EXAMPLE 17: Full.Coupled Diffusion 312 SUPREM-IV.GS – 2D ...

EXAMPLE 17: Full.Coupled Diffusion
diffuse time=.00001 temp=800 argon
In this first diffusion step, a short time anneal is done to establish the equi-
librium defect concentrations in case you want to plot the initial values of
these (since they are temperature dependent). In the second diffuse state-
ment:
diffuse time=15 temp=800 continue argon
the continue parameter is used to maintain the initial defect values. There-
fore, the initial diffuse uses the fermi method, and the defect levels are ini-
tially set at their equilibrium values. (Since the interstitials are set at the +1
charge state, the equilibrium interstitial concentration is fermi level depen-
dent and therefore the initial interstitial profile follows the beryllium pro-
file). The main diffusion step uses the full.cpl method. The dopant/defect
pairing parameters are specified by the following statements.
interstitial gaas beryllium neu.0=0 pos.0=0 neg.0=0
dneg.0=0 dpos.0=0
interstitial gaas beryllium tneg.0=0 tpos.0=0
These are all normally set to 0 if one assumes that the pair concentration is
much less than the defect concentration (non-equilibrium levels of defects
will still occur, since the defect flux equation in the full-coupled mode still
takes into account the extra defects produced by the diffusion process). In
Figure 1, the as-implanted and diffused beryllium and interstitial profiles
generated are shown. One can see that, compared to Example 13, a kink in
the profile occurs due to this defect non-equilibrium effect. If one increas-
es the interstitial diffusivity in the statement:
interstitial gaas D.0=5e-14 D.E= 0.
from 5 10 -14 to 1 10 -11 for example, the interstitials are able to diffuse
back to their equilibrium levels everywhere, and normal “Example 13
type” diffusion occurs.
SUPREM-IV.GS – 2D Process Simulation for Si and GaAs 311