Article pubs.acs.org/JPCC
Wetting Layer: The Key Player in Plasma-Assisted Silicon Nanowire Growth Mediated by Tin Soumyadeep Misra, Linwei Yu,* Wanghua Chen, and Pere Roca i Cabarrocas LPICM, Ecole Polytechnique, CNRS, 91128 Palaiseau, France ABSTRACT: Unidirectional growth of silicon nanowires (SiNWs) can be achieved via a vapor−liquid−solid (VLS) mechanism. Interestingly, when adopting low surface tension metals such as tin or indium to mediate the growth of SiNWs in a plasma-assisted VLS process, the standard scenario is challenged by the fact that low surface tension metals tend to coat the high energy sidewalls of SiNWs. Moreover, we show that tin-assisted SiNW growth can continue without any apparent metal droplet on top. To address these issues, we investigate the time evolution of tin-assisted SiNW growth. We suggest that, during the initial growth phase, an ultrathin metal layer wetting the SiNW sidewalls allows silicon adatoms to diffuse toward the top of the nanowires, and thus enhances the axial growth while suppressing the radial one (in a way similar to molecular beam epitaxy growth of SiNWs). Later on, the wetting layer can assist the growth even when the tin droplets are exhausted from the top of SiNWs. We propose that this phenomenon may hold for all low surface tension metal mediated VLS growth of SiNWs and could be helpful to tailor them for specific device applications.
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INTRODUCTION Effective structural and morphological control of silicon nanowires (SiNWs) is critical to promote the use of these onedimensional building blocks in a new generation of transistors, sensors, and photovoltaic devices.1−3 When fabricated via a vapor−liquid−solid (VLS) mode, nanoscale metal particles absorb gas precursors and lead to the precipitation of crystalline silicon at the liquid/solid interface.4 During this VLS growth, the metal liquid droplets are usually at the tip of the SiNWs and dewet their sidewall surfaces, as depicted in Figure 1a. As formulated by Nebol’sin, to carry out a stable VLS growth, the metal droplets should have a higher surface energy than that of the SiNW sidewall.5 Therefore, for a low surface tension metal such as tin (Sn), having a surface tension of 0.575 N/m5, the standard VLS growth is challenged by the fact that the liquid droplets will tend to wet the high energy SiNW sidewalls (typically >1 N/m6) and thus fail to maintain the stability criterion. In contrast to this, our previous studies have shown that low surface tension metals, such as Sn,7 indium (In),8 gallium (Ga),9 and bismuth (Bi)10 can promote VLS growth of SiNWs in a plasma-assisted process. We proposed that an ultrathin metal wetting layer might help to modify the effective surface energy and stabilize the droplets on the top of SiNWs.11 Thus, at least during the initial growth phase of SiNWs, there must be a coexistence of a liquid droplet on top and an ultrathin wetting layer on the sidewall, as depicted schematically in Figure 1b. Nevertheless, the impact of this wetting layer, on the growth dynamics/mechanism and the structural properties of SiNWs, has not been studied so far. We here report an unexpected behavior for Sn-assisted SiNW growth, where gradually tapered nanowires with a diameter smaller than 50 nm can continue to grow without any visible droplets sitting on top. Our purpose is to understand this peculiar growth mechanism by closely examining the morphology of the SiNWs as a function of the growth duration. © XXXX American Chemical Society
Figure 1. (a) Standard VLS nanowire growth with a metal droplet on top, usually observed when a high surface tension metal such as gold is used. (b) Low surface tension metals such as Sn, In, and Bi assisted nanowire growth with the presence of a sidewall wetting layer along with the droplet on top. (c−f) Schematic representation of the Sn assisted nanowire growth in plasma-assisted VLS mode.
Received: March 28, 2013 Revised: July 25, 2013
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dx.doi.org/10.1021/jp403063d | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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EXPERIMENTAL METHODS Sn-assisted SiNW growth was carried out in a capacitively coupled plasma enhanced chemical vapor deposition (PECVD) reactor. The experimental procedure involved the following steps: (i) First, a thin Sn layer of 2 or 6 nm (nominal thickness) was thermally evaporated on top of a ZnO:Al film, sputtered on Corning glass (Cg), as illustrated in Figure 1c. (ii) Then, the substrates were loaded into the PECVD reactor, where Sn droplets were formed by applying a hydrogen (H2) plasma at a temperature of 300 °C. The flow rate, chamber pressure, and radio frequency power density were 100 sccm (standard cubic centimeters per minute), 600 mTorr, and 50 mW/cm2, respectively. During this step, the oxide formed on top of the evaporated Sn layer was reduced and nanoscale Sn liquid droplets were formed on ZnO:Al/Cg substrate, as observed in the scanning electron microscope (SEM, Hitachi S-4800) images shown in Figure 2h and i, corresponding to the initial Sn thickness of 2 and 6 nm, respectively. This step is also described schematically in Figure 1d.
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RESULTS AND DISCUSSION To trace the growth evolution of Sn-assisted SiNWs, a series of samples with growth durations ranging from 4 min to 4 h were prepared, while keeping the other parameters exactly the same. Each sample was characterized by SEM as presented in Figure 2a−g. Up to 12 min of growth, short and straight nanowires with Sn droplets on top were obtained. Further increase of the growth duration to 20 min led to a gradual tapering of the SiNWs, with a thicker base and sharper tip. That was accompanied by a significant shrinking of the Sn droplets, which became hardly detectable in the SEM images. The gradual shrinking of metal droplets in a plasma-assisted VLS process could result from three possible mechanisms: (i) Sn can be incorporated into the SiNW itself. However, considering the extremely low solubility of Sn in Si at the working temperature,14 this can hardly account for the disappearing of Sn. (ii) Sn can be progressively removed from the SiNW top through atomic hydrogen etching by forming a volatile tin hydride.15 (iii) As the Sn liquid has a lower surface energy than the SiNW sidewalls, it tends to spread out in the form of an ultrathin metal liquid wetting layer on the sidewalls,11 thus leading to a gradual loss of Sn from the droplets. Up to 20 min, the growth seems to follow the normal VLS behavior with Sn droplets resting on top of the SiNWs. The average length and the average diameter measured at the base of SiNWs are estimated from the 20 longest wires found in the SEM images and plotted against growth time in Figure 3a and b for two different initial Sn layer thicknesses. As we can see, during this period, the average length of SiNWs is proportional to the growth duration. When the growth duration was extended to 60 min, the SiNWs continued to grow longer but became sharply tapered with a needle-like shape. Interestingly, even though we could not detect Sn droplets on top of the SiNWs, the length followed the same linear trend as observed in Figure 3a. The growth rate extracted by linear fitting (up to 60 min) is around 0.9 and 1 nm/s for initial thicknesses of 2 and 6 nm, respectively. The growth of SiNWs without the assistance of Sn droplets at the tips indicates an unusual phenomenon, different from the standard VLS process where a metal droplet exists at the top of SiNWs. Normally, in the absence of a metal droplet, SiH4 plasma should lead to a hydrogenated amorphous silicon (a-Si:H) coating on the SiNWs, making them larger in diameter with rounded ends. However, when the growth duration was doubled again to reach 120 min, the growth of SiNWs continued, although the axial growth rate gradually slowed down to 0.54 and 0.77 nm/s for initial Sn thicknesses of 2 and 6 nm, respectively, as shown in Figure 3a. This implies that the axial growth of SiNWs continues, regardless of the fact that no Sn droplets can be observed at SiNW tips (Figure 2f). Later on, when the deposition was extended to 240 min, the axial growth rate dropped to 0.22 and 0.5 nm/s for 2 and 6 nm of initial thickness, respectively. From the SEM image shown in Figure 2g, all the SiNWs are found to be coated with a thick layer of a-Si:H, and to become uniform pillars with rounded ends measuring around 700 nm in diameter. This is what one could expect when the Sn droplets are completely exhausted, with any further deposition only leading to a-Si:H coating over the SiNWs. Raman spectroscopy of the 240 min deposited sample has also confirmed that the coating layer on the SiNWs is amorphous (characterized by a broad Raman peak at 478 cm−1). In the meantime, during the initial growth stage (up to 20 min), the diameter of the SiNWs remains constant; i.e., the radial growth is negligible, as shown in Figure 3b. This is somewhat
Figure 2. (a−g) SEM micrographs of silicon nanowires obtained for different growth durations. The insets show zoomed views of single nanowires. (h and i) SEM micrographs of the Sn droplets obtained after applying the H2 plasma in our PECVD reactor, for two different thicknesses of evaporated Sn.
(iii) Next, a gas mixture containing 10 sccm of silane (SiH4) and 100 sccm of H2 was introduced to reach a chamber pressure of 1 Torr at 400 °C and the plasma was ignited with a radio frequency power density of 20 mW/cm2 to trigger the growth of SiNWs, as depicted in Figure 1e and f. During the plasma-assisted VLS process, SiHx (0 ≤ x ≤ 3) radicals are absorbed by the Sn droplets to establish Si supersaturation. Considering that Si solubility in Sn is very low ( λ). As a consequence, Si atoms have to nucleate on the local sidewall surface, leading to an enhancement in the radial growth rate. However, as the SiNWs become thinner and thinner (d decreases), the top ends of SiNWs also shrink and the local curvature increases as well. Eventually, when the tapering approaches to a sharp tip, as found in Figure 4d, there is no longer any energetic driving force (as Δμ ∝ 1/r, where r is the radius of the SiNW) for the Si adatoms to diffuse toward the tip. Therefore, during the tapering of SiNWs, the axial growth of D
dx.doi.org/10.1021/jp403063d | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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ACKNOWLEDGMENTS S.M. thanks the “French Ministry for Higher Education and Research” for his PhD funding. The work has been carried out in the framework of the TOTAL-LPICM joint research team.
SiNW gradually slows down (as observed in Figure 3a), while the lateral growth speeds up (as observed in Figure 3b). Moreover, through high resolution TEM characterization of a SiNW grown in this phase, a unique core−shell structure inside the SiNWs is observed. We have noticed that the Si core is [211] oriented with a diameter of typically 20 nm, while the crystalline Si shell is probably in the [111] direction, as shown in Figure 6. The Si
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Figure 6. High resolution TEM image showing the crystalline core− shell structure of the nanowire. The crystalline core of the SiNW is in the [211] direction.
[211] plane provides parallel edge steps of Si [111] planes, which are energetically more favorable sites for attaching Si adatoms compared to the [111] plane. For this reason, when the nanowires are covered by an ultrathin wetting layer, the adatoms will preferentially precipitate at the solid interface terminating with the Si [211] plane. A detailed TEM analysis on a similar kind of SiNWs has been reported in our previous work.7 (3) Finally, when the Sn liquid wetting layer is exhausted (growth time above 120 min), the axial growth of SiNWs begins to saturate and the subsequent plasma exposure leads to a-Si:H deposition on the SiNWs, transforming them into thicker pillars, as illustrated in Figure 3e. In summary, we have carried out a detailed investigation on Sn-assisted SiNW growth in a plasma environment and revealed its unique growth behavior. The growth of SiNWs can be explained by the presence of an ultrathin Sn wetting layer on their sidewall. This wetting layer helps to enhance the axial growth rate of SiNWs and effectively suppresses their radial expansion in the first phase, in a way similar to the MBE growth. Our results indicate that the diffusion of the Si adatoms plays a crucial role in enhancing the axial growth of Sn-assisted SiNWs. After that, the liquid Sn wetting layer is responsible for the axial and radial growth until its exhaustion. At this stage, the crystalline growth stops and an a-Si:H layer is deposited on the SiNWs.
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The authors declare no competing financial interest. E
dx.doi.org/10.1021/jp403063d | J. Phys. Chem. C XXXX, XXX, XXX−XXX
