Bernd Hähnlein

Side-gate graphene field-effect transistors with high transconductance

We have fabricated epitaxial side-gate graphene field-effect transistors (FETs) with high transconductance. A side-gate graphene FET with 55 × 60 nm2 active channel dimensions and a lateral gate-channel separation of 95 nm showing a high transconductance of 590 mS/mm is presented. An estimation of the electrostatic gate-channel capacitance of epitaxial side-gate graphene FETs shows that it is in the same order as the electrostatic gate capacitance of common top-gate graphene MOSFETs justifying the high transconductances of our devices. The results of the present paper demonstrate the potential of the side-gate architecture for graphene transistors.

Broadband Tunable, Polarization-Selective and Directional Emission of (6,5) Carbon Nanotubes Coupled to Plasmonic Crystals

We demonstrate broadband tunability of light emission from dense (6,5) single-walled carbon nanotube thin films via efficient coupling to periodic arrays of gold nanodisks that support surface lattice resonances (SLRs). We thus eliminate the need to select single-walled carbon nanotubes (SWNTs) with different chiralities to obtain narrow linewidth emission at specific near-infrared wavelengths. Emission from these hybrid films is spectrally narrow (20–40 meV) yet broadly tunable (∼1000–1500 nm) and highly directional (divergence <1.5°). In addition, SLR scattering renders the emission highly polarized, even though the SWNTs are randomly distributed. Numerical simulations are applied to correlate the increased local electric fields around the nanodisks with the observed enhancement of directional emission. The ability to control the emission properties of a single type of near-infrared emitting SWNTs over a wide range of wavelengths will enable application of carbon nanotubes in multifunctional photonic devices.

Size effect of Young's modulus in AlN thin layers

In this work, the size effect of the aluminum nitrides Youngs modulus is demonstrated. It manifests in a decreasing Youngs modulus with decreasing layer thickness. The observed thickness dependence is significant for thickness below 300 nm. The results were demonstrated on AlN grown by metal organic chemical vapor deposition using microelectromechanical structures. Measuring and analyzing the resonator length dependence of the resonance frequency using a modified Euler-Bernoulli description allowed to extract the thickness dependence of the Youngs modulus. The cantilever curvatures were determined using a newly developed model. It is also demonstrated that the current existing models do not reflect the observed thickness dependence of the Young′s modulus in a satisfactory way. A model is derived to describe the deviation in the thin film limit.

Surface Lattice Resonances for Enhanced and Directional Electroluminescence at High Current Densities

Hybrid photonic-plasmonic modes in periodic arrays of metallic nanostructures offer a promising trade-off between high-quality cavities and subdiffraction mode confinement. However, their application in electrically driven light-emitting devices is hindered by their sensitivity to the surrounding environment and to charge injecting metallic electrodes in particular. Here, we demonstrate that the planar structure of light-emitting field-effect transistor (LEFET) ensures undisturbed operation of the characteristic modes. We incorporate a square array of gold nanodisks into the charge transporting and emissive layer of a polymer LEFET in order to tailor directionality and emission efficiency via the Purcell effect and variation of the fractional local density of states in particular. Angle- and polarization-resolved spectra confirm that the enhanced electroluminescence correlates with the dispersion curves of the surface lattice resonances supported by these structures. These LEFETs reach current densities on the order of 10 kA/cm2, which may pave the way toward practical optoelectronic devices with tailored emission patterns and potentially electrically pumped plasmonic lasers.

Mechanical Properties and Residual Stress of Thin 3C-SiC(100) Films Determined Using MEMS Structures

3C-SiC(100) was grown on Si (100) in a thickness range between 40 and 500 nm by low pressure chemical vapor deposition. The mechanical properties and the residual stress were determined using the length dependence of the resonance frequencies of cantilevers and beams. Taking into account the influence of the cantilever bending and the stress gradients the Young’s modulus of the 3C-SiC(100) was obtained. It decreases with decreasing thickness of the epitaxial layer grown on Si (100).

Properties of Graphene Side Gate Transistors

Epitaxial graphene grown on semiinsulating silicon carbide was used to fabricate side gate graphene transistors. The transconductance of the side gate transistors is comparable to top gate designs. The transconductance decreases with increasing gate width independently on the gate to channel distance in agreement with the transconductance reduction in top gate transistor configu¬rations with increasing channel length. The transconductance of the side gate transistors decreases with increasing channel width due to a decreased specific gate capacitance.

Nanostructuring of graphene on semi-insulating SiC

Epitaxial graphene nanoribbons were fabricated and geometrically measured via scanning electron microscope in the width range of 3...45nm in a new approach. The critical dimension measurement was improved using Monte Carlo simulations for analyzing back scattering effects of the semi-insulating substrate and gaussian convolutions. Different bias powers during oxygen plasma etching allowed the identification of under-etching depths.

Amplification in Graphene Nanoribbon Junctions

All carbon three terminal junctions were fabricated from epitaxial graphene grown on semiinsulating Si-face 4H-SiC. It is demonstrated that self-contained gate devices exhibit current modulation characteristics. The obtained current gain depends on the device design and is controlled by the applied gate voltage.

T- and Y-Branched Three-Terminal Junction Graphene Devices

Heteroepitaxial graphene on semiinsulating silicon carbide was used to fabricate nanoelectronic devices. T- and Y-branched graphene three-terminal junction devices were realized. Room temperature electrical measurements demonstrate pronounced nonlinear electrical properties of the devices. Voltage rectification at room temperature was observed. Increasing branch width reduces the curvature of the voltage rectification response curve of the three-terminal junc¬tions.

Radiative Pumping and Propagation of Plexcitons in Diffractive Plasmonic Crystals

Strong coupling between plasmons and excitons leads to the formation of plexcitons: quasiparticles that combine nanoscale energy confinement and pronounced optical nonlinearities. In addition to these localized modes, the enhanced control over the dispersion relation of propagating plexcitons may enable coherent and collective coupling of distant emitters. Here, we experimentally demonstrate strong coupling between carbon nanotube excitons and spatially extended plasmonic modes formed via diffractive coupling of periodically arranged gold nanoparticles (nanodisks, nanorods). Depending on the light-matter composition, the rather long-lived plexcitons (>100 fs) undergo highly directional propagation over 20 μm. Near-field energy distributions calculated with the finite-difference time-domain method fully corroborate our experimental results. The previously demonstrated compatibility of this plexcitonic system with electrical excitation opens the path to the realization of a variety of ultrafast active plasmonic devices, cavity-assisted energy transport and low-power optoelectronic components.