- Email: [email protected]
Rutherford backscattering and channeling studies of a TiO2(100) substrate, epitaxially grown pure and Nb-doped TiO2 films
..,. :;;I>::
::.y.:::a:-l;::p.. ;... .. ........l... ss ,.:.
..:.* .. . .
.L’,‘.,~,
.,.,., :.:+
,.,..
:,I:.:: y,;:y,; ..‘.....
> _.:
applied
surface science ELSEVTER
Applied Surface Science
I I5 (1997) 38 l-385
Rutherford backscattering and channeling studies of a TiO, (100) substrate, epitaxially grown pure and Nb-doped TiO, films S. Thevuthasan a EnGwmental
aP*,N.R. Shivaparan Molecular
Sciences Laboratory,
b Department
b, R.J. Smith b, Y. Gao ‘, S.A. Chambers a
Pacijic
of Physics. Montana
Received 20 September
Northwest
National
Stute Unirersi~.
L_uboratory. Richlund.
Bo:etnan.
1996: accepted 22 December
WA YY35.2. U.SA
MT 5Y717. USA
I996
Abstract We have investigated the crystalline quality of a TiO,(lOO) substrate, homoepitaxially grown TiO, film and Nb-doped TiO, films using Rutherford backscattering (RBS) and channeling experiments. The minimum yields obtained from the aligned and random spectra are 2.4 f 0.2% for the TiO,(lOO) substrate, and 4.0 _t 0.2% for a homoepitaxial TiO, film. The minimum yields for Ti and Nb are 1.6 + 0.2% and 7.0 + l.O%, respectively, for a Nb-doped TiOz film. Also, about 95% of the Nb atoms occupy cation sites in the Nb-doped TiOz film. The angular yield curves for Ti and Nb from the Nb-doped film confirm the good crystalline quality of the film in which most Nb atoms occupy the cation sites. The calculated surface peak areas for Ti and Nb using a model which incorporates Nb surface segregation from the bulk. agree very well with the corresponding surface peak areas for Ti and Nb extracted from the experiment. 0 1997 Pacific Northwest Laboratories, Battelle. Published by Elsevier Science B.V.
1. Introduction There is a growing interest in the synthesis of model oxides as thin films on various oxide and metal substrates to obtain high quality surfaces. The chemical and physical properties of these surfaces and interfaces for some of the more simple oxides have been extensively studied. For instance, titanium dioxide (TiO,), a relatively simple oxide, has been extensively investigated [l-3]. The excellent photocatalytic activity of TiO, makes the material useful for the destruction of organics, and it has been observed that there is an enhancement in the photocatalytic activity when TiO, is mixed with other oxides such as WO,, MOO, and Nb,O, [4-71.
Corresponding 0169.4332/97/$17.00 PI/
author. 0
SO169-4332(97)00005-6
1997 Pacific
Northwest
Laboratories.
We have recently demonstrated the molecular beam epitaxial (MBE) growth of high-quality undoped and Nb-doped TiO, films on various orientations of TiO, [8]. In the case of Nb-doped TiOz films, NbYTi, _ ,O, phases were grown with .Y as large as 0.40. X-ray photoelectron diffraction (XPD) measurements, along with reflection high energy and low energy electron diffraction (RHEED/LEED) on these pure and Nb-doped films show that both kinds of films possess excellent short and long range structural order with high crystalline quality. In addition. these experiments indicate that Nb atoms incorporate substitutionally at cation sites in the Nb-doped films. The goal of the present work is to investigate the crystalline quality of pure and Nb-doped films in more detail, and to compare the quality of these films to that of the substrate, using Rutherford backscattering (RBS) and channeling techniques. In
Battelle. Published
by Elsevier Science B.V
382
S. TheL,uthasan et al. /Applied
addition, we address the question of Nb segregation from the bulk of the film to the surface. From the angular resolved XPS measurements [9], it is believed that Nb segregates to the near-surface region, rather than being distributed uniformly throughout the film. The channeling experiments are well suited to investigate this phenomenon and confirm the angular resolved XPS results. The channeling mode of high energy ion scattering (HEIS) experiments provide a powerful tool to probe the substrate surface structure as well as over layer structures. In addition, HEIS provides a direct means for accurately measuring the percentage of Nb and Ti in the bulk when the ion beam is incident on the film in a random direction. When the ion beam is incident along a low index crystallographic direction of the single crystal sample, most of the beam is channelled through the rows of atoms. In this channeling geometry, the energy spectrum of the backscattered He ions show a surface peak (SP) which is associated with ions backscattered from the topmost layers at the surface. By rotating the sample a few degrees with respect to the angle of incidence away from the low index direction, the channeling mode transforms to a random mode. The variation of the surface peak area with angle results in the angular yield curve or rocking curve, which provides information about the Nb and Ti lattice occupations.
Swjixr
Science I I5 (1997) 381-385
The primary energy of the ions was 0.96 MeV and the incident ion beam was directed along the [TOO] direction in all the samples.
3. Results and discussion Aligned and random RBS spectra for the Ti02(100) substrate, the pure TiO, homoepitaxial film, and a Nb-doped TiO, film on Ti02(100) are shown in Fig. 1, respectively. A small energy window (AE, = 700-730 channels) near the surface region was used to calculate the minimum yield ( x,,~,). The minimum yield is the ratio of the yield in the channeling geometry to that for a random, non-channeling geometry. For the substrate (Fig. la) xmin is calculated to be 2.4 5 0.2%. In general the Random and Aligned Spectra, E = 0.96 MeV
, , T102 Film _
2. Experimental The Nb-doped TiO, films were grown using procedures described elsewhere [S]. After growth the samples were carefully removed from the MBE system at Pacific Northwest National Laboratory (PNNL), packed in clean containers and transported to Montana State University to perform the ion scattering experiments. These samples were cleaned in acetone and methanol and then introduced into the HEIS system described elsewhere [lo]. The experiments were carried out in the pressure range of l-3 X IO-* Torr. All the samples were heated to 200-250°C to desorb hydrocarbons from the surface. The standard dose of He+ ions for one spectrum was 1.6 X lOI ions/cm’. Energy analysis of the backscattered He+ ions was performed using a bakable silicon detector at a scattering angle of 105”.
800
400
h1.U Allgned
0
t
200
400
11 600
I
II 800
1000
Channel Number Fig. I. Aligned and random RBS spectra for (a) Ti02(100) substrate, (b) homoepitaxially grown TiO, on TiO, (100). and Cc) epitaxially grown Nb-doped TiO, film on TiO, (100). 0.96 MeV incident He+ ions aligned along the [TOO] direction were used.
S. Theruthasan
et al. /Applird
crystalline quality of the substrate seems to be very good and the minimum yield is accordingly very low. From Fig. l(b) the minimum yield for the TiO, homoepitaxial film grown on a Ti02(100) substrate was calculated to be 4.0 + 0.2%. The crystalline quality of this film appears to be a little lower than that of the substrate. Fig. I(c) shows the aligned and random spectra from the Nb-doped film on the TiO,(lOO) substrate. The stoichiometry of this film was determined to be Nb,,. ,,,Ti o,so02 by X-ray photoelectron spectroscopy measurements [8]. The minimum yield for Ti is 1.6 * 0.2% and for Nb is 7.0 & 1.O%. The latter value was determined using the channel numbers from 807-832. The film appears to be well ordered and the crystalline quality of the film is better than that of the pure TiO, film. The fraction of impurity incorporation on substitutional sites in the TiO, lattice can be calculated from the following equation [l I]: s = ( ’ - XimpurllyM
1 - Xtmt 1’
(‘1
where S is the fractional substitution, ximpurlty is the minimum yield of the impurity and xhoht is the minimum yield of the host atoms. Using the values of Xllll” given above in this equation, the fraction of Nb substitution was calculated as 95 f 1%. This shows that about 95% of the Nb occupy substitutionally at cation sites, and there is no evidence that Nb incorporation into the film leads to a secondary phase. This result agrees very well with the X-ray photoelectron diffraction results [8], in which the azimuthal scans at several different polar angles for Ti 2p and Nb 3p,,, p hotoemission show similar modulation. establishing the same site occupation for Nb and Ti atoms. From the random spectrum for the Nb-doped film. the bulk stoichiometry of the film is calculated to be Nb (I017Ti o,4s307,(,. That is. the Nb atoms represent 4.7% of the cation concentration. and the Nb concentration is essentially uniform throughout the bulk of the film except the near surface region. The thickness of the Nb-doped film is calculated as 413 A using the random RBS spectrum. Although Nb substitutes for Ti at lattice sites and does not form any secondary phases, there is compelling evidence that Nb segregates to lattice sites in the near-surface region as reported in Ref. [9]. Since XPS is most
Sutjacr
Srirncr
383
I I.5 (1997) 381-385
sensitive to the surface region of the film the stoichiometry calculated from XPS will be an average over the surface region. The stoichiometry determined using XPS (Nb,,,,Ti,,,,,02). is different from that determined from RBS (Nb,,,,,,Ti,,,,,sxO, ). We have extracted the Nb depth distribution using the experimental surface peak area and appropriate simulations. This analysis is discussed in the later part of the paper. We show in Fig. 2 the normalized Nb and Ti angular yield curves with respect to [loo] direction for the Nb-doped TiOz film. The energy regions shown in Fig. I for calculating the minimum yields were also used to extract the angular yield curves. The Nb and Ti angular yields are normalized to the maximum Nb t 1542 counts) and Ti (10420 counts) yields for the respective energy regions. During the whole angular scan, aligned spectra were collected at the beginning, in the middle and at the end of the angular scan to check for sample damage. These three aligned scans were essentially identical within the experimental uncertainties and, as a result, the sample damage is negligible. In general, both curves for Ti and Nb show very narrow angular widths. The full width at half maximum (FWHM) for Ti is I .32” and for Nb is 1.35”. The variation of the yield as a function of the polar angle is similar for both Ti and Nb which indicates that both occupy similar lattice sites in the film. Fig. 3 shows the experimental surface peak area along with the corresponding simulated values using the Vegas code as a function of polar angle. The
-2.48
-1.56
-0.64
0.28
1.20
2.12
Angle (9 Fig. 2. Angular yield curves with respect to [ IO01 direction for Ti and Nb for the Nb-doped TiO, film on a TiO,( 100) substrate.
384
S. Thecuthasan et al/Applied Surface Peak Area as a Function of Angle 6.00
I - I - I . 1 (a) Ti
The vibrational amplitude and the minimum yield are related through the following equation [14]:
I . I . I
11= (2Nd?r)
Calculated /I
\ Experiment
0.321
I . I -0.6 -0.4
Fig. 3. The experimental
i 1 . I . I . I , I I -0.2 0 0.2 0.4 0.6 Angle (“) and simulated
angular
yield curves for
(a) Ti, and (b) Nb for Nb-doped TiO, film on TiO,(lOO)
Surface Science 115 (I9971 381-385
sub-
strate.
Vegas code uses Monte Carlo calculations and a Moliere screened potential to simulate the ion scattering experiments and is described elsewhere [12]. In our simulations we used 12 unit cells in a slab with the dimensions of the slab bfing 3 X 4.59 A along the [OIO] direckion, 4 X 2.96 A along the [OOl] direction and 4.59 A along the [loo] direction as shown by the insert in Fig. 4. Twenty slabs stacked along the [IOO] direction were used for the simulations. There are twenty four cations in each slab. The calculations were carried out using this model and varying the number of Nb atoms in the slabs near the surface. Our best agreement with the experiment was obtained by substituting seven Nb for Ti in the first slab, three Nb in the second slab and one Nb in the third and deeper slabs into the bulk as shown in Fig. 4. One of the important parameters which is used in the simulations is the vibrational amplitude of the atoms in the slabs. The bulk Debye temperature for TiO, single crystal is reported to be 670 K [13]. The vibrational amplitude for Ti atoms is about 0.045 A at this Debye temperature. Since the vibrational amplitudes of the atoms near the surface region can be significantly different from the vibrational amplitudes of the atoms in the bulk, we used the minimum yield for Ti to calculate the vibrational amplitudes.
- I’?( j&” - N&rrr*)“z,
(2)
where N is the number of atoms per unit volume, d is the interplanar distance, and a is the ThomasFermi screening radius. The yibration amplitude is calculated as 0.126 f 0.009 A using the minimum yield of 2.4% for Ti from the TiO,(lOOl substrate. This is nearly three times the value calculated from the bulk Debye temperature, and may reflect more the quality of the substrate than the Ti vibrations. For the Nb-doped film, the minimum yield of 1.6% for Ti results in a, calculated vibration amplitude of 0.084 + 0.009 A. As an independent check on this value, the FWHM of the angular yield curve for Ti was calculated using this vibrational amplitude. The calculated 6’WHM is 1.40” for a vibration amplitude of 0.084 A, and 1.60” for a vibration amplitude of 0.045 A. Since the value of 1.40” is close to the experimental FWHM of 1.32” for Ti, we assumed that the vibrational amplitude of 0.084 A is correct, and that the vibrations are isotropic. The experimental angular yield curve and the corresponding calculated angular yield curve using our model for Ti are shown in Fig. 3(a). The range for the angle of incidence was selected to be between +0.70” and -0.70” with respect to the channeling geometry. Although both of these curves are shifted by a constant yield, the behavior of these curves as a function of polar angle is similar. Furthermore, since the curves represent the absolute angular yield, the close agreement is more supportive of our assumed Nb Atoms as a Function of Depth from the Surface L
--
0’
0
3+4.59A
““““‘I
’
’ 1
Fig. 4. Nb concentration from the model.
.
Bulk -
Surface ’
’ ’ ’ 2 3 No. of Slabs profile as a function
’
’ 4
’
J 5
of depth extracted
S. Thelurhasan
et al. /Applied
Surface Sciencr
115
f 1997)
383-385
385
model. Thus we conclude that approximately 30% of the Ti are replaced by Nb in the surface slab. Fig. 3(b) shows the results for Nb. Again the experimental curve and the simulated curve using our model as function of polar angle are very similar. Finally, we relate the Nb concentration profile from the ion scattering results to the Nb concentration reported by XPS for the near surface region [8]. The attenuation length of the Nb 3p,,, photoelectrons (1122 eV kinetic energy with Al K,) in TiO, material is calculated to be 20 A [ 151. Within this depth limit, the average concentration of Nb is calculated as 0.08 with appropriate exponential decay due to attenuation. This results agrees with the Nb concentration measurement of 0.10 by XPS within the experimental uncertainties.
bulk. The vibrational amplitudes of Ti atoms in these films are estimated to be nearly twice that calculated from the Debye temperature reported for TiO,.
4. Conclusions
References
Rutherford backscattering and channeling techniques were used to determine the crystalline quality of a TiO& 100) substrate, homoepitaxial TiO,( 1001, and Nb-doped TiO, films. The minimum yield obtained from the aligned and random spectra for the substrate is 2.4 + 0.2% which shows that the quality of the crystal is very good. The minimum yield from the pure TiO,(lOO) homoepitaxial film (4.0% 5 0.2) appears to be a little higher than that of the substrate. The minimum yield for Ti from the Nb-doped film is I .6 f 0.2% and the minimum yield for Nb from the same film is 7.0 _t 1.0%. This film appears to be somewhat better ordered than the pure TiO, film and the substrate. However, at this point it is not clear whether the higher quality of the Nb-doped film is due to the incorporation of the Nb in the film, or to subtle changes in conditions under which these films were grown. The fraction of the Nb substitution for Ti in the film is calculated to be N 95% from the minimum yields of Ti and Nb. The angular yield curves for Ti and Nb from the Nb doped film also show that the crystalline quality of the film is good and that Nb atoms occupy mostly cation sites. The calculated surface peak area for Ti and Nb using the model described in the paper agree very well with the corresponding surface peak area for Ti and Nb extracted from the experiment. The model is derived assuming Nb segregation to the surface from the
Acknowledgements Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the US Department of Energy by Battelle Memorial Institute under contract No. DE-AC06-76RL0 1830. The authors gratefully acknowledge partial support from the US Department of Energy, Office of Basic Energy Sciences, Materials Science Division. The authors from the Department of Physics, Montana State University were supported by National Science Foundation under contract DMR 940925.
Ill A. Fujishima. K. Honda, Nature 238 (1973,) 37. 121 V.E. Henrich, P.A. Cox. The Surface Science of Metal Oxides (Cambridge University Press, Cambridge, 1994). 131 G.C. Bond, R. Burch, in: G.C. Bond, G. Webb (Eds.), Catalysis 6 (Royal Society of Chemistry. London, 1983) p. 71.
141 W. Lee. Y.R. Do. K. Dwight. A. Weld. Mater. Res. Bull. 18 (1993) 1127. [51 Y.R. Do, W. Lee. K. Dwight. A. Weld. J. Solid State Chem. IOX (1994) 198. l61 S. Okazaki. T. Okuyama. Bull. Chem. Sot. Jpn. 65 (1983) 913. [71 Y. Matsumoto, T. Shimizu. A. Toyoda. E. Sato. J. Phya. Chem. X6 (1982) 35x1. Y.Gao. S. Thevuthasan. Y. Liang, N.R. 181 S.A. Chambers. Shivaparan. R.J. Smith, I. Vat. Sci. Technol. A I4 (3) (1996) 1387: Y. Gao. Y. Liang, S.A. Chambers. Surf. Sci. 365 ( 1996) 638. Y. Gao. S. Thevuthasan, S. Wen. K.L. 191 S.A. Chambers, Merkle, N.R. Shivaparan. R.J. Smith. Mater. Res. Bull. ( 1996). to appear. C. 1101 R.J. Smith, C.N. Whang. Xu Mingde, M. Worthington. Hennesy, M. Kim. N. Holland, Rev. Sci. Instr. 58 t 1987) 2284. I1 11 L.C. Feldman, J.W. Mayer, S.T. Picraux, Materials Analysis by Ion Channeling (Academic Press. New York, 1982). [I?1 J.W. Frenken. R.M. Tromp. J.F. van der Veen. Nucl. Instrum. Methods B 17 (1986) 334. (131 American Institute of Physics Handbook. 3rd ed. (McGraw Hill, New York, 1972). [I41 Wei-Kan Chu. J.W. Mayor. M.A. Nicolet. Backscattering Spectroscopy (Academic Press, New York, 1978). 1151 S. Tanuma. C.J. Powell, D.R. Penn. Surf. Interf. Anal. 17 (1991) 911.
::.y.:::a:-l;::p.. ;... .. ........l... ss ,.:.
..:.* .. . .
.L’,‘.,~,
.,.,., :.:+
,.,..
:,I:.:: y,;:y,; ..‘.....
> _.:
applied
surface science ELSEVTER
Applied Surface Science
I I5 (1997) 38 l-385
Rutherford backscattering and channeling studies of a TiO, (100) substrate, epitaxially grown pure and Nb-doped TiO, films S. Thevuthasan a EnGwmental
aP*,N.R. Shivaparan Molecular
Sciences Laboratory,
b Department
b, R.J. Smith b, Y. Gao ‘, S.A. Chambers a
Pacijic
of Physics. Montana
Received 20 September
Northwest
National
Stute Unirersi~.
L_uboratory. Richlund.
Bo:etnan.
1996: accepted 22 December
WA YY35.2. U.SA
MT 5Y717. USA
I996
Abstract We have investigated the crystalline quality of a TiO,(lOO) substrate, homoepitaxially grown TiO, film and Nb-doped TiO, films using Rutherford backscattering (RBS) and channeling experiments. The minimum yields obtained from the aligned and random spectra are 2.4 f 0.2% for the TiO,(lOO) substrate, and 4.0 _t 0.2% for a homoepitaxial TiO, film. The minimum yields for Ti and Nb are 1.6 + 0.2% and 7.0 + l.O%, respectively, for a Nb-doped TiOz film. Also, about 95% of the Nb atoms occupy cation sites in the Nb-doped TiOz film. The angular yield curves for Ti and Nb from the Nb-doped film confirm the good crystalline quality of the film in which most Nb atoms occupy the cation sites. The calculated surface peak areas for Ti and Nb using a model which incorporates Nb surface segregation from the bulk. agree very well with the corresponding surface peak areas for Ti and Nb extracted from the experiment. 0 1997 Pacific Northwest Laboratories, Battelle. Published by Elsevier Science B.V.
1. Introduction There is a growing interest in the synthesis of model oxides as thin films on various oxide and metal substrates to obtain high quality surfaces. The chemical and physical properties of these surfaces and interfaces for some of the more simple oxides have been extensively studied. For instance, titanium dioxide (TiO,), a relatively simple oxide, has been extensively investigated [l-3]. The excellent photocatalytic activity of TiO, makes the material useful for the destruction of organics, and it has been observed that there is an enhancement in the photocatalytic activity when TiO, is mixed with other oxides such as WO,, MOO, and Nb,O, [4-71.
Corresponding 0169.4332/97/$17.00 PI/
author. 0
SO169-4332(97)00005-6
1997 Pacific
Northwest
Laboratories.
We have recently demonstrated the molecular beam epitaxial (MBE) growth of high-quality undoped and Nb-doped TiO, films on various orientations of TiO, [8]. In the case of Nb-doped TiOz films, NbYTi, _ ,O, phases were grown with .Y as large as 0.40. X-ray photoelectron diffraction (XPD) measurements, along with reflection high energy and low energy electron diffraction (RHEED/LEED) on these pure and Nb-doped films show that both kinds of films possess excellent short and long range structural order with high crystalline quality. In addition. these experiments indicate that Nb atoms incorporate substitutionally at cation sites in the Nb-doped films. The goal of the present work is to investigate the crystalline quality of pure and Nb-doped films in more detail, and to compare the quality of these films to that of the substrate, using Rutherford backscattering (RBS) and channeling techniques. In
Battelle. Published
by Elsevier Science B.V
382
S. TheL,uthasan et al. /Applied
addition, we address the question of Nb segregation from the bulk of the film to the surface. From the angular resolved XPS measurements [9], it is believed that Nb segregates to the near-surface region, rather than being distributed uniformly throughout the film. The channeling experiments are well suited to investigate this phenomenon and confirm the angular resolved XPS results. The channeling mode of high energy ion scattering (HEIS) experiments provide a powerful tool to probe the substrate surface structure as well as over layer structures. In addition, HEIS provides a direct means for accurately measuring the percentage of Nb and Ti in the bulk when the ion beam is incident on the film in a random direction. When the ion beam is incident along a low index crystallographic direction of the single crystal sample, most of the beam is channelled through the rows of atoms. In this channeling geometry, the energy spectrum of the backscattered He ions show a surface peak (SP) which is associated with ions backscattered from the topmost layers at the surface. By rotating the sample a few degrees with respect to the angle of incidence away from the low index direction, the channeling mode transforms to a random mode. The variation of the surface peak area with angle results in the angular yield curve or rocking curve, which provides information about the Nb and Ti lattice occupations.
Swjixr
Science I I5 (1997) 381-385
The primary energy of the ions was 0.96 MeV and the incident ion beam was directed along the [TOO] direction in all the samples.
3. Results and discussion Aligned and random RBS spectra for the Ti02(100) substrate, the pure TiO, homoepitaxial film, and a Nb-doped TiO, film on Ti02(100) are shown in Fig. 1, respectively. A small energy window (AE, = 700-730 channels) near the surface region was used to calculate the minimum yield ( x,,~,). The minimum yield is the ratio of the yield in the channeling geometry to that for a random, non-channeling geometry. For the substrate (Fig. la) xmin is calculated to be 2.4 5 0.2%. In general the Random and Aligned Spectra, E = 0.96 MeV
, , T102 Film _
2. Experimental The Nb-doped TiO, films were grown using procedures described elsewhere [S]. After growth the samples were carefully removed from the MBE system at Pacific Northwest National Laboratory (PNNL), packed in clean containers and transported to Montana State University to perform the ion scattering experiments. These samples were cleaned in acetone and methanol and then introduced into the HEIS system described elsewhere [lo]. The experiments were carried out in the pressure range of l-3 X IO-* Torr. All the samples were heated to 200-250°C to desorb hydrocarbons from the surface. The standard dose of He+ ions for one spectrum was 1.6 X lOI ions/cm’. Energy analysis of the backscattered He+ ions was performed using a bakable silicon detector at a scattering angle of 105”.
800
400
h1.U Allgned
0
t
200
400
11 600
I
II 800
1000
Channel Number Fig. I. Aligned and random RBS spectra for (a) Ti02(100) substrate, (b) homoepitaxially grown TiO, on TiO, (100). and Cc) epitaxially grown Nb-doped TiO, film on TiO, (100). 0.96 MeV incident He+ ions aligned along the [TOO] direction were used.
S. Theruthasan
et al. /Applird
crystalline quality of the substrate seems to be very good and the minimum yield is accordingly very low. From Fig. l(b) the minimum yield for the TiO, homoepitaxial film grown on a Ti02(100) substrate was calculated to be 4.0 + 0.2%. The crystalline quality of this film appears to be a little lower than that of the substrate. Fig. I(c) shows the aligned and random spectra from the Nb-doped film on the TiO,(lOO) substrate. The stoichiometry of this film was determined to be Nb,,. ,,,Ti o,so02 by X-ray photoelectron spectroscopy measurements [8]. The minimum yield for Ti is 1.6 * 0.2% and for Nb is 7.0 & 1.O%. The latter value was determined using the channel numbers from 807-832. The film appears to be well ordered and the crystalline quality of the film is better than that of the pure TiO, film. The fraction of impurity incorporation on substitutional sites in the TiO, lattice can be calculated from the following equation [l I]: s = ( ’ - XimpurllyM
1 - Xtmt 1’
(‘1
where S is the fractional substitution, ximpurlty is the minimum yield of the impurity and xhoht is the minimum yield of the host atoms. Using the values of Xllll” given above in this equation, the fraction of Nb substitution was calculated as 95 f 1%. This shows that about 95% of the Nb occupy substitutionally at cation sites, and there is no evidence that Nb incorporation into the film leads to a secondary phase. This result agrees very well with the X-ray photoelectron diffraction results [8], in which the azimuthal scans at several different polar angles for Ti 2p and Nb 3p,,, p hotoemission show similar modulation. establishing the same site occupation for Nb and Ti atoms. From the random spectrum for the Nb-doped film. the bulk stoichiometry of the film is calculated to be Nb (I017Ti o,4s307,(,. That is. the Nb atoms represent 4.7% of the cation concentration. and the Nb concentration is essentially uniform throughout the bulk of the film except the near surface region. The thickness of the Nb-doped film is calculated as 413 A using the random RBS spectrum. Although Nb substitutes for Ti at lattice sites and does not form any secondary phases, there is compelling evidence that Nb segregates to lattice sites in the near-surface region as reported in Ref. [9]. Since XPS is most
Sutjacr
Srirncr
383
I I.5 (1997) 381-385
sensitive to the surface region of the film the stoichiometry calculated from XPS will be an average over the surface region. The stoichiometry determined using XPS (Nb,,,,Ti,,,,,02). is different from that determined from RBS (Nb,,,,,,Ti,,,,,sxO, ). We have extracted the Nb depth distribution using the experimental surface peak area and appropriate simulations. This analysis is discussed in the later part of the paper. We show in Fig. 2 the normalized Nb and Ti angular yield curves with respect to [loo] direction for the Nb-doped TiOz film. The energy regions shown in Fig. I for calculating the minimum yields were also used to extract the angular yield curves. The Nb and Ti angular yields are normalized to the maximum Nb t 1542 counts) and Ti (10420 counts) yields for the respective energy regions. During the whole angular scan, aligned spectra were collected at the beginning, in the middle and at the end of the angular scan to check for sample damage. These three aligned scans were essentially identical within the experimental uncertainties and, as a result, the sample damage is negligible. In general, both curves for Ti and Nb show very narrow angular widths. The full width at half maximum (FWHM) for Ti is I .32” and for Nb is 1.35”. The variation of the yield as a function of the polar angle is similar for both Ti and Nb which indicates that both occupy similar lattice sites in the film. Fig. 3 shows the experimental surface peak area along with the corresponding simulated values using the Vegas code as a function of polar angle. The
-2.48
-1.56
-0.64
0.28
1.20
2.12
Angle (9 Fig. 2. Angular yield curves with respect to [ IO01 direction for Ti and Nb for the Nb-doped TiO, film on a TiO,( 100) substrate.
384
S. Thecuthasan et al/Applied Surface Peak Area as a Function of Angle 6.00
I - I - I . 1 (a) Ti
The vibrational amplitude and the minimum yield are related through the following equation [14]:
I . I . I
11= (2Nd?r)
Calculated /I
\ Experiment
0.321
I . I -0.6 -0.4
Fig. 3. The experimental
i 1 . I . I . I , I I -0.2 0 0.2 0.4 0.6 Angle (“) and simulated
angular
yield curves for
(a) Ti, and (b) Nb for Nb-doped TiO, film on TiO,(lOO)
Surface Science 115 (I9971 381-385
sub-
strate.
Vegas code uses Monte Carlo calculations and a Moliere screened potential to simulate the ion scattering experiments and is described elsewhere [12]. In our simulations we used 12 unit cells in a slab with the dimensions of the slab bfing 3 X 4.59 A along the [OIO] direckion, 4 X 2.96 A along the [OOl] direction and 4.59 A along the [loo] direction as shown by the insert in Fig. 4. Twenty slabs stacked along the [IOO] direction were used for the simulations. There are twenty four cations in each slab. The calculations were carried out using this model and varying the number of Nb atoms in the slabs near the surface. Our best agreement with the experiment was obtained by substituting seven Nb for Ti in the first slab, three Nb in the second slab and one Nb in the third and deeper slabs into the bulk as shown in Fig. 4. One of the important parameters which is used in the simulations is the vibrational amplitude of the atoms in the slabs. The bulk Debye temperature for TiO, single crystal is reported to be 670 K [13]. The vibrational amplitude for Ti atoms is about 0.045 A at this Debye temperature. Since the vibrational amplitudes of the atoms near the surface region can be significantly different from the vibrational amplitudes of the atoms in the bulk, we used the minimum yield for Ti to calculate the vibrational amplitudes.
- I’?( j&” - N&rrr*)“z,
(2)
where N is the number of atoms per unit volume, d is the interplanar distance, and a is the ThomasFermi screening radius. The yibration amplitude is calculated as 0.126 f 0.009 A using the minimum yield of 2.4% for Ti from the TiO,(lOOl substrate. This is nearly three times the value calculated from the bulk Debye temperature, and may reflect more the quality of the substrate than the Ti vibrations. For the Nb-doped film, the minimum yield of 1.6% for Ti results in a, calculated vibration amplitude of 0.084 + 0.009 A. As an independent check on this value, the FWHM of the angular yield curve for Ti was calculated using this vibrational amplitude. The calculated 6’WHM is 1.40” for a vibration amplitude of 0.084 A, and 1.60” for a vibration amplitude of 0.045 A. Since the value of 1.40” is close to the experimental FWHM of 1.32” for Ti, we assumed that the vibrational amplitude of 0.084 A is correct, and that the vibrations are isotropic. The experimental angular yield curve and the corresponding calculated angular yield curve using our model for Ti are shown in Fig. 3(a). The range for the angle of incidence was selected to be between +0.70” and -0.70” with respect to the channeling geometry. Although both of these curves are shifted by a constant yield, the behavior of these curves as a function of polar angle is similar. Furthermore, since the curves represent the absolute angular yield, the close agreement is more supportive of our assumed Nb Atoms as a Function of Depth from the Surface L
--
0’
0
3+4.59A
““““‘I
’
’ 1
Fig. 4. Nb concentration from the model.
.
Bulk -
Surface ’
’ ’ ’ 2 3 No. of Slabs profile as a function
’
’ 4
’
J 5
of depth extracted
S. Thelurhasan
et al. /Applied
Surface Sciencr
115
f 1997)
383-385
385
model. Thus we conclude that approximately 30% of the Ti are replaced by Nb in the surface slab. Fig. 3(b) shows the results for Nb. Again the experimental curve and the simulated curve using our model as function of polar angle are very similar. Finally, we relate the Nb concentration profile from the ion scattering results to the Nb concentration reported by XPS for the near surface region [8]. The attenuation length of the Nb 3p,,, photoelectrons (1122 eV kinetic energy with Al K,) in TiO, material is calculated to be 20 A [ 151. Within this depth limit, the average concentration of Nb is calculated as 0.08 with appropriate exponential decay due to attenuation. This results agrees with the Nb concentration measurement of 0.10 by XPS within the experimental uncertainties.
bulk. The vibrational amplitudes of Ti atoms in these films are estimated to be nearly twice that calculated from the Debye temperature reported for TiO,.
4. Conclusions
References
Rutherford backscattering and channeling techniques were used to determine the crystalline quality of a TiO& 100) substrate, homoepitaxial TiO,( 1001, and Nb-doped TiO, films. The minimum yield obtained from the aligned and random spectra for the substrate is 2.4 + 0.2% which shows that the quality of the crystal is very good. The minimum yield from the pure TiO,(lOO) homoepitaxial film (4.0% 5 0.2) appears to be a little higher than that of the substrate. The minimum yield for Ti from the Nb-doped film is I .6 f 0.2% and the minimum yield for Nb from the same film is 7.0 _t 1.0%. This film appears to be somewhat better ordered than the pure TiO, film and the substrate. However, at this point it is not clear whether the higher quality of the Nb-doped film is due to the incorporation of the Nb in the film, or to subtle changes in conditions under which these films were grown. The fraction of the Nb substitution for Ti in the film is calculated to be N 95% from the minimum yields of Ti and Nb. The angular yield curves for Ti and Nb from the Nb doped film also show that the crystalline quality of the film is good and that Nb atoms occupy mostly cation sites. The calculated surface peak area for Ti and Nb using the model described in the paper agree very well with the corresponding surface peak area for Ti and Nb extracted from the experiment. The model is derived assuming Nb segregation to the surface from the
Acknowledgements Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the US Department of Energy by Battelle Memorial Institute under contract No. DE-AC06-76RL0 1830. The authors gratefully acknowledge partial support from the US Department of Energy, Office of Basic Energy Sciences, Materials Science Division. The authors from the Department of Physics, Montana State University were supported by National Science Foundation under contract DMR 940925.
Ill A. Fujishima. K. Honda, Nature 238 (1973,) 37. 121 V.E. Henrich, P.A. Cox. The Surface Science of Metal Oxides (Cambridge University Press, Cambridge, 1994). 131 G.C. Bond, R. Burch, in: G.C. Bond, G. Webb (Eds.), Catalysis 6 (Royal Society of Chemistry. London, 1983) p. 71.
141 W. Lee. Y.R. Do. K. Dwight. A. Weld. Mater. Res. Bull. 18 (1993) 1127. [51 Y.R. Do, W. Lee. K. Dwight. A. Weld. J. Solid State Chem. IOX (1994) 198. l61 S. Okazaki. T. Okuyama. Bull. Chem. Sot. Jpn. 65 (1983) 913. [71 Y. Matsumoto, T. Shimizu. A. Toyoda. E. Sato. J. Phya. Chem. X6 (1982) 35x1. Y.Gao. S. Thevuthasan. Y. Liang, N.R. 181 S.A. Chambers. Shivaparan. R.J. Smith, I. Vat. Sci. Technol. A I4 (3) (1996) 1387: Y. Gao. Y. Liang, S.A. Chambers. Surf. Sci. 365 ( 1996) 638. Y. Gao. S. Thevuthasan, S. Wen. K.L. 191 S.A. Chambers, Merkle, N.R. Shivaparan. R.J. Smith. Mater. Res. Bull. ( 1996). to appear. C. 1101 R.J. Smith, C.N. Whang. Xu Mingde, M. Worthington. Hennesy, M. Kim. N. Holland, Rev. Sci. Instr. 58 t 1987) 2284. I1 11 L.C. Feldman, J.W. Mayer, S.T. Picraux, Materials Analysis by Ion Channeling (Academic Press. New York, 1982). [I?1 J.W. Frenken. R.M. Tromp. J.F. van der Veen. Nucl. Instrum. Methods B 17 (1986) 334. (131 American Institute of Physics Handbook. 3rd ed. (McGraw Hill, New York, 1972). [I41 Wei-Kan Chu. J.W. Mayor. M.A. Nicolet. Backscattering Spectroscopy (Academic Press, New York, 1978). 1151 S. Tanuma. C.J. Powell, D.R. Penn. Surf. Interf. Anal. 17 (1991) 911.