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

MBE Growth of Self-Assembled InAs and InGaAs Quantum Dots Embedded in
Silicon Matrix
and V-SF formation above and around the InAs QDs have been observed after
the rst PGA cycle at 700 ◦ C. Nevertheless the QDs in Si voids will also evaporate,
but the vapor is conned in the voids, evolved nally to nearly spherical
QD shape. A further Si capping at 700 ◦ C has inuence on Si surface reconstruction
and circular-voids kinetics. A localized circular-voids formation above the
semi-coherent InAs QDs layer only has been observed after the nal Si capping
step. The second PGA cycle at 800 ◦ C proved to have strong impact on circularvoids
kinetics and Si growth quality, through total re-crystallization and melting
of the whole structure and new atomic arrangement. A high quality (defect-free)
Si matrix growth with voids and V-SF free has been observed after the second
PGA cycle at 800 ◦ C conrming the proposed growth model in Fig. 5.21. Strain
analysis conrmed almost fully relaxed QDs. However, These QDs are relaxed via
the creation of mist dislocation loops that are restricted in the InAs/Si interface
region, whereas the Si matrix is (extended) defect-free and with no detectable
strain gradient. However, in some cases few SFs and MTs are observed to form
only inside the dots.
The inuence of InAs coverage has been investigated with 1 ML, 2 and 4
MLs nominal thicknesses grown on 5 ◦ ocut Si (100) substrate. Fig. 5.22 shows
the cross-section HR-TEM images of embedded InAs QDs in silicon matrix with
dierent InAs nominal thicknesses. A clear size, density, and shape evolution has
been observed in this study. The cross-section HR-TEM image in Fig. 5.22(a)
with low InAs coverage of 1 ML shows a small at-coherent InAs QDs. This observation
led to further strain and optical studies as shown in Fig. 5.23 and Fig. 5.24.
A further increase in InAs QDs coverage to 2 MLs resulted in a very homogeneous
size distribution (5-8 nm) and spherical to faceted QDs shape formation
(Fig. 5.22(b)). The deposition of 4 MLs InAs QDs found to have a signicant
inuence on density, size and shape of the QDs. A high density of semi-coherent
InAs QDs with dierent shapes and wider dots size distribution (8-20 nm) has
been observed compared to low InAs covearge of 1 ML. However, other authors
reported a larger and more dislocated InAs clusters compared to our observation
in this investigation [88]. Fig. 5.22(c) illustrates a HR-TEM image of a semicoherent
single QD with lateral size of 18 nm observed in 4 MLs InAs sample.
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