Petrucci, Mario; (1990) Molecular beam epitaxy in the lithium-niobium-oxygen system. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The Nb-Li-O materials system offers an extensive combination of properties useful to integrated optics. Nb2O5 and LiNbO3 already have applications in waveguiding devices, such as electro-optic modulators, Fresnel lenses, and SAW transducers. In several cases, however, it would be desirable to grow these oxides on lattice-matched substrates as epitaxial thin films of controlled composition, crystallinity, and thickness: this thesis describes work aimed at achieving this goal. For the first time. Molecular Beam Epitaxy (MBE) was employed in depositing polycrystalline and single-crystal niobium oxide layers on z-cut LiNbO3 and z-cut sapphire. Two new phases were grown epitaxially: a variant of niobium monoxide (termed "NbO-6C fcc"), and a hexagonal phase (composition NbO1.25, termed "hex-π"). Thin-film niobium metal was also deposited on z-cut LiNbO3. The post-deposition oxidation of oxygen-deficient, MBE- grown niobium oxide was studied, and optical waveguiding achieved in the resulting pentoxide films. The problem of oxygen deficiency in the as-grown material was addressed, leading to the design, construction and investigation of a novel oxygen radical source. Structural, compositional, and optical data are presented for all layers. Process control was greatly enhanced, giving reproducible growth in this system for the first time. A 50keV RHEED unit was constructed, capable of probing insulating substrate and oxide-layer surfaces. Using this, original data was obtained for LiNbO3 substrates and their pre-deposition surface behaviour, the crystal inner potential (V0) was deduced for LiNbO3, and improved electron diffraction data was obtained for grown layers. An understanding was achieved of the limitations and promise of the MBE technique as applied to the Nb-Li-O system. The kinetics of the growth process were examined: this included the first detailed study of oxygen sticking coefficients for growth with and without Li flux. The prospects of MBE were assessed for the deposition of Nb2O5/LiNbO3 layers that can form the basis of device structures.

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