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

Heteroepitaxial Growth of III-V Semiconductor on Silicon Substrates
Where T and T 0 are the nal and initial temperatures of the growth system
respectively, α S and α S are the temperature dependent coecients of thermal
expansion of substrate and layer [14, 50]. The actual dependence of the thermal
expansion coecients on temperature of GaAs are small enough that can be
ignored to rst order, leading to simplied expression for the thermal mismatch
strain as a function of the total change in reactor temperature ∆T [14].
ɛ t = (α S − α L ) ∆T (3.17)
For GaAs lms on Si substrates at typical reactor growth temperature of 600
◦ C, the total tensile strain developed during the cool-down process to room temperature
will be 0.26 %. This strain is signicantly less than the total material
mismatch strain for this material system, but still signicant enough to cause
some important eects in the heteroepitaxial layers. Because this strain develops
as the epitaxial layers are cooled from the growth temperature , dislocation relaxation
mechanisms are much less ecient at relieving the resulting strain [14].
In addition, the dierence in thermal expansion coecients also aects the rate
in which the two crystals contract during post growth or cool-down. Because the
silicon substrate contracts less than the GaP epilayer, tensile strain may accumulate.
Dislocation glide velocity decreases exponentially with decreasing temperature,
and thus the residual thermal expansion strain that remains in a heteroepitaxial
layer at room temperature can be as high as 90 % of the total thermal
mismatch [51]. However, this trapped tensile strain can lead to the formation of
micro-cracks in the epitaxial layer, with micro-crack nucleation behavior governed
by an eective critical cracking thickness, thermal mismatch strain can also act
to reduce the critical thickness of the grown layer [52]. Some practical ways to
account for thermal mismatch strain in the III-V/Si material system have been
proposed. For example, slower cooling rates after growth can encourage additional
tensile strain reduction by pre-existing mist dislocations, although practical considerations
limit how slowly temperature can be reduced [50]. Growth or device
fabrication on reduced areas can increase the total thickness of the epitaxial layers
that can be grown without crack formation [53, 54].
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