NANO LETTERS
Ordered Arrays of Silicon Nanowires Produced by Nanosphere Lithography and Molecular Beam Epitaxy
2005 Vol. 5, No. 12 2524-2527
Bodo Fuhrmann,* Hartmut S. Leipner, and Hans-Reiner Ho1 che Interdisziplina¨res Zentrum fu¨r Materialwissenschaften, Martin-Luther-UniVersita¨t, Hoher Weg 8, D-06120 Halle, Germany
Luise Schubert, Peter Werner, and Ulrich Go1 sele Max-Planck-Institut fu¨r Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany Received September 16, 2005; Revised Manuscript Received October 18, 2005
ABSTRACT Because of their importance in fundamental research and possible applications in nanotechnology and nanoelectronics, semiconductor nanowires have attracted much interest. In addition to the growth itself, the control of the size and location is an essential problem. Here we show the growth of ordered arrays of vertically aligned silicon nanowires by molecular beam epitaxy using prepatterned arrays of gold droplets on Si(111) substrates. The ordered arrays of gold particles were produced by nanosphere lithography.
The vapor-liquid-solid (VLS) process is regarded as the fundamental mechanism of the growth of crystalline Si nanowires.1 Different growth techniques have been developed for the fabrication of silicon nanowires: chemical vapor deposition,2 pulsed laser deposition,3 and molecular beam epitaxy (MBE).4 The growth process is based on the control of small liquid droplets of a metal or alloy where the supersaturation of one component leads to a one-dimensional growth of a nanowire. The liquid alloy remains on the tip of the growing nanowire, and it is fed by the surrounding gas phase with new material. In the case of Si nanowire synthesis, the liquid droplet is an Si-Au eutectic. Recently, we have demonstrated the fabrication of Si nanowires by MBE,5 where gold droplets with significant size distribution and random location were used to promote the VLS growth. The Au droplets were formed by thermal treatment of a thin gold layer deposited in situ. However, an important requirement for device applications is the precise control of the location and the size of the nanowires. While the length of the Si nanowires is determined mainly by the MBE conditions (Si flux, growth temperature, and time), the diameter and the location of the nanowires are controlled by the diameter and the location of the gold droplets. Therefore, the application of lithographic methods for regularly arranged Si nanowires seems to be obvious. In addition to conventional lithography, more and more alternative inexpensive nanolithographic methods are used to * Corresponding Author. E-mail:
[email protected]. 10.1021/nl051856a CCC: $30.25 Published on Web 11/24/2005
© 2005 American Chemical Society
fabricate nanosized patterns.6 One of them is the “natural lithography”,7,8 later renamed to nanosphere lithography,9 in which close-packed monolayers or bilayers of monodisperse spherical particles are used as a lithographic mask. This method has already been applied successfully for the prepatterning of substrates, for example, for the fabrication of magnetic nanoparticles10 or carbon nanofibers.11 Hochbaum et al.12 used a soft lithography method to determine regions on the sample where the nanowires should grow and controlled the nanowire diameter and density by using gold colloids. In previous experiments,5 attempting the self-organized growth of Si nanowires, a random distribution of gold droplets with diameters between 40 and 200 nm was used as a template for the growth. However, in this case the nanowires had no sharp size distribution. The utilization of nanosphere lithography in this work overcomes this problem. In the combination with MBE growth, two main preparation features have to be realized: (i) any oxide layer has to be removed before the deposition of Si atoms and (ii) the regular arrangement of the Au droplets should not be disordered, for example, by coalescence. The first feature should guarantee a direct contact between the droplet and the substrate and subsequent epitaxial growth. Even a native oxide layer of about 2 nm would prevent the formation of nanowires. For all experiments, (111)-oriented Si wafers (As-doped, resistivity of
