Oxide thin films deposited by means of laser ablation
– structure and properties
Jan Kusinski*a, Agnieszka Kopia a, Magdalena Chmielowska a, Slawomir Kac a,
Jan Marczak b, Jozef Firak ba AGH – Technical University of Science and Technology, 30 Mickiewicza Ave.,
30-059 Krakow, Poland
b Military University of Technology, 2 Kaliskiego Str, 00-908 Warsaw, Poland
ABSTRACT
Thin films of CeO2 doped with Cu or Nd and Bi2O3 doped with Y were elaborated by Pulsed Laser Deposition technique from sintered Cu-CeO2, Nd-CeO2 and Bi2O3or Y- Bi2O3 targets. Two types of laser ablation equipment have been applied: one worked with a KrF excimer and second with an Q-switched Nd:YAG. The films were deposited on (100) and (110) oriented Si substrates. Scanning and transmission electron microscopy, as well as, x-ray diffraction analyses showed correlation between a copper and neodymium atom fractions and crystalline structure of the (Cu, Ce)O2 and (Nd, Ce)O2 thin films. As demonstrated by x-ray diffraction analysis, with increased quantity of Cu, and Nd the both types of doped CeO2 thin films manufactured by laser ablation show a change of the crystal growth preferential orientation (c-axis-orientation) from strong <111> to a strong <200> ones. In case of pure bismuth oxide deposition, both TEM and X-ray examinations revealed that at applied experimental conditions only α - Bi2O3 (not δ - Bi2O3) crystals were formed during ablation process. Microstructural TEM examinations permitted to show high grain refinement in Y- Bi2O3 thin films. Preliminary measurements show also high level of electrical conductivity in case of this oxide. Due to SiO2 amorphous layer present at the surface of Si substrate, its crystallographic orientation doesn’t influence the thin film structure of all analyzed oxides.
Keywords: nanomaterials, thin films, cerium dioxide, bismuth oxide, gas sensors, pulsed laser deposition
1. INTRODUCTION
1.1. Laser Ablation Technique
Almost in whole industrially developed world, lasers have found many attractive applications in processes of manufacturing and treatment of materials, e.g. in such technologies like: cutting and welding, surface treatment (hardening, glazing, alloying and cladding), as well as, laser forming (bending). Laser technique is also used in processes of new materials synthesis from other materials (gazes, liquids and solids). Laser vaporization of materials is well known for many years (it was observed parallel with development of ruby lasers), however, for materials synthesis, known as LAPVD – laser assisted physical vapor deposition has been applied in 80-thies and is still under expansion for practical applications1-5.
In laser ablation process, an intense, pulsed, with a high frequency, laser beam is focused on the surface of a target material. During early stages of the ablation process several subsequent processes: photon energy absorption, rapid heating and melting, vaporization and vapor ionization, plasma emission, plasma heating, as well as detonative plasma expansion, take place on the laser beam/target interface. The ablated material, in the form of some volatile phases, e.g. a gas, plasma or droplets of liquid, is deposited onto the substrate with the well defined structure and orientation. With increased laser fluency, the ablation process is dominated by the plasma formation rather than by the surface heating and melting. In general, with the high fluency values, higher than the ablation threshold one can significantly reduce the droplet formation in PLD. The thin film grows with velocity reaching of a few dozens of nanometer/pulse. The LAPVD is probably one of the simples techniques for thin films manufacture1-7. Vapor cloud emitted from the target material during ablation process show many singularities, different from these of vapors formed in other PVD techniques8. Actually, it is well experienced that the LAPLD has a unique capability to produce high-quality thin films of various kinds of materials.
* kusinski@https://www.doczj.com/doc/ce17678806.html,.pl; tel/fax: 48 12 617 3344
Laser Technology VIII: Applications of Lasers,
edited by Wieslaw L. Wolinski, Zdzislaw Jankiewicz, Ryszard S. Romaniuk,
Proc. of SPIE Vol. 6598, 659808, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.726531
Laser ablation technique is successfully applied in high quality thin films production for electronic companies. The technique is also frequently used in elaboration of thin films used as a gas sensors and catalytical devices, multilayer coatings for tribological and anticorrosive applications, as well as, thin films for microelectronics (thin films of superconductors and piezoelectrics, like: YBaCuO, SrBiTaO and SrBiNbO, as well as, hard coatings TiC, TiN, CrN and multicoatings WC/C/WC/C/WC/C. 6.
1.2. Oxide gas detectors and catalysts
These days the human body is exposed to several different risks; the polluted environment produces many of them. It is commonly known that nitrogen oxides (NO2 is more dangerous than NO because causes the lung diseases) are especially dangerous for a human9, indeed, its precise detection is very important, (e.g.: in case of cars and many industrial processes). Adamian at al.10 have been analyzed several oxide materials as a potential gas detectors for a NO/NO2 detection and have showed that bismuth oxide - Bi2O3 is a suitable material for the NO detection and differentiation between NO and NO2. It was proved that, when mixed with other semiconductive oxides, Bi2O3 may be a good detector for CO, NO and H2 gases11,12, as well as ethanol13. Also, bismuth oxides (mainly δ-Bi2O3), are well known as a very good ionic conductors14. The δ-Bi2O3 phase, with the fluorite CaF2 structure (FCC), which shows the highest ionic conductivity between other Bi2O3 phases, is stable only at high temperature range (between 729-825o C), what limits its potential applications15. During slow cooling, below 729o C, the δ-Bi2O3 phase transforms into: tetragonal β-Bi2O3, face centered cubic γ- Bi2O3 and monoclinic α-Bi2O3 present at lowest temperatures.
Searching for an answer to the question: how to extend the δ-Bi2O3 phase presence to lower temperature range, have been actually the aim of many research13-17. One of the shown solutions for the δ-Bi2O3 phase stabilisation was bismuth oxide doping with other elements16. There are works in which the authors, by doping with Mo15, W16, Mg and Zn17, showed successful the δ-Bi2O3 phase stabilisation at low temperatures. The other method of the δ-Bi2O3 phase stabilisation at low temperatures basis on the grain refinement, e.g. via galvanic thin film deposition15, 18. Considering the foregoing, it seems that the laser ablation technique could be an appropriate method for obtaining thin films of the δ-Bi2O3 stable at the low temperature range.
Rare earth elements and their compounds (oxides, carbides, nitrides and intermetallics) play important role in development of many technologies, what is connected with their specific, frequently still not well-determined properties. Cerium dioxide CeO2 shows increased practical applications, e.g.: in the form of powders (pigment in ceramics, polishing material) or solids and thin films (catalysts, gas sensors, as well as, materials for optical, optoelectronic and electronic applications)19-22. Since ceria solid solutions are also good oxygen-ion conductors, they are of great technical interest as solid electrolytes and electrodes in high temperature electrochemical devices, especially in solid oxide fuel cells (SOFC). Ceria has also been currently used as an active component in the oxidation of CO and CH4 and reduction of (NO)x for automotive exhaust23, 24. Doped CeO2 oxide with trivalent rare earth cations shows high catalytic properties and oxide ion conductivity at elevated temperatures25, 26. In this context cerium dioxide can be used in gas sensor devices to detect toxic gases in air. Gas sensors are based either on the catalytic reactions between sensitive materials and gases, or by modification of the surface conductivity of active materials in contact with gas. In the case of catalytic gas sensors, the active materials can be in form of powders or thin films.
Indeed, the general aim of this work was to fabricate by pulsed laser deposition technique and investigate the microstructure and properties of Cu- and Nd-doped CeO2, as well as, Bi2O3 and Y-Bi2O3 thin films.
2. EXPERIMENTAL
The pulsed laser deposition was applied to produce thin films of pure and Cu or Nd doped cerium dioxide, as well as, Bi2O3 and Y- Bi2O3 oxide on the (100) and (110) silicon substrates. The targets were (Cu x,Ce1-x)O2, (Nd x, Ce1-x)O2 compounds containing different atomic volume fractions of copper and neodymium (0, 15, 27 % at.), as well as Bi2O3 and Y- Bi2O3 oxide (0 or 15wt% of Y). The targets were prepared by compacting of: CeO2 with Cu or Nd powders or Y2O3 with Bi2O3 powder under pressure of p = 109 Pa for 5 min. Then the pellets were sintered at T=1200 o C for 2 hours. The final diameter and thickness of each target were: 30 mm and 2 mm, respectively. Nanocrystalline CeO2, (Cu,Ce)O2 and (Nd, Ce)O2 thin films were produced by using an excimer laser (system Complex 301 of the Lambda Physics,
Germany), working with the following parameters: the pulse duration of τ = 30ns, the wavelength λ = 248 nm (KrF radiation), the frequency of f = 10 Hz, the laser beam energy density in the laser focus on the target was ε = 1.5 10-4 J m-2, a laser pulse energy E = 1 J. The Bi2O3 and Y- Bi2O3 thin films were produced using the ablation system - equipped with a Nd:YAG laser (operating with first or fourth harmonic), installed recently at AGH in Krakow (Fig. 1).
Fig. 1. Laser ablation system, equipped with a Nd:YAG laser, working at AGH in Krakow.
Films were obtained in the vacuum chamber under the air atmosphere of p = 4 Pa. The characteristics of the deposition process were following: a fixed laser beam parameters and the target-substrate distance of 60 mm. During the deposition process the laser beam hits the target at an incidence angle Θ = 45°. Both the substrate and target were placed parallel. The temperature of the substrate and the deposition time were constant; 750°C and 360 s, respectively. After the deposition, the films were analyzed ex-situ by several techniques: scanning and transmission electron microscopy and X-ray diffraction. The SEM studies were performed with a Hitachi 3500 N SEM coupled with Noran Energy Disspersive Spectroscopy (EDS) and the TEM images were obtained with a JEOL 2010. The thin films for TEM examinations (cross-sections and plain-view) were prepared by using ion milling technique. X-ray patterns were collected using a Siemens – Bruker D5000 diffractometer equipped with a copper X-ray source (λKα1=1.54056?, λKα2=1.54439?), a secondary monochromator (withdraw the Kβ X-ray) and a rotating sample holder, working in a classical coupled θ-2θmode. In addition, in case of ceria oxides, the catalytic ability and electrical conductivity were measured.
3. RESULTS AND DISCUSSION
3.1. CeO2 and (Cu x,Ce1-x)O2 PLD samples
SEM inspection of all (Cu x,Ce1-x)O2 PLD samples showed that the surface morphology of the deposited films varies depending on the atomic volume of copper. Figures 2(a, b) are SEM images showing examples of (Cu x,Ce1-x)O2 thin film surfaces obtained by applying the same process parameters but from targets having different Cu contents of: 0 and 27 %at. The crystal tips in the pure CeO2 thin film presents a triangular morphology (Fig 1a). The linear dimensions of 3 symmetry axis triangles are less than 10 nm. With cooper addition the crystal-tip morphology changed and a rod-like shape appeared. In Fig. 1b (which represents the sample doped with 27 at.% of Cu) only a rod-like crystal-tips are present. The dimensions of these rectangular rods are about 13,5 nm (length) and 3,5 nm (width). Indeed, when examining the thin film external surface by means of SEM, it can be stated that with increasing concentration of copper in (Cu,Ce)O2 compound the crystal morphology changes from a triangular to a rod-like. TEM examinations revealed columnar structure of thin films, with Cu2O film laying at the CeO2 crystal interfaces (Fig. 3a, b and Fig 4a, b). TEM examinations of both thin film sections (transversal and parallel) showed that CeO2 crystals are nano-sized. Figure 3 shows the TEM images of the transversal cross-section of thin film doped with 27 at. % of Cu. This examination indicated columnar growth of (Cu,Ce)O2 crystals.
The growing crystal diameter varying from 30 to 60 nm and the thickness of the deposited film is about 250 nm. Presence of cracks was visible in the samples containing higher Cu quantity. The cracks started from the SiO 2 amorphous layer. The EDS measurements of thin films (as measured on cross-sections prepared for TEM analysis) show mean values of all compound elements (Cu, Ce and O) to be in good agreement with the nominal composition of the target 27. The analysis of the x-ray diffraction patterns showed the presence of the CeO 2 phase in every deposited sample. The major difference, observed between the XRD diagrams obtained from the undoped and Cu doped (with different Cu quantity) thin films, is the inversion of (111) and (200) peak intensities (see Fig. 2 a, b, c).
Fig. 2. SEM micrographs of showing the morphology of the CeO 2 thin film surface: (a) Cu-0%, (b) Cu-27%.
Fig. 3. TEM images showing microstructure (at low (a) and high-(b) magnification) of the PLD (Cu, Ce)O 2 thin film (transversal
cross-section, sample containing 27 at. % of Cu).
Nanocrystalline CeO 2 thin films, pure and doped with 27 at. % of Cu, were investigated as catalyst for CH 4 oxidation 27. It was shown that Cu doped ceria exhibits high catalytic activity for methane (CH 4) oxidation.
3.2. (Nd x , Ce 1-x )O 2 PLD samples
The SEM examination of (Nd x , Ce 1-x )O 2 PLD samples showed that the surface morphology of the deposited films, is different than that of (Cu x ,Ce 1-x )O 2 thin films and did not varies visibly, with the atomic volume of neodymium. Figures 6(a, b) are SEM images showing examples of (Nd x , Ce 1-x )O 2 thin film surfaces obtained by applying the same process parameters but from targets having different Nd contents of: 6,5 and 27 mol%.
a b O
CeO SiO a
CeO b
a CeO 2
CeO 2
CeO 2CeO 2
Cu 2O
b
Fig. 4. TEM images showing microstructure (at low (a) and high-(b) magnification) of the PLD (Cu, Ce)O 2 thin film (longitudinal cross-section, sample containing 27 at. % of Cu).
Fig. 5. X-ray diffraction patterns of (Cu, Ce)O 2 thin films containing different Cu atom fractions: (a) Cu-0%, (b) Cu-15%, (c) Cu-27%; note: changes of intensity of (111) and (200) peaks.
Cu-0-360
Cu-15-360
a
b
c
Fig. 6. SEM micrographs of showing the morphology of the CeO 2 thin film surface: (a) Nd-0%, (b) Nd-27 mol%.
Fig. 7. TEM images showing microstructure (at low (a) and high-(b) magnification) of the PLD (Nd, Ce)O 2 thin film (longitudinal cross-section, sample containing 27 mol % of Nd).
The crystals in the (Nd x ,Ce 1-x )O 2 thin film presents nano-size cell structure (Fig. 7a, b). The analysis of the x-ray diffraction patterns showed the same tendency of crystal growth preferential orientation changes (c-axis-orientation) from strong <111> to a strong <200> ones with Nd addition, as was observed in the case of Cu doping 27. Microstructural (both: EDS and TEM) and EDS examinations revealed presence of Nd-rich phase at the crystal boundaries 27.
3.3. Bi 2O 3 and Y- Bi 2O 3 PLD samples
Figures 8 and 9 show TEM microstructure of the PLD deposited Bi 2O 3 thin films. The film was deposited on (011) Si monocrystalline substrate at room temperature. Depending on deposition time the film thickness varied from 140 – 240 nm. The formed Bi 2O 3 crystals were nanomentric in size.
In contrary to the ceria thin films, the Bi 2O 3 thin films did not show the columnar structure. During Bi 2O 3 and Y- Bi 2O 3 thin film deposition oxygen atmosphere was used, otherwise the pure Bi was deposited. Indeed, due to oxygen presence the amorphous film of SiO 2 was formed at the Si surface (Fig. 9) and there was not coherency between Si substrate and deposited Bi 2O 3 film crystals. Both TEM and X-ray examinations revealed that at applied experimental conditions in the case of pure bismuth oxide deposition only α - Bi 2O 3 (not δ - Bi 2O 3) crystals were formed during ablation process. Microstructural TEM examinations permitted to show high grain refinement in Y- Bi 2O 3 (Figs. 10a and b). Preliminary measurements show also high level of electrical conductivity in case of this oxide, however it structural characterization is still under verification.
a b
a b
Fig. 8. TEM images showing microstructure (at low (a) and higher-(b) magnification) of the PLD Bi 2O 3 thin film; transversal cross-section.
Fig. 9. HRTEM image showing microstructure of the PLD Bi 2O 3 thin film; transversal cross-section.
2O 3 thin film (transversal cross-section, sample containing 15wt. % of Y).
70 nm
a 60 nm
b
1
11_
1
11_
_6 nm
40 nm
3 nm
a b
4. CONCLUSIONS
The results obtained in this work lead to the following conclusions:
?The PLD method allows to produce homogeneous and nanometric pure and Cu or Nd doped CeO2 thin films on (100) Si substrate as well as Bi2O3 thin films on (011) Si substrate,
?With increasing copper additions, the CeO2 thin film surface crystal changes from triangular to rod-like structure,
?For low Cu and Nd contents, the <111> crystal texture is present, while for higher Cu and Nd additions changes to <200>,
?Cu2O and Nd2O3 amorphous films form at the CeO2 crystal surfaces, while the SiO2 amorphous film grows at the Si substrate,
?Catalytic tests in presence of CH4 gas have shown that such nanomaterials present high catalytic effects when thin film is <200> oriented and doped with 27 % at. Cu,
?Neodymium additions to CeO2 oxide (for analyzed values) don’t influence its catalytic properties for CH4, ?Contrary to the ceria thin films, the Bi2O3 thin films did not show the columnar structure,
?Microstructural examinations show high grain refinement in the both: Bi2O3 and Y- Bi2O3 thin films,
?Preliminary measurements of electrical conductivity of Y- Bi2O3 thin films show its high level.
ACKNOWLEDGEMENTS
The authors wish to thank Committee of Scientific Research of Poland for financial assistance (project No.: PBZ-100/3/3/2004) and AGH-UST for technical support (project No.: 11.11.110.566).
REFERENCES
1.J. Singh: “Review: Laser-beam and photon-assisted processed materials and their microstructures”, Journal of
Materials Science, vol. 29, (1994), p. 5232
2.J. Kusinski: Lasery i ich zastosowanie w in?ynierii materia?owej, Wyd. Akapit, Kraków, 2000 (in Polish)
3. F. Beech, I.W. Boyd: Photochemical processing of electronic Materials, Academic Press, New York, 1999, s. 387-
429
4. B. Major: Ablacja i osadzanie laserem impulsowym, Wyd. …Akapit”, Kraków, 2002 (in Polish)
5.J. M. Lackner: Industrially-scaled hybrid Pulsed Laser Deposition at Room Temperature, Orecop SC. Krakow,
(2005),
6. A.A. Voevodin, S.J.P. Laube, S.D. Walck, J.S. Solomon, M.S. Donely, J.S. Zabinski: Journal of Applied Physics,
78, (1995), p. 4123
7. D.B. Chrisey, G.K. Hubler (Eds.): Pulsed Laser Deposition of Thin Films, John Wiley & Sons, New York, 1994
8. A.A Voevodin., M.S. Donely: “Preparation of amorphous diamond-like carbon by pulsed laser deposition: a critical
review”, Surface and Coating Technology, 82, 1996, p. 199-213
9. B. Gaston, J.M. Drazen, J. Loscalzo, J.S. Stamler: “The biology of nitrogen oxides in the airways”, Am. J. Respir.
Crit. Care Med. 149, (1994), p.538–551
10.Z.N. Adamian, H.V. Abovian, V.M. Aroutiounian: “Smoke sensor on the base of Bi2O3 sesquioxide”, Sensors and
Actuators B, 35–36, (1996), p. 241–243
11.G. Sberveglieri, G. Faglia, S. Groppelli, P. Nelli: “Methods for the preparation of NO, NO2 and H2 sensors based on
tin oxide thin films, grown by means of the r.f. magnetron sputtering technique”, Sensors and Actuators B, 8, (1992), p.79–88
12.A. Cabot, A. Marsal, J. Arbiol, J.R. Morante: “Bi2O3 as a selective sensing material for NO detection”, Sensors and
Actuators B, 99, (2004), p.74–89
13.W.M. Sears: “The gas-sensing properties of sintered bismuth iron molybdate catalyst”, Sensors and Actuators, 19,
(1989), p. 351–370
14.P. Shuk, H.D. Wiemh?fer, U. Guth, W. G?pel, M. Greenblatt: “Oxide ion conducting solid electrolytes based on
Bi2O3”, Solid State Ionics, 89, (1996), p.179–196
15.A. Helfen, S. Merkourakis, G. Wang, M.G. Walls, E. Roy, K. Yu-Zhang, Y. Leprince-Wang: “Structure and
stability studies of electrodeposited δ-Bi2O3”, Solid State Ionics 176, (2005), p. 629–633,
16.T. Takeyama, N. Takahashi, T. Nakamura, S. Itoh: “Heteroepitaxial growth of δ-Bi2O3 thin films on CaF2 (111) by
chemical vapour deposition under atmospheric pressure”, Materials Research Bulletin, ibid, 2006.
17.N. Kumada, N. Takahashi, N. Kinomura, A.W. Sleight: “Preparation of ABi2O6 (A = Mg, Zn) with the trirutile-type
structure”, Materials Research Bulletin, Vol. 32, 8, (1997) p. 1003-1008
18.J.A. E.W. Bohannam, C.C. Jaynes, M.G. Shumsky, J.K. Barton, Switzer: Solid State Ionics, 131, (2000), p. 97
19.J.H. Holles, M.A. Switzer, R.J. Davis: “Influence of ceria and lanthana promoters on the kinetics of No and NO2
reduction by Co over alumina supported palladium and rhodium”, Journal of Catalysis, 190, (2000), p. 247-260
20.J. Kowalczyk, Cz. Mazanek: Metale ziem rzadkich, WNT, Warszawa, 1988 (in Polish)
21.M. Morgensen, N. M. Sammes, G. A. Tompsett: “Physical, chemical and electrochemical properties of pure and
doped ceria”, Solid State Ionic, 129, (2000), p. 63-94
22.A. Tschope, W. Liu, M. Fletzani-Stephanopolous, J.Y. Ying: “Redox activity of nonseichiometric cerium oxide-
based nanocrystallized catalysts”, Journal of Catalysis, 157, (1995), p. 42-50
23.S.N. Jacobsen, U. Helmersson, R. Erlandsson, B. Skarman, L.R. Wallenberg: “Sharp microfaceting of (001)-
oriented cerium dioxide thin films and the effect of annealing on surface morphology”, Surface Science 429, (1999) p. 22-33.
24.B. Skarman, L.R. Wallenberg, P.O. Larsson, A. Andersson, S.N. Jacobsen, U. Helmersson: “Carbon Monoxide
Oxidation on Copper Oxide Thin Films Supported on Corrugated Cerium Dioxide {111} and {001} Surfaces”, J. of Catal.181, (1999) p. 6-15.
25.N. Yamada, Y. Oyama, T. Higuchi, S. Yamaguchi: “Physical, chemical and electrochemical properties of pure and
doped ceria”, Solid State Ionic 172, (2004) p. 293-297.
26.L. Aneflous, J.A. Musso, S. Villain, J.R. Gavarri, H. Benyaich: “Effects of temperature and Nd composition on non-
linear transport properties in substituted Ce1?x Nd x O2?δ cerium dioxides”, J. of Solid State Chem 177, (2004) p. 856-865.
27.M. Chmielowska: “Influence of Cu and Nd on microstructure, electrical conductivity and catalytic properties of
CeO2 thin films obtained by means of laser ablation”, PhD Thesis, AGH-UST, Krakow, 2005, (in Polish).