H.-G. Gehrke

Self-aligned nanostructures created by swift heavy ion irradiation

In tetrahedral amorphous carbon (ta-C) swift heavy ions create conducting tracks of about 8 nm in diameter. To apply these nanowires and implement them into nanodevices, they have to be contacted and gated. In the present work, we demonstrate the fabrication of conducting vertical nanostructures in ta-C together with self-aligned gate electrodes. A multilayer assembly is irradiated with GeV heavy ions and subsequently exposed to several selective etching processes. The samples consist of a Si wafer as substrate covered by a thin ta-C layer. On top is deposited a SiNx film for insulation, a Cr layer as electrode, and finally a polycarbonate film as ion track template. Chemical track etching opens nanochannels in the polymer which are self-aligned with the conducting tracks in ta-C because they are produced by the same ions. Through the pores in the polymer template, the Cr and SiNx layers are opened by ion beam sputtering and plasma etching, respectively. The resulting structure consists of nanowires embed...

Tuning the conductivity of vanadium dioxide films on silicon by swift heavy ion irradiation

We demonstrate the generation of a persistent conductivity increase in vanadium dioxide thin films grown on single crystal silicon by irradiation with 1 GeV 238U swift heavy ions at room temperature. VO2 undergoes a temperature driven metal-insulator-transition (MIT) at 67 °C. After room temperature ion irradiation with high electronic energy loss of 50 keV/nm the conductivity of the films below the transition temperature is strongly increased proportional to the ion fluence of 5·109 U/cm2 and 1·1010 U/cm2. At high temperatures the conductivity decreases slightly. The ion irradiation slightly reduces the MIT temperature. This observed conductivity change is persistent and remains after heating the samples above the transition temperature and subsequent cooling. Low temperature measurements down to 15 K show no further MIT below room temperature. Although the conductivity increase after irradiation at such low fluences is due to single ion track effects, atomic force microscopy (AFM) measurements do not sh...

Ion track lithography and graphitic nanowires in diamondlike carbon

Ion track lithography is well established and is based on heavy ions of several hundred MeV energy passing through a polymer film and thereby changing the material properties along the trajectory of each ion. By selective chemical track etching, small channels (down to 20–30nm diameter) are formed which can be filled with another material or can serve as mask for further etching steps. Another application uses unetched ion tracks directly for nanostructuring. The authors’ investigations showed that graphitic nanowires are formed along ion tracks in insulating diamondlike carbon films. The diameter of these conducing filaments is in the order of 8nm. In the present article the authors describe a combination of these two possibilities to create several nanodevices such as cold field emission devices, quantum-based electronics, or interconnections in very-large-scale integrated circuits. Since the lithographic structure and the conducting filament are produced by the same ion track, the two parts of the devi...

Modeling the diode characteristics of boron nitride/silicon carbide heterojunctions

In this work, we investigate metal–amorphous semiconductor–semiconductor diodes made up of boron nitride/silicon carbide (BN/SiC) heterojunctions. We show that a general conduction model can be applied to this system to explain the measured current-voltage diode characteristics. The conduction model is based on a serial arrangement of a voltage dependent Frenkel–Poole resistance and an ideal Schottky diode. This model is refined to reflect the presence of an amorphous interface layer with thicknesses of about 7(2) nm between BN films and SiC substrates obtained from high resolution cross-section transmission electron microscopy measurements. We demonstrate that this results in an extended Frenkel–Poole and ideal diode model leading to an almost perfect agreement with the measured I-V characteristics of BN/SiC heterojunctions.

Self-aligned nanowires in tetrahedral amorphous carbon multilayer structures

In this work we combine ion track techniques to construct self-aligned vertical structures with nanometer dimensions. The main idea is to use multilayer targets and apply various etching techniques to create openings along the ion path. By irradiating such a multilayered sample including a tetrahedral amorphous carbon (ta-C) layer and a polymer film on top of the stack with swift heavy ions, the track in the polymer and the electrically conducting track in the ta-C layer are self-aligned and need no extra adjustment. Additional layers, e.g., metals or insulators which are little or not affected by the passage of the ions, can be inserted between the polymer resist and the ta-C film. Two appropriate device applications are proposed: a nano-sized field emission cathode and a two terminal quantum dot structure which might be completed to a transistor by adding a gate terminal.