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Improved epitaxy of nickel on rocksalt substrates due to electron bombardment
Short Commdcations Improved epitaxy of nickel on rocksalt substrates due to electron bombardment A number of authors (Hill’, Chopra 2, Stirland3) have found that electric fields or electrons present during deposition can affect the growth process of thin films. Unvala and Booker4 have reported very good auto-epitaxy of silicon using a special design of electron bombardment vapour source and they attribute at least part of this good epitaxy to the purity of silicon vapour and the high rate of deposition they used. It is the purpose of this note to report the observation of improved epitaxy of nickel films on rocksalt substrates due to bombardment by relatively high energy (~7 kV) electrons which are emitted together with the atomic beam from an electron bombardment evaporation source of the type described by Unvala and Grigson”. The results suggest that high-energy electron bombardment may have contributed significantly to the necessary conditions for the near-perfect epitaxy obtained by Unvala and Booker. Nickel was evaporated at a rate of 8 Ajsec on to (100) faces of rocksalt at a distance of 10 cm from the vapour source. By evaporating first on to the aircleaved surface and subsequently on to the vacuum-cleaved surface, direct comparisons could be made of the properties of deposits on these two types of surface at a given substrate temperature. In the case of the vacuum-cleaved surface, the time lapse between cleavage and the start of deposition was less than 2 sec. Evaporations were carried out at pressures in the range 5-15 x 10e6 torr. Electrical measurements revealed that in normal operation, an electron flux of about 5 x lOl4 electrons per cm2 per set of 7-kV electrons was intercepted by the substrate. By interposing a simple electrode system the 7-kV supply could be used to deflect the electron beam from its original path reducing the electron flux intercepted by the substrate to about 5 x 1012 electrons per cm2 per set of low-energy electrons (energy < 50 V), which are presumed to be secondaries generated when the deflected primary beam strikes other parts of the system. The deflector plate system therefore affords two modes of operation: one with, the other without, high-energy electron bombardment of the substrate during deposition. A series of evaporations was carried out in which nickel films, 970 A thick, were prepared at various substrate temperatures. At each temperature films were deposited on to air- and vacuum-cleaved surfaces, both with and without highenergy electron bombardment. After the films had been removed from their substrates, they were analysed by transmission electron microscopy and diffraction using the specimen tilt stage of the machine. As expected, the orientation of the deposited films in all cases tends to be (001) [lOO],i parallel to (001) [lOO],,c,. The characteristics of the Thin Solid
Films, 1 (1967) 235-239 - Elsevier, Amsterdam
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TABLE
1
C H A R A C T E R I S T I C S O F N I C K E L FILM,S D E P O S I T E D O N R O f ' K S A L T S U B S T R A T E S . S U B S T R A T E ( O N D I T I O N
Temperature (~C) 38O 3OO 2OO 100
Without high-energy electrotts Air-cleaved Vucttunl-eleaved
With high-energy electrons Air-cleaved Vacultnt-cleal'ed
4 3 3 6
2 2 3 6
n.c. n.c. n.c. n.c.
3 I 2 5
n.c. n.c. n.c. n.c.
c. c. c. c.
Orientation: 1 --
2 -3 = 4 -.. 5 -6 ~
(100) s p o t p a t t e r n . (100) s p o t p a t t e r n + t w i n n i n g spots. (100) s p o t p a t t e r n ÷ t w i n n i n g s p o t s + f a i n t p o l y c r y s t a l l i n e r i n g s : ring intensity ~ spot intensity. A s (3) b u t r i n g i n t e n s i t y < s p o t i n t e n s i t y . p o l y c r y s t a l l i n e r i n g p a t t e r n w i t h f a i n t (100) s p o t s . polycrystalline ring pattern.
Thin Solid Films, I (1967) 2 3 5 - 2 3 9
I I 2 5
c. c. c. c.
Continuity n.c. c.
~ not continuous continuous.
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Fig. 1. Electron diffraction patterns and micrographs showing effects of electron bombardment for a substrate temperature 300 °C. (a) Air-cleaved without high-energy electrons; (b) vacuumcleaved without high-energy electrons; (c) air-cleaved with high-energy electrons; (d)vacuumcleaved with high-energy electrons. (All micrographs x 15,000).
various films are summaried in Table I. For each film there are two entries dealing with orientation and continuity as deduced from the diffraction pattern and micrograph respectively. In Fig. 1, examples typical of the results for a substrate temperature of 300 °C are reproduced. For non-bombarded substrates it was found, as previously reported (Ino et al. 6) that substrate cleavage in vacuum improves the orientation of the deposit. The most interesting comparisons are those which show directly the effects of high-energy electron bombardment. Thus, for the air-cleaved deposits at 300 °C and 380 °C, electron bombardment improved the orientation significantly to give single-crystal spot patterns with no polycrystalline rings. The effect of electron bombardment on vacuum-cleaved samples was less pronounced since vacuumcleavage itself tends to improve the orientation to the point where the diffraction pattern becomes insensitive to further increases in perfection of the film. Thus for substrate temperatures 300 °C, 200 °C, 100 °C, the diffraction patterns of bornThin Solid Films, 1 (1967) 235-239
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Fig. 2. Kikuchi pattern typical o f electron b o m b a r d e d nickel film.
barded and non-bombarded deposits were comparable. For a substrate temperature of 380 °C, there was an improvement to give a single crystal in which twins were not detectable for the bombarded substrate. It is noteworthy that the diffraction patterns of electron bombarded samples frequently show highly developed Kikuchi patterns, indicative of a high degree of orientation perfection, the occurrence of which in vacuum-deposited nickel films is most unusual (BaltzV). A typical Kikuchi pattern is shown in Fig. 2. Examination of the micrographs of these films showed that all the electron bombarded films were continuous while the non-bombarded films, although at an advanced stage of coalesence, were not hole-free. These results indicate that high-energy electron bombardment of the substrate during deposition has effects comparable to vacuum-cleavage of the substrate in improving the orientation of the deposited film. The improved continuity Thin Solid Fihns, I (1967) 235-239
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is consistent with Stirland's observations of increased nucleation density although this point has not been specifically checked in these experiments. The reasons for the observed improvements in the orientation of the bombarded deposits are not clear. Any explanation in terms of electrostatic phenomena would have to take into account that the NaC1 substrate, which is almost completely surrounded by a conducting enclosure at earth potential, does not become charged under electron bombardment because the secondary electron yield is greater than unity for the primary electron energies involved i.e. ~ 7 kV. This may be deduced from data given by Hachenberg and Brauer a. The secondary electron yield for NaC1 becomes greater than unity for primary energies greater than about 35 V, and the absence of surface charge for incident energies greater than this value is demonstrated in the L.E.E.D. experiments of Marklund and Andersson 9 and confirmed in similar experiments by the authors. Direct heating of the substrate surface due to electron bombardment seems an unlikely possible reason for the observed changes in orientation because a generous theoretical estimate suggests that the temperature rise would not be greater than about l0 ° C - - a view that is substantiated by the observation that for Ts = 100 °C and Ts = 200 °C the structures of the bombarded and non-bombarded films are similar. Department o f Physics, University o f York, Heslington, York (Gt. Britain)
1 2 3 4 5 6 7 8 9
A. CHAMBERS M. PRUTTON
R.M. HILL, Nature, 210 (1966) 512. K.L. CHOPRA,J. Appl. Phys., 37 (1966) 2249. D.J. ST1RLAND,Appl. Phys. Letters, 8 (1966) 326. B.A. UNVALAAND G. R. BOOKER,Phil. Mag., 9 (1964) 691. B.A. UNVALAAND C. W. B. GRIGSON,Proc. 5th Electron Beam Symposium, Alloyd Corp., Boston, 1963, p. 168. S. INO, D. WATANABEANDS. OGAWA,J. Phys. Soc. Japan, 19 (1964) 881. A. BALTZ,3. Appl. Phys., 34 (1963) 1575. O. HACHENBERGANDW. BRAUER,Advan. Electron. Electron Phys., 11 (1959) 413. I. MARKLUNDANDS. ANDERSSON,Surface Sci., 5 (1966) 197.
Received May 16, 1967; revised June 19, 1967 Thin Solid Films, 1 (1967) 235-239
Films, 1 (1967) 235-239 - Elsevier, Amsterdam
- Printed in The Netherlands
236
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TABLE
1
C H A R A C T E R I S T I C S O F N I C K E L FILM,S D E P O S I T E D O N R O f ' K S A L T S U B S T R A T E S . S U B S T R A T E ( O N D I T I O N
Temperature (~C) 38O 3OO 2OO 100
Without high-energy electrotts Air-cleaved Vucttunl-eleaved
With high-energy electrons Air-cleaved Vacultnt-cleal'ed
4 3 3 6
2 2 3 6
n.c. n.c. n.c. n.c.
3 I 2 5
n.c. n.c. n.c. n.c.
c. c. c. c.
Orientation: 1 --
2 -3 = 4 -.. 5 -6 ~
(100) s p o t p a t t e r n . (100) s p o t p a t t e r n + t w i n n i n g spots. (100) s p o t p a t t e r n ÷ t w i n n i n g s p o t s + f a i n t p o l y c r y s t a l l i n e r i n g s : ring intensity ~ spot intensity. A s (3) b u t r i n g i n t e n s i t y < s p o t i n t e n s i t y . p o l y c r y s t a l l i n e r i n g p a t t e r n w i t h f a i n t (100) s p o t s . polycrystalline ring pattern.
Thin Solid Films, I (1967) 2 3 5 - 2 3 9
I I 2 5
c. c. c. c.
Continuity n.c. c.
~ not continuous continuous.
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Fig. 1. Electron diffraction patterns and micrographs showing effects of electron bombardment for a substrate temperature 300 °C. (a) Air-cleaved without high-energy electrons; (b) vacuumcleaved without high-energy electrons; (c) air-cleaved with high-energy electrons; (d)vacuumcleaved with high-energy electrons. (All micrographs x 15,000).
various films are summaried in Table I. For each film there are two entries dealing with orientation and continuity as deduced from the diffraction pattern and micrograph respectively. In Fig. 1, examples typical of the results for a substrate temperature of 300 °C are reproduced. For non-bombarded substrates it was found, as previously reported (Ino et al. 6) that substrate cleavage in vacuum improves the orientation of the deposit. The most interesting comparisons are those which show directly the effects of high-energy electron bombardment. Thus, for the air-cleaved deposits at 300 °C and 380 °C, electron bombardment improved the orientation significantly to give single-crystal spot patterns with no polycrystalline rings. The effect of electron bombardment on vacuum-cleaved samples was less pronounced since vacuumcleavage itself tends to improve the orientation to the point where the diffraction pattern becomes insensitive to further increases in perfection of the film. Thus for substrate temperatures 300 °C, 200 °C, 100 °C, the diffraction patterns of bornThin Solid Films, 1 (1967) 235-239
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Fig. 2. Kikuchi pattern typical o f electron b o m b a r d e d nickel film.
barded and non-bombarded deposits were comparable. For a substrate temperature of 380 °C, there was an improvement to give a single crystal in which twins were not detectable for the bombarded substrate. It is noteworthy that the diffraction patterns of electron bombarded samples frequently show highly developed Kikuchi patterns, indicative of a high degree of orientation perfection, the occurrence of which in vacuum-deposited nickel films is most unusual (BaltzV). A typical Kikuchi pattern is shown in Fig. 2. Examination of the micrographs of these films showed that all the electron bombarded films were continuous while the non-bombarded films, although at an advanced stage of coalesence, were not hole-free. These results indicate that high-energy electron bombardment of the substrate during deposition has effects comparable to vacuum-cleavage of the substrate in improving the orientation of the deposited film. The improved continuity Thin Solid Fihns, I (1967) 235-239
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239
is consistent with Stirland's observations of increased nucleation density although this point has not been specifically checked in these experiments. The reasons for the observed improvements in the orientation of the bombarded deposits are not clear. Any explanation in terms of electrostatic phenomena would have to take into account that the NaC1 substrate, which is almost completely surrounded by a conducting enclosure at earth potential, does not become charged under electron bombardment because the secondary electron yield is greater than unity for the primary electron energies involved i.e. ~ 7 kV. This may be deduced from data given by Hachenberg and Brauer a. The secondary electron yield for NaC1 becomes greater than unity for primary energies greater than about 35 V, and the absence of surface charge for incident energies greater than this value is demonstrated in the L.E.E.D. experiments of Marklund and Andersson 9 and confirmed in similar experiments by the authors. Direct heating of the substrate surface due to electron bombardment seems an unlikely possible reason for the observed changes in orientation because a generous theoretical estimate suggests that the temperature rise would not be greater than about l0 ° C - - a view that is substantiated by the observation that for Ts = 100 °C and Ts = 200 °C the structures of the bombarded and non-bombarded films are similar. Department o f Physics, University o f York, Heslington, York (Gt. Britain)
1 2 3 4 5 6 7 8 9
A. CHAMBERS M. PRUTTON
R.M. HILL, Nature, 210 (1966) 512. K.L. CHOPRA,J. Appl. Phys., 37 (1966) 2249. D.J. ST1RLAND,Appl. Phys. Letters, 8 (1966) 326. B.A. UNVALAAND G. R. BOOKER,Phil. Mag., 9 (1964) 691. B.A. UNVALAAND C. W. B. GRIGSON,Proc. 5th Electron Beam Symposium, Alloyd Corp., Boston, 1963, p. 168. S. INO, D. WATANABEANDS. OGAWA,J. Phys. Soc. Japan, 19 (1964) 881. A. BALTZ,3. Appl. Phys., 34 (1963) 1575. O. HACHENBERGANDW. BRAUER,Advan. Electron. Electron Phys., 11 (1959) 413. I. MARKLUNDANDS. ANDERSSON,Surface Sci., 5 (1966) 197.
Received May 16, 1967; revised June 19, 1967 Thin Solid Films, 1 (1967) 235-239