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

5.3 Results and Discussion
5.3.1 Surface Preparation
Defects are known to be related to the cleanliness of the substrate surface. Surface
cleaning to remove surface impurities and surface preparation to establish a welldened
substrate surface are rst steps of the growth for high-quality III/V lms
on silicon. Adsorbed carbon and surface oxide are the main contaminants to be
considered, since other contaminants such as metals (Cu, Fe, Na, etc.) can be
suppressed to negligible levels through careful substrate chemical treatments [75,
76]. Thermal treatments before, during and after the lm growth are widely used
to improve the material quality of III-V on Si. In-situ thermal cyclic annealing
(TCA) [77], and post growth annealing (PGA) have been proposed in this regards
[78]. PGA was performed by using conventional furnaces and rapid thermal
annealing (RTA). Defect reduction by the PGA was reported [79].
To improve the surface quality of the silicon substrates, which have a profound
inuence on the structural and optical properties of QDs. The silicon substrate
surface was prepared ex-situ using buered HF (NH 4 F/HF : H 2 O) (1:1) (according
to Eq. 5.1 or Eq. 5.2) for 4 minutes as a pre-removal step of the surface native
oxide followed by an in-situ thermal desorption at 900 ◦ C (described by Eq 5.3)
for 15 minutes as inferred from single crystalline surface according to RHEED
pattern, which was observed above 840 ◦ C [80]. In the next approach, the surface
of the Si substrate was rst exposed to hydrogen ux with P H = 3 × 10 −7
Torr at 500 ◦ C for 45 minutes in the buer chamber [6]. The substrate was then
transferred to the growth chamber for in-situ thermal desorption and for RHEED
investigations before, during and after oxide desorption.
SiO 2 + 4HF + 2NH 4 F =⇒ (NH 4 ) 2 SiF 6 + 2H 2 O (5.1)
SiO 2 + 6HF =⇒ H 2 SiF 6 + 2H 2 O (5.2)
SiO 2 + Si =⇒ 2SiO (5.3)
RHEED streak patterns with hemispherical shapes were observed after AHcleaning
followed by thermal oxide desorption at 700 ◦ C as shown in Fig. 5.1(c).
This is an indication of a clean 2D crystalline surface compared to diused
RHEED pattern (amorphous surface) obtained before the treatment (see Fig. 5.1(a)).
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