Jun 21 2024

III-V semiconductor nanowires (NW) form an integral part of future electronic and optoelectronic devices, such as NW-field effect transistors (NW-FET), light emitting devices and solar cells. In these devices accurate doping control is mandatory to tailor the carrier transport; and, indeed, the last decade has seen intense research into doping studies of III-V NWs, especially of GaAs-based NWs where reliable n-type doping has remained challenging. This is because Si impurities, the most common n-type dopants in III-V materials, are amphoteric in nature and may cause undesired p-type conductivity, as often seen in GaAs-based NWs.

Recently. we have shown that this situation can be mitigated by employing proper catalyst-free growth approaches; however, the dynamics of Si dopant incorporation and the spatially controlled electrical activity have remained unknown. In a new study, T. Schreitmüller and co-workers were able to address this missing link by correlating the spatial distribution of Si dopants and electrical transport characteristics on a local scale, using a combination of atom probe tomography, Raman scattering, and single NW-FET device characterization. In this collaborative work, that involved partners at Northwestern University and PDI Berlin and as just published in Physical Review Materials (2024), it was also found that Si segregation at the NW surface causes a thin parasitic p-type layer. Successful wet chemical etching methods were further developed to recover the underlying n-type conduction of the bulk NW. These results are instrumental in designing various future III-V NW-device architectures.

Jun 14 2024

Boosting the efficiency of photovoltaic solar cells beyond the theoretical limit requires suppressing one of the main energy loss mechanisms in these devices, which is the thermalization of hot carriers. Understanding the origin of hot carrier relaxation in solar cells and inhibiting thermalizaiton pathways is essential to realize efficient hot carrier solar cells. So far, it has been speculated that non-radiative processes, such as Auger or Shockley-Read-Hall recombination could be strong and conversely affect hot carrier effects in nanostructured materials, such as 1D-nanowires - but a clear experimental verification has been missing.

In a recent article, published in Physical Review B (2024), H. Esmaielpour and N. Isaev performed detailed optical investigations of the hot carrier temperature and recombination dynamics of InGaAs-InAlAs core-shell nanowires using steady-state and time-resolved photoluminescence (TRPL) spectroscopy. The analysis evidences that at low lattice temperature, strong Auger heating processes are beneficial in creating substantial hot carrier effects. At higher temperature, hot carrier relaxation rates are found to increase due to enhanced Shockley-Read-Hall and surface recombination, while other effects mediated by phonon and impurity scattering were also quantitatively captured. The findings presented in this work provide a clear pathway for optimizing the hot carrier effect in 1D-nanostructures under the influence of the various non-radiative processes and scattering dynamics at play.

May 3 2024

We are happy to welcome Benjamin Haubmann as a new PhD student candidate to our Semiconductor Quantum Nanomaterials Group. Benjamin has recently completed his M.Sc. studies in Physics at TUM, where in his final thesis project he obtained already first hands-on experience in simulation, fabrication and optical spectroscopy of Si-integrated nanolasers emitting at mid-infrared wavelengths.

Benjamin will continue in the direction of on-chip integrated nanolasers and particularly strengthen our efforts in developing new low-threshold telecom-band nanolasers from semiconductor nanowire heterostructures. He will engange substantially in developing new material combinations through molecular beam epitaxial approaches, and explore the functional nanolasers in on-chip integrated settings using advanced optical spectroscopy methods.

Mar 8 2024

Emerging applications in silicon (Si) photonics and telecom-band optical data communication are increasingly targeting at monolithically integrated nanoscale light sources, including III-V nanowire (NW) lasers that emit at Si-transparent wavelengths (>1.1 µm). Conventional III-V NW-lasers are mainly based on GaAs or InP materials, and therefore have unsuitable band-gaps to enable lasing emission in the desired Si-transparency region. Only through the integration of quantum heterostructures the emission of the active gain media in GaAs and InP NWs can be tailored towards the telecom-band, but competing strain effects make this a very tedious task.

An alternative and more straightforward approach is now demonstrated by P. Schmiedeke, et al., in Applied Physics Letters (2024), by developing the first, true ternary single NW-lasers with band-gap tuned to energies below the cutoff energy of Si. In particular, ternary GaAsSb NWs were realized with proper resonator geometries and high radiative efficiencies using suitable surface passivation, to achieve optically pumped lasing emission from single NW-lasers. Lasing was observed at low-threshold (< 5 µJ/cm2) and even up to elevated temperatures close to room temperature at Si-transparent wavelenghts of 1.1-1.2 µm. The results presented in this work also point out future directions in further reducing lasing threshold to make this new generation of nanolasers excellent candidates for integrated on-chip coherent light sources on Si photonic platform. These research highlights were recently also selected by Semiconductor Today magazine in a dedicated News article: https://semiconductor-today.com/news_items/2024/mar/tum-010324.shtml

Jan 14 2024

The unique one-dimensional optical cavity structure of semiconductor nanowires (NW) opens a new paradigm for efficient optical confinement and low-loss waveguides in nanophotonic device applications. In particular, combining such photonic waveguides with heterogeneously integrated quantum light sources offers a highly desired architecture for deterministic and scalable quantum integrated photonic circuits (QPIC). However, growth of deterministic active emitters such as e.g. InGaAs quantum dots embedded axially within III-V NWs has remained challenging and requires well selected growth schemes.

Exploiting a unique selective area epitaxy process, doctoral student H. W. Jeong and co-workers have now developed controlled growth of axial InGaAs/GaAs NW heterostructures directly on silicon (Si), with size dimensions of the InGaAs emitters approaching the quantum-confined regime. This work just published in ACS Applied Nano Materials illustrates the physics of heterostructure formation and facet-dependent shape evolution and their effects on the optical emission properties of embedded InGaAs emitters with varying In-contents up to ca. 20%. Besides showing correlated micro-PL and cathodoluminescence (CL) data, advanced 3D STEM tomographic imaging was employed for the first time on such structures to map the full compositional and dimensional nature of the embedded emitter. These insights are expected to drive further advancements in NW-quantum emitters for QPICs as well as integrated Si-photonics, such as high-performance NW-lasers with 0D gain media.

Jan 5 2024

Recent advances in III-V nanowires (NW) have shown promising applications in advanced-concept photovoltaic (PV) solar cells, such as hot carrier solar cells. The 1D geometry of these nanostructures together with spatially confined charge carriers can facilitate the suppression of hot carrier thermalization, which is considered to be one of the main loss mechanisms in PV cells. If hot carrier can be efficiently harvested before cooling, these types of solar cells may overcome the upper theoretical limit of single-junction devices, known as the Shockley-Queisser limit.

An important step towards this goal is to provide fundamental understanding of the role of dimensional properties on the hot carrier effects of the most suitable NW materials. In a recent publication published in the ACS journal Applied Nano Materials, Dr. H. Esmaielpour and co-workers have now demonstrated the strong impact of NW size effects on hot carrier temperature in site-selective InGaAs NWs, which are amongst the perfect candidate materials due to their low thermalization coefficient and low band-gap energy. Using steady-state photoluminescence spectroscopy and modelling, the team could show that decreasing the NW-diameter results in the desired increases in carrier temperature, due to the phonon bottleneck effect and Auger-induced carrier heating. The study further illustrates the competing processes by crystal defects, which scatter hot carriers and thereby lower hot carrier temperature. These findings are instrumental for the further design of efficient hot carrier solar cells and to ultimately enhance the performance and efficiency of such photovoltaic devices.

Nov 23 2023

III-V nanowire (NW) lasers are excellent candidates for miniaturized light sources in on-chip optical interconnects, thanks to their unique sub-wavelength geometry and the direct monolithic integration capabilities on silicon (Si) photonic circuits. Over the years of intense research and optimization of resonator design, surface passivation, and minimization of mirror losses, it became clear that the ultimate performance limits will most likely depend on key intrinsic material properties, such as native- and impurity-induced point defects and their impact on non-radiative recombination.

To interrogate this, we have now demonstrated the impact of impurity-induced point defects on the lasing properties of low-threshold GaAs(Sb)-AlGaAs core-shell NW-lasers, by exploring Si-dopants and their associated vacancy complexes. As reported in a recent article of Advanced Functional Materials (2023), T. Schreitmüller and co-workers used a combination of methods to identify and correlate type and density of Si-induced point defect complexes with the resulting radiative recombination dynamics and lasing threshold behavior of these NW-lasers. Under very large point defect densities, strong non-radiative Shockley-Read-Hall recombination, reduced carrier lifetime, and lowered IQE (internal quantum efficiencies) was found, which was also reflected by nearly an order of magnitude increased lasing threshold at cryogenic temperatures. However, for intermediate to low point defect densities, the lasing threshold was insensitive to these defects, yielding overall very low threshold < 10 µJ/cm2. Most surprisingly, NW-lasers with high vacancy-defect densities were further found to show superior threshold performance at elevated temperatures with much higher temperature stability and room temperature lasing. These unique characteristics were explained by a suppressed Auger recombination, likely due to the acceptor-type states introduced by the vacancy complexes, which points to a very large tolerance of lasing metrics to impurity defects in these classes of nanolasers.

Oct 26 2023

Ternary GaAsSb nanowires (NW) are key materials for long-wavelength, integrated high-speed photonic applications on silicon (Si), such as ultrafast photodetectors, infrared lasers, and optical fiber communications systems. Unfortunately, the growth of such NWs with increased antimony (Sb)-content is notoriously difficult, due to the surfactant nature of Sb that limits Sb incorporation, Ga diffusion and axial growth. In conventional self-catalyzed growth, the situation is particularly severe, where early growth termination, inhomogeneities in morphology and composition have hindered the development of many photonic applications, including GaAsSb NW-lasers.

In a recent article, published in Nanotechnology (2023), P. Schmiedeke and co-workers succeeded in developing a new growth approach that solves the previous limitations. By exploring a unique high-temperature MBE growth procedure, we realized self-catalyzed GaAsSb NWs site-selectively on Si with high aspect-ratio and fully non-tapered morphology under Sb-saturated conditions, where the Sb incorporation is insensitive to any variations in group-III supply. Remarkably, this way, very long (> 7 µm) GaAsSb NWs are achieved with homogeneously high Sb-content (ca. 30%) along the entire NW. The excellent properties of these NWs are also reflected by a completely phase-pure, twin-free zincblende (ZB) crystal structure, and very narrow photoluminescence linewidth (< 15 meV) at wavelengths of 1100-1200 nm. As will be shown in a forthcoming article, these NW materials form the basis of the first GaAsSb NW-lasers on Si.

Oct 6 2023

Semiconductor nanowires (NW) with low bandgap energy are considered as very attractive materials for advanced photoenergy conversion applications, such as hot carrier solar cells (HCSCs). Especially, when the cooling of photoexcited hot carriers in such photovoltaic devices can be slowed down to time scales where extraction is feasible, excess energy can be used and larger efficiencies for light conversion are expected. Developing suitable NW materials and exploring the ultrafast carrier dynamics with unprecedented time-resolution are key to advancing this research field.

In a recent collaborative work between the SQNM-group at WSI and experts in laser- and x-ray phyics at TUM, InAs-based NWs were identified as promising materials for HCSCs due to their high absorption coefficient, high carrier mobility, and large effective electron-to-hole-mass difference. As just published in ACS Applied Energy Materials (2023), the authors reveal key insights into the cooling and recombination dynamics of photoexcited hot carriers in pure and passivated InAs NWs by using ultrafast near-infrared pump-probe spectroscopy. The results show that prevalent Auger heating effects and associated carrier cooling dynamics are strongly dependent on charge-carrier separation and the surface/interface properties induced by the AlAsSb passivation. In addition, a separate charge-carrier population was found in the AlAsSb passivation with surprisingly long lifetime (several nanoseconds), which may further open avenues for core-shell NW multijunction solar cells. This work was supported by the Cluster of Excellence e-conversion.

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.

Feb 23 2023

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.

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.

Jan 5 2023

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.