M. Totaro

IS THE REGIME WITH SHOT NOISE SUPPRESSION BY A FACTOR 1/3 ACHIEVABLE IN SEMICONDUCTOR DEVICES WITH MESOSCOPIC DIMENSIONS?

We discuss the possibility of diffusive conduction and thus of suppression of shot noise by a factor 1/3 in mesoscopic semiconductor devices with two-dimensional and one-dimensional potential disorder, for which existing experimental data do not provide a conclusive result. On the basis of our numerical analysis, we conclude that it is quite difficult to achieve diffusive transport over a reasonably wide parameter range, unless the device dimensions are increased up to the macroscopic scale where, however, shot noise disappears because the device length exceeds the Debye length. In addition, in the case of one-dimensional disorder, some mechanism capable of mode-mixing has to be present in order to reach or even approach the diffusive regime.

Effect of potential fluctuations on shot noise suppression in mesoscopic cavities

We perform a numerical investigation of the effect of the disorder associated with randomly located impurities on shot noise in mesoscopic cavities. We show that such a disorder becomes dominant in determining the noise behavior when the amplitude of the potential fluctuations is comparable to the value of the Fermi energy and for a large enough density of impurities. In contrast to existing conjectures, random potential fluctuations are shown not to contribute to achieving the chaotic regime whose signature is a Fano factor of 1/4, but, rather, to the diffusive behavior typical of disordered conductors. In particular, the 1/4 suppression factor expected for a symmetric cavity can be achieved only in high-quality material, with a very low density of impurities. As the disorder strength is increased, a relatively rapid transition of the suppression factor from 1/4 to values typical of diffusive or quasi-diffusive transport is observed. Finally, on the basis of a comparison between a hard-wall and a realist...

Effect of imperfections on the tunneling enhancement phenomenon in symmetric double quantum dots

The conductance of a pair of quantum dots, coupled through a tunnel barrier and connected to two external leads, exceeds the conductance of the tunnel barrier alone (tunneling enhancement effect) if the device is symmetrical, while it strongly decreases if the symmetry is destroyed. This device could then be used to implement a sensitive detector of symmetry breaking quantities, such as magnetic fields. We present a numerical study of the robustness of this phenomenon to the presence of imperfections. We find that, while a realistic amount of edge roughness in the depletion gates defining the structure does not compromise the enhancement effect significantly, and also lithographic errors can be compensated by properly tuning the voltages applied to the gates, the presence of randomly located ionized dopants in the heterostructure can strongly degrade the conductance enhancement and thus particular care has to be taken in terms of cleanliness and mobility of the heterostructure.

Selective doping of silicon nanowires by means of electron beam stimulated oxide etching.

Direct patterning of silicon dioxide by means of electron beam stimulated etching is shown, and a full characterization of exposure dose is presented. For its high dose, this technique is unsuitable for large areas but can be usefully employed like a precision scalpel for removing silicon dioxide by well-localized points. In this work, this technique is applied to the definition of windows through the oxide surrounding top down fabricated n-doped silicon nanowires. These windows will be employed for a selective doping of the nanowire by boron diffusion. In this way, pn junctions can be fabricated in well-localized points in the longitudinal direction of the nanowire, and an electrical contact to the different junctions can be provided. Electrical I-V characteristics of a nanowire with pn longitudinal junctions are reported and discussed.

Discussion on the possibility of diffusive transport in mesoscopic conductors

We numerically investigate the conductance and the Fano factor in mesoscopic conductors, obtained by modulation doping in GaAs/AlGaAs heterostructures containing randomly located scatterers. In our simulations we represent these scatterers, deriving from the presence of randomly located impurities and dopants, either with hard-wall obstacles or with realistic potential fluctuations. Our results show that in mesoscopic devices it is quite unlikely to reach the diffusive regime, mainly due to the insufficient number of propagating modes.

SCOUT: small chamber for optical UV tests

SCOUT is the acronym of the new facility developed within the XUVLab laboratory of the Department of Physics and Astronomy of the University of Florence. SCOUT stands for “Small Chamber for Optical UV Tests” and has been designed to perform practical and fast measurements for those experiments requiring an evacuated environment. SCOUT has been thought, designed and manufactured by paying a particular attention to its flexibility and adaptability. The functionality and the capabilities of SCOUT have been recently tested in a measurement campaign to characterize an innovative wire-grid polarizer optimized to work in transmission in the UV band. This paper provides a description of the overall manufactured system and its performance and shows the additional resources available at the XUVLab laboratory in Florence that make SCOUT exploitable by whatever compact (within 1 m) optical experiment that investigates the UV band of the spectrum.

Silicon Nanostructures for Thermoelectric Applications

In this chapter, an overview on silicon nanostructures for thermoelectric applications is presented. After an introduction on the key concepts of thermoelectricity, we show that nanostructuring is one of the most promising solutions for making high efficient thermoelectric devices. In particular, we discuss the use of nanostructured silicon as a good thermoelectric material, due to its abundance, its nontoxicity, and its technological pervasiveness in the society, compared to other materials often proposed in the literature. Furthermore, a top-down process for the reliable fabrication of very complex and large area arrays of silicon nanowires (SiNWs) is shown and discussed. Finally, we show that these networks can be employed for the fabrication of high efficiency thermoelectric generators, and the high reliability and the high tolerance with respect to SiNW width dispersion are demonstrated by means of numerical simulations.

SCOUT: a small vacuum chamber for nano-wire grid polarizer tests in the ultraviolet band

Within the Section of Astronomy of the Department of Physics and Astronomy of the University of Firenze (Italy), the XUVLab laboratory is active since 1998 dedicated to technological development, mainly UV oriented. The technological research is focused both on electronics and optics. Our last approach is dedicated to the development of innovative wiregrid polarizers optimized to work in transmission at 121.6 nm. The manufacturing of such optical devices requires advanced technological expertise and suitable experimental structures. First, nanotechnology capability is necessary, in order to build several tiny parallel conductive lines separated by tens of nanometers on wide areas to be macroscopically exploitable in an optical laboratory. Moreover, the characterization of such an advanced optical device has to be performed in vacuum, being air absorptive at 121.6 nm. A dedicated small vacuum chamber, SCOUT (Small Chamber for Optical UV Tests) was developed within our laboratory in order to perform practical and fast measurements. SCOUT hosts an optical bench and is equipped with several opening flanges, in order to be as flexible as possible. The flexibility that has been reached with SCOUT allows us to use the chamber beyond the goals it was thought for. It is exploitable by whatever compact (within 1 m) optical experiment that investigates the UV band of the spectrum.

Ionized donor reordering in typical heterostructures

The mobility of the 2-dimensional electron gas (2DEG) in AlGaAs/GaAs heterostructures is limited by the presence of ionized donors in the doped region, which determines random potential fluctuations in the 2DEG. However, especially at very low temperatures, only a fraction F of all dopants is ionized. When this fraction is significantly less than unity, redistribution of the ionized sites through hopping can lead to reordering of the donor layer charge and thus to larger than otherwise expected 2DEG mobility. Here we evaluate the relevance of this effect in some typical heterostructures, and look for criteria for the achievement of the largest possible mobility.

Using gate voltages to tune the noise properties of a mesoscopic cavity

We propose a layout for a tunable mesoscopic cavity that allows to probe the conductance and noise properties of direct transmission channels (“noiseless scattering states”). Our numerical simulations demonstrate how the variation of different gate voltages in the cavity leads to characteristic signatures of such non‐universal processes. Using realistic assumptions about scattering in two‐dimensional heterostructures, our proposed layout should define a viable protocol for an experimental realization.