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

MBE Growth of Self-Assembled InAs and InGaAs Quantum Dots Embedded in
Silicon Matrix
The structure of 50 nm Si/2 MLs InAs/ 10 nm Si cap (without PGA) is shown
in Fig. 5.15. The AFM image with surface roughness of 0.9 nm is illustrated in
Fig. 5.15(a). However, the top surface of the grown structure dose not look
perfectly connected. A high density of pits or (voids) with abrupt Si wall were
observed, after capping with 10 nm of Si in both, SEM image in Fig. 5.15(b) as
well as dark eld cross section TEM image in Fig. 5.15(c). The observation that
InAs islands commonly occupy shallow depressions or (voids) shows that, during
silicon overgrowth Si does not grow in the immediate vicinity of InAs islands.
Figure 5.15: TEM cross-section image of InAs/Si QDs sample (50 nm Si/2 MLs InAs
/10 nm Si cap) showing; (a) (3 × 3 µm 2 ) AFM image of top surface with RMS value
of 0.9 nm. (b) SE-SEM image with high density of voids, InAs islands prefer to locate
in Si voids (magnied image) pointed with white arrow. (c) Dark eld cross-section
TEM image of high density voids consistent with the SEM image. (TEM & SEM
performed@PDI-Berlin).
This observation has been also detected in similar structures [87]. A possible
reason for this can be related to the highly strained InAs QDs and surface energies
balance which will prevent the Si overgrowth on the top of the dots, leading to
growth scenario in the areas between the dots forming the observed voids.
Fig. 5.16(a) shows a cross-sectional HR-TEM image of InAs QD inside a
Si void with abrupt walls. Geometric phase analysis (GPA) strain mapping in
e yy and e xx directions Fig 5.16(b,c) conrmed highly strained QDs. In the third
sample (Table 5.2), the rst PGA step was investigated at 700 ◦ C for 5 minutes for
the structure of sample 2 (50 nm Si/2 MLs InAs /10 nm Si cap). The AFM image
82