Ordered Si-Ge nanostructures by glancing angle deposition via ion beam sputtering
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Ordered Si-Ge nanostructures by glancing angle deposition via ion beam sputtering
Jens Bauer, Michael Weise, Chinmay Khare, and Bernd Rauschenbach Leibniz Institute of Surface Modification, Leipzig, Germany
ABSTRACT Glancing angle deposition (GLAD) was used to deposit ordered arrangements of Si/Genanocolumns applying the ion beam sputter technique. After substrate preparation by electron beam lithography as well as nanosphere lithography the deposition behavior of GLAD nanocolumns in regular arrangements with different symmetries was studied. The nanocolumns exhibited distinct morphology regions which are correlated to their temporal evolution during deposition. Furthermore, the customization of the column morphology by non-uniform substrate rotation is considered. Axial Si/Ge-heterojunctions were incorporated by sequential deposition. INTRODUCTION The preparation of efficient thermoelectric nanomaterials is a big challenge in modern material science. One promising approach to increase the efficiency of thermoelectric materials is the usage of semiconductor nanowire structures with a small diameter (< 50 nm) and rough surfaces [1, 2]. A further improvement is expected by incorporation of interfaces along the wire axis to reduce the phonon propagation and thus the thermal conductivity in this direction [3]. Latter can be technically realized by planar lattice defects or heterojunction superlattices as well. Hence, a special focus is set on the defined adjustment of nanostructure geometry, size, orientation, and material combination. For this purpose we present physical vapor depositions investigations on self-organized a-Si and a-Ge nanostructures by a recent nanofabrication technique, the glancing angle deposition (GLAD) [4-6]. In particular, a Si or Ge particle beam provided by ion-beam sputtering is directed under an extremely flat angle towards the substrate surface (glancing angle < 20 deg). At low substrate temperatures and thus reduced surface diffusion a ballistic deposition process occurs. As a result of shadowing separated nanoneedles evolve competitively with a defined inclination towards the particle incidence direction. The additional use of substrate rotation allows the shaping of this nanostructure, i.e. with continuous substrate rotation vertical posts, screws or even spirals can be realized with respect to the rotation speed [7]. With an azimuthally modulated particle flux the morphology can be further customized [6]. This paper focuses on the morphology evolution of GLAD-nanocolumns in different twodimensional arrangements and the incorporation of Si/Ge nanoheterojunctions. For that reason, first GLAD experiments on pre-patterned substrates are presented. Then we show results on the temporal evolution and the particular nanostructure morphology. In the second part the impact of non-uniform substrate rotation (ω-modulation) on the formed column shape is discussed. The capability of GLAD to realize axial nanoheterojunctions within nanocolumns is considered in the third part.
EXPERIMENT Si (100) su
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