Introduction

Silicon, the second most abundant element (after oxygen) in the earth's crust, making up 25.7% of the crust by mass, is one of most (probably the absolute) striking material for electronic and technological applications. It has thus become the leading and most prominent building-block material for electronics. Silicon has an indirect band gap of 1.12 eV that is ideal for room temperature operation. It allows the processing flexibility to place today more than 109 devices on a single chip. However, all the single transistors and electronic devices have transferred information to length scale which is relevant with respect to their nanometric scale. Pushing down the dimension of silicon-based devices toward the nanometric scale determines a high concentration of transistors on a single silicon crystal (around 200 mm for the moment) that have permitted high integration level high-speed device performances and unprecedented interconnection levels.

Amorphous semiconductors represent an important area in materials science, which is interesting both from the technological and the theoretical point of view. The simplest way to describe the structure of an amorphous material like a disordered solid is short-range coordination. Extended atomic network of amorphous silicon (a-Si) is random[1]. In other words, the local coordination is tetrahedral (sp3 hybridization) as in crystalline Si (c-Si). The first nearest-neighbor distance, coordination number, or binding energy remains more or less same as in the amorphous and crystalline phases. Moreover, the covalent nature of the bonds means that short-range interactions play a prominent role. Density of electronic or vibrational states is similar in amorphous Si (a-Si) and c-Si[2, 3]. Semiconducting properties are also preserved in the a-Si. As a matter of fact, the presence of dangling bonds in the a-Si affects the performance of electronic devices. In practice, this can be circumvented by hydrogenation of the amorphous material, since hydrogen is a good terminator for insaturated bonds. From the theoretical point of view[2], the main features of the density of states can be accounted for by a molecular description (tight binding or valence force field models). Effect of the topological disorder mainly broadens linewidth of the Raman modes.

Semiconductor nanostructures have also been investigated in recent years due to their immense applications in electronic and optoelectronic devices[4, 5]. Electronic and optical properties of semiconductor nanostructures are strongly affected by the quantum confinement effect due to the reduced dimensions of these systems.