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Systematic investigation of the properties of TiO2 films grown by reactive ion beam sputter deposition
Accepted Manuscript Title: Systematic investigation of the properties of TiO2 films grown by reactive ion beam sputter deposition Author: C. Bundesmann T. Lautenschl¨ager D. Spemann A. Finzel E. Thelander M. Mensing F. Frost PII: DOI: Reference:
S0169-4332(16)31698-1 http://dx.doi.org/doi:10.1016/j.apsusc.2016.08.056 APSUSC 33804
To appear in:
APSUSC
Received date: Revised date: Accepted date:
11-7-2016 9-8-2016 10-8-2016
Please cite this article as: C.Bundesmann, T.Lautenschl¨ager, D.Spemann, A.Finzel, E.Thelander, M.Mensing, F.Frost, Systematic investigation of the properties of TiO2 films grown by reactive ion beam sputter deposition, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.08.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
C. Bundesmann et al., Manuscript for possible publication in Appl. Surf. Sci.
Systematic investigation of the properties of TiO2 films grown by reactive ion beam sputter deposition C. Bundesmanna1
[email protected],
T. Lautenschlägera,
D. Spemanna,b,
A. Finzela, E. Thelandera, M. Mensinga, F. Frosta a
Leibniz-Institut für Oberflächenmodifizierung e.V., Permoserstraße 15, 04318 Leipzig,
Germany b
Universität Leipzig, Fakultät für Physik und Geowissenschaften, Institut für Experimentelle
Physik II, Linnéstraße 5, 04103 Leipzig, Germany 1
Corresponding author. Phone: +49-341-2353354; Fax: +49-341-2352595; Postal address:
Permoserstr. 15, 04318 Leipzig, Germany.
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Highlights TiO2 films grown by ion beam sputter deposition Structure, surface topography, composition, optical properties, mass density Films are amorphous and surface roughness is correlated with scattering angle Incorporation of inert process gas correlated with scattering geometry Correlation of mass density and optical properties, both also with scattering angle
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Abstract TiO2 films were grown by ion beam sputter deposition under systematic variation of ion beam parameters (ion species, ion energy) and geometrical parameters (ion incidence angle, polar emission angle) and characterized with respect to film thickness, growth rate, structural properties, surface topography, composition, optical properties, and mass density. The angular distributions of film thickness and growth rate show an over-cosine shape, which is tilted in forward direction. All films are amorphous and the surface roughness is below 0.22 nm. The investigation of the composition revealed stoichiometric TiO2 with implanted backscattered primary particles. The optical properties were analysed using the Tauc Lorentz (TL) model. The amplitude parameter of the TL model was found to vary systematically with the scattering angle, whereas the impact on the other TL parameters is negligible. Mass density follows the same trends as the optical properties, i.e. optical properties and mass density are correlated. Surface roughness, atomic fraction of implanted primary particles, optical properties and mass density show similar systematic variations with process parameters, especially, with the scattering geometry (i.e. scattering angle). Ion species, ion energy and ion incidence angle have no or only a small impact. The variations in the film properties are tentatively assigned to changes in the angular and energy distribution of the sputtered target particles and backscattered primary particles.
Keywords: Ion beam sputter deposition, TiO2, optical properties, mass density, composition, implantation, root mean square roughness
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1. Introduction Ion beam sputter deposition (IBSD) is a widely used physical vapour deposition (PVD) technique for thin film production. In contrast to other PVD techniques, for instance, magnetron sputtering or evaporation techniques, IBSD offers more degrees of freedom for tailoring the properties of the secondary, film-forming particles and, hence, the properties of the films [1]. Though widely used, the capabilities of IBSD are not completely understood and utilized yet. The focus of this work is the investigation of the IBSD process of TiO2 films, i.e. the exploration of the correlation between ion beam parameters (ion species, ion energy) and geometrical parameters (ion incidence angle and polar emission angle), and film properties. TiO2 films grown by PVD techniques are typically amorphous or poly-crystalline, and are commonly used for optical applications, e.g. multilayer coatings for highly reflective or anti-reflective applications. These applications exploit the high index of refraction of TiO2 (n ~ 2.4). When combining high-index TiO2 films with low-index materials, e.g. SiO2, interference effects can be used. TiO2 films grown with substrate heating or treated by postgrowth annealing are crystalline. Crystalline TiO2 is a semiconductor with a wide band gap which is used in solar cells, but can also exhibit photocatalysis or photoinduced superhydrophilicity [2-5]. Several groups reported on the ion beam sputter deposition of TiO2 films [6-23], mixed films of TiO2-SnO2 [24,25], TiO2-SiO2 [26,27], TiO2-Ta2O5 [28] or TiO2-xNx [29], or even Ge quantum dots in TiO2 [30]. Some of them also consider the influence of substrate temperature [9-11,15,18,19],
thermal
annealing
[13,14,17,21,22]
or
assisting
ion
bombardment [9,14,16,20,23]. The main focus of these works was the investigation of structural and optical properties of reactively sputtered films with unheated substrates. All groups used a fixed sputter geometry (one set of ion incidence angle and polar emission - 4 / 37 -
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angle), and, typically, only a single ion energy of about Eion = 1000 eV and Ar as sputter gas. There exist only two reports on the investigation of the influence of the energy of sputtering and assisting ions (Sputter source: Ar ions, Eion = 800 eV – 1500 eV; assist source: Ar ions, Eion = 200 eV – 300 eV [9]; Sputter source: Ar or Xe ions, Eion = 600 eV – 1200 eV; assist source: Ar or Xe ions, Eion = 600 eV – 1600 eV [20]). The lack of reference data is very likely related to the following facts: (i) The experimental setup does not allow to vary the scattering geometry and/or ion beam parameters. (ii) The angular and energy distribution of the sputtered target particles is commonly believed to be well described by Thompson’s formula [31,32]. This model postulates a cosine-shaped angular distribution and an energy distribution that is independent from the energy of the primary ions. It also does not consider the ion incidence angle, because it is based on the assumption of normally incident ions. Thompson’s formula was reported often to describe experimental findings quite well. However, there are also results that deviate from Thompson’s model due to anisotropy effects that are caused by ions hitting the target at oblique incidence or with low energy [33-44]. These anisotropy effects are strongly related to ion energy, ion species (or the ratio of the mass of the projectile to the mass of the target particle) and ion incidence angle [41-47], but also to the surface topography of the target [48]. (iii) The influence of backscattered primary particles is underestimated or neglected. Recently, it was shown that these particles can retain a large energy and have a significant impact on film properties [44-47,49,50]. Therefore, an experiment was set up which offers the opportunity for a systematic variation of ion beam and geometrical parameters for film growth. The capabilities of the setup were already demonstrated for the elemental materials Ag and Ge [44-47,49,50]. First experimental results for TiO2 films grown by sputtering with Ar were published in Ref. 51. Now the investigations are extended to TiO2 grown by sputtering with Xe. Furthermore, - 5 / 37 -
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additional characterization results, and a more sophisticated analysis and discussion of the optical data are presented. Results concerning the angular and energy distribution of secondary, film-forming particles are described and discussed elsewhere [52].
2. Experimental details Figure 1 shows a schematic sketch of the IBSD setup inside the deposition chamber. It consists of a broad beam ion source, a target holder and a substrate holder. The ion beam source and the target holder are placed on rotary tables, which have their centre of rotation at the centre of the target surface plane. Additionally, an energy-selective mass spectrometer can be used to measure the energy distribution of sputtered target ions and backscattered primary ions. The deposition chamber is of rectangular shape with a dimension of 1 x 1 x 0.7 m3. It is pumped by a 2200 l/s turbo molecular pump to a base pressure of 2x10-6 mbar. The working pressure during sputtering is about 5 x 10-5 mbar. The ion beam source is of radio-frequency (RF) type with a three-grid multi-aperture extraction system with an open diameter of 16 mm [53]. The process gases were Ar or Xe with a volumetric flow rate of 3.5 sccm or 1.1 sccm, respectively. The RF power was set to 70 W for all experiments resulting in a total ion current of about 7 mA. The distance between the exit plane of the ion beam source and the target centre is about 0.15 m, i.e. it is much smaller than mean free path length of the primary ions (Ar: 1.28 m; Xe: 0.72 m) The substrate holder is of semi-circular shape with a radius of curvature of 0.15 m. The holder is segmented such that substrates up to 15 x 15 mm2 can be placed at different polar emission angles in steps of 10°. Several sets of TiO2 films were grown under variation of ion energy, ion species and ion incidence angle on unheated Si (100) substrates with a size of about 12 x 12 mm2. Tab. 1 - 6 / 37 -
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summarizes the main growth parameters for all sample sets. A poly-crystalline Ti target with a purity of 99.99% was used. The deposition was done in oxygen atmosphere with an oxygen volumetric flow rate of 2 sccm. The resulting oxygen partial pressure was about 1.5 x 10-5 mbar. The TiO2 films were characterized with respect to film thickness, growth rate, structural properties, surface roughness, composition, optical properties, and mass density. Film thickness was measured by spectroscopic ellipsometer (and x-ray reflectometry). The growth rate was calculated upon dividing film thickness by the sputter time listed in Table 1. Spectra of the ellipsometric parameters Ψ and Δ were measured in the photon energy range from Ephoton = 0.73 eV to Ephoton = 6.4 eV (or wavelength range from λ = 193 nm to λ = 1700 nm) at four angles of incidence (60°, 65°, 70° and 75°) with an ellipsometer RC2-DI® (J.A. Woollam Co., Inc.) of dual-rotating-compensator type. A four-layer optical model (Air/TiO2/native SiO2/Si) was used for the data analysis. The optical properties of the native SiO2 layer and the Si substrate were taken from Ref. 54, the optical properties of the TiO2 films were modelled using the following complex dielectric function (DF) ̃ It consists of the constant term Tauc-Lorentz (TL) term ̃
̃
(1)
(high-frequency dielectric constant) and a complex . The TL model is commonly used to model the
spectral region of the interband transitions of amorphous semiconductors [55,56], as for instance, for amorphous TiO2 [57-63].
accounts for electronic contributions outside the
spectral region of investigation. The TL model is a combination of the Tauc joint density of states and the classical Lorentz oscillator [55]. The imaginary part of the complex DF reads
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∑
(
)
(2)
ATL,i, BTL,i and E0,i are amplitudes, broadening terms and peak transition energies, respectively. A set of two TL terms was used here (i = 1,2). The optical band gap Eg was assumed to be the same for both TL terms. The real part of the DF ε1,TL is obtained via Kramers-Kronig integration. The reader is referred to Ref. 55 for an analytical expression of ε1,TL. The complex DF ̃ is related to the complex index of refraction ̃ by ̃
̃
(3)
n and k are the (real) index of refraction and the absorption coefficient, respectively. Thickness d and TL parameters of each TiO2 film were fit parameters during the model lineshape analysis of the measured spectra. Two multi sample analyses (MSA) were done, i.e. the ellipsometric spectra of all samples grown by sputtering with Ar (MSA_Ar) or Xe (MSA_Xe) were modelled simultaneously. There were 50 data sets for each multi sample analysis. In order to avoid unwanted correlation of fit parameters, only the film thickness and the TL parameter ATL were allowed to vary independently for all samples, whereas other parameters were coupled, i.e. they were assumed to be the same for all simultaneously analysed data sets. Mueller-Matrix measurements on selected samples have shown no sign of optical anisotropy. Therefore, the use of an isotropic model dielectric function is justified. Structural analysis of the samples was done by X-ray diffraction (XRD). For this purpose, a diffractometer (Ultima IV Type III, Rigaku Corp., Japan) was used. It is equipped with a Cu anode X-ray tube, a graded multilayer mirror to obtain a parallel beam, a theta-theta goniometer, a parallel slit analyser and a scintillation detector.
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The surface topography was measured by scanning atomic force microscopy (AFM) with a large sample scanning force microscope from Bruker (Dimension icon®). The device was operated in Tapping ModeTM and in xy-closed loop configuration. For measuring the surface topography the z-sensor signal is utilized. The measurements were performed in air using silicon probes with a nominal tip radius smaller than 5 nm. A scanning size of 10 µm x 10 µm was used with a pixel resolution of 1024 x 1024. The AFM raw data were processed by a line-wise levelling (flatten) using a polynomial of 1st order in order to realize a sample tilt / offset compensation and to remove noise along the slow scan direction. Finally, from the processed images the root mean square (rms) roughness was calculated. The noise floor level during the measurements was ≤ 0.05 nm rms. Rutherford backscattering spectrometry (RBS) measurements were performed in order to investigate the composition of the TiO2 films, especially the stoichiometry and the amount of implanted inert gas particles. The experiments were done with single charged He ions with an energy of 2 MeV at the LIPSION facility [64]. Mass density was measured using x-ray reflectometry (XRR) using a Seifert XRD 3003 PTS. A parallelized Cu-Kα1 beam (λ = 0.15406 nm) with low divergence (0.005°) was used. The measured curves were analysed with the Rayflex Analyze software using a nonlinear least square fit routine under the assumption of stoichiometric TiO2. In this way, it was possible to extract mass density values and film thickness with an error less than 0.1 g/cm3 and 1.0 nm, respectively.
3. Results and discussion The film thickness data of the TiO2 films versus polar emission angle are summarized in Fig. 2. The presented data were taken from the analyses of the ellipsometric data. The results from the XRR measurements agree excellently with those from the multi sample - 9 / 37 -
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analyses. The growth time was chosen such that the maximum thickness of the sample sets is about 100 nm in order to make the following results comparable. The thickness distributions are of over-cosine shape and tilted in forward direction (βmax > 40°). The principal shape, especially the emission angle βmax of the thickness maximum, is very similar for sputtering with Ar or Xe with same growth parameters. The thickness maximum varies between βmax ~ 40° for sample sets Ar_1 and Xe_1 ( = 60°, Eion = 1000 eV), and βmax ~ 60° for sample sets Ar_4 and Xe_4 ( = 30°, Eion = 500 eV). The over-cosine shape is related to anisotropy effects caused by an incomplete evolution of collision cascade inside the target [33,41-43]. This anisotropy increases with decreasing ion energy and increasing ion incidence angle. It is also affected by the ratio of the mass of the projectile to the mass of the target particle. However, the shape of the thickness distributions in Fig. 2 is not symmetric, which is related to isotropic contributions [52]. Figure 3 depicts the growth rate data versus polar emission angle. As expected, the growth rate increases with increasing ion energy and increasing ion incidence angle, and is higher for sputtering with Xe than for sputtering with Ar. These observations can be explained by the corresponding behaviour of the total sputter yield [1]. XRD revealed that all TiO2 films are amorphous. The surface was found to be very smooth, i.e. the root mean square (rms) roughness was σ ≤ 0.22 nm. The rms roughness (see Fig. 4) increases with increasing scattering angle = 180° - β - . It seems to be unaffected by the ion incidence angle (Fig. 4 (a,c)), decreases slightly with increasing ion energy, and is higher for sputtering with Xe than for sputtering with Ar. All this can be explained by considering the energy distribution of the secondary particles, especially, of the backscattered primary particles [44,46,47,52]. The average energy of the backscattered primary particles increases with decreasing polar emission angle, increasing ion energy and is higher for Ar
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particles than for Xe particles. A higher particle energy results in a higher surface mobility causing a ballistic drift and, hence, in smoother surfaces [65]. The analyses of RBS measurements showed that the films are stoichiometric, i.e. the ratio Ti:O equals 1:2. Additionally, Ar or Xe particles were found inside the TiO2 films grown by sputtering with Ar ions or Xe ions, respectively. Figure 5 depicts the atomic fraction of primary particles versus scattering angle. The curves of all samples sets, which were grown by sputtering with Ar, match perfectly (Fig 5 (a,b)). The same holds for the sample sets grown by sputtering with Xe (Fig. 5 (c,d)), even though the numbers are smaller than those in Fig. 5 (a,b). The incorporated inert gas particles are assigned to implantation of energetic particles (see above), because SRIM calculations showed that an energy of about 40 eV for Ar or 14 eV for Xe is sufficient for a projected range of 0.5 nm in TiO2 [66]. The atomic fraction of implanted primary particles depends mainly on the scattering geometry but also on primary ion species. The same observation was made for Ge films [50] and can be explained by a dynamic equilibrium of competing processes at the growing film (implantation, diffusion, resputtering) and by the fact that the maximum scattering angle for direct backscattering of Xe particles at Ti particles ( ≤ 21°) is smaller than that of direct backscattering of Ar at Ti ( ≤ 180°) due to the different mass ratio of the interacting particles (mAr = 39.9 amu, mTi = 47.9 amu, mXe = 131.3 amu). The relation between the number of implanted primary particles versus scattering angle can be, in parts, qualitatively described by the differential cross section of Rutherford scattering which is proportional to sin(/2)-4 (see Fig. 5). There are some deviations for small scattering angles, which can be explained by the corresponding larger distance of closest approach in the scattering event which results in stronger shielding of the nuclear charges Z1e, Z2e of the colliding atoms by their respective electron shells. As the Rutherford cross section is proportional to (Z1Z2)2 an increased shielding leads to a reduction of the scattering cross section and hence to lower concentrations of the scattered - 11 / 37 -
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primary particles in the film than predicted from the sin(/2)-4 angular dependence alone. Furthermore, there are not only contributions by primary particles backscattered at target and implanted primary particles but also by sputtered, previously implanted primary particles with sufficiently high energy. The proposed TL model with two terms described the experimental ellipsometric spectra very well. Figure 6 shows selected dielectric function spectra of sample sets Ar_1 and Xe_1 as extracted from the multi sample analyses. The data analysis revealed a strong influence of the TL parameter ATL on the quality of the fit results, whereas the uncoupled variation of all other parameters has no or only a negligible impact. Therefore, these parameters were coupled. Figure 7 summarizes the best-fit TL parameter ATL,1 of all sample sets as extracted from the multi sample analyses of the ellipsometric data. The other best-fit parameters are given in Table 2. The curves in Fig. 7 look very similar for all sample sets grown by sputtering with Ar or Xe: an increase with increasing scattering angle followed by a slight decrease. Only for the sample sets grown at the lowest ion energy (Eion = 500 eV, sample sets Ar_4 and Xe_4) or the smallest angle of incidence ( = 0°, sample sets Ar_5 and Xe_5) the TL parameters are smaller than those of the other sample sets. The variation of the TL parameter ATL,1 is tentatively assigned to changes in the film density. The data in Fig. 7 suggest that the main impact comes again from the scattering geometry. The other best-fit parameters (Table 2) are comparable for both multi sample analyses, which indicates a similar electronic structure of all TiO2 films. Table 2 lists also TL parameters of TiO2 films grown by various techniques [58-60,62,63]. There is a good agreement with the data presented here, especially, for the band gap energy and the peak transition energy. The differences in the amplitude parameter are assigned to changes in the film density, which are related to the different energy of the film-forming particles, or the - 12 / 37 -
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model approach (one or two TL terms). For instance, the energy of the film-forming particles in atomic layer deposition (ALD) or
sol-gel technique (SGT) process is known to be
considerably smaller than those in magnetron sputtering (MS) or IBSD, which results in a smaller amplitude parameter as it is documented in Table 2. The supplementary material in Table A.1 summarizes also reference data of the band gap energy and transition energy of TiO2 bulk samples [67-69] and films [57,70-79]. Most of the groups documented a band gap energy between 3.2 eV and 3.4 eV for TiO2 films, without any systematic relation to the growth technique. Hence, the band gap energy parameter is very likely less affected by the properties of the film-forming particles, which is in agreement with the result of the ellipsometry analyses presented here (see above). Figure 8 shows selected spectra of the index of refraction in the spectral region of transparency (λ ≥ 390 nm), which illustrate a considerable variation of the optical properties. The index of refraction was found to be smaller than that of single-crystalline TiO2 (rutile [78,80]; anatase [68,69,78]) and poly-crystalline TiO2 (rutile [77,78,81]; anatase [69,77,78,81]). However, it is comparable to the data of amorphous TiO2 grown by sputter deposition techniques [7-9,14,17,20,21,72,73,81,82], and is larger than that of TiO2 grown by evaporation techniques [9,21,83-86]. Similarly, a former study of SiO2 and TiO2 films grown by reactive dual IBSD [20] described that an assisting Ar or Xe ion bombardment resulted in a decrease of the index of refraction. When increasing the ion energy, the index was reported to decrease. Hence, the growth parameters have to be chosen well when using TiO2 films for optical applications. The variations in the optical properties can be also be explained by considering film density variations caused by variations in the energy distribution of the secondary particles. Recently, it could be shown that the energy of the backscattered primary particles (and sputtered target particles) decreases systematically with increasing scattering angle and have a - 13 / 37 -
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systematic impact on film properties [44-47,49,50]. In general, increasing the particle energy is usually assumed to result in a higher film density [87,88], whereas a decrease of the film density, i.e. a decrease of the TL parameter ATL,1 or index of refraction, with increasing scattering angle was observed here. However, this behaviour is in agreement with results reported by Mohan et al. [88], who investigated and discussed the influence of the energy and current density of an assisting ion beam on the film properties of several oxides, including TiO2. A threshold energy was described above which the films become less dense. The threshold energy was reported to be about Eion = 600 eV for TiO2. Below that threshold energy the index of refraction of TiO2 films was reported to increase with increasing ion energy, above that to decrease with increasing ion energy. Similarly, a decrease of the index of refraction of SiO2 and TiO2 films with increasing ion energy (Eion ≥ 600 eV) of an assisting Ar or Xe ion beam was reported in Ref. 20. The decrease of film density with increasing particle energy could be explained, for instance, by the generation of vacancies. Mass density data versus scattering angle as obtained from XRR measurements are summarized in Fig. 9. The measured values range from 3.45 g/cm3 to 3.85 g/cm3, which is comparable to reference data of amorphous TiO2 films grown by sputter deposition techniques (ρ = (3.4 -3.8) g/cm3, [83]). Amorphous TiO2 films grown by sputter deposition techniques have a lower mass density than crystalline TiO2 (rutile: 4.24 g/cm3, anatase: 3.90 g/cm3, [89]), but a higher mass density than evaporated TiO2 films (ρ = (2.4 -3.3) g/cm3, [83-85]). The reason for that is related to the growth conditions or particle properties. Crystalline TiO2 is grown with substrate heating or is obtained upon post-growth annealing, which leads to a higher crystal quality and a higher film density, whereas evaporated films are less dense because of the lower energy of the film-forming particles. The curves in Fig. 9 look very similar for all sample sets grown by sputtering with Ar and those grown by sputtering with Xe: First, the mass density increases and then decreases - 14 / 37 -
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slightly with increasing scattering angle. However, some curves are slightly different from the others for scattering angles ≥ 120°, i.e. the curves of the sample sets grown with the lowest ion energy (Eion = 500 eV, sample set Ar_4,) or the smallest ion incidence angle ( = 0°, sample sets Ar_5 and Xe_5). All these facts are again assigned to the properties of the filmforming, secondary particles. Following the discussion above, mass density data should be correlated with optical properties, because a smaller packaging density results in a smaller mass density and a smaller optical density. In Fig. 10 the TL parameter ATL,1 is plotted over the mass density data. There is good correlation. Several groups reported before about the correlation between optical properties and mass density [83-85,90]. Typically, the index of refraction at a wavelength of λ = 550 nm is considered. Laube et al. [83] and Bendavid et al. [90] reported an almost linear relation for data of TiO2 films grown by different deposition techniques (see Fig. 11). These data fit quite well with results presented by Mergel at al. [85] for TiO2 films grown by evaporation and the data of this work, even though the absolute numbers are slightly smaller.
4. Summary TiO2 films were grown by ion beam sputter deposition and the influence of ion beam (ion energy, ion species) and geometrical parameters (angle of incidence, polar emission angle, and scattering angle) on film properties was studied. A strong correlation between film properties and growth parameters was observed. Film thickness and growth rate show an over-cosine shape, which is an indication of anisotropy effects in the evolution of the collision cascades inside the target. All TiO2 films are amorphous and stoichiometric. However, the films contain a considerable amount of implanted primary particles depending on the scattering geometry. There are also systematic variations in optical properties and mass - 15 / 37 -
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density, which are correlated with each other. The variations in the film properties are mainly influenced by the scattering geometry whereas there is only a small influence of ion energy, ion species, or ion incidence angle. The observations are assigned to changes in the angular and energy distribution of the film-forming, secondary particles.
Acknowledgements The authors are deeply grateful to G.E. Jellison Jr. (Oak Ridge National Laboratory, retired) for fruitful discussion related to application and interpretation of the TL model. Financial support by the Deutsche Forschungsgemeinschaft (DFG) within project BU2625/1-2 is gratefully acknowledged. The authors also thank I. Herold, M. Müller, F. Scholze, R. Woyciechowski (all IOM) and the IOM workshop for technical support.
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Appendices Supplementary material Table A.1: Reference data of band gap and transition energy of TiO2 bulk samples and films. Method1
Eg
E0
[eV]
[eV]
3.04 / 3.10 3
-
-
3.0 / 3.2 3
-
3.57 / 3.6 3
3.8, 4.8 /
Type
Crystal
Growth
Ref.
structure
technique2
Bulk
Anatase
CTR
[63]
R
Bulk
Anatase
CTR
[64]
SE
Bulk
Anatase
-
[65]
4.3, 5.05 3 2.90 / 2.88 3
-
-
Bulk
Rutile
CTR
[63]
3.84 / 3.51 3
4.1 / 4.3 3
SE
Bulk
Rutile
-
[65]
3.33
-
T
Film
Amorphous
ALD
[66]
3.30
-
SE/T
Film
Amorphous
MS
[67]
3.25
4.88
SE
Film
Amorphous
MS
[68]
3.38
-
SE
Film
Amorphous
MS
[69]
3.20
-
SE
Film
Amorphous
PVD
[53]
3.24 – 3.33
-
T/R
Film
Amorphous
VAD
[70]
3.30
-
SE
Film
Amorphous
VAD
[75]
3.26 – 3.29
-
T
Film
Amorphous
VAD
[71]
T
Film
Anatase
ALD
[66]
3.26 3.30 – 3.37
7.62 – 9.72
T
Film
Anatase
MS
[72]
3.75
-
SE
Film
Anatase
MS
[73]
3.34 / 3.13 4
-
SE
Film
Anatase
MS
[74]
3.40 – 3.48
-
T/R
Film
Anatase
VAD
[70]
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3.15
-
SE
Film
Anatase
VAD
[75]
3.39
5.09
SE
Film
Anatase/-
MS
[68]
Rutile mix 3.25
-
SE
Film
Rutile
MS
[73]
2.84 / 3.03 4
-
SE
Film
Rutile
MS
[74]
3.05
-
SE
Film
Rutile
VAD
[75]
1
Reflection (R) or transmission spectroscopy (T), spectroscopic ellipsometry (SE)
2
Atomic layer deposition (ALD), chemical transport reaction (CTR), magnetron sputtering
(MS), physical vapour deposition (PVD), vacuum arc deposition (VAD) 3
Given for polarization perpendicular/parallel to the optical axis.
4
Given for epitaxial/polycrystalline TiO2 films.
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Figure captions Figure 1
Figure 1: Schematic drawing of the ion beam sputter deposition setup.
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Figure 2 160 140 120
Ar_1 (60°, 1000 eV) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
(a)
Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
(b)
(c)
Xe_2 (30°, 1500 eV) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV)
(d)
dFilm [nm]
100 80 60 40 20 160 140 120
Xe_1 (60°, 1000 eV) Xe_3 (30°, 1000 eV) Xe_5 (0°, 1000 eV)
dFilm [nm]
100 80 60 40 20 0 -60
-30
0
30
60
[°]
90
-30
0
30
60
90
[°]
Figure 2: Film thickness versus polar emission angle of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d). Panels (a,c) and (b,d) summarize the results of the sample sets, which were grown under variation of the ion incidence angle or of the ion energy, respectively.
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Growth rate [nm/min]
Figure 3
0.6
(a)
-30 -40
-20 -10
0 10 20
(b)
30 40
0.4 0.2
50 [°] 60 70
-30 -40
0 10 20
30 40
80 90
0.0
(c) 1.0 0.8
-30 -40
-20 -10
0 10 20
50 [°] 60 70 80 90
Ar_1 (60°, 1000 eV) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
Growth rate [nm/min]
-20 -10
Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
(d) 30 40
0.6 0.4
50 [°] 60 70
-30 -40
-20 -10
0 10 20
30 40
50 [°] 60 70
0.2
80
80
0.0
90
90
Xe_1 (60°, 1000 eV) Xe_3 (30°, 1000 eV) Xe_5 (0°, 1000 eV)
Xe_2 (30°, 1500 eV) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV)
Figure 3: Growth rates versus polar emission angle of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d). The data were calculated considering the film thickness from Fig. 2 and the growth time given in Table 1. Please note the different scales in panels (a,b) and (c,d).
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Figure 4 0.22 (a)
0.20
(b)
0.18 y = 0.00062 x + 0.063
[nm]
0.16 0.14 0.12 0.10
Ar_1 (60°, 1000 eV) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
0.08
Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
0.22 0.20
(c)
y = 0.00095 x + 0.060
(d)
0.18
[nm]
0.16 0.14 0.12 0.10
Xe_1 (60°, 1000 eV) Xe_3 (30°, 1000 eV)
0.08 0.06 30
60
90
120
150
180
[°]
Xe_2 (30°, 1500 eV) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV) 60
90
120
150
180
[°]
Figure 4: Root mean square roughness versus scattering angle of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d). The dashed and solid lines are linear fits to all data in panels (a,b) and (c,d), respectively. The best-fit equations are given in panel (a) and (c).
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Figure 5 5
xAr [at. %]
4
Ar_1 (60°, 1000 eV) (a) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
Xe_1 (60°, 1000 eV) (c) Xe_3 (30°, 1000 eV) Xe_5 (0°, 1000 eV)
Xe_2 (30°, 1500 eV) (d) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV)
(b)
3 2 1 5
xXe [at. %]
4 3 2 1 0 30
60
90
120
150
180
[°]
60
90
120
150
180
[°]
Figure 5: Atomic fraction of inert gas atoms versus scattering angle of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d). The dotted and dashed lines are guides to the eye representing an approximation of the differential cross section of Rutherford scattering (~ sin(/2)-4).
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Figure 6
Figure 6: Selected spectra of the real (a,b) and imaginary part (c,d) of the dielectric function versus photon energy of selected samples of sample sets Ar_1 (a,c) and Xe_1 (b,d).
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Figure 7 240 220
Ar_1 (60°, 1000 eV) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
(a)
Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
(b)
(c)
Xe_2 (30°, 1500 eV) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV)
(d)
ATL,1
200 180 160 140 240 220
Xe_1 (60°, 1000 eV) Xe_3 (30°, 1000 eV) Xe_5 (0°, 1000 eV)
ATL,1
200 180 160 140 120 30
60
90
120
150
180
[°]
60
90
120
150
180
[°]
Figure 7: Best-fit TL-Parameter ATL,1 versus scattering angle of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d).
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C. Bundesmann et al., Manuscript for possible publication in Appl. Surf. Sci.
Figure 8 2.7 Ar_1 (60°, 1000 eV)
(a)
2.6
= 0° = 20° = 40° = 60° = 80°
n
2.5 2.4 2.3 2.2 2.7
Xe_1 (60°, 1000 eV)
(b) 2.6
= 0° = 20° = 40° = 60° = 80°
n
2.5 2.4 2.3 2.2 2.1 400
600
800
1000 1200 [nm]
1400
1600
Figure 8: Selected spectra of the index of refraction versus wavelength of selected samples of sample sets Ar_1 (a) and Xe_1 (b) in the spectral region of transparency.
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Figure 9 4.0 3.9
[g/cm3]
3.8
Ar_1 (60°, 1000 eV) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
(a)
(b)
Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
3.7 3.6 3.5 3.4 4.0 (c)
3.9
(d)
[g/cm3]
3.8 3.7 3.6 3.5 3.4 3.3 30
Xe_1 (60°, 1000 eV) Xe_3 (30°, 1000 eV) Xe_5 (0°, 1000 eV) 60
90
120
Xe_2 (30°, 1500 eV) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV) 150
180
[°]
60
90
120
150
180
[°]
Figure 9: Mass density versus scattering angle of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d).
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Figure 10 210 (a)
200
(b)
190
ATL,1 [eV]
180 170 160 150 Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
Ar_1 (60°, 1000 eV) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
140 130 210
(c)
200
(d)
190
ATL,1 [eV]
180
y = 111.5 x - 232.8
170 160 150 140 130 120 3.4
Xe_1 (60°, 1000 eV) Xe_3 (30°, 1000 eV) Xe_5 (0°, 1000 eV) 3.5
3.6
Xe_2 (30°, 1500 eV) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV)
3.7
3.8
3.9
4.0
[g/cm ]
3.5
3.6
3.7
3.8
3.9
4.0
[g/cm ]
3
3
Figure 10: TL parameter ATL,1 versus mass density of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d). The solid lines are a guide to the eye representing a linear fit to all data points. The best-fit equation is given in panel (d).
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Figure 11 2.8 2.7
n @ = 550 nm
2.6 2.5 2.4 2.3
Filtered arc deposition (Bendavid et al.) Sputtering (Laube et al.) Ion plating (Laube et al.) Plasma impulse CVD (Laube et al.) Evaporation (Laube et al.) Evaporation (Mergel et al.) IBSD (Ar, This work) IBSD (Xe, This work) Laube et al. Mergel et al.
2.2 2.1 2.0 1.9 2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
[g/cm3]
Figure 11: Index of refraction at λ = 550 nm versus mass density of TiO2 films grown by various deposition techniques. The reference data (open symbols) are taken from Ottermann et al. [84], Laube et al. [83], Bendavid et al. [90], Mergel at al. [85]. The lines are redrawn from Refs. 85 and 90.
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Tables Table 1: List of sample sets and main growth parameters. Sample set
Ion
Ion incidence
Ion
Polar emission
Growth
notation*
species*
angle
energy
angles**
time*
[°]
[eV]
[°]
[min]
Ar_1 / Xe_1
Ar / Xe
60
1000
-40 / 90 / 10
140 / 150
Ar_2 / Xe_2
Ar / Xe
30
1500
-10 / 90 / 10
190 / 150
Ar_3 / Xe_3
Ar / Xe
30
1000
-10 / 90 / 10
240 / 240
Ar_4 / Xe_4
Ar / Xe
30
500
-10 / 90 / 10
360 / 480
Ar_5 / Xe_5
Ar / Xe
0
1000
20 / 90 / 10
473 / 1440
* Given for sputtering with Ar/Xe ions ** Minimum / Maximum / Increment
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Table 2: Best-fit TL parameters of the multi sample analyses and reference data from amorphous TiO2 films. Error bars in parentheses represent the 90% confidence limits. ε∞
Eg,i
ATL,i
E0,i
BTL,i
[eV]
[eV]
[eV]
[eV]
0.90 (0.02) 3.25 (0.01) 139 – 200 (1) 2 4.17 (0.01) 1.43 (0.01) 96 (3)
0.3
Growth
Ref.
TL terms technique1 2
IBSD
MSA_Ar
2
IBSD
MSA_Xe
2
ALD
[55]
7.92 (0.05) 13.3 (0.5)
1.10 (0.02) 3.23 (0.01) 134 – 204 (1) 2 4.15 (0.01) 1.44 (0.01) 93 (3)
Number of
7.22 (0.05) 11.5 (0.4)
3.1
150
4.2
1.5
3.4
197
5.5
11
2.33
3.28
245
4.18
1.75
1
MS
[59]
2.55
3.26
226
4.08
1.55
1
MS
[58]
2.19 3
3.26 3
248 3
4.13 3
1.86 3
1
MS
[58]
2.52
2.32
36
4.51
0.5
1
PLD
[54]
3.11
2.58
57
4.29
1.0
1
PLD
[54]
2.1 – 3.2
2.8 – 3.1
45 – 105
4.2 – 4.5
0.4 – 1.1
1
SGT
[56]
1
Atomic layer deposition (ALD), magnetron sputtering (MS), pulsed laser deposition (PLD),
sol-gel technique (SGT). 2
See Fig. 7.
3
Results of the data analysis including a surface layer.
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S0169-4332(16)31698-1 http://dx.doi.org/doi:10.1016/j.apsusc.2016.08.056 APSUSC 33804
To appear in:
APSUSC
Received date: Revised date: Accepted date:
11-7-2016 9-8-2016 10-8-2016
Please cite this article as: C.Bundesmann, T.Lautenschl¨ager, D.Spemann, A.Finzel, E.Thelander, M.Mensing, F.Frost, Systematic investigation of the properties of TiO2 films grown by reactive ion beam sputter deposition, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.08.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
C. Bundesmann et al., Manuscript for possible publication in Appl. Surf. Sci.
Systematic investigation of the properties of TiO2 films grown by reactive ion beam sputter deposition C. Bundesmanna1
[email protected],
T. Lautenschlägera,
D. Spemanna,b,
A. Finzela, E. Thelandera, M. Mensinga, F. Frosta a
Leibniz-Institut für Oberflächenmodifizierung e.V., Permoserstraße 15, 04318 Leipzig,
Germany b
Universität Leipzig, Fakultät für Physik und Geowissenschaften, Institut für Experimentelle
Physik II, Linnéstraße 5, 04103 Leipzig, Germany 1
Corresponding author. Phone: +49-341-2353354; Fax: +49-341-2352595; Postal address:
Permoserstr. 15, 04318 Leipzig, Germany.
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Highlights TiO2 films grown by ion beam sputter deposition Structure, surface topography, composition, optical properties, mass density Films are amorphous and surface roughness is correlated with scattering angle Incorporation of inert process gas correlated with scattering geometry Correlation of mass density and optical properties, both also with scattering angle
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Abstract TiO2 films were grown by ion beam sputter deposition under systematic variation of ion beam parameters (ion species, ion energy) and geometrical parameters (ion incidence angle, polar emission angle) and characterized with respect to film thickness, growth rate, structural properties, surface topography, composition, optical properties, and mass density. The angular distributions of film thickness and growth rate show an over-cosine shape, which is tilted in forward direction. All films are amorphous and the surface roughness is below 0.22 nm. The investigation of the composition revealed stoichiometric TiO2 with implanted backscattered primary particles. The optical properties were analysed using the Tauc Lorentz (TL) model. The amplitude parameter of the TL model was found to vary systematically with the scattering angle, whereas the impact on the other TL parameters is negligible. Mass density follows the same trends as the optical properties, i.e. optical properties and mass density are correlated. Surface roughness, atomic fraction of implanted primary particles, optical properties and mass density show similar systematic variations with process parameters, especially, with the scattering geometry (i.e. scattering angle). Ion species, ion energy and ion incidence angle have no or only a small impact. The variations in the film properties are tentatively assigned to changes in the angular and energy distribution of the sputtered target particles and backscattered primary particles.
Keywords: Ion beam sputter deposition, TiO2, optical properties, mass density, composition, implantation, root mean square roughness
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1. Introduction Ion beam sputter deposition (IBSD) is a widely used physical vapour deposition (PVD) technique for thin film production. In contrast to other PVD techniques, for instance, magnetron sputtering or evaporation techniques, IBSD offers more degrees of freedom for tailoring the properties of the secondary, film-forming particles and, hence, the properties of the films [1]. Though widely used, the capabilities of IBSD are not completely understood and utilized yet. The focus of this work is the investigation of the IBSD process of TiO2 films, i.e. the exploration of the correlation between ion beam parameters (ion species, ion energy) and geometrical parameters (ion incidence angle and polar emission angle), and film properties. TiO2 films grown by PVD techniques are typically amorphous or poly-crystalline, and are commonly used for optical applications, e.g. multilayer coatings for highly reflective or anti-reflective applications. These applications exploit the high index of refraction of TiO2 (n ~ 2.4). When combining high-index TiO2 films with low-index materials, e.g. SiO2, interference effects can be used. TiO2 films grown with substrate heating or treated by postgrowth annealing are crystalline. Crystalline TiO2 is a semiconductor with a wide band gap which is used in solar cells, but can also exhibit photocatalysis or photoinduced superhydrophilicity [2-5]. Several groups reported on the ion beam sputter deposition of TiO2 films [6-23], mixed films of TiO2-SnO2 [24,25], TiO2-SiO2 [26,27], TiO2-Ta2O5 [28] or TiO2-xNx [29], or even Ge quantum dots in TiO2 [30]. Some of them also consider the influence of substrate temperature [9-11,15,18,19],
thermal
annealing
[13,14,17,21,22]
or
assisting
ion
bombardment [9,14,16,20,23]. The main focus of these works was the investigation of structural and optical properties of reactively sputtered films with unheated substrates. All groups used a fixed sputter geometry (one set of ion incidence angle and polar emission - 4 / 37 -
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angle), and, typically, only a single ion energy of about Eion = 1000 eV and Ar as sputter gas. There exist only two reports on the investigation of the influence of the energy of sputtering and assisting ions (Sputter source: Ar ions, Eion = 800 eV – 1500 eV; assist source: Ar ions, Eion = 200 eV – 300 eV [9]; Sputter source: Ar or Xe ions, Eion = 600 eV – 1200 eV; assist source: Ar or Xe ions, Eion = 600 eV – 1600 eV [20]). The lack of reference data is very likely related to the following facts: (i) The experimental setup does not allow to vary the scattering geometry and/or ion beam parameters. (ii) The angular and energy distribution of the sputtered target particles is commonly believed to be well described by Thompson’s formula [31,32]. This model postulates a cosine-shaped angular distribution and an energy distribution that is independent from the energy of the primary ions. It also does not consider the ion incidence angle, because it is based on the assumption of normally incident ions. Thompson’s formula was reported often to describe experimental findings quite well. However, there are also results that deviate from Thompson’s model due to anisotropy effects that are caused by ions hitting the target at oblique incidence or with low energy [33-44]. These anisotropy effects are strongly related to ion energy, ion species (or the ratio of the mass of the projectile to the mass of the target particle) and ion incidence angle [41-47], but also to the surface topography of the target [48]. (iii) The influence of backscattered primary particles is underestimated or neglected. Recently, it was shown that these particles can retain a large energy and have a significant impact on film properties [44-47,49,50]. Therefore, an experiment was set up which offers the opportunity for a systematic variation of ion beam and geometrical parameters for film growth. The capabilities of the setup were already demonstrated for the elemental materials Ag and Ge [44-47,49,50]. First experimental results for TiO2 films grown by sputtering with Ar were published in Ref. 51. Now the investigations are extended to TiO2 grown by sputtering with Xe. Furthermore, - 5 / 37 -
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additional characterization results, and a more sophisticated analysis and discussion of the optical data are presented. Results concerning the angular and energy distribution of secondary, film-forming particles are described and discussed elsewhere [52].
2. Experimental details Figure 1 shows a schematic sketch of the IBSD setup inside the deposition chamber. It consists of a broad beam ion source, a target holder and a substrate holder. The ion beam source and the target holder are placed on rotary tables, which have their centre of rotation at the centre of the target surface plane. Additionally, an energy-selective mass spectrometer can be used to measure the energy distribution of sputtered target ions and backscattered primary ions. The deposition chamber is of rectangular shape with a dimension of 1 x 1 x 0.7 m3. It is pumped by a 2200 l/s turbo molecular pump to a base pressure of 2x10-6 mbar. The working pressure during sputtering is about 5 x 10-5 mbar. The ion beam source is of radio-frequency (RF) type with a three-grid multi-aperture extraction system with an open diameter of 16 mm [53]. The process gases were Ar or Xe with a volumetric flow rate of 3.5 sccm or 1.1 sccm, respectively. The RF power was set to 70 W for all experiments resulting in a total ion current of about 7 mA. The distance between the exit plane of the ion beam source and the target centre is about 0.15 m, i.e. it is much smaller than mean free path length of the primary ions (Ar: 1.28 m; Xe: 0.72 m) The substrate holder is of semi-circular shape with a radius of curvature of 0.15 m. The holder is segmented such that substrates up to 15 x 15 mm2 can be placed at different polar emission angles in steps of 10°. Several sets of TiO2 films were grown under variation of ion energy, ion species and ion incidence angle on unheated Si (100) substrates with a size of about 12 x 12 mm2. Tab. 1 - 6 / 37 -
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summarizes the main growth parameters for all sample sets. A poly-crystalline Ti target with a purity of 99.99% was used. The deposition was done in oxygen atmosphere with an oxygen volumetric flow rate of 2 sccm. The resulting oxygen partial pressure was about 1.5 x 10-5 mbar. The TiO2 films were characterized with respect to film thickness, growth rate, structural properties, surface roughness, composition, optical properties, and mass density. Film thickness was measured by spectroscopic ellipsometer (and x-ray reflectometry). The growth rate was calculated upon dividing film thickness by the sputter time listed in Table 1. Spectra of the ellipsometric parameters Ψ and Δ were measured in the photon energy range from Ephoton = 0.73 eV to Ephoton = 6.4 eV (or wavelength range from λ = 193 nm to λ = 1700 nm) at four angles of incidence (60°, 65°, 70° and 75°) with an ellipsometer RC2-DI® (J.A. Woollam Co., Inc.) of dual-rotating-compensator type. A four-layer optical model (Air/TiO2/native SiO2/Si) was used for the data analysis. The optical properties of the native SiO2 layer and the Si substrate were taken from Ref. 54, the optical properties of the TiO2 films were modelled using the following complex dielectric function (DF) ̃ It consists of the constant term Tauc-Lorentz (TL) term ̃
̃
(1)
(high-frequency dielectric constant) and a complex . The TL model is commonly used to model the
spectral region of the interband transitions of amorphous semiconductors [55,56], as for instance, for amorphous TiO2 [57-63].
accounts for electronic contributions outside the
spectral region of investigation. The TL model is a combination of the Tauc joint density of states and the classical Lorentz oscillator [55]. The imaginary part of the complex DF reads
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∑
(
)
(2)
ATL,i, BTL,i and E0,i are amplitudes, broadening terms and peak transition energies, respectively. A set of two TL terms was used here (i = 1,2). The optical band gap Eg was assumed to be the same for both TL terms. The real part of the DF ε1,TL is obtained via Kramers-Kronig integration. The reader is referred to Ref. 55 for an analytical expression of ε1,TL. The complex DF ̃ is related to the complex index of refraction ̃ by ̃
̃
(3)
n and k are the (real) index of refraction and the absorption coefficient, respectively. Thickness d and TL parameters of each TiO2 film were fit parameters during the model lineshape analysis of the measured spectra. Two multi sample analyses (MSA) were done, i.e. the ellipsometric spectra of all samples grown by sputtering with Ar (MSA_Ar) or Xe (MSA_Xe) were modelled simultaneously. There were 50 data sets for each multi sample analysis. In order to avoid unwanted correlation of fit parameters, only the film thickness and the TL parameter ATL were allowed to vary independently for all samples, whereas other parameters were coupled, i.e. they were assumed to be the same for all simultaneously analysed data sets. Mueller-Matrix measurements on selected samples have shown no sign of optical anisotropy. Therefore, the use of an isotropic model dielectric function is justified. Structural analysis of the samples was done by X-ray diffraction (XRD). For this purpose, a diffractometer (Ultima IV Type III, Rigaku Corp., Japan) was used. It is equipped with a Cu anode X-ray tube, a graded multilayer mirror to obtain a parallel beam, a theta-theta goniometer, a parallel slit analyser and a scintillation detector.
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The surface topography was measured by scanning atomic force microscopy (AFM) with a large sample scanning force microscope from Bruker (Dimension icon®). The device was operated in Tapping ModeTM and in xy-closed loop configuration. For measuring the surface topography the z-sensor signal is utilized. The measurements were performed in air using silicon probes with a nominal tip radius smaller than 5 nm. A scanning size of 10 µm x 10 µm was used with a pixel resolution of 1024 x 1024. The AFM raw data were processed by a line-wise levelling (flatten) using a polynomial of 1st order in order to realize a sample tilt / offset compensation and to remove noise along the slow scan direction. Finally, from the processed images the root mean square (rms) roughness was calculated. The noise floor level during the measurements was ≤ 0.05 nm rms. Rutherford backscattering spectrometry (RBS) measurements were performed in order to investigate the composition of the TiO2 films, especially the stoichiometry and the amount of implanted inert gas particles. The experiments were done with single charged He ions with an energy of 2 MeV at the LIPSION facility [64]. Mass density was measured using x-ray reflectometry (XRR) using a Seifert XRD 3003 PTS. A parallelized Cu-Kα1 beam (λ = 0.15406 nm) with low divergence (0.005°) was used. The measured curves were analysed with the Rayflex Analyze software using a nonlinear least square fit routine under the assumption of stoichiometric TiO2. In this way, it was possible to extract mass density values and film thickness with an error less than 0.1 g/cm3 and 1.0 nm, respectively.
3. Results and discussion The film thickness data of the TiO2 films versus polar emission angle are summarized in Fig. 2. The presented data were taken from the analyses of the ellipsometric data. The results from the XRR measurements agree excellently with those from the multi sample - 9 / 37 -
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analyses. The growth time was chosen such that the maximum thickness of the sample sets is about 100 nm in order to make the following results comparable. The thickness distributions are of over-cosine shape and tilted in forward direction (βmax > 40°). The principal shape, especially the emission angle βmax of the thickness maximum, is very similar for sputtering with Ar or Xe with same growth parameters. The thickness maximum varies between βmax ~ 40° for sample sets Ar_1 and Xe_1 ( = 60°, Eion = 1000 eV), and βmax ~ 60° for sample sets Ar_4 and Xe_4 ( = 30°, Eion = 500 eV). The over-cosine shape is related to anisotropy effects caused by an incomplete evolution of collision cascade inside the target [33,41-43]. This anisotropy increases with decreasing ion energy and increasing ion incidence angle. It is also affected by the ratio of the mass of the projectile to the mass of the target particle. However, the shape of the thickness distributions in Fig. 2 is not symmetric, which is related to isotropic contributions [52]. Figure 3 depicts the growth rate data versus polar emission angle. As expected, the growth rate increases with increasing ion energy and increasing ion incidence angle, and is higher for sputtering with Xe than for sputtering with Ar. These observations can be explained by the corresponding behaviour of the total sputter yield [1]. XRD revealed that all TiO2 films are amorphous. The surface was found to be very smooth, i.e. the root mean square (rms) roughness was σ ≤ 0.22 nm. The rms roughness (see Fig. 4) increases with increasing scattering angle = 180° - β - . It seems to be unaffected by the ion incidence angle (Fig. 4 (a,c)), decreases slightly with increasing ion energy, and is higher for sputtering with Xe than for sputtering with Ar. All this can be explained by considering the energy distribution of the secondary particles, especially, of the backscattered primary particles [44,46,47,52]. The average energy of the backscattered primary particles increases with decreasing polar emission angle, increasing ion energy and is higher for Ar
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particles than for Xe particles. A higher particle energy results in a higher surface mobility causing a ballistic drift and, hence, in smoother surfaces [65]. The analyses of RBS measurements showed that the films are stoichiometric, i.e. the ratio Ti:O equals 1:2. Additionally, Ar or Xe particles were found inside the TiO2 films grown by sputtering with Ar ions or Xe ions, respectively. Figure 5 depicts the atomic fraction of primary particles versus scattering angle. The curves of all samples sets, which were grown by sputtering with Ar, match perfectly (Fig 5 (a,b)). The same holds for the sample sets grown by sputtering with Xe (Fig. 5 (c,d)), even though the numbers are smaller than those in Fig. 5 (a,b). The incorporated inert gas particles are assigned to implantation of energetic particles (see above), because SRIM calculations showed that an energy of about 40 eV for Ar or 14 eV for Xe is sufficient for a projected range of 0.5 nm in TiO2 [66]. The atomic fraction of implanted primary particles depends mainly on the scattering geometry but also on primary ion species. The same observation was made for Ge films [50] and can be explained by a dynamic equilibrium of competing processes at the growing film (implantation, diffusion, resputtering) and by the fact that the maximum scattering angle for direct backscattering of Xe particles at Ti particles ( ≤ 21°) is smaller than that of direct backscattering of Ar at Ti ( ≤ 180°) due to the different mass ratio of the interacting particles (mAr = 39.9 amu, mTi = 47.9 amu, mXe = 131.3 amu). The relation between the number of implanted primary particles versus scattering angle can be, in parts, qualitatively described by the differential cross section of Rutherford scattering which is proportional to sin(/2)-4 (see Fig. 5). There are some deviations for small scattering angles, which can be explained by the corresponding larger distance of closest approach in the scattering event which results in stronger shielding of the nuclear charges Z1e, Z2e of the colliding atoms by their respective electron shells. As the Rutherford cross section is proportional to (Z1Z2)2 an increased shielding leads to a reduction of the scattering cross section and hence to lower concentrations of the scattered - 11 / 37 -
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primary particles in the film than predicted from the sin(/2)-4 angular dependence alone. Furthermore, there are not only contributions by primary particles backscattered at target and implanted primary particles but also by sputtered, previously implanted primary particles with sufficiently high energy. The proposed TL model with two terms described the experimental ellipsometric spectra very well. Figure 6 shows selected dielectric function spectra of sample sets Ar_1 and Xe_1 as extracted from the multi sample analyses. The data analysis revealed a strong influence of the TL parameter ATL on the quality of the fit results, whereas the uncoupled variation of all other parameters has no or only a negligible impact. Therefore, these parameters were coupled. Figure 7 summarizes the best-fit TL parameter ATL,1 of all sample sets as extracted from the multi sample analyses of the ellipsometric data. The other best-fit parameters are given in Table 2. The curves in Fig. 7 look very similar for all sample sets grown by sputtering with Ar or Xe: an increase with increasing scattering angle followed by a slight decrease. Only for the sample sets grown at the lowest ion energy (Eion = 500 eV, sample sets Ar_4 and Xe_4) or the smallest angle of incidence ( = 0°, sample sets Ar_5 and Xe_5) the TL parameters are smaller than those of the other sample sets. The variation of the TL parameter ATL,1 is tentatively assigned to changes in the film density. The data in Fig. 7 suggest that the main impact comes again from the scattering geometry. The other best-fit parameters (Table 2) are comparable for both multi sample analyses, which indicates a similar electronic structure of all TiO2 films. Table 2 lists also TL parameters of TiO2 films grown by various techniques [58-60,62,63]. There is a good agreement with the data presented here, especially, for the band gap energy and the peak transition energy. The differences in the amplitude parameter are assigned to changes in the film density, which are related to the different energy of the film-forming particles, or the - 12 / 37 -
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model approach (one or two TL terms). For instance, the energy of the film-forming particles in atomic layer deposition (ALD) or
sol-gel technique (SGT) process is known to be
considerably smaller than those in magnetron sputtering (MS) or IBSD, which results in a smaller amplitude parameter as it is documented in Table 2. The supplementary material in Table A.1 summarizes also reference data of the band gap energy and transition energy of TiO2 bulk samples [67-69] and films [57,70-79]. Most of the groups documented a band gap energy between 3.2 eV and 3.4 eV for TiO2 films, without any systematic relation to the growth technique. Hence, the band gap energy parameter is very likely less affected by the properties of the film-forming particles, which is in agreement with the result of the ellipsometry analyses presented here (see above). Figure 8 shows selected spectra of the index of refraction in the spectral region of transparency (λ ≥ 390 nm), which illustrate a considerable variation of the optical properties. The index of refraction was found to be smaller than that of single-crystalline TiO2 (rutile [78,80]; anatase [68,69,78]) and poly-crystalline TiO2 (rutile [77,78,81]; anatase [69,77,78,81]). However, it is comparable to the data of amorphous TiO2 grown by sputter deposition techniques [7-9,14,17,20,21,72,73,81,82], and is larger than that of TiO2 grown by evaporation techniques [9,21,83-86]. Similarly, a former study of SiO2 and TiO2 films grown by reactive dual IBSD [20] described that an assisting Ar or Xe ion bombardment resulted in a decrease of the index of refraction. When increasing the ion energy, the index was reported to decrease. Hence, the growth parameters have to be chosen well when using TiO2 films for optical applications. The variations in the optical properties can be also be explained by considering film density variations caused by variations in the energy distribution of the secondary particles. Recently, it could be shown that the energy of the backscattered primary particles (and sputtered target particles) decreases systematically with increasing scattering angle and have a - 13 / 37 -
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systematic impact on film properties [44-47,49,50]. In general, increasing the particle energy is usually assumed to result in a higher film density [87,88], whereas a decrease of the film density, i.e. a decrease of the TL parameter ATL,1 or index of refraction, with increasing scattering angle was observed here. However, this behaviour is in agreement with results reported by Mohan et al. [88], who investigated and discussed the influence of the energy and current density of an assisting ion beam on the film properties of several oxides, including TiO2. A threshold energy was described above which the films become less dense. The threshold energy was reported to be about Eion = 600 eV for TiO2. Below that threshold energy the index of refraction of TiO2 films was reported to increase with increasing ion energy, above that to decrease with increasing ion energy. Similarly, a decrease of the index of refraction of SiO2 and TiO2 films with increasing ion energy (Eion ≥ 600 eV) of an assisting Ar or Xe ion beam was reported in Ref. 20. The decrease of film density with increasing particle energy could be explained, for instance, by the generation of vacancies. Mass density data versus scattering angle as obtained from XRR measurements are summarized in Fig. 9. The measured values range from 3.45 g/cm3 to 3.85 g/cm3, which is comparable to reference data of amorphous TiO2 films grown by sputter deposition techniques (ρ = (3.4 -3.8) g/cm3, [83]). Amorphous TiO2 films grown by sputter deposition techniques have a lower mass density than crystalline TiO2 (rutile: 4.24 g/cm3, anatase: 3.90 g/cm3, [89]), but a higher mass density than evaporated TiO2 films (ρ = (2.4 -3.3) g/cm3, [83-85]). The reason for that is related to the growth conditions or particle properties. Crystalline TiO2 is grown with substrate heating or is obtained upon post-growth annealing, which leads to a higher crystal quality and a higher film density, whereas evaporated films are less dense because of the lower energy of the film-forming particles. The curves in Fig. 9 look very similar for all sample sets grown by sputtering with Ar and those grown by sputtering with Xe: First, the mass density increases and then decreases - 14 / 37 -
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slightly with increasing scattering angle. However, some curves are slightly different from the others for scattering angles ≥ 120°, i.e. the curves of the sample sets grown with the lowest ion energy (Eion = 500 eV, sample set Ar_4,) or the smallest ion incidence angle ( = 0°, sample sets Ar_5 and Xe_5). All these facts are again assigned to the properties of the filmforming, secondary particles. Following the discussion above, mass density data should be correlated with optical properties, because a smaller packaging density results in a smaller mass density and a smaller optical density. In Fig. 10 the TL parameter ATL,1 is plotted over the mass density data. There is good correlation. Several groups reported before about the correlation between optical properties and mass density [83-85,90]. Typically, the index of refraction at a wavelength of λ = 550 nm is considered. Laube et al. [83] and Bendavid et al. [90] reported an almost linear relation for data of TiO2 films grown by different deposition techniques (see Fig. 11). These data fit quite well with results presented by Mergel at al. [85] for TiO2 films grown by evaporation and the data of this work, even though the absolute numbers are slightly smaller.
4. Summary TiO2 films were grown by ion beam sputter deposition and the influence of ion beam (ion energy, ion species) and geometrical parameters (angle of incidence, polar emission angle, and scattering angle) on film properties was studied. A strong correlation between film properties and growth parameters was observed. Film thickness and growth rate show an over-cosine shape, which is an indication of anisotropy effects in the evolution of the collision cascades inside the target. All TiO2 films are amorphous and stoichiometric. However, the films contain a considerable amount of implanted primary particles depending on the scattering geometry. There are also systematic variations in optical properties and mass - 15 / 37 -
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density, which are correlated with each other. The variations in the film properties are mainly influenced by the scattering geometry whereas there is only a small influence of ion energy, ion species, or ion incidence angle. The observations are assigned to changes in the angular and energy distribution of the film-forming, secondary particles.
Acknowledgements The authors are deeply grateful to G.E. Jellison Jr. (Oak Ridge National Laboratory, retired) for fruitful discussion related to application and interpretation of the TL model. Financial support by the Deutsche Forschungsgemeinschaft (DFG) within project BU2625/1-2 is gratefully acknowledged. The authors also thank I. Herold, M. Müller, F. Scholze, R. Woyciechowski (all IOM) and the IOM workshop for technical support.
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Appendices Supplementary material Table A.1: Reference data of band gap and transition energy of TiO2 bulk samples and films. Method1
Eg
E0
[eV]
[eV]
3.04 / 3.10 3
-
-
3.0 / 3.2 3
-
3.57 / 3.6 3
3.8, 4.8 /
Type
Crystal
Growth
Ref.
structure
technique2
Bulk
Anatase
CTR
[63]
R
Bulk
Anatase
CTR
[64]
SE
Bulk
Anatase
-
[65]
4.3, 5.05 3 2.90 / 2.88 3
-
-
Bulk
Rutile
CTR
[63]
3.84 / 3.51 3
4.1 / 4.3 3
SE
Bulk
Rutile
-
[65]
3.33
-
T
Film
Amorphous
ALD
[66]
3.30
-
SE/T
Film
Amorphous
MS
[67]
3.25
4.88
SE
Film
Amorphous
MS
[68]
3.38
-
SE
Film
Amorphous
MS
[69]
3.20
-
SE
Film
Amorphous
PVD
[53]
3.24 – 3.33
-
T/R
Film
Amorphous
VAD
[70]
3.30
-
SE
Film
Amorphous
VAD
[75]
3.26 – 3.29
-
T
Film
Amorphous
VAD
[71]
T
Film
Anatase
ALD
[66]
3.26 3.30 – 3.37
7.62 – 9.72
T
Film
Anatase
MS
[72]
3.75
-
SE
Film
Anatase
MS
[73]
3.34 / 3.13 4
-
SE
Film
Anatase
MS
[74]
3.40 – 3.48
-
T/R
Film
Anatase
VAD
[70]
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3.15
-
SE
Film
Anatase
VAD
[75]
3.39
5.09
SE
Film
Anatase/-
MS
[68]
Rutile mix 3.25
-
SE
Film
Rutile
MS
[73]
2.84 / 3.03 4
-
SE
Film
Rutile
MS
[74]
3.05
-
SE
Film
Rutile
VAD
[75]
1
Reflection (R) or transmission spectroscopy (T), spectroscopic ellipsometry (SE)
2
Atomic layer deposition (ALD), chemical transport reaction (CTR), magnetron sputtering
(MS), physical vapour deposition (PVD), vacuum arc deposition (VAD) 3
Given for polarization perpendicular/parallel to the optical axis.
4
Given for epitaxial/polycrystalline TiO2 films.
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Figure captions Figure 1
Figure 1: Schematic drawing of the ion beam sputter deposition setup.
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Figure 2 160 140 120
Ar_1 (60°, 1000 eV) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
(a)
Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
(b)
(c)
Xe_2 (30°, 1500 eV) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV)
(d)
dFilm [nm]
100 80 60 40 20 160 140 120
Xe_1 (60°, 1000 eV) Xe_3 (30°, 1000 eV) Xe_5 (0°, 1000 eV)
dFilm [nm]
100 80 60 40 20 0 -60
-30
0
30
60
[°]
90
-30
0
30
60
90
[°]
Figure 2: Film thickness versus polar emission angle of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d). Panels (a,c) and (b,d) summarize the results of the sample sets, which were grown under variation of the ion incidence angle or of the ion energy, respectively.
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Growth rate [nm/min]
Figure 3
0.6
(a)
-30 -40
-20 -10
0 10 20
(b)
30 40
0.4 0.2
50 [°] 60 70
-30 -40
0 10 20
30 40
80 90
0.0
(c) 1.0 0.8
-30 -40
-20 -10
0 10 20
50 [°] 60 70 80 90
Ar_1 (60°, 1000 eV) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
Growth rate [nm/min]
-20 -10
Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
(d) 30 40
0.6 0.4
50 [°] 60 70
-30 -40
-20 -10
0 10 20
30 40
50 [°] 60 70
0.2
80
80
0.0
90
90
Xe_1 (60°, 1000 eV) Xe_3 (30°, 1000 eV) Xe_5 (0°, 1000 eV)
Xe_2 (30°, 1500 eV) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV)
Figure 3: Growth rates versus polar emission angle of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d). The data were calculated considering the film thickness from Fig. 2 and the growth time given in Table 1. Please note the different scales in panels (a,b) and (c,d).
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Figure 4 0.22 (a)
0.20
(b)
0.18 y = 0.00062 x + 0.063
[nm]
0.16 0.14 0.12 0.10
Ar_1 (60°, 1000 eV) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
0.08
Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
0.22 0.20
(c)
y = 0.00095 x + 0.060
(d)
0.18
[nm]
0.16 0.14 0.12 0.10
Xe_1 (60°, 1000 eV) Xe_3 (30°, 1000 eV)
0.08 0.06 30
60
90
120
150
180
[°]
Xe_2 (30°, 1500 eV) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV) 60
90
120
150
180
[°]
Figure 4: Root mean square roughness versus scattering angle of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d). The dashed and solid lines are linear fits to all data in panels (a,b) and (c,d), respectively. The best-fit equations are given in panel (a) and (c).
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Figure 5 5
xAr [at. %]
4
Ar_1 (60°, 1000 eV) (a) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
Xe_1 (60°, 1000 eV) (c) Xe_3 (30°, 1000 eV) Xe_5 (0°, 1000 eV)
Xe_2 (30°, 1500 eV) (d) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV)
(b)
3 2 1 5
xXe [at. %]
4 3 2 1 0 30
60
90
120
150
180
[°]
60
90
120
150
180
[°]
Figure 5: Atomic fraction of inert gas atoms versus scattering angle of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d). The dotted and dashed lines are guides to the eye representing an approximation of the differential cross section of Rutherford scattering (~ sin(/2)-4).
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Figure 6
Figure 6: Selected spectra of the real (a,b) and imaginary part (c,d) of the dielectric function versus photon energy of selected samples of sample sets Ar_1 (a,c) and Xe_1 (b,d).
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Figure 7 240 220
Ar_1 (60°, 1000 eV) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
(a)
Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
(b)
(c)
Xe_2 (30°, 1500 eV) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV)
(d)
ATL,1
200 180 160 140 240 220
Xe_1 (60°, 1000 eV) Xe_3 (30°, 1000 eV) Xe_5 (0°, 1000 eV)
ATL,1
200 180 160 140 120 30
60
90
120
150
180
[°]
60
90
120
150
180
[°]
Figure 7: Best-fit TL-Parameter ATL,1 versus scattering angle of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d).
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Figure 8 2.7 Ar_1 (60°, 1000 eV)
(a)
2.6
= 0° = 20° = 40° = 60° = 80°
n
2.5 2.4 2.3 2.2 2.7
Xe_1 (60°, 1000 eV)
(b) 2.6
= 0° = 20° = 40° = 60° = 80°
n
2.5 2.4 2.3 2.2 2.1 400
600
800
1000 1200 [nm]
1400
1600
Figure 8: Selected spectra of the index of refraction versus wavelength of selected samples of sample sets Ar_1 (a) and Xe_1 (b) in the spectral region of transparency.
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Figure 9 4.0 3.9
[g/cm3]
3.8
Ar_1 (60°, 1000 eV) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
(a)
(b)
Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
3.7 3.6 3.5 3.4 4.0 (c)
3.9
(d)
[g/cm3]
3.8 3.7 3.6 3.5 3.4 3.3 30
Xe_1 (60°, 1000 eV) Xe_3 (30°, 1000 eV) Xe_5 (0°, 1000 eV) 60
90
120
Xe_2 (30°, 1500 eV) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV) 150
180
[°]
60
90
120
150
180
[°]
Figure 9: Mass density versus scattering angle of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d).
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Figure 10 210 (a)
200
(b)
190
ATL,1 [eV]
180 170 160 150 Ar_2 (30°, 1500 eV) Ar_3 (30°, 1000 eV) Ar_4 (30°, 500 eV)
Ar_1 (60°, 1000 eV) Ar_3 (30°, 1000 eV) Ar_5 (0°, 1000 eV)
140 130 210
(c)
200
(d)
190
ATL,1 [eV]
180
y = 111.5 x - 232.8
170 160 150 140 130 120 3.4
Xe_1 (60°, 1000 eV) Xe_3 (30°, 1000 eV) Xe_5 (0°, 1000 eV) 3.5
3.6
Xe_2 (30°, 1500 eV) Xe_3 (30°, 1000 eV) Xe_4 (30°, 500 eV)
3.7
3.8
3.9
4.0
[g/cm ]
3.5
3.6
3.7
3.8
3.9
4.0
[g/cm ]
3
3
Figure 10: TL parameter ATL,1 versus mass density of the sample sets grown by sputtering with Ar ions (a,b) and Xe ions (c,d). The solid lines are a guide to the eye representing a linear fit to all data points. The best-fit equation is given in panel (d).
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Figure 11 2.8 2.7
n @ = 550 nm
2.6 2.5 2.4 2.3
Filtered arc deposition (Bendavid et al.) Sputtering (Laube et al.) Ion plating (Laube et al.) Plasma impulse CVD (Laube et al.) Evaporation (Laube et al.) Evaporation (Mergel et al.) IBSD (Ar, This work) IBSD (Xe, This work) Laube et al. Mergel et al.
2.2 2.1 2.0 1.9 2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
[g/cm3]
Figure 11: Index of refraction at λ = 550 nm versus mass density of TiO2 films grown by various deposition techniques. The reference data (open symbols) are taken from Ottermann et al. [84], Laube et al. [83], Bendavid et al. [90], Mergel at al. [85]. The lines are redrawn from Refs. 85 and 90.
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Tables Table 1: List of sample sets and main growth parameters. Sample set
Ion
Ion incidence
Ion
Polar emission
Growth
notation*
species*
angle
energy
angles**
time*
[°]
[eV]
[°]
[min]
Ar_1 / Xe_1
Ar / Xe
60
1000
-40 / 90 / 10
140 / 150
Ar_2 / Xe_2
Ar / Xe
30
1500
-10 / 90 / 10
190 / 150
Ar_3 / Xe_3
Ar / Xe
30
1000
-10 / 90 / 10
240 / 240
Ar_4 / Xe_4
Ar / Xe
30
500
-10 / 90 / 10
360 / 480
Ar_5 / Xe_5
Ar / Xe
0
1000
20 / 90 / 10
473 / 1440
* Given for sputtering with Ar/Xe ions ** Minimum / Maximum / Increment
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Table 2: Best-fit TL parameters of the multi sample analyses and reference data from amorphous TiO2 films. Error bars in parentheses represent the 90% confidence limits. ε∞
Eg,i
ATL,i
E0,i
BTL,i
[eV]
[eV]
[eV]
[eV]
0.90 (0.02) 3.25 (0.01) 139 – 200 (1) 2 4.17 (0.01) 1.43 (0.01) 96 (3)
0.3
Growth
Ref.
TL terms technique1 2
IBSD
MSA_Ar
2
IBSD
MSA_Xe
2
ALD
[55]
7.92 (0.05) 13.3 (0.5)
1.10 (0.02) 3.23 (0.01) 134 – 204 (1) 2 4.15 (0.01) 1.44 (0.01) 93 (3)
Number of
7.22 (0.05) 11.5 (0.4)
3.1
150
4.2
1.5
3.4
197
5.5
11
2.33
3.28
245
4.18
1.75
1
MS
[59]
2.55
3.26
226
4.08
1.55
1
MS
[58]
2.19 3
3.26 3
248 3
4.13 3
1.86 3
1
MS
[58]
2.52
2.32
36
4.51
0.5
1
PLD
[54]
3.11
2.58
57
4.29
1.0
1
PLD
[54]
2.1 – 3.2
2.8 – 3.1
45 – 105
4.2 – 4.5
0.4 – 1.1
1
SGT
[56]
1
Atomic layer deposition (ALD), magnetron sputtering (MS), pulsed laser deposition (PLD),
sol-gel technique (SGT). 2
See Fig. 7.
3
Results of the data analysis including a surface layer.
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