What sheet resistance does ZnO deposited and treated similarly show?

My first guess is dopant level is quite high and you might have obtained kind of segregation at the surface

ZnO prepared by electroless solution method will exhibit good optical property but poor conductivity Except spray pyrolysis which claims some conductivity many papers are silent about this aspect. However, they can be improved by combination of surface modifications and annealing. Its an interesting problem. Try an etching. All the best

Yes. The surface modification may be done by dipping in a very dilute etchant or irradiation followed by a short time annealing. This may introduce a smooth surface composed of conducting grains. While there is a chance of amorphization due to irradiation dip in a suitable etchant may help reduce resistivity.

Even EDX is having 5% error associated with its measurements and hence may not yield a depth profile. Only it can indicate some inclusion.

Dear Zhu,

As you are measuring on the surface, the problem is certainly on surface. I agree with Rathinam, a surface treatment is mandatory to you get some conductivity. I suggest you also map the surface to look for cracks. Are you measuring four points by point electrodes or stripes? Perhaps, four stripes to put the electrodes work better than point electrodes!

What is the thickness of your film? If the films is too thin, then, gas adsoption at the ZnO surface will greatly increase the resistance. ZnO and other metal oxides such as SnO2 are good gas sening materials.

I'm not sure 500 C is high enough to dopes Y3+ into ZnO crystal sturcture. I used 1500 C to allow 8% mol of Y3+ doped into ZrO2 and forming 8-YSZ (Y0.08Zr0.92O2-y). You may check your prepared material by XRD follow by crystal structure refinement (Le Bail or rietveld method). If there is no phase transformation on your crystal, it is suggested that your doping process was unsuccesfull

I think you should be carry out comprehensive studies your materials. It is very likely that in your case there is a cluster formation of Y atoms. If it does not, then you should observe an increase in the lattice parameter for ZnO:Y in comparison with undoped ZnO. This is due to the fact that the ionic radii Zn2+ and Y3+ are 0.74 A and 0.92 A, respectively.

Of course, a high-temperature annealing can influence the cluster formation. You have not noticed this effect. However, the temperature of 200 -500 C, in my opinion, are too low. First of all, I would suggest you spend the morphological study of the surface by obtaining of the SEM and AFM imaging. You should also carry out measurements of photoluminescence spectra, which will provide you information about the optical quality of the investigated films.Next, based on the obtained results to decide to etching the sample surface or the performance of its high-temperature annealing.

Also, be aware that rare earth elements in semiconductor materials act as getters for residual impurities, which are usually responsible for the formation of shallow donor and acceptor levels. Gettering improves optical quality of semiconductor materials. However, at higher concentrations of rare earth elements, this also affects the electronic properties of semiconductors. The presence of deep levels of defects leads to the formation of high-resistance materials. As example of complex studies of ZnO:Y nanorods may be following article : http://www.nanoscalereslett.com/content/7/1/372