Molecular beam epitaxial growth of III-V semiconductor ... - KOBRA

Heteroepitaxial Growth of III-V Semiconductor on Silicon Substrates
Where K s is the site-to-site hoping rate, a is the eective hopping distance
between sites, E d is the diusion energy and T is the substrate temperature [33].
Because of the complicated nature of the most surfaces, diusion is an complicated
process. It should be pointed out that diusion is the process responsible for the
degree of smoothness of the grown lm for a xed growth rate. Diusion atoms
meet and bond with each other forming clusters that dier in size, depending on
the deposit vapor or growth rate.
Another crucial factor is the temperature of the substrate T . Increasing the
temperature beyond certain limits leads to desorption of the molecules back into
the reactor vacuum. Simply they do not contribute to the growth, but are lost.
The desorption grows exponentially and the actual growth rate shrinks correspondingly
(Eq. 3.5).
R des ∝ f(θ) exp −E des
K B T
(3.5)
The desorption rate R des is dependent on the degree of coverage (θ) and the
desorption energy E des [33]. However, the small growth rates allow the adsorbed
molecules to migrate on the substrate to a proper nucleation site for growth.
The so called migration length is a measure of the distance the molecules can
travel on the growing surface before being taken up by an attractive lattice site.
Nucleation on smooth surfaces is not energetically favored. The most attractive
sites are those on terraces and step edges on the growing surface as more chemical
bonds to neighboring sites are possible.
Growth parameters like substrate temperature T, growth rate (g) and the ratio
of group V excess to group III ux (V/III ratio) have to be chosen appropriately
for the desired application. The V/III ratio has a similar impact on the layer
growth as the substrate temperature.
Too high values shorten the migration
length of the group III species, because they can nd a bonding partner more
easily and the incorporation into the crystal is faster.
deteriorate the layer quality as described above.
However, this can also
The desorption of group III
species can also be countered by the V/III ratio, which eectively shifts desorption
temperatures [34, 20]. The sticking coecient is also dependent on the V/III
ratio. Too low values on the other hand increase the migration length, but also
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