Papers by Author: Wolfgang Jantsch

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Authors: Victor Tapio Rangel-Kuoppa, Alexander Tonkikh, Nikolay Zakharov, Peter Werner, Wolfgang Jantsch
Abstract: We investigate self-assembled pyramid-shaped Ge Quantum Dots (QDs) with lateral dimensions of 15 nm, and heights of 2.5-3 nm. These Ge QDs were grown by Molecular Beam Epitaxy (MBE) on n-type Si(100) substrates using the Sb-mediated growth mode. The resistivity of the substrates was about 5 Ωcm. The Si buffer layer below the QDs and the Si capping layer above them were doped up to 1018 cm-3 by Sb. Cross-section transmission electron microscopy shows the QDs and the Sb delta-doped layers. Using standard photolithographic techniques, a 0.3 mm2 Au Schottky contact was applied to the epilayer, while an Ohmic contact was formed on the back side of the substrate. Plotting C-2 vs. V plot reveals the nominal doping of 1018 cm-3. DLTS studies revealed two levels with fitted activation energies of 49 meV and 360-390 meV. They are related to the Sb doping and the Pb interface states, respectively. The simulation suggests a deep level with a volumetric concentration of 2.55×1015 cm-3. Multiplying this value by the thickness of the depletion region obtained from the CV measurements, we find that the deep level capture about 5.8×109 electrons per cm2.
Authors: Alexander A. Tonkikh, Victor Tapio Rangel-Kuoppa, Nikolay D. Zakharov, Wolfgang Jantsch, Peter Werner
Abstract: We report on a specific defect, which may form during the growth of Stranski-Krastanov surfactant-mediated Ge/Si (100) islands. Transmission electron microscopy reveals that these loop-like defects are local and could be represented by a missing plane of Ge atoms inside some of Ge islands. This specific defect may generate an electrically active trap within the Si band gap at about 0.3 eV above the Si valence band edge. Deep level transient spectroscopy reveals that at least 1 % of Ge islands may include such defects.
Authors: Victor Tapio Rangel-Kuoppa, Gang Chen, Wolfgang Jantsch
Abstract: The electrical properties of dome-shaped and pyramid-shaped Ge Quantum Dots (QDs) embedded in p-type Silicon are reported. Capacitance-Voltage (T-CV) characteristics are reported for the temperature range of 35 K to 296 K. The T-CV results showed the desired charge carrier density of the Silicon, on the order of 1016 cm-3, at room temperature. Two shoulders are observed in the CV curves between 270 K and 175 K. They are explained as charge stored in the dome- and pyramid-shaped QDs. Below 175 K, only one shoulder is observed in the CV measurements, attributed to charge trapped in dome-shaped QDs. The DLTS study confirms these results. Using a reverse bias between -0.1 V and -1 V two peaks are seen at 50 and 70 K. They are explained in terms of the boron state (the one at 50 K) and charged stored on pyramid-shaped Ge QDs (the one at 70 K). Increasing the reverse bias from -1 V to -1.4 V shows the appearance of a peak around 60 K, attributed to dome-shaped Ge QDs. At the same time, a shoulder appears around 100 K for -1 V, which extends to larger temperatures as the reverse bias magnitude is increased. The activation energies found are around 50 meV (due to Boron), 150 to 250 meV (due to pyramid-shaped Ge QDs), 300 to 350 meV (due to dome-shaped Ge QDs) and 425 meV (due to both dome- and pyramid-shaped Ge QDs).
Authors: Wolfgang Wille, Ralph Rothemund, Gerald Meinhardt, Wolfgang Jantsch
Abstract: Instead of selective emitter technology we investigate an alternative way to optimize contact formation and increased blue responsivity of highly resistive emitter solar cells using screen print technology for the deposition of the frontside metallization grid. We show with the aid of an inline doping/diffusion set-up at Blue Chip Energy that tuning the emitter doping profile is an alternative way to reduce the effect of Auger recombination in the spectral range from 300 nm to 600 nm. By properly choosing the process conditions we were able to minimize the detrimental effect of the low surface concentration of the dopant on the contact resistance. Due to improved blue light responsivity a significant gain in short circuit current Jsc was achieved. This and a reduced reverse saturation current I00E yielded a higher open circuit voltage VOC and an increase of cell efficiency from 17.6 %-avg to more than 17.9 %-avg.
Authors: Ralph Rothemund, Thomas Umundum, Gerald Meinhardt, Kurt Hingerl, Thomas Fromherz, Wolfgang Jantsch
Abstract: Back–side diffraction gratings enhance a solar cell’s near–band–gap response by diffracting light into higher orders and thereby reducing front–side escape losses. The resulting increased photon absorption and carrier generation improves short–circuit current densities and solar cell efficiencies. Combining rigorous coupled–wave analysis and ray tracing yields a three–dimensional, polarization sensitive optical model to calculate Si absorbance, front–side and back–side losses. For industrially used, pyramidally textured, 180 μm Si solar cells with 85 nm SiNx anti–reflection coating, the application of an optimized back–side grating enhances the short–circuit current density by ≈ +1 mA/cm2, a relative increase of ≈ +2.7 %.
Authors: Ralph Rothemund, Susanne Kreuzer, Thomas Fromherz, Wolfgang Jantsch
Abstract: We fabricated and characterized NIR-active Schottky-contact solar cells with PbS nanocrystals (NCs) as the active medium. The photovoltaic e ffect is due to carrier generation in the PbS NCs as proven by the comparison of the spectrally-resolved external quantum effciency of the devices and absorbance spectra of the PbS NCs. The operative regime is extended well beyond the Si bandgap into the infrared spectral region up to 1500 nm limited by our measurement setup. One sun I-V and time-resolved photocurrent measurements help to identify critical solar cell parameters for the further improvement of PbS NC Schottky-contact solar cells.
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