Strontium Barium Niobate (SBN) thin films

for electro-optic and nonlinear photonic applications.

M. Cuniot-Ponsard

Laboratoire Charles Fabry, IOGS, CNRS, Univ Paris Sud

2 avenue Augustin Fresnel, 91127 Palaiseau cedex, France

Abstract of the presentation

Epitaxial ferroelectric Srx Ba1-x Nb2 O6 thin films have been grown on conductive substrates and fully characterized. The electro-optic coefficient is measured larger than that of a LN crystal. The ability of AFM to write and read domains in the films has been investigated in view of nonlinear applications and is also presented. More details :

1. Introduction

Strontium barium niobate (SrxBa1-xNb2O6, noted SBN : 100x) is a ferroelectric material that crystallizes in the region 0.25 < x < 0.75 with the tetragonal tungsten bronze structure. SBN crystals exhibit an exceptionally high linear electro-optic coefficient, greatly exceeding that of the primary electro-optic material LiNbO3 [1]. The drive towards miniaturization and the development of new photonic components have oriented researchers towards the preparation of electro-optic thin films. These open the path to the realization of future photonic devices like waveguide modulators with a voltage-length product lowered by orders of magnitude, or electrically-tunable photonic crystals.

A second motivation to prepare SBN in the form of a thin film is it makes it easier to engineer ferroelectric domains using an atomic force microscope. Domain patterns may be designed in view of optimizing the quasi-phase-matching regime used for wavelength conversion.

This presentation describes successively the preparation of epitaxial SBN thin films, the characterization of electro-optic film properties using a novel method that determines simultaneously the converse piezoelectric and electro-absorptive coefficients, then an investigation of the ability of Atomic Force Microscopy to write and read domains in SBN films in view of nonlinear applications.

2. Preparation of SBN thin films

The implementation of SBN electro-optic properties requires a successful hetero-epitaxial growth of the film with (001) SBN direction perpendicular to a lattice-matched conductive substrate plane. A (001) SBN // (001) Pt // (001) MgO double epitaxial growth has been achieved by using the RF magnetron sputtering of Pt and ceramic SBN targets. Parameters of Pt and SBN deposition have been fixed from the results of previous investigations in which we established conditions for a stoichiometric composition of the amorphous SBN deposit [2], and for an epitaxial crystallization of a single phase SBN on Pt covered MgO substrates [3]. Amorphous SBN films are crystallized in a rapid thermal annealing furnace at 1160 °C during a few seconds. In-plane orientations of SBN are found mirror symmetric (±18.4°) to the axes of (001) Pt and (001) MgO cell axes. Top platinum dots deposited on the surface enable dielectric and electro-optic characterizations. When prepared in the optimized deposition and crystallization conditions, the films exhibit ferroelectric properties and a strong dielectric non linearity at room temperature [3].

3. Electro-optic characterization

Based on a Fabry-Perot reflective configuration (Fig.1-a), the method presented here enables characterizing simultaneously the electro-optic (EO), converse-piezoelectric and electro-absorptive effects in a film. It provides the magnitude and sign of each of the involved coefficients. The method exploits interferences obtained by reflection onto the EO film sandwiched between bottom and top electrodes. The reflectivity and the variation in reflectivity induced by an AC modulating voltage are recorded versus incident angle, successively for transverse electric TE and transverse magnetic TM polarizations (Fig.1-b). The procedure allows accounting for the whole of these experimental data[4]. The results, at l=633 nm and room temperature, obtained with an epitaxial (SrxBa1-xNb2O6, x=0.60) ferroelectric thin film, are :

, , ,

where r13 and r33 are two linear EO coefficients, d33 is a converse-piezoelectric coefficient, and e, ko, V, E represent, respectively, the film thickness, film ordinary extinction coefficient, applied voltage, and applied electric field. Converse-piezoelectric and electro-absorptive effects are found significant in the film response at a frequency below piezoelectric resonance and cannot be neglected. Diagonal and effective EO coefficients of the SBN film are measured larger than those of a crystal of lithium niobate (LN) at the same wavelength l=633 nm.

(a) (b)

Fig. 1.(a)Principle of the electro-optic characterization, (b)Reflectivity and electric-field-induced variation in reflectivity of a Pt/SBN/Pt structure for TE and TM polarizations. Calculated and experimental data are compared. Calculation was carried out by using the results displayed above.

4. Domain engineering using Atomic Force Microscopy

Atomic Force Microscopy provides the means to write, read, and investigate local polarization. Detection of the ferroelectric domains is usually performed by exploiting the piezoelectric response (PFM) but the PFM contact mode is not suited for imaging ferroelectric materials with a low coercive field like SBN (< 6 kV/ cm). The SBN response to a poling process is investigated here using the Kelvin Probe Force Microscopy (KPFM). Figure 2 shows the KPFM images of two areas of a SBN film where the three capital letters LCF have been previously written by moving the biased tip in contact with the surface. The relationship between the Kelvin voltage that nullifies the harmonic force on the tip and the polarization in a poled film area below the tip is established. It is exploited to quantitatively investigate the role of poling parameters and the stability of the written polarization as a function of the poled area size [5].

Topography KPFM image Topography KPFM image

Fig. 2. KPFM images of two areas of a SBN film where the three capital letters LCF have been previously poled by moving the biased tip in contact with the surface. The topography acquired during the first pass is shown on the left of each KPFM image. The poling voltages applied to the tip were -10 V (left image) and +10V (right image) so that the written polarization had opposite directions.

[1] A. M. Prokhorov and Yu S. Kuz’minov, Ferroelectric Crystals for Laser Radiation Control ( IOP Publishing Ltd, Bristol, 1990), Chap.4.

[2] M. Cuniot-Ponsard, J.M. Desvignes, B. Ea-Kim and E. Leroy, “RF magnetron sputtering deposition of hetero-epitaxial SBN:x thin films”,

J. Appl. Phys. 93(3), 1718-1724 (2003)

[3] M. Cuniot-Ponsard, " Strontium Barium Niobate Thin Films for Dielectric and Electro-Optic Applications " in Ferroelectrics - Material Aspects (InTech, www.intechopen.com, 2011), Chap. 23, p 497-518.

[4] M. Cuniot-Ponsard, J.M. Desvignes, A. Bellemain, F. Bridou, " Simultaneous characterization of the electro-optic, converse-piezoelectric, and electro-absorptive effects in epitaxial (Sr, Ba)Nb2O6 (SBN) thin films ", J. Appl. Phys. 109, 014107 ( 2011).

[5] M. Cuniot-Ponsard,” Kelvin probe force microscopy and electrostatic force microscopy responses to the polarization in a ferroelectric thin film: Theoretical and experimental investigations», J. Appl. Phys. 114, 014302 (2013).