symmetry of the boundary condition

Hi all,
I'm designing a pin diode to calculate the quantum efficiency. Before calculating the electric field, I need to idetify the forts of the structure. I have chosen the bottom and top surface as scattering boundar conditions but I'm not sure the lateral sides. At the end my structure will be formed of 2d array of pin diodes so lateral sides are kind of symmetry. Which type of boundary must be used for this case?
Thank you in advence.