Silicon nanowires grown on Si(1 0 0) substrates via thermal reactions with carbon nanoparticles

https://doi.org/10.1016/S0009-2614(03)00090-3Get rights and content

Abstract

The effect of thermal processing at 1050 °C of a dispersed film of carbon nanoparticles deposited on a Si substrate with a native SiO2 layer has been studied by scanning electron microscopy and scanning photoelectron spectromicroscopy. It has been found that the thermal processing results in formation of pyramidal-shaped defects of 2–7 μm with strongly reduced SiO2 content with silicon wires of diameter ranging between 30 and 50 nm decorating the pyramid walls. The nucleation of the Si nanowires occurs via reduction of the native oxide layer by the nanosized carbon particles, without the need of metal catalysts and at temperatures relatively lower than that used in similar techniques.

Introduction

Silicon nanowires with diameter of several tens of nanometers and length of tens of micrometers, exhibit unusual quantum confinement effects and interesting electrical and optical properties with promising technological impact in the microelectronic field [1], [2], [3], [4], [5], [6], [7]. Several techniques have already been used for synthesis of silicon nanowires, such as vapour–liquid–solid growth (VLS) catalysed by a gold layer on Si [1], laser ablation of metal containing targets [7] and, most recently, Ni-assisted solid–liquid–solid process [3] and carbothermal reduction of Fe-catalysed SiO2 particles [4]. Some of the proposed growth models show that the formation of Si nanowires does not require the presence of a metal, because the top SiO2 layer can play the role of a catalyst in the wire nucleation [2], [3], [4], [5].

We have already shown that Si nanowires can be synthesised without metal catalyst by simple carbothermal reduction of the native oxide layer on a Si(1 0 0) substrate, assisted by C nanoparticles (CNPs) deposited by laser pyrolysis [8], [9]. Annealing of the Si substrate covered by a dispersed layer of CNPs produces square-like pyramidal voids which act as nucleation sites for growth of Si nanowires with diameter of 40–50 nm. The Si nanowire production was favoured at low CNPs coverage, when the carbothermal reduction occurring at the CNPs/substrate interface is terminated after complete consumption of CNPs in contact with Si. On the contrary, when the CNP density is high, the excess of nanoparticles reorganise into self-assembled carbon nanotubes [10], [11], [12].

Here we used two laterally resolved techniques, scanning electron microscopy (SEM) and synchrotron radiation scanning photoelectron microscopy (SPEM), to obtain deeper insight on the morphology, composition and the lateral distribution of the chemical species on the surface obtained under conditions that we believe to favour Si nanowire formation, i.e., annealing of the low density CNPs/Si(1 0 0) interface at 1050 °C.

Section snippets

Experimental

The CNPs were prepared by CO2 laser-induced decomposition of acetylene–ethylene mixtures in a flow reactor [8], [9]. The CNPs were amorphous and largely hydrogenated with a mean size of 50±30 nm. After synthesis, the CNPs were sprayed on the Si(1 0 0) substrates and resistively heated up to 1050 °C at 10−6 atm. The annealed samples were analysed ex situ by SEM (Jeol 5400, resolution of 3 nm with an accelerating voltage of 30 kV) and SPEM. The SPEM measurements were carried out with the

Results

Figs. 1a and b shows SEM images obtained after annealing a Si substrate covered with a low density CNPs at 1050 °C. They reveal the presence of nanostructured filaments and tubules (with diameter ranging between 30 and 50 nm) inside square-like features (‘squares’). These ‘squares’ are the basal plane of void defects, often observed during carbonisation of Si surfaces [15], [16], [17]. They are hollow inverted pyramids (base at the interface, vertex in the substrate) formed by {1 1 1} Si planes

Discussion

The SEM and SPEM analysis of the samples annealed for a short time (Fig. 1, Fig. 2) demonstrates that the formation of the pyramidal-shaped defects plays a crucial role in nucleation of Si nanowires, which grow mostly along the pyramid walls. These defects are typical features appearing on the Si side of the SiC/Si interfaces [15], [16], [17] during Si carbonisation, their origin being attributed to presence of oxygen and oxygen-related defects in bulk Si wafers [21], [22]. However, microscopic

Conclusions

Silicon nanowires can be successfully synthesised without metal catalysts by annealing the Si substrates at 1050 °C in the presence of active carbon nanoparticles. The Si nanowire formation is well described by the oxide-assisted growth model. The present results have proved that in the initial reaction stage the produced Si nanowires decorate the walls of pyramidal voids, formed during carbothermal reduction of the native SiO2 layer covering Si substrates.

Acknowledgments

The authors wish to thank D. Lonza for his technical assistance and C. Cepek for fruitful discussions.

References (25)

  • Z. Zhang et al.

    Chem. Phys. Lett.

    (2001)
  • H.F. Yan et al.

    Chem. Phys. Lett.

    (2000)
  • X.C. Wu et al.

    Chem. Phys. Lett.

    (2001)
  • N. Wang et al.

    Chem. Phys. Lett.

    (1999)
  • S. Botti et al.

    Chem. Phys. Lett.

    (2002)
  • L.-O. Bjorketun et al.

    J. Cryst. Growth

    (1997)
  • S. Gunther et al.

    Ultramicroscopy

    (1998)
  • J.D. Holmes et al.

    Science

    (2000)
  • Y.F. Zhang et al.

    Appl. Phys. Lett.

    (1999)
  • Y.F. Zhang et al.

    Appl. Phys. Lett.

    (1998)
  • S. Botti et al.

    J. Appl. Phys.

    (2000)
  • S. Botti et al.

    Appl. Phys. A

    (1998)
  • Cited by (11)

    • Amorphous SiO<inf>x</inf> nanowires grown on silicon (100) substrates via rapid thermal process of nanodiamond films

      2006, Thin Solid Films
      Citation Excerpt :

      Another possible growth mechanism involves the formation and subsequent oxidation of Si nanowires [27]. There have been reports of carbon-assisted synthesis of silicon nanowires [6,7]. Nevertheless, we suggest that the SiOx nanowires synthesized in our experiment is less likely to follow this mechanism for the following reasons: (1) No crystalline silicon embedded in the amorphous SiOx nanowires were detected in the sample.

    • Polymer-assisted synthesis of aligned amorphous silicon nanowires and their core/shell structures with Au nanoparticles

      2004, Chemical Physics Letters
      Citation Excerpt :

      Upto now, several methods have been employed to produce SiNWs, including chemical vapor deposition (CVD) [3], thermal evaporation of Si powder [4], metal catalyzed vapor–liquid–solid method [5], laser ablation [6], oxygen-assisted synthesis [7], heating SiO2–Si mixtures or pure SiO powders [8,9], and solution etching [10]. Furthermore, it has been recently reported that enhanced yields of SiNWs are obtained by heating a Si substrate coated with carbon nanoparticles at 1050 °C under vacuum [11]. Wherein, the role of carbon is to react with the oxide probably producing a suboxide-type species.

    • Carbon-assisted synthesis of silicon nanowires

      2003, Chemical Physics Letters
      Citation Excerpt :

      SiO2-sheathed crystalline SiNWs have been obtained by heating Si–SiO2 mixtures [10]. It has been recently reported that enhanced yields of SiNWs are obtained by heating a Si substrate coated with carbon nanoparticles at 1050 °C under vacuum [11]. We consider the role of carbon to be as in other carbothermal methods of synthesizing nanowires of oxides, nitrides and other materials, involving a vapor–solid mechanism wherein carbon reacts with the oxide probably producing a suboxide-type species.

    View all citing articles on Scopus
    View full text