Optical absorption modulation by selective codoping of SiGe core-shell nanowires

Progress in Surface Science

Degoli

Ossicini

2017

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Si nanocrystals have been extensively studied because of their novel properties and their potential applications in electronic, optoelectronic, photovoltaic, thermoelectric and biological devices. These new properties are achieved through the combination of the quantum confinement of carriers and the strong influence of surface chemistry. As in the case of bulk Si the tuning of the electronic, optical and transport properties is related to the possibility of doping, in a controlled way, the nanocrystals. This is a big challenge since several studies have revealed that doping in Si nanocrystals differs from the one of the bulk. Theory and experiments have underlined that doping and codoping are influenced by a large number of parameters such as size, shape, passivation and chemical environment of the silicon nanocrystals. However, the connection between these parameters and dopant localization as well as the occurrence of self-purification effects are still not clear. In this review we summarize the latest progress in this fascinating research field considering free-standing and matrix-embedded Si nanocrystals both from the theoretical and experimental point of view, with special attention given to the results obtained by ab-initio calculations and to size-, surface- and interface-induced effects

Section: Codopingsupporting

confidence: 83%

“…6.1. Ab-initio calculations: codoped free-standing and matrix-embedded nanocrystals In order to interpret experimental results, we will report the outcomes of ab-initio simulations for B and P simultaneously codoped Si-NCs [65,82,92,145,146,147,166,177,232,233,234,235,236,237].…”

Section: Codopingmentioning

confidence: 99%

Doped and codoped silicon nanocrystals: The role of surfaces and interfaces

Progress in Surface Science

Degoli

Ossicini

2017

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“…The analysis shows that the simultaneous addition of B and P impurities in Ge core /Si shell NWs can result in a gain of energy with respect to the single‐doping cases, as already demonstrated in the case of co‐doped Si NCs . The DFT calculations performed to investigate the electronic structure of such co‐doped systems revealed that the simultaneous presence of B and P impurities can induce a modification in both the band structure and electronic transitions if compared to pristine NWs . In particular, for co‐doped Si and Ge NWs, we found that: i) contrary to the behavior observed for co‐doped NCs, the impurity states only rather modify the band gap with respect to the undoped wire and ii) this reduction is influenced by the relative positions of dopants (if both in the core, both in edge or in the configuration core‐edge) as well as the distance between impurities.…”

Section: Si/ge Core–shell Nws: Dopingsupporting

confidence: 61%

“…This effect has negative consequences on the efficiency of the doping process because dopants in the surface interact with surface states becoming recombination centers for electrons and holes (thus reducing the carrier concentration). As for Si/Ge core–shell NWs, results of DFT formation energy are reported in Figure . The analysis shows that the simultaneous addition of B and P impurities in Ge core /Si shell NWs can result in a gain of energy with respect to the single‐doping cases, as already demonstrated in the case of co‐doped Si NCs .…”

Section: Si/ge Core–shell Nws: Dopingmentioning

confidence: 99%

First Principles Modeling of Si/Ge Nanostructures for Photovoltaic and Optoelectronic Applications

Physica Status Solidi (b)

Amato

Guerra

et al. 2018

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We discuss results of ab initio calculations for Si, Ge, and Si/Ge nanowires and nanocrystals showing that theory can improve the comprehension of the properties of these systems. First, we consider doped and undoped freestanding hydrogenated nanowires and we explore their properties as a function of the size, geometry, and composition. Secondly, we focus the discussion on the electronic properties of matrix embedded Si, Ge, and Si/Ge nanocrystals by pointing out the role played by composition, quantum confinement, and strain. The discussed results show that, for Si/Ge nanowires, the interface between Si and Ge region plays an important role determining, in some case, the formation of a type II band offset, which is essential for photovoltaic applications. Moreover, for Si/Ge core-shell nanowires, it is shown that: i) selective doping results in the formation of hole or electron accumulation, with interesting consequences for the use of these materials in thermoelectrics and ii) through compensated doping, it is possible to tune the optical properties of these systems. For the embedded nanocrystals, the outcomes suggest that Ge nanocrystals can be suitable as optical absorption centers and that Si/Ge nanocrystals, owing to the localization of the band edge states, are interesting for photovoltaic cells.Dr. I. Marri, Prof. S. Ossicini Centro S3 CNR-Istituto Nanoscienze via Campi 213/a,

“…The co‐doped Si‐NCs exhibit PL energies red‐shifted with respect to those of the corresponding undoped Si‐NCs, moreover these energies can range from the visible to below the bandgap energy of the bulk Si . Thus, we will report here the outcomes of a systematic study of the structural, electronic and optical properties of B and P simultaneously doped Si‐NCs using ab initio DFT calculations . Starting from the hydrogenated Si‐NCs we have co‐doped the Si‐NCs by locating the B and P impurities in all the possible substitutional positions allowing full relaxation with respect to all the atomic positions.…”

Section: Co‐dopingmentioning

confidence: 99%

First Principle Studies of B and P Doped Si Nanocrystals

Physica Status Solidi (a)

Degoli

Ossicini

2017

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The properties of n- and p-doped silicon nanocrystals obtained through ab initio calculations are reviewed here. The aim is the understanding of the effects induced by substitutional doping on the structural, electronic and optical properties of free-standing and matrix-embedded Si nanocrystals. The preferential positioning of the dopants and their effects on the structural properties with respect to the undoped case, as a function of the nanocrystals diameter and termination, are identified through total-energy considerations. The localization of the acceptor and donor related levels in the band gap of the Si nanocrystals, together with the impurity activation energy, are discussed as a function of the nanocrystals size. The dopant induced differences in the optical properties with respect to the undoped case are presented. Finally, the case of B and P co-doped nanocrystals is discussed showing that if carriers are perfectly compensated, the Si nanocrystals undergo a minor structural distortion around the impurities inducing a significant decrease of the impurities formation energies with respect to the single doped case. Due to co-doping, additional peaks are introduced in the absorption spectra, giving rise to a size-dependent red shift of the absorption spectra