Electronic, structural and optical properties of hydrogenated silicon nanocrystals: the role of the excited states

Cantele, Giovanni; Degoli, Elena; Luppi, E.; Magri, Rita; Ninno, D.; Bisi, O.; Ossicini, Stefano; Iadonisi, G.

doi:10.1002/pssc.200461139

Cited by 21 publications

(30 citation statements)

References 8 publications

(13 reference statements)

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“…We first consider only dopants substituting for the atom at the center of the nanocrystals. Donor-doped nanocrystals with hydride-terminated surface have electron binding energies between 2.2 and 2.5 eV (Table ), in agreement with previous results. ,, Note that this is about half the I UD (0/+) – I UD (−/0) gap, and 2 orders of magnitude higher than the activation energies for the same group V dopants in bulk silicon. Similar to bulk silicon, As has a slightly higher E b than P and Sb.…”

Section: Ionization Energy and Electron Affinitysupporting

confidence: 90%

“…The fraction of dopant atoms at the surface of nanocrystals has been related to the energetics of the substitutional dopant, in particular the dependence of the formation energy on its distance to the surface. ,− Additionally, it was found that the formation energy of substitutional P and B is higher the smaller the nanocrystal diameter. , In the basis of these findings, a self-purification mechanism whereby impurities are expelled was proposed . Another possibility is that the distribution of dopant atoms is controlled by the kinetics of adsorption of impurities on the nanocrystal surface during growth …”

Section: Introductionmentioning

confidence: 99%

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Effect of Oxidation on the Doping of Silicon Nanocrystals with Group III and Group V Elements

2012

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Substitutional group III and group V elements, though commonly used as shallow dopants in bulk silicon, have a limited efficiency in silicon nanocrystals. In this work, we use firstprinciples models of 1.5 nm nanocrystals with hydride-and silanolterminated surfaces to understand how oxidation influences the segregation and deactivation of dopants at the surface and the dopant binding energies. We show that the surface oxygen layer changes drastically the radial dependence of the dopant formation energy both for donors and for acceptors, but that, independently from the oxidation, dopant diffusion does not take place at operating conditions. Additionally, we show that the oxidation increases the electron binding energy of the P, As, and Sb and decreases the hole binding energy of B, Al, Ga, and In. ■ INTRODUCTIONThe ability to control the type and concentration of charge carriers in silicon by doping with extrinsic impurities has been central to its success in electronics. It has recently been demonstrated that silicon nanocrystals (NCs) can also be doped n-type or p-type with phosphorus and boron, respectively. 1−3 Growth of doped nanocrystals by nonthermal plasma synthesis can be achieved by addition of phosphine and diborane to the precursor gas mixture, and incorporation efficiencies close to 100% have been reported.Still, the doping of nanocrystals is hindered by some difficulties not present in their bulk counterparts. One of them is the higher dopant activation energy. If the nanocrystal radius is smaller than the Bohr radius of the electron or hole in bulk silicon, the excess electron or hole is considered to be in the strong quantum confinement regime. 4 Due to the quantum confinement, the binding energy of the electron or hole to the ionized dopant can be more than 1 order of magnitude larger than in bulk. 5,6 The smaller the nanocrystal the smaller the effective radius a e characterizing the exponential decay of the hydrogen-like wave function of the bound electron or hole s state, ψ(r) ∼ exp(−r/a e ), where r is the distance between the two. 5 In medium-sized nanocrystals (with ψ(r) ∼ 0 at the surface), even though the quantum confinement is weak, the screening by image charges at the surface of the nanocrystal or at the interface between the nanocrystal and the outer oxide shell may also contribute to increase the dopant activation energy. 7,8 Another problem is to guaranty that dopant incorporation occurs at substitutional positions in the nanocrystal core, where the dopant becomes active. Depending on the nanocrystal size and specific growth method, 80−95% of the P atoms can stay on the nanocrystal surface. 2,3 If the nanocrystals are covered by an oxide layer, as much as 95% can be located there, presumably because P atoms segregate to the surface during growth, becoming incorporated in the oxide during the subsequent surface oxidation process. 1 Once at the surface, P (and B) may assume three-fold coordination and become electrically inactive. 9,10 In this way, surface dangling bonds can be passiv...

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Section: Ionization Energy and Electron Affinitysupporting

confidence: 90%

Section: Introductionmentioning

confidence: 99%

Effect of Oxidation on the Doping of Silicon Nanocrystals with Group III and Group V Elements

2012

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“…Any progress therefore has to rely upon bold simplifications, especially with regard to the size and geometry of the model. Hence, a common approach is to limit the problem to a particular region of interest, either by (i) using finite boundary conditions, , or by (ii) adopting a periodic slab-, wire-, or particle-in-a-box approach, − hoping that within such a limited region the model is still able to adequately capture the electronic local density of states. In such supercell calculations, it is also common to impose a sufficiently large vacuum space to separate the structure replicas, and by this way to mitigate unwanted periodicity effects .…”

Section: Methodsmentioning

confidence: 99%

Resonant Electronic Coupling Enabled by Small Molecules in Nanocrystal Solids

et al. 2014

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The future exploitation of the exceptional properties of nanocrystal (NC) thin films deposited from liquid dispersions of nanoparticles relies upon our ability to produce films with improved electrical properties by simple and inexpensive means. Here, we demonstrate that the electronic conduction of solution-processed NC films can be strongly enhanced without the need of postdeposition treatments, via specific molecules adsorbed at the surfaces of adjacent NCs. This effect is demonstrated for Si NC films doped with the strong molecular oxidizing agent tetrafluoro-tetracyanoquinodimethane (F4-TCNQ). Density functional calculations were carried out with molecule-doped superlattice solid models. It is shown that, when populated by electrons, hybrid molecule/NC states edge (and may actually resonate with) the conduction-band states of the NC solid. This provides extra electronic connectivity across the NC network as the molecules effectively flatten the electronic potential barriers for electron transfer across the otherwise vacuum-filled network interstitialcies.

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“…First-principles modeling studies of doped Si NCs have shown that dopant atoms are energetically preferenced toward the outer surface region; 40,41 however, experimental evidence has been mixed. Oxidation-etching experiments of doped Si NCs from low-pressure plasmas indicated a preference for boron to be found in the core of the Si NCs with phosphorus found in the near-surface region.…”

mentioning

confidence: 99%

Probing Dopant Locations in Silicon Nanocrystals via High Energy X-ray Diffraction and Reverse Monte Carlo Simulation

2019

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Understanding the locations of dopant atoms in ensembles of nanocrystals is crucial to controlling the dopants’ function. While electron microscopy and atom probe tomography methods allow investigation of dopant location for small numbers of nanocrystals, assessing large ensembles has remained a challenge. Here, we are using high energy X-ray diffraction (HE-XRD) and structure reconstruction via reverse Monte Carlo simulation to characterize nanocrystal structure and dopant locations in ensembles of highly boron and phosphorus doped silicon nanocrystals (Si NCs). These plasma-synthesized NCs are a particularly intriguing test system for such an investigation, as elemental analysis suggests that Si NCs can be “hyperdoped” beyond the thermodynamic solubility limit in bulk silicon. Yet, free carrier densities derived from local surface plasmon resonances suggest that only a fraction of dopants are active. We demonstrate that the structural characteristics garnered from HE-XRD and structure reconstruction elucidate dopant location and doping efficacy for doped Si NCs from an atomic-scale perspective.

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Electronic, structural and optical properties of hydrogenated silicon nanocrystals: the role of the excited states

Cited by 21 publications

References 8 publications

Effect of Oxidation on the Doping of Silicon Nanocrystals with Group III and Group V Elements

Effect of Oxidation on the Doping of Silicon Nanocrystals with Group III and Group V Elements

Resonant Electronic Coupling Enabled by Small Molecules in Nanocrystal Solids

Probing Dopant Locations in Silicon Nanocrystals via High Energy X-ray Diffraction and Reverse Monte Carlo Simulation

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