Академия наук республики узбекистан

Fig. 2.1. Ionization in the silicon electron 
beam evaporator
 
The
probability for a silicon atom to 
be ionized by electron impact is given 
by the product of the 
flux
density of 
the electron beam 
Fe 
and the cross 
section for ionization 
a
. 
This
probability multiplied with the density 
of evaporated silicon atoms 
n
Si
gives 
the number of ionizations per time and volume 
g
ion
,
see 
Eq.
(1)
and 
Fig.
2.2.
Fig. 2.2. Impact ionization of silicon atoms 
w
ith the 
density 
n
Si
 and the cross section 
a 
by an electron flux 
F
e
 
 
g
ion
=F
e
·a·r
Si
 
. 
(1)
 
The
design and the operating conditions of the 
e 
gun 
(e
-beam 
focusing,
acceleration 
voltage,
condition of the silicon 
crucible)
influence evaporation and 
the generation of ions but the 
major
parameter is the emission current 
I
ESV
affecting the 
flux
density of the electron beam 
Fe 
as well as the temperature and 
size of the melt and so the density of evaporated silicon atoms 
n
Si
. 
The
e gun 
also produces different kinds of radiation and a high number of reflected and 
secondary electrons. 
These
electrons,
caught in the magnetic field above the 
melt,
can affect ion mobility and 
lifetime. 
To
protect the growth 
chamber from evaporation and 
radiation the e gun and crucible 
are enclosed by a grounded 
shielding with an aperture 
pointing towards the 
substrate,
see 
Fig.
2.3.