Thin Solid Films 429 (2003) 55–62 0040-6090/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00030-0 Defect structure of pulsed laser deposited LiNbO y Al O layers 3 2 3 determined by X-ray diffraction reciprocal space mapping A. Boulle , L. Canale , R. Guinebretiere *, C. Girault-Di Bin , A. Dauger a b a, b a ` Science des Procedes Ceramiques et de Traitements de Surface (SPCTS) umr 6638 ENSCI, 47–73 avenue Albert Thomas, a ´´ ´ Limoges Cedex 87065, France Institut de Recherches en Communications Optiques et Microondes (IRCOM) UMR 6615, Universite de Limoges, 123 avenue A. Thomas, b ´ Limoges Cedex 87060, France Received 13 August 2002; received in revised form 10 December 2002; accepted 8 January 2003 Abstract Epitaxial LiNbO films have been deposited on (0001) Al O substrates by pulsed laser deposition. The microstructure of the 3 2 3 samples has been investigated by X-ray diffraction reciprocal space mapping. Data acquisition is performed on a high-resolution set-up suited to the study of oxide materials. The (000l) planes of the layer are found parallel to the (000l) planes of the substrate. The large in-plane misfit gives rise to two in-plane orientations. In the direction perpendicular to the surface the crystallite size (163 nm) is much smaller than the film thickness (510 nm) suggesting the presence of planar defects. Moreover, the values of the mosaicity (1.28) and root-mean-squared strain (0.17%) cannot be solely explained by misfit dislocations. Conversely the high- thermal expansion coefficient mismatch is at the origin of a high compressive strain level (y0.18%) and may also be responsible for the large column length distribution, root-mean-squared strain and mosaicity.  2003 Elsevier Science B.V. All rights reserved. Keywords: Oxides; X-ray diffraction; Pulsed laser deposition; Reciprocal space mapping 1. Introduction Lithium niobate (LiNbO ) is a well-known ferroelec- 3 tric material with unique piezoelectric, electro-optic and non-linear optical properties. It is widely used in optical devices based on waveguides, like optical phase and intensity modulators. Device integration implies to elab- orate high-quality-thin films. Thanks to greater confine- ment it could yield, for example, electro-optic modulators with lower drive voltage and higher band- width. For such optical applications it is necessary to grow epitaxial LiNbO films on a suitable substrate with 3 reduced optical losses. Among various substrates already used for this goal, a-Al O is a good candidate because 2 3 of its same crystalline structure and the large difference of the refractive indices between the film and the substrate. *Corresponding author. Tel.: q33-5-55-45-22-21; fax: q33-5-55- 79-09-98. E-mail address: r.guinebretiere@ensci.fr (R. Guinebretiere). ` In the past few years, lithium niobate films have been prepared using a large variety of techniques namely chemical vapor deposition w1x, liquid phase epitaxy w2x, sputtering w3–7x, sol–gel w8x, molecular beam epitaxy w9x and pulsed laser deposition (PLD) w10–13x. Among these techniques, the PLD appears to be one of the most promising because of its ability to reproduce the com- position of the target in the film. Many studies have revealed that the crystal quality of ferroelectric films is modified with the film thickness, and the macroscopical properties (such as optical) are known to be very sensitive to the crystal structure and microstructure. For example, if there are many bounda- ries or if the grain boundaries represent a large surface, they can become scattering centers and increase the optical losses. Therefore, microstructural analysis of pulsed laser deposited LiNbO films on sapphire sub- 3 strates is necessary to improve the quality of the devices. X-ray diffraction (XRD) is an efficient tool to inves- tigate the microstructure of layered samples w14x. Indeed, it allows non-destructive characterization of the samples