Epitaxial growth of cerium oxide thin films by pulsed laser deposition

Highlights

• Films deposited at an optimized pressure 3 × 10^− 2 mbar for stoichiometric.

• At a low temperature of 673 K itself, epitaxial CeO₂ films are formed.

• Influences of repetition rate and fluence on epitaxy are studied systematically

• Growth mode and surface morphology are studied systematically

Abstract

The epitaxial cerium oxide (CeO₂) thin films were deposited on yttria stabilized zirconia (YSZ) (100) substrates at various substrate temperatures (673–973 K), energy densities (1–5 J/cm²) and repetition rates (5–30 Hz) with an optimized oxygen partial pressure of 3 Pa, by pulsed laser deposition technique. The films were characterized by X-ray diffraction and atomic force microscopy to study the influence of substrate temperature, laser fluence and repetition rate on epitaxy, growth mode and surface morphology. The X-ray diffraction studies revealed the epitaxial nature of CeO₂ (200) films on yttria stabilized zirconia (100) substrate (CeO₂ (200) ‖ YSZ (100)) deposited in the temperature range 673–973 K. The films prepared at low energy densities (1–3 J/cm²) and low repetition rates (1–25 Hz) also indicated the fully epitaxial nature, whereas the films prepared at higher energy density (≥ 4 J/cm²) and repetition rate (30 Hz) indicated deviation from epitaxy. The atomic force microscopy studies showed the formation of dense and uniform nanocrystallites with smooth morphology. The root mean square surface roughness of the films increased with the increase of substrate temperature, increase of energy density and repetition rate.

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.