Different metal oxides exhibit a wide variety of useful properties including high electrical resistivities, ferroelectricity, pyroelectricity, electro-optical and nonlinear optical (NLO) properties. A potential application cited for ceramic materials exhibiting NLO properties is blue or green light sources useful for optical data storage, high speed laser printers, large screen displays and undersea communications. One approach is to use a crystal having nonlinear optical susceptibility to produce light at twice the frequency of an infrared diode laser through second harmonic generation (SHG). Lithium niobates and tantalates having the general formula: LiNb.sub.x Ta.sub.1-x O.sub.3, where x is a number from 0 to 1 (referred to herein as "LiNb.sub.x Ta.sub.1-x O.sub.3 ") have large nonlinear susceptibilities and transparency from 350 nm-4000 nm and these materials are commonly used for blue light generation in the form of bulk crystals. LiNb.sub.x Ta.sub.1-x O.sub.3 can also be provided in the form of thin films. For blue light generation with high efficiency, the LiNb.sub.x Ta.sub.1-x O.sub.3 NLO film and adjoining materials should form a waveguide, in which layers adjoining the nonlinear optical thin film, have lower refractive indexes than the film. Kanata, T. et al, Journal of Applied Physics, Vol. 62, (1987), pp 2989-2993, teaches the production of waveguides having c-oriented epitaxial films of LiNbO.sub.3 and LiTaO.sub.3 grown on substrates of sapphire: Al.sub.2 O.sub.3.
A major shortcoming presented by bulk NLO crystals is the difficulty of integration in multilayer structures. LiNbO.sub.3 and LiTaO.sub.3 films on sapphire substrates provide a multilayer structure, however, that structure lacks a semiconductor suitable for monolithic integration. In monolithic integration; a single chip would include, for example; diode lasers, frequency doublers, light detectors and necessary electronics. Substrates suitable for monolithic integration include Si and GaAs.
It is therefore desirable to provide a multilayer structure including an epitaxial NLO film overlaying a semiconductor substrate. Semiconductor substrates, however, have a number of characteristics which limit easy application to multilayer structures for second harmonic generation. GaAs is not suitable for use as an optical buffer, since it has a very high refractive index and absorbs in the visible range. GaAs reacts with LiNb.sub.x Ta.sub.l-x O.sub.3 at interfaces to produce undesirable phases.
Problems are also presented by the inherent limitations of epitaxy. Even where materials have the same crystal structure and orientation, epitaxial growth requires that the misfit between lattice constants of the two layers be 15 percent or less. GaAs has a Zinc Blende structure with the lattice parameter 0.5673 nm, while LiNb.sub.x Ta.sub.1-x O.sub.3 has a trigonal structure (for LiTaO.sub.3 a=0.5153 nm and c=1.3755 nm).
Sinharoy, S., Thin Solid Films, Vol. 187, (1990), pp 231-243 teaches the use of one or more epitaxial alkaline earth fluoride buffer layers on semiconductors including GaAs to provide lattice matching. Tiwari, A. N. et al, Journal of Applied Physics, Vol. 71, (1992), pp. 5095-5098, teaches the use of epitaxial fluoride layers as a buffer for the growth of high-temperature superconducting oxides. Epitaxial alkaline earth fluoride buffer layers have the shortcoming of high reactivity with some oxides and deteriorated crystal qualities when processed in oxygen.
Metal oxides such as ZrO.sub.2, PrO.sub.2, CeO.sub.2, Al.sub.2 0.sub.3, MgAl.sub.2 O.sub.4 and MgO have been reported to grow epitaxially on Si substrates. ZrO.sub.2, PrO.sub.2, CeO.sub.2, and MgAl.sub.2 O.sub.4 are unsuitable for use as a buffer layer between GaAs and an NLO film of LiTaO.sub.3 or the like. CeO.sub.2 and PrO.sub.2 have excessive optical absorption. ZrO.sub.2 is transparent, but has a large refractive index. MgAl.sub.2 O.sub.4 is colorless and has a low refractive index, but has too high a deposition temperature for use with GaAs. Orientation of epitaxial metal oxide buffer layers matches the orientation of the substrate in some cases, but not in others. Inoue, T., Applied Physics Letters, Vol. 59, (1991), pp 3604-3606, teaches epitaxial growth of (111)CeO.sub.2 on (111)Si; but epitaxial growth of (110) CeO.sub.2 films on (100)Si. Osaka, Y. et al, Journal of Applied Physics, Vol. 63, (1988), pp 581- 582; teaches epitaxial growth of (100)ZrO.sub.2 on (100)Si, but the growth of polycrystalline films on (111)Si, by the same technique.
Fork, D. K. et al, Applied Physics Letters, Vol. 60, (1992), pp 1621-1623 teaches epitaxial growth of a (100) MgO buffer layer on (100) GaAs by using pulsed laser ablation of Mg metal in an oxygen ambient. This (100)MgO buffer layer is not suitable for the growth of c-axis oriented ((0001) oriented, referred to a hexagonal system) LiNbO.sub.3 or LiTaO.sub.3 films because the (100) planes of GaAs and MgO have four-fold rotation symmetry about &lt;100&gt;, while the (0001) planes of LiTaO.sub.3 have three fold rotational symmetry about &lt;0001&gt;. The epitaxial growth of (100)MgO is also favored by the preference of MgO films for a (100)-oriented state and the fact that the (100) planes are electrically neutral, while the (111) planes are charged.
Hung, L. S. et al, Applied Physics Letters, Vol. 60, (1992), pp 3129-3131, teaches epitaxial growth of a (110)MgO buffer layer on (100)GaAs using ultrahigh vacuum electron beam evaporation of MgO. This (110)MgO buffer layer is not suitable for the growth of c-axis oriented ((0001) oriented) LiNb.sub.x Ta.sub.1-x O.sub.3 films because the (110) plane of MgO has 2-fold rotational symmetry about &lt;110&gt;, while the (0001) planes of LiTaO.sub.3 have three fold rotation symmetry about &lt;0001&gt;.
It is therefore desirable to provide a multilayer structure which includes a semiconductor substrate and can provide for a c-axis oriented non-linear optical film.