A. van der Hart

Hard x-ray nanoprobe based on refractive x-ray lenses

Based on nanofocusing refractive x-ray lenses a hard x-ray scanning microscope is currently being developed and is being implemented at beamline ID13 of the European Synchrotron Radiation Facility (Grenoble, France). It can be operated in transmission, fluorescence, and diffraction mode. Tomographic scanning allows one to determine the inner structure of a specimen. In this device, a monochromatic (E=21keV) hard x-ray nanobeam with a lateral extension of 47×55nm2 was generated. Further reduction of the beam size to below 20 nm is targeted.

Si/Si1-xGex dots grown by selective epitaxy

Using low-pressure chemical vapour deposition, single-layer SiGe and single-quantum-well (SAW) Si/SiGe dots were deposited by selective epitaxy in SiO2 windows using a SiCl2H2/GeH2/H2-based chemistry. The uniformity of selective deposition was investigated over a range of pads from 300 mu m down to 0.14 mu m. No change of growth rate and Ge fraction in SiGe was observed for patterns down to 2 mu m. Only for 0.18 mu m wide dots was a slight increase in Si height measured as compared with the unpatterned area. However, the SiGe layer in the saw dots was 5-4 times thicker, which could be dye to islands or to an enhanced growth of SiGe in the small openings. Arrays with hundreds of millions of 0.15 mu m single-quantum-well Si/SiGe dots very uniform in height could be deposited.

Traveling-wave photomixer with recessed interdigitated contacts on low-temperature-grown GaAs

We have fabricated and characterized novel traveling-wave photomixers with recessed interdigitated metal-semiconductor-metal (MSM) contacts based on low-temperature-grown GaAs. The new recessed MSM geometry led to an improved electric-field distribution inside the photomixer structure and resulted in an up-to-100% increase in the output power of continuously operated devices, compared to conventional MSM devices with standard surface electrodes fabricated on an identical material. The recessed electrode structure also resulted in lower saturation of output power at higher input powers, enabling it to take advantage of higher input powers.

Demonstration of a current-controlled three-terminal Nb–InxGa1−xAs/InP Josephson contact

The supercurrent in a Nb–In0.53Ga0.47As/In0.77Ga0.23As/InP weak link structure is controlled by means of a current injected into the two-dimensional electron gas. For small injection currents the critical current to control current ratio is as large as 20. The measured features can be qualitively explained in terms of a modification of the Andreev level occupation by the injected carriers.

Josephson effect in Nb/two-dimensional electron gas structures using a pseudomorphic InxGa1−xAs/InP heterostructure

Superconducting Nb–In0.53Ga0.47As/In0.77Ga0.23As/InP–Nb contacts with an electrode separation of 450 nm were fabricated. Due to the high mobility of the two-dimensional electron gas, the current transport can be described within the clean limit regime. At 0.3 K a critical current of 3.8 μA was obtained for 6 μm wide contacts leading to a characteristic voltage of 190 μV. The decrease of the critical current with increasing temperature can be explained by a theory developed by A. Chrestin, T. Matsuyama, and U. Merkt [Phys. Rev. B 49, 498 (1994)], which takes δ-shaped barriers at the superconductor/semiconductor interfaces into account.

Ultrafast and highly sensitive photodetectors with recessed electrodes fabricated on low-temperature-grown GaAs

We have fabricated and characterized ultrafast metal-semiconductor-metal (MSM) photodetectors with recessed electrodes, based on low-temperature-grown GaAs. The new recessed-electrode MSM geometry led to an improved electric-field distribution inside the photodetector structure and resulted in a 25% breakdown voltage and sensitivity increase with simultaneous four-fold reduction of capacitance, as compared to the identical MSM devices with planar electrodes. Time-resolved studies performed using 100-fs-duration laser pulses showed that recessed-electrode MSMs exhibited 1.0-ps-wide photoresponse transients with no slow after-pulse tails and their photoresponse time was 0.9 ps. The improved transient photoresponse parameters are the main advantages of the recessed-electrode geometry.

Patterning of 25-nm-wide silicon webs with an aspect ratio of 13

Some of the main problems associated with downscaling of devices are short channel effects and lithography limitations. The double gate concept is known to improve short channel behaviour of MOSFETs. One vertical double gate MOSFET concept requires for the active region silicon webs 300 nm in height and less than 20 nm in width. Subsequent processing means the webs should originally be 10 nm wider than this. Electron beam lithography and reactive ion etching were used to obtain these 25-30-nm-wide silicon webs. To define the structures, hydrogen silsesquioxane (HSQ) was used as electron beam resist; 23-nm-wide and 110-nm-high lines in HSQ were obtained. These structures were tranferred by dry etching with a HBr/O2 plasma and an inductive coupled plasma (ICP) source. This resulted in 25-nm-wide and 330-nm-high silicon webs.

Magnetically refined tips for scanning force microscopy

Abstract Cantilever probes with integrated tips are commercially available. They are routinely used for atomic force microscopy (AFM). For such probes a magnetic refinement of the silicon tip has been developed which results in a deposition of ferromagnetic materials like Nickel or CrCoTa at the top area of the tip. This method consists of essentially three steps: 1. 1. a broad-area sputter-deposition of a ferromagnetic material of any kind, 2. 2. a selective electron-beam induced carbon deposition at the top of the tip of adjustable extension and thickness and 3. 3. a broad-area ion beam sputter-etching which removes the magnetic layer everywhere except underneath the carbon cap. Experiments indicate that the method is reliable and improves the resolution of megnetic force microscopy (MFM). The best lateral spatial resolution amounts 25 nm. Moreover, the domain structure of weak magnetic materials is not destroyed using such probes.

Traveling-wave photomixers fabricated on high energy nitrogen-ion-implanted GaAs

The authors report on fabrication and measurement of traveling-wave photomixers based on high energy and low dose nitrogen-ion-implanted GaAs. They used 3MeV energy to implant N+ ions into GaAs substrates with an ion concentration dose of 3×1012cm−2. The N+-implanted GaAs photomixers exhibit improvements in the output power in comparison with their counterparts, photomixers fabricated on low-temperature-grown GaAs. The maximal output power was 2.64μW at 850GHz. No saturation of the output power with increased bias voltage and optical input power was observed. These characteristics make N+-implanted GaAs the material of choice for efficient high power sources of terahertz radiation.

Normal and inverse current-induced magnetization switching in a single nanopillar

The authors report on current-induced magnetization switching (CIMS) in single-crystalline nanopillars. Fe(14nm)∕Cr(0.9nm)∕Fe(10nm)∕Ag(6nm)∕Fe(2nm) multilayers are deposited by molecular-beam epitaxy. The central Fe layer is coupled to the thick one by interlayer exchange coupling over Cr, while the topmost Fe layer is decoupled. Nanopillars with 150nm diameter are prepared by optical and e-beam lithographies. The opposite spin scattering asymmetries of the Fe∕Cr and Fe∕Ag interfaces enabled the authors to observe normal and inverse CIMS for the two subsystems, which are combined in a single device. At high magnetic fields, steplike resistance changes are measured at positive currents and are attributed to current-driven magnetic excitations.