High resolution electron microscopy and nanodiffraction study of the MgOAl interface

Thin Solid Films, 148 (1987) 301-310 PREPARATION AND CHARACTERIZATION

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HIGH RESOLUTION ELECTRON MICROSCOPY AND N A N O D I F F R A C T I O N S T U D Y OF T H E MgO-A1 I N T E R F A C E S.-Y. ZHANG* AND J. M. COWLEY Department of Physics, Arizona State University, Tempe, A Z 85287 (U.S.A.) (Received March 10, 1986; revised June 2, 1986; accepted September 17, 1986)

The interfaces between MgO crystals and evaporated aluminum layers were studied using high resolution electron microscopy and diffraction from nanometersize areas (nanodiffraction). For evaporation under a wide range of vacuum conditions, an interface layer of thickness about 3 nm is formed. This is initially amorphous but crystallizes to form a spinel structure on electron beam irradiation. For very small amounts of evaporated aluminum, the MgO surface is roughened to form a layer, about 3 nm thick, of pits and protrusions which appear to be pure MgO, continuous with the substrate crystal. The formation of this layer is seen as an intermediate stage in the formation of the spinel interface layer.

1. INTRODUCTION Ceramic-metal interfaces are of interest in relation to many structural, electronic and high temperature applications, and the structural characteristics of such interfaces strongly affect the properties which are critical for such applications. High resolution electron microscopy (HREM) provides a powerful means for the study of the structure of the interfaces in detail, particularly when the incident beam is parallel to the interface and the specimen thickness in the beam direction can be made sufficiently small to allow interpretation of the H R E M image (see the collection of papers in ref. 1). The limitations of the resolution available in electron microscopes can still be important. It is not always possible to make a unique determination of the crystal structures present from the observation of fringe images due to the few prominent crystal lattice plane spacings. In such cases it is useful to supplement the H R E M images with nanodiffraction patterns from identified regions of the image. The nanodiffraction pattern is obtained from regions about 1 nm or less in diameter 2 and allows the structure and orientation of small regions to be determined. In this work we study the interface between MgO crystals and an overlayer of * On leave from Structureand ElementAnalysis Laboratory, Universityof Scienceand Technologyof China, Hefei,Anhui, China. 0040-6090/87/$3.50

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aluminum by using H R E M and nanodiffraction with the incident beam direction parallel to the interface. 2. EXPERIMENTAL PROCEDURES

The interfaces were formed by evaporating aluminum onto the freshly prepared cubic MgO smoke crystals which were made by burning MgO ribbon in a low humidity atmosphere and collecting the smoke crystals on grids with holey carbon films. The deposition of aluminum was carried out under various vacuum conditions, at 10 -2, 10 - 3 and 10 - 4 Pa in a conventional evaporator and at 10-5, 10 -6 and 10 - 7 Pa in an ultrahigh vacuum evaporator. Before the evaporation of aluminum the MgO crystals were heated to 100 °C or so to remove the adsorbed species. H R E M images of MgO smoke crystals, similarly treated, have shown no evidence of any absorbed species on the surface 3 but the presence of a monolayer or less of O H ions (for example) cannot be ruled out. The aluminum layers evaporated onto the MgO{001} surface/ were labout 100-200 ~ thick when,the MgO face was directly exposed to the incident aluminum atoms. Some MgO faces were turned away from the evaporator filament and so received little or no deposited aluminum. Thus one specimen preparation provided all film thicknesses from zero up to the maximum. The high resolution transmission electron microscopy observations were performed with a JEM-200CX microscope at 200 kV with a spherical aberration coefficient Cs of 1.2 mm; the point-to-point resolution was about 0.25 nm. The crystals were tilted so that the incident beam ran along the MgO[100] or MgO[110] direction so that the interface could be revealed clearly. The nanodiffraction patterns were obtained with the electron beam of diameter 1.5nm parallel to the interface in an HB5 scanning transmission electron microscopy instrument from VG Microscopes Ltd., modified by the addition of an optical analyzer system 4 for the convenient observation of the nanodiffraction patterns. 3. RESULTS AND DISCUSSION

3.1. The formation and determination of an intermediate phase at the interface H R E M observations show that an intermediate phase appears at the interface between MgO and aluminum for all the specimens which were made under the different vacuum conditions, as shown Figs. l(a)-l(c). The thickness of the intermediate phase is in general about 3 nm. It is of almost the same thickness for all the specimens. This means that the formation and the thickness of the intermediate phase is independent of the vacuum conditions used for making the specimens. Figures 2(a)-2(c) show a sequence of H R E M images taken from the same crystal at intervals of about 1 min. The sequence shows that the intermediate phase initially appeared to be amorphous, but it was gradually crystallized during the electron beam radiation. Figure 2(d) was taken a few minutes later; the intermediate phase had almost approached a steady crystalline structure. From these results it could be concluded that the electron beam accelerates the development of the

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Fig. 1. High resolution electron micrographs taken in the MgO[ll0] direction, showing that the thickness of the intermediatephase for all specimensunder variousvacuumconditionsis about 3 nm: (a) 10- 3Pa specimen;(b) 10- 5 Pa specimen;(c) 10- 7Pa specimen. equilibrium, fully crystalline, state of the intermediate layer either by enhancing the diffusion to form the stable concentration in the region or else by promoting the crystallization after the equilibrium concentration has been attained. The steady structure of the intermediate phase is formed under the level of electron radiation normally associated with the observation and recording of high resolution images, but the mechanism of the formation of the intermediate phase is worth further investigation. Figure 3 shows the nanodiffraction patterns and an H R E M image taken along the MgO[100] direction. Measurement of the lattice spacing fringes of the intermediate phase suggests that it may have a spinel structure. The fringe spacings correspond to the {220} reflections of spinel. Figure 4 is the [100] direction projection of the spinel structure belonging to the space group Fd3m. It can be seen that there are many electron channels separated by a/2v/'2 = 0.286 nm which is just the {220} lattice plane spacing. The {220} lattice planes are the most prominent of those having spacings within the microscope resolution and so give the predominant fringes in H R E M images. Figures 3(b) and 3(c) show the nanodiffraction patterns obtained from the intermediate phase area and from the interface area of the MgO with the intermediate phase respectively, using an electron beam of diameter about 1.5 nm parallel to the MgO[100] direction. The strong spots in the pattern of Fig. 3(c) come from MgO, and the weaker spots are the same as those in the pattern of Fig. 3(b) which represents the spinel structure. Thus the judgment according to the H R E M images is consistent with the result from the nanodiffraction patterns, and the structure of the intermediate phase is further confirmed. Figure 5 shows a through-focus series of images of the spinel computed by using

Fig. 2. A series of HREM images which were taken from the same crystal, showing that the intermediate phase was gradually crystallized during the electron beam radiation: (a) initially the intermediate phase appeared to be amorphous; (b), (c) the intermediate phase began to be crystallized; (d) the intermediate phase was crystallized completely.

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the standard Arizona State University multislice program. The crystal thickness was assumed to be 12.1 nm, and the defocus values are (a) - 5 0 rim, (b) - 6 0 nm and (c) - 7 0 nm. Comparison of the computed images with the HREM images shows agreement which is satisfactory in view of the imperfect nature of the crystals involved. This result is therefore in agreement with the conclusion suggested by HREM images and the nanodiffraction patterns, namely that the intermediate phase has a spinel structure and presumably has the composition MgA120,. QO

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Fig. 5. H R E M images of the interface region and a through-focus series of computed images of MgAI20 4. The crystal thickness is assumed to be 12.1 nm. The defocus values are (a) - 50 nm, (b) - 60 n m and (c) - 70 nm.

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3.2. The orientational relationship of the three phases According to the HREM images, the nanodiffraction patterns and optical diffraction patterns obtained from the images, the orientation relationships among the three phases were found most often to be of the kind shown in Fig. 3, namely [ 100]~t,o//[100]ugAho ,//[110]A1 [010]~,o//[010]MgAI~O,//[ll2]Ai

[0013~,o // [001 ]M,.,20.//[il 1]. 1 where the MgO[001] direction is taken as perpendicular to the interface. This orientational relationship would not be expected if one considered the growth of aluminum directly on MgO because the unit cell sizes of MgO and aluminum are similar 5. In our case the epitaxial relationship is undoubtedly influenced by the presence of additional oxygen, either trapped by the aluminum during evaporation or pre-existing on the MgO surface. From Fig. 2 it is seen that the orientational relationship of the aluminum with the MgO is present even when the intermediate layer appears to be amorphous. There are exceptional cases when the aluminum layer appears as a small particle. Then the aluminum and MgO appear to have an approximately coherent relationship. In this case the MgO, MgA1204 and aluminum have roughly the same orientation as shown in Fig. 6.

3.3. The roughening of the surface of MgO The HREM observations show that the surface of MgO is very mobile when only a very small amount of aluminum is deposited on it. Figure 7(a) is an MgO crystal which had no evaporated aluminum. The edge looks quite flat. Figure 7(b) shows another MgO crystal to which is attached a small amount of aluminum. As a result there are many small square crystalline projections and pits generated on the surface. The sizes of these pits and projections are about 1-4 nm square. The lattice fringes in the projections are continuations of those in the base MgO crystal. The optical diffraction patterns suggest that the square features still have the MgO structure. This means that there is considerable migration on the surface of the MgO crystal. Because the surface energy of the f.c.c. MgO(100) plane is lowest, the surfaces of the pits and small crystals tend to be bounded by {100} planes, maintaining the square outlines. This roughening takes place even when the amount of aluminum present is so small that no sign of aluminum or its oxide can be detected in the image. It does not take place if no aluminum is evaporated. One aspect of this phenomenon should be noticed. The thickness of the layer made up of the square features on the MgO surface is about 3 nm. This is similar to the thickness of the intermediate phase formed between MgO and aluminum. In spite of the mobilization and redistribution of the surface of MgO which takes place during the electron beam radiation, there is no change in the thickness of the perturbed layer. This implies that the formation of the square features may be an essential initial step in the formation of the intermediate phase. It is reported in the literature that the epitaxial growth of the spinel occurs between MgO and AI20 3 at high temperature (1500-1900 °C) and over a long period of time by the solid state reaction of counterdiffusion of AI3+ and Mg 2+

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Fig. 6. The aluminum and MgO have the approximate coherent relationship if the aluminum evaporated onto the MgO appears as a small particle whose diameter is about 10 nm.

ions 6-8. In our case, the spinel is formed between MgO and aluminum at a much lower temperature and in a shorter period of time. This may be explained by associating it with the roughening of the surface layer of MgO. When the aluminum is evaporated, the first small amount of aluminum deposited onto the MgO gives rise to the many small pits and projections on the MgO surface to a thickness of about 3 nm. These square features increase the area of contact for the aluminum deposited later onto the MgO. Also the formation of the pits and projections suggests that the MgO surface energy has been greatly reduced. Thus it becomes much easier for aluminum and magnesium ions to diffuse across the interface. The increase in

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Fig. 7. HREM image showing that many small square crystal projections and pits are generated on the surface of MgO if there is only a little aluminum evaporated onto the MgO: (a) the image of the MgO crystal with no evaporated aluminum; (b) the image of the MgO crystal with a little aluminum.

surface area also presumably makes it easier for the additional oxygen needed to form the spinel to be absorbed either from the atmosphere (10- 5 to below 10 - 6 Pa) or from the MgO. Electron beam irradiation is known to enhance diffusion processes 9 and so allows the formation and crystallization of the spinel at a low temperature. After the spinel layer about 3 nm in thickness is formed, it may be difficult for aluminum and magnesium ions to counterdiffuse further through the relatively rigid oxygen lattice of the spinel at a low temperature, so the H R E M images show that the spinel layer does not grow beyond a thickness of 3 nm. 4. CONCLUSIONS The evidence suggests that the interface reaction occurs when aluminum is evaporated onto MgO under various vacuum conditions. The intermediate phase formed between MgO and aluminum appears to be nearly amorphous at room temperature, and it is crystallized rapidly under the intense electron radiation normally associated with the observation and recording of high resolution images. The thickness of the intermediate phase is independent of the vacuum conditions used in making the specimens. According to the lattice images and nanodiffraction patterns the intermediate phase has the spinel structure. The roughening of the MgO surface is a prominent characteristic when a small

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a m o u n t of aluminum is deposited onto it. The square features generated on the M g O p r o m o t e the counterdiffusion reaction of aluminum and magnesium ions so that the spinel forms between M g O and a l u m i n u m at a lower temperature. The determination of the intermediate phase demonstrates that the combination of H R E M and nanodiffraction techniques can provide a powerful means for characterizing interface structures. ACKNOWLEDGMENTS This work was supported by the D e p a r t m e n t of Energy under C o n t r a c t D E - A C 0 2 - 7 6 ERO2995 and made use of the resources of the Arizona State University Facility for High Resolution Electron Microscopy within the Center for Solid State Science supported by National Science F o u n d a t i o n G r a n t DMR-8306501. REFERENCES 1 Ultramicroscopy, 14 (1-2) (1984). 2 J.M. Cowley, J. Microsc., 129(1983) 253. 3 T. Tanji and J. M. Cowley, Ultramicroscopy, 17 (1985) 287. 4 J.M. Cowley, in O. Johari (ed.), Scanning Electron Microscopy~1980, Vol. 1, SEM, Chicago, IL, 1980, p. 61. 5 J.W. Matthews, EpitaxialGrowth, AcademicPress, NewYork(1975). R.E. Carter, J. Am. Ceram. Soc., 44 (1961) 116. R.C. Rossi and R. M. Fulrath, J. Am. Ceram. Soc., 46 (1963) 145. F.S. Pettit, E. H. Randklev and E. J. Felton, J. Am. Ceram. Soc., 49 (1966) 199. L.W. Hobbs, in J. J. Hren, J. I. Goldstein and D. C. Joy (eds.), Introduction to Analytical Electron Microscopy, Plenum, New York, 1979,p. 437.

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