M. Boero

Fabrication of Coulomb blockade devices by combination of high resolution electron beam lithography and deposition of granular films

Abstract A reproducible fabrication method for Coulomb blockade devices which combines Electron Beam Lithography and granular gold film deposition is presented. The deposition parameters are optimised to produce gold grains of n nm diameter with distances between grains around 1 nm. Nanofabrication techniques developed to fully control the size of the dot array deposited between two contact electrodes are presented. In good agreement with previous numerical predictions, we find that the Coulomb gap of a two-dimensional disordered array increases with the linear array size.

Simulation and growth of gold on silicon oxide in one-dimensional and quasi-one-dimensional arrays

The granular film deposition of gold atoms by means of thermal evaporation on silicon oxide surfaces is known to produce nanometer-scale islands which show electrical characteristics dominated by Coulomb blockade. Here we report on the growth and the simulation of structures produced by a combination of evaporation and lithography. A lithographic technique is used to define a region of a width of several nm that is surrounded by polymethylmethacrylate (PMMA). The PMMA confines the diffusion of the atoms, and the growth process is characterized by different boundary conditions compared with the case where atoms are deposited on a macroscopic surface. This method enables us to create quasi-one-dimensional (quasi-1D) and 1D structures in which the gold islands are arranged in a single row where the lateral size is only a few nanometers. Such structures offer the possibility of studying Coulomb blockade in 1D arrays and a signature of self-organized growth is observed in such structures.

Origin of yield problems of single electron devices based on evaporated granular films

An evaporated nanometer scale granular film provides a simple system for studying Coulomb blockade effects. This technique has often been used during the last few decades. However with respect to potential devices, specific problems continue to obstruct broader application. It is virtually impossible to observe Coulomb blockade in one–dimensional structures, and even for wide two–dimensional systems the yield is frustratingly low. We study these problems using a comprehensive theoretical framework that enables us to model both the growth aspects, and the electrical characteristics. In particular, we study how the morphology of the islands influences their electrical properties. Explanations for the observed behavior are put forward.

Monte Carlo simulation of the growth of metallic quantum dots

Abstract The phenomenon of Coulomb Blockade allows to control the flow of individual electrons and has raised hopes of building electronics devices based on such phenomenon. In order to observe Coulomb Blockade, islands, or dots, on the nanometre scale must be fabricated in order to obtain a small enough capacitance for the electrostatic energy associated with adding a single electron to the island to exceed thermal fluctuations. Coulomb Blockade has been observed both in semiconductor and in metallic structures. In particular irregular 2D arrays of metallic dots of typical size less than 10nm have been fabricated by means of Electron Beam Lithography combined with granular film deposition. In this paper we present a theoretical study of the grain deposition and subsequent island formation based on a Monte Carlo technique. The model allows to account for a variety of important parameters such as temperature, diffusion vs. deposition rate, and in particular the effect of contacts. Optimum parameters to obtain reproducible Coulomb Blockade devices are suggested.

A detailed theory of excitons in quantum dots

Quantum-dot systems are confined semiconductor structures which exhibit a fully discrete spectrum due to the size confinement in all directions. The position of the energy levels inside such structures can be changed by adjusting their geometrical dimensions. Such structures are particularly interesting for optical applications for two reasons: (i) both the electrons and holes are confined in the same small physical region, and therefore the strength of recombination processes is increased, and (ii) by changing the position of the energy levels, one can in principle tune quantum-dot lasers over a wide range of wavelengths. The presence of size confinement gives rise to two competing effects: on one hand it causes an upward shift of the energy levels, and on the other it enhances the Coulomb attraction between electrons and holes. These effects tend to shift the position of the exciton energies in opposite directions, so that a careful modelling of such structures is required in order to understand which is the dominant effect and how the excitons behave as a function of confinement. While there have been several studies on ideal systems, we attempt to model a system more closely aligned to experiment. In this study we investigate: (i) the effect of the shape of the lateral potential of a quantum disk, i.e. parabolic and hard-wall; (ii) the effect of wave-function leakage in the barries; and (iii) the effect of the light-heavy hole mixing on the effective masses.

Theoretical and Experimental Studies of Magneto-tunneling in Quantum Dots

We study the magneto-transport in vertical quantum dots boththeoretically and experimentally. We focus our attention onthe non-linear transport regime. We demonstrate that the peakamplitudes in the I–V characteristics show a dramaticallydifferent behaviour as a function of the magnetic field,depending on the value of the angular momentum of the dotstate through which tunnelling occurs. The investigationallows us to probe details of the quantum dot wavefunctionsand to distinguish tunnelling through localised states relatedto impurities or to quantum dots.

A study of ion-implanted semiconductor nanostructures

We report on the experimental evidence for quantum confinement in the transport properties of quantum dots fabricated by a novel technique. This technique allows us to confine the electron motion only in the quantum dot, leading to 3D-0D-3D resonant tunnelling transport. A theoretical framework has been developed at the same time to investigate 3D-0D-3D transport devices. The theoretical results agree well with the experimental observations, opening the possibility of directly probing the wavefunctions of 0D structures.

Surface acoustic wave scattering in quantum wire arrays: theory and experiment

Abstract We have investigated anisotropic surface acoustic wave (SAW) scattering from quantum wire arrays as a function of electron concentration and magnetic field, and we have Fermis golden rule to model the SAW scattering. With the SAW perpendicular to the wires we observe oscillations in the transmitted intensity, reflecting the magnetic depopulation of ID subbands, and this agrees well with the theory. With the SAW parallel to the wires the theory predicts negligible attenuation, and this is confirmed experimentally at low electron concentrations. Surprisingly, at high electron concentrations the SAW attenuation increases sharply in this orientation to the point where it is three times larger than in the unstructured 2DEG. We speculate that this is due to the excitation of intraband 2D plasmon-like modes.

Calculated I-V characteristics of low-dimensional structures

Abstract In this work we calculate the I–V characteristics of quantum dot devices. The I–V curves of such structures are dependent not only upon the dot spectrum, but also on whether they are connected to bulk contacts or to contacts confined in one or more direction. Here we concentrate on 1D-0D-1D and 3D-0D-3D devices, using two different techniques: a transfer-matrix approach and a non-equilibrium Greens function formalism. The Coulomb interaction and single particle aspects are included in the theory. For 1D-0D-1D structures the I–V curve consists of a series of upward and downward steps, the latter being present only if the Coulomb repulsion inside the dot is important. These downward steps prove to be thermally robust. For 3D-0D-3D devices the I–V curve is dominated by peaks rather than steps, reflecting the difference in density of states in the contacts. Comparison with experiments shows very good agreement.