Electronic and Optical Properties of Silicon Nanocrystals

Abstract

Several strategies have been researched over the last few years for light generation and amplification in silicon. One of the most promising one is based on silicon nanocrystals (Si NCs) with the aim of taking advantage of the reduced dimensionality of the nanocrystalline phase (1-5 nm in size) where quantum confinement, band folding, and surface effects play a crucial role [1,2]. Indeed, it has been found that the Si NC bandgap increases with decrease in size and a photoluminescence (PL) external efficiency in excess of 23% is obtained [3]. Si NC-based LED with high efficiency have been obtained by using Si NC active layers [4] and achieving separate injection of electrons and holes [5]. Moreover, optical gain under optical pumping has been already demonstrated in a large variety of experimental conditions [6][7][8][9][10][11].After the initial impulse given by the pioneering work of Canham on photoluminescence from porous Si [12], nanostructured silicon has received extensive attention. This activity is mainly centered on the possibility of getting relevant optoelectronic properties from nanocrystalline Si. The huge efforts made toward matter manipulation at the nanometer scale have been motivated by the fact that desirable properties can be generated by just changing the system dimension and shape. Investigation of phenomena such as the Stokes shift (difference between absorption and emission energies), the PL emission energy versus nanocrystals size, the doping properties, the radiative lifetimes, the nonlinear optical properties, the quantum-confined Stark effect (QCSE), and so on can give a fundamental contribution to the understanding of how the optical response of such systems can be tuned. A considerable amount of work has been done on excited Si NCs [2, 13-23], but a clear understanding of some aspects is still lacking. The question of surface effects, in particular oxidation, has been addressed in the last few years. Both theoretical calculations and experimental observations have been applied to investigate the possible active role of the interface on the optoelectronic properties of Si NCs. Different models have been proposed: Baierle et al. [24] have considered the role of the surface geometry distortion of small hydrogenated Si clusters in the excited state.

Insight into the effects of surface oxidation and carbonization on the electronic properties of silicon quantum dots and silicon slabs: a density functional study

“…1 reduced. Surface oxidation reduced energy band gap, which was in agreement with the existing calculation results [24][25][26][27] and experiment evident. 28 However, there were two inuences of surface C-substitution on energy gap.…”

Photoluminescence Enhancement of Adsorbed Species on Si Nanoparticles

“…Si nanoparticles possess significant properties such as wide-bandgap energies due to the quantum confinement effect [ 1 , 2 ], large surface area [ 3 ], and high reactivity [ 4 ]. Moreover, Si is a safe material, possible to apply to the bio-medical field [ 5 ].…”

“…PL from Si nanoparticles is differently attributed to several causes. In cases where the PL energy strongly depends on the size of Si nanoparticles, it arises most likely from band-to-band transition of Si nanoparticles because the bandgap energy is determined by its size due to the quantum confinement effect [ 1 , 2 ]. On the other hand, Zhu et al [ 19 ] and Inada et al [ 20 ] have attributed PL from Si nanoparticles to energy states of SiO 2 (most probably interface states) formed on Si nanoparticles, and in this case, the PL energy does not depend on the size of Si nanoparticles.…”

Tight-binding calculations of SiGe alloy nanocrystals in SiO2 matrix

“…In solid line we show the results obtained with current parametrsization, in dashed line the results obtained in the same method, but the parameters of SiO 2 matrix taken from from [15]. We also show the levels energy calculated using the pseudopotential method [24] (dotted line) and effective mass approach [23] (dash-dot line). As seen from [15].…”