Request on boundary conditions; laser ablation of Ti

Hey everyone,
Am runx a simulation on Ti using laser ablation. The lammps input script requiries periodic boundary conditions for this purpose but my simulation need to have fixed boundary condition in the z axis. How do I overcome this problem?. Secondly, how do I Am create a half filled simulation cell? I can't find my way around since am a new user. Can some one help me out?
Best regards

Why does it require PBC? If it is b/c you are using
a Kspace solver like PPPM, then there is a slab option
for the solvers to allow periodic in x-y, non-periodic in z.
Although in general it will be a bad idea to lose charged
atoms using PPPM, b/c you will not preserve charge-neutrality.

The create_atoms command can fill any portion of the simulation
box if you use it with a region you define.