Controlled faceting in 〈110〉 germanium nanowire growth by switching between vapor-liquid-solid and vapor-solid-solid growth

Abstract

We show that the hexagonal cross-section of germanium nanowires grown in the 〈110〉 direction by physical vapor deposition is a consequence of minimization of surface energy of the collector droplet. If the droplet is lost or solidified, two {001} sidewall facets are quickly overgrown and the nanowire exhibits a rhomboidal cross-section. This process can be controlled by switching between the liquid and solid state of the droplet, enabling the growth of nanowires with segments having different cross-sections. These experiments are supported by in-situ microscopic observations and theoretical model.

Real-Time Observation of Collector Droplet Oscillations during Growth of Straight Nanowires

“…25 The formation of this crosssection is rather easy to explain because the simulations show that the very sharp corners cannot be wetted by the droplet. 16,26 Therefore, in the absence of noncatalyzed sidewall growth (resulting from our in situ observations), the previously trigonal facet quickly grows into an irregular hexagon, and the cross-section becomes symmetric. To explain the formation of the zigzag trench, we have followed the growth of such a nanowire (Figure 4b, ⟨110⟩-viewing direction), concluding that the shape evolution is again accompanied by cyclic droplet motion.…”

Concurrent Growth of Kirkendall Pores and Vapor–Solid–Solid Protuberances on Ni Wires During Mo Vapor-Phase Deposition

“…Additional features, such as faceted tips and tapered sides, also support the VSS growth mode, as emphasized by previous studies. [23][24][25]36,37] Although mentioned by other researchers, the possibility concerning the size effect on the melting temperature depression is not applicable to our case because all protuberances have tip sizes far exceeding the size threshold (<100 nm). [19,20,25] Further compelling evidence for the operation of VSS is the presence of surface terraces and steps in sizeable quantities.…”

“…[19,21,[23][24][25][35][36][37][38] Since the eutectic temperature of Ni-Mo binary system is 1323°C, [33] markedly above the processing temperature of 1273 K (1000°C), and since there are no other substances in liquid form available that could function as effective catalyst, the VLS option can be eliminated, leaving VSS as the active mechanism for the growth of protuberance. Additional features, such as faceted tips and tapered sides, also support the VSS growth mode, as emphasized by previous studies.…”

Vapor-solid-solid grown Ge nanowires at integrated circuit compatible temperature by molecular beam epitaxy

“…13,14 To date, a few reports covered the VSS growth of Ge NWs, in the catalyst-Ge systems as follows: Ni-Ge [at 300-375 C via chemical vapor deposition (CVD)], 15 Mn-Ge (at 350 C via CVD), 16 Cu-Ge (at 200-300 C via CVD), 14 and Au-Ge [at 255-330 C via CVD or physical vapor deposition (PVD)]. 17,18 To meet the temperature compatibility of current ICs when the Ge NWs are required to be grown on processed circuits, 19 a low synthetic temperature of Ge NWs should be utilized. The growth with a catalyst such as Ni or Mn will occur above 300 C, which can considerably damage the processed circuit.…”

“…On the other hand, as an established steady catalyst, Au exhibits a significant position in NW growth. Although the VSS growth of Au-assisted Ge NWs has been achieved at a temperature lower than T e by utilizing ultra-high vacuum (UHV) CVD 17 or PVD, 18 neither of these reports produced a detailed analysis of VSS grown Ge NWs. Furthermore, the growth temperature ranged from 255 to 330 C, which is difficult to deal with present IC technique.…”