Tuning the Liquid–Vapour Interface of VLS Epitaxy for Creating Novel Semiconductor Nanostructures

Abstract

Controlling the morphology and composition of semiconductor nano- and micro-structures is crucial for fundamental studies and applications. Here, Si-Ge semiconductor nanostructures were fabricated using photolithographically defined micro-crucibles on Si substrates. Interestingly, the nanostructure morphology and composition of these structures are strongly dependent on the size of the liquid–vapour interface (i.e., the opening of the micro-crucible) in the CVD deposition step of Ge. In particular, Ge crystallites nucleate in micro-crucibles with larger opening sizes (3.74–4.73 μm2), while no such crystallites are found in micro-crucibles with smaller openings of 1.15 μm2. This interface area tuning also results in the formation of unique semiconductor nanostructures: lateral nano-trees (for smaller openings) and nano-rods (for larger openings). Further TEM imaging reveals that these nanostructures have an epitaxial relationship with the underlying Si substrate. This geometrical dependence on the micro-scale vapour–liquid–solid (VLS) nucleation and growth is explained within a dedicated model, where the incubation time for the VLS Ge nucleation is inversely proportional to the opening size. The geometric effect on the VLS nucleation can be used for the fine tuning of the morphology and composition of different lateral nano- and micro-structures by simply changing the area of the liquid–vapour interface.

Nucleation of InP on Si under micro-crucibles at ultra-high vacuum using a two-step VLS process

“…In order to characterize the micro-crucibles better with SEM, capping layers made of SiO 2 /Si 3 N 4 can be removed via DRIE using SF 6 gas [11,14]. Figure 2(a) shows a planview SEM micrograph of the same micro-crucibles shown in figure 1(h) (rotated 90 • clockwise) after a long DRIE of 18 mins with 25 sccm of SF 6 and 100 W of plasma power at 20 mTorr.…”

“…∼5 Pa), that smaller Au catalysts are easier to start a VLS growth because it requires a smaller precursor dose to reach a supersaturation state [13]. This shows that the adatom surface diffusion under the capping layers is the dominant growth mechanism in CBE, where the growth pressures are orders of magnitude smaller than used in CVD [10][11][12][13][14].…”

“…∼5 Pa), that smaller Au catalysts are easier to start a VLS growth because it requires a smaller precursor dose to reach a supersaturation state[13]. This shows that the adatom surface diffusion under the capping layers is the dominant growth mechanism in CBE, where the growth pressures are orders of magnitude smaller than used in CVD[10][11][12][13][14].The good alignment of Au, In, and P peaks in figure 4(d) is not observed in figure4(e), where In and P peaks do not with any apparent Au peaks. This finding suggests that these In P signals, shown in figure4(e-iv, v), from an unintentionally nucleated that was grown on top of the capping layer.…”