Gözde Tütüncüoglu

Photonic–Plasmonic Coupling of GaAs Single Nanowires to Optical Nanoantennas

We successfully demonstrate the plasmonic coupling between metal nanoantennas and individual GaAs nanowires (NWs). In particular, by using dark-field scattering and second harmonic excitation spectroscopy in partnership with analytical and full-vector FDTD modeling, we demonstrate controlled electromagnetic coupling between individual NWs and plasmonic nanoantennas with gap sizes varied between 90 and 500 nm. The significant electric field enhancement values (up to 20×) achieved inside the NW-nanoantennas gap regions allowed us to tailor the nonlinear optical response of NWs by engineering the plasmonic near-field coupling regime. These findings represent an initial step toward the development of coupled metal-semiconductor resonant nanostructures for the realization of next generation solar cells, detectors, and nonlinear optical devices with reduced footprints and energy consumption.

III–V nanowire arrays: growth and light interaction

Semiconductor nanowire arrays are reproducible and rational platforms for the realization of high performing designs of light emitting diodes and photovoltaic devices. In this paper we present an overview of the growth challenges of III-V nanowire arrays obtained by molecular beam epitaxy and the design of III-V nanowire arrays on silicon for solar cells. While InAs tends to grow in a relatively straightforward manner on patterned (111)Si substrates, GaAs nanowires remain more challenging; success depends on the cleaning steps, annealing procedure, pattern design and mask thickness. Nanowire arrays might also be used for next generation solar cells. We discuss the photonic effects derived from the vertical configuration of nanowires standing on a substrate and how these are beneficial for photovoltaics. Finally, due to the special interaction of light with standing nanowires we also show that the Raman scattering properties of standing nanowires are modified. This result is important for fundamental studies on the structural and functional properties of nanowires.

Modulation doping of GaAs/AlGaAs core-shell nanowires with effective defect passivation and high electron mobility.

Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 ± 0.06 × 10(16) cm(-3) was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 ± 300 cm(2) V(-1) s(-1) and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 ± 0.3 and 2.4 ± 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.

From Twinning to Pure Zincblende Catalyst-Free InAs(Sb) Nanowires

III-V nanowires are candidate building blocks for next generation electronic and optoelectronic platforms. Low bandgap semiconductors such as InAs and InSb are interesting because of their high electron mobility. Fine control of the structure, morphology, and composition are key to the control of their physical properties. In this work, we present how to grow catalyst-free InAs1-xSbx nanowires, which are stacking fault and twin defect-free over several hundreds of nanometers. We evaluate the impact of their crystal phase purity by probing their electrical properties in a transistor-like configuration and by measuring the phonon-plasmon interaction by Raman spectroscopy. We also highlight the importance of high-quality dielectric coating for the reduction of hysteresis in the electrical characteristics of the nanowire transistors. High channel carrier mobilities and reduced hysteresis open the path for high-frequency devices fabricated using InAs1-xSbx nanowires.

Increased Photoconductivity Lifetime in GaAs Nanowires by Controlled n-Type and p-Type Doping

Controlled doping of GaAs nanowires is crucial for the development of nanowire-based electronic and optoelectronic devices. Here, we present a noncontact method based on time-resolved terahertz photoconductivity for assessing n- and p-type doping efficiency in nanowires. Using this technique, we measure extrinsic electron and hole concentrations in excess of 10(18) cm(-3) for GaAs nanowires with n-type and p-type doped shells. Furthermore, we show that controlled doping can significantly increase the photoconductivity lifetime of GaAs nanowires by over an order of magnitude: from 0.13 ns in undoped nanowires to 3.8 and 2.5 ns in n-doped and p-doped nanowires, respectively. Thus, controlled doping can be used to reduce the effects of parasitic surface recombination in optoelectronic nanowire devices, which is promising for nanowire devices, such as solar cells and nanowire lasers.

Ga-assisted growth of GaAs nanowires on silicon, comparison of surface SiOx of different nature

The influence of the oxide in Ga-assisted growth of GaAs nanowires on Si substrates is investigated. Three different types of oxides with different structure and chemistry are considered. We observe that the critical oxide thicknesses needed for achieving nanowire growth depends on the nature of oxide and how it is processed. Additionally, we find that different growth conditions such as temperature and Ga rate are needed for successful nanowire growth on different oxides. We generalize the results in terms of the characteristics of the oxides such as surface roughness, stoichiometry and thickness. These results constitute a step further towards the integration of GaAs technology on the Si platform.

Vectorial scanning force microscopy using a nanowire sensor

Self-assembled nanowire (NW) crystals can be grown into nearly defect-free nanomechanical resonators with exceptional properties, including small motional mass, high resonant frequency and low dissipation. Furthermore, by virtue of slight asymmetries in geometry, a NWs flexural modes are split into doublets oscillating along orthogonal axes. These characteristics make bottom-up grown NWs extremely sensitive vectorial force sensors. Here, taking advantage of its adaptability as a scanning probe, we use a single NW to image a sample surface. By monitoring the frequency shift and direction of oscillation of both modes as we scan above the surface, we construct a map of all spatial tip-sample force derivatives in the plane. Finally, we use the NW to image electric force fields distinguishing between forces arising from the NW charge and polarizability. This universally applicable technique enables a form of atomic force microscopy particularly suited to mapping the size and direction of weak tip-sample forces.

Characterization and analysis of InAs/p-Si heterojunction nanowire-based solar cell

The growth of compound semiconductor nanowires on the silicon platform has opened many new perspectives in the area of electronics, optoelectronics and photovoltaics. We have grown a 1 x 1 mm(2) array of InAs nanowires on p-type silicon for the fabrication of a solar cell. Even though the nanowires are spaced by a distance of 800 nm with a 3.3% filling volume, they absorb most of the incoming light resulting in an efficiency of 1.4%. Due to the unfavourable band alignment, carrier separation at the junction is poor. Photocurrent increases sharply at the surrounding edge with the silicon, where the nanowires do not absorb anymore. This is further proof of the enhanced absorption of semiconductors in nanowire form. This work brings further elements in the design of nanowire-based solar cells.

Plasmonic Waveguide-Integrated Nanowire Laser

Next-generation optoelectronic devices and photonic circuitry will have to incorporate on-chip compatible nanolaser sources. Semiconductor nanowire lasers have emerged as strong candidates for integrated systems with applications ranging from ultrasensitive sensing to data communication technologies. Despite significant advances in their fundamental aspects, the integration within scalable photonic circuitry remains challenging. Here we report on the realization of hybrid photonic devices consisting of nanowire lasers integrated with wafer-scale lithographically designed V-groove plasmonic waveguides. We present experimental evidence of the lasing emission and coupling into the propagating modes of the V-grooves, enabling on-chip routing of coherent and subdiffraction confined light with room-temperature operation. Theoretical considerations suggest that the observed lasing is enabled by a waveguide hybrid photonic-plasmonic mode. This work represents a major advance toward the realization of application-oriented photonic circuits with integrated nanolaser sources.

High Yield of GaAs Nanowire Arrays on Si Mediated by the Pinning and Contact Angle of Ga

GaAs nanowire arrays on silicon offer great perspectives in the optoelectronics and solar cell industry. To fulfill this potential, gold-free growth in predetermined positions should be achieved. Ga-assisted growth of GaAs nanowires in the form of array has been shown to be challenging and difficult to reproduce. In this work, we provide some of the key elements for obtaining a high yield of GaAs nanowires on patterned Si in a reproducible way: contact angle and pinning of the Ga droplet inside the apertures achieved by the modification of the surface properties of the nanoscale areas exposed to growth. As an example, an amorphous silicon layer between the crystalline substrate and the oxide mask results in a contact angle around 90°, leading to a high yield of vertical nanowires. Another example for tuning the contact angle is anticipated, native oxide with controlled thickness. This work opens new perspectives for the rational and reproducible growth of GaAs nanowire arrays on silicon.