Bernhard Basnar

Diagnosing viruses by the rolling circle amplified synthesis of DNAzymes

Circular DNA is used as a template for the amplified detection of M13 phage ssDNA by a rolling circle amplification (RCA) process that synthesizes DNAzyme chains, thus enabling the colorimetric or chemiluminescent detection of the analyte.

Nanoparticle–enzyme hybrid systems for nanobiotechnology

Biomolecule–nanoparticle (NP) [or quantum‐dot (QD)] hybrid systems combine the recognition and biocatalytic properties of biomolecules with the unique electronic, optical, and catalytic features of NPs and yield composite materials with new functionalities. The biomolecule–NP hybrid systems allow the development of new biosensors, the synthesis of metallic nanowires, and the fabrication of nanostructured patterns of metallic or magnetic NPs on surfaces. These advances in nanobiotechnology are exemplified by the development of amperometric glucose sensors by the electrical contacting of redox enzymes by means of AuNPs, and the design of an optical glucose sensor by the biocatalytic growth of AuNPs. The biocatalytic growth of metallic NPs is used to fabricate Au and Ag nanowires on surfaces. The fluorescence properties of semiconductor QDs are used to develop competitive maltose biosensors and to probe the biocatalytic functions of proteases. Similarly, semiconductor NPs, associated with electrodes, are used to photoactivate bioelectrocatalytic cascades while generating photocurrents.

Ultrathin InAlN/AlN Barrier HEMT With High Performance in Normally Off Operation

We present GaN-based high electron mobility transistors (HEMTs) with a 2-nm-thin InAlN/AlN barrier capped with highly doped n++ GaN. Selective etching of the cap layer results in a well-controllable ultrathin barrier enhancement-mode device with a threshold voltage of +0.7 V. The n++ GaN layer provides a 290-Omega/\square sheet resistance in the HEMT access region and eliminates current dispersion measured by pulsed IV without requiring additional surface passivation. Devices with a gate length of 0.5-mum exhibit maximum drain current of 800 mA/mm, maximum transconductance of 400 mS/mm, and current cutoff frequency fT of 33.7 GHz. In addition, we demonstrate depletion-mode devices on the same wafer, opening up perspectives for reproducible high-performance InAlN-based digital integrated circuits.

Analytical evaluation of tapping mode atomic force microscopy for chemical imaging of surfaces

Scanning probe methods like atomic force microscopy (AFM) and related techniques are promising candidates for morphological, physical, and chemical characterization of surfaces on the sub-micrometer scale. In order to evaluate the analytical potential of tapping mode AFM for obtaining material specific information on surface structures along with topography, we have studied the influence of various experimental parameters on height and phase contrast using self-assembled monolayers (SAMs) as well defined model systems. The organic films were deposited onto silicon substrates starting from alkyltrichlorosilanes with methyl-, ester-, and hydroxyl-end groups, respectively. As a result it was found that reproducibility suffers from the fact that even small changes in parameters determining the force interaction between tip and sample can lead to pronounced changes in image contrast. Nevertheless it has been possible to identify comparatively stable regions for the imaging parameters allowing to distinguish different sample systems by their specific pattern of height and phase contrasts, which can be seen as a valuable analytical contribution towards sub-micrometer chemical imaging with scanning probe microscopy.

Proposal and Performance Analysis of Normally Off

Design considerations and performance of n++ GaN/InAlN/AlN/GaN normally off high-electron mobility transistors (HEMTs) are analyzed. Selective and damage-free dry etching of the gate recess through the GaN cap down to a 1-nm-thick InAlN barrier secures positive threshold voltage, while the thickness and the doping of the GaN cap influence the HEMT direct current and microwave performance. The cap doping density was suggested to be 2 × 1020 cm-3. To screen the channel from the surface traps, the needed cap thickness was estimated to be only 6 nm. Design is proved by an experiment showing a constant value of the HEMT dynamical access resistance, while a single-pulse experiment indicated almost collapse-free performance. On the other hand, it is found that the n++ GaN cap does not contribute to the HEMT drain current conduction, nor does it provide a path for the off-state breakdown. HEMTs with a gate length of 0.25 μm and a 4-μm source-to-drain distance show a drain-to-source current of 0.8 A/mm, a transconductance of 440 mS/mm, a threshold voltage of ~0.4 V, and a cutoff frequency of 50 GHz. A thin and highly doped GaN cap is also found to be suitable for the processing of normally on HEMTs by adopting the nonrecessed gate separated from the cap by insulation.

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We report on an annealing-induced “gate sinking” effect in a 2-nm-thin In0.17Al0.83N/AlN barrier high electron mobility transistor with Ir gate. Investigations by transmission electron microscopy linked the effect to an oxygen containing interlayer between the gate metal and the InAlN layer and revealed diffusion of oxygen into iridium during annealing. Below 700 °C the diffusion is inhomogeneous and seems to occur along grain boundaries, which is consistent with the capacitance-voltage analysis. Annealing at 700 °C increased the gate capacitance over a factor 2, shifted the threshold voltage from +0.3 to +1 V and increased the transconductance from 400 to 640 mS/mm.

GaN/InAlN/AlN/GaN HEMTs With 1-nm-Thick InAlN Barrier

We present a novel approach for the reversible switching of the emission wavelength of a quantum cascade laser (QCL) using a halochromic cladding. An air-waveguide laser ridge is coated with a thin layer of polyacrylic acid. This cladding introduces losses corresponding to the absorption spectrum of the polymer. By changing the state of the polymer, the absorption spectrum and losses change, inducing a shift of 7 cm(-1) in the emission wavelength. This change is induced by exposure to acidic or alkaline vapors under ambient conditions and is fully reversible. Such lasers can be used as multi-color light source and as sensor for atmospheric pH.

Metal-related gate sinking due to interfacial oxygen layer in Ir/InAlN high electron mobility transistors

We have investigated an inductively coupled plasma etching recipe using SiCl4 and SF6 with a resulting selectivity >10 for GaN in respect to InAlN. The formation of an etch-resistant layer of AlF3 on InAlN required about 1 min and was noticed by a 4-times-higher initial etch rate on bare InAlN barrier high electron mobility transistors (HEMTs). Comparing devices with and without plasma-treatment below the gate showed no degradation in drain current and gate leakage current for plasma exposure durations shorter than 30 s, indicating no plasma-induced damage of the InAlN barrier. Devices etched longer than the required time for the formation of the etch-resistant barrier exhibited a slight decrease in drain current and an increase in gate leakage current which saturated for longer etching-time durations. Finally, we could prove the quality of the recipe by recessing the highly doped 6 nm GaN cap layer of a GaN/InAlN/AlN/GaN heterostructure down to the 2 nm thin InAlN/AlN barrier layer.

Reversible switching of quantum cascade laser-modes using a pH-responsive polymeric cladding as transducer

The fabrication of bottom-up nanostructures is a crucial step for the advancement of nanotechnology. Dip-pen nanolithography has started off as a method for the transfer of small organic molecules and has matured over the years to one of the most versatile patterning techniques available in the nanoscale. Three-dimensional structures made from organic or inorganic materials on a large variety of different substrates and length scales have been fabricated. This review highlights the techniques used for the fabrication of these structures together with their practical applications. Furthermore, the physical mechanisms involved in the dip-pen process are discussed by summarizing the experimental and theoretical results obtained so far.

Characterization of Plasma-Induced Damage of Selectively Recessed GaN/InAlN/AlN/GaN Heterostructures Using SiCl4 and SF6

A method for tuning the emission of a midinfrared quantum cascade laser using ultraviolet light is presented. The method uses a quantum cascade laser where a photochromic material is deposited as the top waveguide cladding. Changing the state of the cladding causes variations in the absorbance of the cladding. Wavelength dependent changes in the waveguide losses occur, forcing the laser to emit at a wavelength correlated with an absorbance minimum of the cladding. A blueshift by about 6 cm−1 was obtained at room temperature under UV irradiation which was fully reversed by exposure to visible light or room temperature relaxation.