xmpl3d90, 90th 'example' data file for CPO3DS
Field emission semi-conductor cathode with dielectrics.
A simplified example of an array of 3 field-emission tips mounted in a system that includes dielectric materials. This is the first example of the space-charge and cathode option combined with the dielectrics option.
The 3 field-emission tips are all hemispherical caps on cylindrical posts. The radii are 20nm and the lengths of the posts are 200nm. The centres of the posts are separated by 40nm. This array of 3 posts is mounted on a cathode block, opposite the anode block, both mounted on a dielectric base
of dielectric constant 10.
Arbitrary work functions and potentials are used to give a field emission current.
One space-charge iteration is called. In fact in this simulation the space-charge of the rays is not sufficient to affect the fields significantly. The iteration is useful though because it allows the contour options for space-charge and current density to be used. It can be seen that current from the central emitter is approximately half that from the end emitters. This is due to the fact that fields at the central emitter are approximatly 10% lower than at the end emitters, because the central emitter is partially screened by the end ones.
Here are some important technical points:
(1) The maximum step length for ray tracing is given a small value (3nm) but an 'advanced' option is used to increase this to 20nm when the electron energy becomes greater than 100eV, to give faster tracing.
(2) The 'reference' point for the cathode is placed at a large negative z, to ensure that it applies to all 3 emitters.
This is intended only as a quick demonstration -for a serious simulation the parameters would need to be more carefully thought about and experimented with.