Walter Schottky Institute
Center for Nanotechnology and Nanomaterials

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


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RECENT & UPCOMING EVENTS
(2019 - 2020)



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 (2019) 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. 






Cover feature in APL in December 2019 issue
Dec 9 2019

[research]

Mapping individual radiative recombination channels at the nanoscale in direct correlation with the underlying structural and compositional properties of semiconductor nanostructures requires unprecedented highly resolved spectroscopy methods. This is particularly a challenge in novel nanomaterials with ultrasmall dimensions down to below 10 nm, where the limited spatial resolution of conventional characterization techniques start to fail.


In a recent collaboration with the group of Prof. Jürgen Christen at the Otto-von-Guericke University Magdeburg, low-temperature cathodoluminescence in a scanning transmission electron microscope (STEM-CL) was developed to probe highly resolved luminescence channels in GaAs nanowire heterostructures. The results published in Applied Physics Letters (2019) illustrate a direct one-by-one correlation between the complex radial structure and the distinct carrier recombination channels in a thin slice of a single GaAs-AlGaAs core-multishell nanowire heterostructure prepared by focused ion beam. The radial luminescence profile is decomposed into four different emission features, associated with the GaAs core and defect luminescence, the emission of a single embedded GaAs quantum well and shell luminescence correlated with allow fluctuations. The appealing nature of the powerful STEM-CL characterization was highlighted by the Editor's choice as cover feature in the Dec. 9 (2019) issue of Applied Physics Letters.
















Akhil Ajay joins as Postdoctoral Research Associate to support ERC project QUANtIC
Nov 15 2019

[research]

We warmly welcome Dr. Akhil Ajay as a newly appointed Postdoctoral Research Associate to the Semiconductor Quantum Nanomaterials Group. Akhil will lead his research efforts towards deterministically integrated single photon sources in nanowire waveguides that form part of the ERC Consolidator project "Quantum Nanowire Integrated Photonic Circuits (QUANtIC)".

Akhil has recently completed his PhD studies in Nanophysics at the Commisariat à l'énergie atomique et aux énergies alternatives (CEA) and Université Grenoble Alpes, France. After a brief research stay at Cardiff University, Wales, he joined our group in the fall of 2019. He brings large expertise in the growth and characterization of nanowire-based quantum heterostructures and devices, and is well prepared to contribute greatly to the exciting ERC project.















TUM-IAS Focus Workshop on "Hot Carrier Dynamics of Advanced Concept Solar Cells"
Oct 30 2019

[research]

The Koblmüller Group has recently hosted the TUM-IAS Focus Workshop on "Hot Carrier Dynamics in Advanced Concept Solar Cells" from October 28-29, 2019 at the TUM Institute for Advanced Study. The workshop formed part of the "Focal Periods 2019" program that aims at promoting interdisciplinary and sustainable collaboration between TUM-IAS Focus Groups and leading scientists worldwide working on various aspects of hot carrier dynamics in low-dimensional semiconductors and their related solar energy conversion devices. A particular emphasis was placed on theoretical and experimental understanding of non-equilibrium carrier relaxation dynamics and multi-exciton generation in a variety of nanostructured materials (quantum wells, dots, wires, perovskites, 2D materials, etc.).


The workshop consisted of 20 keynote talks drawn from leading groups across Europe, US, and Asia-Pacific, as well as local groups at TUM and Ludwig-Maximilians-University Munich, hosted in seven focus sessions. The talks were complemented by a poster session of more than 20 posters presented by graduate student and postdoctoral researchers. For further information please see the workshop website at:


https://www.wsi.tum.de/hot-carrier-dynamics






New insights into optical absorption behavior of composition-tuned InGaAs nanowire arrays
Aug 28 2019

[research]

Semiconductor nanowire arrays are promising architectures for solar-energy harvesting applications due to their unique 3D-structured geometry. To achieve competitive conversion efficiencies, the nanowires must absorb sunlight over a broad range of wavelengths despite occupying only a small fraction of the array‘s volume. InGaAs nanowires are considered as attractive materials, since they offer excellent electronic properties and widely tunable composition.


Using UV-Vis-NIR absorption spectroscopy and correlated modeling, the optical absorption characteristics of InGaAs nanowire arrays were recently explored by systematically tuning alloy composition, nanowire dimensions and array geometry. In a manuscript just published in Nanotechnology (2019), J. Treu, et al. illustrate how nanowire arrays with intermediate diameters and comparatively small area fill factors can lead to enhanced optical absorption beyond the equivalent thin film case. Most interestingly, the studies also show that large variations in alloy composition and electronic band gap have almost no influence on the optical absorption spectra, while minute changes in nanowire dimension (diameter) cause very strong shifts in the absorption resonances. These results provide thereby useful guidelines in optimizing the absorption efficiency of nanowire-based solar cells.


Online Link






Head of Group appointed as PI to Excellence Cluster e-conversion
Jul 22 2019

[research]

Gregor Koblmüller, head of the Semiconductor Quantum Nanomaterials group, was appointed as new PI to the Cluster of Excellence e-conversion in one of the recent executive meetings.  The PI has long-standing collaborative efforts with several members of e-conversion and also supported the cluster from the early proposal phase onwards. Aspects of his research, particularly those related to the creation of innovative nanomaterials and semiconductor based photovoltaic and thermoelectric energy conversion systems, will play a central role in the cluster over the coming years. 


e-conversion is one out of four Clusters of Excellence recently awarded to TUM by the German Research Foundation (DFG, Deutsche Forschungsgemeinschaft). The new cluster focuses on investigations of fundamental mechanisms of various energy conversion processes and aims to develop microscopic concepts for more efficient conversion needed in sustainable energy technologies.


Further information about e-conversion can be found at:


https://www.e-conversion.de/






Master Thesis Award for Paul Schmiedeke
Jul 19 2019

[research]

Paul Schmiedeke, PhD student in the Semiconductor Quantum Nanomaterials Group, was recently awarded together with two other students the 2018 Best Master Thesis Award at the Walter Schottky Institute. The award acknowledges outstanding thesis work submitted per year by master students enrolled within any of the several research groups at TUM-WSI. 


The awardee received the prize for his thesis entitled “Continuous-wave (cw) lasing of GaAs nanowires – Experiments and comprehensive modeling” performed under co-supervision by the Chair of Nanoelectronics, ECE-TUM. In this work, Paul combined sophisticated rate equation models, finite difference time domain simulations and confocal PL spectroscopy to evaluate the intrinsic output power and lasing linewidth behavior in bottom-up grown GaAs nanowire lasers. In addition, he developed a very original nanowire-cavity design and mirror-concept for increasing the facet reflectivities that allowed him to explore lasing characteristics unobscured by extrinsic factors. This work sets important foundations for understanding and controlling technologically relevant cw-lasing in III-V based nanolasers.















Comprehensive study of III-V nanowire lasers coupled to Si waveguides published in J. Appl. Phys.
Jun 26 2019

[research] The recent integration of III-V semiconductor nanowire lasers on silicon waveguides marked a key step towards their usage as coherent light sources for future silicon photonics applications. Unfortunately, the low index contrast between III-Vs and silicon results in a weak modal reflectivity, calling for improved design structures that enable both low-threshold lasing and good in-coupling efficiency into waveguides.

In a recent work published in the Journal of Applied Physics 125, 243102 (2019), J. Bissinger, et al. reported a comprehensive investigation of how the alternating refractive index of a silicon waveguide with a thin SiO2 interlayer can be exploited to substantially improve the reflectivity at the III-V/silicon interface. Using numerical simulations, the authors point out that the modal reflectivity in vertical cavity GaAs-(In,Ga)AlAs based core-shell nanowire lasers can exceed 80%, while maximum waveguide coupling efficiencies of approx. 50% can be realized at a central wavelength of 1300 nm. The representation also applies simplified approximations such that these analyses can be applied to a variety of materials systems and serve as guidelines in the construction of optimised nanowire lasers on silicon-on-insulator for on-chip optical interconnects.


Online Link






New PhD student Tobias Schreitmüller joins efforts on III-V nanowire optoelectronic devices
Apr 15 2019

[research]

We welcome Tobias Schreitmüller as a newly appointed PhD student to the Semiconductor Quantum Nanomaterials group. Tobias will focus in his doctoral thesis project on the development of high-quality p-n junctions and realization of electrically driven monolithically integrated III-V nanowire devices.

Tobias has recently completed his M.Sc. studies in Physics at TU Munich, where he worked in his thesis on "Metal contacts to n-GaAs nanowire shells - towards future electrically driven nanowire lasers". Having gained already valuable insights and great experience in processing and characterization of contacted nanowires, Tobias is well prepared to continue his research towards functional nanowire-based optoelectronic devices. 


















Best Poster Award at EuroMBE-2019 Workshop for Daniel Ruhstorfer
Feb 20 2019

[research]

Daniel Ruhstorfer, PhD student at the Walter Schottky Institute received recently a Best Poster Award at the 20th European Workshop on Molecular Beam Epitaxy (EuroMBE-2019) in Lenggries, Germany (Feb. 17-20, 2019). The EuroMBE Workshop is a biannual meeting of the MBE community in Europe, and hosts a few hundred attendees, including both prominent and young scientists.

The awardee received the prize for his contribution entitled “Vapor-solid selective area molecular beam epitaxy and doping of catalyst-free GaAs nanowires on silicon”. In this work, Daniel reports a new growth mode that enables n-type doping by Si dopants in GaAs nanowires which has hitherto been very difficult in conventional MBE growth processes. These findings present an important milestone in the fabrication of future electrically driven light sources on silicon.





















Congratulations to Thomas Stettner for his successful PhD defense
Feb 18 2019

[research]

Thomas Stettner, graduate student in the Semiconductor Quantum Nanomaterials Group, successfully defended his doctoral thesis before his committee. Thomas is amongst some of the first doctoral candidates to graduate from our group, and set a high standard for subsequent students, being highly praised for the quality of his thesis and presentation. His thesis entitled „GaAs-based nanowire lasers on silicon – growth and optical investigations“ demonstrated for the first time direct coupling of lasing emission in monolithically integrated single GaAs nanowire lasers to Si waveguides on photonic circuits. In addition, his work also explored extensively the use of quantum wells as gain material in coaxial nanowire laser heterostructures, enabling tunability of emission wavelength, threshold gain, amongst other lasing metrics.






Fundamental work on dislocation-mediated effects on thermal transport published in Nature Materials
Jan 29 2019

[research]

Extended defects such as dislocations are common in technologically important III-V semiconductors, and affect heat dissipation, for example, in nitride-based high-power electronic devices. For decades, dislocations were predicted to induce anisotropic heat transport depending on their direction, however, experimental observation has been still lacking. 

In a well-concerted collaboration between teams at the National University of Singapore (NUS), Oak Ridge National Laboratory (ORNL, USA) and WSI-TUM, highly oriented threading dislocation (TD) arrays were designed in few-µm thick, epitaxial InN (indium nitride) films for which the strong thermal transport anisotropy was measured for the first time using time-domain thermoreflectance. The results published in B. Sun, et al, Nature Materials (2018) show that the cross-plane thermal conductivity is almost 10-fold higher than the in-plane thermal conductivity for state-of-the-art TD densities of ~1010/cm2. With enhanced understanding of dislocation-phonon interactions obtained in this work, we expect developments for directed heat dissipation in the thermal management of diverse device applications.


Online Link






Best Poster Award at MRS Fall Meeting for Thomas Stettner and Daniel Ruhstorfer
Jan 15 2019

[research]

Thomas Stettner and Daniel Ruhstorfer, both PhD students at the Walter Schottky Institute share a Best Poster Award that was recently awarded at the MRS Fall Meeting 2018 held by the Materials Research Society in Boston, USA (Nov. 25-30, 2018). The MRS Fall Meeting is one of the largest international conferences for materials research with over 50 symposia and ca. 6000 attendees annually.



The awardees received their prize for their contribution entitled “GaAs-AlGaAs core-shell nanowire lasers on silicon” which was presented in the Symposium “Nanowires and Related 1D Nanostructures – New Opportunities and Grand Challenges”. Thomas Stettner and Daniel Ruhstorfer are both students supervised by PD Dr. Gregor Koblmüller at the Semiconductor Quantum Nanosystems Chair of Prof. Jonathan Finley at WSI. 










PI appointed as TUM Junior Fellow
Jan 1 2019

[research]

Based on the recent ERC Consolidator Grant awarded to the PI, Gregor Koblmüller is now nominated by the presidental office as TUM Junior Fellow. The TUM Junior Fellow program was initiated about 10 years ago, and acknowledges independent research group leaders who have obtained substantial single-PI research grants in a highly competitive application process. TUM Junior Fellows take full responsibility of their own employees and are further entitled to various relevant measures within the faculty.


Further information about the TUM Junior Fellow program can be found at:


https://www.professoren.tum.de/en/tum-junior-fellows/





















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