A. Notargiacomo

Conductance quantization in etched Si/SiGe quantum point contacts

We fabricated strongly confined Schottky-gated quantum point contacts by etching Si/SiGe heterostructures and observed intriguing conductance quantization in units of approximately 1e2/h. Non-linear conductance measurements were performed depleting the quantum point contacts at fixed mode-energy separation. We report evidences of the formation of a half 1e2/h plateau, supporting the speculation that adiabatic transmission occurs through 1D modes with complete removal of valley and spin degeneracies.

Single-electron transistor based on modulation-doped SiGe heterostructures

We report the characterization of a single-electron transistor based on bended wires fabricated on modulation-doped SiGe two-dimensional electron gas. Electrical measurements show a diamond-shaped stability plot and a nonperiodic sequence of conductance peaks. The device behavior suggests the presence of disorder-induced multiple islands along the wire. Conductance oscillations remain well pronounced above liquid helium temperature.

Design Concepts, Fabrication and Advanced Characterization Methods of Innovative Piezoelectric Sensors Based on ZnO Nanowires.

Micro- and nano-scale materials and systems based on zinc oxide are expected to explode in their applications in the electronics and photonics, including nano-arrays of addressable optoelectronic devices and sensors, due to their outstanding properties, including semiconductivity and the presence of a direct bandgap, piezoelectricity, pyroelectricity and biocompatibility. Most applications are based on the cooperative and average response of a large number of ZnO micro/nanostructures. However, in order to assess the quality of the materials and their performance, it is fundamental to characterize and then accurately model the specific electrical and piezoelectric properties of single ZnO structures. In this paper, we report on focused ion beam machined high aspect ratio nanowires and their mechanical and electrical (by means of conductive atomic force microscopy) characterization. Then, we investigate the suitability of new power-law design concepts to accurately model the relevant electrical and mechanical size-effects, whose existence has been emphasized in recent reviews.

Current–Voltage Characteristics of ZnO Nanowires Under Uniaxial Loading

The charge transport in zinc-oxide piezosemiconductive nanowires under purely vertical compressive or tensile strains is investigated. For simplicity, only the additional band bending originated by the piezoelectric charges has been accounted for. Moreover, a constant volumetric piezoelectric charge density is assumed, distributed within a maximum distance δpiezo from the two junctions between the metal ends and the nanowire. Examples demonstrate that the carrier concentration, the energy conduction band profile, and the current-voltage characteristics significantly depend on δpiezo. Therefore, we propose the use of current- voltage measurements to obtain information on δpiezo in strained piezosemiconductors.

Three-dimensional shaping of sub-micron GaAs Schottky junctions for zero-bias terahertz rectification

We demonstrate a rectification mechanism based on quantum tunneling through the narrow Schottky barrier of a sub-micrometric Au/Ti/n-GaAs junction, which is capable of efficient power detection of free-space terahertz radiation beams even without an applied dc bias. Three-dimensional shaping of the junction geometry provides an enhanced zero-bias tunneling probability due to increased electric fields at the junction, resulting in cutoff frequencies up to 0.55 THz, responsivity up to 200 V/W, and noise equivalent power better than 10−9 W/Hz0.5 without applied dc bias.

Investigating the CVD Synthesis of Graphene on Ge(100): toward Layer-by-Layer Growth

Germanium is emerging as the substrate of choice for the growth of graphene in CMOS-compatible processes. For future application in next generation devices the accurate control over the properties of high-quality graphene synthesized on Ge surfaces, such as number of layers and domain size, is of paramount importance. Here we investigate the role of the process gas flows on the CVD growth of graphene on Ge(100). The quality and morphology of the deposited material is assessed by using μ-Raman spectroscopy, X-ray photoemission spectroscopy, scanning electron microscopy, and atomic force microscopy. We find that by simply varying the carbon precursor flow different growth regimes yielding to graphene nanoribbons, graphene monolayer, and graphene multilayer are established. We identify the growth conditions yielding to a layer-by-layer growth regime and report on the achievement of homogeneous monolayer graphene with an average intensity ratio of 2D and G bands in the Raman map larger than 3.

Nanofabrication of quantum wires on (100) Si and SiGe by shifted-resist pattern and anisotropic wet etching

A technique based on shifted resist pattern and anisotropic wet etching has been developed for fabrication of quantum wires on (100) silicon and silicon/germanium substrates. The Electron Beam Lithography exposure of a same pattern properly shifted is used to obtain a mask for the subsequent wet etching. This etching takes advantage on the strong rate dependency on plane orientation and allows V groove shaped thin wires formation.

Micromachined silicon slits for beam diagnostics in particle accelerators

We report the fabrication process of a silicon target with a rectangular slit as an instrument for measuring the size and the angular divergence of high charge-density electron beams in particles accelerators. Bulk micromachining of silicon wafers by means of anisotropic etching allowed the definition of slits with parallel straight edges and low disuniformity. The disuniformities of the completed device evaluated by scanning electron microscopy were found to be tolerable with respect to the wavelength used in the experiments. Tests of the fabricated targets are in progress in the injector of ELETTRA, the synchrotron radiation facility in Trieste, Italy.

Low field magnetotransport in strained Si/SiGe cavities

Low field magnetotransport revealing signatures of ballistic transport effects in strained Si/SiGe cavities is investigated. We fabricated strained Si/SiGe cavities by confining a high mobility Si/SiGe 2DEG in a bended nanowire geometry defined by electron-beam lithography and reactive ion etching. The main features observed in the low temperature magnetoresistance curves are the presence of a zero-field magnetoresistance peak and of an oscillatory structure at low fields. By adopting a simple geometrical model we explain the oscillatory structure in terms of electron magnetic focusing. A detailed examination of the zero-field peak lineshape clearly shows deviations from the predictions of ballistic weak localization theory.

X-ray scanning microscope study of strain instabilities in low mismatched SiGe alloys grown on Si(001) substrates

An X-ray scanning microscope study of self-organized morphological defects arising during the growth of thick SiGe virtual substrates grown on Si(001) surface is reported. The 200 nm microscope resolution permitted to probe the local alloy composition. The measured variations of the Ge content in the alloy demonstrate that defect formation and evolution represent a strain relief mechanism. The observed large stress concentration inside the defect and the absence of the cross-hatch pattern on its internal facets are findings in close analogy with the lateral strain relaxation of self-organized islands in heterostructures.