Kolja Kolata

Clustering effects in Ga(AsBi)

The photoluminescence from a Ga(AsBi) sample is investigated as a function of pump power and lattice temperature. The disorder-related features are analyzed using a Monte Carlo simulation technique. A two-scale approach is introduced to separately account for cluster localization and alloy disorder effects. The corresponding characteristic energy scales of 11 and 45 meV are deduced from the detailed comparison between experiment and simulation.

Giant dynamical Stark shift in germanium quantum wells

We report a strong dynamical (ac) Stark shift of the direct gap transitions in Ge quantum wells at both cryogenic and room temperature. A blueshift of 67 meV is observed, exceeding values reported for III-V materials by about an order of magnitude. The fast intervalley scattering in the Ge material system leads to short dephasing times which in return causes larger shifts.

Universal ultrafast detector for short optical pulses based on graphene

Graphene has unique optical and electronic properties that make it attractive as an active material for broadband ultrafast detection. We present here a graphene-based detector that shows 40-picosecond electrical rise time over a spectral range that spans nearly three orders of magnitude, from the visible to the far-infrared. The detector employs a large area graphene active region with interdigitated electrodes that are connected to a log-periodic antenna to improve the long-wavelength collection efficiency, and a silicon carbide substrate that is transparent throughout the visible regime. The detector exhibits a noise-equivalent power of approximately 100 µW·Hz(-½) and is characterized at wavelengths from 780 nm to 500 µm.

Bismuth-containing III–V semiconductors: Epitaxial growth and physical properties

The growth, surface, and bulk properties of GaAsBi and related III-V alloys are examined and the potential benefits of these materials are explored in terms of device applications. The methods used include molecular beam epitaxy growth, scanning tunneling microscopy, scanning electron microscopy, transmission electron microscopy, photoluminescence spectroscopy, deep-level transient spectroscopy, dynamic modeling, and theoretical analysis. The results show that considerable progress has been made in alloying bismuth with GaAs and that the structural, optical, and electronic quality is very good for the alloys investigated.

Towards exploitation of singlet-exciton fission in organic crystals and potential integration with inorganic semiconductors

Molecular semiconductors offer intriguing electronic properties. In particular, singlet-exciton fission, the nonradiative decay of one singlet exciton into two triplet excitons effectively doubles the amount of carriers available for, e.g., photovoltaic current generation, thereby effectively surpassing the Shockley-Queisser-limit. An efficient use of singletexciton fission in actual devices, however, requires a detailed understanding of the decay dynamics in donor-acceptor heterostructures. We present a quantitative study on model single-crystalline perfluropentacene at cryogenic temperature and related heterostructures to reveal the intricate interplay between singlet-exciton fission and the nanoscopic molecular arrangement, the role of charge-transfer into and out of molecular systems and discuss the potential for functionalizing inorganic semiconductors. Finally, the potential implications in heterosystems and for functionalization of inorganic semiconductor devices are discussed.

Singlet-Exciton Fission Dynamics in Single-Crystalline Perfluoropentacene

The dynamics of photoexcited carriers are studied in single-crystalline perfluoropentacene by spectrally and polarization-resolved fs-pump white-light-probe spectroscopy. Our results reveal a correlation between singlet-exciton fission and the slip-stacked arrangements of the individual molecules.

Relaxation and recombination processes in Ge/SiGe multiple quantum wells

The carrier dynamics that occurs in Ge/SiGe QWs when electrons are excited to confined states at Γ is studied by means of optical spectroscopy at different lattice temperatures. The typical times for the different relaxation and recombination processes are given and discussed.

Ultrafast electron-hole scattering monitored by hole cooling in optically excited Germanium quantum wells

The scattering and cooling dynamics in Ge quantum wells are investigated on a picosecond time scale. Time-resolved pump-probe experiments reveal an efficient scattering process between electrons in the L-valley and holes in the Γ-valley.

Giant AC Stark shift in germanium

We observe a strong ultrafast AC Stark shift of the direct band transition in strained germanium quantum wells grown on silicon. At 150 meV, the maximum measured blue shift of the band edge is one order of magnitude larger than typically found in III-V materials. The power dependence shows a linear behavior between the electrical field intensity and the magnitude of the shift which is in good agreement with a dressed-exciton model. The coherent excitation dynamics in germanium are thus mainly governed by the direct transitions.