Jumping-Catalyst Dynamics in Nanowire Growth

Nanowire growth is generally considered a steady-state process, but oscillatory phenomena are known to often play a fundamental role. Here we identify a natural sequence of distinct growth modes, in two of which the catalyst droplet jumps periodically on and off a crystal facet. The oscillatory modes result from a mismatch between catalyst size and wire diameter; they enable growth of straight smooth-sided wires even when the droplet is too small to span the wire tip. Jumping-catalyst growth modes are seen both in computer simulations of vapor-liquid-solid growth, and in movies of Si nanowire growth obtained by in situ microscopy. Our simulations also provide new insight into nanowire kinking.

Figure 1

Growth of a straight uniform wire by periodic droplet jumping (oscillating mode). (a) Sequence of instantaneous morphologies from a simulation (movie M1 in the Supplemental Material [17]), illustrating evolution and droplet jumping; pairs of images show immediately before and after a jump. Here $ϵ=0.35$ and the growth rate is $3\times {10}^{-4}$; see text for other details. (b) Sequence of bright-field images from a UHV-TEM movie (movie M3 in the Supplemental Material [17]), showing oscillating catalyst configurations in a 55 nm diameter Si nanowire grown from a AuSi catalyst at $1.2\times {10}^{-6}\mathrm{Torr}$ disilane and $645\text{}°\text{}\mathrm{C}$. The nanowire growth rate is $0.32\mathrm{nm}{\mathrm{s}}^{-1}$ and oscillations occur approximately every 10 s. Note that varying the electron beam intensity does not have a strong effect on the growth of this nanowire (or on VLS Si nanowire growth in general [18]).

Figure 2

Growth of a straight uniform wire by periodic droplet jumping (alternating mode). (a) Sequence from a growth simulation (movie M2 in the Supplemental Material [17]), same as Fig. 1 except $ϵ=2$ and the growth rate is ${10}^{-4}$. Pairs of images show wire immediately before and after a droplet jump. (b) Bright-field image series from a UHV-TEM movie (movie M4 in the Supplemental Material [17]) showing growth of a 9 nm diameter Si nanowire from an AlAuSi alloy catalyst with base diameter 7 nm at $4.2\times {10}^{-5}\mathrm{Torr}$ disilane and $545\text{}°\mathrm{C}$. The nanowire growth rate is $0.2\mathrm{nm}{\mathrm{s}}^{-1}$ and the droplet resides on each tip facet for roughly 3 s.

Figure 3

(a)–(e) Examples of repeated kinking during growth when ambient noise is much larger than inequivalence between facets. Both are taken to be very small, so the only effect of noise is to randomize the jump direction. Conditions are otherwise as in Fig. 1 except values of $ϵ$ and the growth rate, respectively, are (b) 1.2 and ${10}^{-4}$, (c) 1.35 and $3\times {10}^{-4}$, (d),(e) 1.6 and ${10}^{-3}$ with (e) at a later time. The average segment length between kinks increases with growth rate. (f) Image from a movie showing repeated kinking in a 7 nm diameter Si nanowire growing from an AlAuSi alloy catalyst at $4.2\times {10}^{-5}\mathrm{Torr}$ disilane and $475°\mathrm{C}$.

Figure 4

Illustration of initial stages controlling growth mode. (a)–(e) $ϵ=0.35$, giving initial behavior as in Fig. 1 and Figs. 3, where the droplet jumps between wetting one facet and two facets. (f)–(h) $ϵ=1.0$, giving initial behavior as in Fig. 2, with the droplet never wetting more than one facet. (i) Steady-symmetric mode for $ϵ=0.8$.