Bruno Pellegrini

Shot noise in resonant tunneling structures

We propose a quantum mechanical approach to noise in resonant tunneling structures, that can be applied in the whole range of transport regimes, from completely coherent to completely incoherent. In both limiting cases, well known results which have appeared in the literature are recovered. Shot noise reduction due to both Pauli exclusion and Coulomb repulsion, and their combined effect, are studied as a function of the rate of incoherent processes in the well (which are taken into account by means of a phenomenological relaxation time), and of temperature. Our approach allows the study of noise in a variety of operating conditions (i.e., equilibrium, sub-peak voltages, second resonance voltages), and as a function of temperature, explaining experimental results and predicting interesting new results, such as the dependence of noise on filled emitter states and the prediction of both increasing and decreasing shot noise with increasing temperature, depending on the structure. It also allows the determination of the major contributions to shot noise suppression by performing noise measurements at the second resonance voltage.

A detailed analysis of the metal-semiconductor contact

Abstract A general quantum and electronic theory able to explain the electric and photoelectric experimental properties of the metal-semiconductor contacts is proposed. The theory consists firstly in calculating the electric space charge due to the quantum mechanical tunneling of the electrons from the metal into the semiconductor, and vice-versa, and to the metal and semiconductor bands bending. Then the electric charge so obtained is utilised to solve in an appropriate and complete way the Poisson equation so as to determine the electric field and potential as functions of the abscissa x. The electric field F(x) is employed to obtain a new expression for the junction capacitance C, holding in the general case of a non-uniform charge, whereas the electric potential νi(x) is used to calculate general expressions for the thermionic and photoelectric currents i and iph, respectively, taking into account in this both the tunneling probability through the energy barrier and the many-valley structure of the semiconductor energy bands. Finally, from νi(x), C, i and iph four new expressions of the energy barrier height of the contact are deduced. The theoretical results relative to the barrier height so determined (which hold for both n-and p-type semiconductors) are compared with published experimental values obtained, by means of capacitance and photocurrent measurements: (a) on contacts between n-type CdS and Au, Cu, Ag and Pt; (b) on contacts between n-type GaAs and Au, Ag, Cu, Sn, Al and Pt and; (c) on contacts between p-type GaAs and Au and Al. The agreement between the theoretical and experimental values is very good.

Very sensitive measurement method of electron devices current noise

The problem of measuring very low levels of current noise in bipoles (linear or not) is dealt with, and a measurement technique is proposed. This technique allows the measurement of noise power spectra 6-10 dB lower than the equivalent input power spectrum of the amplified necessary to perform the measurement. An improvement of 16-20 dB in the sensitivity is obtained with respect to the one of conventional methods, which, for an acceptable accuracy, require the noise of the bipole under test to be 10 dB larger than the equivalent input one of the amplifier. The present method is based on the accurate measurement of the amplifier transimpedance with respect to the input current noise sources and on the precise evaluation and subtraction of the contribution from all the spurious sources to the total noise. The whole procedure is implemented by means of a dual-channel signal analyzer and almost completely automated. The technique has been tested by using it to measure the power spectra of the noise given by known generators, of the Nyquist noise produced by bipoles made up of resistors and capacitors, and of shot noise in p-n junctions. The experimental results agree very well with theoretical predictions. >

Fabrication of silicon nanostructures by geometry controlled oxidation

This work presents fabrication processes of nanostructures on a top silicon layer of silicon on insulator substrates. These processes rely on the properties shown by a trapezoidal cross section, as the one obtained by anisotropic wet etching, when reduced by dry oxidation. As demonstrated by numerical simulations, oxide stress limits the oxidation process if the minor base is large enough, compared with the thickness. If instead the minor base is small, a triangular section is generated during the oxidation process and a controlled strong reduction of the cross section, until the nanometer range is possible. The fabrication of a silicon nanowire longer than 1.5 μm and with a cross section of 15 nm, obtained with this technique, is shown and demonstrated. By providing zones with different initial cross section dimensions, constrictions can be fabricated in suitable positions and controlled by oxidation reduction, so that tunnel barriers can be obtained. Room temperature electrical characterization of tunne...

Semiclassical simulation of quantum cellular automaton circuits

We present a simulator for quantum cellular automaton (QCA) circuits, based on a semiclassical model and aimed at the determination of the error probability in the presence of thermal excitations. Different methods are discussed; either a complete exploration of the configuration space, or a partial exploration by means of simulated annealing. Results for the probability of obtaining the ground state and the correct logical output are derived and compared for a chain of cells, for a majority voting gate and for a simple combinatorial network. This approach can be extended to generic implementations of the QCA paradigm.

A new measurement method of MOS transistor parameters

Abstract A new method for the extraction of some of the main physical parameters characterizing an MOS process is presented. This method requires current measurements for small drain voltages to be performed at least on two transistors which differ only for the channel length. In particular it makes it possible to determine the threshold voltage, the channel shortening, the drain and source parasitic resistances and the carrier mobility in the channel. The method is primarily intended for usage with a computerized measuring system.

Quantum analysis of shot noise suppression in a series of tunnel barriers

We report the results of an analysis, based on a straightforward quantum-mechanical model, of shot noise suppression in a structure containing cascaded tunneling barriers. Our results exhibit a behavior that is in sharp contrast with existing semiclassical models for this particular type of structure, which predict a limit of 1/3 for the Fano factor as the number of barriers is increased. The origin of this discrepancy is investigated and attributed to the presence of localization on the length scale of the mean free path, as a consequence of the strictly one-dimensional 1D nature of disorder, which does not create mode mixing, while no localization appears in common semiclassical models. We expect localization to be indeed present in practical situations with prevalent 1D disorder, and the existing experimental evidence appears to be consistent with such a prediction. In the study of low-dimensional devices, suppression of shot noise with respect to the value predicted for the case of a Poissonian noise process by Schottky’s theorem has represented one of the most active fields of investigation in the last two decades. Such a suppression phenomenon, described by means of the Fano factor, i.e., the ratio of the actual shot noise power spectral density to the full value 2qIwhere q is the value of the elementary charge and I is the average value of the current flowing through the device, has been predicted and observed in many different mesoscopic structures and is the result of the presence of correlations between charge carriers, which reduce current fluctuations in the device. From the theoretical point of view, shot noise suppression has been investigated both with quantum-mechanical and with semiclassical approaches. Such activities have led to the discovery of “universal” values for the Fano factor in specific structures; in particular, for disordered conductors a universal suppression factor of 1/3 has been found both with random matrix theory 1,2 and with a semiclassical approach. 3 This result has received further confirmation from numerical

Improved Feedback Theory

A previous cut-insertion theorem for linear circuits and its application to a generalization of the elementary feedback theory are further extended. The new theorem is based on the splitting of an arbitrary node into two nodes and on the insertion between them and another arbitrary ldquoreferencerdquo node, of a three-terminal circuit which keeps the network currents and voltages unchanged. The system analysis is in turn performed on the equivalent cut network by means of the techniques for two-port circuits, using generic variables, i.e., either voltages or currents. The new approach allows one to define a two-port circuit as an open loop around the split and reference nodes, as well to define a new feedback model specifically focused on it. Furthermore, it allows one to retrieve all the results of the previous general feedback theories, as well as several new ones. The obtained expressions of the overall transfer function include, as a particular case, the Blackman formula for the immittances, and relationships without the leakage term, which are useful for circuit synthesis and for dealing with spurious signals. New expressions are proposed for the evaluation of the output and input immittances. The virtual short-circuit method is derived and numerical and design examples are provided. The new approach retains the intuitive feedback vision of the elementary models.

Analysis of shot noise suppression in mesoscopic cavities in a magnetic field

We present a numerical investigation of shot noise suppression in mesoscopic cavities and an intuitive semiclassical explanation of the behavior observed in the presence of an orthogonal magnetic field. In particular, we conclude that the decrease of shot noise for increasing magnetic field is the result of the interplay between the diameter of classical cyclotron orbits and the width of the apertures defining the cavity. Good agreement with published experimental results is obtained, without the need of introducing fitting parameters.

A low cost high resolution pattern generator for electron-beam lithography

A simple, very low cost pattern generator for electron-beam lithography is presented. When it is applied to a scanning electron microscope, the system allows a high precision positioning of the beam for lithography of very small structures. Patterns are generated by a suitable software implemented on a personal computer, by using very simple functions, allowing an easy development of new writing strategies for a great adaptability to different user necessities. Hardware solutions, as optocouplers and battery supply, have been implemented for reduction of noise and disturbs on the voltages controlling the positioning of the beam.