Nanowire Physics and Characterization

To integrate nanowires into electronic components reliably, a thorough understanding of their relevant physical properties is required. The well-established physics of three-dimensional, two-dimensional, and more recently zero-dimensional semiconductor materials must be revised for the specific one-dimensional nanowire properties, such as surface and interface relaxation, carrier confinement (electrons and holes), and the strong dielectric index change at the nanowire environment interface. These new theories must be compared with experiments on actual nanowire structures, such as the verification of electronic structure calculations for nanowire heterostructures by photoluminescence measurements.
Electronic properties   Crucial parameters for the electrical performance of nanowire devices are the carrier concentration (defined by the doping level), carrier mobility, and band gap. Since these parameters can strongly depend on details of the nanowire growth, direct feedback between electrical characterization and growth optimization is necessary. Although future devices are envisioned with direct electrical contacts to the wires on a suitably structured growth substrate, nanowire characterization is usually performed horizontally (Fig. 7) as described in the previous section on nanowire contacts. The mobility can then be estimated based on how the nanowire conductance changes with the back-gate voltage. In parallel, optical techniques with micrometer-range spatial resolution (µ-photoluminescence) can also address individual wires and, in particular, reveal the band gap.
	Figure 7. Three-dimensional representation of an atomic force microscope image (7 µm x 7 µm) of a nanowire test device. The InP nanowire (diameter 70 nm) spans the image from left to right. Three wide metallic contacts are used as Ohmic current leads for two test structures on the same nanowire: the left part shows two additional electrodes (voltage probes for four-point measurements), whereas the right part features several side-gates for local gating.
Structural properties   The intrinsic band structure and density of states of nanowires depend directly on the structural design and quality, in particular on the occurrence of growth-related defects (dislocations, stacking faults, etc.). Consequently, these have to be investigated and understood before the electronic properties can be optimized. The crystalline nanowire structure may be quite different from that of bulk materials or other semiconductor heterostructures (superlattices, quantum dots) because of surface relaxation effects or kinetic limitations.
For example, longitudinal or axial InAs/InP nanowire heterostructures grown on InAs(111)B have a hexagonal lattice (Fig.3b), while both of these materials are cubic in the bulk. In small-diameter nanowire heterostructures, interface strain can be more easily accommodated. This inhibits the formation of misfit dislocations of mismatched materials and allows extended band gap engineering. Additionally, the use of core–shell structures can strongly impact the electrical and optical properties [20]. Understanding the strain in nanowires is thus a demanding problem for device optimization. Here, electron microscopy is a very efficient tool to obtain quantitative information about crystal structure and strain, as well as to identify growth-related defects like surface faceting, twins, and stacking faults. X-ray diffraction techniques also help to gain statistical information about shapes, misorientations, elastic and intermixing strain relaxations, and structural defects of epitaxial samples (Fig. 8). The results can then be compared with atomistic electronic and transport simulations to quantify the impact of important growth defects.
	Figure 8. Grazing incidence X-ray diffraction of epitaxial InAs/InP heterostructure nanowires. The reciprocal units are indexed with the hexagonal surface lattice of the face-centered cubic InAs substrate. The experiment is performed with a grazing incidence of 0.1° and 10 keV photon energy. Note the scattering contributions of the hexagonal close-packed nanowires (NW) as well as twins (TW) in the cubic phase that come from a small substrate overgrowth in-between the NWs. The peak doubling of the NW peaks comes from multiple scattering effects.
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