Daniel Rudolph

Lasing from individual GaAs-AlGaAs core-shell nanowires up to room temperature

Semiconductor nanowires are widely considered to be the next frontier in the drive towards ultra-small, highly efficient coherent light sources. While NW lasers in the visible and ultraviolet have been widely demonstrated, the major role of surface and Auger recombination has hindered their development in the near infrared. Here we report infrared lasing up to room temperature from individual core-shell GaAs-AlGaAs nanowires. When subject to pulsed optical excitation, NWs exhibit lasing, characterized by single-mode emission at 10 K with a linewidth <60 GHz. The major role of non-radiative surface recombination is obviated by the presence of an AlGaAs shell around the GaAs-active region. Remarkably low threshold pump power densities down to ~760 W cm(-2) are observed at 10 K, with a characteristic temperature of T(0)=109±12 K and lasing operation up to room temperature. Our results show that, by carefully designing the materials composition profile, high-performance infrared NW lasers can be realised using III/V semiconductors.

Growth kinetics in position-controlled and catalyst-free InAs nanowire arrays on Si"111… grown by selective area molecular beam epitaxy

We investigated the interwire distance dependence on the growth kinetics of vertical, high-yield InAs nanowire arrays on Si(111) grown by catalyst-free selective area molecular beam epitaxy (MBE). Utilizing lithographically defined SiO2 nanomasks on Si(111) with regular hole patterns, catalyst-free and site-selective growth of vertically (111)-oriented InAs nanowires was achieved with very high yields of ∼90 percent. Interestingly, the yield of vertically ordered nanowires was independent of the interwire distance and the initial growth stages. Significant size variation in the nanowires was found to depend critically on the interwire distance and growth time. Two growth regimes were identified—(i) a competitive growth regime with shorter and thinner nanowires for narrow interwire distances and (ii) a diffusion-limited growth regime for wider distances, providing good estimates for the surface diffusion lengths. Surprisingly, despite these size-dependent effects the nanowire geometries remained unaltered ...

Direct observation of a noncatalytic growth regime for GaAs nanowires.

We identify a new noncatalytic growth regime for molecular beam epitaxially grown GaAs nanowires (NWs) that may provide a route toward axial heterostructures with discrete material boundaries and atomically sharp doping profiles. Upon increase of the As/Ga flux ratio, the growth mode of self-induced GaAs NWs on SiO(2)-masked Si(111) is found to exhibit a surprising discontinuous transition in morphology and aspect ratio. For effective As/Ga ratios <1, in situ reflection high-energy electron diffraction measurements reveal clear NW growth delay due to formation of liquid Ga droplets since the growth proceeds via the vapor-liquid-solid mechanism. In contrast, for effective As/Ga ratios >1 an immediate onset of NW growth is observed indicating a transition to droplet-free, facet-driven selective area growth with low vertical growth rates. Distinctly different microstructures, facet formation and either the presence or absence of Ga droplets at the apex of NWs, are further elucidated by transmission electron microscopy. The results show that the growth mode transition is caused by an abrupt change from As- to Ga-limited conditions at the (111)-oriented NW growth front, allowing precise tuning of the dominant growth mode.

Spontaneous Alloy Composition Ordering in GaAs-AlGaAs Core–Shell Nanowires

By employing various high-resolution metrology techniques we directly probe the material composition profile within GaAs-Al0.3Ga0.7As core-shell nanowires grown by molecular beam epitaxy on silicon. Micro Raman measurements performed along the entire (>10 μm) length of the [111]-oriented nanowires reveal excellent average compositional homogeneity of the nominally Al0.3Ga0.7As shell. In strong contrast, along the radial direction cross-sectional scanning transmission electron microscopy and associated chemical analysis reveal rich structure in the AlGaAs alloy composition due to interface segregation, nanofaceting, and local alloy fluctuations. Most strikingly, we observe a 6-fold Al-rich substructure along the corners of the hexagonal AlGaAs shell where the Al-content is up to x ~ 0.6, a factor of 2 larger than the body of the AlGaAs shell. This is associated with facet-dependent capillarity diffusion due to the nonplanarity of shell growth. A modulation of the Al-content is also found along the radial [110] growth directions of the AlGaAs shell. Besides the ~10(3)-fold enhancement of the photoluminescence yield due to inhibition of nonradiative surface recombination, the AlGaAs shell gives rise to a broadened band of sharp-line luminescence features extending ~150-30 meV below the band gap of Al0.3Ga0.7As. These features are attributed to deep level defects under influence of the observed local alloy fluctuations in the shell.

Absence of vapor-liquid-solid growth during molecular beam epitaxy of self-induced InAs nanowires on Si

The growth mechanism of self-induced InAs nanowires (NWs) grown on Si (111) by molecular beam epitaxy was investigated by in situ reflection high energy electron diffraction and ex situ scanning and transmission electron microscopy. Abrupt morphology transition and in-plane strain relaxation revealed that InAs NWs nucleate without any significant delay and under the absence of indium (In) droplets. These findings are independent of the As/In-flux ratio, revealing entirely linear vertical growth rate and nontapered NWs. No evidence of In droplets nor associated change in the NW apex morphology was observed for various growth termination procedures. These results highlight the absence of vapor-liquid-solid growth, providing substantial benefits for realization of atomically abrupt doping and composition profiles in future axial InAs-based NW heterostructures on Si.

High compositional homogeneity in In-rich InGaAs nanowire arrays on nanoimprinted SiO2/Si (111)

We report improved homogeneity control of composition-tuned In1−xGaxAs (x < 0.4) nanowire (NW) arrays grown by catalyst-free molecular beam epitaxy(MBE) on nanoimprinted SiO2/Si (111) substrates. Using very high As/(Ga+In) ratios at growth temperatures of 550 °C enabled uniform incorporation of the respective group-III elements (In,Ga) over the investigated composition range, confirmed by high-resolution x-ray diffraction (HRXRD) and energy dispersive x-ray spectroscopy. Low-temperature (20 K) photoluminescence of these In-rich In1−xGaxAs NW ensembles reveal state-of-the-art linewidths of ∼29–33 meV. These are independent of Ga content, suggesting an overall low degree of phase separation. In contrast, self-assembled, non-periodic In1−xGaxAs NW arrays show larger inhomogeneity with increased peakwidths in 2θ−ω HRXRD scans as well as broadened Raman modes. These results demonstrate the excellent potential of site-selective MBEgrowth of high-periodicity non-tapered In1−xGaxAs NW arrays with low size and composition dispersion for optimized device integration on Si.

Dynamic Acoustic Control of Individual Optically Active Quantum Dot-like Emission Centers in Heterostructure Nanowires

We probe and control the optical properties of emission centers forming in radial heterostructure GaAs-Al0.3Ga0.7As nanowires and show that these emitters, located in Al0.3Ga0.7As layers, can exhibit quantum-dot like characteristics. We employ a radio frequency surface acoustic wave to dynamically control their emission energy, and occupancy state on a nanosecond time scale. In the spectral oscillations, we identify unambiguous signatures arising from both the mechanical and electrical component of the surface acoustic wave. In addition, different emission lines of a single emission center exhibit pronounced anticorrelated intensity oscillations during the acoustic cycle. These arise from a dynamically triggered carrier extraction out of the emission center to a continuum in the radial heterostructure. Using finite element modeling and Wentzel-Kramers-Brillouin theory we identify quantum tunneling as the underlying mechanism. These simulation results quantitatively reproduce the observed switching and show that in our systems these emission centers are spatially separated from the continuum by >10.5 nm.

High mobility one- and two-dimensional electron systems in nanowire-based quantum heterostructures

Free-standing semiconductor nanowires in combination with advanced gate-architectures hold an exceptional promise as miniaturized building blocks in future integrated circuits. However, semiconductor nanowires are often corrupted by an increased number of close-by surface states, which are detrimental with respect to their optical and electronic properties. This conceptual challenge hampers their potentials in high-speed electronics and therefore new concepts are needed in order to enhance carrier mobilities. We have introduced a novel type of core-shell nanowire heterostructures that incorporate modulation or remote doping and hence may lead to high-mobility electrons. We demonstrate the validity of such concepts using inelastic light scattering to study single modulation-doped GaAs/Al0.16Ga0.84As core-multishell nanowires grown on silicon. We conclude from a detailed experimental study and theoretical analysis of the observed spin and charge density fluctuations that one- and two-dimensional electron channels are formed in a GaAs coaxial quantum well spatially separated from the donor ions. A total carrier density of about 3 × 10(7) cm(-1) and an electron mobility in the order of 50,000 cm(2)/(V s) are estimated. Spatial mappings of individual GaAs/Al0.16Ga0.84As core-multishell nanowires show inhomogeneous properties along the wires probably related to structural defects. The first demonstration of such unambiguous 1D- and 2D-electron channels and the respective charge carrier properties in these advanced nanowire-based quantum heterostructures is the basis for various novel nanoelectronic and photonic devices.

Directional and dynamic modulation of the optical emission of an individual GaAs nanowire using surface acoustic waves.

We report on optical experiments performed on individual GaAs nanowires and the manipulation of their temporal emission characteristics using a surface acoustic wave. We find a pronounced, characteristic suppression of the emission intensity for the surface acoustic wave propagation aligned with the axis of the nanowire. Furthermore, we demonstrate that this quenching is dynamical as it shows a pronounced modulation as the local phase of the surface acoustic wave is tuned. These effects are strongly reduced for a surface acoustic wave applied in the direction perpendicular to the axis of the nanowire due to their inherent one-dimensional geometry. We resolve a fully dynamic modulation of the nanowire emission up to 678 MHz not limited by the physical properties of the nanowires.

Diameter dependent optical emission properties of InAs nanowires grown on Si

We report on the optical emission properties of catalyst-free, molecular beam epitaxy grown InAs nanowires (NW) on Si (111) using photoluminescence spectroscopy. InAs NW ensembles with similar density, length, and crystal structure (wurtzite-phase with stacking faults) but substantially different NW diameter (40–135 nm) are investigated, and the role of diameter on band-edge emission elucidated. Thick (>100 nm) as-grown NWs show relatively strong emission efficiency with emission up to >130 K, red-shift with temperature (T) and low-T band‐edge energy of ∼0.41 eV similar to bulk zincblende InAs. Reduction in NW diameter yields a characteristic blue‐shift (∼0.435 eV for 40-nm thin NWs), which is related to quantum confinement effects and confirmed by simulations.