Quantum wire
The term quantum wire (engl. Quantum wire ) describes a spatial potential structure in which the freedom of movement of charge carriers is limited to one dimension. For real existing quantum wires it is required that the typical singularities of the one-dimensional electronic density of states can be demonstrated. Such quantum wires allow the experimental study of the one-dimensional electron gas.
Like quantum wells and quantum dots, quantum wires belong to the quantum structure .
Crystalline quantum wires
Quantum wires on a crystalline basis can only be produced if the De Broglie wavelength of the charge carriers in the material under consideration is well above atomic dimensions. This condition is not met for metals at room temperature. In contrast, the De Broglie wavelength for electrons in many semiconductor materials is in the range of a few nanometers.
In the case of crystalline quantum wires, a distinction is still made between diffusive and ballistic quantum wires.
- In diffusive quantum wires, the mean free path of the electrons is smaller than the length of the quantum wire. The quantum wire has many lattice defects , and there are many scattering processes of the electrons at the defects . The electron movement in such a system is described by the Boltzmann equation .
- In ballistic quantum wires, the mean free path of the electrons is greater than the length of the quantum wire. The quantum wire has few lattice defects and consequently few scattering processes occur. If a quantum wire has no lattice defects, its intrinsic resistance is zero. However, with ballistic quantum wires an electrical resistance can very well be measured. However, this resistance is due to a contact resistance that was first demonstrated experimentally by Rafael de Picciotto.
Manufacture in semiconductor heterostructures
The procedure is complicated compared to the production of quantum wells, which can be produced by epitaxial growth on a sufficiently smooth semiconductor substrate.
Typically, a thin layer with a thickening along a line is embedded in a material with a larger band gap . Due to their spatial expansion in the solid , the charge carriers experience a lower potential within the thickening than in the adjacent thinner quantum well and are therefore localized.
For the production were z. B. used the following methods:
- Overgrowth of cleavage surfaces ( cleaved edged overgrowth ): Here, a quantum well is first produced by epitaxial growth of a double heterostructure . The material is then cleaved parallel to the direction of growth, and the cleavage surface is overgrown with a second quantum well. The line of intersection of the mutually perpendicular quantum wells forms the quantum wire.
- Growth on incorrectly inclined surfaces (English. Growth on vicinal substrates ): A incorrectly inclined semiconductor substrate is used, on the surface of which regular steps from the height of a few atomic layers have formed. If a double heterostructure is grown on the substrate, quantum wires form along the steps.
- Growth (engl. On non-planar substrates growth on non-planar substrate ): There is used a semiconductor substrate on whose surface a V-shaped trenches are etched using lithographic techniques. These can be up to several micrometers wide. During the subsequent growth of a double heterostructure using MOVPE , a quantum wire crescent-shaped cross-section is formed at the bottom of the V-trench due to the capillarity .