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

Optimization of MEE and MBE growth of GaP Buer on Silicon Substrate
signicantly less concern regarding methods to reduce nucleation of mist dislocations
and managing the threading dislocation density via methods such as graded
buers. The critical thickness for GaP on Si(001) is approximately 70 nm [126],
beyond which the energy necessary to nucleate a mist dislocation becomes lower
than the energy buildup of increasing lattice strain as the GaP layer thickness
increases. In comparison to GaAs, the Matthews-Blakeslee critical thickness of
GaAs/Si system is about ∼ 1 nm [127].
Another critical theoretical issue mentioned in literature is the potential growth
problems associated with a large dierence in the coecients of thermal expansion
for GaP and Si. However, the thermal expansion coecient of a given material
is related to the expansion or contraction of the crystal lattice as the material's
temperature changes. Commonly, the lattice expands under increased temperature,
and returns to its original dimensions as the temperature is reduced. Si has
a coecient of thermal expansion of 2.6 × 10 −6 K −1 , while GaP has a coecient
of 4.7×10 −6 K −1 . The dierence in thermal expansion coecients also aects the
rate in which the two crystals contract during post-growth and cool-down steps.
Because the silicon substrate contracts more slowly than the GaP epilayer, tensile
strain may accumulate. If the epilayer is thick, the additional tensile strain can
result in micro-cracking. In GaAs/Si, such cracks do not typically appear until
the GaAs layer is 3-5 µm thick [55]. This dierence has the potential to cause
cracking during cooling after growth at high temperatures [128]. It is unknown
at what thickness micro-cracking will occur in the GaP/Si system, but it is not
expected to be an issue when GaP thickness do not exceed 1 µm, as is the case
in the work reported here. In this chapter, the optimization of GaP growth using
dierent growth modes will be discussed in details. In agreement with thesis
approach, the results of GaP growth reported here are also below the critical
thickness, with an ultimate goal to achieve the planar growth of GaP on silicon
substrate with uniform smooth surface and 2D morphology prole. The successful
epitaxial integration of GaP/Si structure will be very benecial to many
optoelectronic applications.
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