Gain theory and models in silicon nanostructures

The main goal in the information technology is to have the possibility of integrating low-dimensional structures showing appropriate optoelectronic properties with the well established and highly advanced silicon microelectronics present technology. Therefore, after the initial impulse given by the work of Canham on visible luminescence from porous Si, nanostructured Si has received extensive attention both from experimental and theoretical point of view during the last ten years. This activity is mainly centered on the possibility of getting relevant optoelectronic properties from nanocrystalline Si, which in the bulk crystalline form is an indirect band gap semiconductor, with very inefficient light emission in the infrared. Although some controversial interpretations of the visible light emission from low-dimensional Si structures still exist, it is generally accepted that the quantum confinement, caused by the restricted size, and the surface passivation are essential for this phenomenon.Here we will review our activity in the field of the theoretical determination of the structural, electronic and optical properties of Si nanocrystals (Si-nc). The present work aims at answer a very important question related to the origin of the enhanced photoluminescence in Si-nc embedded in SiO2. In fact, optical gain has been recently observed in ion implanted Si-nc and in Si-nc formed by plasma enhanced chemical vapour deposition and annealing treatments. We propose, here, an analysis of the experimental findings based on an effective rate equation model for a four level system; moreover looking at our theoretical results for the optical properties of Si-nc we search for structural model that can be linked to the four level scheme. As final outcome, due to the results for the optoelectronic properties of Si-nc in different interface bond configurations, we demonstrate that in order to account for the striking photoluminescence properties of Si-nc it is necessary to take carefully into account not only the role of quantum confinement, but also the role of the interface region surrounding the Si-nc.Silicon, the leading material in microelectronics during the last four decades, also promises to be the key material in the future. Despite many claims that silicon technology has reached fundamental limits, the performance of silicon microelectronics continues to improve steadily. The same holds for almost all the applications for which Si was considered to be unsuitable. The main exception to this positive trend is the silicon laser, which has not been demonstrated to date. The main reason for this comes from a fundamental limitation related to the indirect nature of the Si band-gap. In the recent past, many different approaches have been taken to achieve this goal: dislocated silicon, extremely pure silicon, silicon nanocrystals, porous silicon, Er doped Si-Ge, SiGe alloys and multiquantum wells, SiGe quantum dots, SiGe quantum cascade structures, shallow impurity centers in silicon and Er doped silicon. In this contest the present work aims at answer a very important question related to the origin of the enhanced photoluminescence in Si-nc embedded in SiO2. In fact, optical gain has been recently observed in ion implanted Si-nc and in Si-nc formed by plasma enhanced chemical vapour deposition and annealing treatments. We propose, here, an analysis of the experimental findings based on an effective rate equation model for a four level system; moreover looking at our theoretical results for the optical properties of Si-nc we search for structural model that can be linked to the four level scheme. As final outcome, due to the results for the optoelectronic properties of Si-nc in different interface bond configurations, we demonstrate that in order to account for the striking photoluminescence properties of Si-nc it is necessary to take carefully into account not only the role of quantum confinement, but also the role of the interface region surrounding the Si-nc