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.

Nov 5 2024

Large-scale ensembles of deterministically integrated quantum emitters play a key role for on-chip photonic quantum communication and computation applications. In this regard, semiconductor nanowire (NW) arrays emerged as promising platform for hosting such integrated emitters, since they offer direct monolithic growth on silicon (Si) and precise placement for the active emitter. In these devices, predicting the optical properties of large numbers of integrated emitters is, however, a tedious task due to the vast geometrical parameter space and variations at the single object level.

Therefore intensive statistical studies employing high-throughput approaches are needed to evaluate significant numbers of individual emitters within large NW arrays in a fully automated way. In a recent work just published in Nano Letters (2024), such approach has now been demonstrated by spectroscopically mapping more than 10.000 individual InGaAs quantum heterostructures embedded in GaAsSb/AlGaAs NW arrays on Si. The studies performed in collaboration with the University of Manchester were able to assess key optical properties such as uniformity and yield in luminescence efficiency, as well as trends in emission wavelength dependent on geometrical parameters. Notably, structural defects such as rotational twins were found to have the most pronounced influence on shifts in the emission wavelength due to quantum confinement effects. These findings provide us now with useful guidelines for tailoring the emission of axial NW quantum heterostructures for next-generation quantum light sources in on-chip quantum photonic integrated circuits.

Sep 26 2024

Mid-infrared (MIR) photonics is an attractive application field with strong potential for infrared imaging, sensing, metrology, and high bandwidth optical data communication in Si photonic integrated circuits. Hence, development of on-chip intrated nanoscale laser sources emitting at MIR wavelengths (2-12 µm) is of paramount importance. However, nanolasers in the MIR spectral range have remained scarce due to significant material physics challenges, and intrinsic effects such as Auger recombination that limit the radiative efficiency.

A key breakthrough is now reported by our team using indium arsenide (InAs) nanowires as viable MIR-nanolaser sources with emission under continuous-wave (CW) operation. Lead doctoral student S. Meder and co-workers successfully developed site-selective InAs nanowires on Si with resonator geometries designed for minimal threshold gain, combining systematic simulations with epitaxial growth and optical pumping experiments. As just reported in the journal Advanced Functional Materials (2024), CW lasing was thereby realized for the low-order TE01 mode at resonator widths >730 nm and at wavelengths of 2.4-2.7 µm. The lasing action was also supported by clear signatures of positive net optical gain in a dedicated Hakki-Paoli analysis. Record low lasing threshold of 1.4 kWcm-2 was observed in a NW-laser with nearly sepcular end facets, that maintained up to temperatures close to 100K. These findings mark an important advancement in the development of nanolasers for integrated MIR photonics.

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.