Growth of inversion domains in oxygen-rich aluminium nitride

Materials Science and Engineering, A 154 (1992) L 19-L24

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Letter

Growth of inversion domains in oxygen-rich aluminium nitride O. Massler, K.-U. Senftleben and H. G. Sockel lnstitut fiir Werkstoffwissenschafien, Lehrstuhl 1, Universitiit Erlangen-Niirnberg, Martensstr. 5, D-8520 Erlangen (Germany) (Received January 24, 1992; in revised form February 17, 1992)

sites for AI, 6 of the tetrahedra pointing upwards and 6 downwards. Generally the AI atoms fill only one type of these tetrahedra in the defect-free lattice. If a certain region in a grain is built up with Al-filled upward pointing tetrahedra, and the rest with Al-filled downward pointing tetrahedra, the regions are separated by an inversion domain boundary (IDB). The orientations of the Al-filled tetrahedra on both sides of the IDB are related by an inversion operation and therefore the lattices on both sides have opposite orientation.

Abstract Inversion domain boundaries (IDBs) have been investigated by transmission electron microscopy (TEM) in hot-pressed, oxygen-rich AIN. The observed microstructures are discussed with regard to the reasons for the formation of IDBs, transport of oxygen, and further ordering phenomena in oxygen-containing AIN.

1. Introduction

3. Experimental details

The investigated hot-pressed A1N, provided by ESK, Kempten, Germany, had an oxygen content of 1.7 wt.% and a heat conductivity of 55 W mK-1. The phases in the material were studied by X-ray diffraction ( X R D ) ( D 5 0 0 , Siemens). The transmission electron microscopy (TEM) investigations were carried out in a Philips EM400T with an energy-dispersive

A1N was put forward by Borom, Slack and Szymaszek [1] as early as 1972 as a material with high thermal conductivity~ Since that time many investigations have been carried out on the properties and the production of this material [2-5]. Slack [6] deduced a value for the heat conductivity r of 320 W inK-1 for pure AIN from measured heat conductivities for oxygen-containing AIN. Studies of the influence of the microstructure on the heat conductivity in the last few years [7-14] allow the conclusion that the oxygen content in the AIN grains is responsible for the measured low values of r (150-200 W mK -1) in sintered AIN. To increase these values, one must have a detailed understanding of the incorporation of oxygen in the lattice and of oxygen transport through the lattice during the production of the material.

2. AIN crystal structure

AIN crystallizes in the wurtzite structure belonging to the P63mc space group 186. This is one of the noncentrosymmetric space groups. The polar direction is (0001). The structure consists of hexagonal sublattices of AI and of N, displaced by 0.385c (c is the lattice constant) in the (0001) direction [15]. Each atom is tetrahedrally coordinated by the other species. In the unit cell the N-sublattice offers 12 possible tetrahedral 0921-5093/92/$5.00

Fig. 1. TEM image of the microstructure of hot-pressed AIN with IDBs, 27R (at A) and AION (at B) phases. © 1992 - Elsevier Sequoia. All rights reserved

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Fig. 2. Identification of the IDBs: contrast experiments with (1120) zone axis: (a) bright field image with particles at the curved defects; (b) CBED (convergent beam electron diffraction) pattern exhibiting the polarity of the (1120) zone axis: the different intensity distribution for g = (0002) and g = (0002) should be noted; (c) dark field image with g = (0002), exhibiting opposite contrast on both sides of the boundary; (d) dark field image with g = (0002): the opposite contrast on both sides of the boundary and the inverse contrast appearance in comparison to image (c) are emphasized here.

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X-ray analysis unit (EDS) (PV 9760/66 Model EM400T 154-10, EDAX) equipped with a super ultrathin window. For the identification of the IDBs a zone axis of the type {uvt0) was orientated parallel to the transmitted beam under multibeam conditions. Dark field images using the g = _+(0002) reflections of the polar direction {0001) should exhibit opposite contrast on both sides of the boundary and between the dark field images in comparison with each other. This behaviour is owing to the violation of Friedel's law [16].

B S : 2 6 z 2 1 EDA× B5--DEC--91 TIME= RSTE= 4484CPS 1337 PRST= FS = 1315/ : Ml;'l-Mat r l A B = p l a n a r IDB

4. Results

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X-ray diffraction measurements indicated that there could be other phases present besides AIN, but due to their low concentration a clear identification was not possible. TEM investigations showed precipitates (Ferich particles, A1203) and various lattice defects (extended defects, a few dislocations). Figure 1 represents a typical image of the microstructure with several lattice defects. In addition there are grains of 27R (Fig. 1, A) and A1ON phase (Fig. 1, B) present. Almost every grain exhibited one or more extended defects appearing in planar and curved form as well as in complex combinations of both types. The planar defects were observed to lie only in the basal plane. The curved defects are observed to have no preferred orientation, but they sometimes showed in certain parts planar forms in different orientations. TEM dark field images with g = _+(0002) reflections of the {l120)-zone axis clearly identify the extended defects as IDBs (Fig. 2). A segregation of oxygen is observed at planar, but not at curved IDBs (Fig. 3). Some areas, whole grains or regions within a grain, show a stripe-like contrast (Fig. 4). These regions could be identified by electron diffraction and by EDSanalysis as 27R stacking fault polytype. If only a part of a grain consists of 27R polytype, then the lattices of the two phases (AIN and 27R) in the grain exhibit the same orientation and the 27R phase is often found to be in contact with an IDB (Fig. 4). Sometimes many very small particles are found near the IDBs and can be detected only by their strain contrast in the TEM image (Figs. 2(c)-2(d)). A greater magnification of Fig. 2(c) showed such strain contrast on the curved parts of the IDB. In general these particles occur on planar as well as curved IDBs.

5. Discussion

From earlier investigations on oxygen-containing AIN crystals, a strong tendency for the formation of

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Fig. 3. Measurements of oxygen concentrations by EDS analysis at a planar IDB (---) and in the lattice ("') near it.

associates and ordered structures with increasing oxygen content becomes apparent. The formation of dissociated vacancies is assumed in the range below 0.75 wt.% of oxygen [9]. Their formation leads to a reduction of the heat conductivity due to the phonon scattering cross-section of the oxygen-vacancy complex. For oxygen contents above 0.75 wt.% the formation of defects consisting of AI atoms octahedrally coordinated by six O atoms is assumed [9]. These defects can coalesce to form two-dimensional extended defects. The surroundings are organized in such a way that N and AI atoms exchange their lattice sites on opposite sides of the extended defect and an IDB is formed. These defects have been considered by Serneels et al. [16], who calculated dark field intensities at inversion domains due to the violation of Friedel's law. Blank et al. [17] have already suggested the occurrence of IDBs in materials with wurtzite structures. The investigations of IDBs in the present work focused on their nucleation and growth. The conclusions are obtained from evaluation of the observed IDB shapes and the further developing microstructure. The principal idea behind our investigations is the assumption that the segregation of oxygen at planar IDBs is driven by a strong decrease in energy compared to the state with dissolved oxygen in the A1N lattice. This energy decrease is expected to be much greater at planar rather than curved IDBs. The idea is supported by the observation of the exactly planar IDBs in the basal plane, which are connected to curved IDBs in general with large angles. Additional support is given by the observation of segregated oxygen by EDS analysis at planar but not at curved IDBs. The supersaturation of oxygen in the lattice in the observed crystals is, under the present experimental conditions, not high enough to induce homogeneous nucleation of

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Fig. 4. (a) Dark field image of (1120) orientated A1N grain using g = (0002): in the lower part of the image a 27R phase is growing from the grain boundaries into the grain; (b) diffraction pattern from the AIN grain; (c) diffraction pattern from the 27R phase showing that the lattices of A1N and 27R have the same orientation.

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Fig. 5. Morphologies of IDBs which illustrate schematically the model for their growth; (a) dome-like 1DBs, nucleated at a split basal dislocation; (b) spike-like IDBs at an early stage of growth in an oxygen-supersaturated grain with a quickly expanding planar IDB; (c) morphology at a later stage when the growth rate has decreased.

inversion domains. Therefore heterogeneous nucleation of inversion domains is observed at internal surfaces as grain boundaries or phase boundaries of precipitates. If such surfaces are not present, the nucleation is assumed to occur at split basal dislocations forming a stacking fault of the type a / 3 ( l i 0 0 )

[18]. According to our assumptions, the formed nucleus and the larger forms of the inversion domains should always possess a planar IDB in order to achieve an energy decrease. A curved IDB is also necessary to enclose the already formed inversion domain with three-dimensional extension. Therefore, after some period of growth, inversion domains appear with characteristic shapes such as spikes of different thickness at grain boundaries or as dome-like defects in the grain (Fig. 5). In these forms one observes that the curved IDB has not remained near the planar IDB but has moved away into the lattice. The driving force for this process is provided by the energy decrease for the segregation of oxygen from the surroundings of the curved IDB. This could be understood by a transport of oxygen through the curved IDB to the energetically more attractive planar IDB. The oxygen remaining in the curved IDB could be under the detection limit of the EDS analysis unit. If under certain conditions slower transport occurs or higher oxygen contents exist within regions into which the curved IDB is growing, the oxygen content in the curved IDB will increase. After exceeding a certain limit, this will cause the formation of new planar IDBs, connected with the curved IDB. This process leads to

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the frequently observed zigzag forms of IDBs. If the oxygen content cannot become high enough at the moving curved IDBs to give rise to the formation of planar IDBs only curved IDBs are observed [7, 19]. The fast transport of oxygen to the planar IDB causes this boundary to grow quickly. This leads to a small angle between the planar and the curved IDB at the fast growing boundary of the planar defect because there is too little time for the adjacent part of the curved IDB to expand into the oxygen-containing region and to form a larger angle with the planar IDB. This last process happens when the growth of the planar IDB slows down at a later stage of growth. If the nucleation of inversion domains or the movement of the curved IDBs occurs in a region with higher oxygen content compared to other regions the conditions will give rise to a narrower arrangement of the growing planar IDBs in order to absorb the segregating oxygen. This explains different densities of planar IDBs in the same material, caused by an inhomogeneous distribution of oxygen or in samples of different origin with different oxygen contents. For extremely high concentrations of oxygen, one could expect the density of the planar IDBs to reach values which would correspond to oxygen contents of the 27R, the 12H or other polytypes. On this assumption, these phases should be formed by very dense zigzag arrangements of IDBs, the very narrow planar IDBs being connected by a curved IDB between them. Investigations aiming at the confirmation of these conclusions from the present model are in progress.

6. Conclusions A model for the nucleation and growth of inversion domains in A1N, based on the strong difference of the segregation energies of oxygen at planar and curved IDBs, has been developed. It is confirmed by several phenomena in the morphologies of observed IDBs in high oxygen content AIN and by conclusions from results in other AIN materials. If results of further investigations support this model, then the study of the associated energetics and kinetics as a function of temperature and oxygen content in the AIN grains will be of fundamental importance for the improvement of the heat conductivities of sintered and appropriately annealed AIN.

Acknowledgments The authors thank Prof. H. Mughrabi and Dr. R. Keller for critical reviews of the manuscript, and Dr. W. Grellner, ESK, Kempten, for providing the AIN.

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References 1 M. P. Borom, G. A. Slack, and J. W. Szymaszek, Ceramic Bulletin, 51 (1972) 852. 2 K. Komeya, H. Inoue and A. Tsuge, J. Am. Ceram. Soc., 37 (1974)411. 3 M. Kulig, T. Hofmann, E Ansorge and C. Riissel, cfi/Ber DKG, 66(1989)65. 4 R. Riedel and K.-U. Gaudl, J. Am. Ceram. Soc., 74 (1991) 1331. 5 T.B. Troczynski and E S. Nicholson, J. Am. Ceram. Soc., 72 (8)(1989) 1488. 6 G.A. Slack, J. Phys. Chem. Solids, 34(1973) 321. 7 A. S. Westwood and M. R. Notis, Mat. Res. Soc. Syrup. Proc., 167(1990) 265. 8 A. D. Westwood and M. R. Notis, J. Am. Ceram. Soc., 74 (1991) 1226.

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9 R. A. Youngman and J. H. Harris, J. Am. Ceram. Soc., 73 (1990) 271. 10 E Rat and R. Hamminger, Pract. Met., 28 ( 1991 ) 115. 11 S. McKernan, Mat. Res. Soc. Symp. Proc., 167 (1990) 259. 12 J. H. Harris, R. A. Youngman and R. G. Teller, J. Mat. Res., 5 (8)(1990) 1763. 13 M. E Denanot and J. Rabier, J. Mat. Sci., 24 (1989) 1594. 14 C. Riissel, T. Hofmann and G. Limmer, cfi/Ber DKG, 68 (1991)22. 15 G. A. Jeffrey and G. S. Parry, J. Chem. Phys., 23 (1955) 406. 16 R. Serneels, M. Snykers, E Delavignette, R. Gevers and S. Amelinckx, Phys. Status Solidi B, 58 ( 1973) 277. 17 H. Blank, P. Delavignette, R. Gevers and S. Amelinckx, Phys. Status Solidi, 7(1964) 747. 18 A. Berger, J. Am. Ceram. Soc., 74 ( 1991 ) 1148. 19 K.-U. Senftleben, Ph.D. Thesis, Universit~it ErlangenNiirnberg, 1992.