Hubert Brückl

Wavelength-tunable microbolometers with metamaterial absorbers

Microbolometers are modified by metallic resonant absorber elements, leading to an enhanced responsivity at selectable wavelengths. The dissipative energy absorption of tailored metamaterials allows for engineering the response of conventional bolometer microbridges. The absorption peak position and height are determined by the geometry of the metamaterial. Square-shaped metal/dielectric/metal stacks as absorber elements show spectral resonances at wavelengths between 4.8 and 7.0 microm in accordance with numerical simulations. Total peak absorptions of 0.8 are obtained. The metamaterial modified bolometers are suitable for multispectral thermal imaging systems in the mid-IR and terahertz regime.

Photovoltaic properties of thin film heterojunctions with cupric oxide absorber

In this work, we report on the fabrication, characterization, and photovoltaic properties of sputter-deposited, thin film heterojunctions combining p-type cupric oxide (CuO) absorber with n-type ZnO. The structural investigation reveals highly crystalline, columnar growth of the layers and confirms that the absorbers phase is purely CuO, with only negligible traces of Cu2O. The optical characterization yields for CuO an indirect bandgap of 1.2 eV and a direct optical transition at approximately 3 eV. The short circuit current, open circuit voltage, fill factor, and power conversion efficiency of the heterojunction solar cells were extracted as a function of the CuO thickness under AM1.5 G (1 kW/m2) illumination. From the observed dependencies, we conclude that the photovoltaic performance is compromised by a restricted carrier collection efficiency, caused by the small carrier lifetime in CuO. Indeed, the carrier population is found to decay with time constants of 40 and 460 ps. A maximum power conversio...

Characterization of embedded MgO/ferromagnet contacts for spin injection in silicon

In this work we present the structural and electrical characterization of sputter-deposited CoFe(B)/MgO/Si metal-insulator-semiconductor tunneling junctions for injection and detection of spin polarized current in silicon. The multilayers have been deposited in 700 nm deep trenches, patterned in thick SiO2 dielectric, on n- and p-doped wafers. The films inside the trenches are continuous with a correlated and low roughness. The MgO barrier grows amorphous without indication of pinholes. The dc and ac transport properties of the junctions were studied as a function of temperature and frequency. A relatively high interface trap density at the MgO/Si-interface is extracted from admittance spectra measurements. Transport is dominated by majority carriers in the case of n-doped and by minority carriers for the p-doped wafers. This leads to distinct rectification characteristics for the two wafer types, which would significantly influence the spin injection efficiency of the tunneling junctions.

Artificial cilia of magnetically tagged polymer nanowires for biomimetic mechanosensing

Polymeric nanowires of polypyrrole have been implemented as artificial cilia on giant-magneto-resistive multilayer sensors for a biomimetic sensing approach. The arrays were tagged with a magnetic material, the stray field of which changes relative to the underlying sensor as a consequence of mechanical stimuli which are delivered by a piezoactuator. The principle resembles balance sensing in mammals. Measurements of the sensor output voltage suggest a proof of concept at frequencies of around 190 kHz and a tag thickness of ∼300 nm. Characterization was performed by scanning electron microscopy and magnetic force microscopy. Micromagnetic and finite-element simulations were conducted to assess basic sensing aspects.

Structural and electrical characterization of SiO2/MgO(001) barriers on Si for a magnetic transistor

We report a structural and electrical study of sputter-deposited SiO2/MgO barriers for developing magnetic Si-based transistors. We propose that SiO2/MgO tunneling barriers may utilize spin-filtering by achieving crystalline MgO (001) while reducing spin-scattering due to the Si/SiO2 interface. We find that MgO (<3 nm thick) crystallizes with (001) preferred orientation on thermally oxidized Si(<2 nm). Typical processing temperatures do not cause significant intermixing with SiO2 or ferromagnetic electrode. Conversely, MgO on Si is amorphous up to 2 nm thick. Capacitance-voltage characteristics of MgO capacitors are influenced significantly by the density of interface-states, as high as 5×1013 cm−2 eV−1 while Si/SiO2/MgO structures are electrically beneficial by reducing to 6×1012 cm−2 eV−1.

TEM Study on Diffusion Process of NiFe Schottky and MgO/NiFe Tunneling Diodes for Spin Injection in Silicon

Analytical electron microscopy is employed to study the structural properties of NiFe Schottky diodes and NiFe/MgO tunneling diodes after annealing up to 400° C. Electrical characterization revealed a drop of the barrier height for the Schottky diodes, while the tunneling diodes are thermally stable. From the cross-sectional TEM images of the Schottky diodes, metal diffusion into Si substrate was found. Investigations of the diffusion using energy dispersive spectroscopy and energy filtered transmission electron microscopy revealed that Ni diffused into the Si substrate to form nickel silicide. Moreover, in some cases, the gold capping layer also diffused into the substrate even deeper than Ni. In the case of the tunneling diodes, metal diffusion was inhibited by the presence of MgO.

Fluxgate Principle Applied to a Magnetic Tunnel Junction for Weak Magnetic Field Sensing

A method is presented for measuring weak magnetic fields by applying a measurement principle known from fluxgate magnetometers to a magnetic tunnel junction. The fluxgate measurement principle based on second harmonic detection is transferred to magnetic tunnel junctions embedded in current lines. Based on Fourier analysis, we developed an analytical model to describe the response of a magnetic tunnel junction fluxgate sensor. The analytical result is compared to MATLAB simulations using Fast Fourier Transformation. Experimental results are obtained with a sensor prototype using lock-in amplification to detect the second harmonic component of the signal generated by the resistance change of the junction. A linear sensor characteristic with a sensitivity in the order of mV/mT is detected in an unshielded setup. Design improvements of the sensor layout as well as a low RA product of the magnetic tunnel junctions should make it possible in the future to detect pT-fields. A cheap sensing technology using magnetic tunnel junctions with the ability of measuring fields in the order of pT could promote novel diagnostic methods such as magnetocardiography into daily clinical routine.

Antireflective surface structures in glass by self-assembly of SiO 2 nanoparticles and wet etching

We describe the fabrication of an antireflective surface structure with sub-wavelength dimensions on a glass surface using scalable low-cost techniques involving sol-gel coating, thermal annealing, and wet chemical etching. The glass surface structure consists of sand dune like protrusions with 250 nm periodicity and a maximum peak-to-valley height of 120 nm. The antireflective structure increases the transmission of the glass up to 0.9% at 700 nm, and the transmission remains enhanced over a wide spectral range and for a wide range of incident angles. Our measurements reveal a strong polarization dependence of the transmission change.

Effect of AZO substrates on self-seeded electrochemical growth of vertically aligned ZnO nanorod arrays and their optical properties

We present a single step and an electrochemical synthesis of vertically aligned ZnO nanorod (NR) arrays, directly on transparent aluminium-doped zinc oxide (AZO) electrodes. The NRs grow from mild, aqueous-based solution at low temperature, with no need for catalysts or additional seed layer. The use of textured AZO as substrate allows for highly effective growth of hexagonally faceted, single-crystalline ZnO NRs along the wurtzite c-axis. The matching of the crystal lattices initiates a self-seeding route, thus the inherent growth habit of the AZO crystallites advances the vertical growth and alignment of NRs. Moreover, the thickness dependant grain size of the AZO layer provides a valuable feature for tuning the diameter of ZnO NRs grown atop. In the absence of any seed mediator, the interfacial quality is expected to improve significantly. This should enhance the thermal and electrical transport throughout the whole nanostructured transparent electrode. The NR growth was investigated under systematic manipulation of the synthesis variables in order to optimize growth conditions for highly aligned, single-crystalline NRs with a large aspect ratio and a good optical quality. The structure and optical property of the AZO/ZnO NR ensembles were characterized by atomic force microscopy, scanning electron microscopy, X-ray diffraction, photoluminescence, and ultravioletvisible transmission spectroscopy.

Ballistic electron emission microscopy on spin valve structures

Spin valve structures, as employed in base layers of spin valve transistor devices, are characterized by ballistic electron emission microscopy (BEEM). In detail, Co–Cu–Permalloy–Au layers sputtered onto n-type GaAs bulk substrates were studied. BEEM spectra taken on these multilayers show that magnetocurrents on the order of 600% can be achieved even at room temperature. Small area images (400nm×400nm) show that the spin filtering effect of the spin valves is quite homogeneous on the submicron scale. On larger scales, magnetic domains were imaged close to the switching field of the spin valve structure.