Bulk and surface acoustic wave (SAW) devices are a convenient solution for high frequency filters in mobile communication systems due to their stability, reliability and compactness. In addition, SAW filters are ideal for gas sensing, with many industrial, environmental and biomedical applications. Furthermore, SAW sensors offer the additional feature of wireless transmission, and therefore the feasibility of operation in harsh environments. Whereas acoustic wave devices are typically fabricated on quartz or LiNbO₃, other piezoelectric materials where devices could exhibit novel or improved characteristics are highly desired. III-nitrides, particularly AlN, combine high SAW velocity and electromechanical coupling coefficients, while showing excellent thermal and chemical stability. These properties pave the way for the development of SAW devices operating at frequencies higher than 2 GHz, a requirement of modern telecomm applications. Moreover, the semiconductor character and direct bandgap of AlGaN enables the integration of SAW devices with electronics and optoelectronics.

In this work a review of the fabrication techniques of nitrides for SAW applications and the device properties will be done. Several acoustic modes are observed in the transfer functions of the filters: besides the Rayleigh wave, guided modes (Sezawa) arise when the sound velocity in the nitride layer is lower than in the substrate (like in GaN/sapphire), and new pseudobulk modes may also appear. The effect of the substrate characteristics (sound velocity and thickness) will be addressed, including the anisotropy found in devices grown on sapphire due to the crystal structure mismatch. Finally, two novel devices based on surface acoustic waves will be discussed: a SAW-assisted photodetector, based in the integration of a SAW generator and a UV photodetector, and a DC controlled SAW filter, obtained by the fabrication of a SAW device on a 2-DEG AlGaN/GaN heterostructure.

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3. Low frequency noise measurements in InN films