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Lateral electrical transport, optical properties and photocurrent measurements in two-dimensional arrays of silicon nanocrystals embedded in SiO2.

Gardelis S, Manousiadis P, Nassiopoulou AG - Nanoscale Res Lett (2011)

Bottom Line: Electronic transport is determined by the collective effect of Coulomb blockade gaps in the Si NCs.Our results show that Si NCs are useful building blocks of photovoltaic devices for use as better absorbers than bulk Si in the visible and ultraviolet spectral range.However, when strong quantum confinement effects come into play, carrier transport is significantly reduced due to strong exciton localization and Coulomb blockade effects, thus leading to limited photocurrent.

View Article: PubMed Central - HTML - PubMed

Affiliation: IMEL/NCSR Demokritos, Terma Patriarchou Grigoriou, Aghia Paraskevi, 15310 Athens, Greece. S.Gardelis@imel.demokritos.gr.

ABSTRACT
In this study we investigate the electronic transport, the optical properties, and photocurrent in two-dimensional arrays of silicon nanocrystals (Si NCs) embedded in silicon dioxide, grown on quartz and having sizes in the range between less than 2 and 20 nm. Electronic transport is determined by the collective effect of Coulomb blockade gaps in the Si NCs. Absorption spectra show the well-known upshift of the energy bandgap with decreasing NC size. Photocurrent follows the absorption spectra confirming that it is composed of photo-generated carriers within the Si NCs. In films containing Si NCs with sizes less than 2 nm, strong quantum confinement and exciton localization are observed, resulting in light emission and absence of photocurrent. Our results show that Si NCs are useful building blocks of photovoltaic devices for use as better absorbers than bulk Si in the visible and ultraviolet spectral range. However, when strong quantum confinement effects come into play, carrier transport is significantly reduced due to strong exciton localization and Coulomb blockade effects, thus leading to limited photocurrent.

No MeSH data available.


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Arrhenius plot of current, I for film B at a bias of 5 V.
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Figure 4: Arrhenius plot of current, I for film B at a bias of 5 V.

Mentions: Temperature dependence of the current in all films revealed two main temperature regimes for transport. At low temperatures, carriers can tunnel through the SiO2 potential barriers between the Si NCs, whereas at higher temperatures thermionic emission is more pronounced than tunnel transport. A similar behavior has been observed, in general, in the transport of polycrystalline Si films [32-34] and of other granular semiconductors which consist of grains separated by grain boundaries [35,36]. Figure 4 shows an example of an Arrhenius plot of the current obtained from film B at a bias of 5 V as a function of 1/kT, where k is the Boltzmann constant and T is the temperature. Activation energies E1 and E2, corresponding to thermionic emission and tunneling respectively, were extracted from these plots. For film B, E1 was calculated to be 563 meV when the applied voltage between the electrodes was 5 V. This value was reduced to 463 meV at an applied voltage of 20 V. This is expected to occur as carriers at higher applied voltages acquire higher energies, resulting in lower activation energies for thermionic emission. E2 was calculated to be 78 meV with small fluctuations around this value for different applied voltages. Similar values for E1 were extracted from the Arrhenius plots for films C and D, whereas E2 was reduced for films C to a value of 35 meV and for film D to 25 meV. We have proven elsewhere [37] that E2 is the charging energy which is needed by a carrier to overcome the Coulomb blockade gap of a nanocrystal. The larger values of E2 for film B compared to those of films C and D agree very well with the fact that larger Coulomb blockade gaps are expected for the smaller Si NCs consisting film B.


Lateral electrical transport, optical properties and photocurrent measurements in two-dimensional arrays of silicon nanocrystals embedded in SiO2.

Gardelis S, Manousiadis P, Nassiopoulou AG - Nanoscale Res Lett (2011)

Arrhenius plot of current, I for film B at a bias of 5 V.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211285&req=5

Figure 4: Arrhenius plot of current, I for film B at a bias of 5 V.
Mentions: Temperature dependence of the current in all films revealed two main temperature regimes for transport. At low temperatures, carriers can tunnel through the SiO2 potential barriers between the Si NCs, whereas at higher temperatures thermionic emission is more pronounced than tunnel transport. A similar behavior has been observed, in general, in the transport of polycrystalline Si films [32-34] and of other granular semiconductors which consist of grains separated by grain boundaries [35,36]. Figure 4 shows an example of an Arrhenius plot of the current obtained from film B at a bias of 5 V as a function of 1/kT, where k is the Boltzmann constant and T is the temperature. Activation energies E1 and E2, corresponding to thermionic emission and tunneling respectively, were extracted from these plots. For film B, E1 was calculated to be 563 meV when the applied voltage between the electrodes was 5 V. This value was reduced to 463 meV at an applied voltage of 20 V. This is expected to occur as carriers at higher applied voltages acquire higher energies, resulting in lower activation energies for thermionic emission. E2 was calculated to be 78 meV with small fluctuations around this value for different applied voltages. Similar values for E1 were extracted from the Arrhenius plots for films C and D, whereas E2 was reduced for films C to a value of 35 meV and for film D to 25 meV. We have proven elsewhere [37] that E2 is the charging energy which is needed by a carrier to overcome the Coulomb blockade gap of a nanocrystal. The larger values of E2 for film B compared to those of films C and D agree very well with the fact that larger Coulomb blockade gaps are expected for the smaller Si NCs consisting film B.

Bottom Line: Electronic transport is determined by the collective effect of Coulomb blockade gaps in the Si NCs.Our results show that Si NCs are useful building blocks of photovoltaic devices for use as better absorbers than bulk Si in the visible and ultraviolet spectral range.However, when strong quantum confinement effects come into play, carrier transport is significantly reduced due to strong exciton localization and Coulomb blockade effects, thus leading to limited photocurrent.

View Article: PubMed Central - HTML - PubMed

Affiliation: IMEL/NCSR Demokritos, Terma Patriarchou Grigoriou, Aghia Paraskevi, 15310 Athens, Greece. S.Gardelis@imel.demokritos.gr.

ABSTRACT
In this study we investigate the electronic transport, the optical properties, and photocurrent in two-dimensional arrays of silicon nanocrystals (Si NCs) embedded in silicon dioxide, grown on quartz and having sizes in the range between less than 2 and 20 nm. Electronic transport is determined by the collective effect of Coulomb blockade gaps in the Si NCs. Absorption spectra show the well-known upshift of the energy bandgap with decreasing NC size. Photocurrent follows the absorption spectra confirming that it is composed of photo-generated carriers within the Si NCs. In films containing Si NCs with sizes less than 2 nm, strong quantum confinement and exciton localization are observed, resulting in light emission and absence of photocurrent. Our results show that Si NCs are useful building blocks of photovoltaic devices for use as better absorbers than bulk Si in the visible and ultraviolet spectral range. However, when strong quantum confinement effects come into play, carrier transport is significantly reduced due to strong exciton localization and Coulomb blockade effects, thus leading to limited photocurrent.

No MeSH data available.


Related in: MedlinePlus