Mapping active dopants in single silicon nanowires using off-axis electron holography.

We demonstrate that state-of-the-art off-axis electron holography can be used to map active dopants in silicon nanowires as thin as 60 nm with 10 nm spatial resolution. Experiment and simulation demonstrate that doping concentrations of 10(19) and 10(20) cm(-3) can be measured with a detection threshold of 10(18) cm(-3) with respect to intrinsic silicon. Comparison of experimental data and simulations allows an estimation of the charge density at the wire-oxide interface of -1 x 10(12) electron charges cm(-2). Off-axis electron holography thus offers unique capabilities for a detailed analysis of active dopant concentrations in nanostructures.

Donor deactivation in silicon nanostructures.

The operation of electronic devices relies on the density of free charge carriers available in the semiconductor; in most semiconductor devices this density is controlled by the addition of doping atoms. As dimensions are scaled down to achieve economic and performance benefits, the presence of interfaces and materials adjacent to the semiconductor will become more important and will eventually completely determine the electronic properties of the device. To sustain further improvements in performance, novel field-effect transistor architectures, such as FinFETs and nanowire field-effect transistors, have been proposed as replacements for the planar devices used today, and also for applications in biosensing and power generation. The successful operation of such devices will depend on our ability to precisely control the location and number of active impurity atoms in the host semiconductor during the fabrication process. Here, we demonstrate that the free carrier density in semiconductor nanowires is dependent on the size of the nanowires. By measuring the electrical conduction of doped silicon nanowires as a function of nanowire radius, temperature and dielectric surrounding, we show that the donor ionization energy increases with decreasing nanowire radius, and that it profoundly modifies the attainable free carrier density at values of the radius much larger than those at which quantum and dopant surface segregation effects set in. At a nanowire radius of 15 nm the carrier density is already 50% lower than in bulk silicon due to the dielectric mismatch between the conducting channel and its surroundings.

Doping limits of grown in situ doped silicon nanowires using phosphine.

Structural characterization and electrical measurements of silicon nanowires (SiNWs) synthesized by Au catalyzed vapor-liquid-solid growth using silane and axially doped in situ with phosphine are reported. We demonstrate that highly n-doped SiNWs can be grown without structural defects and high selectivity and find that addition of the dopant reduces the growth rate by less than 8% irrespective of the radius. This indicates that also the dopant incorporation is radius-independent. On the basis of electrical measurements on individual wires, contact resistivities as low as 1.2 x 10(-7) omega cm(-2) were extracted. Resistivity measurements reveal a reproducible donor incorporation of up to 1.5 x 1020 cm-3 using a gas phase ratios of Si/P = 1.5 x 10(-2). Higher dopant gas concentrations did not lead to an increase of the doping concentration beyond 1.5 x10(20) cm(-3).

Nanoparticle printing with single-particle resolution.

Bulk syntheses of colloids efficiently produce nanoparticles with unique and useful properties. Their integration onto surfaces is a prerequisite for exploiting these properties in practice. Ideally, the integration would be compatible with a variety of surfaces and particles, while also enabling the fabrication of large areas and arbitrarily high-accuracy patterns. Whereas printing routinely meets these demands at larger length scales, we have developed a novel printing process that enables positioning of sub-100-nm particles individually with high placement accuracy. A colloidal suspension is inked directly onto printing plates, whose wetting properties and geometry ensure that the nanoparticles only fill predefined topographical features. The dry particle assembly is subsequently printed from the plate onto plain substrates through tailored adhesion. We demonstrate that the process can create a variety of particle arrangements including lines, arrays and bitmaps, while preserving the catalytic and optical activity of the individual nanoparticles.