Dear Bhawana,

1) You need high quality insulating ZnO films, and must optimize your preparation conditions either for ZnO thin films, screen printed layers, or thin ceramic slabs.

1A) A good quality ZnO insulating layer means, you must achieve a high dc resistance, or low ac conductivity, no defects, no porosity, no impurities, reduced strain.

       However the qualityof the ZnO layer leads to many internal effects.

       and the electroding leads to external effects, and these should be minimised.

       The properties sof ZnO leading to intrinsic effects (crystallographic structure, strain, microstructure (grains, grain size, grain boundaries) etc., also contribute, and has to distinguish the origin.

2) Apply appropriate metal electrodes layers on either side of the thin ZnO layer.

     Some initial studies indicate Cr electrodes are good, Au/Cr is also good. Nickel is know to produce ohmic contact to ZnO.

2A) You must optimise your ZnO thinckness you need firstly, and then apply the electrodes, and measure the dielectric response (measure complex impedance) as a function of frequency. A careful analysis would reveal the origin for ac and dc conduction mechanisms of the (metal/ZnO/metal)  structure which you have prepared. Remember the dielectric response will be directly related to the type of MIM (metal-insulator-metal) structure which have prepared.

3) You must have measured your ac properties as a function of frequency, in the frequency range of your interest for your acoustic sensor application, and many a times  you will see the dielectric constant and tangent delta loss factor vary with frequency. They will be very high in the low frequency region (< One kilo hertz), and show significant dispersion.

You have to minimize this dispersion by making changes in your preparation conditions, and the selection of the electrodes for your structure. 

4) The total conductivity of the system includes ac conductivity + dc conductivity.

     ac conductivity is frequency dependent, unlike dc conductivity.

5) To conclude that you have a good ZnO layer, and the appropopriate electrodes, you must achieve a dispersion free (dielectric constant and tan delta).

   - The tangent delta should be low ( the lower the better).

6) The increase in tan delta (at any frequency, especially the lower frequency range), is related to different mechanisms.

7) Coming directly to your question, yes tan delta will be high if the ZnO layer resistance is low.  The higher the dc resistance ( more than 10 to the power  of 11 or  12 ohm-cm),  would result in lower dc conductivity, and the measured tan delta will be lowered.

Thank you for valuable contribution 

The main problem with ZnO as a piezo electric material that it is a semiconductor with metal like characteristics. So , its conductivityis relatively large rendering the piezoelectric layer with relatively large leakage resistance. The piezo voltage resulting from the mechanical vibration will be shunted by the internal resistance of the ZnO.

You have to reduce the conductivity by using fine grained ZnO such that the grain boundaries impede the motion of electrons in the materials and thereby increase its Resistance appreciably.

Tan delta= the loss tangent= G/wC, where G is the conductance parallel to C. For ideal operation G=0 which means completely insulating material.

Best wishes