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Pulsed laser deposition of tungsten and tungsten oxide thin films with tailored structure at the nano- and mesoscale
Applied Surface Science 253 (2007) 8130–8135 www.elsevier.com/locate/apsusc
Pulsed laser deposition of tungsten and tungsten oxide thin films with tailored structure at the nano- and mesoscale A. Bailini, F. Di Fonzo, M. Fusi, C.S. Casari *, A. Li Bassi, V. Russo, A. Baserga, C.E. Bottani Center for Nanoengineered Materials and Surfaces (NEMAS), CNISM and Dipartimento di Ingegneria Nucleare, Politecnico di Milano, Via Ponzio 34/3, I-20133 Milano, Italy Available online 28 February 2007
Abstract Nanostructured thin films synthesized by assembling atoms or clusters present a structure characterized by a modulation at the nanoscale and by a large effective area, which can be exploited for the tailoring of specific structural or electronic properties. These systems are appealing for functional applications, e.g. in sensing and catalysis. We have investigated the deposition of tungsten and tungsten oxide thin films with a wide range of morphologies by exploiting nanosecond pulsed laser deposition (PLD) in an inert background atmosphere (He, Ar and Kr). We show that the non-dimensional ratio of the target-to-substrate distance to the time integrated visible plume length, which depends on the gas mass and pressure and on the substrate position, permits to select morphologies ranging from a compact structure with a density similar to bulk, to a film with an open, low density foam-like mesostructure and a high fraction of voids. # 2007 Elsevier B.V. All rights reserved. Keywords: PLD; Nanostructured thin films; Tungsten; Tungsten oxide
1. Introduction Transition metal and metal oxide films are interesting materials in view of the possibility of growing functional coatings with tailored properties [1–8]. Among these, tungsten and tungsten oxide films have been investigated for applications in the fields of microelectronics, sensing and catalysis. For instance, tungsten thin films have been studied for the development of electrical contacts [3,4], while tungsten oxide films present gas sensing (via molecule adsorption and subsequent surface conductivity variation), electrochromic [5–7] and catalytic properties [8]. For these applications the increase of the specific effective surface and the comprehension of the transport properties is of great importance. In particular, deposition of tungsten oxide films by cluster assembling offers the possibility of tailoring meso- and nanostructure, effective surface, oxidation and degree of crystallinity. Pulsed laser deposition (PLD) in a background gas allows the synthesis of cluster-assembled films with tailored surface * Corresponding author. E-mail address: [email protected] (C.S. Casari). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.145
properties since the ablation plume is confined by the interaction with the surrounding gas and the increased collision rate of the ablated species favors cluster formation in the plume [9,10]. Moreover, the energy distribution of clusters impinging onto the substrate is affected by plume characteristics, e.g. their kinetic energy is reduced after diffusion in the background atmosphere [11], leading to the possibility of low energy cluster deposition and memory effects in the so-formed nanostructured materials [12,13]. Due to interaction with a background gas, the plume undergoes a transition to a stable shock front as the ablated material scatters off the background gas [14]. During plume expansion through the gas, the pressure within the plume progressively lowers and the shock wave front slows down until it stops at a definite stopping distance which depends on the type and pressure of the background gas [14]. Plume expansion and shock front stopping distance in various atmospheres have also been studied by means of different models, see e.g. [15,16]. Thus, when considering thin film deposition, target-to-substrate distance plays a fundamental role as a deposition parameter together with the gas pressure, since both contribute to determine the substrate relative position with respect to the plume shock front.
A. Bailini et al. / Applied Surface Science 253 (2007) 8130–8135
We recently demonstrated that tungsten and tungsten oxide films with a wide range of morphologies can be deposited by means of PLD exploiting a background inert gas (He, Ar) pressure, as revealed by SEM, AFM and Raman observations [17,18]. In particular, films deposited at the same target-tosubstrate distance (50 mm) appear compact with a smooth surface as long as the gas pressure remains below a few Pa. At a few tens Pa the roughness increases and the morphology begins to show a nano- and mesostructure which evolves towards a porous structure. Finally, for gas pressure greater than a few hundreds Pa the film has an open and apparently soft, foam-like structure. Similar trends are observed using He or Ar as background gas, while pressure thresholds between different morphologies (smooth, nanostructured and foam-like) depend on the gas mass. In addition, we have found (by means of Raman spectroscopy, as reported in detail in Refs. [17,18]) a tendency towards oxidation when the samples are exposed to ambient air (ex situ oxidation) leading to the formation of amorphous or nanocrystalline tungsten oxide. Such ex situ oxidation appears strictly related to the film morphology since the degree of oxidation increases with increasing the background gas pressure (compact films at low pressure are metallic, more open structures deposited at higher pressures show the presence of amorphous tungsten oxide) [17,18]. We here extend our previous works and show the possibility of tuning the morphology of tungsten and tungsten oxide films at the nano/mesoscale by varying different deposition parameters, namely the gas mass and pressure and the target-to-substrate distance. In particular, one can tune the morphology of the deposited films by varying the non-dimensional parameter L, defined as the ratio of the target-to-substrate distance dTS to the time integrated visible plume length lp (L = dTS/lp), and thus controlling the aggregation/deposition mechanisms. Varying L, the film morphology ranges from compact (L < 1) to nanostructured (L 1), to foam-like (L > 1).
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visible part) are captured. Scanning electron microscopy (SEM) images were acquired with a Zeiss Supra 40 field emission SEM. 3. Results and discussion In order to investigate the role of both the background gas mass and pressure and the target-to-substrate distance (dTS) on the film morphology we first observed plasma plumes in different background gas conditions. Plume dynamics in vacuum is characterized by a practically free and collisionless propagation regime. The spatial distribution is strongly forward directed and the observed light emission is weak [14]. When a background gas is present in the deposition chamber the light emission increases due to particle collisions producing radiative de-excitation of the ablated species, both in the body of the plume and particularly in the expansion front; the plume edge is better defined due to the presence of a shock wave front and the plume is slowed down and spatially confined [19]. The time integrated visible plume length (lp) is related with the maximum position reached by the shock wave front (i.e. the stopping distance) and in Fig. 1 we plot lp values as a function of gas pressure for different gas mass (He: 4 a.m.u., Ar: 40 a.m.u., Kr: 84 a.m.u.). Ablated particles confinement increases and indeed lp decreases when background gas mass and pressure increase in agreement with recent results by Amoruso et al. [20]. The time integrated picture of a typical plasma plume in 100 Pa He where lp = 50 mm (dTS = 65 mm) is reported in the inset of Fig. 1, corresponding to the L > 1 condition. We want here to show that different film morphologies at the nano/mesoscale, which are the result of the variation of film growth mechanisms, can be related to the non-dimensional ratio L = dTS/lp. SEM images of films deposited in He, Ar and Kr background gas, with L = 0.7, 1 and 1.5 are reported in
2. Experimental details Tungsten and tungsten oxide films have been grown by PLD on Si substrates at room temperature in a UHV compatible deposition chamber. UV laser pulses (duration 10–15 ns) from a KrF excimer laser (l = 248 nm, 10 Hz repetition rate) were focused on a W target (purity 99.99%) with an energy density on the target (fluence) of about 4.5 J/cm2. These fluence value, laser energy and focusing conditions were chosen to minimize droplet production. The system is equipped with a gas inlet system allowing pressure control from high vacuum (HV, 10 6 Pa) to atmospheric pressure and a quartz microbalance to measure film mass and thickness. Films of different thickness were grown (several thousands laser pulses) with a target-tosubstrate distance dTS in the range 30–100 mm. The deposition rate varies in the 0.03–1.10 nm/s range. Plasma plume pictures are taken through a viewport (orthogonal to the plume axis) with a digital camera with exposure time of 0.5 s corresponding to an average over five plumes (at 10 Hz laser repetition rate). With this choice and assuming different plumes as equivalent, the time integrated shape and dimension of the plume (i.e. its
Fig. 1. Time integrated length lp (i.e. stopping distance of the shock wave front) of visible ablation plumes as a function of background gas pressure, for He, Ar and Kr. Inset: time integrated picture of W plume at 100 Pa He where lp = 50 mm and dTS = 65 mm are reported.
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Fig. 2. SEM images of tungsten and tungsten oxide films deposited in (first column) He; (second column) Ar; (third column) Kr; at different non-dimensional distances (first line) L = 0.7; (second line) L = 1; (third line) L = 1.5; obtained by changing the gas pressure at fixed dTS = 50 mm (according to lp data of Fig. 1).
Fig. 2. We varied L by simply changing the gas pressure (according to data presented in Fig. 1) while keeping a constant target-to-substrate distance (dTS = 50 mm). In these depositions the condition L = 1 (for dTS = 50 mm) is obtained at about 20 Pa Kr, 40 Pa Ar and 100 Pa He, corresponding to the formation of porous nanostructured films and to the crossover from compact to foam-like structures. SEM images (Fig. 2) reveal that the surface structure, at fixed L, is quite similar for different gases. Moreover, as already reported, we have found a tendency towards spontaneous oxidation when films are exposed to the ambient atmosphere [17,18]. Here, at fixed dTS = 50 mm, metallic films are obtained for pressure values up to about 10 Pa Kr, 20 Pa Ar and 50 Pa He, i.e. for L < 1; the transition from metallic to amorphous oxide takes place over the same pressure range where film morphology begins to change from compact to nanostructured [17], i.e. for L 1. For L > 1 surface oxidation is favored by the nanostructuring and opening of the structure and by the corresponding increase of the effective surface area of the deposited material.
The different film growth mechanisms and regimes are more clear in cross-sectional SEM images (Fig. 3) of films deposited at different L values by varying the He pressure at fixed dTS = 50 mm (and variable number of pulses): compact atomby-atom growth for L < 1 (Fig. 3a and b: vacuum, L 1, and He 50 Pa, lp = 75 mm, L = 0.7); columnar growth, characterized by a porous mesostructure, for L = 1 (Fig. 3c: He 100 Pa, lp = 50 mm); cauliflower-like growth, characterized by a large number of voids and low adhesion of the film probably due to low kinetic energy of the deposited particles for L > 1 (Fig. 3d: He 200 Pa, lp = 40 mm, L = 1.25). We estimated the film density by combining film thickness, measured from SEM cross-sections, and mass from quartz microbalance measurements. For the films of Fig. 3 the corresponding material density varies over one order of magnitude (from 19 to 0.4 g/ cm3), as shown in Fig. 4. An abrupt decrease in the film density occurs when crossing the L 1 condition, in agreement with the change in morphology and structure already discussed. It is worth noticing that even considering oxidation, occurring for
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Fig. 3. SEM cross-sectional images of tungsten and tungsten oxide films deposited in (a) vacuum and in He at (b) 50 Pa; (c) 100 Pa; (d) 200 Pa at fixed dTS = 50 mm corresponding to (a) L 1; (b) L = 0.7; (c) L = 1; (d) L = 1.25.
L > 1, the film density at such L values is still substantially lower than the bulk WO3 density (r = 7.3 g/cm3). We then investigated the effect of changing both gas pressure and dTS while keeping L constant. SEM images in Fig. 5 show the morphology of films deposited at L = 0.7 and 1.3, at two different He gas pressures (50 and 100 Pa). At 50 Pa
Fig. 4. Estimated density values of films of Fig. 3 as a function of background He pressure. Films were deposited at fixed dTS = 50 mm in order to vary the non-dimensional L value. Density values of bulk W and WO3 are reported for reference.
He, being lp = 75 mm, we have chosen dTS = 52.5 mm (Fig. 5a) and dTS = 97.5 mm (Fig. 5c), respectively; at 100 Pa He, being lp = 50 mm, the values of target-to-substrate distance are dTS = 35 mm (Fig. 5b) and dTS = 65 mm (Fig. 5d), respectively. At L = 0.7, the film surface is more compact and looks similar even if the gas pressure changes. An increase of the porosity and disordering for L = 1.3 is observed, corresponding to the change of regime in the film formation mechanisms. Our observations permit to identify the non-dimensional distance L as a guiding parameter to select distinct deposition conditions when PLD is operated in the presence of a background gas: (a) L < 1 (substrate ‘in plume’), at which we think that a reduced scattering of the ablated species and a limited cluster growth may occur; (b) L 1, substrate placed near the plume shock wave front; (c) L > 1 (substrate ’out of plume’), diffusion of the formed particles in the gas. The observed morphology trends seem to be related to the decreasing length lp of the visible ablation plume with respect to the target-to-substrate distance dTS. When lp > dTS (L < 1) extremely smooth surfaces are probably related to an atom-byatom growth or to small cluster fragmentation on impact. When the length of the ablation plume is reduced or dTS is increased, so that lp < dTS (L > 1), cluster formation is favored by plume confinement [21,22] and the kinetic energy of the deposited particles is reduced due to diffusion in the background gas before landing onto the substrate (low energy
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Fig. 5. SEM images of films deposited in He at (first column) 50 Pa; (second column) 100 Pa, with non-dimensional distance (first line) L = 0.7; (second line) L = 1.3; at variable dTS (a) 52.5 mm; (b) 35 mm; (c) 97.5 mm; (d) 65 mm.
cluster deposition regime). This leads to a cluster-assembled material where clusters nucleated in the plume can be deposited with a limited fragmentation, thus allowing the formation of very soft and open film structures. We note that for L 1, atomic force microscopy measurements were almost impossible, due to the presence of very soft structures with a roughness of the same order of magnitude of the film thickness. Indeed the value of L can be selected by varying different experimental parameters, not only gas pressure/mass and target-to-substrate distance, but also laser energy density. All these factors can affect film morphology and their effects could be somehow related to the non-dimensional ratio L, setting a relationship between them. Of course plume dynamics, and thus cluster nucleation, kinetic energy and size distribution, as well as deposition, surface and coalescence, do not simply depend on the non-dimensional distance L only, but specifically depend on all the parameters which concur to determine L, and also on other parameters, like laser energy and energy density, as well as, of course, on the specific material being ablated. The effect of changing all these parameters and the ablated material certainly needs further investigations to understand how film morphology is consequently affected. Anyway we propose that the non-dimensional distance L can be used as a rule of thumb in the design of the morphology properties of new materials produced by PLD.
4. Conclusions In summary, the surface morphology of tungsten and tungsten oxide films deposited in an inert background gas is characterized by a general trend between different morphologies (smooth, nanostructured and sponge-like) as a result of the variation of film growth mechanisms, related to plume expansion dynamics. In particular, we compared the film nanoand mesostructure with the non-dimensional distance L defined as the ratio of target-to-substrate distance dTS to the time integrated visible plume length lp. This parameter couples the effect of gas pressure and target-to-substrate distance and permits to qualitatively select the film growth regime (i.e. L < 1: compact, L 1: nanostructured and L > 1: foam-like). Ranging L from 0.7 to 1.3, deposited film density varies over one order of magnitude. Conversely, morphologies qualitatively similar are obtained by changing type of gas, gas pressure (indeed lp) and dTS, while maintaining the same value of L. The possibility of tuning the morphology, the specific surface and the oxidation degree of tungsten films is of great importance for sensing applications. We want to point out that the surface morphology and film properties are far to be completely determined by the value of L, since other phenomena (such as cluster growth in the plume, or mobility and aggregation of clusters at the surface) which do not simply depend on L, must
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be considered in the mechanisms of film growth. Anyway, simple and easy-to-check deposition conditions are often required in material synthesis and we suggest the use of the non-dimensional L parameter as a guiding tool to make PLD a extremely versatile technique for the growth of nanostructured films with tailored morphology and structural properties. Acknowledgment Authors are grateful to M.F. Bressanelli for the valuable contribution given during his degree thesis work. References [1] [2] [3] [4]
A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1. M. Gratzel, Nature 414 (2001) 338. A.M. Dhote, S.B. Ogale, Appl. Phys. Lett. 64 (21) (1994) 2809. G.M. Mikhailov, A.V. Chernykh, V.T. Petrashov, J. Appl. Phys. 80 (2) (1996) 948. [5] J.L. Solis, S. Saukko, L. Kish, C.G. Granqvist, V. Lantto, Thin Solid Films 391 (2001) 255. [6] P.R. Bueno, F.M. Pontes, E.R. Leite, L.O.S. Bulhoes, P.S. Pizani, P.N. Lisboa-Filho, W.H. Schreiner, J. Appl. Phys. 94 (2004) 2102. [7] L. Meda, R.C. Breitkopf, T.E. Haas, R.U. Kirss, Thin Solid Films 402 (2002) 126.
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[8] J. Engweiler, J. Harf, A. Baiker, J. Catal. 159 (1996) 259. [9] D.B. Geohegan, A.A. Puretzky, G. Duscher, S.J. Pennycook, Appl. Phys. Lett. 72 (1998) 2987. [10] A.V. Rode, E.G. Gamaly, B. Luther-Davies, Appl. Phys. A 70 (2000) 135. [11] R. Dolbec, E. Irissou, M. Chaker, D. Guay, F. Rosei, M.A. El Khakhani, Phys. Rev. B 70 (2004), 201406-1. [12] A. Pereira, L. Cultrera, A. Dima, M. Susu, A. Perrone, H.L. Du, A.O. Volkov, R. Cutting, P.K. Datta, Thin Solid Films 497 (2006) 142. [13] P. Jensen, Rev. Mod. Phys. 71 (1999) 1695. [14] D.B. Geohegan, in: D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, John Wiley & Sons, New York, 1994 , pp. 145–148. [15] A.V. Bulgakov, N.M. Bulgakova, J. Phys. D: Appl. Phys. 31 (1998) 693. [16] S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, M. Vitiello, X. Wang, Phys. Rev. B 67 (2003) 224503. [17] F. Di Fonzo, A. Bailini, V. Russo, A. Baserga, D. Cattaneo, M.G. Beghi, P.M. Ossi, C.S. Casari, A. Li Bassi, C.E. Bottani, Catal. Today 116 (2006) 69. [18] A. Li Bassi, C.S. Casari, F. Di Fonzo, A. Bailini, D. Dellasega, V. Russo, A. Baserga, D. Cattaneo, C.E. Bottani, Mater. Res. Soc. Symp. Proc. 0901 (2006) Ra24-04.1. [19] S. Amoruso, B. Toftmann, J. Schou, Phys. Rev. E 69 (2004) 056403. [20] S. Amoruso, A. Sambri, M. Vitiello, X. Wang, Appl. Surf. Sci. 252 (2006) 4712. [21] X.Y. Chen, S.B. Xiong, Z.S. Sha, Z.G. Liu, Appl. Surf. Sci. 115 (1997) 279. [22] Y. Kunii, T. Makimura, K. Murakami, Jpn. J. Appl. Phys. 35 (1996) 4780.
Pulsed laser deposition of tungsten and tungsten oxide thin films with tailored structure at the nano- and mesoscale A. Bailini, F. Di Fonzo, M. Fusi, C.S. Casari *, A. Li Bassi, V. Russo, A. Baserga, C.E. Bottani Center for Nanoengineered Materials and Surfaces (NEMAS), CNISM and Dipartimento di Ingegneria Nucleare, Politecnico di Milano, Via Ponzio 34/3, I-20133 Milano, Italy Available online 28 February 2007
Abstract Nanostructured thin films synthesized by assembling atoms or clusters present a structure characterized by a modulation at the nanoscale and by a large effective area, which can be exploited for the tailoring of specific structural or electronic properties. These systems are appealing for functional applications, e.g. in sensing and catalysis. We have investigated the deposition of tungsten and tungsten oxide thin films with a wide range of morphologies by exploiting nanosecond pulsed laser deposition (PLD) in an inert background atmosphere (He, Ar and Kr). We show that the non-dimensional ratio of the target-to-substrate distance to the time integrated visible plume length, which depends on the gas mass and pressure and on the substrate position, permits to select morphologies ranging from a compact structure with a density similar to bulk, to a film with an open, low density foam-like mesostructure and a high fraction of voids. # 2007 Elsevier B.V. All rights reserved. Keywords: PLD; Nanostructured thin films; Tungsten; Tungsten oxide
1. Introduction Transition metal and metal oxide films are interesting materials in view of the possibility of growing functional coatings with tailored properties [1–8]. Among these, tungsten and tungsten oxide films have been investigated for applications in the fields of microelectronics, sensing and catalysis. For instance, tungsten thin films have been studied for the development of electrical contacts [3,4], while tungsten oxide films present gas sensing (via molecule adsorption and subsequent surface conductivity variation), electrochromic [5–7] and catalytic properties [8]. For these applications the increase of the specific effective surface and the comprehension of the transport properties is of great importance. In particular, deposition of tungsten oxide films by cluster assembling offers the possibility of tailoring meso- and nanostructure, effective surface, oxidation and degree of crystallinity. Pulsed laser deposition (PLD) in a background gas allows the synthesis of cluster-assembled films with tailored surface * Corresponding author. E-mail address: [email protected] (C.S. Casari). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.145
properties since the ablation plume is confined by the interaction with the surrounding gas and the increased collision rate of the ablated species favors cluster formation in the plume [9,10]. Moreover, the energy distribution of clusters impinging onto the substrate is affected by plume characteristics, e.g. their kinetic energy is reduced after diffusion in the background atmosphere [11], leading to the possibility of low energy cluster deposition and memory effects in the so-formed nanostructured materials [12,13]. Due to interaction with a background gas, the plume undergoes a transition to a stable shock front as the ablated material scatters off the background gas [14]. During plume expansion through the gas, the pressure within the plume progressively lowers and the shock wave front slows down until it stops at a definite stopping distance which depends on the type and pressure of the background gas [14]. Plume expansion and shock front stopping distance in various atmospheres have also been studied by means of different models, see e.g. [15,16]. Thus, when considering thin film deposition, target-to-substrate distance plays a fundamental role as a deposition parameter together with the gas pressure, since both contribute to determine the substrate relative position with respect to the plume shock front.
A. Bailini et al. / Applied Surface Science 253 (2007) 8130–8135
We recently demonstrated that tungsten and tungsten oxide films with a wide range of morphologies can be deposited by means of PLD exploiting a background inert gas (He, Ar) pressure, as revealed by SEM, AFM and Raman observations [17,18]. In particular, films deposited at the same target-tosubstrate distance (50 mm) appear compact with a smooth surface as long as the gas pressure remains below a few Pa. At a few tens Pa the roughness increases and the morphology begins to show a nano- and mesostructure which evolves towards a porous structure. Finally, for gas pressure greater than a few hundreds Pa the film has an open and apparently soft, foam-like structure. Similar trends are observed using He or Ar as background gas, while pressure thresholds between different morphologies (smooth, nanostructured and foam-like) depend on the gas mass. In addition, we have found (by means of Raman spectroscopy, as reported in detail in Refs. [17,18]) a tendency towards oxidation when the samples are exposed to ambient air (ex situ oxidation) leading to the formation of amorphous or nanocrystalline tungsten oxide. Such ex situ oxidation appears strictly related to the film morphology since the degree of oxidation increases with increasing the background gas pressure (compact films at low pressure are metallic, more open structures deposited at higher pressures show the presence of amorphous tungsten oxide) [17,18]. We here extend our previous works and show the possibility of tuning the morphology of tungsten and tungsten oxide films at the nano/mesoscale by varying different deposition parameters, namely the gas mass and pressure and the target-to-substrate distance. In particular, one can tune the morphology of the deposited films by varying the non-dimensional parameter L, defined as the ratio of the target-to-substrate distance dTS to the time integrated visible plume length lp (L = dTS/lp), and thus controlling the aggregation/deposition mechanisms. Varying L, the film morphology ranges from compact (L < 1) to nanostructured (L 1), to foam-like (L > 1).
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visible part) are captured. Scanning electron microscopy (SEM) images were acquired with a Zeiss Supra 40 field emission SEM. 3. Results and discussion In order to investigate the role of both the background gas mass and pressure and the target-to-substrate distance (dTS) on the film morphology we first observed plasma plumes in different background gas conditions. Plume dynamics in vacuum is characterized by a practically free and collisionless propagation regime. The spatial distribution is strongly forward directed and the observed light emission is weak [14]. When a background gas is present in the deposition chamber the light emission increases due to particle collisions producing radiative de-excitation of the ablated species, both in the body of the plume and particularly in the expansion front; the plume edge is better defined due to the presence of a shock wave front and the plume is slowed down and spatially confined [19]. The time integrated visible plume length (lp) is related with the maximum position reached by the shock wave front (i.e. the stopping distance) and in Fig. 1 we plot lp values as a function of gas pressure for different gas mass (He: 4 a.m.u., Ar: 40 a.m.u., Kr: 84 a.m.u.). Ablated particles confinement increases and indeed lp decreases when background gas mass and pressure increase in agreement with recent results by Amoruso et al. [20]. The time integrated picture of a typical plasma plume in 100 Pa He where lp = 50 mm (dTS = 65 mm) is reported in the inset of Fig. 1, corresponding to the L > 1 condition. We want here to show that different film morphologies at the nano/mesoscale, which are the result of the variation of film growth mechanisms, can be related to the non-dimensional ratio L = dTS/lp. SEM images of films deposited in He, Ar and Kr background gas, with L = 0.7, 1 and 1.5 are reported in
2. Experimental details Tungsten and tungsten oxide films have been grown by PLD on Si substrates at room temperature in a UHV compatible deposition chamber. UV laser pulses (duration 10–15 ns) from a KrF excimer laser (l = 248 nm, 10 Hz repetition rate) were focused on a W target (purity 99.99%) with an energy density on the target (fluence) of about 4.5 J/cm2. These fluence value, laser energy and focusing conditions were chosen to minimize droplet production. The system is equipped with a gas inlet system allowing pressure control from high vacuum (HV, 10 6 Pa) to atmospheric pressure and a quartz microbalance to measure film mass and thickness. Films of different thickness were grown (several thousands laser pulses) with a target-tosubstrate distance dTS in the range 30–100 mm. The deposition rate varies in the 0.03–1.10 nm/s range. Plasma plume pictures are taken through a viewport (orthogonal to the plume axis) with a digital camera with exposure time of 0.5 s corresponding to an average over five plumes (at 10 Hz laser repetition rate). With this choice and assuming different plumes as equivalent, the time integrated shape and dimension of the plume (i.e. its
Fig. 1. Time integrated length lp (i.e. stopping distance of the shock wave front) of visible ablation plumes as a function of background gas pressure, for He, Ar and Kr. Inset: time integrated picture of W plume at 100 Pa He where lp = 50 mm and dTS = 65 mm are reported.
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Fig. 2. SEM images of tungsten and tungsten oxide films deposited in (first column) He; (second column) Ar; (third column) Kr; at different non-dimensional distances (first line) L = 0.7; (second line) L = 1; (third line) L = 1.5; obtained by changing the gas pressure at fixed dTS = 50 mm (according to lp data of Fig. 1).
Fig. 2. We varied L by simply changing the gas pressure (according to data presented in Fig. 1) while keeping a constant target-to-substrate distance (dTS = 50 mm). In these depositions the condition L = 1 (for dTS = 50 mm) is obtained at about 20 Pa Kr, 40 Pa Ar and 100 Pa He, corresponding to the formation of porous nanostructured films and to the crossover from compact to foam-like structures. SEM images (Fig. 2) reveal that the surface structure, at fixed L, is quite similar for different gases. Moreover, as already reported, we have found a tendency towards spontaneous oxidation when films are exposed to the ambient atmosphere [17,18]. Here, at fixed dTS = 50 mm, metallic films are obtained for pressure values up to about 10 Pa Kr, 20 Pa Ar and 50 Pa He, i.e. for L < 1; the transition from metallic to amorphous oxide takes place over the same pressure range where film morphology begins to change from compact to nanostructured [17], i.e. for L 1. For L > 1 surface oxidation is favored by the nanostructuring and opening of the structure and by the corresponding increase of the effective surface area of the deposited material.
The different film growth mechanisms and regimes are more clear in cross-sectional SEM images (Fig. 3) of films deposited at different L values by varying the He pressure at fixed dTS = 50 mm (and variable number of pulses): compact atomby-atom growth for L < 1 (Fig. 3a and b: vacuum, L 1, and He 50 Pa, lp = 75 mm, L = 0.7); columnar growth, characterized by a porous mesostructure, for L = 1 (Fig. 3c: He 100 Pa, lp = 50 mm); cauliflower-like growth, characterized by a large number of voids and low adhesion of the film probably due to low kinetic energy of the deposited particles for L > 1 (Fig. 3d: He 200 Pa, lp = 40 mm, L = 1.25). We estimated the film density by combining film thickness, measured from SEM cross-sections, and mass from quartz microbalance measurements. For the films of Fig. 3 the corresponding material density varies over one order of magnitude (from 19 to 0.4 g/ cm3), as shown in Fig. 4. An abrupt decrease in the film density occurs when crossing the L 1 condition, in agreement with the change in morphology and structure already discussed. It is worth noticing that even considering oxidation, occurring for
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Fig. 3. SEM cross-sectional images of tungsten and tungsten oxide films deposited in (a) vacuum and in He at (b) 50 Pa; (c) 100 Pa; (d) 200 Pa at fixed dTS = 50 mm corresponding to (a) L 1; (b) L = 0.7; (c) L = 1; (d) L = 1.25.
L > 1, the film density at such L values is still substantially lower than the bulk WO3 density (r = 7.3 g/cm3). We then investigated the effect of changing both gas pressure and dTS while keeping L constant. SEM images in Fig. 5 show the morphology of films deposited at L = 0.7 and 1.3, at two different He gas pressures (50 and 100 Pa). At 50 Pa
Fig. 4. Estimated density values of films of Fig. 3 as a function of background He pressure. Films were deposited at fixed dTS = 50 mm in order to vary the non-dimensional L value. Density values of bulk W and WO3 are reported for reference.
He, being lp = 75 mm, we have chosen dTS = 52.5 mm (Fig. 5a) and dTS = 97.5 mm (Fig. 5c), respectively; at 100 Pa He, being lp = 50 mm, the values of target-to-substrate distance are dTS = 35 mm (Fig. 5b) and dTS = 65 mm (Fig. 5d), respectively. At L = 0.7, the film surface is more compact and looks similar even if the gas pressure changes. An increase of the porosity and disordering for L = 1.3 is observed, corresponding to the change of regime in the film formation mechanisms. Our observations permit to identify the non-dimensional distance L as a guiding parameter to select distinct deposition conditions when PLD is operated in the presence of a background gas: (a) L < 1 (substrate ‘in plume’), at which we think that a reduced scattering of the ablated species and a limited cluster growth may occur; (b) L 1, substrate placed near the plume shock wave front; (c) L > 1 (substrate ’out of plume’), diffusion of the formed particles in the gas. The observed morphology trends seem to be related to the decreasing length lp of the visible ablation plume with respect to the target-to-substrate distance dTS. When lp > dTS (L < 1) extremely smooth surfaces are probably related to an atom-byatom growth or to small cluster fragmentation on impact. When the length of the ablation plume is reduced or dTS is increased, so that lp < dTS (L > 1), cluster formation is favored by plume confinement [21,22] and the kinetic energy of the deposited particles is reduced due to diffusion in the background gas before landing onto the substrate (low energy
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Fig. 5. SEM images of films deposited in He at (first column) 50 Pa; (second column) 100 Pa, with non-dimensional distance (first line) L = 0.7; (second line) L = 1.3; at variable dTS (a) 52.5 mm; (b) 35 mm; (c) 97.5 mm; (d) 65 mm.
cluster deposition regime). This leads to a cluster-assembled material where clusters nucleated in the plume can be deposited with a limited fragmentation, thus allowing the formation of very soft and open film structures. We note that for L 1, atomic force microscopy measurements were almost impossible, due to the presence of very soft structures with a roughness of the same order of magnitude of the film thickness. Indeed the value of L can be selected by varying different experimental parameters, not only gas pressure/mass and target-to-substrate distance, but also laser energy density. All these factors can affect film morphology and their effects could be somehow related to the non-dimensional ratio L, setting a relationship between them. Of course plume dynamics, and thus cluster nucleation, kinetic energy and size distribution, as well as deposition, surface and coalescence, do not simply depend on the non-dimensional distance L only, but specifically depend on all the parameters which concur to determine L, and also on other parameters, like laser energy and energy density, as well as, of course, on the specific material being ablated. The effect of changing all these parameters and the ablated material certainly needs further investigations to understand how film morphology is consequently affected. Anyway we propose that the non-dimensional distance L can be used as a rule of thumb in the design of the morphology properties of new materials produced by PLD.
4. Conclusions In summary, the surface morphology of tungsten and tungsten oxide films deposited in an inert background gas is characterized by a general trend between different morphologies (smooth, nanostructured and sponge-like) as a result of the variation of film growth mechanisms, related to plume expansion dynamics. In particular, we compared the film nanoand mesostructure with the non-dimensional distance L defined as the ratio of target-to-substrate distance dTS to the time integrated visible plume length lp. This parameter couples the effect of gas pressure and target-to-substrate distance and permits to qualitatively select the film growth regime (i.e. L < 1: compact, L 1: nanostructured and L > 1: foam-like). Ranging L from 0.7 to 1.3, deposited film density varies over one order of magnitude. Conversely, morphologies qualitatively similar are obtained by changing type of gas, gas pressure (indeed lp) and dTS, while maintaining the same value of L. The possibility of tuning the morphology, the specific surface and the oxidation degree of tungsten films is of great importance for sensing applications. We want to point out that the surface morphology and film properties are far to be completely determined by the value of L, since other phenomena (such as cluster growth in the plume, or mobility and aggregation of clusters at the surface) which do not simply depend on L, must
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be considered in the mechanisms of film growth. Anyway, simple and easy-to-check deposition conditions are often required in material synthesis and we suggest the use of the non-dimensional L parameter as a guiding tool to make PLD a extremely versatile technique for the growth of nanostructured films with tailored morphology and structural properties. Acknowledgment Authors are grateful to M.F. Bressanelli for the valuable contribution given during his degree thesis work. References [1] [2] [3] [4]
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