Schottky and cold field-emission guns comparing in electron microscopes.

 

A field emission gun is based on a release of electrons from surface of sharply pointed tungsten tip as a result of application of a strong electric field(>10⁷ Vm^-1). The field emission is caused by applying a voltage between the tip and an aperture metal anode.

Higher brightness is achieved from the gun that consists of two electrodes: the first electrode has the extraction voltage and the second one has the final accelerating voltage, required for the electron gun. There are two types of field emission gun differ mainly by their tips:

• CFE - cold field emitter is the base type. Application of the very strong electric field causes the surface energy barrier to deform to such an extent that it becomes physically very narrow. There becomes a finite probability of electrons from the Fermi level penetrating the barrier even at ambient temperature. Usually made of single crystal tungsten sharpened tip with radius of about 100 nm, the needle is so sharp that electrons are extracted directly from the tip.
• SE – Schottky emitter. The Schottky emitter combines the high brightness and low energy spread of the cold field emitter with the high stability and low beam noise of thermal emitters. A commercial Schottky emitting cathode is less sharp than the others; there a monatomic layer of ZrO is formed on the tungsten surface. This coating reduces the work function of the tungsten from 4.54 to 2.8 eV. At an operating temperature of about 1800 K, this emitter may not be as bright as the cold field emitter but it delivers stable high currents and is less demanding in operation.

The follow table summarizes the properties of common sources:

Source type	Brightness[A/cm2/sr]	Source size [nm]	Energy spread (eV)	Vacuum requirement (Torr)
SE	~108	20	0.9	10-9
CFE	~109	5	0.22	10-10