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Structure and growth morphology of SiOx films deposited on NaCl (001) faces
Vacuum/volume 37/numbers 1/2/pages 97 to 99/1987
O042-207X/87S3.00+.00 Pergamon Journals Ltd
Printed in Great Britain
Structure and growth morphology of SiOx films deposited on NaCI (001) faces M El H i t i , P B Barna and F M R e i e h a ' , Research Institute for Technical Physics of the Hungarian Academy of Sciences, PO Box 76, H- 1325 Budapest, Hungary
The very early stages of SiO x film formation were studied on NaCI (100) surfaces by evaporating SiO. The selective nucleation of SiO x on the surface defects resulted in decoration patterns by developing threedimensional SiO x grains. The experiments were extended to get information both on the morphological peculiarities of SiO x grains and on the activated condensation of Au on a NaCI surface covered by a thin SiO X film.
Introduction
The processes of polycrystalline thin film formation are generally considered to start by nucleation and growth of individual grains 1. However the possibility of nucleation is neglected in some cases when discussing the atomic processes building the amorphous structures 2. Experiments carried out on a-Ge, a-Sb and a-Se films3'4 unambiguously proved that individual two- or three-dimensional grains can still develop when an amorphous structure forms upon vapour condensation. Consequently we have to consider the migration of the impinging species on the substrate and the self surface or edge mobility of adatoms when studying the formation of amorphous thin films too. On the other hand, the existence of the nucleation of these films is a direct indication that the impinging vapour species are migrating on the substrate, while the development of three-dimensional grains strongly suggests that the self-surface mobility of adatoms should have some real values. The formation of SiOx films on a NaC1 (100) surface was studied in the present experiments to collect additional information both on the nucleation experiments and the growth of SiOx islands. The study of surface defect structure of SiO x grains was possible by depositing Au on their surface and developing a secondary decoration pattern. Experimental
Air-cleaved NaCi (I00) surfaces were used as substrates, heated using Krohn's method 5. The substrate temperature was measured by a Ni-NiCr thermocouple embedded in the crystal. A Ta boat was used to evaporate SiO, and a hairpin-type W wire source to evaporate gold respectively. The evaporation rate and the thickness of the films were controlled by a quartz crystal microbalance (QCM). The pressure during film deposition was
* Mansoura University, Faculty of Sciences, Physics Department, Mansoura, Egypt.
5 • 10=* Pa. The SiOx-Au layer system covered by a 5-10 nm thick C film was floated off from the NaCI crystal in distilled water and supported on a microgrid. Both phase contrast 6 and dark field techniques in the TEM proved to be effective methods to study the SiO x films and to reveal their peculiarities. The scattered beam near to the central spot was used for taking dark field images (Figure l(b)).
Results Selective condensation o f S i O x on the surface defects of NaCI (100).
The contrast differences observed in TEM images of the amorphous SiO x films (Figures l(a) and (b)) are due to the differences in their mass thickness. The structural details appearing as dark spots and lines in the bright field and as bright ones in the dark field correspond to the well-known decoration patterns developing upon gold deposition 7 (Figure 2). In this experiment half of the NaC1 surface was shadowed during Au deposition while the whole surface was exposed to the SiO x vapour beam. In this way the area on the left-hand side was decorated only by SiO x while that on the right-hand side by Au at first and also by SiO x. This figure clearly shows that the same sites are preferential condensation sites both for Au and SiO x promoting the formation of three-dimensional grains also in the case of SiOx film deposition (Figure 3). Direct measurements of Barna et al 8 have determined thickness differences of about + 2 nm between the top of the hills and the fiat areas within similar films of 7 nm average thickness. The details of the morphology of amorphous SiO x grains are clearly seen in Figure 4. These exhibit 'dendrite-like' morphology analogous to the two-dimensional islands found in a-Ge films according to a model based on a computer simulation9. The development of these morphologies can be described by the limited self-edge mobility of adatoms. Activated condensation of gold on the SiO x grains formed on a NaCI surface. A thin SiO x film (5 nm nominal thickness measured
97
M El Hiti, P B Barna and FM Reicha. Structure and growth morphology of SiO~ films
(a)
Figure 3. Pt shadowed TEM image of a SiO~ film deposited on NaC1 (1130} at 250°C. Deposition rate 0.1 n m s 1 thickness 7 nm (with the permission of Dr ,& Barna8).
(b) Figure l. Bright (a) and dark (b) field images ofa SiO x film on air cleaved NaCI(100). Deposition parameters: substrate temperature 7~-250~C. deposition rate 0.005 nm s- ', thickness 7 nm.
Figure 2. Decoration pattern of a NaCI (100) surface: left-hand side developed for SiO~ deposition, right-hand side developed for Au and a successive SiO x deposition. Deposition parameters: Ts=250L'C; SiOx deposition rate 0.1 n m s -~, thickness 10nm: Au deposition rate 0.05 nm s 1 thickness 2 nm.
by a Q C M ) , deposited at 0.005 n m s - , at 250°C, develops grains on the defect sites of the NaCI surface as s h o w n before. The SiO x grains a n d hills developing on the NaCI surface activate the c o n d e n s a t i o n of gold resulting in secondary decoration p a t t e r n s (Figure 5). G r o u p s of a few gold crystals (mostly dimers or trimers) 98
Figure 4. TEM image ofSiO x grains on NaC1 (100), T~= 250 'C, deposition rate0.1 n m s i thickness7nm.
are to be seen o n the SiO x grains developed on the defects while the hills on the steps of the NaCI surface are decorated by b r o a d e r lines of gold crystals. The [-100] o r i e n t a t i o n of the Au crystals is p r o n o u n c e d , however a large n u m b e r of crystals are r a n d o m l y oriented. The [100] oriented gold crystals are situated at the edge of the SiO~ islands. The selective c o n d e n s a t i o n of Au on the SiO x films takes place also when a thicker a n d c o n t i n u o u s SiO x film was deposited: a 7.5 n m thick film was deposited first at 0.005 n m s - ~ a n d then a 5 n m thick film at 0.1 nm s - 1 (Figure 6). In the areas between these hills no nucleation can be found. This indicates that the SiO x film exhibits n u m e r o u s defects on the hills while the SiO~ film developed on the defect-free area of the NaC1 surface might have a more ordered structure. These SiO x film d o m a i n s do not contain active surface sites for Au nucleation. Selected area diffraction (SAD) proves the development of a r a t h e r strong [100] orientation of Au crystals also in that case.
Conclusions The nucleation taking place at the very early stages of SiO x deposition proves that the impinging v a p o u r species are mobile on the NaCI crystal surface at higher temperatures. The SiO x
M El Hiti, P B Barna and F M Reicha: Structure and growth morphology of SiO x films
Figure 5. Activated condensation of Au by the grains of a 5 nm thick SiO x
(a)
film deposited on NaCI (100) at 250°C and 0.005 nm s- 1, thickness of Au film is 1 nm, deposition rate 0.01 nm s- 1.
nuclei are preferentially formed at the steps a n d point defects of a NaCI surface in a similar way to t h a t for the c o n d e n s a t i o n of Au. Both the bright a n d dark field T~EM studies of these films revealed the presence of three-dimensional SiO x grains grown from the nuclei. The grains have a dendrite-like m o r p h o l o g y indicating a limited self-edge mobility of SiOx species. The SiO x grains developing o n the NaC1 surface activate the c o n d e n s a t i o n of Au a n d p r o m o t e the [100] epitaxial o r i e n t a t i o n of the Au crystals. These experiments a n d the Au c o n d e n s a t i o n on thicker SiO x films carried out at 250°C indicate the existence of n u m e r o u s defects o n the surface of SiO x grains developing on the defects of a NaCI surface. However the surface of SiO x film d o m a i n s growing on the defect-free areas of NaC1 seem to be free from active sites for Au nucleation, i.e. one can expect a more complete a m o r p h o u s structure in these d o m a i n s a p p r o a c h i n g the ideal a m o r p h o u s network structure 1°. Consequently different kinds of structures can exist in v a p o u r deposited SiO x films depending on the local structure of the substrate surface.
References 1 K L Chopra, Thin Film Phenomena, McGraw-Hill, NY (1969). 2 A G Dirks and H J Leamy, Thin Solid Films, 47, 219 (1977). 3 K L Chopra, A C Rastogi and D K Pandya, Phil Mag, 30, 935 (1974). 4 ,~ Barna, I Nagy, G Radn6czi, P Thomas and L T6th, Proc Amorphous Semiconductors (Edited by I K6sa-Somogyi), p 449, Akad6miai Kiad6, Budapest (1976).
(b) Figure 6. Secondary Au decoration of a 12.5 nm thick SiO~ film deposited at 250°C on NaC1 (100) (a) bright field image, (b) dark field image taken by the scattered beam near to the central spot.
s M Krohn and ,A Barna, Proc Second Colloquium on Thin Films (Edited by E Hahn), p 45, Akad6miai Kiad6, Budapest (1968). 6 F Thon, ZeitschrfNaturforschun#, 21A, 476 (1966). 7 G A Bassett, Phil Ma#, 3, 1042 (1958). 8 ,~ Barna, P B Barna, J F P6cza and I Pozsgai, Acta phys Hung, 33, 399 (1973). 9 ,~ Barna, P B Barna, G Radn6czi, H Sugawara and P Thomas, Thin Solid Films, 48, 136 (1978). lo De Polk, J Non-Cryst Solids, 5, 365 (1971).
99
O042-207X/87S3.00+.00 Pergamon Journals Ltd
Printed in Great Britain
Structure and growth morphology of SiOx films deposited on NaCI (001) faces M El H i t i , P B Barna and F M R e i e h a ' , Research Institute for Technical Physics of the Hungarian Academy of Sciences, PO Box 76, H- 1325 Budapest, Hungary
The very early stages of SiO x film formation were studied on NaCI (100) surfaces by evaporating SiO. The selective nucleation of SiO x on the surface defects resulted in decoration patterns by developing threedimensional SiO x grains. The experiments were extended to get information both on the morphological peculiarities of SiO x grains and on the activated condensation of Au on a NaCI surface covered by a thin SiO X film.
Introduction
The processes of polycrystalline thin film formation are generally considered to start by nucleation and growth of individual grains 1. However the possibility of nucleation is neglected in some cases when discussing the atomic processes building the amorphous structures 2. Experiments carried out on a-Ge, a-Sb and a-Se films3'4 unambiguously proved that individual two- or three-dimensional grains can still develop when an amorphous structure forms upon vapour condensation. Consequently we have to consider the migration of the impinging species on the substrate and the self surface or edge mobility of adatoms when studying the formation of amorphous thin films too. On the other hand, the existence of the nucleation of these films is a direct indication that the impinging vapour species are migrating on the substrate, while the development of three-dimensional grains strongly suggests that the self-surface mobility of adatoms should have some real values. The formation of SiOx films on a NaC1 (100) surface was studied in the present experiments to collect additional information both on the nucleation experiments and the growth of SiOx islands. The study of surface defect structure of SiO x grains was possible by depositing Au on their surface and developing a secondary decoration pattern. Experimental
Air-cleaved NaCi (I00) surfaces were used as substrates, heated using Krohn's method 5. The substrate temperature was measured by a Ni-NiCr thermocouple embedded in the crystal. A Ta boat was used to evaporate SiO, and a hairpin-type W wire source to evaporate gold respectively. The evaporation rate and the thickness of the films were controlled by a quartz crystal microbalance (QCM). The pressure during film deposition was
* Mansoura University, Faculty of Sciences, Physics Department, Mansoura, Egypt.
5 • 10=* Pa. The SiOx-Au layer system covered by a 5-10 nm thick C film was floated off from the NaCI crystal in distilled water and supported on a microgrid. Both phase contrast 6 and dark field techniques in the TEM proved to be effective methods to study the SiO x films and to reveal their peculiarities. The scattered beam near to the central spot was used for taking dark field images (Figure l(b)).
Results Selective condensation o f S i O x on the surface defects of NaCI (100).
The contrast differences observed in TEM images of the amorphous SiO x films (Figures l(a) and (b)) are due to the differences in their mass thickness. The structural details appearing as dark spots and lines in the bright field and as bright ones in the dark field correspond to the well-known decoration patterns developing upon gold deposition 7 (Figure 2). In this experiment half of the NaC1 surface was shadowed during Au deposition while the whole surface was exposed to the SiO x vapour beam. In this way the area on the left-hand side was decorated only by SiO x while that on the right-hand side by Au at first and also by SiO x. This figure clearly shows that the same sites are preferential condensation sites both for Au and SiO x promoting the formation of three-dimensional grains also in the case of SiOx film deposition (Figure 3). Direct measurements of Barna et al 8 have determined thickness differences of about + 2 nm between the top of the hills and the fiat areas within similar films of 7 nm average thickness. The details of the morphology of amorphous SiO x grains are clearly seen in Figure 4. These exhibit 'dendrite-like' morphology analogous to the two-dimensional islands found in a-Ge films according to a model based on a computer simulation9. The development of these morphologies can be described by the limited self-edge mobility of adatoms. Activated condensation of gold on the SiO x grains formed on a NaCI surface. A thin SiO x film (5 nm nominal thickness measured
97
M El Hiti, P B Barna and FM Reicha. Structure and growth morphology of SiO~ films
(a)
Figure 3. Pt shadowed TEM image of a SiO~ film deposited on NaC1 (1130} at 250°C. Deposition rate 0.1 n m s 1 thickness 7 nm (with the permission of Dr ,& Barna8).
(b) Figure l. Bright (a) and dark (b) field images ofa SiO x film on air cleaved NaCI(100). Deposition parameters: substrate temperature 7~-250~C. deposition rate 0.005 nm s- ', thickness 7 nm.
Figure 2. Decoration pattern of a NaCI (100) surface: left-hand side developed for SiO~ deposition, right-hand side developed for Au and a successive SiO x deposition. Deposition parameters: Ts=250L'C; SiOx deposition rate 0.1 n m s -~, thickness 10nm: Au deposition rate 0.05 nm s 1 thickness 2 nm.
by a Q C M ) , deposited at 0.005 n m s - , at 250°C, develops grains on the defect sites of the NaCI surface as s h o w n before. The SiO x grains a n d hills developing on the NaCI surface activate the c o n d e n s a t i o n of gold resulting in secondary decoration p a t t e r n s (Figure 5). G r o u p s of a few gold crystals (mostly dimers or trimers) 98
Figure 4. TEM image ofSiO x grains on NaC1 (100), T~= 250 'C, deposition rate0.1 n m s i thickness7nm.
are to be seen o n the SiO x grains developed on the defects while the hills on the steps of the NaCI surface are decorated by b r o a d e r lines of gold crystals. The [-100] o r i e n t a t i o n of the Au crystals is p r o n o u n c e d , however a large n u m b e r of crystals are r a n d o m l y oriented. The [100] oriented gold crystals are situated at the edge of the SiO~ islands. The selective c o n d e n s a t i o n of Au on the SiO x films takes place also when a thicker a n d c o n t i n u o u s SiO x film was deposited: a 7.5 n m thick film was deposited first at 0.005 n m s - ~ a n d then a 5 n m thick film at 0.1 nm s - 1 (Figure 6). In the areas between these hills no nucleation can be found. This indicates that the SiO x film exhibits n u m e r o u s defects on the hills while the SiO~ film developed on the defect-free area of the NaC1 surface might have a more ordered structure. These SiO x film d o m a i n s do not contain active surface sites for Au nucleation. Selected area diffraction (SAD) proves the development of a r a t h e r strong [100] orientation of Au crystals also in that case.
Conclusions The nucleation taking place at the very early stages of SiO x deposition proves that the impinging v a p o u r species are mobile on the NaCI crystal surface at higher temperatures. The SiO x
M El Hiti, P B Barna and F M Reicha: Structure and growth morphology of SiO x films
Figure 5. Activated condensation of Au by the grains of a 5 nm thick SiO x
(a)
film deposited on NaCI (100) at 250°C and 0.005 nm s- 1, thickness of Au film is 1 nm, deposition rate 0.01 nm s- 1.
nuclei are preferentially formed at the steps a n d point defects of a NaCI surface in a similar way to t h a t for the c o n d e n s a t i o n of Au. Both the bright a n d dark field T~EM studies of these films revealed the presence of three-dimensional SiO x grains grown from the nuclei. The grains have a dendrite-like m o r p h o l o g y indicating a limited self-edge mobility of SiOx species. The SiO x grains developing o n the NaC1 surface activate the c o n d e n s a t i o n of Au a n d p r o m o t e the [100] epitaxial o r i e n t a t i o n of the Au crystals. These experiments a n d the Au c o n d e n s a t i o n on thicker SiO x films carried out at 250°C indicate the existence of n u m e r o u s defects o n the surface of SiO x grains developing on the defects of a NaCI surface. However the surface of SiO x film d o m a i n s growing on the defect-free areas of NaC1 seem to be free from active sites for Au nucleation, i.e. one can expect a more complete a m o r p h o u s structure in these d o m a i n s a p p r o a c h i n g the ideal a m o r p h o u s network structure 1°. Consequently different kinds of structures can exist in v a p o u r deposited SiO x films depending on the local structure of the substrate surface.
References 1 K L Chopra, Thin Film Phenomena, McGraw-Hill, NY (1969). 2 A G Dirks and H J Leamy, Thin Solid Films, 47, 219 (1977). 3 K L Chopra, A C Rastogi and D K Pandya, Phil Mag, 30, 935 (1974). 4 ,~ Barna, I Nagy, G Radn6czi, P Thomas and L T6th, Proc Amorphous Semiconductors (Edited by I K6sa-Somogyi), p 449, Akad6miai Kiad6, Budapest (1976).
(b) Figure 6. Secondary Au decoration of a 12.5 nm thick SiO~ film deposited at 250°C on NaC1 (100) (a) bright field image, (b) dark field image taken by the scattered beam near to the central spot.
s M Krohn and ,A Barna, Proc Second Colloquium on Thin Films (Edited by E Hahn), p 45, Akad6miai Kiad6, Budapest (1968). 6 F Thon, ZeitschrfNaturforschun#, 21A, 476 (1966). 7 G A Bassett, Phil Ma#, 3, 1042 (1958). 8 ,~ Barna, P B Barna, J F P6cza and I Pozsgai, Acta phys Hung, 33, 399 (1973). 9 ,~ Barna, P B Barna, G Radn6czi, H Sugawara and P Thomas, Thin Solid Films, 48, 136 (1978). lo De Polk, J Non-Cryst Solids, 5, 365 (1971).
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