Elsevier

Applied Surface Science

Volume 257, Issue 22, 1 September 2011, Pages 9485-9489
Applied Surface Science

Mg diffusion in K(Ta0.65Nb0.35)O3 thin films grown on MgO evidenced by Auger electron spectroscopy investigation

https://doi.org/10.1016/j.apsusc.2011.06.041Get rights and content

Abstract

The diffusion of Mg in pulsed laser deposited K(Ta0.65Nb0.35)O3 thin films epitaxially grown on (1 0 0) MgO single crystal substrate were investigated by Auger electron spectroscopy (AES). A diffusion of Mg from the substrate into the whole thickness (400 nm) of the as-deposited K(Ta0.65Nb0.35)O3 films was observed with an accumulation of Mg at the surface. Ex situ post-annealing (750 °C/2 h) has led to a homogeneous distribution of Mg in all the ferroelectric coating. This strong reaction between film and substrate promotes a doping effect, responsible for the reduction of K(Ta0.65Nb0.35)O3 dielectric losses in comparison with films grown on other substrates.

Highlights

► Evidence of long range Mg diffusion from MgO substrate in as deposited KTN films. ► Investigation of Mg diffusion by Auger electron spectroscopy. ► Effect of annealing on homogenization of diffused Mg in KTa0.65Nb0.35O3 films. ► KTa0.65Nb0.35O3 thin films grown by pulsed laser deposition. ► KTa0.65Nb0.35O3 epitaxially grown on MgO.

Introduction

Nowadays, more and more interest is devoted to multifunctional oxides, which present a large variety of properties such as ferroelectricity, ferromagnetism, superconductivity, etc. [1], [2]. This class of materials, usually composed of more than two or three elements in addition to oxygen, presents relatively complex crystalline structures. The key point for the integration of these materials as thin films is the rigorous control of their composition and crystalline orientation onto substrates suitable for the targeted applications.

In the same time, the monitoring of the orientation of thin film and the achievement of an epitaxial growth require a judicious choice of the substrate according to its crystalline structure, leading to a low lattice parameter mismatch. Epitaxial growth is of first importance either to control the behaviour of anisotropic compounds or to improve structural properties, in terms of grain boundaries for instance. Possible chemical film/substrate interactions that can affect the physical properties have also to be considered, especially when high deposition temperature process is required. KTa1−xNbxO3 (KTN) family is a solid solution extended from KTaO3 (KT) to KNbO3 (KN). It presents a perovskite type structure with different symmetries depending on temperature and composition [3], [4]. At room temperature, KN is orthorhombic and presents a ferroelectric ordering, while KT is cubic and paraelectric. Hence, the paraelectric–ferroelectric transition temperature, named as Curie temperature TC, can be tuned by controlling the Ta/Nb ratio [3], [4]. All compositions along the solid solution could be described in a pseudo-cubic structure with very small lattice parameter changes [5]. This family of materials possesses strong electro-optic effect [6], [7], [8], strong piezoelectric coefficient [9], [10] and promising tunable dielectric properties [11] for microwave applications, which is the main interest in the present study.

Indeed, in the field of telecommunications, ferroelectric materials are potential candidates to achieve electrically reconfigurable microwave devices due to a high tunability, i.e. strong variation of their dielectric permittivity under a D.C. bias voltage [12], [13], [14]. However, their main drawback for such application is their quite high dielectric losses in the microwave range. The integration of ferroelectric thin film in coplanar microwave devices requires the use of a dielectric substrate presenting: (i) a relatively low dielectric permittivity ɛr in order to confine the electric field in the ferroelectric layer and to enhance its tunable properties; (ii) low dielectric loss tan δ to minimize the global losses of the device. Among the suitable substrates (1 0 0) MgO single crystal substrate has been chosen in numerous studies because of its lattice parameter close to that of KTN (at room temperature aMgO = 4.21 Å [15] and aKTN = 3.99 Å for x = 0.35 [16] corresponding to a mismatch of −4.8%) and its dielectric properties (at 10 GHz and room temperature, ɛr = 9.8 and tan δ < 2.10−5 [15]).

In the present paper, the study was focused on the deposition of KTa0.65Nb0.35O3, labelled hereafter as KTN(65/35), by pulsed laser deposition (PLD) on (1 0 0) MgO single crystal substrate. KTN(65/35) is expected to be in its paraelectric phase, i.e. cubic structure, in regards of the reported Curie temperature value (TC = −4 °C) of the bulk compound [3], [4]. Comparison of results obtained by different groups shows that routinely KTN films exhibit lower dielectric losses measured in microwave range when deposited on MgO substrate, compared to other substrates such as LaAlO3 and sapphire [17], [18], [19], [20]. It was a strong motivation to pay attention to possible Mg diffusion from the substrate into the film through Auger electron spectroscopy (AES) analyses.

Section snippets

Experimental

KTN(65/35) thin films (400 nm thick) were deposited on (1 0 0) MgO single crystal substrate by PLD using a KrF excimer laser (Tuilaser Excistar, pulse duration of 20 ns, λ = 248 nm) operating at 2 Hz with an energy of 210 mJ (corresponding to a fluence of 2 J cm−2). During the deposition, the substrate temperature and the oxygen pressure were kept constant at 700 °C and 0.3 mbar, respectively. The undesired non-ferroelectric pyrochlore phase, often observed with a slight deviation of potassium stoichiometry

As-deposited films

The θ XRD pattern of KTN(65/35) film on MgO, displayed in Fig. 1a, only exhibits peaks of the perovskite phase with a (1 0 0) orientation (indexation is given in the cubic system). The quality of the orientation is highlighted by the (1 0 0) rocking curve (see inset in Fig. 1a) with a full width at half maximum (FWHM) Δω = 1.5°. The in-plane ordering was probed by performing the {1 1 0} KTN φ-scan and the {2 2 0} MgO φ-scan. These patterns, presented in Fig. 1b, show an epitaxial cube-on-cube growth

Discussion

The AES study of the KTN films deposited on MgO substrate has revealed a diffusion of Mg all along the film as thick as 400 nm. It is worth noting that a careful analysis by AES of KTN thin films deposited on sapphire substrates (Mg-free) did not reveal any trace of Mg, discarding thus any external magnesium contamination coming for instance from substrate heater or deposition chamber. These results confirmed a previous hypothesis suggesting a possible Mg diffusion from secondary ion mass

Conclusion

In conclusion, we have observed by AES a diffusion of Mg coming from MgO single crystal substrate in the whole thickness of KTN films (400 nm thick). For the as-deposited films, a higher concentration of Mg in rounded islands of 1 μm in diameter is present at the surface of the film. The annealing treatment, often performed on the films intended to microwave applications, contribute to make the Mg distribution homogeneous in the bulk film. There are still open questions about the mechanism of

Acknowledgements

The authors want especially to thank Dr A. Perrin for helpful and fruitful scientific discussions. Work was financially supported by Region Bretagne and Thalès TRT (projects PRIR Discotec and MATCOM).

References (43)

  • L.W. Martin et al.

    Mater. Sci. Eng. R

    (2010)
  • D. Rytz et al.

    J. Cryst. Growth

    (1982)
  • M. Sasaura et al.

    J. Cryst. Growth

    (2005)
  • A. Rousseau et al.

    Thin Solid Films

    (2008)
  • W.-C. Zheng et al.

    Physica B

    (1996)
  • F. Freund et al.

    J. Cryst. Growth

    (1977)
  • D.C. Sun et al.

    J. Eur. Ceram. Soc.

    (2004)
  • H.-j. Bae et al.

    Mater. Sci. Eng. B

    (2005)
  • Q. Simon et al.

    Thin Solid Films

    (2009)
  • Q. Xu et al.

    J. Alloys Compd.

    (2009)
  • J. Wang et al.

    J. Alloys Compd.

    (2010)
  • M.W. Cole et al.

    Thin Solid Films

    (2000)
  • M. Jain et al.

    Thin Solid Films

    (2004)
  • N. Izyumskaya et al.

    Crit. Rev. Solid State Mater. Sci.

    (2009)
  • S. Triebwasser

    Phys. Rev.

    (1959)
  • V. Gopalan et al.

    J. Appl. Phys.

    (1997)
  • J.E. Geusic et al.

    Appl. Phys. Lett.

    (1964)
  • F.S. Chen et al.

    J. Appl. Phys.

    (1966)
  • M. Demartin Maeder et al.

    J. Electroceram.

    (2004)
  • M. Zgonik et al.

    J. Appl. Phys.

    (1993)
  • J. Venkatesh et al.

    J. Am. Ceram. Soc.

    (2005)
  • Cited by (0)

    View full text