There are two types of Differential Charging.
Horizontal Differential Charging is the most common type of differential charging. A similar type of charging is due to Vertical Differential Charging.
Horizontal Differential Charging
When you analyze materials that are thick non-conductive materials that have a rough surface (eg powder), have an odd shape (eg a toothpick), or have a mixture of conductive and non-conductive material side-by-side, then the surface might develop Horizontal Differential Charging.
If the charge neutralizer (low voltage beam of electrons, flood gun) is not properly aligned or does not have enough voltage or emission current, then the surface will also show horizontal differential charging. An example of horizontal differential charging is shown here. This drawing also shows that you can use a Charge-Control Mesh-Screen to level out the non-uniform electric field by having a grounded mesh-screen sitting 0.5-1.0 mm just above the surface of the sample.
This overlay shows the effect of differential charging that is only 30 eV in strength. The SiO2 coating is thick, but has some channels in the film, that allows electrons from below to propagate along grain boundaries to reach the surface, but the number of electrons is not enough to fully neutralize the surface.
This overlay shows the total effect of having a surface that suffers differential charging. There is not only a positive (+) charge shift tof ~30 eV, but the number of photoelectrons that can escape are reduced by 4X the full emission from a properly charge compensated surface.
Vertical Differential Charging
When you analyze materials that have a thin layer (<5 nm) of non-conducting material (eg native oxides) on top of a conductive material (eg a metal) that is grounded, we should normally turn the Flood Gun OFF to avoid the type of Vertical Differential Charging shown below. If you do need to keep the Flood Gun turns ON, then it is best to use as low a voltage as possible with low current, again to avoid or minimize vertical differential charging