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Electromodulated reflectance study of self-assembled Ge/Si quantum dots.

Yakimov A, Nikiforov A, Bloshkin A, Dvurechenskii A - Nanoscale Res Lett (2011)

Bottom Line: Up to three optical transitions are observed.The low-energy resonance is proposed to correspond to a band-to-continuum hole transition in the Ge valence band.The other two modulation signals are attributed to the spatially direct transitions between the electrons confined in the L and Δ(4) valleys of the Ge conduction band, and the localized hole states at the Γ point.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia. yakimov@isp.nsc.ru.

ABSTRACT
We perform an electroreflectance spectroscopy of Ge/Si self-assembled quantum dots in the near-infrared and in the mid-infrared spectral range. Up to three optical transitions are observed. The low-energy resonance is proposed to correspond to a band-to-continuum hole transition in the Ge valence band. The other two modulation signals are attributed to the spatially direct transitions between the electrons confined in the L and Δ(4) valleys of the Ge conduction band, and the localized hole states at the Γ point.

No MeSH data available.


Related in: MedlinePlus

Calculated band-edge diagram of the strained Ge pyramid in Si(001) along the growth axis with the relevant interband transitions. For CB Δ and L points are shown. The electron and hole energy levels are indicated by horizontal dashed lines.
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Figure 5: Calculated band-edge diagram of the strained Ge pyramid in Si(001) along the growth axis with the relevant interband transitions. For CB Δ and L points are shown. The electron and hole energy levels are indicated by horizontal dashed lines.

Mentions: The band diagram for the three lowest interband transitions is shown in Figure 5. Due to the tensile strain, the sixfold-degenerate conduction-band minimum at the Δ point of Si around the Ge dot splits into the fourfold-degenerate in-plane Δ(4) valleys and the twofold-degenerate Δ(2) valleys along the [001] growth direction. The lowest CB edge just above and below the Ge island is formed by the Δ(2) valleys [10]. In the valence band, there is a large offset and the holes are confined inside the Ge dot, yielding type-II band-edge alignment. Since the electron and hole are spatially separated the electron-hole overlap (f ) is as small as 0.18. The oscillator strength is greatly increased for the spatially direct interband transitions. The lowest direct transition inside the dot involves the Δ(4) CB (f = 0.56) and the second one includes the L valley (f = 0.86). The calculated energy difference between the spatially direct Δ(4) - Γ and spatially indirect Δ(2) - Γ transitions, 35 meV, is consistent with that obtained previously from PL spectroscopy, 34-52 meV [4]. It is worth to note that although the energies of these transitions are close to each other (0.7 eV), the oscillator strength (∝ f 2) of the direct transition is larger by one order of magnitude to dominate in the absorption spectra. Therefore, it is unlikely to observe the indirect transition in absorption or reflectance experiments. However, it can be easily detected in PL measurements as they can probe selectively the ground-state emission energy.


Electromodulated reflectance study of self-assembled Ge/Si quantum dots.

Yakimov A, Nikiforov A, Bloshkin A, Dvurechenskii A - Nanoscale Res Lett (2011)

Calculated band-edge diagram of the strained Ge pyramid in Si(001) along the growth axis with the relevant interband transitions. For CB Δ and L points are shown. The electron and hole energy levels are indicated by horizontal dashed lines.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211264&req=5

Figure 5: Calculated band-edge diagram of the strained Ge pyramid in Si(001) along the growth axis with the relevant interband transitions. For CB Δ and L points are shown. The electron and hole energy levels are indicated by horizontal dashed lines.
Mentions: The band diagram for the three lowest interband transitions is shown in Figure 5. Due to the tensile strain, the sixfold-degenerate conduction-band minimum at the Δ point of Si around the Ge dot splits into the fourfold-degenerate in-plane Δ(4) valleys and the twofold-degenerate Δ(2) valleys along the [001] growth direction. The lowest CB edge just above and below the Ge island is formed by the Δ(2) valleys [10]. In the valence band, there is a large offset and the holes are confined inside the Ge dot, yielding type-II band-edge alignment. Since the electron and hole are spatially separated the electron-hole overlap (f ) is as small as 0.18. The oscillator strength is greatly increased for the spatially direct interband transitions. The lowest direct transition inside the dot involves the Δ(4) CB (f = 0.56) and the second one includes the L valley (f = 0.86). The calculated energy difference between the spatially direct Δ(4) - Γ and spatially indirect Δ(2) - Γ transitions, 35 meV, is consistent with that obtained previously from PL spectroscopy, 34-52 meV [4]. It is worth to note that although the energies of these transitions are close to each other (0.7 eV), the oscillator strength (∝ f 2) of the direct transition is larger by one order of magnitude to dominate in the absorption spectra. Therefore, it is unlikely to observe the indirect transition in absorption or reflectance experiments. However, it can be easily detected in PL measurements as they can probe selectively the ground-state emission energy.

Bottom Line: Up to three optical transitions are observed.The low-energy resonance is proposed to correspond to a band-to-continuum hole transition in the Ge valence band.The other two modulation signals are attributed to the spatially direct transitions between the electrons confined in the L and Δ(4) valleys of the Ge conduction band, and the localized hole states at the Γ point.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia. yakimov@isp.nsc.ru.

ABSTRACT
We perform an electroreflectance spectroscopy of Ge/Si self-assembled quantum dots in the near-infrared and in the mid-infrared spectral range. Up to three optical transitions are observed. The low-energy resonance is proposed to correspond to a band-to-continuum hole transition in the Ge valence band. The other two modulation signals are attributed to the spatially direct transitions between the electrons confined in the L and Δ(4) valleys of the Ge conduction band, and the localized hole states at the Γ point.

No MeSH data available.


Related in: MedlinePlus