Watkins, Rebecca; (2024) Diamond Power Electronics and Radiation Detectors. Doctoral thesis (Ph.D), UCL (University College London).

Text WatkinsR_Thesis.pdf - Accepted Version Access restricted to UCL open access staff until 1 October 2027. Download (28MB)

Abstract

This work studies the application of diamond, a material known for its extreme hardness, to the fields of power electronics and radiation detection. Diamond’s unmatched power handling capabilities, carrier mobility and thermal conductivity lend it to high power applications. However, conventionally boron-doped diamond power devices are reaching their efficiency limit. A trade-off between ON-state Resistance, RON and breakdown voltage, BV is reached, depending on the doping concentration. Another key limitation in the formation of vertical power devices is the requirement for very large boron concentrations (> 1020cm−3) in the p+ layer, to achieve effective Ohmic contacts. Boron and dislocation incorporation into subsequent lightly boron-doped layers leads to difficulty achieving control over the boron concentration and quality of the layer. This work presents two methods for the production of diamond Schottky diodes, without the requirement for the p+ layer, simplifying the fabrication process. The first approach utilises surface transfer doped diamond, achieved by encapsulating H-terminated diamond with HfO2. No bulk doping is needed, enabling low dislocation densities. Additionally, both the Ohmic and Schottky contacts can be formed directly on the H-terminated diamond. The devices showed excellent blocking capabilities, demonstrating no catastrophic breakdown under the maximum applied field (0.167MVcm−1) and a large rectification ratio up to 108. The second approach replaces the p+ layer with internal laser written electrodes. The electrodes form conductive wires made up of various forms of carbon, thus they have been termed ‘Nano-Carbon Network Wires’ (NCNWs). The incorporation of NCNWs within a vertical diamond Schottky diode resulted in a reduction in RON and leakage current. The final device boasts a rectification ratio of 106 and RONS = 17 mΩcm2 at 200oC and did not breakdown at the maximum applied field 0.56 MVcm−1 at 100 V. With further optimisation these diodes could find use in high power converters required by a wide range of sectors including transportation, energy transmission, energy storage, aerospace, defence etc . Diamond’s superior carrier transport properties and unparalleled radiation tolerance also make it an ideal material for alpha/ neutron detection. High performing diamond detectors are already commercially available. However, they experience degradation on exposure to large radiation doses. It is known that reducing the electrode spacing in diamond detectors makes them less susceptible to the effects of mean free path reduction, and therefore more resilient to high radiation doses. In this work internal NCN electrodes are fabricated, permitting detector electrode spacing as small as 20μm. Planar diamond detectors were fabricated with and without internal NCN electrodes. Incorporation of NCN electrodes resulted in a marked increase in efficiency (> 9%) and resolution. A vertical configured diamond detector was also fabricated, consisting of closely spaced NCN vias, contacted at the back of the 2 mm thick diamond substrate, which extend up to 20 μm below the alpha facing surface. Close to 100 % efficiency and ns rise times were demonstrated in both detector designs. This work demonstrates the versatility of femto-second laser writing within the field of radiation detection. Reduced detector electrode spacing can be achieved in thick diamond substrates, enabling more durable detectors for deployment in extreme nuclear environments.

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