As the defocus of the incident beam is changed from under-focus to over-focus, i.e. increasing the objective lens current, the position of the bright fringe, or group of two or three bright fringes, is seen to move from the interior of the crystal to the edge and beyond it. The m a x i m u m intensity of the fringes is greatest when they are within a few a
lattice planes of the edge. This direction of ment is opposite to that to be expected 1 bright Fresnel fringe at the edge of a crystal is outside the crystal for under-focus and for over-focus. The bright "channelled" frir fact, appear to have m a x i m u m intensity wh~ coincide with the bright Fresnel fringe. The variation of the bright fringe intensi thickness is illustrated in figs. 6a and 6b. TI magnification image, fig. 6a, was obtaine the incident beam in the [110] direction. "I little indication of any enhancement of the sity near the crystal edge. In the image tained with five times lower magnification, line is seen near the edge. In this case the cl ing is not very strong and the bright frinl less intense than the Fresnel fringe at the edge. Their intensity is seen to vary w: thickness of the wedge-shaped crystal (90 o The thickness varies from zero at the right .~ at the left in the picture. The bright frin varies in intensity with a periodicity of ab~ in thickness. This is in contrast to t proximate periodicity of about 300 .~ for tl~ field in the crystal, away from the edge, flected in the contrast variations of the fringe images for 100 keV electrons [22]. The minor indentation in the crystal e suited from removal of one or two layers ol from the surface by intense electron ra
Fig. 6. (a) High resolution image of the edge of a MgO crystal with the incident beam axis in the [110] direction for a thi~ about 500 .~. (b) Image with five times lower magnification and different defocus showing the variation of the bright fringe for thicknesses from zero (right-hand side) to 630 .~ (left-hand side). Marker = 10 nm.
J.M. Cowley/ Observations of surface-channellingphenomena with STEM instrument
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when the incident beam was held stationary at that position for focussing and stigmation of the objective lens.
5. Energy loss for surface-channelled electrons The low-loss portion (0-50 eV) of the EELS spectrum was obtained for electrons channelled along the surfaces of MgO crystals for each of the modes of observation used to demonstrate the channelling phenomena in the diffraction patterns or images. For diffraction patterns such as those of fig. 1 the surface-channelled spots close to the shadow edge were passed through the aperture to the EELS spectrometer. With the incident beam parallel to the (100) crystal face, spectra were obtained from both the high-angle bright parabolas and from the forward-scattered electrons on the axis. The peaks observed in the EELS spectra were much the same in all cases. The most clearly defined sets of peaks were obtained with an axial detector and the incident beam parallel to the (100) crystal face. Spectra obtained with the beam in the [001] and [011] azimuths are compared in fig. 7 with the spectrum obtained by transmission through a thin crystal. The transmission spectrum shows the prominent bulk plasmon peak at 22 eV with weak maxima at 8.2, 11.3, 14.5, 19.6 and 25 eV. The spectra, figs. 7b and 7c, were selected from series of spectra, obtained as the beam axis was moved in steps of a few ~, towards the crystal edge, as was done in earlier experiments [23]. Fig. 7b shows four spectra obtained in such a series for an MgO crystal in the [001] orientation. The exact positions Of the beam axis, relative to the edge, could not be determined with any accuracy because of the small amount of drift of the specimen position over the period taken to collect the series of spectra. It may be assumed, however, that the maximum intensity of the energy loss peaks corresponds to the beam position giving the maximum intensity in the image, i.e. for the strongest surface channelling Cbndition. For both figs. 7b and 7c, the bulk plasmon has negligible intensity. The surfacerelated peaks, weak in fig. 7a, are strong. For the
Fig. 7. EELS spectra in the low-loss region obtained from a MgO smoke crystal: (a) for transmission, showing the bulk plasmon peak at 22 eV; (b) for surface-channelled electrons with incident beam parallel to the (100) surface in the [001] direction, and (c) as for (b) but with the incident beam in the [011] direction.
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[001] orientation, fig. 7b, the peak positions are 8.2, 11.0, 14.0 and 19.2 eV. For the [011] orientation, fig. 7c, the peaks are at 8.2, 10.6, 13.6 and 19.0 eV. The differences between these values and the values for the minor peaks of fig. 7a are probably not significant, firstly because the spectra are rather noisy and, secondly, because the peak positions appear to vary with crystal orientation and thickness and with the position of the beam relative to the crystal edge. The zero-loss peaks, recorded with reduced intensity in fig. 7, indicate a peak width of 1.5 eV so that energy loss values may be determined to within approximately 0.5 eV. The peak appearing at about 8.2 eV for the [001] orientation varies from 8 to 9.3 eV for the [011] and [012] orientations. The peak at about 11 eV has a value averaging 10.5 eV for [001] and 11.5 for the [011] and [012] orientation. The peak at around 14 eV for the [011] orientation appears around 14.0 eV for [001] and 13.5 eV for [012]. The peak at 19-20 eV appears around 19.5 eV for [001] and [012], but for [011] appears over a wide range, from 18.0 to almost 22 eV, with a bi-modal distribution having preferred values of around 19.0 and 20.5 eV. This peak is commonly seen to be broader for the [011] orientation than for other orientations, as is evident from fig. 7c. N o clear evidence has been found for systematic differences in energy-loss values for EELS spectra obtained from the surface-resonance diffraction spots, the high-angle bright parabolas or the axial positions on the shadow image patterns. The correlation of the surface-related energy-loss peaks with the surface-channelling effects, giving bright fringes in the images such as fig. 5, is confirmed by obtaining similar images using electrons which have lost the corresponding amounts of energy. Fig. 8 shows intensity profiles across the crystal edge obtained in the same manner as for fig. 5b except that the EELS spectrometer is used as an energy filter with an energy window of approximately 3 eV. The zero-loss trace shows a group of stron~ fringes, with the lattice-plane periodicity of 2.1 A, at the crystal edge. For an energy loss of around 10 eV the fringes are relatively more prominent. The signal strength falls slowly in the region outside the crystal. This observation
Fig. 8. Intensity profiles across the edge of a Mgq crystal as for fig. 5b but with energy filtering, for enel of 0, 10 and 16 eV for (a), (b) and (c), respective is consistent with the evidence of fig. 6b ir the lower curves, obtained with the beam the crystal, show relatively strong excitati low energy losses [23,24]. For the energ 3 around 16 eV the intensity in the surface fr much higher than the intensity in the inside or outside the crystal.
6. Discussion and conclusions
It has been demonstrated that surfw nance-channelling effects may have signifi~ fluence on the observations of surfaces the STEM instrument in the imaging, difJ and EELS modes. In the SREM ima! surfaces, detection of the surface-channell, trons emerging from step edges allows hi,,
J.M. Cowley/ Observationsof surface-channellingphenomena with STEM instrument
trast to be obtained from the down-steps which appear with low contrast in images obtained with specular reflections from the crystal surface. Thus a comparison of SREM images obtained with the various diffracted beams allows a more complete analysis of the surface morphology. The imaging of the down-steps is not strongly dependent on the incident beam orientation because, with a convergent incident beam, the intense diffraction spots in the reflection diffraction pattern were produced by the surface-channelled electrons, and detection of these intense features ensures that the resonance conditions are satisfied. For the crystals viewed in transmission with one flat face parallel to the incident beam axis, the crystal orientation can readily be determined by observation of the shadow images. The pattern of K-lines produced by the convergent beam allows the crystal orientation to be adjusted. The appearance of the bright parabolas in the vacuum region of the image indicates that the surface resonance condition is established in the surface planes of atoms at the crystal edge. The high-intensity fringes appearing at the crystal edge in high resolution STEM images of a properly aligned crystal are attributed to surface channelling effects. The form and intensity distribution of these fringes are strongly dependent on the experimental conditions, including the crystal orientation and thickness, the defocus and aberrations of the objective lens and the detector position. It is also to be expected that the surfacechannelling effect will be dependent on the form and composition of the surface layers, since it has been shown that REM and REELS observations can be strongly affected by the presence of an absorbed monolayer on a MgO (100) surface [3]. The intense irradiation of the surface by the incident electron beam has an unknown effect on absorbed layers of gas and may, in some cases, remove surface layers of atoms from the MgO crystals itself. Hence it is not surprising that great difficulty was experienced in reproducing exactly the intensity distributions in the profiles such as fig. 5b. Although many similar profiles were obtitined, it was not possible to correlate the intensity distributions in the profiles with values for the various experimental parameters.
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Similar uncertainties exist in the correlation of the energy-loss values observed in the EELS spectra with the experimental parameters. There appear to be differences in the energy-loss peak positions or differences in the ranges of energy-loss values, for spectra obtained in different azimuthal orientations, close to th~ various principal axial directions, but no correlations with other experimental variables can be made. It may. be anticipated that differences in surface channelling conditions, corresponding to the channelling of electrons along different rows of atoms in the surface, might give large differences in the energy-loss values. In REELS observations from the (110) cleavage surfaces of GaAs, strong additional energy-loss peaks have been found at 6.3 eV [25] and, for a different orientation, at 4.5 eV [10]. These peaks occur over very narrow ranges (less than 10 -3 rad) of the incident beam orientation. They are attributed to transitions taking place between the possible lateral energy states of the electrons channelled along lines or planes of atoms. For MgO it has been shown by Kambe et al. [26] than for channelling along the [011] direction in the bulk crystal, bound channelling states differ for channelling along the rows of Mg atoms and the rows of O atoms. It is to be expected that for channelling along rows of alternate Mg and O atoms in the [001] direction, some bound states with intermediate energy values may exist. Hence, if the existence of these energy states is reflected in the EELS spectra for the surface layers, some clear differences should be observed for the spectra obtained in the [001] and [011] azimuths. The observed differences in the EELS peak positions could be explained in this way if the energies associated with channelling in the (011) direction along Mg and O atoms were about 20.5 and 9 eV and if the energy for channelling in the [001] direction was about 10.5 eV, but no basis exists for assuming that the energies could have these values.
Acknowledgements This work was supported by N H S F grants DMR-8510059 and DMR-8810238, and made use
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of the resources of the ASU Faculty for High Resolution Electron Microscopy, supported by NSF grant DMR-8611609.
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[12] L.-M. Peng and J.M. Cowley, J. Electron Micros (1987) 43. [13] L.-M. Peng, J.M. Cowley and Nan Yao, Ultram 26 (1988) 189. [14] G. Lehmpfuhl and W.C.T. Dowell, Acta Cryst. ? 569. [15] L.-M. Peng and J.M. Cowley, Surface Sci. 201 (1 [16] Z.L. Wang, J. Liu, Ping Lu and J.M. Cowley, croscopy 27 (1989) 101. [17] J.M. Cowley, Ultramicroscopy 4 (1979) 413. [18] J.M. Cowley, Ultramicroscopy 4 (1979) 435. [19] J.A. Lin and J.M. Cowley, Ultramicroseopy 19 (] [20] J.M. Cowley and M. Disko, Ultramicroscopy 469. [21] J.M. Cowley and J.C.H. Spence, Ultramicroscop: 433. [22] J.C.H. Spence and H. Kolar, Phil. Mag. A39 (l [23] J.M. Cowley, Surface Sci. 114 (1982) 587. [24] L.D. Marks, in: Electron Microscopy and Anal3 Inst. Phys. Conf. Ser. 61, Ed. M.J. Goringe (I~ London-Bristol, 1981) p. 259. [25] L.-M. Peng and J.M. Cowley, Surface Sci. 204 (1 [26] K. Kambe, G. Lehmpfuhl and F. Fujimoto, , forsch. 29a (1974) 1034.