Preparation and properties of epitaxially grown LiNbO(3) thin films on sapphire substrates by metallo-organic decomposition process

Rong-Chein Richard Wu, Purdue University

Abstract

A preliminary investigation of applying MOD technology to grow epitaxial thin films of LiNbO$\sb3$ on sapphire for maximum birefringence for potential applications in photonic thin film devices demonstrated that the MOD process can be a quick, cost effective process without forming a gel, or powder or processing in an expensive vacuum system. The films were crack-free for single layer fired thickness 250 nm or less, which corresponded to 11% or less weight oxide concentration (after firing) formulation solution for spinning, and had good transparency for grain size 250 nm or less. The oxygen lattice mismatch between LiNbO$\sb3$ film and sapphire is about 7% in (110) plane. In this study, a method to overcome this difficulty was developed for epitaxial growth by creating a vacancy mechanism to shrink the LiNbO$\sb3$ lattice to fit the sapphire substrate. Nearly 100% preferred (110) grain orientation was achieved by carrying out the thermal processing at a low oxygen partial pressure and the films could be reoxidized without effecting the preferred orientation. The dielectric constants and dissipation factors were measured as a function of frequency for three different grain sizes, 85 nm, 120 nm and 300 nm. The divergence of the dielectric constant from single crystal results for smaller grain sizes 85 nm and 120 nm was explained on the basis of interfacial polarization. The lower dielectric constant for grain size 120 nm compared to that of grain size 85 nm was due to the orientation dependence. The linear and quadratic electro-optic coefficients for the epitaxial LiNbO$\sb3$ thin film with grain size 120 nm grown on (110) sapphire substrate were 0.122 $\times$ 10$\sp{-12}$ (m/V) and 0.14 $\times$ 10$\sp{-19}$ (m/V)$\sp2$, respectively. The small linear electro-optic coefficient, which is about 0.4% of single crystal r$\sb{33}$ value, is caused partially by impurities and porosity, and mainly by space charge fields induced at the grain boundaries, the unpoled ferroelectric 180$\sp\circ$ LiNbO$\sb3$ domains inside each of the grain resulting in the additional space charge layers built up, and the misoriented grains.

Degree

Ph.D.

Advisors

Vest, Purdue University.

Subject Area

Materials science

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