Heteroepitaxy and Waveguide Formation for Solution Deposited LiNbO 3 Thin Layers
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THIN LAYER PROCESSING Alkoxide precursors are advantageous for solution deposition due to the low processing temperatures and flexibility with respect to doping. An alkoxide route similar to that of Eichorst and Payne 4 was used. Lithium ethoxide and niobium ethoxide were synthesized, combined in ethanolic solutions, and refluxed 5 . Distillation of excess ethanol results in a crystalline double metal alkoxide, LiNb(OCH 2 CH 3 )6 , thereby ensuring stoichiometry in later solutions 6 . The crystalline product was separated, and dissolved in dry ethanol to form 0.1-0.3M solutions for spin coating. Solutions were spin coated on (001), (110) and (012) sapphire substrates, which had been optically polished, annealed and cleaned by trichloroethane, acetone and isopropanol before spin coating. The as-cast layers were processed as shown in figure 1. After rapid heating to 300'C, FTIR analysis indicates elimination of organic groups, leaving an amorphous lithium niobium oxide, as observed by x-ray diffraction. Crystallization occurred after rapid heating to 450-650'C. Atomic force microscopy revealed surface roughness to be a strong function of crystallization conditions 7 . X-ray diffraction results from rapid heat treatments to 650'C, shown in figure 2, show heteroepitaxial layers of (006), (110) and (012) LiNb0 3 on (001), (110) and (012) 2 sapphire, respectively. LiNbO 3 layers were built up to 320-500nm in thickness on lcm substrates. The temperature of crystallization could be decreased by water additions to the ethanolic solution, but this generally was at the expense of the quality of epitaxy 8 ,9 . This has been attributed to hydrolysis and condensation reactions in the sol prior to spin coating, resulting in possible multiple nucleation sites. While base additions to solutions seem to retard these effects, the solutions used in the present work were unhydrolyzed.
S0.1-0.3M LiNb(OEt)6
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Spin coat at 3000rpm [300 0 C pyrolysis 450-650°C crystallization
Repeat -40-80nm/layer Figure 1: Flow diagram for solution deposition 202
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20 Figure 2: Cu Koy x-ray diffraction data for LiNbO 3 on (a) (006), (b) (1 10) and (c) (012) sapphire
For device applications, the ability to produce heteroepitaxial thin layers of low optical loss in specific geometries is important. For signal modulation, orientations with the polar axis lying in the plane such as (110) LiNbO 3 are desired, while quasi-phase matched second harmonic generation requires a polar axis normal to the plane, such as (006) LiNbO 3 . The ability to avoid twinning and high angle grain boundaries is extremely important. An indication of grain boundary orientation in (006) films is the rotational symmetry observed in phi-scans 7 , which show a single variant for (006) LiNbO3/(001)A12 0 3 . Erbium and neodymium doping was achieved by adding solutions of their resp
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