Ch. Deneke, C. M¨uller, and O.G. Schmidt Two different approaches are generally used to
fabricate nanometer structures. The bottom-up approach relies on the self-formation pro-cess of self-assembling nanostructures, such as semiconductor quantum dots or chemi-cally synthesized nanoparticles, whereas the top-down approach utilizes lithographic and etching techniques to create well-defined and well-positioned nano-electromechanical sys-tems (NEMS). A huge impact is expected if these two approaches could be merged into one single technology.
Recently, it was shown that semiconductor lay-ers form a new class of free-standing nano-objects if they are released from their sub-strate [V.Y. Prinz et al., Physica E 6, 828 (2000);
O.G. Schmidt et al., Nature 410, 168 (2001);
O.G. Schmidt et al., Advanced Materials 13, 756 (2001)]. The fabrication process combines self-formation with standard semiconductor pro-cessing techniques and therefore establishes a powerful integration of top-down and bottom-up approach.
A layer structure consisting of a thin sacrificial layer followed by one or more top layers are grown pseudomorphically onto a substrate by solid source molecular beam epitaxy. The sac-rificial layer is than removed ex-situ with selec-tive etching. As a result the top layer structure is released from the substrate and can form a novel kind of nanoobject. We can distinguish two different methods for nanotube formation.
For the first method a single layer is grown on top of a sacrificial layer. After the ex-situ etching step the top layer wraps up and folds back onto its own substrate (see Fig. 1).
This method requires not necessarily any spe-cial features of the layer structure and is therefore general. The second method in-volves a special design of the layer structures.
Two different and lattice mismatched materials
Figure 1: Folding (Method I) an rolling up (Method II) nanotubes.
Figure 2: Multi-wall InGaAs nanotube on a GaAs surface. (a) The tube has rolled up over a distance of 50µm and has a length of 9µm (b). (c) Magnifi-cation of the lower opening, demonstrating at least eight full rotations.
are grown pseudomorphically on top of each other introducing thus strain in the structure.
The strain results in a momentum that forces the top layers to roll up when released from the sub-strate. The position of the nanotube is defined by the starting point and the etching time, while the diameter can be scaled by changing the layer thicknesses and the built-in strain.
A layer sequence of 2.83 nm AlAs, 1.9 nm In033Ga067As and 3.5 nm GaAs was grown on top of a GaAs (001) substrate. The InGaAs/GaAs bilayer is inherently strained due
to the lattice mismatch between the two mate-rials. During the ex-situ etching step the AlAs layer is removed by HF solution. As a result the bilayer is released from its substrate. A scan-ning electron microscope of the resulting nano-tube is presented in Fig. 2. The nano-tube has rolled up over a distance of 50 µm and has a diame-ter of approximately 500 nm. We therefore con-clude that the nanotube has performed about 30 rotations.
Figure 3: Nanotube with a diameter of 15 nm. The tube rolled over a distance of 6µm performing 30 rotations.
Figure 3 shows a nanotube obtained from a layer structure consisting of 2.83 nm AlAs/
0.41 nm In033Ga067As / 1.25 nm GaAs. This nanotube has a diameter of 15 nm and has rolled
Figure 4: (a) Completely free-standing nanotube. (b), (c) Schematic illustration of the fabrication of the nanotube.
Figure 5: (a) Ring-like vertical membrane based on a strained bilayer SiGe/Si system. (b) Schematic illus-tration of the formation process of the free-standing membrane. (c) Nanopipeline based on a folded back SiGe layer (method I). (d) Tube from semiconductor/metal hybrid layer system.
up over 6 µm, again performing about 30 rota-tions. Our results demonstrate that the inner to outer diameter ratio can be tuned over a wide range. Whereas the nanotube in Fig. 2 has a ra-tio of nearly 1, the nanotube in Fig. 3 exhibits a ratio of only 0.125. This special design free-dom makes the nanotubes ideal candidates for fundamental investigations as well as for possi-ble applications.
An entirely free-standing nanotubes is pre-sented in Fig. 4(a). The sample was cleaved per-pendicular to the nanotube so that the tube ex-tends over the cleaved edge (see Fig. 4(b) and (c)).
The technique introduced here is not restricted to III-V semiconductors nor to free-standing nanotubes. Other nanometer-size objects can be produced such as ultra-thin vertical membranes or helical coils.
Figure 5(a) shows a ring-like free-standing membrane formed out of a strained SiGe layer structure. The formation of the membrane fol-lows a two step procedure (see Fig. 5(b)). In the first step the bilayer bends up perpendicularly to the surface. In a second step the bilayer starts to curl horizontally giving rise to the ring-like membrane in Fig. 5(a).
Figure 5(c) shows a nanotube from a layer struc-ture consisting of a semiconductor layer, an ox-ide and a metal layer. On top of a Ge sacri-ficial layer a SiGe layer sequence was grown, starting with a Ge content of 70% and ending with a pure Si layer. The Si layer was oxidized and Ti was finally deposited on top of the struc-ture. After selective etching a nanotube formed on the surface – integrating different materials.
The object shown in Fig. 5(d) represents a Si-based nanopipeline formed after method I. A layer with 40% average Ge concentration was grown on top of a 70 nm thick Ge sacrificial buffer layer. Creases formed perpendicular to the main tube, showing potential for inlet and outlet channels in a more complex nanopipeline system.
In conclusion, the release of semiconduc-tor layers from their substrate offers a new route to form well-defined and well-positioned nanometer-sized objects on substrate surfaces.
We have applied this technique to created nan-otubes with different sizes and geometries and out of different material systems. Some of the tubes performed 30 rotations on the surface.
Other objects like ultra-thin ring-like mem-branes and nanopipeline systems have been pre-sented.