Formation and properties of proton-exchanged and annealed LiNbO3 waveguides for surface acoustic wave

The proton-exchanged (PE) and annealed PE (APE) z-cut LiNbO3 waveguides were fabricated using H4P2O7. The positive strain, c-axis lattice constant change (delta C/C), was calculated to be about +0.43%, which was almost independent of the exchanged conditions. The penetration depth of H measured by secondary ion mass spectrometry (SIMS) exhibited a step-like profile, which was assumed to be equal to the waveguide depth (d). The surface acoustic wave (SAW) properties of PE and APE z-cut LiNbO3 samples were investigated. The phase velocity (Vp) and electromechanical coupling coefficient (K2) of PE samples were significantly decreased by the increase of kd, where k was the wavenumber (2 pi/lambda). The insertion loss (IL) of PE samples was increased by the increase of kd and became nearly constant at kd > 0.064. The temperature coefficient of frequency (TCF) of PE samples showed an apparent increase with kd, reaching a maximum at kd = 0.292, then slightly decreased at higher kd. The effects of annealing resulted in a restoration of Vp and an improvement of IL.