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A combinatorial chemical beam vapour deposition approach to tune the electrical conductivity of Nb:TiO2 films via Si co-doping
Thin Solid Films 615 (2016) 265–270
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
A combinatorial chemical beam vapour deposition approach to tune the electrical conductivity of Nb:TiO2 films via Si co-doping C.S. Sandu a,⁎, E. Wagner a, S. Harada a, G. Benvenuti a, W. Maudez a, M. Jobin b, C. Pellodi b, P. Muralt c a b c
3D-OXIDES, 130 Rue Gustave Eiffel, Saint Genis Pouilly 01630, France HEPIA, University of Applied Sciences (HES-SO), 4 rue de la Prairie, CH-1202 Genève, Switzerland Laboratoire de Céramique, Ecole Polytechnique Fédérale de Lausanne, Station 12, CH-1015 Lausanne, Switzerland
a r t i c l e
i n f o
Article history: Received 29 April 2016 Received in revised form 11 July 2016 Accepted 13 July 2016 Available online 15 July 2016 Keywords: Ternary oxide Nanocomposite Combinatorial Chemical beam vapour deposition Electrical conductivity
a b s t r a c t Chemical beam vapour deposition (CBVD) is a thin film deposition technique operated under high vacuum conditions in which film growth occurs through the thermally activated chemical decomposition of precursor molecules at the substrate surface. This technique was used in a combinatorial mode to investigate the influence of Si doping on the properties of Nb-doped TiO2 films, a well-known transparent conductive oxide material. The ternary oxide system (Si, Nb, Ti, O) displays good intermiscibility between the elements. By adding a Si precursor flow gradient to homogeneous Ti and Nb precursor flows, it was demonstrated that the resistivity of deposited films increased by over 5 orders of magnitude (from 1 to 100,000 Ω·cm) with Si doping levels between 2 and 21at.%. Meanwhile, only a slight variation of the refractive index of about 10% was observed. A fundamental film morphology study showed that the conductivity variation was due to Si segregation at the grain boundaries of the conductive Nb:TiO2 structure. © 2016 Published by Elsevier B.V.
1. Introduction The field of complex metal oxides is increasingly of interest to researchers as they seek to enhance and expand the already vast array of exciting physical properties associated with these materials [1–3]. Key to exploiting these benefits is the development of techniques capable of the requisite level of control when it comes to the growth of quaternary oxide thin films. Chemical beam vapour deposition (CBVD) is a vapour phase technique operated under high vacuum conditions in which film growth occurs through the chemical decomposition of precursor molecules at the substrate surface [4–6]. One of its main advantages is that several elements are co-deposited from independent, oriented chemical precursor beams without any gas phase reaction. This offers the opportunity to generate a variety of flow gradients from several different precursors across the surface of a substrate and thus obtain a material map over large areas. The desired functional property can be measured in order to identify the region of the film that exhibits the most favourable value. A good example of a target material suitable for the combinatorial approach is the transparent conductive oxide (Si, Nb):TiO2 because of the good intermiscibility between the elements. By adding a few atomic percent of Nb to anatase TiO2 thin films, their electrical conductivity was enhanced by orders of magnitude [7–10]. The optimum values reported for 40 nm epitaxial films approach 2– 3 × 10−4 Ω·cm [7]. In order to obtain materials with intermediate values of resistivity in the range 1–100,000 Ω·cm we doped Nb:TiO2 films with Si in the range 2–21 at.% (the values correspond to the ratios
http://dx.doi.org/10.1016/j.tsf.2016.07.032 0040-6090/© 2016 Published by Elsevier B.V.
100 × [Si] / ([Si] + [Nb] + [Ti])). In this way, the electrical conductivity mirrors the chemical patterning of the thin film (Si content). When employed in electrode arrays, such films with elevated resistivity can potentially act as localised heaters for biological applications. For example, the ability to generate local temperature gradients can have an impact on cell growth. Some works report the achievement of intermediate electrical resistivity values in TiSiOx compounds by varying the oxygen content [11], but in this case the films lost their optical transparency and thermal stability. In our case, because of the formation of an insulating-conductive nanocomposite SiO2/Nb:TiO2 material, the films remain optically transparent and exhibit a refractive index variation of just 10% for a 105 variation in electrical conductivity. We will show that the material can change from crystalline to nanocomposite and then to amorphous, depending on the level of Si-doping. Similar research on the influence of nanocomposite metallic-insulating materials on the electrical properties of thin films were reported for binary nitrides like NbN, TiN and ZrN doped with Si [12–15], but no report on (Si, Nb):TiO2 has yet been published, to the best of our knowledge. In this paper we investigate the growth and electrical conductivity of (Si, Nb):TiO2 thin films deposited using the combinatorial capability of the CBVD technique with Ti and Nb precursors in homogeneous flow and a Si precursor in gradient flow. The goal of this paper is to emphasize the efficiency of combinatorial CBVD technology to develop complex materials (quaternary and nanocomposite). Another objective of the present article is to describe the growth process of nanocomposite (Si, Nb):TiO2 thin films and to explain the conductivity behaviour through mechanisms at the nanoscale level. In order to prove that Si
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relevant in relative terms. From cross-sectional SEM we can estimate the average thickness of the deposited films and their growth morphology well. The electrical conductivity was measured by a 4 point probe configuration. 3. Results A summary of the deposition conditions and sample characteristics is given in Table 1. We have to emphasize that anatase was the only crystalline phase present in the deposited films. The Nb content in the films reported in Table 1 was in the range 5–10 at.%. In our previous research on Nb:TiO2 films (currently unreported), any phase, morphology or electrical conductivity differences were observed in this compositional range. The variation of the O content across all samples was b0.5 at.%. The C content in the films was lower than 1 at.%. 3.1. Combinatorial deposition and various gradients Fig. 1. A sketch of the combinatorial configuration used during the depositions.
segregation occurs at the grain boundaries of conductive Nb:TiO2, detailed structural and morphological investigations have been carried out. 2. Experimental Oxide thin films were deposited onto 4″ thermally-oxidised (500 nm SiO2 layer) silicon wafers using an ABCD Technologies Sybilla 150 CBVD machine. More detailed information concerning the capabilities of the CBVD machine is presented in [16,17]. The liquid precursors used during the investigation were titanium tetraisopropoxide (Ti(OiPr)4, CAS 546-68-9, M = 284.22 g·mol− 1), tetraethoxy(dimethylaminoethoxy)niobium (Nb(OEt)4(dmae), SAFC research compound, M = 361.13 g·mol− 1) and tetrabutoxysilane (Si(OnBu)4, CAS 4766-57-8, M = 320.54 g·mol−1). The Ti and Nb precursor flows were homogeneously distributed (6 sources for each) while the Si precursor flow was unidirectional (1 source) in order to create a gradient in the Si-content across the wafer (see Fig. 1) and to obtain multiple compositions in a single run. The Ti and Nb precursor reservoirs were held at a temperature of 32 °C and 60 °C, respectively for all films and the Si precursor reservoir temperature was varied between 50 and 65 °C. The substrate heater temperature was varied between 360 and 500 °C. Before starting the deposition, the chamber was pumped to a base pressure of 5 × 10−4 Pa. A liquid nitrogencooled cryo-panel helped to maintain a pressure below 2 × 10−3 Pa during the deposition. The morphology and the chemical composition of the thin films were investigated by scanning electron microscopy equipped with energy dispersive X-ray analysis (SEM-EDX) using a Merlin SEM and by transmission electron microscopy (TEM) in cross-section using a Tecnai Osiris microscope. Because the SEM-EDX spectra were taken under similar conditions, differences greater than ±0.2 at.% can be considered as
All samples present a high colour uniformity which confirms small thickness variation. A typical deposited sample is shown in Fig. 2a. The variation of the chemical composition and 2 point sheet resistance is indicated at 3 positions along the Si-gradient direction. The optical thickness of the film varies by b 5% as estimated by the high colour uniformity. SEM cross-sectional images (Fig. 2b) taken from the two extreme points of the Si gradient direction confirm the Si rich region is b2% thinner. The SEM and EDX data demonstrate the ability of the CBVD technique to deposit thin films with a controlled chemical gradient in a single step. As we can observe in Fig. 2c the Si-content decreases linearly from 15 to 9 at.%, which corresponds to a gradient of 0.6 at.% per cm. In this compositional range the electrical resistivity strongly depends on the Si content, but its variation is not linear (Fig. 2c). We must point out that the increase in resistivity with Si content is not a general behaviour, but specific to the compositional range where the material is nanocomposite in nature, consisting of conducting Nb:TiO2 crystallites surrounded by an insulating SiO2 tissue. As was observed in other TCOs [10] or NbSiN [12] thin films, the electrical conductivity depends not only on the doping level, but also on the nanostructure. The shape and size of grains together with their boundaries play a key role in the electrical conductivity. The influence of Si doping on the nanostructure and hence the electrical conductivity will be developed in the following sections. 3.2. Microstructure and chemical composition Both increasing the Si content and the substrate temperature have the effect of reducing grain size (Figs. 3 and 4). The mechanism responsible for this effect is the segregation of Si atoms to the grain boundaries which inhibits the growth of the Nb:TiO2 crystallites. Three distinct regimes have been identified by observing the evolution of morphology with Si content (Fig. 3). Similar behaviour was observed in the case of Me-Si-N system [12–15]. For low Si contents (CSi ≤ 6 at.%) the film morphology is unchanged and in this concentration range the Si atoms probably substitute Ti atoms in the TiO2 lattice. Exceeding the solubility
Table 1 Deposition conditions and sample characteristics. Sample ID
Heater temp. (°C)
Si precursor temp. (°C)
Si-precursor pressure (Pa)
Film thickness (nm)
Growth rate (nm·min−1)
Si content range (at.%)
2 points resistance at the wafer-center (kΩ)
1 2 3 4 5 6 7
500 500 500 500 440 420 360
65 60 55 50 60 60 60
8 5.5 3.5 2.5 5.3 5 5.5
540 690 690 690 480 1100 250
8.6 10.3 10.6 11.0 5.3 6.1 2
15–21 9.2–15 3.6–5.7 2–2.2 9.3–10.7 7.2–9.1 2–4.2
40,000 100 32 10 35 2.5 0.8
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Fig. 2. Sample 2: a) optical picture; b) cross-sectional SEM images at the extremities of the sample and c) resistivity variation in the Si gradient direction.
Fig. 3. SEM images corresponding to films deposited at 500 °C with varying Si-content.
limit leads to the growth of a nanocomposite film that is composed of Nb:TiO2 nanocrystallites surrounded by an amorphous SiO2 layer. In this intermediate regime the grain sizes decrease until the film becomes amorphous. Further increases of the Si content (CSi ≥ 15 at.%) lead to an increase in the quantity of amorphous SiO2 in the films. The surface morphology of the films does not change in this regime. Similar behaviour has been observed in the case of NbSiN [12] and ZrSiN [14] composite materials.
Increasing the substrate temperature reduces the solubility limit of Si in the Nb:TiO2 phase and then increases the volume of the insulating SiO2 material situated at the grain boundaries. The accumulation of Si atoms on the TiO2 crystallite surface will stop the growth of crystallites, the consequence being the reduction of the grain size in the film [14]. The situation is illustrated in Fig. 4 for films doped with approximately 9 at.% Si deposited at substrate temperatures of 420, 440 and 500 °C. The highest deposition temperature leads to the smallest grain size.
Fig. 4. SEM images corresponding to films having containing approximately 9 at.% Si and deposited at 500 (a), 440 (b) and 420 °C (c).
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Fig. 5. Cross-sectional Bright Field TEM images on samples deposited at 500 °C with 5 at.% (a), and 21 at.% Si (b).
Fig. 7. Cross-sectional Dark Field TEM images of samples containing around 2 at.% of Si deposited at 500 °C (a) and 360 °C (b).
Cross-sectional TEM images provide insights concerning the crystallinity and growth of the films deposited under various conditions and direct evidence of the segregation of Si atoms at the grain boundaries. The degradation of crystallinity with increasing Si content is clearly illustrated in Fig. 5. The films containing 5 at.% Si (Fig. 5 a) are columnar, polycrystalline and textured and present high roughness on the film surface. The films containing 21 at.% Si are amorphous and present low roughness on the film surface. In the case of the sample containing 15 at.% Si, it was possible to reveal the segregation of Si atoms at the Nb:TiO2 grain boundaries by EDX-mapping (Fig. 6). The EDX image is obtained by measuring an EDX spectrum in each point and then the
corresponding number of counts for each element is converted in colour intensity. In order to better visualize the Si-segregation (in green) at the grain boundaries, an integrated (across 100 points) line-scan on the intensity profile of the Si (in green) and Ti (in red) elements is shown. The segregation of Si at the grain boundaries is proven by the opposing trends in the Ti and Si intensity profiles. In the case of the samples containing around 2 at.% of Si, where Si segregation did not occur, the grain size was determined by the substrate temperature via the precursor decomposition rate and ad-atom mobility. Then the lateral size of grains increases from 100 nm to 500 nm as the deposition temperature decreases from 500 °C to
Fig. 6. a) HyperMap EDX image showing the elemental distribution in the film no 2 (Si (green), Ti (red), Nb (blue)). b) An elemental line profile (right) for Ti (red) and Si (green) across the grains. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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3.3. Resistivity versus Si content and morphology
Fig. 8. Resistivity as a function of Si content for samples deposited at 500 °C.
360 °C. The selected dark field images illustrate well the situation (Fig. 7). A similar behaviour for Si-doped materials was reported in the case of ZrN/SiN thin films [14]. To summarize the evolution of film morphology as a function of Si content: at low Si contents the films are well-crystallized; as the Si content rises, films become first poorly crystallized and then amorphous. The decrease of the crystallite size in the films due to Si incorporation should be very rapid according to the relationship CSi ~1/d (where CSi is Si-content and d is the grain size), as was found for Ti-Si-N [13], ZrSi-N [14] and Nb-Si-N [12] nanocomposite films. The decrease of the crystallite size and the formation of an insulating SiOy layer between conducting Nb:TiO2 crystallites has a significant influence on the electrical conductivity as it will be presented in the next chapter.
For clarity we would mention that all Si-doped TiO2 (without Nb) films deposited by us using CBVD were insulating. The Nb-doped TiO2 films (without Si) deposited under similar conditions displayed resistivities between 10−2 and 1 Ω·cm which are 100 times higher than that of epitaxial films deposited by PLD reported in [7] and 10 times higher than that of Aerosol-CVD films reported in [7] or annealed PVD films reported in [18]. The higher resistivities of Nb:TiO2 CBVD films could be explained by the open, columnar growth and relatively small crystallite size obtained in these particular conditions. This article does not focus on the electrical performance of transparent conducting oxides. Instead, the electrical resistivity behaviour was measured in order to verify the nanocomposite structure of the Si, Nb:TiO2 material. The behaviour of the resistivity (measured by 4 points probe) as a function of Si content shown in Fig. 8 seems strange at first sight, but it can easily be explained by considering the influence of the film morphology on electron transport. The relatively high resistivity of the films containing only 2 at.% of Si grown at 500 °C is explained by the open, columnar grain morphology associated with the anatase phase under these conditions. The high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging mode being sensitive to density variation is appropriate for revealing the porosity present at the grain boundaries in the film. In this kind of feather-like structure the quality of contact between grains is poor (Fig. 9). The mixture of straight and inclined columns (see also Fig. 7a), together with the rough column walls define a particular morphology. In this type of thin film, the limitation of conductivity is probably not related to the grain size, but on the quality of contact between the grains. The film grown at lower temperature shows a denser morphology (Fig. 7b) and has a conductivity that is 30× higher. This could be explained by the superior crystalline quality: wider crystallites and better inter-grain conductivity. In terms of the dependence of resistivity on Si content (Fig. 8) we can distinguish two regimes for films deposited at 500 °C. We do not observe a clear trend for Si contents up to 6 at.%, most probably because the conductivity of the films depends strongly on the grain morphology rather than the formation of the insulating SiO2 layer surrounding the crystallites, in this regime. Under the second regime (N6 at.% Si), the resistivity increases exponentially with Si content. It is in this regime that the nanocomposite material is formed: insulating-SiO2/conductingNb:TiO2. Such behaviour is typical for nanocomposite insulating/conductive materials [15].
4. Summary and conclusions We have shown that by using the combinatorial CBVD technique it is possible to deposit (Si,Nb):TiO2 thin films with a wide-range of Sicontents and to understand the correlation between nanostructure and electrical properties of such complex materials from just 7 deposition experiments. The influence of substrate temperature and of the incorporation of Si on the morphology and electrical conductivity of Nb:TiO2 films was well-addressed. Across the 7 samples, varying [Si] from 2 to 21 at.% on the metal side ([Si] + [Ti] + [Nb] = 100%), the electrical conductivity varied by 5 orders of magnitude. The insights at the nanoscale level provided by TEM investigations afforded us the possibility to confirm the segregation of Si atoms at the Nb:TiO2 grain boundaries and hence the formation of a conducting-insulating nanocomposite material. The correlation of the electrical resistivity with film nanostructure provides information concerning the factors limiting carrier transport in such conductinginsulating nanocomposite films and therefore offers the opportunity to control electrical conductivity as macroscopic property. Fig. 9. HAADF-STEM image on sample deposited at 500 °C and containing 2 at.% of Si.
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Acknowledgements The authors wish to thank the CIME-EPFL team for their TEM investigation facilities. The authors wish to acknowledge the FEDER (Fonds Européen de Développement Economique et Régional) (5887/20132011) for financing the Nanobium project through the Interreg IVA programme. References [1] H. Koinuma, Chemistry and electronics of oxides from carbon dioxide to perovskite, Thin Solid Films 486 (1–2) (2005) 2–10. [2] H.U. Habermeier, Thin films of perovskite-type complex oxides, Mater. Today 10 (10) (2007) 34–43. [3] J. Heber, Materials science: Enter the oxides, Nature 459 (2009) 28–30. [4] G. Benvenuti, C.S. Sandu, E. Wagner, TiO2 laser and electron beam assisted chemical deposition, IOP Conf. Ser.: Mater. Sci. Eng. 8 (2010), 012006. [5] A. Dabirian, Y. Kuzminykh, C.S. Sandu, S. Harada, E. Wagner, P. Brodard, G. Benvenuti, P. Hoffmann, Combinatorial high-vacuum chemical vapor deposition of textured hafnium-doped lithium niobate thin films on sapphire, Cryst. Growth Des. 11 (1) (2011) 203–209. [6] A. Dabirian, Y. Kuzminykh, B. Afra, S. Harada, E. Wagner, C.S. Sandu, G. Benvenuti, S. Rushworth, P. Muralt, P. Hoffmann, Combinatorial discovery and optimization of amorphous HfO2-Nb2O5 mixture with improved transparency, Electrochem. SolidState Lett. 13 (2010) G60–G63. [7] Y. Furubayashi, T. Hitosugi, Y. Yamamoto, K. Inaba, G. Kinoda, et al., A transparent metal: Nb-doped anatase TiO2, Appl. Phys. Lett. 86 (2005) 252101. [8] Y. Furubayashi, N. Yamada, Y. Hirose, Y. Yamamoto, M. Otani, T. Hitosugi, T. Shimada, T. Hasegawa, Transport properties of d-electron-based transparent conducting oxide: anatase Ti1-xNbxO2, J. Appl. Phys. 101 (2007) 093705.
[9] D.S. Bhachu, S. Sathasivam, G. Sankar, D.O. Scanlon, G. Cibin, C.J. Carmalt, I.P. Parkin, G.W. Watson, S.M. Bawaked, A.Y. Obaid, S. Al-Thabaiti, S.N. Basahel, Solution processing route to multifunctional titania thin films: highly conductive and photcatalytically active Nb:TiO2, Adv. Funct. Mater. 24 (2014) 5075–5085. [10] J.P. Niemela, Y. Hirose, K. Shigematsu, M. Sano, T. Hasegawa, M. Karppinen, Transition in electron scattering mechanism in atomic layer deposited Nb:TiO2 thin films, Appl. Phys. Lett. 107 (2015) 192102. [11] D. Brassard, D.K. Sarkar, M.A. El Khakani, L. Ouellet, Tuning the electrical resistivity of pulsed laser deposited TiSiOx thin films from highly insulating to conductive behaviors Appl, Phys. Lett. 84 (2004) 2304–2306. [12] C.S. Sandu, M. Benkahoul, R. Sanjinés, F. Lévy, Model for the evolution of Nb-Si-N thin films as a function of Si content relating the nanostructure to electrical and mechanical properties, Surf. Coat. Technol. 201 (2006) 2897–2903. [13] P. Jedrzejowski, B. Baloukas, J.E. Klemberg-Sapieha, L. Martinu, Optical characteristics and color of TiN/SiN1.3 nanocomposite coatings, J. Vac. Sci. Technol. A 22 (2004) 725–733. [14] C.S. Sandu, F. Medjani, R. Sanjinés, A. Karimi, F. Lévy, Structure, morphology and electrical properties of sputtered Zr-Si-N thin films: From solid solution to nanocomposite, Surf. Coat. Technol. 201 (2006) 4219–4223. [15] C.S. Sandu, S. Harada, R. Sanjinés, A. Cavaleiro, A unique approach to reveal the nanocomposite nc-MN/SiN-layer architecture of thin films via electrical measurements, Surf. Coat. Technol. 204 (2010) 1907–1913. [16] E. Wagner, C.S. Sandu, S. Harada, G. Benvenuti, V. Savu, P. Muralt, Fabrication of complex oxide microstructures by combinatorial chemical beam vapour deposition through stencil masks, Thin Solid Films 586 (2015) 64–69. [17] E. Wagner, C.S. Sandu, S. Harada, P. Muralt, G. Benvenuti, Geometry of chemical beam vapor deposition system for efficient combinatorial investigations of thin oxide films: deposited film properties versus precursor flow simulations, ACS Comb. Sci. 18 (2016) 154–161. [18] S. Seeger, K. Ellmer, M. Weise, D. Gogova, D. Abou-Ras, R. Mientus, Reactive magnetron sputtering of Nb-doped TiO2 films: relationships between structure, composition and electrical properties, Thin Solid Films 605 (2016) 44–52.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
A combinatorial chemical beam vapour deposition approach to tune the electrical conductivity of Nb:TiO2 films via Si co-doping C.S. Sandu a,⁎, E. Wagner a, S. Harada a, G. Benvenuti a, W. Maudez a, M. Jobin b, C. Pellodi b, P. Muralt c a b c
3D-OXIDES, 130 Rue Gustave Eiffel, Saint Genis Pouilly 01630, France HEPIA, University of Applied Sciences (HES-SO), 4 rue de la Prairie, CH-1202 Genève, Switzerland Laboratoire de Céramique, Ecole Polytechnique Fédérale de Lausanne, Station 12, CH-1015 Lausanne, Switzerland
a r t i c l e
i n f o
Article history: Received 29 April 2016 Received in revised form 11 July 2016 Accepted 13 July 2016 Available online 15 July 2016 Keywords: Ternary oxide Nanocomposite Combinatorial Chemical beam vapour deposition Electrical conductivity
a b s t r a c t Chemical beam vapour deposition (CBVD) is a thin film deposition technique operated under high vacuum conditions in which film growth occurs through the thermally activated chemical decomposition of precursor molecules at the substrate surface. This technique was used in a combinatorial mode to investigate the influence of Si doping on the properties of Nb-doped TiO2 films, a well-known transparent conductive oxide material. The ternary oxide system (Si, Nb, Ti, O) displays good intermiscibility between the elements. By adding a Si precursor flow gradient to homogeneous Ti and Nb precursor flows, it was demonstrated that the resistivity of deposited films increased by over 5 orders of magnitude (from 1 to 100,000 Ω·cm) with Si doping levels between 2 and 21at.%. Meanwhile, only a slight variation of the refractive index of about 10% was observed. A fundamental film morphology study showed that the conductivity variation was due to Si segregation at the grain boundaries of the conductive Nb:TiO2 structure. © 2016 Published by Elsevier B.V.
1. Introduction The field of complex metal oxides is increasingly of interest to researchers as they seek to enhance and expand the already vast array of exciting physical properties associated with these materials [1–3]. Key to exploiting these benefits is the development of techniques capable of the requisite level of control when it comes to the growth of quaternary oxide thin films. Chemical beam vapour deposition (CBVD) is a vapour phase technique operated under high vacuum conditions in which film growth occurs through the chemical decomposition of precursor molecules at the substrate surface [4–6]. One of its main advantages is that several elements are co-deposited from independent, oriented chemical precursor beams without any gas phase reaction. This offers the opportunity to generate a variety of flow gradients from several different precursors across the surface of a substrate and thus obtain a material map over large areas. The desired functional property can be measured in order to identify the region of the film that exhibits the most favourable value. A good example of a target material suitable for the combinatorial approach is the transparent conductive oxide (Si, Nb):TiO2 because of the good intermiscibility between the elements. By adding a few atomic percent of Nb to anatase TiO2 thin films, their electrical conductivity was enhanced by orders of magnitude [7–10]. The optimum values reported for 40 nm epitaxial films approach 2– 3 × 10−4 Ω·cm [7]. In order to obtain materials with intermediate values of resistivity in the range 1–100,000 Ω·cm we doped Nb:TiO2 films with Si in the range 2–21 at.% (the values correspond to the ratios
http://dx.doi.org/10.1016/j.tsf.2016.07.032 0040-6090/© 2016 Published by Elsevier B.V.
100 × [Si] / ([Si] + [Nb] + [Ti])). In this way, the electrical conductivity mirrors the chemical patterning of the thin film (Si content). When employed in electrode arrays, such films with elevated resistivity can potentially act as localised heaters for biological applications. For example, the ability to generate local temperature gradients can have an impact on cell growth. Some works report the achievement of intermediate electrical resistivity values in TiSiOx compounds by varying the oxygen content [11], but in this case the films lost their optical transparency and thermal stability. In our case, because of the formation of an insulating-conductive nanocomposite SiO2/Nb:TiO2 material, the films remain optically transparent and exhibit a refractive index variation of just 10% for a 105 variation in electrical conductivity. We will show that the material can change from crystalline to nanocomposite and then to amorphous, depending on the level of Si-doping. Similar research on the influence of nanocomposite metallic-insulating materials on the electrical properties of thin films were reported for binary nitrides like NbN, TiN and ZrN doped with Si [12–15], but no report on (Si, Nb):TiO2 has yet been published, to the best of our knowledge. In this paper we investigate the growth and electrical conductivity of (Si, Nb):TiO2 thin films deposited using the combinatorial capability of the CBVD technique with Ti and Nb precursors in homogeneous flow and a Si precursor in gradient flow. The goal of this paper is to emphasize the efficiency of combinatorial CBVD technology to develop complex materials (quaternary and nanocomposite). Another objective of the present article is to describe the growth process of nanocomposite (Si, Nb):TiO2 thin films and to explain the conductivity behaviour through mechanisms at the nanoscale level. In order to prove that Si
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relevant in relative terms. From cross-sectional SEM we can estimate the average thickness of the deposited films and their growth morphology well. The electrical conductivity was measured by a 4 point probe configuration. 3. Results A summary of the deposition conditions and sample characteristics is given in Table 1. We have to emphasize that anatase was the only crystalline phase present in the deposited films. The Nb content in the films reported in Table 1 was in the range 5–10 at.%. In our previous research on Nb:TiO2 films (currently unreported), any phase, morphology or electrical conductivity differences were observed in this compositional range. The variation of the O content across all samples was b0.5 at.%. The C content in the films was lower than 1 at.%. 3.1. Combinatorial deposition and various gradients Fig. 1. A sketch of the combinatorial configuration used during the depositions.
segregation occurs at the grain boundaries of conductive Nb:TiO2, detailed structural and morphological investigations have been carried out. 2. Experimental Oxide thin films were deposited onto 4″ thermally-oxidised (500 nm SiO2 layer) silicon wafers using an ABCD Technologies Sybilla 150 CBVD machine. More detailed information concerning the capabilities of the CBVD machine is presented in [16,17]. The liquid precursors used during the investigation were titanium tetraisopropoxide (Ti(OiPr)4, CAS 546-68-9, M = 284.22 g·mol− 1), tetraethoxy(dimethylaminoethoxy)niobium (Nb(OEt)4(dmae), SAFC research compound, M = 361.13 g·mol− 1) and tetrabutoxysilane (Si(OnBu)4, CAS 4766-57-8, M = 320.54 g·mol−1). The Ti and Nb precursor flows were homogeneously distributed (6 sources for each) while the Si precursor flow was unidirectional (1 source) in order to create a gradient in the Si-content across the wafer (see Fig. 1) and to obtain multiple compositions in a single run. The Ti and Nb precursor reservoirs were held at a temperature of 32 °C and 60 °C, respectively for all films and the Si precursor reservoir temperature was varied between 50 and 65 °C. The substrate heater temperature was varied between 360 and 500 °C. Before starting the deposition, the chamber was pumped to a base pressure of 5 × 10−4 Pa. A liquid nitrogencooled cryo-panel helped to maintain a pressure below 2 × 10−3 Pa during the deposition. The morphology and the chemical composition of the thin films were investigated by scanning electron microscopy equipped with energy dispersive X-ray analysis (SEM-EDX) using a Merlin SEM and by transmission electron microscopy (TEM) in cross-section using a Tecnai Osiris microscope. Because the SEM-EDX spectra were taken under similar conditions, differences greater than ±0.2 at.% can be considered as
All samples present a high colour uniformity which confirms small thickness variation. A typical deposited sample is shown in Fig. 2a. The variation of the chemical composition and 2 point sheet resistance is indicated at 3 positions along the Si-gradient direction. The optical thickness of the film varies by b 5% as estimated by the high colour uniformity. SEM cross-sectional images (Fig. 2b) taken from the two extreme points of the Si gradient direction confirm the Si rich region is b2% thinner. The SEM and EDX data demonstrate the ability of the CBVD technique to deposit thin films with a controlled chemical gradient in a single step. As we can observe in Fig. 2c the Si-content decreases linearly from 15 to 9 at.%, which corresponds to a gradient of 0.6 at.% per cm. In this compositional range the electrical resistivity strongly depends on the Si content, but its variation is not linear (Fig. 2c). We must point out that the increase in resistivity with Si content is not a general behaviour, but specific to the compositional range where the material is nanocomposite in nature, consisting of conducting Nb:TiO2 crystallites surrounded by an insulating SiO2 tissue. As was observed in other TCOs [10] or NbSiN [12] thin films, the electrical conductivity depends not only on the doping level, but also on the nanostructure. The shape and size of grains together with their boundaries play a key role in the electrical conductivity. The influence of Si doping on the nanostructure and hence the electrical conductivity will be developed in the following sections. 3.2. Microstructure and chemical composition Both increasing the Si content and the substrate temperature have the effect of reducing grain size (Figs. 3 and 4). The mechanism responsible for this effect is the segregation of Si atoms to the grain boundaries which inhibits the growth of the Nb:TiO2 crystallites. Three distinct regimes have been identified by observing the evolution of morphology with Si content (Fig. 3). Similar behaviour was observed in the case of Me-Si-N system [12–15]. For low Si contents (CSi ≤ 6 at.%) the film morphology is unchanged and in this concentration range the Si atoms probably substitute Ti atoms in the TiO2 lattice. Exceeding the solubility
Table 1 Deposition conditions and sample characteristics. Sample ID
Heater temp. (°C)
Si precursor temp. (°C)
Si-precursor pressure (Pa)
Film thickness (nm)
Growth rate (nm·min−1)
Si content range (at.%)
2 points resistance at the wafer-center (kΩ)
1 2 3 4 5 6 7
500 500 500 500 440 420 360
65 60 55 50 60 60 60
8 5.5 3.5 2.5 5.3 5 5.5
540 690 690 690 480 1100 250
8.6 10.3 10.6 11.0 5.3 6.1 2
15–21 9.2–15 3.6–5.7 2–2.2 9.3–10.7 7.2–9.1 2–4.2
40,000 100 32 10 35 2.5 0.8
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Fig. 2. Sample 2: a) optical picture; b) cross-sectional SEM images at the extremities of the sample and c) resistivity variation in the Si gradient direction.
Fig. 3. SEM images corresponding to films deposited at 500 °C with varying Si-content.
limit leads to the growth of a nanocomposite film that is composed of Nb:TiO2 nanocrystallites surrounded by an amorphous SiO2 layer. In this intermediate regime the grain sizes decrease until the film becomes amorphous. Further increases of the Si content (CSi ≥ 15 at.%) lead to an increase in the quantity of amorphous SiO2 in the films. The surface morphology of the films does not change in this regime. Similar behaviour has been observed in the case of NbSiN [12] and ZrSiN [14] composite materials.
Increasing the substrate temperature reduces the solubility limit of Si in the Nb:TiO2 phase and then increases the volume of the insulating SiO2 material situated at the grain boundaries. The accumulation of Si atoms on the TiO2 crystallite surface will stop the growth of crystallites, the consequence being the reduction of the grain size in the film [14]. The situation is illustrated in Fig. 4 for films doped with approximately 9 at.% Si deposited at substrate temperatures of 420, 440 and 500 °C. The highest deposition temperature leads to the smallest grain size.
Fig. 4. SEM images corresponding to films having containing approximately 9 at.% Si and deposited at 500 (a), 440 (b) and 420 °C (c).
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Fig. 5. Cross-sectional Bright Field TEM images on samples deposited at 500 °C with 5 at.% (a), and 21 at.% Si (b).
Fig. 7. Cross-sectional Dark Field TEM images of samples containing around 2 at.% of Si deposited at 500 °C (a) and 360 °C (b).
Cross-sectional TEM images provide insights concerning the crystallinity and growth of the films deposited under various conditions and direct evidence of the segregation of Si atoms at the grain boundaries. The degradation of crystallinity with increasing Si content is clearly illustrated in Fig. 5. The films containing 5 at.% Si (Fig. 5 a) are columnar, polycrystalline and textured and present high roughness on the film surface. The films containing 21 at.% Si are amorphous and present low roughness on the film surface. In the case of the sample containing 15 at.% Si, it was possible to reveal the segregation of Si atoms at the Nb:TiO2 grain boundaries by EDX-mapping (Fig. 6). The EDX image is obtained by measuring an EDX spectrum in each point and then the
corresponding number of counts for each element is converted in colour intensity. In order to better visualize the Si-segregation (in green) at the grain boundaries, an integrated (across 100 points) line-scan on the intensity profile of the Si (in green) and Ti (in red) elements is shown. The segregation of Si at the grain boundaries is proven by the opposing trends in the Ti and Si intensity profiles. In the case of the samples containing around 2 at.% of Si, where Si segregation did not occur, the grain size was determined by the substrate temperature via the precursor decomposition rate and ad-atom mobility. Then the lateral size of grains increases from 100 nm to 500 nm as the deposition temperature decreases from 500 °C to
Fig. 6. a) HyperMap EDX image showing the elemental distribution in the film no 2 (Si (green), Ti (red), Nb (blue)). b) An elemental line profile (right) for Ti (red) and Si (green) across the grains. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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3.3. Resistivity versus Si content and morphology
Fig. 8. Resistivity as a function of Si content for samples deposited at 500 °C.
360 °C. The selected dark field images illustrate well the situation (Fig. 7). A similar behaviour for Si-doped materials was reported in the case of ZrN/SiN thin films [14]. To summarize the evolution of film morphology as a function of Si content: at low Si contents the films are well-crystallized; as the Si content rises, films become first poorly crystallized and then amorphous. The decrease of the crystallite size in the films due to Si incorporation should be very rapid according to the relationship CSi ~1/d (where CSi is Si-content and d is the grain size), as was found for Ti-Si-N [13], ZrSi-N [14] and Nb-Si-N [12] nanocomposite films. The decrease of the crystallite size and the formation of an insulating SiOy layer between conducting Nb:TiO2 crystallites has a significant influence on the electrical conductivity as it will be presented in the next chapter.
For clarity we would mention that all Si-doped TiO2 (without Nb) films deposited by us using CBVD were insulating. The Nb-doped TiO2 films (without Si) deposited under similar conditions displayed resistivities between 10−2 and 1 Ω·cm which are 100 times higher than that of epitaxial films deposited by PLD reported in [7] and 10 times higher than that of Aerosol-CVD films reported in [7] or annealed PVD films reported in [18]. The higher resistivities of Nb:TiO2 CBVD films could be explained by the open, columnar growth and relatively small crystallite size obtained in these particular conditions. This article does not focus on the electrical performance of transparent conducting oxides. Instead, the electrical resistivity behaviour was measured in order to verify the nanocomposite structure of the Si, Nb:TiO2 material. The behaviour of the resistivity (measured by 4 points probe) as a function of Si content shown in Fig. 8 seems strange at first sight, but it can easily be explained by considering the influence of the film morphology on electron transport. The relatively high resistivity of the films containing only 2 at.% of Si grown at 500 °C is explained by the open, columnar grain morphology associated with the anatase phase under these conditions. The high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging mode being sensitive to density variation is appropriate for revealing the porosity present at the grain boundaries in the film. In this kind of feather-like structure the quality of contact between grains is poor (Fig. 9). The mixture of straight and inclined columns (see also Fig. 7a), together with the rough column walls define a particular morphology. In this type of thin film, the limitation of conductivity is probably not related to the grain size, but on the quality of contact between the grains. The film grown at lower temperature shows a denser morphology (Fig. 7b) and has a conductivity that is 30× higher. This could be explained by the superior crystalline quality: wider crystallites and better inter-grain conductivity. In terms of the dependence of resistivity on Si content (Fig. 8) we can distinguish two regimes for films deposited at 500 °C. We do not observe a clear trend for Si contents up to 6 at.%, most probably because the conductivity of the films depends strongly on the grain morphology rather than the formation of the insulating SiO2 layer surrounding the crystallites, in this regime. Under the second regime (N6 at.% Si), the resistivity increases exponentially with Si content. It is in this regime that the nanocomposite material is formed: insulating-SiO2/conductingNb:TiO2. Such behaviour is typical for nanocomposite insulating/conductive materials [15].
4. Summary and conclusions We have shown that by using the combinatorial CBVD technique it is possible to deposit (Si,Nb):TiO2 thin films with a wide-range of Sicontents and to understand the correlation between nanostructure and electrical properties of such complex materials from just 7 deposition experiments. The influence of substrate temperature and of the incorporation of Si on the morphology and electrical conductivity of Nb:TiO2 films was well-addressed. Across the 7 samples, varying [Si] from 2 to 21 at.% on the metal side ([Si] + [Ti] + [Nb] = 100%), the electrical conductivity varied by 5 orders of magnitude. The insights at the nanoscale level provided by TEM investigations afforded us the possibility to confirm the segregation of Si atoms at the Nb:TiO2 grain boundaries and hence the formation of a conducting-insulating nanocomposite material. The correlation of the electrical resistivity with film nanostructure provides information concerning the factors limiting carrier transport in such conductinginsulating nanocomposite films and therefore offers the opportunity to control electrical conductivity as macroscopic property. Fig. 9. HAADF-STEM image on sample deposited at 500 °C and containing 2 at.% of Si.
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