# Silvaco TCAD: Constant Impurity Concentration In the Allocation Profile

#### Stuff1161

##### Newbie
Hi everyone! I'm just trying to learn how to user the Silvaco TCAD and I have a point, that I can't understand. I'm using the following directives to create a structure and implant an impurity:

Code:
go athena

# GRID
line x loc=0.00 spac=0.10
line x loc=1 spac=0.10

line y loc=0.00 spac=0.001
line y loc=1 spac=0.001

# SUBSTRATE
init silicon c.arsenic=2.3e15 orientation=100 two.d

# IMPLANT
implant boron dose=3.2e15 energy=125 tilt=0 rotation=0 crystal
#

STRUCTURE OUTFILE=zad2.str
Then I'm open the structure in the Tony Plot and use a cutline to see the allocation profile. On the allocation profile, we can see that impurity concentration is decreasing with increasing the depth, but for some reason when impurity concentration becomes equal to 10^5 cm^-3 (concentration of Boron after ~0.55 mkm depth), it starts to be constant with increasing of the depth. And I can't understand why, because from the technology's point of view impurity concentration must decreasing, not being constant. Could someone explain it? Thank you in advance!

Last edited by a moderator:

#### timof

I see an increasing implanted impurity (boron) concentration up to 0.5 um, then it falls off.

Arsenic concentration was defined to be constant, equal to 2.3e15 cm-3 - why are you expecting to see anything but constant (as a function of depth)?

Read the description of the "init" command in the Athena user manual, what do its parameters mean.

#### Stuff1161

##### Newbie
I see an increasing implanted impurity (boron) concentration up to 0.5 um, then it falls off.

Arsenic concentration was defined to be constant, equal to 2.3e15 cm-3 - why are you expecting to see anything but constant (as a function of depth)?

Read the description of the "init" command in the Athena user manual, what do its parameters mean.
Maybe I'm should be more clear. I'm interested in only about boron. As you can see on the Boron graph, concentration up to 0,5 um, then it falls off. My question is why it falls off till 1e5 cm-3 and becomes constant (concentration is 1e5 cm-3 for all depth that >0,5 um), not to zero as I expecting.

#### timof

Oh, I see now, what you are thinking about.

Well, electrically, a doping concentration of 1e5 cm-3 is essentially zero, it does not have any electrical effect whatsoever.
Why they set it at exactly this level - I have no idea.
Maybe you can find this info in the manual.
But if you change this constant level by 1-2-3-4 orders of magnitude up or down - nothing should happen, all the electrical results would be exactly teh same.

#### Stuff1161

##### Newbie
Oh, I see now, what you are thinking about.

Well, electrically, a doping concentration of 1e5 cm-3 is essentially zero, it does not have any electrical effect whatsoever.
Why they set it at exactly this level - I have no idea.
Maybe you can find this info in the manual.
But if you change this constant level by 1-2-3-4 orders of magnitude up or down - nothing should happen, all the electrical results would be exactly teh same.
I know that is too much low value, but in my university it's a principal question why it's not 0. But anyway, thanks, still trying to find an answer

#### timof

There is a mesh, along the "depth" (Y) direction.
The doping is defined on the mesh, in every point.
Having doping equal to zero (rather than a small, irrelevant value) may cause some computational problems, e.g. when evaluating various mathematical functions, like log.
That's the reason "why" they set some minimum value.

Now, the question of "how" they (Silvaco) do that - try to read the manual.
If you cannot find it - ask Silvaco, if you can do that.
I am nto aware of any forums specializing in TCAD, where you would get an immediate answer.

#### Stuff1161

##### Newbie
There is a mesh, along the "depth" (Y) direction.
The doping is defined on the mesh, in every point.
Having doping equal to zero (rather than a small, irrelevant value) may cause some computational problems, e.g. when evaluating various mathematical functions, like log.
That's the reason "why" they set some minimum value.

Now, the question of "how" they (Silvaco) do that - try to read the manual.
If you cannot find it - ask Silvaco, if you can do that.
I am nto aware of any forums specializing in TCAD, where you would get an immediate answer.
The version of computational problems is great! Thank you !!!!