A letter for beginners ask some questions

Dear Lammps users:

I am a master student from China, now I have met a difficult question to deal with, which is about the surface growth simulation. So, I write this letter to you for some help. I really appreciate it if you could give me some useful advice.

Now I will talk about the question.

My research area is atom lithography. This technology’s description is as the following pictures.


Fig.1 Fig.2

Fig.1 describes the schematic overview of atom lithography. In a high vacuum(<1e-5 Pa) environment, the atom beam (Cr, 1650℃,925m/s) move to the substrate(Si,25℃) vertically. When the atom beam passes through the standing wave, the light field will change the atom trajectories to focus on the antinodes of the standing wave. Then the when the atoms arrive at the substrate, they have a density distribution, which forms uniform lines. The deposition process lasts about 1 hour. And the distance between the adjacent lines is 213 nm.

What I need to simulate is the process of surface growth of Cr on the Si substrate. And I want to get the visible process of the Cr deposition and the last picture of the deposition, just like the Fig.2.

And I have some question to ask you for help:

1. Whether the Molecular Dynamics or the KMC method is proper for the simulation;

2. If MD is proper, which software is the best choice. Does the Lammps work?

3. Based on your rich experience, is there any other useful method for this simulation?

The question has troubled me for a few days. I am looking forward to your help. Many thanks.

Yours Sincerely

Deng Xiao

Shanghai, P.R. China

In principle, MD could model this behavior
and you could enhance the fix deposit command
in LAMMPS to put atoms down onto a large
surface with the directional focusing in your

However, your system is so large that
I doubt this is do-able with MD. You should
do a back-of-the-envelope calculation
of the number of atoms involved in your
diagram on the right, and the timescale
required given the deposition rate.

I’m guessing atoms * time will be
too large for MD.

Off-lattice (or possible even on-lattice)
kMC could be used for this since a “step”
is the deposition of one atom and you
could add surface diffusion events.
You could look at our SPPARKS code
(google it) for a parallel kMC that is
on-lattice which can do deposition
(again you’d have to add the directional
dependence). For off-lattice with
surface relaxation, I’d look in the literature.
I’ve seen off-lattice KMC papers on deposition