Sep 17 2023

III-V semiconductor nanowire (NW) lasers hold large potential for telecom-band optical data communication and sensing applications due to their ultracompact size and direct site-selective integration on silicon. Recent obervations also point to ultrafast lasing dynamics, with high-frequency oscillations up to >250 GHz. The microscopy mechanisms and dynamic processes that govern such emission, and thus determine their potential for ultrafast opto-electronic devices, have remained fully unexplored.

In a recent report published in Physical Review Applied (2023), A. Thurn, et al. combined non-resonant degenerate pump-probe spectroscopy on single GaAs-AlGaAs core-shell NW-lasers, together with numerical simulations using k-resolved Bloch equation model and quantum statistical models, to unveil fundamentally new insights into the non-equilibrium dynamics of these nanolasers. Supported by groups at TU Berlin, Cardiff University and Sandia National Laboratories, these studies evidenced that the ultrafast intensity and phase oscillations of an optically pumped NW-laser are a result of competing carrier heating and cooling during lasing operation. Such characteristics are feasible thanks to the miniaturized dimensions of these lasers, which sets important foundations for engineering the temporal emission dynamics in future heterogeneously integrated nanolasers on silicon.

Jul 13 2023

Nitin Srirang Mukhundhan, former Master student in the Semiconductor Quantum Nanomaterials Group, was recently awarded the 2022 Best Master Thesis Award at the Walter Schottky Institute. The award acknowledges outstanding thesis work submitted each year by master students enrolled within any of the several research groups at TUM-WSI.

The awardee received the prize for his thesis entitled "Monolithic Integration of Quantum Emitters on Photonic Integrated Circuits". In this work, funded by the ERC-project QUANtIC, Nitin developed a comprehensive numerical approach to explore the coupling of a deterministic quantum dot (QD) emitter embedded in a vertical-cavity photonic wire to a silicon-on-insulator (SOI) quantum photonic integrated circuit (QPIC). By optimizing the photonic wire cavity, the placement of the QD emitter, and key geometrical parameters of the underlying Si-ridge waveguide on SOI, he was able to demonstrate coupling efficiencies in excess of 80% at telecom-band wavelengths (1.3 µm) via exploitation of strong Purcell enhancements. To prove his concept study, Nitin also preformed first experiments in the fabrication of optimized SOI-ridge waveguides and the monolithic growth of InGaAs/GaAs-based nanowire-QDs. This work sets important foundations for integrating solid-state based single photon sources onto scalable QPICs in future quantum communication applications.

May 26 2023

We warmly congratulate Paul for his very impressive doctoral thesis defense, that was unanimously awarded with highest academic honors "summa cum laude". Paul's thesis work aimed at the development of integrated III-V semiconductor nanowire (NW) lasers on silicon for on-chip integrated coherent light sources in Si photonic applications. Herein, emission wavelength control towards the technologically relevant telecom-band (1.3-1.55 µm) is of key importance, which poses immense challenges in traditional III-As semiconductor systems.

By exploring the vast strain-engineering potentials of III-V NW-systems, Paul has entered a promising terrain in materials engineering that was hitherto untouched in NW-lasers. Starting with rigorous modelling work, he proposed new forms of strain-compensated GaAs- and GaAsSb-based core-shell quantum well NW-lasers to meet the desired emission control, which he then demonstrated in experiment, by combining intense materials growth efforts (using MBE) with novel analytical and spectroscopic methods in the most uncompromising way. In a dissertation of nearly 350 pages, containing six individual chapters of novel results - rich in experiment and modelling alike - Paul has demonstrated highly creative research that led to many seminal findings in the growth and laser physics of III-V NWs. Besides his research work, he managed the IT infrastructure of the entire WSI, which is deeply appreciated by all institute members. We therefore not only congratulate him for his great achievements, but thank him also for his continuous engagements for the common good. We all wish Paul many successes in his next endeavors.

May 12 2023

The SQNM group warmly congratulates Sergej for his successful doctoral thesis defense. During his past few years, Sergej developed new concepts for nano-thermoelectric materials to break the unfavorable interdependencies of thermoelectric parameters observed in typical bulk-like materials. By exploring high-mobility core-shell nanowire systems with 1D-electronic properties, he proposed to decouple Seebeck coefficient and thermopower from carrier density by utilizing the enhanced 1D density of states in ultrathin nanowire core channels.

In a rigorous approach, Sergej combined thermoelectric characterization on gated GaAs-AlGaAs core-shell nanowire field-effect transistors (NW-FETs) with micro-Raman spectroscopy for thermal conductivity measurements. Hereby, he demonstrated strong enhancements in the thermopower correlated with the 1D-subband structure, while further reducing thermal conductivity via phonon scattering at the core-shell interfaces. Ultimately, he applied this concept also to prototype InAs-AlAsSb NW-FETs, which are even more promising in terms of material and transport properties. After all this pioneering work at WSI, we wish Sergej all the best for his future professional career.

May 5 2023

Various applications in nano- and quantum technologies require functional materials scaled to the smallest possible dimensions to host strong quantum confinement effects or to allow unprecedented strain engineering capabilities. Semiconductor nanowires (NW) are such ultra-scalable materials due to their intrinsic one-dimensional (1D) geometry, where both the direct bottom-up growth and reverse-reaction, i.e., crystal decomposition, provide powerful means to tune 1D-dimensions to the few-nm scale. To date, relatively little attention has been paid to the reverse, decomposition process, despite its larger flexibility in creating ultrathin NWs. Especially, understanding of microscopic and atomistic processes during crystal decomposition is key to tuning shape and size, but demands sophisticated experiments, ideally in the native in-situ environment.

In a recent work, just published in Nanoscale Advances (2023), doctoral candidate P. Schmiedeke performed in-situ transmission electron microscopy (in-situ TEM) experiments at the Universite Paris-Saclay (NanoMax-TEMPOS) to answer some of the most pressing questions of decomposition dynamics of III-V semiconductor NWs. In particular, he investigated the decomposition kinetics of clean, ultrathin GaAs NWs and the role of distinctly different crystal polytype (zincblende vs. wurtzite) in real-time and on the atomic scale. The whole process, from the NW growth to the decomposition, was conducted in-situ without breaking vacuum to maintian pristine crystal surfaces. From these studies we found that radial decomposition occurs much faster for zincblende compared to wurtzite phase NWs, due to the development of nano-faceted sidewall morphology and sublimation along the entire NW length. In contrast, wurtzite NWs form single-faceted, vertical sidewalls with decomposition proceeding only via step-flow mechanism from the NW tip, but with competing axial rates exceeding by far the radial ones. This new knowledge allows us to accurately fine-tune NW dimensions selectively based on the underlying crystal structure.

Mar 16 2023

We congratulate Fabio to pass his recent doctoral thesis defense. Fabio's work aimed at developing advanced InAs-based core-shell nanowire systems for applications in mid-IR optoelectronics and hot-carrier harvesting. Prior to his thesis, typical InAs-based nanowire heterostructures were hampered by large lattice mismatch issues, preventing the realization of functional devices where extended active regions via core-shell design were desired.

Fabio explored therefore a new class of closely lattice-matched InAs-AlAsSb core-shell NW heterostructures, and further tailored their electronic properties by controlling strain and quantum confinement effects, making several key contributions to this  active research field. He was a master in molecular beam epitaxy (or as he called himself magically 'the wizard' - see picture) - a method by which he produced a large set of interesting materials to support his studies. We warmly congratulate Fabio, and wish him every success in his future endeavors.

Jan 20 2023

Ternary GaAsSb semiconductor nanowires (NW) offer many unique physical properties that make them attractive for integrated nano-electronic and -photonic applications. Mainstream synthesis methods, such as the common vapor-liquid-solid (VLS) growth process, have however hindered their full exploitation, due to morphological instabilities, phase segregation and inhibited growth arising from the VLS growth mode.

Only very recently, alternative methods were proposed to create GaAsSb NW with strongly altered growth dynamics using selective area epitaxy - yet, a full understanding of the key principles has remained elusive. Fundamental advances are now reported in the most recent work by H. W. Jeong, et al., Small (2023), which identifies the role of composition and the nature of the Sb-mediated growth surface on the resulting growth dynamics and physical properties of catalyst-free GaAsSb NW. Interestingly, depending on composition and the Sb-As exchange reaction both surfactant and anti-surfactant action is found leading to growth acceleration and deceleration behaviour. Microstructural analysis further revealed that within even narrow compositional range (1.5-6% Sb) the phase-pure crystal domain lengths increased by nearly an order of magnitude, showing much greater tunability than in VLS-type NW. The work also demonstrates 3-fold increases in exciton lifetimes upon increasing the phase-purity in the NW. The results therefore show important implications for utilizing the GaAsSb NW in nano-photonic and optoelectronic devices.

Dec 22 2022

We welcome three new PhD student candidates joining our SQNM research group at the turn of the year. Several former candidates are currently finishing up, and our group is eager to expand research activities along several core topics. First, Genet Hirpessa, a former M.Sc. student in Electrical Engineering at the University of Bologna will develop new nanothermoelectric materials and devices via a DFG-based project “Advanced thermoelectric properties in 1D quantum-confined core-shell nanowire heterostructures” (KO-4005/10-1). Further, Cem Doganlar with a M.Sc. degree in Materials Science from the Vienna University of Technology (TU Wien) will pursue monolithically integrated nanolaser sources on silicon in his doctoral research. His project will be supported by the ERC Consolidator grant QUANtIC. Finally, Abhilash Uhle, a recent M.Sc. student in Materials Science through the MaMaSelf program, will work towards novel 2D-layered group-III nitride materials and heterostructures, supported by the Cluster of Excellence e-conversion and in close collaboration with the MWM group. All new candidates will work extensively in MBE growth, state-of-the-art nanofabrication, and advanced optical and electrical transport spectroscopy. We wish them best of luck in their research career.





Dec 18 2022

In a recent call for new focus projects, the Cluster of Excellence e-conversion granted our proposed project entitled “Ultrafast hot carrier harvesting concepts in sustainable 2D-nitride layered materials”. The aim of this funding call was to establish thematically oriented consortia projects in preparation for the next funding round of e-conversion 2.0. Our selected focus project builds upon a consortium that brings together four PIs with key expertise in the fields of ultrafast spectroscopy (A. Holleitner / R. Kienberger, both TUM), computational materials science (H. Ebert, LMU), and materials engineering of advanced energy materials (G. Koblmüller, TUM). Via the employment of several new doctoral student positions, the team will work together closely with associated expert researchers within e-conversion, involving H. Esmaielpour, H. Iglev, and E. Zallo (all at TUM).

The goal of this consortium is to exploit hot carrier dynamics in suitable, sustainable energy materials for the development of ultrahigh-efficiency solar cells. In particular, hot carrier solar cells are envisioned as such exceptional class of photovoltaic cells, offering the potential for efficiencies well beyond the Shockley-Queisser limit. Thereby, efficiency increases are predicted by exploiting phonon engineering practices to prevent carrier thermalization of photo-absorbed carriers, either by extraction of hot carriers via energy selective contacts, or multi-carrier generation. The materials of choice will be nanostructured group-III nitride materials, since these robust materials offer a great combination of suitable properties, such as wide tunable bandgaps across the entire solar spectrum, excellent absorption coefficients, as well as large phonon bandgaps which help to suppress thermalization of hot carriers. We look forward to this exciting research project over the next several years thanks to the excellent support by e-conversion.

Nov 22 2022

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.

Nov 15 2022

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.

Oct 10 2022

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.

Aug 19 2022

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.

Jun 23 2022

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.

Feb 15 2022

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.