Novel UV methods for patterning micro-scale conductive structures on insulating diamond surfaces
thesisposted on 28.03.2022, 11:02 by Matthew Allen Clarke
This thesis reports an investigation into laser direct-write manipulation of the terminating layer on diamond surfaces. A novel two-photon etching process is used to partially or completely oxidize the diamond surface to create functional surfaces with engineered regions of oxygen and hydrogen termination. Hydrogen-terminated diamond has a sub-surface two-dimensional hole sheet, with a carrier density of ~2×10¹³ cm⁻² which results in conduction when the surface is exposed to a mildly acidic water layer. This is contrasted by an oxygen-terminated surface that is highly insulating. Two-photon UV etching of oxygen-terminated regions on a hydrogen-terminated surface thus has the potential, for example, to enable a highly practical method for developing micro-scale surface electronic systems. To this end, hydrogen-terminated surfaces were prepared by exposure to a high-power hydrogen plasma source and electrically characterised after evaporation of Au contacts onto the surface. A method of producing consistent, low-resistivity, and smooth hydrogen-terminated surfaces has been developed empirically. Conducting tracks of H-termination were electrically isolated by direct-write two-photon UV laser etching. This technique has been used to electrically isolate individual, and pairs of, Au contacts from the bulk surface whilst allowing conduction inside the region defined by the etched path. Also, partial removal of H-termination was demonstrated enabling the resistance (hole doping) on the surface to be modified in a highly controllable fashion over many orders of magnitude. This result demonstrates an important feature of the laser technique compared with traditional chemical doping techniques. Patterning of hydrogen and oxygen terminated regions was used to structure planar field-effect transistors (FETs). Although the FETs displayed significant resistance and were sub-optimal devices nevertheless the drain-source current was influenced by the gate voltage. These results provide the basis towards a functioning p-type surface conduction diamond FET fabricated by a direct-write laser technique, and is the first demonstration of the ability to write complex patterns into the surface termination of diamond by way of pulsed UV laser techniques. This work reveals the challenges ahead to developing practical devices, and highlights the potential for the UV laser direct-write technique for rapid-prototyping of functionalized diamond surface structures for electronics as well as other possible interesting applications including quantum devices and biosubstrates.