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Epitaxy in the aqueous oxidation of (001) single crystal copper films
Surface Technology, 8 (1979) 399 - 404
399
© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
EPITAXY IN THE AQUEOUS OXIDATION OF (001) SINGLE CRYSTAL COPPER FILMS
J. P. G. FARR and A. J. S. McNEIL Department of Industrial Metallurgy, The University, Birmingham B15 2TT (Gt. Britain)
(Received February 2, 1979)
Summary Observations are reported on the aqueous oxidation of copper single crystal films. A third copper/cuprous oxide epitaxial relationship has been found, which is related to the two epitaxial relationships already reported in the literature.
1. Introduction In a study of the electrocrystallization of nickel, described elsewhere [1], (001)-oriented single crystal copper films about 100 nm thick were prepared as substrates. These copper films were produced by vacuum evaporation, at pressures better than 10-5 Torr, on to cleaved (001) single crystal faces of rock-salt, following the procedure of Brockway and Marcus [2, 3]. Subsequently the films were annealed in hydrogen (1 atm, 540 °C, 5 min) while still on their rock-salt substrates. The copper films were then prepared for nickel plating by stripping them from the rock-salt in distilled water. As in the findings of Brockway and Marcus [3] and also those of Gaigher and van Wyk [4] these water-stripped copper films were susceptible to extensive oxidation. The cuprous oxide (Cu20) growths did n o t appear either on films stripped in glycerol, in which rock-salt is slightly soluble, or on films stripped in water with a small addition of hydrochloric acid. They could be removed by a second hydrogen annealing treatment, after which they did not readily re-form in the atmosphere.
2. Experimental observations Figures l(a), 2 and 3 show typical electron diffraction patterns, produced by copper/cuprous oxide bicrystals in an AEI EM6G electron microscope working at 100 kV. Three epitaxial relationships are evident, all of which are manifest in Fig. l(a), and each can be distinguished with the help of the simplified nets in Figs. l(b) - (d).
400
O
O
•
i,,{220} • .------
- {110)
J200} •
O
(a)
o /
\o
e j
(b) 0
•
0
©
O
0
0
+---1111x/
,,(1101
•
°\
/
\
\~_~"I200l
\
0
(c)
!
©
0
/ 0
(d)
Fig. 1. The diffraction pattern (a) displays the three epitaxial relationships Cu20/Cu. The basic diffraction nets for the oxide are given in (b) (001), (c) (111) and (d) (110), with the nets in (c) and (d) appearing doubly positioned in the diffraction pattern, and all the nets being multiplied by double diffraction (see ref. 6). The effects of double diffraction are more clearly demonstrated in the simpler patterns of Figs. 2 and 3. The diffraction spots in the nets are indicated as follows: ©, copper spots; e, Cu20 spots in the (001) and ( 111 ) patterns; -- Cu 20 spots in the (110) pattern. T h e t h r e e epitaxial relationships are as follows: (1) ( 0 0 1 ) Cu2OIJ(001 ) Cu, with [ 1 1 0 ] Cu2OH[li-0 ] Cu, a straightforward parallel relationship; (2) (111) Cu2Of](001 ) Cu, with d o u b l e positioning so t h a t [1i-0] C u 2 0 H[ll-0] Cu and 110 Cu2OJf [ 1 1 0 ] Cu (in fact these triangular o x i d e growths e x h i b i t q u a d r u p l e positioning [ 5 ] , b y d o u b l e positioning a b o u t each o f the t w o (110) directions in the c o p p e r surface (see Fig. 4), t h o u g h the d i f f r a c t i o n p a t t e r n distinguishes o n l y t w o m u t u a l l y p e r p e n d i c u l a r types); (3) (110) Cu2OIl(001) Cu, with d o u b l e positioning so t h a t [11-0] C u 2 0 N [ l l - 0 ] C u and [ 1 1 0 ] C u 2 O H [ l l 0 ] Cu.
401
Fig. 2. This diffraction pattern was produced almost entirely by Cu20 in the (111) orientation. The faintness of the streaked (111} oxide spots contrasts with Fig. l(a). Very little oxide in the parallel (001) orientation is present, as indicated by the weakness of the (200} diffraction spots. Fig. 3. This diffraction pattern was produced almost entirely by oxide in the (001) orientation, with little (110) oxide, as shown by the faintness of the (111} spots, and with almost none in the (111) orientation. Y
z,
(110) (111)-(001)--
[ 110 ]'~
x
Fig. 4. A diagram showing a block of Cu20 grown in parallel orientation on copper, with the (001) plane of Cu20 parallel to the (001) copper surface, and the [1:10] directions coincident in both lattices. This is the first epitaxial relationship listed in the text. To produce the other two, the Cu20 is tilted about the [1:10] direction, first through about 55 ° to obtain the second relationship, (111 )Cu2OII (001) Cu, and then through a total of 90 ° to obtain the third relationship, (110) Cu2Oll(001 ) Cu. T h e c o n n e c t i o n b e t w e e n t h e s e t h r e e r e l a t i o n s h i p s is i l l u s t r a t e d in F i g . 4. T h i s s h o w s a b l o c k o f o x i d e r e s t i n g w i t h its ( 0 0 1 ) b a s e p l a n e o n t h e ( 0 0 1 ) s u r f a c e o f t h e c o p p e r . T h i s is t h e f i r s t , p a r a l l e l o x i d e o r i e n t a t i o n . N o w i f t h e o x i d e l a t t i c e is t i l t e d t h r o u g h a p p r o x i m a t e l y 55 ° a b o u t t h e c o m m o n [ 1 1 0 ] d i r e c t i o n , t o b r i n g t h e ( 1 1 1 ) o x i d e p l a n e d o w n t o m e e t t h e ( 0 0 1 } c o p p e r sur-
402 face, this produces the second epitaxial relationship. Then if the oxide lattice is tilted further a b o u t this [150] direction, through a total of 90 °, the (110) oxide plane is brought parallel to Cu (001) and we have the third epitaxial relationship. All the spots in the (110) oxide diffraction pattern (see Figs. l(a) and l(d)) are streaked in an arc of approximately 16 ° about the (0,0,0) pattern origin, a feature which is not found in the (001) and (111) oxide patterns. This streaking can be seen most clearly in the {111} spots, which are unique to the (110) oxide pattern, and is less obvious b u t still apparent in the other {200}, (110} and {220} spots, which are shared with the other patterns. This form of streaking is consistent with the ( l l 0 ) - o r i e n t e d oxide taking up a range of misorientations a b o u t the [001 ] direction normal to the (001) copper surface. The nature of the oxide growths produced during water stripping was variable. Not only did the relative amounts of the different orientations change from film to film, but sometimes one growth type was absent. The diffraction pattern in Fig. 2 was produced almost entirely by cuprous oxide in the (111) orientation, with very little contribution from oxide in the (001) and (110) orientations, as shown by the faintness of the (200} and (111} spots, respectively. The pattern is built up solely by a pair of hexagonal nets of {220} and {110} spots, such as are shown in Fig. l(c), which have undergone multiplication first by double positioning, and then by double diffraction [6] from each of the surrounding copper spots. Figure 3 shows a different balance of oxide orientations, where the pattern is largely produced by oxide in the (001) orientation, with a small contribution from the (110) oxide and almost none at all from the (111) oxide.
3. Discussion The growth of crystalline cuprous oxide on copper substrates in both gaseous and aqueous environments has been the subject of extensive study [2, 3, 5, 7 - 10]. Only two epitaxial relationships have been found between copper and its oxide. Type (2) (as numbered above) predominates, with type (1) appearing infrequently, but no mention of type (3) has been found in the literature. However, in a study of the selective oxidation of copper in a-brass, Takahashi and Trillat [11] appear to have produced the third Cu/Cu20 relationship without realising it. These authors presented an electron diffraction pattern which displays all three oxide orientations. They also gave a corresponding diagram which shows the numerous diffraction spots that can be produced by cuprous oxide in orientations (1) and (2), but which omits the third orientation. Crude measurements on their diffraction patterns suggest the presence of streaked {111} cuprous oxide spots, which are very similar in appearance to those found in the present study (see Figs. l(a), l(d) and 3).
403 We have found three reports of epitaxy in the aqueous oxidation of copper, by Lawless and Mitchell [7], by Lawless and Miller [8], and by Kruger [9]. Though Kruger's work showed that significant differences exist between aqueous and gaseous oxidation systems, all three studies found the same pair of copper/cuprous oxide epitaxial relationships, types (1) and (2) given above. From the examination of oxide growth on several single crystal copper faces, Lawless and Mitchell [ 7 ] concluded that while the formation of some form of oriented overgrowth depends on a small misfit in the interface plane, the nucleation process is important in determining the nature of that overgrowth. Nucleation is influenced by such factors as the condition of the copper surface, the presence and nature of contaminants and the temperature and nature of the oxidizing environment. Lawless and Gwathmey [5] found that traces of contaminants, such as grease and chloride from wash water, produced marked and consistent changes in the oxide patterns on their spherical copper crystals. Certainly, in the work reported here, the nature of the oxide produced on the water-stripped copper films, as shown by the balance of the different oxide orientations, was variable (see Figs. 1 3). It is n o t e w o r t h y that Lee and Farnsworth [12] concluded from their LEED experiments t h a t the growth of cuprous oxide on {001) copper at room temperature required the presence of a foreign substance such as water. The work of Lawless and his associates [5, 7] indicates that in the oriented growth of cuprous oxide on copper at least one pair of ~110) directions must be parallel in the mating crystal faces. Figure 4 shows how all three oxide orientations observed in the present work are related through the rotation of the cuprous oxide lattice about such a (110~ direction in the (001) copper surface. The third oxide orientation, (110) Cu20[[(001 ) Cu, thus appears as a natural extension to the reported behaviour of this system. To conclude, most of the work on copper oxidation, such as that of Lawless [5, 7, 8] and Kruger [9], has concerned bulk crystals in purified environments. The oxide growths examined in the present work have formed on vapour deposited films under uncontrolled conditions, where adventitious yet persistent impurities may have been present. Under these conditions it appears that a new copper/cuprous oxide epitaxial relationship has been found.
Acknowledgments We thank W. Canning and Co. Ltd. for the award of a research scholarship (A.J.S.M.) and Professor D. V. Wilson for the provision of laboratory facilities and for his continuing interest. References
1 J. P. G. Farr and A. J. S. McNeil, Chem. Soc. Faraday Syrup., 12 (1977) 143. 2 L. O. Brockway and R. B. Marcus, J. Appl. Phys., 34 (1963) 921.
404 3 L. O. Brockway, R. B. Marcus and A. P. Rowe, in M. H. Francombe and H. Sato (eds.), Single Crystal Films, Pergamon, 1964, p. 231 e t seq. 4 H. L. Gaigher and G. N. van Wyk, Electrochim. Acta, 18 (1973) 849. 5 K. R. Lawless and A. T. Gwathmey, Acta Metali., 4 (1956) 153. 6 J. W. Edington, Electron Diffraction in the Electron Microscope, Monograph 2 in the Monographs in Practical Electron Microscopy in Materials Science Series, Macmillan, London, 1975. 7 K. R. Lawless and D. F. Mitchell, M~m. Sci. Rev. M6tall., 62 (1965) pp. 27 and 39. 8 K. R. Lawless and G. T. Miller, Acta Crystallogr., 12 (1959) 594. 9 J. Kruger, J. Electrochem. Soc., 106 (1959) 847. 10 R. F. Mehl, E. L. McCandless and F. N. Rhines, Nature (London), 134 (1934) 1009. 11 N. Takahashi and J. J. Trillat, Acta Metall., 4 (1956) 201. 12 R. N. Lee and H. E. Farnsworth, Surf. Sci., 3 (1965) 461.
399
© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
EPITAXY IN THE AQUEOUS OXIDATION OF (001) SINGLE CRYSTAL COPPER FILMS
J. P. G. FARR and A. J. S. McNEIL Department of Industrial Metallurgy, The University, Birmingham B15 2TT (Gt. Britain)
(Received February 2, 1979)
Summary Observations are reported on the aqueous oxidation of copper single crystal films. A third copper/cuprous oxide epitaxial relationship has been found, which is related to the two epitaxial relationships already reported in the literature.
1. Introduction In a study of the electrocrystallization of nickel, described elsewhere [1], (001)-oriented single crystal copper films about 100 nm thick were prepared as substrates. These copper films were produced by vacuum evaporation, at pressures better than 10-5 Torr, on to cleaved (001) single crystal faces of rock-salt, following the procedure of Brockway and Marcus [2, 3]. Subsequently the films were annealed in hydrogen (1 atm, 540 °C, 5 min) while still on their rock-salt substrates. The copper films were then prepared for nickel plating by stripping them from the rock-salt in distilled water. As in the findings of Brockway and Marcus [3] and also those of Gaigher and van Wyk [4] these water-stripped copper films were susceptible to extensive oxidation. The cuprous oxide (Cu20) growths did n o t appear either on films stripped in glycerol, in which rock-salt is slightly soluble, or on films stripped in water with a small addition of hydrochloric acid. They could be removed by a second hydrogen annealing treatment, after which they did not readily re-form in the atmosphere.
2. Experimental observations Figures l(a), 2 and 3 show typical electron diffraction patterns, produced by copper/cuprous oxide bicrystals in an AEI EM6G electron microscope working at 100 kV. Three epitaxial relationships are evident, all of which are manifest in Fig. l(a), and each can be distinguished with the help of the simplified nets in Figs. l(b) - (d).
400
O
O
•
i,,{220} • .------
- {110)
J200} •
O
(a)
o /
\o
e j
(b) 0
•
0
©
O
0
0
+---1111x/
,,(1101
•
°\
/
\
\~_~"I200l
\
0
(c)
!
©
0
/ 0
(d)
Fig. 1. The diffraction pattern (a) displays the three epitaxial relationships Cu20/Cu. The basic diffraction nets for the oxide are given in (b) (001), (c) (111) and (d) (110), with the nets in (c) and (d) appearing doubly positioned in the diffraction pattern, and all the nets being multiplied by double diffraction (see ref. 6). The effects of double diffraction are more clearly demonstrated in the simpler patterns of Figs. 2 and 3. The diffraction spots in the nets are indicated as follows: ©, copper spots; e, Cu20 spots in the (001) and ( 111 ) patterns; -- Cu 20 spots in the (110) pattern. T h e t h r e e epitaxial relationships are as follows: (1) ( 0 0 1 ) Cu2OIJ(001 ) Cu, with [ 1 1 0 ] Cu2OH[li-0 ] Cu, a straightforward parallel relationship; (2) (111) Cu2Of](001 ) Cu, with d o u b l e positioning so t h a t [1i-0] C u 2 0 H[ll-0] Cu and 110 Cu2OJf [ 1 1 0 ] Cu (in fact these triangular o x i d e growths e x h i b i t q u a d r u p l e positioning [ 5 ] , b y d o u b l e positioning a b o u t each o f the t w o (110) directions in the c o p p e r surface (see Fig. 4), t h o u g h the d i f f r a c t i o n p a t t e r n distinguishes o n l y t w o m u t u a l l y p e r p e n d i c u l a r types); (3) (110) Cu2OIl(001) Cu, with d o u b l e positioning so t h a t [11-0] C u 2 0 N [ l l - 0 ] C u and [ 1 1 0 ] C u 2 O H [ l l 0 ] Cu.
401
Fig. 2. This diffraction pattern was produced almost entirely by Cu20 in the (111) orientation. The faintness of the streaked (111} oxide spots contrasts with Fig. l(a). Very little oxide in the parallel (001) orientation is present, as indicated by the weakness of the (200} diffraction spots. Fig. 3. This diffraction pattern was produced almost entirely by oxide in the (001) orientation, with little (110) oxide, as shown by the faintness of the (111} spots, and with almost none in the (111) orientation. Y
z,
(110) (111)-(001)--
[ 110 ]'~
x
Fig. 4. A diagram showing a block of Cu20 grown in parallel orientation on copper, with the (001) plane of Cu20 parallel to the (001) copper surface, and the [1:10] directions coincident in both lattices. This is the first epitaxial relationship listed in the text. To produce the other two, the Cu20 is tilted about the [1:10] direction, first through about 55 ° to obtain the second relationship, (111 )Cu2OII (001) Cu, and then through a total of 90 ° to obtain the third relationship, (110) Cu2Oll(001 ) Cu. T h e c o n n e c t i o n b e t w e e n t h e s e t h r e e r e l a t i o n s h i p s is i l l u s t r a t e d in F i g . 4. T h i s s h o w s a b l o c k o f o x i d e r e s t i n g w i t h its ( 0 0 1 ) b a s e p l a n e o n t h e ( 0 0 1 ) s u r f a c e o f t h e c o p p e r . T h i s is t h e f i r s t , p a r a l l e l o x i d e o r i e n t a t i o n . N o w i f t h e o x i d e l a t t i c e is t i l t e d t h r o u g h a p p r o x i m a t e l y 55 ° a b o u t t h e c o m m o n [ 1 1 0 ] d i r e c t i o n , t o b r i n g t h e ( 1 1 1 ) o x i d e p l a n e d o w n t o m e e t t h e ( 0 0 1 } c o p p e r sur-
402 face, this produces the second epitaxial relationship. Then if the oxide lattice is tilted further a b o u t this [150] direction, through a total of 90 °, the (110) oxide plane is brought parallel to Cu (001) and we have the third epitaxial relationship. All the spots in the (110) oxide diffraction pattern (see Figs. l(a) and l(d)) are streaked in an arc of approximately 16 ° about the (0,0,0) pattern origin, a feature which is not found in the (001) and (111) oxide patterns. This streaking can be seen most clearly in the {111} spots, which are unique to the (110) oxide pattern, and is less obvious b u t still apparent in the other {200}, (110} and {220} spots, which are shared with the other patterns. This form of streaking is consistent with the ( l l 0 ) - o r i e n t e d oxide taking up a range of misorientations a b o u t the [001 ] direction normal to the (001) copper surface. The nature of the oxide growths produced during water stripping was variable. Not only did the relative amounts of the different orientations change from film to film, but sometimes one growth type was absent. The diffraction pattern in Fig. 2 was produced almost entirely by cuprous oxide in the (111) orientation, with very little contribution from oxide in the (001) and (110) orientations, as shown by the faintness of the (200} and (111} spots, respectively. The pattern is built up solely by a pair of hexagonal nets of {220} and {110} spots, such as are shown in Fig. l(c), which have undergone multiplication first by double positioning, and then by double diffraction [6] from each of the surrounding copper spots. Figure 3 shows a different balance of oxide orientations, where the pattern is largely produced by oxide in the (001) orientation, with a small contribution from the (110) oxide and almost none at all from the (111) oxide.
3. Discussion The growth of crystalline cuprous oxide on copper substrates in both gaseous and aqueous environments has been the subject of extensive study [2, 3, 5, 7 - 10]. Only two epitaxial relationships have been found between copper and its oxide. Type (2) (as numbered above) predominates, with type (1) appearing infrequently, but no mention of type (3) has been found in the literature. However, in a study of the selective oxidation of copper in a-brass, Takahashi and Trillat [11] appear to have produced the third Cu/Cu20 relationship without realising it. These authors presented an electron diffraction pattern which displays all three oxide orientations. They also gave a corresponding diagram which shows the numerous diffraction spots that can be produced by cuprous oxide in orientations (1) and (2), but which omits the third orientation. Crude measurements on their diffraction patterns suggest the presence of streaked {111} cuprous oxide spots, which are very similar in appearance to those found in the present study (see Figs. l(a), l(d) and 3).
403 We have found three reports of epitaxy in the aqueous oxidation of copper, by Lawless and Mitchell [7], by Lawless and Miller [8], and by Kruger [9]. Though Kruger's work showed that significant differences exist between aqueous and gaseous oxidation systems, all three studies found the same pair of copper/cuprous oxide epitaxial relationships, types (1) and (2) given above. From the examination of oxide growth on several single crystal copper faces, Lawless and Mitchell [ 7 ] concluded that while the formation of some form of oriented overgrowth depends on a small misfit in the interface plane, the nucleation process is important in determining the nature of that overgrowth. Nucleation is influenced by such factors as the condition of the copper surface, the presence and nature of contaminants and the temperature and nature of the oxidizing environment. Lawless and Gwathmey [5] found that traces of contaminants, such as grease and chloride from wash water, produced marked and consistent changes in the oxide patterns on their spherical copper crystals. Certainly, in the work reported here, the nature of the oxide produced on the water-stripped copper films, as shown by the balance of the different oxide orientations, was variable (see Figs. 1 3). It is n o t e w o r t h y that Lee and Farnsworth [12] concluded from their LEED experiments t h a t the growth of cuprous oxide on {001) copper at room temperature required the presence of a foreign substance such as water. The work of Lawless and his associates [5, 7] indicates that in the oriented growth of cuprous oxide on copper at least one pair of ~110) directions must be parallel in the mating crystal faces. Figure 4 shows how all three oxide orientations observed in the present work are related through the rotation of the cuprous oxide lattice about such a (110~ direction in the (001) copper surface. The third oxide orientation, (110) Cu20[[(001 ) Cu, thus appears as a natural extension to the reported behaviour of this system. To conclude, most of the work on copper oxidation, such as that of Lawless [5, 7, 8] and Kruger [9], has concerned bulk crystals in purified environments. The oxide growths examined in the present work have formed on vapour deposited films under uncontrolled conditions, where adventitious yet persistent impurities may have been present. Under these conditions it appears that a new copper/cuprous oxide epitaxial relationship has been found.
Acknowledgments We thank W. Canning and Co. Ltd. for the award of a research scholarship (A.J.S.M.) and Professor D. V. Wilson for the provision of laboratory facilities and for his continuing interest. References
1 J. P. G. Farr and A. J. S. McNeil, Chem. Soc. Faraday Syrup., 12 (1977) 143. 2 L. O. Brockway and R. B. Marcus, J. Appl. Phys., 34 (1963) 921.
404 3 L. O. Brockway, R. B. Marcus and A. P. Rowe, in M. H. Francombe and H. Sato (eds.), Single Crystal Films, Pergamon, 1964, p. 231 e t seq. 4 H. L. Gaigher and G. N. van Wyk, Electrochim. Acta, 18 (1973) 849. 5 K. R. Lawless and A. T. Gwathmey, Acta Metali., 4 (1956) 153. 6 J. W. Edington, Electron Diffraction in the Electron Microscope, Monograph 2 in the Monographs in Practical Electron Microscopy in Materials Science Series, Macmillan, London, 1975. 7 K. R. Lawless and D. F. Mitchell, M~m. Sci. Rev. M6tall., 62 (1965) pp. 27 and 39. 8 K. R. Lawless and G. T. Miller, Acta Crystallogr., 12 (1959) 594. 9 J. Kruger, J. Electrochem. Soc., 106 (1959) 847. 10 R. F. Mehl, E. L. McCandless and F. N. Rhines, Nature (London), 134 (1934) 1009. 11 N. Takahashi and J. J. Trillat, Acta Metall., 4 (1956) 201. 12 R. N. Lee and H. E. Farnsworth, Surf. Sci., 3 (1965) 461.