NANO LETTERS
Structures and Energetics of Indium-Catalyzed Silicon Nanowires
2009 Vol. 9, No. 4 1467-1471
Z. W. Wang and Z. Y. Li* Nanoscale Physics Research Laboratory, School of Physics and Astronomy, UniVersity of Birmingham, Birmingham, B15 2TT U.K. Received November 5, 2008; Revised Manuscript Received January 14, 2009
ABSTRACT Strong size dependent structures of Si nanowires, grown via indium nanoparticles, have been revealed by high-resolution transmission electron microscopy studies. It is found that, below the critical value of particle diameter of ∼100 nm, the growth changes from 〈111〉 to predominantly 〈211〉 direction and the formation of multiple {111} twins changes from perpendicular to the 〈111〉 growth direction to parallel to the 〈211〉 axial direction. The growth mechanisms are discussed in terms of relative surface/interface energy, using Au catalyzed Si nanowires as a comparative benchmark.
Silicon nanowires (SiNW) have been continuously drawing attention in the science community for many years due to their potential in applications in semiconductorbased technology.1,2 The vapor-liquid-solid (VLS) growth mechanism, proposed by Wagner and Ellis,3 has been widely employed in synthesizing these nanowires. In recent years, significant progress has been made in controlling their size, morphology, and crystallinity.4,5 However, contamination of metal nanocatalysts in VLS grown nanowires remains a great concern. For example, the most common catalyst material, Au, has been shown diffusing and clustering on sidewalls of SiNW6 as well as being trapped inside the nanowires.7,8 Incorporation of Au is well known to affect electronic properties of Si and thus the performance of devices if they are included in electronic components. This has motivated active exploration of new growth techniques as well as searching for alternative catalyst materials, such as aluminum (Al) and indium (In) that are compatible with the existing semiconductor-based technology.9,10 Since these alternative catalysts are not widely used, there is very limited data on the systematic characterization of crystallinity and defects of wires, issues directly related to electron transport in SiNW11 and understanding of underlying growth mechanism. In this study, we conducted a detailed high-resolution transmission electron microscopy (TEM) study of SiNW grown from In catalysts using a plasma-enhanced chemical vapor deposition (CVD) method. The detailed synthesis method has been addressed in recent publications.10 Here we concentrate on crystallinity of SiNWs. We demonstrate that the growth orientation and twinning formation are strongly dependent on the size of In catalysts. To gain insight * To whom correspondence should be addressed. E-mail: ziyouli@ nprl.ph.bham.ac.uk. Telephone: +44-121-4144593. 10.1021/nl803345u CCC: $40.75 Published on Web 03/04/2009
2009 American Chemical Society
to our understanding of the growth mechanism of SiNWs, we apply the most extensively studied Au-mediate Si nanowire as a model system for comparative study. Indium droplets were prepared by electrodeposition on Si(001) wafer that was treated by H2 plasma cleaning. Coldwall plasma-enhanced chemical vapor deposition system (PlasmaLab 100 by Oxford instruments) was used to grow SiNW. The growth temperature was at 600 °C with silane (SiH4) used as source gas mixed with Ar and/or H2. Silane partial pressure was kept at 0.12-0.25 torr.10 The Si wafer was immersed in toluene and a drop of the solution containing nanowires was then placed on standard TEM copper grids coated with amorphous carbon film. All imaging was performed in a FEI Tecnai F20 Transmission Electron Microscope with a field emission gun operating at 200 KV accelerating voltage. Double-tilting sampler holder was used to align the nanowires with respect to the electron beam. Selected-area electron diffraction and the fast Fourier transforms of the lattice-resolved images of SiNWs were combined to determine the growth direction of the nanowires. The SiNWs under investigation were synthesized using In nanoparticles with size range of 20-200 nm in diameter.10 Figure 1a shows two typical SiNWs imaged using TEM when they are supported by amorphous carbon film: one is straight with a large diameter and the other is slightly curved with a smaller diameter. The SiNWs have characteristic tapering shape and the indium droplets can be seen at the tip of the SiNWs with spherical shape. Figure 1b plots the wire diameter, d1, at the liquid/solid interface as a function of In droplet diameter, φ, showing a monotonic relation for the whole size range. In order to provide a quantitative description of the morphological wire tapering
shape, we assume the wire being a regular cone and define the geometric factor κ as κ)
d2 - d1 L
(1)
where d1 and d2 are the wire diameters at the interface and base, respectively, and L is the wire length. In Figure 1c, the κ is plotted as a function of In droplet diameter. The measurements are done by taking a fixed length L at 500 nm for all wires to eliminate effects due to any irregularity of the wire shape. One can see that the data points scattered around a mean value (indicated by a dashed line as guide for eyes) do not seem to show strong dependency on the droplet diameter over the extended range that we have investigated. One possibility for the scattering is that the nanowires have faceted sidewalls, thus d2-d1 in eq 1 would vary with the viewing direction. The tapering of the wires could be a result of the gradual reduction of the droplet during the growth.6 Because of the extremely low In solid solubility in Si (0.004%),12 the loss of indium atoms through the incorporation into SiNW is expected to be insignificant. Instead, we do find a small loss of indium through diffusion on nanowire sidewalls. However, the diffusion is limited only to ∼30 nm away from the In droplet.13 There is another possibility resulting in the tapered morphology of the SiNW, that is a result of continuous uncatalyzed deposition of Si on side-walls of the nanowires.14,15 Although we cannot completely rule out this possibility, our diameter-independence of the κ factor for In/SiNW seems to imply that this cannot be the main reason unless there is a constant sticking coefficient for silane radicals on the Si nanowire sidewalls without dependence on the crystalline planes. We will demonstrate below that the growth direction, hence crystal orientation of sidewall surfaces, is clearly size-dependent. We believe that the thermal evaporation of In is the most likely cause due to the much lower melting temperature of In compared with that of Au. The In droplet has a distinct spherical shape at the tip of SiNW, as shown in Figure 2a and 2b. It has a large contact angle of 140∼160° with the wire, much larger than that in the Au-catalyst Si nanowire (95∼115°).4,16 A close-up view of the droplet/nanowire interface is shown in Figure 2c where a step with four atomic layers (highlighted with dashed line) can be clearly identified. Such a step is very common. The growth direction of the nanowire has been characterized from 2D Fourier transforms of the lattice-resolved images of SiNWs. The inset of Figure 2c shows the fractional 1/3{422} reflections, indicating that the wire growth direction is 〈211〉.17 We conducted a systematic investigation on growth direction of nanowires with varied size. Figure 3 complies results of the growth orientation as a function of diameters of In droplets. It is found that the growth orientation has a strong size-dependence. SiNWs grown from large droplets prefer the 〈111〉 growth direction. This preference changes from the 〈111〉 direction to 〈211〉 as the droplet diameter reduces to around 100 nm that corresponds to the wire diameter of 45 nm. We also find that the crossover from 1468
Figure 1. Morphology of Si-nanowires. (a) A transmission electron microscopy image showing typical morphology of indium-catalyzed SiNW on amorphous carbon film. (b) The diameter of the nanowire at the In/Si interface as a function of indium droplet diameter. The solid line is a polynomial fit to data. (c) The tapering factor, defined by (d2 - d1)/L (see the schematic drawing in the inset for definition) as a function of indium droplet diameter. The dashed line is a guide for eyes. The error bars are calculated by standard deviation.
〈111〉 to 〈211〉 occurs at a much larger In droplet size than that observed for Au-mediated SiNWs. The latter is usually below 20 nm.4 Two other growth directions, 〈311〉 and 〈100〉, have also been observed, but only once each among 19 nanowires examined and both with small size. Although these growth directions have been reported previously on Au or Pt catalyzed SiNWs,18,19 this is the first time we have unambiguously identified them in In-catalyzed SiNWs. For Au-catalyzed SiNW, 〈110〉 is the preferential growth direction for nanowire diameter smaller than 10 nm.4 However, no nanowires with the 〈110〉 direction have been identified in this Nano Lett., Vol. 9, No. 4, 2009
Figure 2. Indium-Si alloy and Si-nanowire interface. (a) A TEM image of In-catalyzed nanowires, showing the characteristic spherical shape of In droplet at the tip of the wire. (b) A schematic drawing of the nanowire showing the tapering angle R and the contact angle β as discussed in the text. (c) A lattice-resolved magnified view of the In droplet /Si interface area marked in panel a. The image has been pass-filtered. The dashed line marks the interface where a growth step can be seen. The inset shows the fractional 1/3{422} reflections, indicating that the wire growth direction is 〈211〉.
Figure 3. Si-nanowire growth direction. The nanowire growth directions of 〈311〉, 〈100〉, 〈211〉, and 〈111〉 is shown changing with the indium-droplet diameter. A pie diagram in the inset shows the statistical distribution of growth directions in all 19 wires we investigated.
study, probably because the nanowires prepared here all have a diameter larger than 10 nm. The statistical data on the growth direction of In-SiNWs is shown in the inset of Figure 3. Nano Lett., Vol. 9, No. 4, 2009
High-resolution TEM revealed that twinning defects are common in In-catalyzed SiNWs. Figure 4a shows a nanowire grown along 〈211〉 direction with the In droplet of 72 nm in diameter. Multiple twins, marked by the solid line, with the twinning mirror planes parallel to the axis of the nanowire are clearly seen. Figure 4b shows a nanowire grown along the 〈111〉 direction with the In droplet of 128 nm in diameter. The top inset displays the lattice-resolved twin image, showing clearly that these twins have both sets of {111} planes at an angle of 70.5° to each other (see the markers A and B). The electron diffraction provides further evidence of the existence of two {111} twin mirror planes. The diffraction spots shown in the bottom inset of Figure 4b can be indexed by two sets of {111} twins highlighted by two dashed line frames. It is most interesting to note that this type of twins was only observed in the transition range where the growth direction changes from the 〈211〉 to 〈111〉 when the In droplet diameter is around 100 nm. The multiple-twins perpendicular to the 〈111〉 growth direction has been reported previously for In-SiNW when the droplet diameter is above 200 nm.10 For very small sized In-SiNWs (
