Vapor-liquid-solid (VLS) growth of semiconductor nanowires has been extensively studied as an avenue to control composition, crystallinity, strain and doping of nanowires for potential applications in Si device processing. As evidenced by studies of the Ge nanowire/Au catalyst system, an advantage of nanowire growth as compared with bulk synthesis is the generally large departure from equilibrium, which allows for supersaturation and undercooling during growth^1,2 and formation of metastable structures and compositions.^3,4,5  Non-equilibrium growth also offers possiblities for metastable solute trapping of dopants. Here we show results using Sn both as the growth catalyst, and as a dopant in the nanowires using SnCl₄ as the dopant gas. Incorporating Sn into the nanowires offers the possibility of increasing the carrier mobility, and of achieving a direct band-gap for efficient light absorption and emission by pushing the concentration of Sn in Ge beyond the equilibrium value.⁶  Fig. 1 shows the morphology and single crystallinity of nanowires grown using Sn as the catalyst. The catalyst, formed by evaporation of thin Sn layers on Ge substrates, produces wires with typical diameters <10 nm and a <110> growth axis. The Ge-Sn binary eutectic liquid occurs at a composition that is close to pure Sn so that the expected composition of the liquid droplet from which the Ge NW grows is very tin-rich. However, the tip that remains after growth degrades rapidly in the electron beam, preventing reliable EDS analysis. Nanowires that are grown using Au catalysts, with Sn added via the introduction of SnCl₄ gas partway through the growth process, are shown in Fig. 2.  Sn is incorporated into the liquid catalyst droplet, which enlarges the catalyst and the nanowire diameter.  During end-of-growth cool-down Sn and Ge are rejected from the catalyst resulting in tapered ends, with Au remaining at the tip.  Fig. 3 and 4 show the effect of higher and lower concentrations of SnCl₄ gas flow respectively. Higher Sn concentration causes the Sn to precipitate out at the surface of the nanowires. Lower Sn concentration allows the wire to form a coreshell structure with stacking faults, presumably dislocation loops, forming in the outer region. Preliminary EDS results suggest a Sn concentration of 2-5 at%, well above the <1 at% equilibrium value. We are currently investigating the nature of the stacking faults, and the growth parameters needed to achieve high Sn concentrations while minimizing defects.

Acknowlegements:  Financial support is provided by National Science Foundation grant DMR-1206511. Part of this work was performed at the Stanford Nano Shared Facilities.

1. Kodambaka, S., Tersoff, J., Reuter, M. C., and Ross, F. M., Science 316, 729 (2007).

2. Adhikari, H., Marshall, A. F., Goldthorpe, I. A., Chidsey, C. E. D., and McIntyre, P. C., ACS Nano 1, 415 (2007).

3.  Marshall, A. F., Goldthorpe, I. A., Adhikari, H., Koto, M., Wang, Y.-C., Fu, L., Olsson, E., and McIntyre, P. C., Nano Lett. 10, 3302 (2010).

4. Sutter, E. and Sutter, P., Nanotechnology 22, 295605 (2011).

5. Gamalski, A. D., Tersoff, J., Sharma, R., Ducati, C., and Hofmann, S., Phys. Rev. Lett. 108, 255702 (2012).

6. Kouvetakis, J., Menendez, J., and Chizmeshya, A. V. G.,  Annu. Rev. Mater. Res. 36, 497 (2006).

Figures:

Fig. 1: A Ge NW grown with a Sn catalyst: the NWs are approximately 5-10 nm in diameter and have a <110> growth direction.

Fig. 2: An SEM image of Ge NWs grown with Au catalysts and doped with Sn for the second half of growth.

Fig. 3: High SnCl4 flow results in Sn precipitates along the surface of the NW.

Fig. 4: Lower SnCl4 flow results in stacking faults across the nanowire.

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