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One-dimensionally disordered structure and polytypism in SiC
Mat. Res. Bull. Vol. in the United State s.
I0, pp. Z57-Z60,
1975.
Pergamon
Press,
Inc.
Printed
ONE-DIMENSIONALLY DISORDERED STRUCTURE AND POLYTYPISM IN SiC
H. Sato * School of Materials Engineering, Purdue University, W. Lafayette, IN. 47907 and S. Shinozaki Scientific Research Staff, Ford Motor Company, Dearborn, MI. 48121
(Received F e b r u a r y 6, 1975; C o m m u n i c a t e d
by R. A. Huggins)
ABSTRACT Almost perfect, one-dimensionally disordered structures in SiC are found by transmission electron microscopy in a CVD material and in an annealed reaction-sintered material at 1900°C. The observed common existence of one-dimensionally disordered structure in SiC supports the concept that the structure of SiC can be conveniently represented by a close packing of spheres with nearest neighbor interactions.
SiC is well known for its extensive polytypical behavior and takes many different forms of stacking variants of close-packed structure (i, 2). The existence of many polytypes, however, is roughly understood from the structure and its bonding characteristics. Since the bonding between Si and C is mostly covalent and is short range, the characteristic of the structure can be understood conveniently in terms of a close packed stacking of spheres with radius ~ , where a is the edge length of the unit tetrahedron, centered either at Si atoms or at C atoms, with nearest neighbor central force interactions (3). In fact, under the ideal condition of the close packing of spheres of nearest neighbor interactions, the energy of a crystal does not depend on the stacking order. Hence, any close packed structure is equally probable. Although this would roughly explain the origin of polytypism in SiC, the most probable structure would then be a non periodic, one-dimensionally disordered structure. Although a certain degree of one dimensional disorder is common in SiC (4), no structure with a completely disordered arrangement of layers has yet been reported. Instead, a predominant existence of a small number of structures of relatively short period (6H, 4H, 15R, etc. in Ramsdell notation (5)) indicates the deviation from the
*Formerly with the Scientific Research Staff, Ford Motor Co. 257
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SiC
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idealized condition. Therefore, the central problem of polytypism in SiC is to find what such a deviation from the ideal condition should be. The present report presents the case that such a one-dimensionally (if not perfectly) disordered structure (or a structure with a non-periodic stacking order) is, nevertheless, rather frequently found in SiC, and that the condition of close packing of spheres with nearest neighbor interactions is still a good approximation. One-dimensionally disordered structures are commonly observed in CVD (chemical vapor deposition) materials by transmission electron microscopy (6). When SiC is deposited from a vapor phase at substrate temperatures of 1200°C ~ 1400°C, SiC crystals grow with the c-axis (the axis normal to the close packed planes) perpendicular to the substrate, although a small deviation in angle from place to place exists. The structure is mostly 3C in the Ramsdell notation (fcc form Qf SiC or B-SiC) and the growth pattern is dendritic. For thicker deposits the growth pattern changes to columnar as shown in the transmission electron micrograph in the (01"0) orientation (indexing based on the hexagonal lattice) in Fig. la. Here, each crystallite shows dense, parallel lines perpendicular to the c-axis of the crystal, indicating a high density of stacking faults. A corresponding electron diffraction pattern from such a crystallite is shown in Fig. lb. It shows a very sharp spot on the c-axis which corresponds to the (ill) spot of the fcc structure, indicating that the interplaner distance between successive close packed layers is not at all disturbed. (The superposed diffused line is due to a multiple diffraction effect and needs not be concerned in the present arguments.) Otherwise, the diffraction pattern indicates almost continuous reciprocal lattice lines. In other words, the stacking order of the crystal is almost completely random while the structure is perfect in the other two dimensions. Another example of one dimensionally disordered structures can be found when reaction-sintered SiC materials fabricated near 1500°C are heated to temperatures between 1900°C and 2000°C. In reaction-sintered material, SiC crystals newly formed by a reaction of carbon and molten Si at around 1500°C are always of the 3C structure (7). The 3C structure or 8-SIC is generally a low temperature form of polytypes and tends to transform into s-SiC (6H, 4H, etc.) at higher temperatures (8). One-dimensionally disordered structures appear at an intermediate stage of the transformation. The electron micrograph again in the (01"0) orientation in Fig. 2a shows ( b y an arrow) a crystallite which is in such a transition stage. Although such a crystallite looks similar to other B-SiC grains in the electron-micrograph with the exception of a high density of stripes, the diffraction pattern obtained from the crystallite in Fig. 2b is practically identical to that shown in Fig. ib and indicates that the crystallite has a typical onedimensionally disordered structure. These examples indicate that the existence of almost perfect one-dimensionally disordered structures is rather common in SiC if examined by transmission electron microscopy. Some further comments concerning the existence of one-dimensionally disordered structures in these materials are in order. In identifying structures, X-ray diffraction methods with powder specimens is commonly used. Since CVD material includes both perfect 3C crystals and one dimensionally disordered crystals together, powder diffraction patterns inevitably indicate diffraction lines of 3C structure with an extensive diffuseness. Such X-ray results have often been erroneously interpreted that the material is either extremely fine-grained or very heavily strained. In fact, transmission electron microscopy of individual grains in reaction-sintered materials
Vol. I0, No. 4
STRUCTURE
AND
POLYTYPISM
iN SiC
FIG. 1
a)
Transmission electron micrograph of a CVD material in (01-0) orientation. Each band in the micrograph corresponds to a columnar, one-dimensionally disordered crystal.
b)
Electron diffraction pattern obtained from the central band shown above. The origin and the cubic (iii) type spot on the c-axis are indicated by arrows.
shows clearly that these grains are either heavily twinned or strongly faulted as a result of polytypism but are not strained (7). In other words, the existence of stacking faults, and the resulting diffuseness in diffraction patterns are not necessarily the sign of the existence of strain. On the other hand, the existence of a one-dimensionally disordered structure in annealed materials as shown in Fig. 2 indicates that the stacking order of 3C crystals becomes disordered before they transform into one of e-SiC structures, indicating a possibility of solid state transformation in SiC. It is commonly accepted that a transformation in structure in SiC is due to a process of recrystallization through surface diffusion and not due to solid state transformations through rearrangements of layers (8). The possibility of solid state transformations from one polytype to another as is indicated here, however, has a very important implication in understanding the existence of some of the long period polytypes in SiC (3, 9) and requires a further close examination. Powell and Will (i0) reported such possibilities of solid state transformations for 2H and 3C crystals. Helpful comments of Professor G. L. Liedl of Purdue University are greatly appreciated.
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IN SiC
Volo I0, No. 4
FIG. 2
a)
b)
Transmission electron micrograph of a B-SiC grain which has almost perfect one-dimensionally disordered (IDDO) structure in (01-0) orientation. The right hand corner of the grain separated by a sharp line from the disordered 8-SIC crystal has a perfect 3C structure. Electron diffraction pattern obtained from the central part of the above micrograph. The origin and the cubic (iii) type spot on the c-axis are indicated by arrows.
References i. 2. 3. 4. 5. 6. 7. 8. 9. I0.
A, R. Verma and K. P. Krishna, Polymorphlsm and Polytypism in Crystals. John Wiley & Sons, Inc., New York (1966). P. T. B. Shaffer, Acta Cryst. B25 2477 (1969). H. Sato, S. Shinozaki and M. Yessik, J. Appl. Phys. 45 1630 (1974). H. Jagodzinski, Neues Jahrb. Miner., Monatsch. ~ 49 (-1954). L. S. Ramsdell, Am. Mineral. 3 2 6 4 (1947). S. Shinozaki and H. Sato, to be published. H. Sato, S. Shinozaki, M. Yessik and J. E. Noakes, Proc. 3rd International Conference on SiC, R. C. Marshall. J. W. Faust and C. E. Ryan, eds. Univ South Carolina Press (1974) P. 222. W. F. Knippenberg, Philips Research Report, 18 161 (1963). M. Yessik, S. Shinozaki and H. Sato, to be pu-blished. J. H. Powell and H. A. Will, J. Appl. Phys. 43 1400 (1972).
Work supported in part by the National Science Foundation under Contract No. DMR7203018.
I0, pp. Z57-Z60,
1975.
Pergamon
Press,
Inc.
Printed
ONE-DIMENSIONALLY DISORDERED STRUCTURE AND POLYTYPISM IN SiC
H. Sato * School of Materials Engineering, Purdue University, W. Lafayette, IN. 47907 and S. Shinozaki Scientific Research Staff, Ford Motor Company, Dearborn, MI. 48121
(Received F e b r u a r y 6, 1975; C o m m u n i c a t e d
by R. A. Huggins)
ABSTRACT Almost perfect, one-dimensionally disordered structures in SiC are found by transmission electron microscopy in a CVD material and in an annealed reaction-sintered material at 1900°C. The observed common existence of one-dimensionally disordered structure in SiC supports the concept that the structure of SiC can be conveniently represented by a close packing of spheres with nearest neighbor interactions.
SiC is well known for its extensive polytypical behavior and takes many different forms of stacking variants of close-packed structure (i, 2). The existence of many polytypes, however, is roughly understood from the structure and its bonding characteristics. Since the bonding between Si and C is mostly covalent and is short range, the characteristic of the structure can be understood conveniently in terms of a close packed stacking of spheres with radius ~ , where a is the edge length of the unit tetrahedron, centered either at Si atoms or at C atoms, with nearest neighbor central force interactions (3). In fact, under the ideal condition of the close packing of spheres of nearest neighbor interactions, the energy of a crystal does not depend on the stacking order. Hence, any close packed structure is equally probable. Although this would roughly explain the origin of polytypism in SiC, the most probable structure would then be a non periodic, one-dimensionally disordered structure. Although a certain degree of one dimensional disorder is common in SiC (4), no structure with a completely disordered arrangement of layers has yet been reported. Instead, a predominant existence of a small number of structures of relatively short period (6H, 4H, 15R, etc. in Ramsdell notation (5)) indicates the deviation from the
*Formerly with the Scientific Research Staff, Ford Motor Co. 257
Z58
STRUCTURE
AND
POLYTYPISMIN
SiC
Volo
I0, No. 4
idealized condition. Therefore, the central problem of polytypism in SiC is to find what such a deviation from the ideal condition should be. The present report presents the case that such a one-dimensionally (if not perfectly) disordered structure (or a structure with a non-periodic stacking order) is, nevertheless, rather frequently found in SiC, and that the condition of close packing of spheres with nearest neighbor interactions is still a good approximation. One-dimensionally disordered structures are commonly observed in CVD (chemical vapor deposition) materials by transmission electron microscopy (6). When SiC is deposited from a vapor phase at substrate temperatures of 1200°C ~ 1400°C, SiC crystals grow with the c-axis (the axis normal to the close packed planes) perpendicular to the substrate, although a small deviation in angle from place to place exists. The structure is mostly 3C in the Ramsdell notation (fcc form Qf SiC or B-SiC) and the growth pattern is dendritic. For thicker deposits the growth pattern changes to columnar as shown in the transmission electron micrograph in the (01"0) orientation (indexing based on the hexagonal lattice) in Fig. la. Here, each crystallite shows dense, parallel lines perpendicular to the c-axis of the crystal, indicating a high density of stacking faults. A corresponding electron diffraction pattern from such a crystallite is shown in Fig. lb. It shows a very sharp spot on the c-axis which corresponds to the (ill) spot of the fcc structure, indicating that the interplaner distance between successive close packed layers is not at all disturbed. (The superposed diffused line is due to a multiple diffraction effect and needs not be concerned in the present arguments.) Otherwise, the diffraction pattern indicates almost continuous reciprocal lattice lines. In other words, the stacking order of the crystal is almost completely random while the structure is perfect in the other two dimensions. Another example of one dimensionally disordered structures can be found when reaction-sintered SiC materials fabricated near 1500°C are heated to temperatures between 1900°C and 2000°C. In reaction-sintered material, SiC crystals newly formed by a reaction of carbon and molten Si at around 1500°C are always of the 3C structure (7). The 3C structure or 8-SIC is generally a low temperature form of polytypes and tends to transform into s-SiC (6H, 4H, etc.) at higher temperatures (8). One-dimensionally disordered structures appear at an intermediate stage of the transformation. The electron micrograph again in the (01"0) orientation in Fig. 2a shows ( b y an arrow) a crystallite which is in such a transition stage. Although such a crystallite looks similar to other B-SiC grains in the electron-micrograph with the exception of a high density of stripes, the diffraction pattern obtained from the crystallite in Fig. 2b is practically identical to that shown in Fig. ib and indicates that the crystallite has a typical onedimensionally disordered structure. These examples indicate that the existence of almost perfect one-dimensionally disordered structures is rather common in SiC if examined by transmission electron microscopy. Some further comments concerning the existence of one-dimensionally disordered structures in these materials are in order. In identifying structures, X-ray diffraction methods with powder specimens is commonly used. Since CVD material includes both perfect 3C crystals and one dimensionally disordered crystals together, powder diffraction patterns inevitably indicate diffraction lines of 3C structure with an extensive diffuseness. Such X-ray results have often been erroneously interpreted that the material is either extremely fine-grained or very heavily strained. In fact, transmission electron microscopy of individual grains in reaction-sintered materials
Vol. I0, No. 4
STRUCTURE
AND
POLYTYPISM
iN SiC
FIG. 1
a)
Transmission electron micrograph of a CVD material in (01-0) orientation. Each band in the micrograph corresponds to a columnar, one-dimensionally disordered crystal.
b)
Electron diffraction pattern obtained from the central band shown above. The origin and the cubic (iii) type spot on the c-axis are indicated by arrows.
shows clearly that these grains are either heavily twinned or strongly faulted as a result of polytypism but are not strained (7). In other words, the existence of stacking faults, and the resulting diffuseness in diffraction patterns are not necessarily the sign of the existence of strain. On the other hand, the existence of a one-dimensionally disordered structure in annealed materials as shown in Fig. 2 indicates that the stacking order of 3C crystals becomes disordered before they transform into one of e-SiC structures, indicating a possibility of solid state transformation in SiC. It is commonly accepted that a transformation in structure in SiC is due to a process of recrystallization through surface diffusion and not due to solid state transformations through rearrangements of layers (8). The possibility of solid state transformations from one polytype to another as is indicated here, however, has a very important implication in understanding the existence of some of the long period polytypes in SiC (3, 9) and requires a further close examination. Powell and Will (i0) reported such possibilities of solid state transformations for 2H and 3C crystals. Helpful comments of Professor G. L. Liedl of Purdue University are greatly appreciated.
Z59
Z60
STRUCTURE
AND
POLYTYPISM
IN SiC
Volo I0, No. 4
FIG. 2
a)
b)
Transmission electron micrograph of a B-SiC grain which has almost perfect one-dimensionally disordered (IDDO) structure in (01-0) orientation. The right hand corner of the grain separated by a sharp line from the disordered 8-SIC crystal has a perfect 3C structure. Electron diffraction pattern obtained from the central part of the above micrograph. The origin and the cubic (iii) type spot on the c-axis are indicated by arrows.
References i. 2. 3. 4. 5. 6. 7. 8. 9. I0.
A, R. Verma and K. P. Krishna, Polymorphlsm and Polytypism in Crystals. John Wiley & Sons, Inc., New York (1966). P. T. B. Shaffer, Acta Cryst. B25 2477 (1969). H. Sato, S. Shinozaki and M. Yessik, J. Appl. Phys. 45 1630 (1974). H. Jagodzinski, Neues Jahrb. Miner., Monatsch. ~ 49 (-1954). L. S. Ramsdell, Am. Mineral. 3 2 6 4 (1947). S. Shinozaki and H. Sato, to be published. H. Sato, S. Shinozaki, M. Yessik and J. E. Noakes, Proc. 3rd International Conference on SiC, R. C. Marshall. J. W. Faust and C. E. Ryan, eds. Univ South Carolina Press (1974) P. 222. W. F. Knippenberg, Philips Research Report, 18 161 (1963). M. Yessik, S. Shinozaki and H. Sato, to be pu-blished. J. H. Powell and H. A. Will, J. Appl. Phys. 43 1400 (1972).
Work supported in part by the National Science Foundation under Contract No. DMR7203018.