Pulsed laser deposition of KNbO 3 thin films
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Pulsed laser deposition of KNbO3 thin films M. J. Mart´ın, J. E. Alfonso,a) J. Mendiola, and C. Zaldob) Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cient´ıficas, Cantoblanco, 28049 Madrid, Spain
D. S. Gillc) and R. W. Eason Department of Physics and Optoelectronics Research Centre, University of Southampton, Southampton S017 1BJ, United Kingdom
P. J. Chandler School of Mathematical and Physical Sciences, University of Sussex, Brighton BN1 9QH, United Kingdom (Received 19 August 1996; accepted 17 March 1997)
The laser ablation of stationary KNbO3 single crystal targets induces a Nb enrichment of the target surface. In rotated targets this effect is observed only in those areas irradiated with low laser fluence. The composition of the plasma formed close to the target surface is congruent with the target composition; however, at further distances K-deficient films are formed due to the preferential backscattering of K in the plasma. This loss may be compensated for by using K-rich ceramic targets. Best results so far have been obtained with fKgyfNbg 2.85 target composition, and crystalline KNbO3 films are formed when heating the substrates to 650 ±C. Films formed on (100)MgO single crystals are usually single phase and oriented with the (110) film plane parallel to the (100) substrate surface. (100)NbO may coexist with KNbO3 on (100)MgO. At substrate temperatures higher than 650 ±C, niobium diffuses into MgO forming Mg4 Nb2 O9 and NbO, leading to K evaporation from the film. Films formed on (001) a –Al2 O3 (sapphire) show the coexistence of (111), (110), and (001) orientations of KNbO3 , and the presence of NbO2 is also observed. KNbO3 films deposited on (001)LiNbO3 crystallize with the (111) plane of the film parallel to the substrate surface. For the latter two substrates the Nb diffusion into the substrate is lower than in MgO and consequently the K concentration retained in the film is comparatively larger.
I. INTRODUCTION
In the last few years, increasing attention has been paid to the preparation of ferroelectric thin films. This interest is due to the possibility of producing integrated ferroelectric memories, pyroelectric sensors with fast response, miniaturized piezoelectric elements for mechanical switching and integrated optical waveguides.1 Niobate oxides may play a major role in some of these applications and in particular the use of LiNbO3 single crystals is widespread as a bulk optoelectronic medium, mainly due to the availability of high quality large size single crystals.2 KNbO3 is also well known for its optical properties as a photorefractive material3 and as a second harmonic generator for Alx Ga12x As diode lasers.4 Lithium and potassium niobate thin films have been deposited using several techniques.5 In particular, they a)
Permanent address: Centro Internacional de F´ısica, Santa F´e de Bogot´a, Apdo. A´ereo 49490, Colombia. b) Author to whom correspondence should be addressed. c) Present address: Foundat
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