ZnO surface distorted after doing nvt

I am doing simulation of ZnO surface using reaxff in lammps. But after doing nvt my zno surface is distorted its become valley type. Is there anything wrong of doing nvt? any help will be appreciated.

There is nothing wrong with NVT but there may be other reasons for the failure: surface too small, time step size too large, damping to strong/weak, bad initial geometry, and perhaps a bad force field (using the wrong one), etc)