Electrical and Mechanical Properties of Three-Dimensional Tungsten Nanostructures

A focussed ion beam (FIB) can be used to grow thin films of tungsten. This is done by scanning the ion beam (containing gallium ions accelerated to 30 kV) over a tungsten hexacarbonyl precursor gas. The gas molecules are cracked by the ions with the result that tungsten is deposited onto a substrate. The resulting tungsten film is superconducting, with a critical transition temperature of around 5 Kelvin. We will extend the FIB-tungsten growth technique to make a variety of three-dimensional nanostructures and devices. Specifically:(i) We will make superconducting tungsten nanowires of radius less than 10 nm. We will measure the electronic properties of the nanowires to determine if they are behaving as quantum phase slip centres (QPSC). A QPSC is a local non-superconducting region in a nanowire which cannot be suppressed no matter how low in temperature the nanowire is cooled to. (ii) We will make Josephson junctions and superconducting quantum interference devices (SQUIDs). The Josephson junction is the basic building block of all superconducting electronic circuits. Two Josephson junctions in parallel are a SQUID, which is the most sensitive detector of magnetic flux. We will use our three-dimensional FIB growth technique to make a highly sensitive gradiometric pick-up coil coupled to a SQUID. (iii) We will make tungsten thin films of thickness less than 10 nm. These will be patterned into meander lines and used as superconducting single photon detectors (SSPDs). The SSPD is a strong candidate technology for photon detection in quantum communication and quantum computation experiments.(iv) We will study the mechanical properties of vertically-grown nanorods. By oscillating them on a piezo-stage mounted in a scanning electron microscope we will measure their Young's modulus and spring constant. This will allow us to determine their suitability for nanomechanics experiments, including the development of an ultra-sensitive mass balance for single molecule mass detection.