Multilayered Ge nanodots have drawn much attention due to their potential applications in optoelectronics, such as photodetectors and lasers. Many groups studied multilayered Ge nanodots with Si spacers on Si(001) grown by Stranski-Krastanov (SK) growth mode and vertically aligned by local tensile strain induced by buried Ge nanodots (1-2). However, to avoid plastic relaxation caused by a 4.2% lattice mismatch between Si and Ge, thick nanodots and/or large layer numbers are challenging. Additionally, laterally-aligned Ge nanodots without pre-structuring have not been reported. In this study, we demonstrate 3-dimensional (3D) self-ordered Ge nanodots on SiGe virtual substrate (VS) by SiGe/Ge cyclic epitaxial growth and show the effects of fabrication parameters.

The 3D self-ordered Ge nanodots were fabricated by reduced-pressure chemical vapor deposition. A Si_0.4Ge_0.6 VS with step-graded buffer deposited on Si(001) wafer was used. This VS was post-annealed at 1000°C, followed by chemical-mechanical polishing. After HF dip, the substrate was baked at 850°C in H₂, then cooled down to 550°C for epitaxial growth. A 52-82 nm thick Si_0.48Ge_0.52 layer was deposited using a H₂-SiH₄-GeH₄ gas mixture, then a self-assembled Ge nanodots layer via SK growth mechanism was deposited with 7.5-15.0 nm Ge coverage using a H₂-GeH₄ gas mixture. This Si_0.48Ge_0.52/Ge deposition cycle was repeated 5 to 20 times to fabricate the 3D self-ordered Ge nanodot stack. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to analyze the morphology and alignment of the Ge nanodots. The facets of the Ge nanodots were studied by analyzing the cross-section cuts of AFM images and confirmed by scanning transmission electron microscopy (STEM). Nano-beam diffraction (NBD) was used to study strain in the superlattice.

Fig. 1(a-c) show the AFM images of the 5-cycle superlattices with Ge coverage 7.5 to 15.0 nm. In fig. 1(a), we can see mainly two types of nanodots, diamond (32%) and dome (68%). The height of the diamond-like nanodots is 9 nm with a standard deviation (s) of 2.6 nm while that of the dome-like nanodots is 23 nm with s=1.8 nm. With increasing Ge coverage, dome-like nanodots dominate (fig. 1(b)) and some nanodots merge with the adjacent nanodots (fig. 1(c)). Since the dome-like nanodots show a lower s in height than the diamond-like nanodots do, engineering of self-ordering is more feasible with the dome-like nanodot.

Fig. 2(a-b) show the angle view of SEM images of Ge nanodots on 5- and 20-cycle superlattice of 12.5 nm-Ge/52 nm-SiGe. The dome-like nanodots are dominant and laterally aligned. The alignment and the uniformity improve with the increasing cycle number of the superlattice. However, when the thickness of Si_0.48Ge_0.52 spacer increases, the lateral- and vertical alignment of nanodots become random, and the amount of diamond-like nanodots increases (not shown).

To study facets of two types of nanodots, the tilt of each facet was calculated from the cross-section cuts of AFM images. Fig. 3(a) shows an AFM image of the dome-like nanodot. Fig. 3(b) shows the cross-section cuts of fig. 3(a) along <110>. These cross-section cuts are well-overlapped with the high-angle annular dark-field (HAADF) STEM image as shown in the background of fig. 3(b). Therefore, it is possible and reliable to estimate the facets from the cross-section cuts of our AFM images. By this method, we confirmed that the diamond-like nanodot is composed of {105} and the dome-like nanodot is composed of {113} and {159}. This is consistent with a study of Ge dot on Si substrate except for {159} facet (3). Instead of {159} facet, a relatively similar facet {3 15 23} was reported. This difference may result from the less compressive strain in our Ge nanodots because SiGe VS was used.

To explain the vertical correlation of Ge nanodots, a HAADF STEM image and in- and out-of-plane strain distributions measured by NBD are shown in fig. 4(a-c). The nanodots are vertically aligned (fig. 4(a)). The SiGe on the nanodot shows a relatively higher lattice parameter along <110> (fig. 4(b)) and lower lattice parameter along <001> (fig. 4(c)) compared to that on Ge wetting layer, indicating tensile strain. This tensile strain area is the preferred position for Ge nanodot formation because of less lateral lattice mismatch. Consequently, the nanodots tend to grow above the buried nanodots.

3D self-ordered multilayered Ge nanodots on SiGe VS were successfully fabricated, and the facets and the vertical correlation of Ge nanodots were studied.

References

1. P.S. Chen et al. Materials Science and Engineering B 108 (2004) 213-218
2. K.L. Wang et al. Proceeding of the IEEE 95 (2007) 1866
3. J.T. Robinson et al. Nanotechnology 20 (2009) 085708