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
(2021 - 2022)



Origins of array in-/homogeneity in bottom-up grown GaAs nanowires revealed
Nov 22 2022

[research]

The synthesis of entirely catalyst-free GaAs nanowires (NW) using state-of-the-art epitaxial methods, such as molecular beam epitaxy, has remained a notoriously difficult task. Despite the possibilities in realizing position-controlled, selective area growth, typical NW-arrays exhibit large size and shape inhomogeneities and even nucleation events that do not turn into successful nanowire growth. This obviously poses large limitations for scalable NW-array devices, such as solar cells or bottom-up photonic crystal cavities, where perfect homogeneity matters.


In a recent article, published in the Journal of Applied Physics (2022), we succeeded in tracking the origins of such inhomogeneity issues, and further proposed modifications to the growth kinetics processes to solve them. Essentially, we found that the formation rate of rotational twins, which are specific crystalline defects stabilizing the growth of GaAs NWs, defines the key parameter deciding if NW-arrays turn out homogeneous or inhomogeneous. By comparing growth of undoped with Si-doped GaAs NWs, nearly 3-fold increased twin formation probabilities were found for the doped case, proving quint-essential for excellent NW-array homogeneity with close to 100% NW growth yield. To fully describe this twin-enhanced growth process as a function of doping, complementary Monte-Carlo simulations were performed to reproduce the experimentally observed size and shape distributions in large-scale arrays.






Holistic approach developed for exploring elastically strained III-V nanowire quantum heterostructures
Nov 15 2022

[research]

III-V semiconductor nanowire (NW) heterostructures have become promising materials to integrate high-quality, optically active regions, such as quantum wells, with small footprint onto technologically relevant, e.g. silicon (Si), platforms. Despite much progress, the practical limits of integration, and in particular of strain accomodation in quantum wells within such 3D-structured NW arrays, are far from being fully understood. This is because correlations of the precise microstructure, strain, and the resulting emission characteristics have remained elusive on the single-object level.

Now, in an extensive collaboration with Northwestern University, doctoral student P. Schmiedeke (WSI-TUM) together with M. O. Hill (Northwestern) developed a unique approach to correlating morphology, strain, defects, and emission of InGaAs/GaAs based NW quantum wells to understand the limits of elastic strain accomodation specific to their geometry. As just reported in the journal ACS Nano (2022), the two doctoral students realized non-destructive, full 3D Bragg coherent diffraction imaging (BCDI) of intact quantum wells directly on vertically integrated nanowires on Si, to enable immediate correlation with photoluminescence properties. Key findings addressed critical thickness evaluation, the formation of strained and partially relaxed regions, and their roles on the emission characteristics via experiment and modelling. Such holistic approach allows to develop predictive models that enable the design of new compact, nanoscale integrated light sources.






Young Scientist Award for Hyowon Jeong
Oct 10 2022

[research] Hyowon Jeong, PhD candidate in the Semiconductor Quantum Nanomaterials Group, was recently awarded the Young Scientist Award at the Conference of Korean Scientist and Engineers Association that took place in Essen, Germany from October 07-09, 2022. The Annual Conference, which takes place every Fall, covers a broad range of research and engineering topics across various disciplines and hosts about more than 200 participants from all research areas.

 

The awardee received the prize for his contribution entitled “Heterogeneous III-V Nanowire Quantum Emitters on Silicon Photonic Circuits”. In this work, Hyowon presented recent developments towards on-chip quantum dot emitters in nanowire waveguides deterministically integrated onto Si photonic platform. Being part of a research team focusing on numerical modelling, epitaxial growth and extensive structural and optical characterization through the ERC Consolidator project QUANtIC, Hyowon has made excellent progress in the understanding and optimization of InGaAs/GaAs(Sb) based nanowire emitters. This work defines relevant milestones in the realization of advanced light sources on quantum integrated photonic circuits for future quantum communication applications.






Unexpected enhancements in GaAs nanowire growth discovered by Sb self-surfactants
Aug 19 2022

[research]

The use of surfactants has become a powerful means to intentionally modify growth dynamics and resulting structural and electronic properties of many materials. For GaAs nanowires (NW), surfactant mediated growth was so far investigated only under traditional vapor-liquid-solid processes – primarily by use of antimony (Sb), which however rather inhibited growth.


Surprising differences were now found by turning to unconventional vapor-solid growth methods, where the presence of Sb led to strong enhancements in the NW growth dynamics. In a report just published in Applied Physics Letters (2022), Dr. Akhil Ajay and co-workers recognized that miniscule addition of Sb (1-2%) resulted in a hitherto unexpected self-surfactant effect, further yielding very high aspect ratio NWs with high homogeneity. The presence of Sb proved also beneficial for the microstructure, by overcoming the heavy polytype intermixing of different crystal phases commonly seen in Sb-free GaAs NWs. The modified microstructure was further reflected in the observation of single-peak emission characteristics and few-ns long carrier lifetimes probed by steady-state and time-resolved photoluminescence spectroscopy. Our findings will serve the development of high-quality GaAs(Sb) NW arrays with very high aspect ratios for NW photonic applications.






Recognition for PhD Student Andreas Thurn at CSW-2022
Jun 23 2022

[research]

Andreas Thurn, PhD candidate at the Chair of Semiconductor Nanostructures and Quantum Systems, has once more been recognized for his research work on ultrafast spectroscopy of GaAs nanowire lasers. He was awarded the CSW-2022 "Best Student Paper Award - Honorable Mention" at the recent Compound Semiconductor Week 2022 in Ann Arbor, Michigan, USA.


The awardee received the recognition for his oral presentation entitled "Self-induced ultrafast electron-hole plasma temperature oscillations in GaAs-based nanowire lasers", presented in Session "Intersubband and Nanoscale Lasers". This work is based on a collaboration between groups at the WSI-TUM, TU Berlin, Cardiff University and Sandia National Laboratories, and aims to probe and understand hitherto unobserved carrier temperature oscillations in such novel nanolasers by combining both experiment with different numerical modelling approaches.









New PhD students Steffen Meder and Jona Zöllner join our team
Feb 15 2022

[research]

We are happy to welcome Steffen Meder and Jona Zöllner as new PhD student candidates to our Semiconductor Quantum Nanomaterials Group. Both Steffen and Jona completed their recent M.Sc. studies in Physics at TUM, where through their final thesis projects they obtained already first hands-on experience in fabrication, optical and transport spectroscopy of advanced semiconductor nanomaterials.


Steffen and Jona will particularly strengthen our efforts in developing new Si-integrated mid-infrared (MIR) coherent light sources from semiconductor nanowire heterostructures, via the EU-based FET Open "OptoSilicon" project. They will engange substantially in state-of-the-art nanofabrication and advanced optical spectroscopy, collaborating closely with all of our team members.






Pioneering nanowire-based quantum emitters for efficient quantum photonic integrated circuits
Dec 18 2021

[research]

Quantum photonic integrated circuits (QPIC) have emerged as a powerful platform to incorporate quantum optical elements on a single chip for driving quantum technological applications in secure data communication and distributed quantum computing. Practical QPICs pose, however, many challenges, most critically the need for arrays of deterministic, heterogeneously integrated, efficient photon sources, such as quantum dots (QD) that can generate single or entangled photon pairs on demand.


In a recent report just published in Optics Express (2021), we proposed a new design concept of a monolithically integrated, vertical-cavity nanowire-based quantum emitter (QD) that offers highly efficient coupling to Si-based QPICs. Using finite-difference time domain (FDTD) simulations, lead author Nitin Mukhundhan designed a broadband QD emitter with a central wavelength of 1.3 µm that is embedded in a vertical-cavity GaAs NW directly on a Si-ridge waveguide (WG). Hereby, key geometrical parameters of the NW and Si-WG dimensions were tuned to explore the coupling mechanisms to the NW and Si-WG modes. Interestingly, Fabry-Perot resonances were observed in the Purcell enhancement of the QD emission in the finite NW cavity that govern the outcoupled power into the Si-WG modes. With an optimized geometry that places the QD emitter in a finite NW in close proximity to the Si-WG, we were able to achieve peak out-coupling efficiencies as high as 80%.


The interesting new results will soon be also presented at the SPIE-Photonics West 2022 Conference in San Francisco by our lead author Nitin Mukhundan.






e-conversion funds new tandem project on atomically-thin 2D-nitride materials
Dec 1 2021

[research]

Group-III nitride materials are well known for their important roles in energy-efficient solid-state lighting, high-power electronics and light-harvesting applications, because of their various unique properties. What makes these classes of materials now even more fascinating is that recent predictions propose new and enhanced properties in the limit of only few atomic layers, where strong quantum confinement and excitonic effects come into play. Giant absorption properties along with huge tunability of exciton binding energies and carrier lifetimes are to be expected, but experimental demonstrations are largely missing.

In a recent open call for new tandem-projects, the Cluster of Excellence e-conversion granted our successful project application on the proposed topic of "Exploring atomically thin 2D-nitride materials for solid-state lighting and light harvesting applications". The project runs over the next three years (2022-2025) and involves both experimental groups (G. Koblmüller / E. Zallo at WSI) and a computational materials group (H. Ebert, LMU Chemistry). Hereby, our goals are to use first-principles calculations to guide the development of ultra-confined 2D-nitride materials at the single-monolayer or few-layer level. Using our newly established MBE-Analytics Cluster we will synthesize the materials in the purest possible environment, and further establish insights into structure-property relationships correlating electronic properties with photo-physical effects and carrier dynamics. The project will be a rewarding experience for a new Ph.D. student to be funded by e-conversion. Applications for hiring are now open!






Dr. Hamidreza Esmaielpour joins as new research associate via Marie-Curie fellowship program
Nov 12 2021

[research]

We warmly welcome Dr. Hamidreza Esmaielpour as a newly appointed Postdoctoral Research Associate to the Semiconductor Quantum Nanomaterials Group. Hamidreza was recently awarded the prestigious Marie Sklodowska Curie Action (MSCA) Fellowship via the EuroTechPostdoc2-Programme, which allows him to pursue research at TUM in conjunction with the Ècole Polytechnique de Paris (L'X) acting as Co-Host University.


Hamidreza brings large expertise in the field of hot-carrier solar cells (HCSC), their device fabrication and advanced optical spectroscopy and modelling. In his project, he will combine his research strengths with our group's focus on nanowire systems to realize new forms of high-efficiency, low-dimensional HCSCs over the coming years.


Hamidreza has recently completed his PhD studies in Solid State Physics at the University of Oklahoma, USA with academic distinction, as recognized by the Nielsen Prize Award. Before joining our group at TUM, he held an appointment as Postdoctoral Fellow at the CNRS, Institut Photovoltaique d'Ille-de-France (IPVF) in Palaiseau, France.






Novel InAs-AlAsSb core-shell nanowire system demonstrated
Nov 8 2021

[research]

Low-bandgap InAs nanowires (NW) have drawn large attention in recent years due to potential applications in electronics, photonics and mid-infrared (MIR) optoelectronics. For these, functional core-multishell heterostructures are much desired, but such device structures suffered so far from the lack of proper lattice-matched shell systems.


Now, in a report just published in Applied Physics Letters (2021), Fabio del Giudice and co-workers pioneered a novel InAs-AlAsSb core-shell system to solve previous limitations. As a member of the 6.1A-family of materials, the InAs-AlAsSb system exploits not only close lattice-matching conditions, but also exceptional conduction band offsets, electron confinement, and tunability of band line-up, with no analogue amongst all III-V semiconductors. In our work, we also developed direct monolithic integration of composition-tunable InAs-AlAsSb NW arrays on silicon, and explored the effects of shell composition on the morphological, strain and MIR emission properties via correlated Raman and PL spectroscopy. Based on strain and electronic structure simulations, we further emphasized the unique tunability between type-I and type-II band line-up at the core-shell interface. These findings will enable large versatility of emerging nanoscale Si-photonic and optoelectronic devices.






Get-together after major lab reconstructions
Oct 27 2021

[research]

The year 2021 has been coined by substantial laboratory reconstructions and overhauling / maintenance activities. Not only did we undergo a major MBE opening campaign to introduce new materials and system components, after 1154 consecutive sample growths over the last 6 years. Also, both our heavily used low-noise electrical and magneto-transport setup and the mid-IR confocal PL laboratory had to move to new locations. Thanks to the tireless and patient efforts made by all team members, we finally came back in operation in the last few weeks.

To celebrate the completed lab reconstructions, the SQNM group held a joyful get-together with drinks and food under Munich's late October sunshine. We took the opportunity to take a new group photo that was long overdue, and toasted on new research endeavors to come.






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

[research]

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

[research]

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








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