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Atomic force microscopy study of titanium dioxide thin films grown by atomic layer epitaxy
32
Thin Solid Films, 228 (1993) 32-35
Atomic force microscopy study of titanium dioxide thin films grown by atomic layer epitaxy Mikko
Ritala and Markku
Leskel~i
Department of Chemistry, University of Helsinki, SF-O0100 Helsinki (Finland)
Leena-Sisko Johansson Department of Applied Physics, University of Turku, SF-20500 Turku (Finland)
Lauri Niinist6 Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, SF-02150 Espoo (Finland)
Abstract Atomic force microscopy was used to study the morphological development of TiO 2 thin films deposited on mica and soda-lime substrates by atomic layer epitaxy using TiC14 and water as reactants. TiO 2 was observed to agglomerate, causing increasing surface roughening with increasing number of deposition cycles. The reasons for the agglomeration are discussed.
1. Introduction In our previous paper [1] we have reported on the growth o f TiO2 thin films with reproducible and uniform thickness using the atomic layer epitaxy (ALE) process [2], where TiCI4 and water were pulsed alternately on a heated substrate and the reactor was purged with nitrogen between the reactant pulses. According to the measured film properties, i.e. refractive index, density, etchability and hydrogen content in the bulk of the film, the films obtained were of high quality. However, some discrepancies with the ideal ALE growth were observed. According to the ideal layer-by-layer growth mechanism the growth rate per cycle should be independent of the number of reaction cycles. However, from optical film thickness measurements the growth rate was observed to decrease as the number o f reaction cycles was increased. The observed decrease can be related either to the decrease of the number of reactive sites or to the morphological changes taking place during the film growth. The latter explanation was supported by scanning electron microscopy micrographs which showed that films were not completely uniform but consisted of grains whose lateral diameters were in the order of 100 nm. Furthermore, the results of Lakomaa et al. [3], who have used a similar ALE process to deposit TiO2 on the surface of silica powder, have given strong evidence of agglomeration; even after one reaction cycle crystalline TiO2 was observed by X-ray diffraction. Another striking observation in our work was an interplay between the films on substrates located face to
0040-6090/93/$6.00
face at a distance of 2 mm. The growth rate was observed to be dependent, as well as on the material of the substrate itself, also on the material of the opposite substrate, suggesting that some kind of gas phase migration takes place during the film growth. In the present work the development of the surface morphology has been studied in more detail by means of atomic force microscopy (AFM). In order to evaluate the substrate coverage, supplementary chemical information was obtained with X-ray photoelectron spectroscopy (XPS).
2. Experimental details The deposition and characterization of the TiO 2 films has been described earlier [ 1]. Samples for A F M measurements were prepared by varying the number of reaction cycles between 0 and 4000 using 0.2 s reactant pulses and 0.5 s purging sequences. The growth rate determined from films grown using 2000 and 4000 cycles was 0.4/~ cycle -1. The A F M images were taken at least 30 mm from the leading edge of the substrate in order to avoid the profile region [ 1]. A major emphasis in the present study was given to films deposited on phlogopite mica because the cleavage planes of mica provide substrate surfaces which are atomically smooth over large areas. Also, films on soda-lime glass were studied because of the technological importance of this substrate material. Using these two substrate materials it was also possible to study the effects of the film
© 1993 -- Elsevier Sequoia. All rights reserved
M. Ritala et al. / A F M study o f A L E TiO:
crystallinity on the morphology since the films on sodalime are amorphous whereas on mica they are polycrystalline futile. The AFM measurements were carried out e x situ, i.e. the deposited films were cooled down and transferred in air to the microscope. The measurements were also performed in air. The atomic force microscope used was a Nanoscope II with both 0.7 and 12 ~tm scanners. Scan sizes varied from 10 to 10 000 nm. XPS measurements were carried out with a PHI Small Spot ESCA 5400 electron spectrometer. XPS wide scan
33
spectra were excited by non-monochromatized Mg K0t radiation and measured using an 89 eV analyser energy and a 0.5 eV channel width.
3. Results and discussion
Figure 1 depicts a series of AFM images taken from the phlogopite after the deposition of TiO2 films using various number of reaction cycles at 500 °C. Even
(a)
(d)
(b)
(e)
(c)
(0
Fig. I. AFM images taken (a) from pure phlogopite and after (b) 20, (c) 100, (d) 400, (e) 2000 and (f) 4000 TiO2 deposition cycles. Different z scales in the first two images should be noted.
M. Ritala et al. / AFM study of ALE Ti02
34
though the cleavage plane of the pure phlogopite contained some small steps it was smooth over large areas. At a higher magnification the atomic structure of phlogopite could be observed. This structure was lost already after four reaction cycles and the mica appeared to be coated with an amorphous film with no structural details. However, this did not necessarily mean that the substrate was totally covered because small particles can travel with the tip of the microscope, destroying the high resolution. Also, the thermal vibrations of the atoms in small clusters can be detrimental to the resolution. When the number of reaction cycles was increased to 20 small aggregates were observed. As the growth proceeded further these aggregates grew both vertically and horizontally. Initially the aggregate density increased (cf 100 and 400 reaction cycles) but when the neighbouring aggregates came into a contact they coalesced and the aggregate density decreased whereas the aggregate size increased (cf 400, 2000 and 4000 reaction cycles). The development of the roughness was further studied by means of the standard deviations of the z values which were obtained from 460 × 460 nm 2 image areas (Table 1). A clear trend of increasing roughness was observed. The standard deviation divided by the number of reaction cycles obviously showed that the rate of roughening decreased as the growth proceeded further. Because AFM does not give chemical information about the surface we used XPS to study the substrate coverage. The relative atomic ratio of mica cations (Si + A1 + K + Mg + Na) to titanium was 2.2 after 100 cycles but only 0.05 after 200 cycles. If a steady layerby-layer growth were assumed the corresponding film thicknesses would be 4-5 and 8-10rim respectively. Thus it can be concluded that the film growth took place by the island mode and the total substrate coverage was obtained between 100 and 200 cycles. The morphological development of the amorphous films on soda-lime was seen to be very similar to that of the polycrystalline films on phlogopite. Furthermore, T A B L E 1. Standard deviation o f z values taken from an area o f 460 x 460 n m 2 Number of reaction cycles
Standard deviation o f z (nm)
Standard deviation o f z divided by n u m b e r o f reaction cycles (nm)
0 4 20 100 400 2000 4000
0.25 0.50 0.66 5.0 6.3 6.9 19
-0.13 0.033 0.050 9.016 0.0035 0.0048
the morphologies of films grown at 300 °C on sodalime were comparable with those grown at 500 °C. There is no unambiguous explanation for the aggregation. It can be a result of either gas phase reactions or migration processes of the adsorbed species as well as their joint effect. As discussed in our earlier paper [ I] the chemistry behind the growth process can be very complicated. Ideally, the growth would proceed through alternate reactions of surface hydroxyls and chlorines with gaseous T i C I 4 and water: n(-OH)(s) + TiC14(g) ~ (-O-)n TiC!,_n(s) + nHCl(g)
(1) (-O-)n TiCla_,(s) + (4 - n)H20(g) (--O-)n Ti(OH)4_ n(s) + (4 - n)HCl(g)
(2)
where n = 1-3. Without any migration or gas phase nucleation processes the film would grow uniformly over the whole substrate. The high free energy associated with the interface between the film and the substrate can act as a driving force for the aggregation. However, in addition to the driving force, also an effective mechanism for material transportation is needed to achieve the observed level of aggregation. On the basis of the stability of the substrate materials at the studied temperatures it is reasonable to assume that the aggregation mechanism is comparable with the particle growth of unsupported TiO2 powder. Pijolat and coworkers have shown that both HC1 [4] and water [5] vapours increase the rate of surface area reduction of anatase powder at relatively low temperatures (417 and 550 °C) where no densification occurs. The accelerating effect of water was attributed to an enhanced surface migration of oxygen in the form of hydroxyl ions instead of oxide ions [5]. The effect of HCI was stronger than that of water and it was attributed to a formation of a volatile species Ti(OH)2C12 preceded by a dissociative adsorption of HC1 [6]. By means of this species the migration process is enhanced by converting it from the surface to the gas phase. The present study has been carried out at comparable temperatures and both of the mechanisms are possible. According to reaction (2) the water pulse transforms the surface into a hydroxyl-terminated surface and, consequently, the hydroxyl density is high enough to promote aggregation. In contrast, the byproduct of reactions (1) and (2) is HC1 which can interact with the surface, producing Ti(OH)2CI2. An alternative mechanism for the formation of Ti(OH)2CI2 is the interaction between the intermediate surface species (-O-)nTiC14_ n and (-O-)~Ti(OH)4_~, especially when n = 2. Also, a ligand exchange reaction of TIC14 can produce Ti(OH)2C12; 2(-OH)(s) + TiCla(g) --+2(-Cl)(s) + Ti(OH)2CI2(g ) (3)
M. Ritala et al. ] AFM study of A L E 7702
The narrow flow channel between the two substrates means that, once formed and desorbed, Ti(OH)2C12 will undergo a number of collisions with the substrates before leaving the reactor chamber [7]. Consequently, the probability for the readsorption of Ti(OH)2CI2 is high enough to cause agglomeration. Unfortunately, estimation of the rates of formation, desorption and adsorption of Ti(OH)2C12 are prevented by the lack of thermodynamic data. It should be noted that when the interface area is reduced by migration processes the film growth is likely to exhibit the island growth mode which is in accordance with our results. In addition, the formation of Ti(OH)2CI 2 can explain also the observed interplay between the substrates. In our previous paper we suggested that the dependence of the growth rate on the substrate material is caused by the number of reactive sites, i.e. hydroxyl groups, on the substrate surface [1]. Consequently, the effect of the opposite substrate can be understood in terms of the gas phase migration of hydroxyl groups from one substrate to another, leveling out the difference between them. An alternative explanation for the formation of aggregates is three-dimensional nucleation in the gas phase provided that there are, somehow, simultaneously both TiCI4 and water in the gas phase. It has been observed that a strong thickness profile is formed at the leading edge of the substrate as a consequence of desorption of one reactant from the walls upstream from the substrate during the pulse sequence of another reactant [1]. However, the desorbed reactant is rapidly exhausted and in the studied area films have uniform thicknesses, suggesting that the contribution of this mechanism cannot be remarkable. Furthermore, this process would lead to a film consisting of three-dimensional nuclei over uniformly distributed "real" ALE film and it would be thus contradictory with our results. Another origin for the gas phase nucleation might be the liberation of water during the TiC14 pulse as a result of the adsorption of HCI [8]: Ti-OH(s) + HCI(g)~-Ti(H20)-CI(s) ~Wi-Cl(s) + H20(g )
(4)
35
However, because of the higher TIC14 concentration relative to the HC1 concentration reaction (1) is likely to dominate over reaction (4). Consequently, nearly layer-by-layer growth should be achieved instead of the observed island growth.
4. Conclusions The AFM study shows that the surfaces of ALEgrown TiO2 films roughen as the growth proceeds. The roughening rate is at its highest during the first reaction cycles and, according to the XPS results, film growth takes place by the island mode. The roughening seems to be caused by migration processes driven by the minimization of the film-substrate interface energy because this mechanism leads to the observed island growth whereas gas phase nucleation processes would lead to Stranski-Krastanov growth. No remarkable differences in the morphology between crystalline and amorphous films were observed.
Acknowledgment The work was supported in part by the Academy of Finland and Technology Development Centre (TEKES), Helsinki, Finland.
References 1 M. Ritala, M. Leskel/i., E. NykS.nen, P. Soininen and L. Niinist6, Thin Solid Films, 225 (1993) 288. 2 T. Suntola and J. Hyviirinen, Annu. Rev. Mater. Sci., 15 (1985) 177. 3 E.-L. Lakomaa, S. Haukka and T. Suntola, Appl. Surf. Sci., 60/61 (1992) 742. 4 F. Gruy and M. Pijolat, J. Am. Ceram. Soc., 75(1992) 657. 5 J.-L. Hrbrard, P. Nortier, M. Pijolat and M. Soustelle, J. Am. Ceram. Soc., 73 (1990) 79. 6 F. Gruy and M. Pijolat, J. Am. Ceram. Sac., 75 (1992) 663. 7 T. Suntola, Thin Solid Films, 216(1992) 84. 8 G. D. Parfitt, J. Ramsbotham and C. H. Rochester, Trans. Faraday Soc., 67 (1971) 3100.
Thin Solid Films, 228 (1993) 32-35
Atomic force microscopy study of titanium dioxide thin films grown by atomic layer epitaxy Mikko
Ritala and Markku
Leskel~i
Department of Chemistry, University of Helsinki, SF-O0100 Helsinki (Finland)
Leena-Sisko Johansson Department of Applied Physics, University of Turku, SF-20500 Turku (Finland)
Lauri Niinist6 Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, SF-02150 Espoo (Finland)
Abstract Atomic force microscopy was used to study the morphological development of TiO 2 thin films deposited on mica and soda-lime substrates by atomic layer epitaxy using TiC14 and water as reactants. TiO 2 was observed to agglomerate, causing increasing surface roughening with increasing number of deposition cycles. The reasons for the agglomeration are discussed.
1. Introduction In our previous paper [1] we have reported on the growth o f TiO2 thin films with reproducible and uniform thickness using the atomic layer epitaxy (ALE) process [2], where TiCI4 and water were pulsed alternately on a heated substrate and the reactor was purged with nitrogen between the reactant pulses. According to the measured film properties, i.e. refractive index, density, etchability and hydrogen content in the bulk of the film, the films obtained were of high quality. However, some discrepancies with the ideal ALE growth were observed. According to the ideal layer-by-layer growth mechanism the growth rate per cycle should be independent of the number of reaction cycles. However, from optical film thickness measurements the growth rate was observed to decrease as the number o f reaction cycles was increased. The observed decrease can be related either to the decrease of the number of reactive sites or to the morphological changes taking place during the film growth. The latter explanation was supported by scanning electron microscopy micrographs which showed that films were not completely uniform but consisted of grains whose lateral diameters were in the order of 100 nm. Furthermore, the results of Lakomaa et al. [3], who have used a similar ALE process to deposit TiO2 on the surface of silica powder, have given strong evidence of agglomeration; even after one reaction cycle crystalline TiO2 was observed by X-ray diffraction. Another striking observation in our work was an interplay between the films on substrates located face to
0040-6090/93/$6.00
face at a distance of 2 mm. The growth rate was observed to be dependent, as well as on the material of the substrate itself, also on the material of the opposite substrate, suggesting that some kind of gas phase migration takes place during the film growth. In the present work the development of the surface morphology has been studied in more detail by means of atomic force microscopy (AFM). In order to evaluate the substrate coverage, supplementary chemical information was obtained with X-ray photoelectron spectroscopy (XPS).
2. Experimental details The deposition and characterization of the TiO 2 films has been described earlier [ 1]. Samples for A F M measurements were prepared by varying the number of reaction cycles between 0 and 4000 using 0.2 s reactant pulses and 0.5 s purging sequences. The growth rate determined from films grown using 2000 and 4000 cycles was 0.4/~ cycle -1. The A F M images were taken at least 30 mm from the leading edge of the substrate in order to avoid the profile region [ 1]. A major emphasis in the present study was given to films deposited on phlogopite mica because the cleavage planes of mica provide substrate surfaces which are atomically smooth over large areas. Also, films on soda-lime glass were studied because of the technological importance of this substrate material. Using these two substrate materials it was also possible to study the effects of the film
© 1993 -- Elsevier Sequoia. All rights reserved
M. Ritala et al. / A F M study o f A L E TiO:
crystallinity on the morphology since the films on sodalime are amorphous whereas on mica they are polycrystalline futile. The AFM measurements were carried out e x situ, i.e. the deposited films were cooled down and transferred in air to the microscope. The measurements were also performed in air. The atomic force microscope used was a Nanoscope II with both 0.7 and 12 ~tm scanners. Scan sizes varied from 10 to 10 000 nm. XPS measurements were carried out with a PHI Small Spot ESCA 5400 electron spectrometer. XPS wide scan
33
spectra were excited by non-monochromatized Mg K0t radiation and measured using an 89 eV analyser energy and a 0.5 eV channel width.
3. Results and discussion
Figure 1 depicts a series of AFM images taken from the phlogopite after the deposition of TiO2 films using various number of reaction cycles at 500 °C. Even
(a)
(d)
(b)
(e)
(c)
(0
Fig. I. AFM images taken (a) from pure phlogopite and after (b) 20, (c) 100, (d) 400, (e) 2000 and (f) 4000 TiO2 deposition cycles. Different z scales in the first two images should be noted.
M. Ritala et al. / AFM study of ALE Ti02
34
though the cleavage plane of the pure phlogopite contained some small steps it was smooth over large areas. At a higher magnification the atomic structure of phlogopite could be observed. This structure was lost already after four reaction cycles and the mica appeared to be coated with an amorphous film with no structural details. However, this did not necessarily mean that the substrate was totally covered because small particles can travel with the tip of the microscope, destroying the high resolution. Also, the thermal vibrations of the atoms in small clusters can be detrimental to the resolution. When the number of reaction cycles was increased to 20 small aggregates were observed. As the growth proceeded further these aggregates grew both vertically and horizontally. Initially the aggregate density increased (cf 100 and 400 reaction cycles) but when the neighbouring aggregates came into a contact they coalesced and the aggregate density decreased whereas the aggregate size increased (cf 400, 2000 and 4000 reaction cycles). The development of the roughness was further studied by means of the standard deviations of the z values which were obtained from 460 × 460 nm 2 image areas (Table 1). A clear trend of increasing roughness was observed. The standard deviation divided by the number of reaction cycles obviously showed that the rate of roughening decreased as the growth proceeded further. Because AFM does not give chemical information about the surface we used XPS to study the substrate coverage. The relative atomic ratio of mica cations (Si + A1 + K + Mg + Na) to titanium was 2.2 after 100 cycles but only 0.05 after 200 cycles. If a steady layerby-layer growth were assumed the corresponding film thicknesses would be 4-5 and 8-10rim respectively. Thus it can be concluded that the film growth took place by the island mode and the total substrate coverage was obtained between 100 and 200 cycles. The morphological development of the amorphous films on soda-lime was seen to be very similar to that of the polycrystalline films on phlogopite. Furthermore, T A B L E 1. Standard deviation o f z values taken from an area o f 460 x 460 n m 2 Number of reaction cycles
Standard deviation o f z (nm)
Standard deviation o f z divided by n u m b e r o f reaction cycles (nm)
0 4 20 100 400 2000 4000
0.25 0.50 0.66 5.0 6.3 6.9 19
-0.13 0.033 0.050 9.016 0.0035 0.0048
the morphologies of films grown at 300 °C on sodalime were comparable with those grown at 500 °C. There is no unambiguous explanation for the aggregation. It can be a result of either gas phase reactions or migration processes of the adsorbed species as well as their joint effect. As discussed in our earlier paper [ I] the chemistry behind the growth process can be very complicated. Ideally, the growth would proceed through alternate reactions of surface hydroxyls and chlorines with gaseous T i C I 4 and water: n(-OH)(s) + TiC14(g) ~ (-O-)n TiC!,_n(s) + nHCl(g)
(1) (-O-)n TiCla_,(s) + (4 - n)H20(g) (--O-)n Ti(OH)4_ n(s) + (4 - n)HCl(g)
(2)
where n = 1-3. Without any migration or gas phase nucleation processes the film would grow uniformly over the whole substrate. The high free energy associated with the interface between the film and the substrate can act as a driving force for the aggregation. However, in addition to the driving force, also an effective mechanism for material transportation is needed to achieve the observed level of aggregation. On the basis of the stability of the substrate materials at the studied temperatures it is reasonable to assume that the aggregation mechanism is comparable with the particle growth of unsupported TiO2 powder. Pijolat and coworkers have shown that both HC1 [4] and water [5] vapours increase the rate of surface area reduction of anatase powder at relatively low temperatures (417 and 550 °C) where no densification occurs. The accelerating effect of water was attributed to an enhanced surface migration of oxygen in the form of hydroxyl ions instead of oxide ions [5]. The effect of HCI was stronger than that of water and it was attributed to a formation of a volatile species Ti(OH)2C12 preceded by a dissociative adsorption of HC1 [6]. By means of this species the migration process is enhanced by converting it from the surface to the gas phase. The present study has been carried out at comparable temperatures and both of the mechanisms are possible. According to reaction (2) the water pulse transforms the surface into a hydroxyl-terminated surface and, consequently, the hydroxyl density is high enough to promote aggregation. In contrast, the byproduct of reactions (1) and (2) is HC1 which can interact with the surface, producing Ti(OH)2CI2. An alternative mechanism for the formation of Ti(OH)2CI2 is the interaction between the intermediate surface species (-O-)nTiC14_ n and (-O-)~Ti(OH)4_~, especially when n = 2. Also, a ligand exchange reaction of TIC14 can produce Ti(OH)2C12; 2(-OH)(s) + TiCla(g) --+2(-Cl)(s) + Ti(OH)2CI2(g ) (3)
M. Ritala et al. ] AFM study of A L E 7702
The narrow flow channel between the two substrates means that, once formed and desorbed, Ti(OH)2C12 will undergo a number of collisions with the substrates before leaving the reactor chamber [7]. Consequently, the probability for the readsorption of Ti(OH)2CI2 is high enough to cause agglomeration. Unfortunately, estimation of the rates of formation, desorption and adsorption of Ti(OH)2C12 are prevented by the lack of thermodynamic data. It should be noted that when the interface area is reduced by migration processes the film growth is likely to exhibit the island growth mode which is in accordance with our results. In addition, the formation of Ti(OH)2CI 2 can explain also the observed interplay between the substrates. In our previous paper we suggested that the dependence of the growth rate on the substrate material is caused by the number of reactive sites, i.e. hydroxyl groups, on the substrate surface [1]. Consequently, the effect of the opposite substrate can be understood in terms of the gas phase migration of hydroxyl groups from one substrate to another, leveling out the difference between them. An alternative explanation for the formation of aggregates is three-dimensional nucleation in the gas phase provided that there are, somehow, simultaneously both TiCI4 and water in the gas phase. It has been observed that a strong thickness profile is formed at the leading edge of the substrate as a consequence of desorption of one reactant from the walls upstream from the substrate during the pulse sequence of another reactant [1]. However, the desorbed reactant is rapidly exhausted and in the studied area films have uniform thicknesses, suggesting that the contribution of this mechanism cannot be remarkable. Furthermore, this process would lead to a film consisting of three-dimensional nuclei over uniformly distributed "real" ALE film and it would be thus contradictory with our results. Another origin for the gas phase nucleation might be the liberation of water during the TiC14 pulse as a result of the adsorption of HCI [8]: Ti-OH(s) + HCI(g)~-Ti(H20)-CI(s) ~Wi-Cl(s) + H20(g )
(4)
35
However, because of the higher TIC14 concentration relative to the HC1 concentration reaction (1) is likely to dominate over reaction (4). Consequently, nearly layer-by-layer growth should be achieved instead of the observed island growth.
4. Conclusions The AFM study shows that the surfaces of ALEgrown TiO2 films roughen as the growth proceeds. The roughening rate is at its highest during the first reaction cycles and, according to the XPS results, film growth takes place by the island mode. The roughening seems to be caused by migration processes driven by the minimization of the film-substrate interface energy because this mechanism leads to the observed island growth whereas gas phase nucleation processes would lead to Stranski-Krastanov growth. No remarkable differences in the morphology between crystalline and amorphous films were observed.
Acknowledgment The work was supported in part by the Academy of Finland and Technology Development Centre (TEKES), Helsinki, Finland.
References 1 M. Ritala, M. Leskel/i., E. NykS.nen, P. Soininen and L. Niinist6, Thin Solid Films, 225 (1993) 288. 2 T. Suntola and J. Hyviirinen, Annu. Rev. Mater. Sci., 15 (1985) 177. 3 E.-L. Lakomaa, S. Haukka and T. Suntola, Appl. Surf. Sci., 60/61 (1992) 742. 4 F. Gruy and M. Pijolat, J. Am. Ceram. Soc., 75(1992) 657. 5 J.-L. Hrbrard, P. Nortier, M. Pijolat and M. Soustelle, J. Am. Ceram. Soc., 73 (1990) 79. 6 F. Gruy and M. Pijolat, J. Am. Ceram. Sac., 75 (1992) 663. 7 T. Suntola, Thin Solid Films, 216(1992) 84. 8 G. D. Parfitt, J. Ramsbotham and C. H. Rochester, Trans. Faraday Soc., 67 (1971) 3100.