Surface acoustic wave ultraviolet photodetectors using epitaxial ZnO multilayers grown on r -plane sapphire

A surface acoustic wave (SAW) ultraviolet (UV) photodetector is made of a zinc oxide ( ZnO ) based epitaxial multilayer structure on an r -plane sapphire ( r - Al 2 O 3 ) substrate. Piezoelectric and semiconducting ZnO layers are used for SAW excitation and photodetection, respectively. A thin Mg 0.2 Zn 0.8 O layer grown between the two ZnO layers isolates the semiconducting layer from the piezoelectric one. In contrast to previously reported SAW UV detectors on GaN and LiNbO 3 , the Sezawa SAW mode in the ZnO ∕ r - Al 2 O 3 system is used for its high acoustic velocity and large maximum effective piezoelectric coupling constant. The interaction of the SAW with the photogenerated carriers in the semiconducting ZnO layer results in a phase shift and an insertion loss change, as functions of light wavelength and power. The ZnO SAW UV detector can be used as a passive zero-power remote wireless UV sensor.