1. Absorption process

First, the model tries to predict where an atom would land on a surface and its rate at particular environmental conditions, such as temperature and vapor pressure. In order to land on a surface, atoms have to overcome the so-called activation energy barrier. The frequency of passing through the activation barrier can by calculated by the Arrhenius equation:

${\displaystyle A_{in}=A_{0,in}\exp \left(-{\frac {E_{a,in}}{kT}}\right)}$

where A is thermal frequency of molecular vibration, k is Boltzmann constant.

2. Desorption process

When atoms land on a surface, there are two possibilities. First, they would diffuse on the surface and find other atoms to make a cluster, which will be discussed below. Second, they could come off of the surface or so-called desorption process. The desorption is described exactly as in the absorption process, with the exception of a different activation energy barrier.

${\displaystyle A_{out}=A_{0,out}\exp \left(-{\frac {E_{a,out}}{kT}}\right)}$

For example, if all positions on the surface of the crystal are energy equivalent, the rate of growth can be calculated from Turnbull formula:

${\displaystyle V_{c}=hC_{0}(A_{out}-A_{0,out})=hC_{0}\exp \left(-{\frac {E_{a,in}}{kT}}\right)\cdot \left(1-\exp \left(-{\frac {\Delta G}{kT}}\right)\right)}$

where, ∆G = E_in – E_out, A_out, A_{o out} are frequencies to go in or out of crystal for any given molecule on the surface, h - height of the molecule in the growth direction, C_o concentration of the molecules in direct distance from the surface.

3. Diffusion process on surface

Diffusion process can also be calculated with Arrhenius equation:

${\displaystyle D=D_{0}\exp \left(-{\frac {E_{d}}{kT}}\right)}$

where, D is diffusion coefficient, E_d is diffusion activation energy.

All three processes strongly depend on surface morphology at a certain time. For example, atoms tend to lend at the edges of a group of connected atoms, the so-called island, rather than on a flat surface, this reduces the total energy. When atoms diffuse and connect to an island, each atom tends to diffuse no further, because activation energy to detach itself out of the island is much higher. Moreover, if an atom landed on top of an island, it would not diffuse fast enough, and the atom would tend to move down the steps and enlarge it.

1. Bond strength and temperature

Bond strength and temperature certainly play important roles in the crystal grow process. For high bond strength, when atoms land on a surface, they tend to be closed to atomic surface clusters, which reduce total energy. This behavior results in many isolated cluster formations with a variety of size yielding a rough surface. Temperature, on the other hand, controls the high of the energy barrier.

Conclusion: high bond strength and low temperature is preferred to grow a smoothed surface.

2. Surface and bulk diffusion effect

Thermodynamically, a smooth surface is the lowest ever configuration, which has the smallest surface area. However, it requires a kinetic process such as surface and bulk diffusion to create a perfectly flat surface.

Conclusion: enhancing surface and bulk diffusion will help create a smoother surface.

3. Supersaturation level

Conclusion: low impingement rate helps creating smoother surface.

4. Morphology at different combination of conditions

With the control of all growth conditions such as temperature, bond strength, diffusion, and saturation level, desired morphology could be formed by choosing the right parameters. Following is the demonstration how to obtain some interesting surface features: