- Email: [email protected]
Production of porous carbon thin films by pulsed laser deposition
Thin Solid Films 350 (1999) 49±52
Production of porous carbon thin ®lms by pulsed laser deposition D. Vick*, Y.Y. Tsui, M.J. Brett, R. Fedosejevs Department of Electrical and Computer Engineering, University of Alberta, Edmonton, AB T6G 2G7, Canada Received 6 February 1999; accepted 12 March 1999
Abstract Pulsed laser deposition (PLD) has been used together with the Glancing Angle Deposition (GLAD) technique [1,2] for the ®rst time to produce highly porous structured ®lms. A laser produced carbon plasma and vapour plume was deposited at a highly oblique incident angle onto rotating Si substrates, resulting in ®lms exhibiting high bulk porosity and controlled columnar microstructure. By varying the substrate rotation rate, the shape of the microcolumns can be tailored. These results extend the versatility of the GLAD process to materials not readily deposited by means of traditional physical vapour deposition techniques. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Pulsed laser desorption; Porous carbon; Carbon; Physical vapour deposition
1. Introduction Recent experimental work has demonstrated that highly porous ®lms with tailored sub-micron structures can be produced by means of physical vapour deposition (PVD) [1±4]. The technique used to deposit these ®lms relies on dynamic in situ control of substrate motion together with highly-oblique vapour incidence. Cross sectional scanning electron microscope (SEM) images of the ®lms reveal isolated columns in the form of chevrons, helices and staircase structures of left or right chirality, posts, and bent nematics [5]. The average column separation and bulk porosity are observed to increase as the angle u between the substrate normal and the trajectory of the incident vapour is made increasingly oblique. Glancing angle deposition (GLAD) has been put forth to describe the speci®c deposition regime delineated by u $ 708C, for which the bulk porosity becomes greatly enhanced [6]. The porosity of GLAD ®lms arises from the self-shadowing effect, which may operate down to atomic scale lengths under conditions of low surface diffusion [7]. Geometric arguments [8] and simulations [1,9] have been used to clarify the role of self-shadowing in producing the observed characteristics of the ®lms. The effectiveness of the selfshadowing mechanism can be varied during deposition (by adjusting u) to control the aerial density of the ®lm as a function of depth, permitting the production, for example, of ®lms exhibiting density layering. A number of possible * Corresponding author. Tel.: 178-492-3332; fax: 178-492-1811. E-mail address: [email protected] (D. Vick)
applications are suggested by the properties of the ®lms, including sensors and catalysis (porosity) and passive optical devices (control of refractive index, chirality) [10]. To date, GLAD ®lms have been produced primarily by means of thermal or electron beam evaporation. The list of evaporants that have been investigated includes MgF2, CaF2, SiO, Si, SiO2, Cu, Cr, Mn, Al, Ti, Pt, and ZrO2. More recently, Ti GLAD ®lms have been deposited by low pressure, long throw (LPLT) sputtering [11]. In this letter we further extend the versatility of the technique by demonstrating the ®rst porous carbon GLAD ®lms grown by means of a pulsed laser deposition (PLD). Pulsed laser deposition has become an increasingly popular technique over the preceding decade for the production of thin ®lms [12] There are a number of attractive characteristics of PLD sources which make them suitable for the GLAD technique. First, the high energy density that may be delivered to the target allows one to evaporate materials with melting or sublimation temperatures above the capabilities of traditional PVD systems. Second, the source size is typically very small, leading to a nearly parallel particle ¯ux at the substrate which facilitates the effectiveness of the self shadowing mechanism. Finally, a typical PLD chamber is readily recon®gured to allow for deposition from multiple sources. A simpli®ed schematic of the experimental set-up used in the present work is shown in Fig. 1. The target-substrate assembly was situated in a vacuum chamber pumped to a base pressure of 7 £ 1025 Torr (9:3 £ 1023 Pa). The plasma source was produced using a Lumonics Excimer-500 krypton ¯ouride laser con®gured as an unstable resonator oscil-
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00274-6
50
D. Vick et al. / Thin Solid Films 350 (1999) 49±52
The wafer pieces were af®xed to rotating mounts which allowed the azimuthal orientation of the substrate to be varied during deposition, while keeping u ®xed. The rate of rotation with respect to the laser shot rate could be controlled by incrementally stepping the rotation throughout the depositions. This allowed us to reproduce by PLD a number of the GLAD ®lm morphologies that have been reported previously.
Fig. 1. The experimental con®guration used to deposit porous carbon ®lms.
lator and operating at a wavelength and repetition rate of 248 nm and 15 Hz. The laser beam was brought to the vacuum chamber and focused by a 50 cm focal length lens onto a graphite tape target at an incident angle of 408. The nominal laser pulse energy and duration were 20±40 mJ and 15 ns. Focal spot measurements taken using a ®ltered CCD camera positioned at the target plane indicate that 90% of the laser energy was incident within an area 7:6 £ 1023 cm 2. These parameters resulted in a nominal intensities of 1:6±3:2 £ 108 W/cm 2 at the target. The same measurements indicate that peak intensities were 3:1±6:3 £ 10 8 W/cm 2. The graphite tape was wound onto a rotating tape drive assembly which was activated during depositions to ensure that an undamaged target area was exposed for each laser pulse in order to minimise the shot to shot variation in the characteristics of the expansion plume. Under the conditions described above, the expansion plume consisted of carbon ions, electrons, neutral molecules, clusters, and particulate ejecta of nanometer scale size. During depositions the chamber pressure climbed to 3:3 £ 1024 Torr (4:4 £ 1022 Pa). This pressure was suf®ciently low to ensure that there was a line of sight ¯ight of the evaporant particles to the substrates after an initial expansion near the target surface. The substrates consisted of cleaved pre-cleaned silicon wafers pieces approximately 2 cm 2, positioned 8.0 cm from the laser plume source and oriented to produce an obliquely-incident ¯ux of u 828 upon the substrates.
Fig. 2. SEM images of a carbons ®lm deposited under conditions of ®xed deposition angle u 82 and substrate rotation. For the ®lm of (a), the substrate was rotated twice during deposition. For the ®lm (b), the substrate was rotated continuously at a rate of 3.2 rev/min during deposition. (c) Top view of the ®lm (b), demonstrating porosity.
D. Vick et al. / Thin Solid Films 350 (1999) 49±52
The combined effect of the self shadowing mechanism and substrate rotation on the sub-structure of the ®lms is illustrated in the SEM images of Fig. 2. For the ®lm of Fig. 2a, the substrate was rotated at a uniform rate with respect to the number of laser shots such that the substrate completed two complete revolutions throughout the course of the deposition. The resulting morphology consisted of isolated helical columns of pitch equal to one half the ®lm thickness. During the deposition, new material is added only at the column tips such that growth is directed towards the source. The handedness of the helix thus opposes the sense of the substrate rotation. The pitch of the helical columns can be controlled by varying the rotation rate of the substrate and deposition rate of the ®lm. If the rotation rate of the substrate is increased, the pitch of the helices diminishes and eventually the columns reach the limiting form of vertical posts, as exempli®ed in Fig. 2b, where a rotation rate of 3.2 turns per minute was employed. The top view of the vertical post ®lm is shown in Fig. 2c which illustrates clearly the porosity of the ®lm and isolated nature of the columns. The structures are very similar to those that have been produced using GLAD by other PVD methods [5,11]. Based on the results of previous work, a greater degree of porosity should readily be achievable in PLD ®lms by increasing the incident angle u . The nominal Ê /s for deposition rates normal to the substrates were 0.8 A Ê /s for the ®lm of Fig. 2b, and the ®lm of Fig. 2a and 1.0 A were thus comparable to those achieved in LPLT sputtering of Ti GLAD ®lms [11]. Additional measurements were performed in order to characterise the expansion plume. The substrate assembly was replaced by a quartz crystal thickness monitor (CTM) and a Faraday cup positioned 6.0 cm away from the target in close proximity to one another in order to collect the expansion plume along a trajectory intercepting the substrate position. The CTM head was oriented such that the expansion plume impinged at normal incidence onto the crystal. A Ê /s was recorded for conditions nominal deposition rate of 5 A similar to the depositions of Fig. 2. From shot-averaged time of ¯ight Faraday cup measurements, a characteristic ion energy of 82 eV was obtained for the temporal peak of the ion current. The kinetic energy of the ion component of the expansion is thus much more energetic than that normally associated with thermal or electron beam evaporators, for which the vapour species typically possess energies of a fraction of an eV. The fraction of the total carbon plume deposited in the form of ions was estimated by integrating the Faraday cup current and comparing this value to mass deposition rate as inferred from the CTM measurements. Preliminary results indicate that the ions comprise about 10% of the mass of the deposition plume for the laser/target conditions of this experiment, assuming that the average charge state of the ions was 1. A more thorough characterisation of the angular distribution and charge state distribution of the ions as a function of incident laser intensity is currently in progress and will be reported at a later date.
51
No attempt was made to characterise the energies or fractions of the neutral species, which comprises the bulk of the expansion. A previous study of PLD produced carbon plasmas, performed at laser intensities comparable to the present work, made use of laser-induced ¯orescence diagnostics to obtain characteristic energies of , 12 eV for the neutral carbon species [13]. These authors also reported typical ion energies of 80 eV based on Langmuir probe measurements, in good agreement with our own measurements. Carbon ®lms produced at normal incidence at such deposition conditions are usually very hard, and contain many diamond-like s-bonds [14,15]. It is expected that each of the columns produced in the present process may also be composed of diamond-like carbon. The remaining component of the expansion plume, namely particulate debris, results from the shock unloading at the laser focus and was evident in top view, low magni®cation SEM images of our ®lms. Particulates are a characteristic and undesirable feature of PLD ®lms [16]. One approach to the debris problem involves the con®nement and guiding of the plasma expansion plume along a curved trajectory through the use of magnetic ®elds. Initial experiments have been conducted towards this end and have demonstrated the feasibility of the process [17]. In summary, it has been shown for the ®rst time that pulsed laser deposition can be employed to produce porous carbon ®lms. Further work devoted to the characterisation of these ®lms and the deposition process is required and will be forthcoming. The pulsed laser deposition technique can be employed for virtually any source material and can easily be used to deposit controlled stoichiometric mixtures of materials either through separate targets or using mixed sources to begin with. These preliminary results con®rm that the GLAD process can be employed with the higher deposition energies typically present in a laser plasma plume and is viable beyond the limits imposed on PVD systems. Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Alberta Microelectronic Corporation. The authors wish to thank George Braybrook for the SEM work. References [1] K. Robbie, L.J. Friedrich, S.K. Dew, T. Smy, M.J. Brett, J. Vac. Sci. Technol. A 13 (1995) 1032. [2] K. Robbie, M. Brett, A. Lakhtakia, Nature 384 (1996) 616. [3] R. Messier, T. Gehrke, C. Frankel, V.C. Venugopal, W. Otano, A. Lakhtakia, J. Vac. Sci. Technol. A 15 (1997) 2148. [4] F. Liu, M.T. Umlor, L. Shen, J. Weston, W. Eads, J.A. Barnard, G.J. Mankey, J. Appl. Phys. 85 (1999) 5486. [5] K. Robbie, J. Sit, M.J. Brett, J. Vac. Sci. Technol. B 16 (1998) 1115. [6] K. Robbie, M.J. Brett, J. Vac. Sci. Technol. A 15 (1997) 1460.
52
D. Vick et al. / Thin Solid Films 350 (1999) 49±52
[7] L. Abelmann, C. Lodder, Thin Solid Films 305 (1997) 1. [8] R.N. Tait, T. Smy, M.J. Brett, Thin Solid Films 226 (1993) 196. [9] D. Vick, L. Friedrich, S.K. Dew, M.J. Brett, K. Robbie, M. Seto, T. Smy, Thin Solid Films 336 (1999) 1. [10] A. Lakhtakia, R. Messier, M.J. Brett, K. Robbie, Innov. Mater. Res. 1 (1996) 165. [11] J. Sit, D. Vick, K. Robbie, M.J. Brett, J. Mater. Res. Comm. 14 (1999) 1197. [12] J.T. Cheung, in: D.B. Chisley, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994 p. 1.
[13] D.L. Pappas, K.L. Saenger, J.J. Cuomo, R.W. Dreyfus, J. Appl. Phys. 72 (1992) 3966. [14] J. Robertson, Pure and Appl, Chem. 66 (1994) 1789. [15] D.L. Pappas, K.L. Saenger, J. Bruley, W. Krakow, J.J. Cuomo, T. Gu, R.W. Collins, J. Appl. Phys. 71 (1992) 5675. [16] L. Chen, in: D.B. Chisley, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994 p. 167. [17] Y.Y. Tsui, D. Vick, R. Fedosejevs, Appl. Phys. Lett. 70 (1997) 1953.
Production of porous carbon thin ®lms by pulsed laser deposition D. Vick*, Y.Y. Tsui, M.J. Brett, R. Fedosejevs Department of Electrical and Computer Engineering, University of Alberta, Edmonton, AB T6G 2G7, Canada Received 6 February 1999; accepted 12 March 1999
Abstract Pulsed laser deposition (PLD) has been used together with the Glancing Angle Deposition (GLAD) technique [1,2] for the ®rst time to produce highly porous structured ®lms. A laser produced carbon plasma and vapour plume was deposited at a highly oblique incident angle onto rotating Si substrates, resulting in ®lms exhibiting high bulk porosity and controlled columnar microstructure. By varying the substrate rotation rate, the shape of the microcolumns can be tailored. These results extend the versatility of the GLAD process to materials not readily deposited by means of traditional physical vapour deposition techniques. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Pulsed laser desorption; Porous carbon; Carbon; Physical vapour deposition
1. Introduction Recent experimental work has demonstrated that highly porous ®lms with tailored sub-micron structures can be produced by means of physical vapour deposition (PVD) [1±4]. The technique used to deposit these ®lms relies on dynamic in situ control of substrate motion together with highly-oblique vapour incidence. Cross sectional scanning electron microscope (SEM) images of the ®lms reveal isolated columns in the form of chevrons, helices and staircase structures of left or right chirality, posts, and bent nematics [5]. The average column separation and bulk porosity are observed to increase as the angle u between the substrate normal and the trajectory of the incident vapour is made increasingly oblique. Glancing angle deposition (GLAD) has been put forth to describe the speci®c deposition regime delineated by u $ 708C, for which the bulk porosity becomes greatly enhanced [6]. The porosity of GLAD ®lms arises from the self-shadowing effect, which may operate down to atomic scale lengths under conditions of low surface diffusion [7]. Geometric arguments [8] and simulations [1,9] have been used to clarify the role of self-shadowing in producing the observed characteristics of the ®lms. The effectiveness of the selfshadowing mechanism can be varied during deposition (by adjusting u) to control the aerial density of the ®lm as a function of depth, permitting the production, for example, of ®lms exhibiting density layering. A number of possible * Corresponding author. Tel.: 178-492-3332; fax: 178-492-1811. E-mail address: [email protected] (D. Vick)
applications are suggested by the properties of the ®lms, including sensors and catalysis (porosity) and passive optical devices (control of refractive index, chirality) [10]. To date, GLAD ®lms have been produced primarily by means of thermal or electron beam evaporation. The list of evaporants that have been investigated includes MgF2, CaF2, SiO, Si, SiO2, Cu, Cr, Mn, Al, Ti, Pt, and ZrO2. More recently, Ti GLAD ®lms have been deposited by low pressure, long throw (LPLT) sputtering [11]. In this letter we further extend the versatility of the technique by demonstrating the ®rst porous carbon GLAD ®lms grown by means of a pulsed laser deposition (PLD). Pulsed laser deposition has become an increasingly popular technique over the preceding decade for the production of thin ®lms [12] There are a number of attractive characteristics of PLD sources which make them suitable for the GLAD technique. First, the high energy density that may be delivered to the target allows one to evaporate materials with melting or sublimation temperatures above the capabilities of traditional PVD systems. Second, the source size is typically very small, leading to a nearly parallel particle ¯ux at the substrate which facilitates the effectiveness of the self shadowing mechanism. Finally, a typical PLD chamber is readily recon®gured to allow for deposition from multiple sources. A simpli®ed schematic of the experimental set-up used in the present work is shown in Fig. 1. The target-substrate assembly was situated in a vacuum chamber pumped to a base pressure of 7 £ 1025 Torr (9:3 £ 1023 Pa). The plasma source was produced using a Lumonics Excimer-500 krypton ¯ouride laser con®gured as an unstable resonator oscil-
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00274-6
50
D. Vick et al. / Thin Solid Films 350 (1999) 49±52
The wafer pieces were af®xed to rotating mounts which allowed the azimuthal orientation of the substrate to be varied during deposition, while keeping u ®xed. The rate of rotation with respect to the laser shot rate could be controlled by incrementally stepping the rotation throughout the depositions. This allowed us to reproduce by PLD a number of the GLAD ®lm morphologies that have been reported previously.
Fig. 1. The experimental con®guration used to deposit porous carbon ®lms.
lator and operating at a wavelength and repetition rate of 248 nm and 15 Hz. The laser beam was brought to the vacuum chamber and focused by a 50 cm focal length lens onto a graphite tape target at an incident angle of 408. The nominal laser pulse energy and duration were 20±40 mJ and 15 ns. Focal spot measurements taken using a ®ltered CCD camera positioned at the target plane indicate that 90% of the laser energy was incident within an area 7:6 £ 1023 cm 2. These parameters resulted in a nominal intensities of 1:6±3:2 £ 108 W/cm 2 at the target. The same measurements indicate that peak intensities were 3:1±6:3 £ 10 8 W/cm 2. The graphite tape was wound onto a rotating tape drive assembly which was activated during depositions to ensure that an undamaged target area was exposed for each laser pulse in order to minimise the shot to shot variation in the characteristics of the expansion plume. Under the conditions described above, the expansion plume consisted of carbon ions, electrons, neutral molecules, clusters, and particulate ejecta of nanometer scale size. During depositions the chamber pressure climbed to 3:3 £ 1024 Torr (4:4 £ 1022 Pa). This pressure was suf®ciently low to ensure that there was a line of sight ¯ight of the evaporant particles to the substrates after an initial expansion near the target surface. The substrates consisted of cleaved pre-cleaned silicon wafers pieces approximately 2 cm 2, positioned 8.0 cm from the laser plume source and oriented to produce an obliquely-incident ¯ux of u 828 upon the substrates.
Fig. 2. SEM images of a carbons ®lm deposited under conditions of ®xed deposition angle u 82 and substrate rotation. For the ®lm of (a), the substrate was rotated twice during deposition. For the ®lm (b), the substrate was rotated continuously at a rate of 3.2 rev/min during deposition. (c) Top view of the ®lm (b), demonstrating porosity.
D. Vick et al. / Thin Solid Films 350 (1999) 49±52
The combined effect of the self shadowing mechanism and substrate rotation on the sub-structure of the ®lms is illustrated in the SEM images of Fig. 2. For the ®lm of Fig. 2a, the substrate was rotated at a uniform rate with respect to the number of laser shots such that the substrate completed two complete revolutions throughout the course of the deposition. The resulting morphology consisted of isolated helical columns of pitch equal to one half the ®lm thickness. During the deposition, new material is added only at the column tips such that growth is directed towards the source. The handedness of the helix thus opposes the sense of the substrate rotation. The pitch of the helical columns can be controlled by varying the rotation rate of the substrate and deposition rate of the ®lm. If the rotation rate of the substrate is increased, the pitch of the helices diminishes and eventually the columns reach the limiting form of vertical posts, as exempli®ed in Fig. 2b, where a rotation rate of 3.2 turns per minute was employed. The top view of the vertical post ®lm is shown in Fig. 2c which illustrates clearly the porosity of the ®lm and isolated nature of the columns. The structures are very similar to those that have been produced using GLAD by other PVD methods [5,11]. Based on the results of previous work, a greater degree of porosity should readily be achievable in PLD ®lms by increasing the incident angle u . The nominal Ê /s for deposition rates normal to the substrates were 0.8 A Ê /s for the ®lm of Fig. 2b, and the ®lm of Fig. 2a and 1.0 A were thus comparable to those achieved in LPLT sputtering of Ti GLAD ®lms [11]. Additional measurements were performed in order to characterise the expansion plume. The substrate assembly was replaced by a quartz crystal thickness monitor (CTM) and a Faraday cup positioned 6.0 cm away from the target in close proximity to one another in order to collect the expansion plume along a trajectory intercepting the substrate position. The CTM head was oriented such that the expansion plume impinged at normal incidence onto the crystal. A Ê /s was recorded for conditions nominal deposition rate of 5 A similar to the depositions of Fig. 2. From shot-averaged time of ¯ight Faraday cup measurements, a characteristic ion energy of 82 eV was obtained for the temporal peak of the ion current. The kinetic energy of the ion component of the expansion is thus much more energetic than that normally associated with thermal or electron beam evaporators, for which the vapour species typically possess energies of a fraction of an eV. The fraction of the total carbon plume deposited in the form of ions was estimated by integrating the Faraday cup current and comparing this value to mass deposition rate as inferred from the CTM measurements. Preliminary results indicate that the ions comprise about 10% of the mass of the deposition plume for the laser/target conditions of this experiment, assuming that the average charge state of the ions was 1. A more thorough characterisation of the angular distribution and charge state distribution of the ions as a function of incident laser intensity is currently in progress and will be reported at a later date.
51
No attempt was made to characterise the energies or fractions of the neutral species, which comprises the bulk of the expansion. A previous study of PLD produced carbon plasmas, performed at laser intensities comparable to the present work, made use of laser-induced ¯orescence diagnostics to obtain characteristic energies of , 12 eV for the neutral carbon species [13]. These authors also reported typical ion energies of 80 eV based on Langmuir probe measurements, in good agreement with our own measurements. Carbon ®lms produced at normal incidence at such deposition conditions are usually very hard, and contain many diamond-like s-bonds [14,15]. It is expected that each of the columns produced in the present process may also be composed of diamond-like carbon. The remaining component of the expansion plume, namely particulate debris, results from the shock unloading at the laser focus and was evident in top view, low magni®cation SEM images of our ®lms. Particulates are a characteristic and undesirable feature of PLD ®lms [16]. One approach to the debris problem involves the con®nement and guiding of the plasma expansion plume along a curved trajectory through the use of magnetic ®elds. Initial experiments have been conducted towards this end and have demonstrated the feasibility of the process [17]. In summary, it has been shown for the ®rst time that pulsed laser deposition can be employed to produce porous carbon ®lms. Further work devoted to the characterisation of these ®lms and the deposition process is required and will be forthcoming. The pulsed laser deposition technique can be employed for virtually any source material and can easily be used to deposit controlled stoichiometric mixtures of materials either through separate targets or using mixed sources to begin with. These preliminary results con®rm that the GLAD process can be employed with the higher deposition energies typically present in a laser plasma plume and is viable beyond the limits imposed on PVD systems. Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Alberta Microelectronic Corporation. The authors wish to thank George Braybrook for the SEM work. References [1] K. Robbie, L.J. Friedrich, S.K. Dew, T. Smy, M.J. Brett, J. Vac. Sci. Technol. A 13 (1995) 1032. [2] K. Robbie, M. Brett, A. Lakhtakia, Nature 384 (1996) 616. [3] R. Messier, T. Gehrke, C. Frankel, V.C. Venugopal, W. Otano, A. Lakhtakia, J. Vac. Sci. Technol. A 15 (1997) 2148. [4] F. Liu, M.T. Umlor, L. Shen, J. Weston, W. Eads, J.A. Barnard, G.J. Mankey, J. Appl. Phys. 85 (1999) 5486. [5] K. Robbie, J. Sit, M.J. Brett, J. Vac. Sci. Technol. B 16 (1998) 1115. [6] K. Robbie, M.J. Brett, J. Vac. Sci. Technol. A 15 (1997) 1460.
52
D. Vick et al. / Thin Solid Films 350 (1999) 49±52
[7] L. Abelmann, C. Lodder, Thin Solid Films 305 (1997) 1. [8] R.N. Tait, T. Smy, M.J. Brett, Thin Solid Films 226 (1993) 196. [9] D. Vick, L. Friedrich, S.K. Dew, M.J. Brett, K. Robbie, M. Seto, T. Smy, Thin Solid Films 336 (1999) 1. [10] A. Lakhtakia, R. Messier, M.J. Brett, K. Robbie, Innov. Mater. Res. 1 (1996) 165. [11] J. Sit, D. Vick, K. Robbie, M.J. Brett, J. Mater. Res. Comm. 14 (1999) 1197. [12] J.T. Cheung, in: D.B. Chisley, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994 p. 1.
[13] D.L. Pappas, K.L. Saenger, J.J. Cuomo, R.W. Dreyfus, J. Appl. Phys. 72 (1992) 3966. [14] J. Robertson, Pure and Appl, Chem. 66 (1994) 1789. [15] D.L. Pappas, K.L. Saenger, J. Bruley, W. Krakow, J.J. Cuomo, T. Gu, R.W. Collins, J. Appl. Phys. 71 (1992) 5675. [16] L. Chen, in: D.B. Chisley, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994 p. 167. [17] Y.Y. Tsui, D. Vick, R. Fedosejevs, Appl. Phys. Lett. 70 (1997) 1953.