Excitons in silicon nanocrystallites: The nature of luminescence

Luppi, E.; Iori, Federico; Magri, Rita; Pulci, Olivia; Ossicini, Stefano; Degoli, Elena; Olevano, Valério

doi:10.1103/physrevb.75.033303

Cited by 73 publications

(74 citation statements)

References 26 publications

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“…The difference between the GW electronic gap and the GW þ BSE optical excitonic gap gives the exciton binding energy E b . We note the presence of exciton binding energies as big as 2.2 eV, which are very large if compared with bulk Si ($15 meV) or with carbon nanotubes [73,74], where E b $ 1 eV, but similar to those calculated for undoped Si NCs [75] of similar size and for Si and Ge small nanowires [76,77].…”

Section: Codoped Silicon Nanocrystalsmentioning

confidence: 93%

Electronic and Optical Properties of Silicon Nanocrystals

Bulutay

Ossicini

2010

Silicon Nanocrystals

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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.

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Section: Codoped Silicon Nanocrystalsmentioning

confidence: 93%

Electronic and Optical Properties of Silicon Nanocrystals

Bulutay

Ossicini

2010

Silicon Nanocrystals

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“…52 However, while the total correction to E g is noticeable in bulk materials, in strongly confined systems the enhanced excitonic interaction is known to compensate the self-energy of about the same amount. 23,52,57,58 As a consequence, in the case of small embedded QD, one deals with "correct" E g values (determined by QD states), but "uncorrect" band offsets due to the systematic error in the SiO 2 -related energy values.…”

Section: Structures and Methodsmentioning

confidence: 99%

Energetics and carrier transport in doped Si/SiO₂quantum dots

et al. 2015

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In the present theoretical work we have considered impurities, either boron or phosphorous, located at different substitutional sites in silicon quantum dots (Si-QDs) with diameters around 1.5 nm, embedded in a SiO2 matrix. Formation energy calculations reveal that the most energetically-favored doping sites are inside the QD and at the Si/SiO2 interface for P and B impurities, respectively. Furthermore, electron and hole transport calculations show in all the cases a strong reduction of the minimum voltage threshold, and a corresponding increase of the total current in the low-voltage regime. At higher voltage, our findings indicate a significant increase of transport only for P-doped Si-QDs, while the electrical response of B-doped ones does not stray from the undoped case. These findings are of support for the employment of doped Si-QDs in a wide range of applications, such as Si-based photonics or photovoltaic solar cells.[ Electronic Supplementary Information (ESI) available. See

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“…Ab-initio models are limited to very small nanocrystal sizes. 6,[24][25][26][27] Semi-empirical approaches like tight-binding 6,14,28 and pseudopotential 29,30 models are a good option in the case of the free-standing or hydrogen-passivated Si NCs but they still lack an appropriate description of the Si/SiO 2 boundary leading to a strong overestimation of the quantization energies in the case of the Si NCs embedded in SiO 2 . In this respect, it seems natural to employ a model based on the multiband effective mass approximation supplemented by the appropriate boundary conditions, along the lines of previous studies.…”

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Multiphonon relaxation of moderately excited carriers in Si/SiO2nanocrystals

et al. 2012

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We study the phonon-assisted intraband relaxation of electrons and holes confined in Si nanocrystals. The rates of relaxation processes are calculated as functions of the nanocrystal size and of the temperature. It occurs that the main contribution to the relaxation is provided by the mechanism where a single acoustic and a number of optical phonons, which is necessary to compensate the energy difference between the quantized charge carrier levels, are involved. We show that the phonon-assisted transitions between neighboring, size-quantized levels occur typically on a picosecond timescale, but vary over several orders of magnitude with the nanocrystal size. This results in a multi-exponential decay of the carrier populations averaged over an ensemble of the nanocrystals with a given size distribution. When the nanocrystal size is reduced and more than two phonons are required for the transition, there is a qualitative difference in the behavior of the transition probabilities between the electrons and the holes. Whereas the electron transition times strongly oscillate around approximately the same mean values in the picosecond range with some drops towards nanoseconds, there is a clearly pronounced tendency of the relaxation time increase into the nanosecond time range for the hole transitions when the nanocrystal size is decreased. The increase of the temperature leads to a moderate decrease of the relaxation times but does not change the picture qualitatively.

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Excitons in silicon nanocrystallites: The nature of luminescence

Cited by 73 publications

References 26 publications

Electronic and Optical Properties of Silicon Nanocrystals

Electronic and Optical Properties of Silicon Nanocrystals

Energetics and carrier transport in doped Si/SiO₂quantum dots

Multiphonon relaxation of moderately excited carriers in Si/SiO2nanocrystals

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Excitons in silicon nanocrystallites: The nature of luminescence

Cited by 73 publications

References 26 publications

Electronic and Optical Properties of Silicon Nanocrystals

Electronic and Optical Properties of Silicon Nanocrystals

Energetics and carrier transport in doped Si/SiO2quantum dots

Multiphonon relaxation of moderately excited carriers in Si/SiO2nanocrystals

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Energetics and carrier transport in doped Si/SiO₂quantum dots