• electron mobility
• electronic transport
• FET
• InAs
• nanotechnology
• nanowire
• photoconductivity
• transistor

Semiconductor nanowires are artificial structures of nanometer size with an approximately cylindric shape. They have an aspect ratio much larger than one so that, in most instances, they can be considered 1D systems. This peculiar property makes nanowires one of the most interesting technologies in the roadmap for the electronic downscaling. At the moment, however, they are still the object of scientific research and, even if nanowire transistor devices start to be investigated and developed, many physical aspects and characteristics are still to be investigated.

In this thesis I have studied the transport properties of single nanowire transistors in a variety of configurations. The lateral-gate and the back-gate geometry were realized, measured and simulated using a finite element formalism. The simulation allowed the computation of electron mobility and charge density within the wires and the comparison among the results of different wire structures.
Photoconductivity measurements were also attempted in order to directly explore the nanowire electronic band structure.

The thesis is divided into four chapters.

- Semiconductor nanowires

In the first part I present the peculiarities of a semiconductor nanowire, discussing the features of the semiconductor material and the effect of the low dimensionality. Then I explain the growth process of the different kinds of nanowires used in this work (homogeneous and radially heterostructured). The nanowires were grown in the NEST laboratory with the Chemical Beam Epitaxy (CBE) technique. The realization of nanowires can be explained with the Vapor-Liquid-Solid (VLS) model for the growth mechanism involving the seed of a gold particle. After the growth an important stage is to check the nanowires produced by Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray spectroscopy (EDAS).

- Transport and optical properties in nanowires

An introduction is given to the transport in the mesoscopic structures distinguishing between ballistic and diffusive transport. The specific case of InAs nanowires transport is taken in account, discussing all the main factors that affect transport (sample contacts, gate contact, temperature and nanowire size). A simple transport model is then presented for the single nanowire transistor configuration: with some approximations it is possible to estimate the electron mobility and the charge density in the wire.
InAs is sensitive to mid-infrared light because of its small direct gap. After an overview on the electronic transitions related to photoconductivity, general properties of photoconductivity are described. Some photoconductivity experiments in the literature are reported.

- Processing and experimental set-up

The fabrication of a nanowire device requires many steps using photo-lithography and electron-lithography techniques. All steps are accurately described establishing a methodology for the single nanowire transistor configuration. Then the set-up for transport measurements is presented as well as the one for optical measurements.

- Transport and photoconductivity measurements

In the last part the transport and optical measurements are described. Transport results are discussed with the support of a finite element model that simulates the nanowire device. The model was developed in the case of a back-gate and a lateral-gate geometry in order to compute a more accurate capacitance of the nanowire-gate system. In this way a more reliable result on the electron mobility and charge density in the nanowire is estimated and compared with the literature results.
First attempts to measure the photoconductive response of individual nanowire are also discussed examining in particular the influence of the substrate.