Scanning probe microscopy with gallium nitride nanowires

Abstract: In scanning probe microscopy the properties of the tip is crucial in determining which information that can be obtained from the resulting images and spectroscopy. Ordinarily metal tips are used for scanning, however by using semiconducting nanowires as tips one would have very well defined probes with more advanced possibilities for spectroscopic measurements. However, currently creating such high precision and advanced scanning probe tips has been expensive and time consuming. GaN nanowires would be excellent candidates for semiconductor tips since they can have sharp end points and are hard. A method for quickly creating STM tips from large arrays of GaN nanowires has been developed. The basic principle of the method is to apply a conductive glue to readily available blunt tungsten tips and bring them in contact with an array of nanowires. A large number of different methods for fabricating the complete tips with the attached nanowires were evaluated. For the initial evaluation Scanning Electron Microscopy (SEM) and optical microscopy was employed in the clean room facilities in Lund. Many process variables were evaluated at room temperature including glue type, tungsten tip shape, wire preparation and method for transferring wires. Finally the yield of useable tips for the STM was found to be one out of three with optimum methods. The time for creating one of these tips was 15 minutes, a significant improvement on other methods for creating nanowire probes. The new tips were then tested in a Scanning Tunneling Microscope (STM) on a thin atomically flat Au metal film. Scanning tunneling spectroscopy measurements gave the expected current to voltage characteristics from a tunneling contact between GaN and Au. These tests showed that imaging with the tips is possible, but compared to measurements performed also with ordinary metal tips on similar substrates, obtaining high resolution was more difficult and the tips were less mechanically stable.