CST studio eigenmode dispersion diagram

Hi,

i am simulating a square lattice 2d photonic crystal and i want to get the dispersion diagram.

For TE mode, i have periodic boundary at xy direction and electric boundary at z. An air box is placed above the substrate.

phase x and y is scanned for the brillouin zone.

The photonic crystal is a air rod of r = 0.3a in a silicon substrate. a = 1. units are meters and hertz.

I can get good result for mode 1. But for higher modes, the results are not matching to correct results at all.

I dont know what is the problem.