Search

We report the effects of Si doping on the growth dynamics and size distribution of entirely catalyst-free GaAs nanowire (NW) arrays grown by selective area molecular beam epitaxy on SiO2-masked Si (111) substrates. Surprising improvements in the NW-array uniformity are found with increasing Si doping, while the growth of undoped NWs appears in a metastable regime, evidenced by large size and shape distributions, and the simultaneous presence of crystallites with tetrahedral termination. Correlating scanning electron microscopy and transmission electron microscopy investigations, we propose that the size and shape distributions are strongly linked to the underlying twin defect formation probabilities that govern the growth. Under the present growth conditions, Si-doping of GaAs NWs leads to a very high twin defect formation probability (∼0.4), while undoped NWs exhibit a nearly threefold decreased probability (∼0.15). By adopting a model for facet-mediated growth, we describe how the altered twin formation probabilities impact the competing growth of the relevant low-index NW facets, and hence, NW size and shape. Our model is further supported by a generic Monte Carlo simulation approach to highlight the role of twin defects in reproducing the experimentally observed size distributions.

We report a comprehensive study of the impact of the structural properties in radial GaAs-Al0.3Ga0.7As nanowire-quantum well heterostructures on the optical recombination dynamics and electrical transport properties, emphasizing particularly the role of the commonly observed variations of the quantum well thickness at different facets. Typical thickness fluctuations of the radial quantum well observed by transmission electron microscopy lead to pronounced localization. Our optical data exhibit clear spectral shifts and a multipeak structure of the emission for such asymmetric ring structures resulting from spatially separated, yet interconnected quantum well systems. Charge carrier dynamics induced by a surface acoustic wave are resolved and prove efficient carrier exchange on native, subnanosecond time scales within the heterostructure. Experimental findings are corroborated by theoretical modeling, which unambiguously show that electrons and holes localize on facets where the quantum well is the thickes...

Nonlocal quasiparticle transport in normal-superconductor-normal (NSN) hybrid structures probes sub-gap states in the proximity region and is especially attractive in the context of Majorana research. Conductance measurement provides only partial information about nonlocal response composed from both electron-like and hole-like quasiparticle excitations. In this work, we show how a nonlocal shot noise measurement delivers a missing puzzle piece in NSN InAs nanowire-based devices. We demonstrate that in a trivial superconducting phase quasiparticle response is practically charge-neutral, dominated by the heat transport component with a thermal conductance being on the order of conductance quantum. This is qualitatively explained by numerous Andreev reflections of a diffusing quasiparticle, that makes its charge completely uncertain. Consistently, strong fluctuations and sign reversal are observed in the sub-gap nonlocal conductance, including occasional Andreev rectification signals. Our results prove conductance and noise as complementary measurements to characterize quasiparticle transport in superconducting proximity devices., accepted in Semiconductor Science and Technology (Letter)

Ultrathin InAs nanowires (NW) with a one-dimensional (1D) sub-band structure are promising materials for advanced quantum-electronic devices, where dimensions in the sub-30 nm diameter limit together with post-CMOS integration scenarios on Si are much desired. Here, we demonstrate two site-selective synthesis methods that achieve epitaxial, high aspect ratio InAs NWs on Si with ultrathin diameters below 20 nm. The first approach exploits direct vapor–solid growth to tune the NW diameter by interwire spacing, mask opening size and growth time. The second scheme explores a unique reverse-reaction growth by which the sidewalls of InAs NWs are thermally decomposed under controlled arsenic flux and annealing time. Interesting kinetically limited dependencies between interwire spacing and thinning dynamics are found, yielding diameters as low as 12 nm for sparse NW arrays. We clearly verify the 1D sub-band structure in ultrathin NWs by pronounced conductance steps in low-temperature transport measurements using back-gated NW-field effect transistors. Correlated simulations reveal single- and double degenerate conductance steps, which highlight the rotational hexagonal symmetry and reproduce the experimental traces in the diffusive 1D transport limit. Modelling under the realistic back-gate configuration further evidences regimes that lead to asymmetric carrier distribution and breakdown of the degeneracy depending on the gate bias.

Ternary III-V alloys of tunable bandgap are a foundation for engineering advanced optoelectronic devices based on quantum-confined structures including quantum wells, nanowires, and dots. In this context, core-shell nanowires provide useful geometric degrees of freedom in heterostructure design, but alloy segregation is frequently observed in epitaxial shells even in the absence of interface strain. High-resolution scanning transmission electron microscopy and laser-assisted atom probe tomography were used to investigate the driving forces of segregation in nonplanar GaAs-AlGaAs core-shell nanowires. Growth-temperature-dependent studies of Al-rich regions growing on radial {112} nanofacets suggest that facet-dependent bonding preferences drive the enrichment, rather than kinetically limited diffusion. Observations of the distinct interface faceting when pure AlAs is grown on GaAs confirm the preferential bonding of Al on {112} facets over {110} facets, explaining the decomposition behavior. Furthermore, three-dimensional composition profiles generated by atom probe tomography reveal the presence of Al-rich nanorings perpendicular to the growth direction; correlated electron microscopy shows that short zincblende insertions in a nanowire segment with predominantly wurtzite structure are enriched in Al, demonstrating that crystal phase engineering can be used to modulate composition. The findings suggest strategies to limit alloy decomposition and promote new geometries of quantum confined structures.

Semiconductor nanowire (NW) lasers are nanoscale coherent light sources that exhibit a small footprint, low-threshold lasing characteristics, and properties suitable for monolithic and site-selective integration onto Si photonic circuits. An important milestone on the way toward novel on-chip photonic functionalities, such as injection locking of laser emission and all-optical switching mediated by coherent optical coupling and feedback, is the integration of individual, deterministically addressable NW lasers on Si waveguides with efficient coupling and mode propagation in the underlying photonic circuit. Here, we demonstrate the monolithic integration of single GaAs-based NW lasers directly onto lithographically defined Si ridge waveguides (WG) with low threshold power densities of 19.8 μJ/cm2 when optically excited. The lasing mode of individual NW lasers is shown to couple efficiently into propagating modes of the underlying orthogonal Si WG, preserving the lasing characteristics during mode propagati...

In vapor-liquid-solid GaAs NWs, doping control has proven to be difficult due to non-uniform incorporation and strong amphoteric behavior of Si dopants in the presence of a liquid droplet catalyst. We present for the first time clear evidence of n-type doping in direct bottom-up growth of GaAs:Si NWs by catalyst-free selective area molecular beam epitaxy.

Recent demonstrations of direct integration of GaAs-based nanowire lasers on Si photonic circuits exhibited efficient in-coupling of lasing emission to proximal waveguides - however, transmission in the Si waveguide suffered from high absorption losses. To overcome these issues, we develop here GaAs-(In, Al)GaAs-based nanowire lasers with lasing emission tunable to longer wavelengths ( $> 1.1\ \mu \mathrm{m}$ ) by using coaxial InGaAs/AlGaAs MQW (multi-quantum well) structures as active gain media. We illustrate also the sensitive interplay between design and growth parameters of the MQW structure on the resulting emission wavelength and lasing threshold.

The recent integration of III-V semiconductor nanowire (NW) lasers on silicon waveguides marked a key step toward their usage as coherent light sources for future silicon photonics applications. However, the low index contrast between III-V semiconductors 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. Here, we perform numerical simulations to explore how the alternating refractive index of a silicon waveguide with a thin SiO2 interlayer can be used to significantly improve the reflectivity at the III-V–silicon interface to values of up to 83%. We further investigate the frequency dependencies of the end-facet reflectivity and in-coupling efficiency as a function of the nanowire and waveguide dimensions. Our results are kept general by the normalization to the nanowire radius R and show for a waveguide width of 2.75⋅ R a maximum coupling efficiency of 50%. Variations in waveguide height or SiO2 interlayer thickness by ± 0.1 ⋅ R increase the coupling efficiency by a factor of 2, with little effect on the end-facet reflectivity. Ultimately, a prototypical NW-laser structure consisting of a 1.3-μm emitting InGaAs MQW active region in a core-multishell structure was simulated, showing an optimized low-threshold gain of

We report a comprehensive study of the growth dynamics in highly periodic, composition tunable InAsSb nanowire (NW) arrays using catalyst-free selective area molecular beam epitaxy. Employing periodically patterned SiO2-masks on Si (111) with various mask opening sizes (20–150 nm) and pitches (0.25–2 μm), high NW yield of >90% (irrespective of the InAsSb alloy composition) is realized by the creation of an As-terminated 1 × 1-Si(111) surface prior to NW nucleation. While the NW aspect ratio decreases continually with increasing Sb content (x Sb from 0% to 30%), we find a remarkable dependence of the aspect ratio on the mask opening size yielding up to ∼8-fold increase for openings decreasing from 150 to 20 nm. The effects of the interwire separation (pitch) on the NW aspect ratio are strongest for pure InAs NWs and gradually vanish for increasing Sb content, suggesting that growth of InAsSb NW arrays is governed by an In surface diffusion limited regime even for the smallest investigated pitches. Compositional analysis using high-resolution x-ray diffraction reveals a substantial impact of the pitch on the alloy composition in homogeneous InAsSb NW arrays, leading to much larger x Sb as the pitch increases due to decreasing competition for Sb adatoms. Scanning transmission electron microscopy and associated energy-dispersive x-ray spectroscopy performed on the cross-sections of individual NWs reveal an interesting growth-axis dependent core–shell like structure with a discontinuous few-nm thick Sb-deficient coaxial boundary layer and six Sb-deficient corner bands. Further analysis evidences the presence of a nanoscale facet at the truncation of the (111)B growth front and {1-10} sidewall surfaces that is found responsible for the formation of the characteristic core–shell structure.

The realization of n-type conduction in directly bottom-up grown Si-doped GaAs nanowires (NWs) by molecular beam epitaxy has remained a long-standing challenge. Unlike the commonly employed vapor–liquid−solid growth, where the amphoteric nature of Si dopants induces p-type conduction, we report a completely catalyst-free, selective area molecular beam epitaxial growth that establishes n-type behavior under Si doping. The vapor–solid selective area growth on prepatterned Si (111) substrates is enabled by an important in situ substrate pretreatment to create an As-terminated 1 × 1-Si(111) substrate necessary for the growth of [111]-oriented GaAs:Si NWs with a large aspect ratio and high yield. Correlated resonant Raman scattering and single-NW micro-photoluminescence (μPL) experiments confirm the n-type nature of the Si-doped GaAs NWs evidenced by a dominant SiGa local vibrational Raman mode, a distinct band filling effect (up to > 10 meV) along with increased PL peak broadening upon increased Si concentration. Excessive Si doping is further found to induce some auto-compensation evidenced by red-shifted PL and the appearance of minor SiAs and SiGa–SiAs pair-like local vibrational Raman modes. Employing excitation power dependent μPL, we further discern signatures in below-gap defect luminescence (∼1.3–1.45 eV) arising from structural defects and Si dopant-point defect complexes.The realization of n-type conduction in directly bottom-up grown Si-doped GaAs nanowires (NWs) by molecular beam epitaxy has remained a long-standing challenge. Unlike the commonly employed vapor–liquid−solid growth, where the amphoteric nature of Si dopants induces p-type conduction, we report a completely catalyst-free, selective area molecular beam epitaxial growth that establishes n-type behavior under Si doping. The vapor–solid selective area growth on prepatterned Si (111) substrates is enabled by an important in situ substrate pretreatment to create an As-terminated 1 × 1-Si(111) substrate necessary for the growth of [111]-oriented GaAs:Si NWs with a large aspect ratio and high yield. Correlated resonant Raman scattering and single-NW micro-photoluminescence (μPL) experiments confirm the n-type nature of the Si-doped GaAs NWs evidenced by a dominant SiGa local vibrational Raman mode, a distinct band filling effect (up to > 10 meV) along with increased PL peak broadening upon increased Si concentrat...

Semiconductor nanowire (NW) lasers are attractive as integrated on-chip coherent light sources with strong potential for applications in optical communication and sensing. Realizing lasers from individual bulk-type NWs with emission tunable from the near-infrared to the telecommunications spectral region is, however, challenging and requires low-dimensional active gain regions with an adjustable band gap and quantum confinement. Here, we demonstrate lasing from GaAs-(InGaAs/AlGaAs) core-shell NWs with multiple InGaAs quantum wells (QW) and lasing wavelengths tunable from ∼0.8 to ∼1.1 μm. Our investigation emphasizes particularly the critical interplay between QW design, growth kinetics, and the control of InGaAs composition in the active region needed for effective tuning of the lasing wavelength. A low shell growth temperature and GaAs interlayers at the QW/barrier interfaces enable In molar fractions up to ∼25% without plastic strain relaxation or alloy intermixing in the QWs. Correlated scanning transmission electron microscopy, atom probe tomography, and confocal PL spectroscopy analyses illustrate the high sensitivity of the optically pumped lasing characteristics on microscopic properties, providing useful guidelines for other III-V-based NW laser systems.

Semiconductor nanowire (NW) lasers are nanoscale coherent light sources that exhibit a small footprint, low-threshold lasing characteristics, and properties suitable for monolithic integration onto Si photonic circuits. An important milestone on the way towards novel on-chip photonic functionalities is the integration of individual, deterministically addressable NW lasers on Si waveguides with efficient coupling and mode propagation in the underlying photonic circuit. Here, we demonstrate the monolithic integration of single GaAs-based NW lasers directly onto lithographically defined Si ridge waveguides (WG). Subject to optical excitation, the observed lasing behavior shows clear “s-shape”-characteristics, linewidth narrowing and threshold values down to 19.8±1 µJ/cm², which is the lowest value reported to date for this kind of integrated lasing structure. The lasing mode of individual NW lasers is shown to couple efficiently into propagating modes of the underlying orthogonal Si WG, preserving the spectral characteristics during mode pro-pagation in the WG, and in good agreement with Finite-Difference Time-Domain (FDTD) simulations. Using a WG structure with a series of mask openings along the central mode propagation axis, we further illustrate the out-coupling properties of both spontaneous and stimulated emission and demonstrate propagation of the lasing mode over distances > 60 µm, despite absorption in the silicon dominating the propagation losses. By replacing the bulk GaAs as the active gain medium by InxGa1-xAs quantum wells, the emission wavelength can be shifted towards telecommunication bands in order to reduce the absorption losses in silicon. These results pave the way for future on-chip monolithic integration of III-V NW lasers onto silicon based optoelectronic circuits.

We demonstrate lasing from individual GaAs-based NWs integrated onto Si ridge waveguides. In addition, proof-of-principle coupling emission to the Si WG is shown, with propagation distances of the lasing mode exceeding > 60 pm

We report on the fabrication and use of deuterated polyethylene (dPE) as a coating material for ultra-cold neutron (UCN) storage and transport. The Fermi potential has been determined to be 214~neV and the wall loss coefficient $\eta$ is 1.3$\cdot$10$^4$ per wall collision. The coating technique allows for a wide range of applications and new possibilities in this field of physics. In particular, flexible and quasi-massless UCN guides with slit-less shutters and slit-less UCN storage volumes become possible. These properties enable the use in next-generation measurements of the electric dipole moment of the neutron., Comment: 4 pages, 3 figures

We investigate the proximity effect in InAs nanowire (NW) junctions with superconducting contacts made of Al. The carrier density in InAs is tuned by means of the back gate voltage $V_g$. At high positive $V_g$ the devices feature transport signatures characteristic of diffusive junctions with highly transparent interfaces - sizable excess current, re-entrant resistance effect and proximity gap values ($\Delta_N$) close to the Al gap ($\Delta_0$). At decreasing $V_g$, we observe a reduction of the proximity gap down to $\Delta_N\approx\Delta_0/2$ at NW conductances $\sim2e^2/h$, which is interpreted in terms of carrier density dependent reduction of the Al/InAs interface transparency. We demonstrate that the experimental behavior of $\Delta_N$ is closely reproduced by a model with shallow potential barrier at the Al/InAs interface.