D. Krapf

Hebrew University of Jerusalem, Jerusalem, Jerusalem District, Israel

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Publications (7)27.12 Total impact

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    ABSTRACT: A striking correlation between infrared photoinduced absorption spectra and the photoluminescence from silicon nanocrystals indicates that quantized electronic sublevels of the nanocrystals are resonantly coupled to surface vibrational modes via a polarization field produced by coherent longitudinal polar vibrations. Our experimental results and model support the assumption that the mechanism responsible for the efficient photoluminescence from silicon nanocrystals should be assigned to inhibition of nonradiative channels rather than enhancement of radiative channels.
    Nano Letters 01/2006; 5(12):2443-7. · 13.03 Impact Factor
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    ABSTRACT: Optical transitions between the quantized sublevels of colloidally synthesized InAs nanocrystals have been revealed using infrared photoinduced absorption spectroscopy. Two different groups of intersublevel transitions were observed. Using a correlation between the measured transition energies and experimental data obtained by scanning tunneling microscopy and photoluminescence excitation spectra, we have identified one group to be related to interconduction transitions while the other is related to intervalence transitions. However, in contrast to conduction intersublevel transitions that appear at all temperatures, we have found that the valence intersublevel transitions are thermally activated and cannot be observed at temperatures below 100 K. This behavior can be explained by assuming a three-level system model for the valence band. We propose two possible explanations for our findings; the first is based on the assumption of well-defined envelope-state symmetry relations while the second attributes the thermal activation process to the presence of shallow surface localized hole states.
    Physical review. B, Condensed matter 02/2004; 69(7). · 3.77 Impact Factor
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    ABSTRACT: The quantum confinement model, which assigns some of the luminescence features in porous silicon to size quantization in Si nano-crystallites, also predicts quantization of both the conduction and valence bands into sub-levels. In order to resolve this effect we have used a new experimental technique called “photo-induced infrared absorption spectroscopy”. Here, a pump, visible laser, optically induces carriers in the conduction/valence band. Optical transitions between the quantized sub-levels are resolved by a probe, infrared beam in the energy range 50–300 meV. A broad photo-induced absorption signal has been observed in the 60–250 meV spectral range, in agreement with the prediction of the quantum confinement model. However, the photo-induced absorption signal decreases with the decreasing temperature, resolving activation energy of about 10 meV. This behavior can be understood if the allowed optical transitions are from the exciton singlet state only. Also, we found additional features in the photo-induced absorption spectrum that are correlated with the Si=O vibrational modes. Our results indicate a strong coupling between bulk excitonic states and surface states in small Si nano-crystallites.
    physica status solidi (a) 05/2003; 197(2):566 - 571. · 1.21 Impact Factor
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    ABSTRACT: A modified p-type Si/SiGe quantum well infrared photodetector for multispectral infrared imaging applications is demonstrated. In order to improve the detector’s performances we have used a SiGe emitter and a low-temperature wet passivation process that give rise to a reduced dark current, even at relatively high bias voltages. Multispectral photoresponse at the long, mid and short wavelength infrared atmospheric windows was observed. The response peaks are assigned to the various classes of intervalence band transitions in the quantum wells and in the SiGe emitter layers. © 2001 American Institute of Physics.
    Applied Physics Letters 01/2001; 78(4):495-497. · 3.79 Impact Factor
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    ABSTRACT: In this paper we present a systematic experimental investigation of the optical properties associated with inter-subband and inter-valence band transitions in p-type pseudomorphic Si1−xGex/Si multiple quantum-wells structure under high-temperature thermal treatments. The structure exhibits two types of optical absorption lines: the first obeys the inter-subband selection rules and is assigned to heavy-hole transitions while the second obeys the inter-valence band selection rules and is assigned to transitions between a heavy hole and a mixed spin split off and light-hole state. Annealing treatments reveal two kinds of thermally activated processes. The first process is assigned to strain relaxation while the second is assigned to Si and Ge inter-diffusion. Raman spectroscopy provides additional support to our interpretation of the activation processes. We propose a quantitative model, based on the Bir–Pikus deformation potential to explain the experimental results.
    Physica E Low-dimensional Systems and Nanostructures 04/2000; 7(s 1–2):255–258. · 1.52 Impact Factor
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    ABSTRACT: Thermal relaxation processes due to strain relaxation and Si/Ge interdiffusion were investigated in pseudomorphic p-type SiGe/Si quantum wells using infrared-polarization-resolved absorption spectroscopy. The samples were annealed from room temperature up to 1060 °C and intersubband transitions between the lowest heavy-hole states and inter-valence-band transitions between heavy-hole and spin-split-off hole states were utilized to probe thermal activation processes. The strain relaxation process is activated at temperatures above 750 °C and causes a decrease of the intersubband absorption and an increase of the inter-valence-band absorption. At temperatures above 940 °C, we found that a second process of Si/Ge interdiffusion causes a reduction of all absorption lines in the spectrum. We proposed a simple model that provides a qualitative explanation to the above results. © 1999 American Institute of Physics.
    Applied Physics Letters 10/1999; 75(15):2232-2234. · 3.79 Impact Factor
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    ABSTRACT: In this paper we present a systematic experimental investigation of the optical properties associated with inter-subband and inter-valence band transitions in p-type pseudomorphic Si1 xGex=Si multiple quantum-wells structure under high-temperature thermal treatments. The structure exhibits two types of optical absorption lines: the rst obeys the inter-subband selection rules and is assigned to heavy-hole transitions while the second obeys the inter-valence band selection rules and is assigned to transitions between a heavy hole and a mixed spin split o and light-hole state. Annealing treatments reveal two kinds of thermally activated processes. The rst process is assigned to strain relaxation while the second is assigned to Si and Ge inter-diusion. Raman spectroscopy provides additional support to our interpretation of the activation processes. We propose a quantitative model, based on the Bir{Pikus deformation potential to explain the experimental results. ? 2000 Elsevier Science B.V. All rights reserved.