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Non-epitaxial and graphoepitaxial growth of tin thin films on an UHV-cleaved sodium chloride substrate
Journal of Crystal Growth 69 (1984) 149-154 North-Holland, A m s t e r d a m
149
NON-EPITAXIAL AND GRAPHOEPITAXIAL G R O W T H OF TIN T H I N FILMS ON AN UHV-CLEAVED S O D I U M C H L O R I D E S U B S T R A T E
T. OSAKA, Y. KASUKABE and H. N A K A M U R A Department of Metallurgical Engineering, Waseda University, Shinjuku-ku, Tokyo 160, Japan Received 28 June 1984; manuscript received in final form 28 August 1984
Thin films of tin were prepared by vacuum deposition on an UHV-cleaved sodium chloride substrate at room temperature in order to study the growth features. The small particles of tin, which were examined by high resolution T E M and electron diffraction, grew graphoepitaxially on steps, whereas those on flat areas were located and oriented almost randomly. The results, therefore, indicate that several works concerning the epitaxial growth of tin that has been reported so far have been performed on surfaces with a high density of steps parallel to the (100> direction of the substrate.
1. Introduction We have recently demonstrated that tin particles deposited on the flat areas of UHV-cleaved potassium chloride substrate surfaces were epitaxial with the (100) direction of tin parallel to the (100) direction of the potassium chloride, while tin particles on areas with a high density of steps were graphoepitaxially grown parallel to the direction of the step, even though the orientation of the step deviated from the potassium chloride crystallographic axes [1]. We have therefore pointed out that experiments related to epitaxy of metals on alkali halides have to be performed on the step-free, flat substrate surfaces, since the cleavage surfaces of alkali halide crystals have fine topographical structures composed of six characteristic features [1,2]. Extensive studies have been carried out on the growth of thin films of tin on a sodium chloride substrate by vacuum deposition [3-9]. Some workers reported that the (100) plane and the (100) direction of tin crystals are aligned parallel to the (100) plane and the [100] or [010] direction of sodium chloride substrates, respectively. In these reports, however, little if any attention has been paid to the fact that thin film growth of tin is strongly influenced by topographical features
which may be unrelated to the crystallographic structure of a sodium chloride crystal. The present paper describes for thin films of tin that epitaxy does not occur on the flat surfaces of UHV-cleaved sodium chloride substrates, though the correlation of the growth characteristics of tin with the surface structures of the substrate.
2. Experimental A detailed description of the experimental apparatus was presented in an earlier paper [1]. The pressures of the working chamber were less than 1 x 10 -1° and 3 x 10 -1° Torr before and during evaporation, respectively. Immediately after a constant evaporation rate of tin (99.9999%) was achieved, single crystal blocks of high purity sodium chloride (12 mm long, 5 mm wide, 1.2 mm thick) were cleaved. In addition, in order to survey more finely detailed substrate surfaces, as soon as tin had been deposited, a small quantity of A u - P d alloy was evaporated onto the as-deposited substrate. The angle of incidence of the A u - P d vapor was about 77 ° with respect to the substrate normal. The evaporation rate and deposit thickness were monitored with a quartz crystal microbalance. Rates of 2.5 × 10 aa a t o m s / c m 2. s (0.4
0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
150
T. Osaka et al. / Non-epitaxial and graphoepitaxial growth of Sn on NaCI
n m / m i n ) and 5.0 x 1012 a t o m s / c m 2. s (0.02 n m / m i n ) were maintained for the tin and the A u - P d depositions, respectively. The mean deposit thicknesses of tin and A u - P d were 7.5 and 0.5 nm, respectively. The substrates were always kept at room temperature during evaporation. For the preparation of T E M specimens, the as-deposited films were backed with an amorphous silicon monoxide film before admitting air, in order to fix the deposits on the substrate. The use of the SiO-backing film allows not only the pressure during the SiO evaporation to be held in the range of 10 -1° Tort but also prevents modifications of the fine structures of the substrate or migration of the deposits on it, when exposed to air. After removal from the deposition system, the films of tin were separated from the sodium chloride substrates, by floating on water, and mounted on T E M specimen grids. These films were examined with selected-area diffraction, bright-field TEM, and dark-field TEM.
3. Results
Fig. 1 shows a typical electron micrograph of tin deposits grown on the tartan zone [1] of an UHV-cleaved substrate. This zone is characterized by transverse and longitudinal steps which run parallel to the <100> directions of the sodium chloride substrate and flat areas (" step-free areas") in between these step lines [1,2]. Comparing the particles located at steps with those on the flat area, those on the step are smaller, more numerous, and are aligned parallel to the step lines. On the other hand, those of the flat area are observed to be almost randomly oriented. 3.1. Growth on the f l a t substrate surfaces
Fig. 2 shows an electron micrograph and a selected-area diffraction (SAD) pattern obtained from the particles of tin grown on the flat area,
Fig. 1. Typical room temperature tin deposits on the tartan zone of an UHV-cleaved sodium chloride substrate which includes flat areas and some surface steps.
T. Osaka et al. / Non- epitaxial and graphoepitaxial growth of Sn on NaCI
151
Fig. 2. Electron micrograph (a) and diffraction pattern (b) of tin particles deposited on the flat area of the tartan zone.
which were taken with the selected-area aperture commensurate with the electron micrograph. The SAD pattern illustrates that many of the individual tin particles are not epitaxial on the flat area of the sodium chloride substrate. 3.2. Growth on the steps
The electron micrograph and the SAD pattern shown in figs. 3a and 3b, which were taken from the particles grown on the high density longitudinal steps of a tartan zone, indicate that the particles were oriented almost completely parallel to the (100) direction of the substrate: (100)Sn Jl (001)NaC1 and [010]Sn rJ [100] or [010] NaCI: In addition, there is very weak additional orientation such as; (001)Sn Jl (001)NaC1 and [100]Sn Jl [100] or [010]NaC1, as shown in fig. 3b. Other weak diffraction spots which are indicated by A and B in fig. 3b stem from the twinning plane of (301): Fig. 4a shows a schematic representation and indexing of diffraction patterns obtained from the twinned crystal of tin as shown in fig. 4b. The
diffraction spots A and B in fig. 3b occur when an incident beam is parallel to the [100] and [010] direction of tin (fig. 4b), respectively. Figs. 1 and 3 evidence pronounced features in the growth rersults on steps. Most of the particles are grown with the elongation perpendicular to the step line, which coincides with the [001] direction of tin, as is apparent from the corresponding SAD pattern in fig. 3b. However, the mechanism for the [001] elongation of tin particles shown in fig. 1 is not understood at present. Fig. 5 shows bright- and dark-field images and a SAD pattern obtained from the tin particles grown on the transition zone that includes some steps inclined with respect to the crystallographic axes. The dark-field image taken with the 200 reflection, which is marked by an arrow in the figure, reveals that the corresponding array of the particles is at a 10 ° angle relative to the (100) direction of the substrate. In other words, tin particles were found to grow parallel to the step regardless of the crystallographic orientation of the step.
152
T. Osaka et aL / Non- epitaxial and graphoepitaxial growth of Sn on NaCI
Fig. 3. Electron micrograph (a) and diffraction pattern (b) of tin particles deposited on a stepped area.
4. Discussion T h e f i n d i n g s i n d i c a t e that the g r o w t h o f t h i n films of tin results i n a l m o s t r a n d o m p a r t i c l e dist r i b u t i o n i n the flat areas, w h e r e a s o n the o t h e r h a n d the g r a p h o e p i t a x i a l o r i e n t a t i o n takes p l a c e o n the surface steps regardless of the crystallo-
011 ,".:. ......
,~ ..................
y'
: ,
/
::
:,
•
....
\
,
,
/
t B-
7,,,
~ ~
~0~ ........
"
211 ,~
.....
: %
.
i - _ - xC . . . . . . .
'
o .................
C1
30~
~ : ,-, '>,'~.
\,,
g r a p h i c axes o n the s o d i u m c h l o r i d e substrates. As was m e n t i o n e d in the i n t r o d u c t i o n , in o u r earlier p a p e r [1], we h a v e r e p o r t e d the e p i t a x i a l a n d g r a p h o e p i t a x i a l g r o w t h of tin v a p o r - d e p o s i t e d o n t o a U H V - c l e a v e d p o t a s s i u m c h l o r i d e s u b s t r a t e at r o o m t e m p e r a t u r e a n d h a v e c o n c l u d e d t h a t the g r a p h o e p i t a x i a l effect at the s u r f a c e steps over-
2, ....
\ -,
,x
i ¢
~i:...............
'rZ
b
Fig. 4. (a) Schematic representation and indexing of diffraction patterns corresponding to the (100} (solid lines) and (101 } (dotted lines) orientations• A-type of diffraction spots (cross) stems from the double diffraction. B-type (open circle) can be obtained by a 180° rotation of the matrix points (filled circle) about the twin axis (301). (b) The twinned crystal of tin.
T. Osaka et al. / Non-epitaxial and graphoepitaxial growth of Sn on NaCI
153
Fig. 5. Bright- (a), dark-field (c) images, and diffraction pattern (b) taken from tin particles grown on a transition zone: The 200 spots marked by an arrow correspond to a 10 ° azimuthal angle relative to the (100) direction of the substrate.
Fig. 6. Additional fine decoration with A u - P d of the tin-decorated tartan zone.
154
T. Osaka et al. / Non -epitaxial and graphoepitaxial growth of Sn on NaCI
powers epitaxial effect on the flat surfaces for this system. The difference in the growth features of tin of the step-free surfaces in the sodium chloride and potassium chloride substrates might be due to the differences in the geometric misfit between the deposit and the substrate: For Sn/NaC1, when the [100] and [001] directions of tin are parallel to the [100] and [010] directions of a sodium chloride, respectively, the corresponding misfits are 3.5% and 13.6%. On the other hand, for Sn/KC1 the geometric misfits with respect to the identical directions as mentioned above are 7.3% and -1.1%. The misfit for Sn/NaC1 is definitely large when compared to S n / K C I . Therefore, non-epitaxy of the tin particles grown on the step-free surfaces in the sodium chloride substrate is believed to be caused by this larger misfit. Judging from the micrograph of fig. 1, some of the tin particles in the step-free areas seem to be parallel to the (100) direction of the substrate. According to Krohn et al. [10], tin is rather poorer than gold, palladium, and platinum with respect to resolution and contrast for decoration of the surface steps on a sodium chloride crystal. The A u - P d that was evaporated immediately following the completion of the tin deposition manifested itself in an additional finer decoration, making the original external shapes of tin particles slightly modified, as shown in fig. 6. By this fine decoration, it was confirmed that some transverse slip lines and a cross-slip zone existed in the flat area, which had not been detected by tin, and that the tin particles were aligned to the (100) direction o~ the substrate because of the presence of such fine steps in the range of 1 to 3 nm in height, which was measured from the A u - P d shadowing angle,
whereas on the flat area (right hand side of fig. 6) within the resolution by the A u - P d decoration no particles were oriented parallel to the substrate axes. It seems reasonable to conclude, therefore, that the studies of nucleation, epitaxy, and film growth phenomena on UHV-cleaved alkali halide crystals (as well as on crystals treated in other ways, such as the air-cleaved [1]) have to be properly performed on the flat surfaces in the tartan zone.
Acknowledgements The authors are indebted to Messrs. K. Fuwa and M. Yata for critical reading of this manuscript and for helpful discussions throughout this work. This work was partially supported by a Grantin-Aid from the Ministry of Education.
References [1] T. Osaka, T. Kawana, T. Nojima and K. Heinemann, J. Crystal Growth 61 (1983) 509. [2] L.S. de Wainer and G.A. Bassett, Phil. Mag. 38 (1978) 707. [3] R.W. Vook, J. Appl. Phys. 32 (1961) 1557. [4] R.W. Vook, J. Appl. Phys. 32 (1961) 2474. [5] A.E. Curzon, Cryogenics2 (1962) 334. [6] G. Shimaokaand G. Komoriya, J. Vacuum Sci. Technol. 7 (1969) 178. [7] H.P. Singh and L.E. Murr, Phil. Mag. 26 (1972) 649. [8} L.E. Murr, Thin Solid Films 20 (1974) 81. [9] G.L. Molhota, S.K. Sharma, S. Choudhuri and A.K. Pal, J. Phys. Soc. Japan 45 (1978) 930. [10] M. Krohn, G. Gerth and H. Stenzel, Phys. Status Solidi (a) 55 (1979) 375. "~
149
NON-EPITAXIAL AND GRAPHOEPITAXIAL G R O W T H OF TIN T H I N FILMS ON AN UHV-CLEAVED S O D I U M C H L O R I D E S U B S T R A T E
T. OSAKA, Y. KASUKABE and H. N A K A M U R A Department of Metallurgical Engineering, Waseda University, Shinjuku-ku, Tokyo 160, Japan Received 28 June 1984; manuscript received in final form 28 August 1984
Thin films of tin were prepared by vacuum deposition on an UHV-cleaved sodium chloride substrate at room temperature in order to study the growth features. The small particles of tin, which were examined by high resolution T E M and electron diffraction, grew graphoepitaxially on steps, whereas those on flat areas were located and oriented almost randomly. The results, therefore, indicate that several works concerning the epitaxial growth of tin that has been reported so far have been performed on surfaces with a high density of steps parallel to the (100> direction of the substrate.
1. Introduction We have recently demonstrated that tin particles deposited on the flat areas of UHV-cleaved potassium chloride substrate surfaces were epitaxial with the (100) direction of tin parallel to the (100) direction of the potassium chloride, while tin particles on areas with a high density of steps were graphoepitaxially grown parallel to the direction of the step, even though the orientation of the step deviated from the potassium chloride crystallographic axes [1]. We have therefore pointed out that experiments related to epitaxy of metals on alkali halides have to be performed on the step-free, flat substrate surfaces, since the cleavage surfaces of alkali halide crystals have fine topographical structures composed of six characteristic features [1,2]. Extensive studies have been carried out on the growth of thin films of tin on a sodium chloride substrate by vacuum deposition [3-9]. Some workers reported that the (100) plane and the (100) direction of tin crystals are aligned parallel to the (100) plane and the [100] or [010] direction of sodium chloride substrates, respectively. In these reports, however, little if any attention has been paid to the fact that thin film growth of tin is strongly influenced by topographical features
which may be unrelated to the crystallographic structure of a sodium chloride crystal. The present paper describes for thin films of tin that epitaxy does not occur on the flat surfaces of UHV-cleaved sodium chloride substrates, though the correlation of the growth characteristics of tin with the surface structures of the substrate.
2. Experimental A detailed description of the experimental apparatus was presented in an earlier paper [1]. The pressures of the working chamber were less than 1 x 10 -1° and 3 x 10 -1° Torr before and during evaporation, respectively. Immediately after a constant evaporation rate of tin (99.9999%) was achieved, single crystal blocks of high purity sodium chloride (12 mm long, 5 mm wide, 1.2 mm thick) were cleaved. In addition, in order to survey more finely detailed substrate surfaces, as soon as tin had been deposited, a small quantity of A u - P d alloy was evaporated onto the as-deposited substrate. The angle of incidence of the A u - P d vapor was about 77 ° with respect to the substrate normal. The evaporation rate and deposit thickness were monitored with a quartz crystal microbalance. Rates of 2.5 × 10 aa a t o m s / c m 2. s (0.4
0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
150
T. Osaka et al. / Non-epitaxial and graphoepitaxial growth of Sn on NaCI
n m / m i n ) and 5.0 x 1012 a t o m s / c m 2. s (0.02 n m / m i n ) were maintained for the tin and the A u - P d depositions, respectively. The mean deposit thicknesses of tin and A u - P d were 7.5 and 0.5 nm, respectively. The substrates were always kept at room temperature during evaporation. For the preparation of T E M specimens, the as-deposited films were backed with an amorphous silicon monoxide film before admitting air, in order to fix the deposits on the substrate. The use of the SiO-backing film allows not only the pressure during the SiO evaporation to be held in the range of 10 -1° Tort but also prevents modifications of the fine structures of the substrate or migration of the deposits on it, when exposed to air. After removal from the deposition system, the films of tin were separated from the sodium chloride substrates, by floating on water, and mounted on T E M specimen grids. These films were examined with selected-area diffraction, bright-field TEM, and dark-field TEM.
3. Results
Fig. 1 shows a typical electron micrograph of tin deposits grown on the tartan zone [1] of an UHV-cleaved substrate. This zone is characterized by transverse and longitudinal steps which run parallel to the <100> directions of the sodium chloride substrate and flat areas (" step-free areas") in between these step lines [1,2]. Comparing the particles located at steps with those on the flat area, those on the step are smaller, more numerous, and are aligned parallel to the step lines. On the other hand, those of the flat area are observed to be almost randomly oriented. 3.1. Growth on the f l a t substrate surfaces
Fig. 2 shows an electron micrograph and a selected-area diffraction (SAD) pattern obtained from the particles of tin grown on the flat area,
Fig. 1. Typical room temperature tin deposits on the tartan zone of an UHV-cleaved sodium chloride substrate which includes flat areas and some surface steps.
T. Osaka et al. / Non- epitaxial and graphoepitaxial growth of Sn on NaCI
151
Fig. 2. Electron micrograph (a) and diffraction pattern (b) of tin particles deposited on the flat area of the tartan zone.
which were taken with the selected-area aperture commensurate with the electron micrograph. The SAD pattern illustrates that many of the individual tin particles are not epitaxial on the flat area of the sodium chloride substrate. 3.2. Growth on the steps
The electron micrograph and the SAD pattern shown in figs. 3a and 3b, which were taken from the particles grown on the high density longitudinal steps of a tartan zone, indicate that the particles were oriented almost completely parallel to the (100) direction of the substrate: (100)Sn Jl (001)NaC1 and [010]Sn rJ [100] or [010] NaCI: In addition, there is very weak additional orientation such as; (001)Sn Jl (001)NaC1 and [100]Sn Jl [100] or [010]NaC1, as shown in fig. 3b. Other weak diffraction spots which are indicated by A and B in fig. 3b stem from the twinning plane of (301): Fig. 4a shows a schematic representation and indexing of diffraction patterns obtained from the twinned crystal of tin as shown in fig. 4b. The
diffraction spots A and B in fig. 3b occur when an incident beam is parallel to the [100] and [010] direction of tin (fig. 4b), respectively. Figs. 1 and 3 evidence pronounced features in the growth rersults on steps. Most of the particles are grown with the elongation perpendicular to the step line, which coincides with the [001] direction of tin, as is apparent from the corresponding SAD pattern in fig. 3b. However, the mechanism for the [001] elongation of tin particles shown in fig. 1 is not understood at present. Fig. 5 shows bright- and dark-field images and a SAD pattern obtained from the tin particles grown on the transition zone that includes some steps inclined with respect to the crystallographic axes. The dark-field image taken with the 200 reflection, which is marked by an arrow in the figure, reveals that the corresponding array of the particles is at a 10 ° angle relative to the (100) direction of the substrate. In other words, tin particles were found to grow parallel to the step regardless of the crystallographic orientation of the step.
152
T. Osaka et aL / Non- epitaxial and graphoepitaxial growth of Sn on NaCI
Fig. 3. Electron micrograph (a) and diffraction pattern (b) of tin particles deposited on a stepped area.
4. Discussion T h e f i n d i n g s i n d i c a t e that the g r o w t h o f t h i n films of tin results i n a l m o s t r a n d o m p a r t i c l e dist r i b u t i o n i n the flat areas, w h e r e a s o n the o t h e r h a n d the g r a p h o e p i t a x i a l o r i e n t a t i o n takes p l a c e o n the surface steps regardless of the crystallo-
011 ,".:. ......
,~ ..................
y'
: ,
/
::
:,
•
....
\
,
,
/
t B-
7,,,
~ ~
~0~ ........
"
211 ,~
.....
: %
.
i - _ - xC . . . . . . .
'
o .................
C1
30~
~ : ,-, '>,'~.
\,,
g r a p h i c axes o n the s o d i u m c h l o r i d e substrates. As was m e n t i o n e d in the i n t r o d u c t i o n , in o u r earlier p a p e r [1], we h a v e r e p o r t e d the e p i t a x i a l a n d g r a p h o e p i t a x i a l g r o w t h of tin v a p o r - d e p o s i t e d o n t o a U H V - c l e a v e d p o t a s s i u m c h l o r i d e s u b s t r a t e at r o o m t e m p e r a t u r e a n d h a v e c o n c l u d e d t h a t the g r a p h o e p i t a x i a l effect at the s u r f a c e steps over-
2, ....
\ -,
,x
i ¢
~i:...............
'rZ
b
Fig. 4. (a) Schematic representation and indexing of diffraction patterns corresponding to the (100} (solid lines) and (101 } (dotted lines) orientations• A-type of diffraction spots (cross) stems from the double diffraction. B-type (open circle) can be obtained by a 180° rotation of the matrix points (filled circle) about the twin axis (301). (b) The twinned crystal of tin.
T. Osaka et al. / Non-epitaxial and graphoepitaxial growth of Sn on NaCI
153
Fig. 5. Bright- (a), dark-field (c) images, and diffraction pattern (b) taken from tin particles grown on a transition zone: The 200 spots marked by an arrow correspond to a 10 ° azimuthal angle relative to the (100) direction of the substrate.
Fig. 6. Additional fine decoration with A u - P d of the tin-decorated tartan zone.
154
T. Osaka et al. / Non -epitaxial and graphoepitaxial growth of Sn on NaCI
powers epitaxial effect on the flat surfaces for this system. The difference in the growth features of tin of the step-free surfaces in the sodium chloride and potassium chloride substrates might be due to the differences in the geometric misfit between the deposit and the substrate: For Sn/NaC1, when the [100] and [001] directions of tin are parallel to the [100] and [010] directions of a sodium chloride, respectively, the corresponding misfits are 3.5% and 13.6%. On the other hand, for Sn/KC1 the geometric misfits with respect to the identical directions as mentioned above are 7.3% and -1.1%. The misfit for Sn/NaC1 is definitely large when compared to S n / K C I . Therefore, non-epitaxy of the tin particles grown on the step-free surfaces in the sodium chloride substrate is believed to be caused by this larger misfit. Judging from the micrograph of fig. 1, some of the tin particles in the step-free areas seem to be parallel to the (100) direction of the substrate. According to Krohn et al. [10], tin is rather poorer than gold, palladium, and platinum with respect to resolution and contrast for decoration of the surface steps on a sodium chloride crystal. The A u - P d that was evaporated immediately following the completion of the tin deposition manifested itself in an additional finer decoration, making the original external shapes of tin particles slightly modified, as shown in fig. 6. By this fine decoration, it was confirmed that some transverse slip lines and a cross-slip zone existed in the flat area, which had not been detected by tin, and that the tin particles were aligned to the (100) direction o~ the substrate because of the presence of such fine steps in the range of 1 to 3 nm in height, which was measured from the A u - P d shadowing angle,
whereas on the flat area (right hand side of fig. 6) within the resolution by the A u - P d decoration no particles were oriented parallel to the substrate axes. It seems reasonable to conclude, therefore, that the studies of nucleation, epitaxy, and film growth phenomena on UHV-cleaved alkali halide crystals (as well as on crystals treated in other ways, such as the air-cleaved [1]) have to be properly performed on the flat surfaces in the tartan zone.
Acknowledgements The authors are indebted to Messrs. K. Fuwa and M. Yata for critical reading of this manuscript and for helpful discussions throughout this work. This work was partially supported by a Grantin-Aid from the Ministry of Education.
References [1] T. Osaka, T. Kawana, T. Nojima and K. Heinemann, J. Crystal Growth 61 (1983) 509. [2] L.S. de Wainer and G.A. Bassett, Phil. Mag. 38 (1978) 707. [3] R.W. Vook, J. Appl. Phys. 32 (1961) 1557. [4] R.W. Vook, J. Appl. Phys. 32 (1961) 2474. [5] A.E. Curzon, Cryogenics2 (1962) 334. [6] G. Shimaokaand G. Komoriya, J. Vacuum Sci. Technol. 7 (1969) 178. [7] H.P. Singh and L.E. Murr, Phil. Mag. 26 (1972) 649. [8} L.E. Murr, Thin Solid Films 20 (1974) 81. [9] G.L. Molhota, S.K. Sharma, S. Choudhuri and A.K. Pal, J. Phys. Soc. Japan 45 (1978) 930. [10] M. Krohn, G. Gerth and H. Stenzel, Phys. Status Solidi (a) 55 (1979) 375. "~