Elsevier

Applied Surface Science

Volume 307, 15 July 2014, Pages 455-460
Applied Surface Science

Effects of substrate and ambient gas on epitaxial growth indium oxide thin films

https://doi.org/10.1016/j.apsusc.2014.04.056Get rights and content

Highlights

  • Epitaxial indium oxide thin films were grown on sapphire and LaAlO3 single crystal substrates.

  • Stoichiometric In2O3 films are formed under oxygen.

  • Nanocomposite In2O2.5 films are formed under argon ambient.

  • Various orientation relationships depend on the substrate symmetry and gas ambient.

  • Domain matching epitaxy describes the in-plane epitaxial relationships.

Abstract

Indium oxide thin films were grown by pulsed electron beam deposition method at 500 °C on c-cut sapphire and (0 0 1) oriented LaAlO3 single crystal substrates in oxygen or argon gas. The effects of ambient gas and substrate symmetry on the growth of indium oxide thin films were studied. Stoichiometric In2O3 films are formed in oxygen, while oxygen deficient In2O2.5 films are grown in argon, with In metallic nanoclusters embedded in a In2O3 matrix (nanocomposite films). In both cases, epitaxial In2O3 films having the bixbyite phase were grown with various orientation relationships, depending upon the substrate symmetry and gas ambient (oxygen or argon). Domain matching epitaxy was used to describe the precise in-plane epitaxial film–substrate relationships. The differences in film texture were correlated to the differences in growth conditions, while the differences in the film properties were correlated to the film oxygen composition.

Introduction

Owing to its specific optical (high transparency in the visible domain) and electrical (high conductivity) properties, indium oxide (In2O3) is used in a lot of applications in thin film form [1], [2], [3], [4]. However, transport properties of this oxide are still a matter of discussion [5], [6], [7], [8], and therefore the growth of In2O3 epitaxial thin films has been studied to determine their intrinsic physical properties [9], [10], [11]. The ideal substrate for the epitaxial growth of In2O3 thin films is cubic Y-stabilized ZrO2 (YSZ) due to the small mismatch (1.7%) between the In2O3 bixbyite and the YSZ fluorite [11]. Such epitaxial In2O3 films on YSZ present interesting transport properties [12], [13] with electron mobility as high as 226 cm2/V s [12]. Epitaxial In2O3 thin films were also obtained on c-cut sapphire substrate [10] despite the higher film–substrate mismatch, but the electron mobility was lower than that for the films grown on YSZ [9]. All these results were obtained on (1 1 1) oriented In2O3 films. From both applied and fundamental aspects, it is important to understand the pertinent factors affecting the structural characteristics and physical properties in epitaxial In2O3 films.

In this paper, we report thus on the study of the growth of In oxide films formed on different substrates and under different gas ambient (oxygen and argon). Two single crystal substrates were used: c-cut sapphire and (0 0 1) oriented cubic LaAlO3. For the latter substrate, epitaxial indium oxide thin films were not reported yet. According to the differences in substrates symmetry, different film textures and microstructures are expected. Moreover, as the precise oxygen composition influences the nature, structure and properties of oxide films [14], [15], [16], the effect of oxygen deficiency in indium oxide films was also checked in this work. Pulsed electron beam deposition (PED) was used to grow such films since it allows the control of the oxygen incorporation in the films [15]. Epitaxial indium oxide films were thus obtained with texture and epitaxial relationships depending upon the substrate symmetry and growth conditions. These structural differences are related to the differences in the indium and oxygen fluxes reaching the surface of the growing film, while physical properties are mainly depending on the oxygen composition of the films.

Section snippets

Experimental

The In oxide films were grown on c-cut sapphire, (1 0 0) Si and (0 0 1) LaAlO3 oriented substrates by the PED method in the experimental setup previously described [8], [9], [10], [11], [12], [13], [14], [15]. The growth system consists of a pulsed-electron beam source in the channel configuration delivering pulses with 100 ns duration and 2.5 J/cm2 fluence. Films in the 50–700 nm thickness range were grown at 500 °C. The pulsed-electron beam ablated a high purity In2O3 target in pure Ar or O2 gas at a

Results

It has been previously reported that in pulsed-energy beam deposition methods like PED, the stoichiometry of the oxide films is controlled by the partial oxygen pressure during the growth [15], [18], [19]. Indeed, the flux of oxygen atoms reaching the surface of the growing film depends upon the oxygen partial pressure (PO2), and as a result the incorporation of oxygen atoms is reduced when PO2 is decreased. In this work, In oxide films have been grown under either Ar at a pressure (2 × 10−2 mbar)

Discussion

The surface morphology of the indium oxide depends upon the growth conditions. The surface of films grown in argon gas present nanostructures whose density and size increase with temperature. The temperature which induces crystallization of the nanoparticles plays a role on the nanostructure formation, but it has been also observed that the films grown at the same temperature in oxygen gas do not present the same surface nanostructures. The temperature is not the sole parameter, and the precise

Conclusion

Indium oxide thin films were grown by pulsed electron beam deposition method on c-cut sapphire and LaAlO3 single crystalline substrates under oxygen or argon pressure, respectively. Stoichiometric In2O3 films are grown under oxygen, while largely oxygen deficient In oxide films are formed under argon, i.e. in fact nanocomposite films with In metallic nanoclusters embedded in a In2O3 matrix are observed in this last case. Whatever the gas ambient during the growth epitaxial films with various

Acknowledgements

M. Nistor would thank a grant of the Romanian National Authority for Scientific Research, CNCS–UEFISCDI, project number PN-II-ID-PCE-2011-3-0566. The cooperative structure around SAFIR (Université Pierre et Marie Curie-Paris 6) is acknowledged for the RBS measurements.

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