Self-Ordered Ge Nanodot Fabrication By Reduced Pressure Chemical Vapor Deposition
Authors: Yamamoto Yuji , Itoh Yuhki , Zaumseil Peter , Schubert Markus Andreas , Capellini Giovanni , Washio Katsuyoshi , Tillack Bernd 
Published: February 2020

Technologies of group IV semiconductor fabrication enable new artificial materials such as superlattice and nanodot structures (1). In particular, three dimensional (3D) ordered SiGe or Ge structures are promising materials to improve quantum dot lasers that can be integrated into existing CMOS platforms (2). In order to control regularly ordered nucleation sites, usually pre-patterning of the substrate is required. In the case of 3D stacking of Ge nanodots on Si substrates, the Ge nanodots tend to grow vertically aligned, due to the elastic strain field of the buried Ge nanodots (3). In a previous study, we reported a self-ordered body-centered tetragonal (BCT) SiGe nanodot formation using reduced pressure (RP) CVD, by repeating SiGe and Si layer stack deposition (4). By repeating several cycles of the SiGe and Si layer stack deposition, checkerboard Si mesas are formed on embedded SiGe dots and next SiGe dots are formed in concave region between the Si mesas. In this study we discuss the influence of Si surface roughness on the Ge nanodot formation and present a self-ordered 3D Ge nanodot formation.
3D Ge nanodot layer stack fabrication is carried out by using a single wafer RPCVD system. Standard Si (100) wafers are used. After H2 bake in the reactor, GeH4 is injected at 550°C to form Ge nanodots. Then a Si cap is deposited using H2-SiH4 at 600°C followed by H2-SiH2Cl2 at 700°C. In order to investigate the influence of surface roughness and strain on the Ge nanodot formation, a checkerboard Si mesa with ~7 nm height is fabricated by BCT Si0.6Ge0.4 nanodot layer stacks (4) for selected samples.
Surface morphology is measured by AFM and SEM. Alignment of the Ge nanodots is measured by cross section SEM and TEM. Periodicity is measured by X-ray diffraction.
By GeH4 exposure at 550°C Ge nanodots (Diameter: ~50, height: 2 nm) with ~6×1010 cm-2 were randomly formed on the Si surface. By postannealing at 600°C, migration of the Ge nanodots occurs resulting in larger Ge nanodots (Diameter: ~60 nm, height: 5 nm) and lower density (~1.5×1010 cm-2) as shown in Fig. 1.
On the other hand, in the case of the checkerboard mesa structured Si surface fabricated by BCT Si0.6Ge0.4 nanodot underneath (RMS: 1.2 nm) (Fig. 2a), Ge nanodots are formed in concave region (Fig. 2b). This phenomenon is also observed for SiGe nanodot (4). The position of the Ge nanodot formation seems to be determined by a balance of strain field over embedded SiGe dots and surface energy. In this case, surface energy reduction seems to be more dominant compared to strain at the Si surface transferred from embedded Si0.6Ge0.4.
In Fig 3, a cross section STEM image of 6 layers of Ge nanodot with Si spacer layer stack on the staggered aligned SiGe nanodot is shown. The Ge nanodots are vertically aligned. As shown in Fig. 2b, the first Ge nanodots are formed in the concave regions of the Si surface, therefore the first Ge nanodots are aligned to embedded SiGe nanodots underneath. By depositing a Si cap layer on the first Ge nanodots, a smoother Si surface (RMS: 0.5 nm) is formed compared to the initial checkerboard structure. By following GeH4 exposure, a second Ge nanodot formation occurred on the embedded Si surface. The aligned Ge nanodot formation seems to be driven by the tensile strain field at the Si surface above embedded Ge nanodots, because of a smooth Si surface.
In Fig. 4a and 4b, nano beam diffraction (NBD) images of out-of-plane and in-plane strain distribution of the vertically aligned Ge nanodot structure are shown, respectively. The positions of the Ge nanodots are visible by higher vertical lattice parameter (Fig. 4a). In the Si part, smaller vertical lattice parameters are observed above the Ge nanodots compared to other area, indicating that the Si on the Ge nanodot is tensile strained. In the lateral lattice parameter distribution (Fig. 4b), only a periodic strain variation is visible, but to identify the position of the Ge nanodot is not possible, indicating that the 3D Ge nanodot stack is pseudomorphically grown. These NBD results support the assumption of Ge nanodot alignment discussed in Fig. 3.
In this study we demonstrated self-ordered and vertically aligned Ge nanodot layer stack fabrication.

Reference

(1) K. L. Wang et al., Mat. Sci. in Semicond. Proc. 8 (2005) 389
(2) G. Capellini et al., Appl. Phys. Lett. 82 (2003) 1772
(3) Y. Nakaumura et al., Nanotechnology 21 (2010) 095305
(4) Y. Yamamoto et al., Nanotechnology 28 (2017) 485303

Figure 1