Walter Schottky Institute
Center for Nanotechnology and Nanomaterials

Semiconductor Quantum Nanomaterials - News
Group leader: PD Dr. Gregor Koblmueller (Chair of Prof. Dr. Jonathan Finley)


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RECENT & UPCOMING EVENTS
(2020 - 2021)



Editor's Pick: Low-threshold nanowire lasers emitting at 1.3 µm at room temperature
Jun 2 2021

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III-V based semiconductor nanowire (NW) lasers hold large potential for telecom-band optical data communication and sensing applications due to their ultracompact size, unique properties, and integrability on silicon (Si) photonic circuits. Realizing telecom-band lasing in GaAs based NW lasers with low bandgap gain media has been, however, notoriously difficult because of high compressive strain built up in the active regions.


In a recent report published in Applied Physics Letters (2021), P. Schmiedeke, et al. proposed and demonstrated an advanced coaxial GaAs-InGaAs multi-quantum well (MQW) NW laser that solves previous limitations. By implementing a crucial strain-compensating InAlGaAs buffer layer to suppress the compressive strain in the MQW, much larger In molar fractions without plastic relaxation are obtained. Thereby, NW-lasers with In-content up to 40% in the MQW were achieved with tuneability of the lasing emission to below the Si transparency region while maintaining exceptional material quality. This way we achieved optically pumped room-temperature lasing operation with a threshold < 50 µJcm-2 in the telecom O-band close to 1.3 µm.


The editors of Appl. Phys. Lett. were favorably impressed by our work, and highlighted this breakthrough result as Editor's Pick in the June-2021 (Vol.118(22)) issue.


Online Link






SIOE Merit Award for Andreas Thurn
Apr 6 2021

[research]

Andreas Thurn, PhD candidate at the Chair of Semiconductor Nanostructures and Quantum Systems, was recently awarded the SIOE Merit Award at the Semiconductor and Integrated Opto-Electronics Conference (SIOE-2021) in Cardiff, United Kingdom. The award acknowledges outstanding student contributions in the fields of nanophotonics and integrated opto-electronics based on advanced semiconductor materials.


The awardee received the prize for his oral presentation entitled “Ultrafast non-equilibrium dynamics in GaAs-based nanowire lasers”, which is grounded on collaborative work between experimental groups at the WSI-TUM and theory groups at TU Berlin, Cardiff University and Sandia National Laboratories. In this work, Andreas combined non-resonant degenerate pump-probe spectroscopy on single GaAs-AlGaAs core-shell nanowire (NW) lasers, together with numerical simulations using k-resolved Bloch equation and quantum statistical models, to unveil new and unexpected NW laser dynamics. In particular, he could observe ultrafast lasing dynamics similar to relaxation oscillations, but which are governed by cyclic heating/cooling processes of the electron-hole plasma in the NW lasers. This work sets important foundations to engineer the ultrafast temporal emission dynamics in advanced nanolasers on silicon.     






New DFG project on advanced nanowire thermoelectrics
Jan 13 2021

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We are excited to start off the new year with a new DFG-project on nanowire thermoelectrics that was granted today by the German Research Foundation (DFG). The single-PI project run by PD G. Koblmüller (WSI-SQNM Group) is entitled "Advanced thermoelectric properties in 1D quantum-confined core-shell nanowire heterostructures" (KO-4005/10-1). It aims to establish core-shell semiconductor nanowire (NW) heterostructures as an experimental platform for advanced nano-thermoelectric materials that exploit several advantages of quantum confinement mediated carrier transport and phonon properties tailored by the core-shell structure. Particular attention will be paid to new forms of InAs-based core-shell NWs with strong 1D-confinement and long quasi-ballistic mean free paths of carriers, as well as large atomic mass contrast to limit thermal conductivity. Hereby, we aim to push the thermoelectric performance of these new types of NW heterostructures to the quantum limit, i.e., the theoretical power factor quantum bound, while independently tuning phonon scattering via size effects in the core-shell structure. With future nano-thermoelectrics requiring both n- and p-channels, we further develop ambipolar systems that take advantage of quantized electron and hole transport in a single thermoelectric device.






Comprehensive insights into growth dynamics of ternary InAsSb nanowire arrays revealed
Jan 8 2021

[research]

InAsSb-based nanowires (NW) are amongst the most interesting semiconductor materials, due to their exceptionally large spin-orbit coupling and lowest band gap of all III-V semiconductors. This makes these 1D nanostructures very promising candidates, for example, in hybrid semiconductor-superconductor devices for topological quantum information applications as well as for mid-infrared (MIR) and long-wavelength infrared (LWIR) optoelectronic devices. Synthesis of this class of NWs has started only recently, where many fundamental aspects in the growth kinetics and structure-property relationships are still unknown.


We have now provided a very comprehensive study of the growth dynamics and correlated structural/compositional properties of periodic catalyst-free InAsSb NW arrays, published in Nanotechnology (2021) by D. Ruhstorfer, et al.. Specifically, we explored the interrelated growth kinetics and Sb incorporation dynamics for a wide set of array geometry parameters and growth conditions, with the goal to realize high aspect-ratio InAsSb NWs and enhanced Sb incorporation. Using correlated HR-XRD, HR- and scanning TEM, we further mapped their compositional structure and found an interesting core-shell sub-structure with Sb deficient interlayers. These findings, which can be applied to diverse ternary III-V based NWs, were supported by models for the evolution of the sub-structure and explained by the formation of a nanoscale facet at the truncation of the (111)B growth front and {1-10} sidewall surfaces.


Online Link





New DFG project on MBE growth and spectroscopy of 2D materials
Dec 2 2020

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Despite the huge upsurge of interests and the many fundamental discoveries in 2D materials over the past decade, the creation of these materials with tunable, reproducible, and intrinsically pure properties has been notoriously difficult. This has motivated us to set up advanced MBE growth facilities combined with unique in situ spectroscopic capabilities, funded by a DFG large-scale equipment grant and recent support from the Clusters of Excellence e-Conversion and MCQST.

Now, in a most recent application to the German Research Foundation (DFG), we successfully acquired collaborative funding for our first scientific project utilizing the newly installed MBE laboratory at WSI. The project grant awarded to both PIs, Prof. Finley (FI 947/8-1) and PD G. Koblmüller (KO 4005/9-1), allows us to hire two new PhD students and aims at the MBE growth of group-III metal chalcogenide based 2D materials and the direct in situ optical spectroscopy in the native UHV environment. Thereby, we expect to obtain unprecedented insights into the very intrinsic electronic and optical properties of ultrapure van-der-Waals epitaxial 2D materials, and further develop advanced heterostructures to steer exciton formation and dissociation dynamics in this class of materials via interface and compositional engineering, relevant for future quantum light technologies and photo-catalysis applications. We are very excited to start this first project and enter the 2D materials business from a new perspective.






New EU FET-Open project Opto-Silicon
Oct 27 2020

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We are excited to embark on a new FET-Open project entitled Opto-Silicon that was recently granted by the EU Horizon2020 program. FET Open supports highly competitive projects in the framework of Europe's research and innovation program, aiming at fundamental breakthroughs and arising new technologies. The project running over the next four years (2021-2025) focuses on developing integrated light sources, based on silicon-germanium (hex-SiGe) materials platform, with existing Si electronics and passive Si-photonics circuitry. Hereby, a European consortium will exploit their diverse interdisciplinary capabilities, from theoretical physics, materials synthesis and characterization, optics and laser physics, to optoelectronic device engineering, in order to establish optoelectronic functionality in this promising class of materials directly on Si. The project is coordinated by Jos Haverkort (TU Eindhoven) and involves the groups of Erik Bakkers (TU Eindhoven), Kirsten Moselund/Heinz Schmid (IBM Zurich Research Labs), Gregor Koblmüller and Jonathan Finley (WSI-TUM), Silvana Botti (Univ. Jena), and Laetitia Vincent (Univ. Paris Saclay). Our role at WSI will contribute particularly to the integration on Si-photonic waveguides and circuits, and development of advanced pump-probe absorption and photocurrent measurement platforms.






Ultrathin InAs nanowires on silicon realized with observations of strong 1D subband characteristics
Oct 1 2020

[research]

The family of small-gap III-V semiconductor nanowires, such as InAs NWs, is a promising class of materials for applications in nanoelectronics, topological quantum computation, and energy harvesting. Especially, their one-dimensional (1D) properties in the ultra-small size limit and carrier transport via 1D-subbands are attractive features for downscaled transistors in CMOS and tunneling FETs, ballistic transistors, nano-thermoelectric devices, as well as hot-carrier type solar cells. Realizing ultrathin InAs NWs in the required CMOS-compatible forms on silicon without the use of foreign catalysts has remained, however, a major challenge till now.

In a recent report published in Nanoscale (2020) by F. del Giudice, et al., we developed two catalyst-free and monolithic growth schemes of ultrathin InAs NWs directly on Si with diameters scaled to the sub-20 nm regime, where strong quantum confinement effects occur. In particular, we report a novel reverse-reaction growth (RRG) method by which the NWs are thinned via in situ thermal decomposition to diameters close to 10 nm. To prove the anticipated confinement behavior, we directly probed the 1D-subband structure by low-temperature measurements in NW-FET devices. We clearly confirmed 1D-conductance steps, and provided further evidence of single and double degenerate states arising from the hexagonal symmetry of the NWs. 


Online Link






A historic day in the MBE lab
Sep 1 2020

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Today we proudly celebrate a historic milestone in our E24 MBE laboratory dedicated to the growth of high-purity III-V semiconductor nano- and quantum materials. As of this week, we have produced at our Veeco GEN-II MBE system more than one thousand sample growths in a continuous, uninterrupted growth campaign lasting for more than 5 years without any mandatory maintenance. MBE is known as a delicate and heavily operated epitaxy system that typically calls for maintenance from time to time with extended down-times due to wear and tear, replacement of evaporation materials, failures in operation, etc.

 

The present achievement is therefore a testimony to the excellent handling of the equipment, the careful and collegial operation by a large number of operators (PhD students, postdocs), and the very skilled and tireless system supervision by our expert technician Hubert Riedl. To eternalize this landmark, we synthesized a special ceremonial sample (EPI-1000) consisting of arrays of InAs nanowires on silicon which as a whole represent the WSI logo in an appealing colorful tone. The vivid colors produced on the wafer surface are a result of resonant light trapping properties of regular nanowire arrays with different periodicities, which was realized by combining top-down electron beam lithography (substrate pre-patterning) and successive bottom-up high-yield growth of InAs nanowires (see also our recent article in Nanotechnology 2019). The ceremonial sample is now open for display and can be viewed in the showcase cabinet in the WSI entrance area by all our visitors coming to WSI.






New PhD student Hyowon Jeong joins our team via the ERC project QUANtIC
May 10 2020

[research]

We are happy to welcome Hyowon Jeong as a new PhD student candidate to our Semiconductor Quantum Nanomaterials Group. Hyowon recently completed his M.Sc. studies in Physics at the Ludwig-Maximilians-University Munich where he conducted research in his final thesis on optical spectroscopy of perovskite nanocrystals.


Hyowon will particularly strengthen our efforts in realizing site-selectively integrated single photon sources in nanowire waveguides that form part of the ERC Consolidator project “Quantum Nanowire Integrated Photonic Circuits (QUANtIC)”. He will engage substantially in state-of-the art nanofabrication and advanced optical spectroscopy, collaborating closely with all of our team members.

 








New DFG-grant on collaborative project between TU Darmstadt and our SQNM group
Apr 1 2020

[research]

In a recent application to the German Research Foundation (DFG) a collaborative project on Terahertz (THz)-based sources and receivers between PIs at TU Darmstadt (Prof. S. Preu) and WSI-TUM (PD G. Koblmüller, KO-4005/8-1) was successfully granted. The project entitled “Rare-earth: Photoconductors for Terahertz generation and detection” aims to build upon the emerging ErAs:In(Al)GaAs based photoconductors that exhibit outstanding performance in terms of peak dynamic range in both continuous-wave and pulsed operation, bandwidth and reduced carrier lifetimes, as recently recognized at TU Darmstadt. In the present project, new photoconductor layer designs as well as tuning of rare-earth ErAs/Sb-precipitates in In(Al,Ga)As-based materials will be investigated in our group at WSI-TUM to further increase resistance and carrier mobility in these active device structures. We are very excited to start this project with our collaboration partners at TU Darmstadt, and hope to push research of 1550-nm compatible photomixers to new performance levels needed for realization of THz source and receiver applications.






Long-desired n-type conductive behavior realized in MBE-grown Si-doped GaAs nanowires
Feb 6 2020

[research]

Control of carrier conductivity by doping is essential for many semiconductor devices, including nanowire (NW) based electronic and optoelectronic devices such as light emitting diodes or lasers. Surprisingly, the realization of n-type conduction in directly bottom-up grown Si-doped GaAs NWs by molecular beam epitaxy (MBE) has remained a long-standing challenge. This is because in common vapor-liquid-solid growth of GaAs NWs, the amphoteric behavior of Si dopants induces p-type conduction instead of n-type conduction.


In a recent report just published in Applied Physics Letters (2020) by D. Ruhstorfer, et al. we developed a completely catalyst-free selective area MBE growth process which allows the realization of n-type behavior under Si doping for the first time. Correlative studies using Raman scattering and single-NW micro-photoluminescence experiments confirmed the n-type nature of the Si-doped GaAs NWs, e.g. by signatures of dominant Si(Ga) local vibrational Raman modes and distinct band filling effects. We also delineated conditions under which excessive Si doping may lead to compensation due to increased Si dopant-point defect complexes.


Online Link






Demonstration of advanced nanowire thermoelectric exploiting quantum confined core-shell structure
Jan 13 2020

[research]

Semiconductor nanowires (NWs) hold great potential in advanced thermoelectrics due to their reduced dimensions and low-dimensional electronic character. In particular, small NW diameters are expected to enhance phonon scattering necessary to suppress thermal conductivity, while 1D-like charge carrier channels in NWs with high carrier mobility may lead to enhanced thermopower.


However, unfavorable links between electrical and thermal conductivity in state-of-the-art unpassivated NWs, have so far prevented the full exploitation of these unique advantages. This has encouraged us to develop a new promising model system based on a sophisticated core-shell NW heterostructure. As just published in Advanced Materials (2020) by S. Fust, et al. ["Quantum confinement enhanced thermoelectric properties in modulation-doped GaAs-AlGaAs core-shell nanowires"] we proposed a surface-passivated 1D-quantum confined NW thermoelectric 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. In particular, high-mobility modulation-doped GaAs/AlGaAs core-shell NWs with thin (sub-40 nm) GaAs NW core channels were employed in thermoelectric device test structures, where the electrical and thermoelectric transport was characterized on the same exact 1D-channel. 1D-subband transport was verified by a discrete stepwise increase in the conductance, which coincided with strong oscillations in the corresponding Seebeck voltage. Peak Seebeck coefficients as high as ~65-85 µV/K were observed for the lowest subbands, resulting in equivalent thermopower of 60 µW/mK2 (S2G ~0.06 pW/K2) within a single subband. As probed by Raman spectroscopy, these core-shell NW heterostructures also exhibit remarkably low thermal conductivity (~ 3 W/m.K), about one order of magnitude lower than state-of-the-art unpassivated GaAs NWs.


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