Sticking coefficient

Sticking coefficient is the term used in surface physics to describe the ratio of the number of adsorbate atoms (or molecules) that adsorb, or "stick", to a surface to the total number of atoms that impinge upon that surface during the same period of time.^[1] Sometimes the symbol S_c is used to denote this coefficient, and its value is between 1 (all impinging atoms stick) and 0 (no atoms stick). The coefficient is a function of surface temperature, surface coverage (θ) and structural details as well as the kinetic energy of the impinging particles.

Derivation

When arriving at a site of a surface, an adatom has three options. There is a probability that it will adsorb to the surface ($P_a$), a probability that it will migrate to another site on the surface ($P_m$), and a probability that it will desorb from the surface and return to the bulk gas ($P_d$). For an empty site (θ=0) the sum of these three options is unity.

$P_a + P_m + P_d=1$

For a site already occupied by an adatom (θ>0), there is no probability of adsorbing, and so the probabilities sum as:

$P_d'+P_m'=1$

For the first site visited, the P of migrating overall is the P of migrating if the site is filled plus the P of migrating if the site is empty. The same is true for the P of desorption. The P of adsorption, however, does not exist for an already filled site.

$P_{m1}=P_m(1-\theta)+P_m'(\theta)$

$P_{d1}=P_d(1-\theta)+P_d'(\theta)$

$P_{a1}=P_m(1-\theta)$

The P of migrating from the second site is the P of migrating from the first site and then migrating from the second site, and so we multiply the two values.

$P_{m2}=P_{m1} \times P_{m1}=P_{m1}^2$

Thus the sticking probability ($s_c$) is the P of sticking of the first site, plus the P of migrating from the first site and then sticking to the second site, plus the P of migrating from the second site and then sticking at the third site etc.

$s=P_a(1-\theta)+P_{m1}P_a(1-\theta)+P_{m1}^2P_a(1-\theta)...$

$s=P_a(1-\theta)\sum_{n=0}^{\infin} P_{m1}^n$

There is an identity we can make use of.

$\sum_{n=0}^{\infin} x^n =\frac{1}{1-x}\forall x<1$

$\therefore s=P_a(1-\theta)\frac{1}{1-P_{m1}}$

The sticking coefficient when the coverage is zero $s_0$ can be obtained by simply setting $\theta=0$. We also remember that

$1-P_{m1}=P_a+P_d$

$s_0=\frac{P_a}{P_a+P_d}$

$\frac{s}{s_0}=\frac{P_a(1-\theta)}{1-P_{m1}}\frac{P_a+P_d}{P_a}$

If we just look at the P of migration at the first site, we see that it is certainty minus all other possibilities.

$P_m1=1-P_d(1-\theta)-P_d'(\theta)-P_a(1-\theta)$

Using this result, and rearranging, we find:

$\frac{s}{s_0}=[1+\frac{P_d'\theta}{(P_a+P_d)(1-\theta)}]^{-1}$

$\frac{s}{s_0}=[1+\frac{K\theta}{1-\theta}]^{-1}$

$K\overset{\underset{\mathrm{def}}{}}{=}\frac{P_d'}{P_a+P_d}$

References

1. sticking coefficient IUPAC Compendium of Chemical Terminology 2nd Edition (1997), Accessed 30 September 2008

• King-Ning Tu, James W. Mayer, and Leonard C. Feldman, in Electronic Thin Film Science for Electrical Engineers and Materials Scientists, Macmillan, New York, 1992, pp. 101–102.