Kris's Research Notes

May 31, 2011

Droplets without Extended Species

Filed under: GaAs Simulations — Kris Reyes @ 2:53 pm

Recall we initially devised the notion of an atom’s extended species — that atom’s species along with nearest-neighbor information — in order to form Ga droplets. The idea was to encourage Ga(0) – Ga(0) bonds by making those bonds stronger than other Ga-Ga bonds. (Recall: Ga(0) means a Ga atom with no As neighbors. This led to droplet formation instead of wetting. In light of the recent problems with extended species to determine Ga-As bonds, we had considered possibly eliminating these different types of bonds in favor of just one type of Ga-As bond. Naturally, we may ask if extended species are even necessary for droplet formation. It turns out that they are not.

We consider adding next-nearest neighbor bonds between Ga atoms, and similarly between As atoms. This has two advantages:

1. Without next-nearst neighbor bonds, if we only have one type of Ga-As bond, then there is no difference in energy barriers between a Ga adatom hopping on an As terminated surface and a Ga atom breaking off a Ga terminated surface. This is because both Ga atoms have two Ga-As bonds:

If we add next-nearest neighbor Ga-Ga bonds, this problem is addressed since the second case now has two next-nearest neighbor bonds to break, increasing the barrier.

2. Ga-Ga next nearest neighbor bonds do indeed encourage droplet formation.

To show the second point, I ran a simulation with nearest-neighbor bond strengths $\gamma_{AA} = 0.10 eV, \gamma_{GG} = 0.3 eV, \gamma_{GA} = 1.0 eV$ and next-nearest neighbor bond strengths $\gamma^\prime_{GG} =\gamma^\prime_{AA} = 0.10 eV.$ All diffusion rates are of the form:

$\displaystyle 10^{13} \times \exp^{-\beta E_L}$,

where $E_L$ is the energy about the diffusing atom. There were no atom-atom exchanges. The simulations were initialized to a flat GaAs substrate. Then Ga atoms were deposited at a rate of 1 monolayer/second for 5 seconds. The temperature was 473 K.

Here is a movie of the simulation:

Here is a moving-average movie of the simulation, to get an idea of what the average shape of the droplet is:

A droplet does indeed form.