Semiconductor Quantum Nanomaterials

Technical University of Munich

Walter Schottky Institute

Am Coulombwall 4

85748 Garching

Tel. +49 89 289 12779


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Research focus: semiconductor quantum nanomaterials, quantum nanowire photonics, quantum transport

Our research activities are focused on innovative semiconductor nano and quantum materials (III-V quantum nanowires, quantum dots and novel 2D heterostructures) and their use within diverse research areas spanning the fields of nano and quantum electronics, nano-optoelectronics and quantum photonics, as well as information technology and sensing. Along these avenues we pursue all stages from synthesis of nano and quantum materials to simulation and characterization of electronic, optical, structural, and charge carrier properties, as well as to device fabrication and probing device functionalities in regimes that are ruled by mesoscopic and quantum effects.

Quantum Nanowire Integrated Light Sources

Novel quantum-nanowire (NW) cavities on SOI-based platform are not only promising systems as integrated nanolaser sources for future on-chip optical interconnects, but may also enable directly integrated single and entangled states of light that can be efficiently coupled and computed on demand in underlying quantum circuits. One central theme of our work is to synthesize and explore the properties of quantum heterostructures (wells and dots) in NW cavities and further integrate these deterministically on 1D-waveguides (WG).

Our vision towards quantum optical circuits is to thereby exploit chiral optical effects in 1D-WG geometries, which will enable coupling of circularly polarized exciton spin states from NW-quantum dot (QD) emitters to chiral points for bi- and uni-directional propagation of exciton spin-states. Such demonstration to map polarization entanglement into path entanglement may enable large arrays of deterministic NW-QD/WG systems and to realize cascaded quantum states much needed for quantum information processing.

Selected Publications

  • B. Loitsch, D. Rudolph, S. Morkötter, M. Döblinger, G. Grimaldi, L. Hanschke, S. Matich, E. Parzinger, U. Wurstbauer, G. Abstreiter, J. J. Finley, and G. Koblmüller: “Tunable quantum confinement in ultrathin, optically active semiconductor nanowires via reverse reaction growth”, Advanced Materials 27, 2195 (2015).
  • B. Loitsch, M. Müller, J. Winnerl, P. Veit, D. Rudolph, G. Abstreiter, J. J. Finley, F. Bertram, J. Christen, and G. Koblmüller: “Microscopic nature of crystal phase quantum dots in ultrathin GaAs nanowires by nanoscale luminescence characterization”, New Journal of Physics 18, 063009 (2016).
  • T. Stettner, A. Thurn, M. Döblinger, M. O. Hill, J. Bissinger, S. Matich, T. Kostenbader, D. Ruhstorfer, H. Riedl, M. Kaniber, L. J. Lauhon, J. J. Finley, and G. Koblmüller: “Tuning lasing emission towards telecommunication wavelengths in GaAs-(In,Al)GaAs core-multishell nanowires”, Nano Letters 18, 6292 (2018).

Quantum Phenomena in Transport

1D-like quantum NWs and their heterostructures play a substantial role in advanced quantum electronics and quantum information technologies. Ongoing research in these quantum electronic systems aims at investigations of charge carrier dynamics and interaction phenomena, and to investigate the highly relevant, hitherto unexplored non-equilibrium transport in 1D-nano­structures and their devices. Hereby, we study ultra-pure III-V-based NWs with intrinsically very high charge carrier mobility as well as systems with large spin-orbit interaction.

Particularly, we aim at control of the relevant transport regimes (ballistic vs. diffusive) – a capability that allows us to capture several important investigations of many-body correlations, spin-orbit interaction effects, as well as inelastic scattering and electron-phonon interaction effects.

Moving further in the direction of hybrid superconducting-semiconductor topological systems for quantum information processing, we explore proximity-induced local barriers defined between superconducting contacts and NWs to provide a platform for Majorana fermion bound states. This opens unique possibilities in studying unusual non-equilibrium quasiparticle distributions.

Selected Publications

  • S. Morkötter, N. Jeon, D. Rudolph, S. Matich, M. Döblinger, D. Spirkoska, E. Hoffman, J. J. Finley, L. J. Lauhon, G. Abstreiter, and G. Koblmüller: “Demonstration of confined electron gas and steep-slope behavior in delta-doped GaAs-AlGaAs core-shell nanowire transistors“, Nano Letters 15, 3295 (2015).
  • D. Irber, J. Seidl, D. J. Carrad, J. Becker, N. Jeon, B. Loitsch, J. Winnerl, S. Matich, M. Döblinger, Y. Tang, S. Morkötter, G. Abstreiter, J. J. Finley, M. Grayson, L. J. Lauhon, and G. Koblmüller: “Quantum transport and sub-band structure of modulation-doped GaAs/AlAs core-superlattice nanowires“, Nano Letters 17, 4886 (2017).
  • A. V. Bubis, A. O. Denisov, S. U. Piatrusha, I. E. Batov, V. S. Khrapai J. Becker, J. Treu, D. Ruhstorfer, and G. Koblmüller: “Proximity effect and interface transparency in Al/InAs-nanowire/Al-diffusive junctions”, Semiconductor Science and Technology 32, 094007 (2017).


Purcell enhanced coupling of nanowire quantum emitters to silicon photonic waveguides

N. Mukhundhan, A. Ajay, J. Bissinger, J.J. Finley, G. Koblmüller

Optics Express 29 (26), 43068-43081 (2021).

Show Abstract

We design a quantum dot (QD) embedded in a vertical-cavity photonic nanowire (NW), deterministically integrated on a silicon-on-insulator (SOI) waveguide (WG), as a novel quantum light source in a quantum photonic integrated circuit (QPIC). Using a broadband QD emitter, we perform finite-difference time domain simulations to systematically tune key geometrical parameters and to explore the coupling mechanisms of the emission to the NW and WG modes. We find distinct Fabry-Perot resonances in the Purcell enhanced emission that govern the outcoupled power into the fundamental TE mode of the SOI-WG. With an optimized geometry that places the QD emitter in a finite NW in close proximity to the WG, we obtain peak outcoupling efficiencies for polarized emission as high as eighty percent. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.

DOI: 10.1364/oe.442527

Charge-neutral nonlocal response in superconductor-InAs nanowire hybrid devices

A.O. Denisov, A.V. Bubis, S.U. Piatrusha, N.A. Titova, A.G. Nasibulin, J. Becker, J. Treu, D. Ruhstorfer, G. Koblmueller, E.S. Tikhonov, V.S. Khrapai

Semiconductor Science and Technology 36, 09LT04 (2021).

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Nonlocal quasiparticle transport in normal-superconductor-normal (NSN) hybrid structures probes sub-gap states in the proximity region and is especially attractive in the context of Majorana research. Conductance measurement provides only partial information about nonlocal response composed from both electron-like and hole-like quasiparticle excitations. In this work, we show how a nonlocal shot noise measurement delivers a missing puzzle piece in NSN InAs nanowire-based devices. We demonstrate that in a trivial superconducting phase quasiparticle response is practically charge-neutral, dominated by the heat transport component with a thermal conductance being on the order of conductance quantum. This is qualitatively explained by numerous Andreev reflections of a diffusing quasiparticle, that makes its charge completely uncertain. Consistently, strong fluctuations and sign reversal are observed in the sub-gap nonlocal conductance, including occasional Andreev rectification signals. Our results prove conductance and noise as complementary measurements to characterize quasiparticle transport in superconducting proximity devices.

DOI: 10.1088/1361-6641/ac187b

Ultrathin catalyst-free InAs nanowires on silicon with distinct 1D sub-band transport properties

F. del Giudice, J. Becker, C. de Rose, M. Doeblinger, D. Ruhstorfer, L. Suomenniemi, J. Treu, H. Riedl, J.J. Finley, G. Koblmueller

Nanoscale 12 (42), 21857-21868 (2020).

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Ultrathin InAs nanowires (NW) with a one-dimensional (1D) sub-band structure are promising materials for advanced quantum-electronic devices, where dimensions in the sub-30 nm diameter limit together with post-CMOS integration scenarios on Si are much desired. Here, we demonstrate two site-selective synthesis methods that achieve epitaxial, high aspect ratio InAs NWs on Si with ultrathin diameters below 20 nm. The first approach exploits direct vapor-solid growth to tune the NW diameter by interwire spacing, mask opening size and growth time. The second scheme explores a unique reverse-reaction growth by which the sidewalls of InAs NWs are thermally decomposed under controlled arsenic flux and annealing time. Interesting kinetically limited dependencies between interwire spacing and thinning dynamics are found, yielding diameters as low as 12 nm for sparse NW arrays. We clearly verify the 1D sub-band structure in ultrathin NWs by pronounced conductance steps in low-temperature transport measurements using back-gated NW-field effect transistors. Correlated simulations reveal single- and double degenerate conductance steps, which highlight the rotational hexagonal symmetry and reproduce the experimental traces in the diffusive 1D transport limit. Modelling under the realistic back-gate configuration further evidences regimes that lead to asymmetric carrier distribution and breakdown of the degeneracy depending on the gate bias.

DOI: 10.1039/d0nr05666a

Quantum-confinement enhanced thermoelectric properties in modulation-doped GaAs-AlGaAs core-shell nanowires

S. Fust, A. Faustmann, D. J. Carrad, J. Bissinger, B. Loitsch, M. Döblinger, J. Becker, G. Abstreiter, J. J. Finley, G. Koblmueller

Advanced Materials 32, 1905458 (2019).

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Nanowires (NWs) hold great potential in advanced thermoelectrics due to their reduced dimensions and low-dimensional electronic character. However, unfavorable links between electrical and thermal conductivity in state-of-the-art unpassivated NWs have, so far, prevented the full exploitation of their distinct advantages. A promising model system for a surface-passivated one-dimensional (1D)-quantum confined NW thermoelectric is developed that enables simultaneously the observation of enhanced thermopower via quantum oscillations in the thermoelectric transport and a strong reduction in thermal conductivity induced by the core–shell heterostructure. High-mobility modulation-doped GaAs/AlGaAs core–shell NWs with thin (sub-40 nm) GaAs NW core channel are employed, where the electrical and thermoelectric transport is characterized on the same exact 1D-channel. 1D-sub-band transport at low temperature is verified by a discrete stepwise increase in the conductance, which coincided with strong oscillations in the corresponding Seebeck voltage that decay with increasing sub-band number. Peak Seebeck coefficients as high as ≈65–85 µV K−1 are observed for the lowest sub-bands, resulting in equivalent thermopower of S2σ ≈ 60 µW m−1 K−2 and S2G ≈ 0.06 pW K−2 within a single sub-band. Remarkably, these core–shell NW heterostructures also exhibit thermal conductivities as low as ≈3 W m−1 K−1, about one order of magnitude lower than state-of-the-art unpassivated GaAs NWs.


Breakdown of corner states and carrier localization by monolayer fluctuations in a radial nanowire quantum wells

M. M. Sonner, A. Sitek, L. Janker, D. Rudolph, D. Ruhstorfer, M. Döblinger, A. Manolescu, G. Abstreiter, J. J. Finley, A. Wixforth, G. Koblmueller, H. J. Krenner

Nano Lett. 19 (5), 3336-3343 (2019).

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We report a comprehensive study of the impact of the structural properties in radial GaAs-Al0.3Ga0.7As nanowire-quantum well heterostructures on the optical recombination dynamics and electrical transport properties, emphasizing particularly the role of the commonly observed variations of the quantum well thickness at different facets. Typical thickness fluctuations of the radial quantum well observed by transmission electron microscopy lead to pronounced localization. Our optical data exhibit clear spectral shifts and a multipeak structure of the emission for such asymmetric ring structures resulting from spatially separated, yet interconnected quantum well systems. Charge carrier dynamics induced by a surface acoustic wave are resolved and prove efficient carrier exchange on native, subnanosecond time scales within the heterostructure. Experimental findings are corroborated by theoretical modeling, which unambiguously show that electrons and holes localize on facets where the quantum well is the thickest and that even minute deviations of the perfect hexagonal shape strongly perturb the commonly assumed 6-fold symmetric ground state.


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