Epitaxial growth of cerium oxide thin films by pulsed laser deposition
Introduction
Epitaxial cerium oxide (CeO2) thin films are technologically important material for various applications, such as semiconductors, superconductors, piezoelectric films and solid oxide fuel cells [1], [2]. CeO2 is a stable rare earth oxide and possesses the properties, such as high refractive index, high melting point, chemical stability, good adhesion, high ionic conductivity and thermal stability [3]. It exhibits simple cubic structure, high dielectric constant with a lattice parameter (a = 0.541 nm) very close to that of silicon (0.543 nm), √2a of YBa2Cu3O7 (YBCO) and good compatibility at its higher deposition temperature. The CeO2 is widely used as ultra thin gate oxide for complementary metal oxide semiconductor technology, stable capacitor devices for large scale integration, dynamic random access memory and high temperature oxidation resistant coatings. Due to its wide band gap and good transparency in the VIS-NIR regions, it is used in UV-blocking filters, single and multilayer coatings for optical devices and electro-chromic windows [4], [5], [6]. It has exceptional catalytic activities for the treatment of exhaust gas from automobiles and for hydrogen storing [7]. It is also used as corrosion resistant coating on metals like aluminium and stainless steel [8], [9].
In order to attain the epitaxial films such as SrTiO3, (Sr,Ba)Nb2O6 and YBCO on Si (100), the use of a buffer layer is indispensable [10]. The yttria stabilized zirconia (YSZ) and CeO2/YSZ epitaxial thin films are used as buffer layers on Si (100) substrates in electronic devices and high temperature superconductors [11], [12], [13]. The YSZ can deoxidize the native SiO2 layer on Si substrates and hence extensively used as a substrate material for making epitaxial YBCO superconducting films. CeO2 is used as a second buffer layer (YBCO/CeO2/YSZ/Si(100)) for YBCO films on both single crystal substrates and flexible metallic substrates due to its small lattice mismatch with YBCO and YSZ and also prevent the chemical reaction between YSZ and YBCO [14].
CeO2 film requires some basic properties, such as appropriate orientation, crystallinity and smooth surface for being used as buffer layer [15], [16] and in most cases, CeO2 film is necessitate to be epitaxial. There are only a few reports that deal with orientation, surface morphology and growth mode of CeO2 films [13], [16]. CeO2 epitaxial film can be prepared by many methods such as sputtering [17], [18], metal-organic chemical vapour deposition [19], molecular beam epitaxy [20], electron beam evaporation [21] and pulsed laser deposition (PLD) [16], [22]. Among these methods, PLD is attractive for growth of epitaxial films, improve layer-by-layer growth, stoichiometry and control of surface morphology. For all the materials, it is necessary to control the thickness and roughness of the thin films down to an atomic scale. The parameters such as target-substrate distance, ambient gas and its pressure, laser energy density (fluence), repetition rate (number of laser pulses per second) and substrate temperature can be varied for the deposition of films. All these parameters influence plasma formation, film growth and film properties. Generally, the properties of highly oriented films approximate the properties of single crystals. The deposited layers must have a large homogeneity with well-defined material properties, smooth surfaces and correct oxygen stoichiometry. A major advantage of PLD is the generation of species with high kinetic energy, which can provide crystal growth at relatively low temperatures preferable for semiconductor based device production [23], [24].
The aim of the present work is to prepare the high quality epitaxial CeO2 thin films on YSZ (100) single crystal substrates at different substrate temperatures, laser energy densities and repetition rates and hence to investigate their influence on epitaxy, growth mode and surface morphology.
Section snippets
Experimental details
Commercially available CeO2 (99.9% purity, Acros, USA) powder was compacted into a pellet of 25 mm diameter and 4 mm thickness. The pellet was sintered at 1473 K for 6 hours. The sintered pellet was used as target for PLD. The epitaxial CeO2 thin films were deposited on YSZ (100) single crystal substrates with one side polished, under an optimized oxygen partial pressure of 3 Pa using a KrF excimer laser (λ = 248 nm). Three different sets of experiments were performed with the variation of
Effect of substrate temperature
The sintered CeO2 pellet was found to be phase pure CeO2 with cubic structure (a = 0.541 nm, JCPDS No. 34–0394) [5], [6] and used for the preparation of epitaxial films. YSZ substrate is the most appropriate for the preparation of CeO2 epitaxial film, because CeO2 and YSZ have similar lattice constants (0.5411 nm and 0.5161 nm), so that the misfit parameter is small (5%). The misfit parameter has a strong influence on the epitaxial formation. In addition, the two materials have the same crystal
Conclusions
The CeO2 (200) epitaxial films were deposited on YSZ (100) single crystal substrates at various temperatures, laser energy densities and repetition rates by PLD. The XRD studies showed the cube-on-cube epitaxy for the films deposited in the substrate temperature range 673–973 K, low energy densities of 1–3 J/cm2 and repetition rates of 5–25 Hz. The films deposited at higher energy density of ≥ 4 J/cm2 and higher repetition rate of 30 Hz showed deviation from epitaxial nature. AFM analysis showed
Acknowledgments
The authors are thankful to the National Research Foundation of Korea (NRF) for the grant funded by the Korea Government (MEST) No. 2012-0009455.
References (33)
- et al.
Thin Solid Films
(1992) - et al.
Mater. Sci. Eng. B
(1996) - et al.
Thin Solid Films
(2011) Surf. Sci. Rep.
(2000)- et al.
Thin Solid Films
(2009) - et al.
Surf. Coat. Technol.
(2006) - et al.
Thin Solid Films
(2003) - et al.
Physica C
(2000) - et al.
J. Cryst. Growth
(2000) - et al.
Physica C
(2001)
Surf. Sci.
Physica C
Thin Solid Films
Thin Solid Films
Mater. Sci. Eng. B
Physica C
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