Introduction
In the last years, particular interest has been raised by the so called
“chaotic cavities”, mesoscopic regions delimited by input and output con- strictions that are much smaller than the cavities themselves, so that, from a semiclassical point of view, an electron entering them from one of the con- strictions will spend a long time bouncing inside, till it will leave the cavity from the same or the other constriction. The purpose of this thesis is to fabricate and characterize a device based on chaotic cavities, that it is more convenient to call mesoscopic cavities to underline the fact that their chaotic- ity is not the result of a classically chaotic shape. In particular, I am going to focus on the issue of shot noise suppression phenomena that characterize these mesoscopic cavities.
All these phenomena of shot noise suppression are the result of cor- relations between charge carriers that decrease the variance of the random process corresponding to the elementary charges crossing the device; such correlations may result either from Fermi exclusion or from Coulomb inter- action.
For this reason, I have fabricated a device with five quantum points contact in series, this means four mesoscopic cavities in series that are
8 µm wide and 5 µm long, defined by constrictions with a lithographic width W =500 nm. This device has been fabricated on a Gallium Arsenide- Aluminium Gallium Arsenide heterostructure with a two- dimensional gas, which guarantees high mobility and high carrier concentration.
The concept of this device draws inspiration from the article of Ober- holzer et al.[1], in which they describe the behavior of shot noise suppression phenomena in two mesoscopic cavities in series that are 8 µm wide and 5 µm long.
In particular I am interested in understanding the discrepancies between some articles present in the literature of mesoscopic cavities.
For example, recently, the noise behavior of chaotic cavities in an or- thogonal magnetic field has been measured[1], observing a linear reduction of the Fano factor as the magnetic field is increased with the reduction of the portion of the cavity area explored by the electrons.
An alternative interpretation[2] has been provided, for the behavior in the presence of a magnetic field, based on the ratio of the classical cyclotron diameter to the constriction width, according to which the shape of the cavity is not relevant to the shot noise suppression.
The device of this thesis and the electrical characterization were made in the Cavendish Laboratory (Physics Department of the University of Cam-
bridge) under the supervision of Dr. C. G. Smith. and the subsequent analysis of the data has been performed in the Department of “Ingegneria dell’Informazione” in Pisa under the supervision of Prof. M. Macucci.
The device has been fabricated using both photolithography and elec- tron beam processes. The electrical characterization has been performed at 4.2 K and 1.3 K both in the dark and under illumination. From these electri- cal tests it was expected to see conductance quantization and, furthermore, whether there is interaction between neighboring couples of split gates. In order to investigate the basic of shot noise suppression phenomena in meso- scopic cavities, a starting point consist in obtaining two consecutive split gates biased at the same time on the first conductance plateau. With the availability of multiple split gates, it will be possible to define cavities of dif- ferent length and also to obtain up to four cascaded cavities. This will allow experimental verification of the theoretical predictions on the dependence of shot noise suppression as a function of cavity length made by Oberholzer et al.[1] or by Macucci et al.[2]. Furthermore, it will be possible to test the- ories on the shot noise behavior of cascaded cavities (Macucci et al.[3] and Oberholzer et al.[4]).
Thesis organization
This thesis consists of three Chapters. First of all, there is an explanation of what a mesoscopic cavity is and of shot noise suppression phenomena. We discuss why this suppression appears and how it can be detected.
In the second Chapter there is a description of the optical and elec- tron beam processes used for lithography and of the associated fabrication procedures.
In the third Chapter all electrical characterizations necessary to prove the correct operation of the device are presented. They have been performed at 4.2 K and 1.3 K both in the dark and under illumination. We also include the characterization of the device under a perpendicular magnetic field at 1.3 K in the dark.
BIBLIOGRAPHY
[1] S. Oberholzer, E. V. Sukhorukov, C. Strunk and C. Sch¨onenberger, Shot Noise by Quantum Scattering in Chaotic Cavities, Phys. Rev. Lett. 86 (2001) 10.
[2] P. Marconcini, M. Macucci, G. Iannaccone, B. Pellegrini and G.Marola, Analysis of Shot Noise Suppression in Mesoscopic Cavities in a Magnetic Field, cond-mat/ 0411691.
[3] M. Macucci, G. Iannaccone and P. Marconcini, Shot noise behavior of cascaded mesoscopic structures.
[4] S. Oberholzer, E. V. Sukhorukov and C. Sch¨onenberger, Crossover be- tween classical and quantum shot noise in chaotic cavities.
