As you mentioned, the ZnO insulating material normally do not have an extinction coefficient k. In this case, you can surely define this dielectric layer using the real part of the refractive index n without k. If you’re assuming that the dispersion effect of the refractive index n over the broadband (i.e., visible range) is not that critical in your simulation model or considering only single frequency, you can simly impose the constant n value in your model as below.
On the other hand, if you want to simulate the dispersion characteristic of ZnO over the specific range, then you can import the refractive index n spectrum (please refer to this KX contents). You can download the refractive index of ZnO material following useful website: http://refractiveindex.info/?shelf=main&book=ZnO&page=Bond-o. In your case you might need to insert k value as 0 in your sampled data set of (n,k vs. wavelength) when you import n, k values via sampled data in Material Database window. After you apply newly generated refractive index information of ZnO into your geometry (i.e., rectangle), you can set the level of the k over the broadband as possible as zero by adjusting the “imaginary weight” parameter in Material window. It’s important to keep in mind that it’s very hard to obtain the k value as “0” in this situation unless the n value is just a flat line (constant). As you fine-tune the imaginary weight parameter of ZnO material to make a better fitting, you can obtain nearly zero level of k (i.e., 10^-10) as a following example.
I hope this would be helpful for you.