3. Results and Discussion
3.1 The sensing mechanisms of TFBAR-based UV sensors
The sensing mechanism for the shifts in the operating frequency of acoustic devices can be attributed to the acoustoelectric effect. When UV light was illuminated on the surface of ZnO thin film, the excess carriers in the ZnO layer would be produced. Then, the photogenerated carriers will increase the sheet conductivity in the ZnO thin film and interact with the electric field generated by acoustic wave devices, i.e. acoustoelectric effect. The photogenerated carriers will decrease the piezoelectric properties and reduce the phase velocity of piezoelectric thin films. The interaction between the acoustic waves and the charge carriers leads to the changes in phase velocity (ǻv) is given by [12]:
22
22 0 2 ( ) s s o s vK vv C V V ' (1)
where vo is the phase velocity on a free surface, K2 is the electromechanical coupling coefficient of the substrate, ı is the sheet conductivity, Cs = s + o, s and o are the dielectric constants of the substrate and vacuum, respectively. The relationship between the sheet conductivity and the change of phase velocity was illustrated in Fig. 6. The influence of UV light illumination on SAW velocity can be monitored by measuring the frequency shifts of TFBAW oscillators.