Kris's Research Notes

October 8, 2010

Ga Droplet crystallization

Filed under: GaAs Simulations — Kris Reyes @ 2:41 pm

Just for fun and to see how our current code would handle it, I tried running the droplet crystallization experiment. Denis provided me details of the experiment, and I wanted to simulate a portion of it. Specifically:

1. I start out with 10 layers of GaAs Crystal and fix temperature to 498 K.
2. I turn on a Ga Flux of 0.1 monolayers Ga/second for 37 seconds.
3. I turn off the Ga Flux and let the crystal anneal for 60 seconds.
4. I turn on an As Flux of 3 monolayers As/second for 90 seconds.

The extended species definitions are the same as the ones detailed in this post. The pairwise energies are also the same, except I changed the $Ga^{(0)}-Ga^{(0)}$ bond strength to 0.28 eV (I thought this would give us shorter, wider droplets — it does not). I set the desorption potentials $\mu_{Ga} = 3$ (effectively $\infty$) and $\mu_{As} = 0.35$. I also had to implement bulk diffusion. I used a very simple (and so incorrect) model for the rates of bulk diffusion. The rate of swapping atoms $A, B$ when both $A, B$ are not vacuum and are of different species was given by

$r_{swap}(A,B) = \Omega \exp[\alpha(E(A) + E(B))],$

where $\alpha$ is a constant and $E(A), E(B)$ are the local bond energies at atom $A, B$, respectively. In this run, I set $\alpha = 0.25$.

Here are the movies sped up twice and four times as fast as real time. Both movies are samples of the system every 0.4 seconds:

• ($2\times$ real time): Movie (25 MB)
• ($4\times$ real time): Movie (25 MB)

I actually sampled the system every 0.1 seconds, but movies at this full resolution are huge — at least for animated gifs. I am looking in to making actual movies instead.