Highest optical gap tetrahedral amorphous carbon

Abstract

The deposition and characterisation of tetrahedral amorphous carbon (ta-C) with an E04 optical gap of 3.5 eV and Tauc gap of 3 eV is presented. This is the highest optical gap reported in literature for ta-C and was directly measured using photothermal deflection spectroscopy (PDS) and UV-Vis spectrophotometry. Independent PDS, UV-Vis and electron energy loss spectroscopy (EELS) measurements confirmed the high gap. A large Urbach slope of 600 meV was measured, which indicated that there are many tail states. Electron spin resonance (ESR) measurements confirmed that this material has a high spin density of 7.5×1020 spins/cm3. Post-deposition vacuum annealing of the samples to 500 °C resulted in a small increase of the optical gap, E04∼3.6 eV, and a decrease of the defects to 2.7×1020 spins/cm3. Post-deposition annealing at this temperature did not significantly change the sp3 fraction, but caused a 2–3 order of magnitude increase of the conductivity and a significant reduction of the stress. Thus, this is the first demonstration that we can engineer the defects in ta-C by increasing the electron delocalisation whilst maintaining the high sp3 and optical gap of the material.