Chapter 5
Electron emission from
solids
Every practical electron beam instrument relies on a stable, longlived electron source. The most commonly used sources extract
electrons from a bulk metal or semiconductor, and accelerate the
particles across a vacuum gap using the electric field of an electrode at a positive potential relative to the source. The beam thus
appears to originate from an apparent source, real or virtual, seen
looking back from the electron optical system.
For practical purposes this apparent source is characterized by
its measurable macroscopic properties. These include beam energy, current, lateral intensity distribution, angular intensity distribution, and energy spread. These quantities are a function of
the physical source properties, including geometry and material.
They also depend on the temperature and applied electric field
as controllable operating parameters. Because of brightness conservation, the macroscopic source properties govern the optical
properties of the entire optical system.
Electron emission is a fundamentally quantum mechanical process. A great deal of insight can be gained by regarding the bulk
emitter material in terms of a relatively simple model originally
proposed by Sommerfeld [82]. In this model, the conduction elec297
Electron emission from
solids
Every practical electron beam instrument relies on a stable, longlived electron source. The most commonly used sources extract
electrons from a bulk metal or semiconductor, and accelerate the
particles across a vacuum gap using the electric field of an electrode at a positive potential relative to the source. The beam thus
appears to originate from an apparent source, real or virtual, seen
looking back from the electron optical system.
For practical purposes this apparent source is characterized by
its measurable macroscopic properties. These include beam energy, current, lateral intensity distribution, angular intensity distribution, and energy spread. These quantities are a function of
the physical source properties, including geometry and material.
They also depend on the temperature and applied electric field
as controllable operating parameters. Because of brightness conservation, the macroscopic source properties govern the optical
properties of the entire optical system.
Electron emission is a fundamentally quantum mechanical process. A great deal of insight can be gained by regarding the bulk
emitter material in terms of a relatively simple model originally
proposed by Sommerfeld [82]. In this model, the conduction elec297