Dec 4 2025

The enhancement of power conversion efficiency beyond the theoretical limit of single-junction solar cells is a key objective in the advancement of hot carrier solar cells. Recent findings indicate that quantum wells (QWs) can effectively generate hot carriers by confining charged carriers within their potential wells and by optimizing material properties.

In a recent work, just published in the Journal of Applied Physics (2025), we explored the impact of quantum confinement on the thermodynamic properties of photogenerated hot carriers in p–i–n InGaAs/InAlAs heterostructure diodes with different QW thicknesses. The experimental findings indicate that the widest QW demonstrates more pronounced hot carrier effects than the thinner quantum wells. This observation aligns with theoretical predictions and underscores the significance of the well width in influencing carrier dynamics. Additionally, the open-circuit voltage of the samples demonstrates a correlation with the degree of quantum confinement, mirroring trends observed in the quasi-Fermi level splitting of hot carriers. The results of this study provide new insights into the impact of quantum confinement on the optical properties of non-equilibrium hot carriers in QW structures and offer pathways for developing efficient hot carrier absorbers for photovoltaic applications.

Sep 19 2025

Semiconductor nanowires (NWs) hold great potential as compact and cost-efficient nanoscale lasers because they offer both a gain medium and a microcavity for efficient optical feedback all in one structure. Especially, telecom-band vertical-cavity nanowire (NW) lasers are promising integrated light sources for silicon (Si) photonics applications, due to their small-size footprint and direct coupling to waveguides and circuits, but have remained elusive within the important GaAs materials system.

In a recent article, published in Nano Letters (2025), PhD student C. Doganlar and co-workers demonstrated the direct site-selective integration of a vertical-cavity GaAs-based NW laser on Si that exhibits lasing emission in its as-grown (monolithic) geometry at the telecom O-band (∼1.3 μm). This achievement relies on an advanced NW heterostructure using an InGaAs/InAlGaAs multiple quantum well (MQW) active gain region coaxially integrated on a vertical GaAsSb NW core with high Sb content to minimize strain energy. Consequently, uniform composition throughout the entire MQW and minimal strain (<1.3 ± 0.2%) with no extended defects are verified by scanning transmission electron microscopy and nanobeam electron diffraction. Single-mode lasing is consistently observed for a range of operation temperatures under optical pumping with lasing thresholds as low as 160 μJ/cm². Mode-dependent threshold gain analyses reveal further that a high-order transverse mode is responsible for lasing. This achievement opens a clear route for establishing monolithically integrated, GaAs-based telecom-band NW lasers with ultracompact footprints on Si photonic circuits.

Aug 3 2025

The SQNM group warmly congratulates Tobias for his successful doctoral thesis defense. In his research work, Tobias developed p-i-n doped III-V nanowire heterostructures as on-chip laser diodes integrated on silicon photonic platform. Nanowire laser diodes operating in the near-infrared spectral range have not been reported before, but are much needed as electrically injected devices for practical applications in on-chip optical interconnects.

The thesis presented by Tobias constructs therefore all the necessary ingredients for p-i-n doped nanowire diode structures; first, he explored the growth, the optical and electrical properties of n-type doped GaAs(Sb) nanowire cores, then the consecutive formation of high-quality InGaAs multi-quantum well active regions as well as low-resistivity p-doped contact layers. Using a variety of advanced analytical methods, he was able to provide comprehensive insights into doping mechanisms and the role of dopants on the lasing properties of these devices. Tobias further demonstrated optically pumped lasing in fully p-i-n doped nanowire diode structures with excellent threshold characteristics, and ultimately described the extensive device fabrication scheme for electrically driven devices. After this pioneering work, we extend our best wishes to Tobias for his future professional career.

May 8 2025

We warmly congratulate Hyowon for his successful doctoral thesis defense. During his past few years, Hyowon developed new concepts of quantum light sources that can be monolithically integrated on Si photonic circuits. The core idea behind this work was to use vertically integrated III-V nanowire (NW) waveguides with deterministically embedded, individual quantum disks (QD) that produce on-chip coupled single photons for quantum communication applications.

To develop such monolithic NW-QD systems (integrated on Si photonic circuitry) requires a whole set of well-matched structural and optical properties of both the NW-host material and the embedded active quantum emitter for optimum light generation and coupling. To this end, the thesis of Hyowon aimed to address these various aspects individually through a subset of different objectives: (i) the optimization of GaAs(Sb) NW resonator structures epitaxially grown on Si, (ii) the incorporation of optically active InGaAs emitters, and (iii) their final site-selective integration on Si waveguide structures on silicon-on-insulator (SOI) platform. Hereby, he was able to pioneer the formation of axially controlled quantum disks in a multi-facetted material structure and develop clear understandings of their structure-property relationships. After all his great work at WSI, we wish Hyowon all the best for his future professional career.

Mar 4 2025

As of Feb. 4, 2025 the PI has started a new position as W3 Professor in  “Experimental Physics/Applied Physics” in the Solid State Physics Division at TU Berlin. For further information please refer to the following links:

https://www.tu.berlin/en/naturwissenschaften/news-details/gregor-koblmueller-becomes-new-professor-at-the-institute-of-solid-state-physics

https://www.tu.berlin/ag-koblmueller

Feb 28 2025

One-dimensional structures such as nanowires (NWs) show great promise in tailoring hot carrier thermalization in semiconductors with important implications for the design of efficient hot carrier absorbers. However, the fabrication of defect-free crystal structures and control of their intrinsic electronic properties can be challenging, raising concerns about the rolfe of competing radiative and non-radiative recombination mechanisms that govern hot carrier effects. 

In a recent article, published in Applied Physics Letters (2025), H. Esmaielpour and co-workers revealed the impact of crystal purity and altered electronic properties on the hot carrier properties by comparing two classes of III-V semiconductor NW arrays with similar bandgap energies and geometries, but different crystal quality: one composed of GaAsSb NWs, which host antisite point defects but are free of planar stacking defects, and the other InGaAs NWs with a very high density of stacking defects. Interestingly, in-depth photoluminescence spectroscopy demonstrated that the InGaAs NWs exhibit stronger hot carrier effects, despite the presence of large densities of macroscopic defects. This result arises from higher rates of Auger recombination in the InGaAs NWs due to their increased n-type conductivity, as compared to GaAsSb NWs that are limited by the acceptor-type point defects. Our findings suggest that while enhancing material properties is crucial for improving the performance of hot carrier absorbers, optimizing conditions to increase the rates of Auger recombination will further boost the efficiency of these devices.

Jan 10 2025

Jona Zöllner, PhD student at the Chair of Semiconductor Nanostructures and Quantum Systems, was recently awarded the ISNTT-2024 Student Poster Award at the International School and Symposium on Nanodevices and quanTum Technologies in Atsugi, Japan. The award acknowledges excellent student contributions in the fields of hybrid quantum systems, integrated nanophotonic and quantum photonic/electronic devices.

The awardee received the prize for his poster presentation entitled "Design and fabrication of hex-SiGe nanowire-induced photonic crystal cavities", which is grounded on a combined computational and experimental effort to realize new integrated nanoscale laser structures on the Si/SiGe plaform. In this work, Jona used numerical simulations to design photonic crystal (PhC) cavities with slot waveguides hosting hexagonal SiGe nanowires with direct band-gap as active region. By further using PDMS-based transfer-stamp methods, single hex-SiGe (and complementary InAs) nanowires were accurately located inside lithographically fabricated slot waveguides of Si-PhC cavities and their emission characteristics verified using polarized reflectance and µPL spectroscopy. Thereby this work sets important foundations to engineer the emission dynamics in advanced nanolasers integrated on silicon.