Influence of gate material and diamond surface termination on current conduction in metal/Al2O3/diamond capacitors

The current conduction in negatively biased metal/Al2O3/diamond capacitors with various gate materials and diamond surface terminations was comprehensively characterized by assuming a space-charge-controlled field emission (SCC-FE) process, yielding the following results. The current-feeding carriers were field-emitted from the electrode that had a barrier height lower than that of the opposing electrode. Specifically, in an Au-gate oxygen-terminated diamond capacitor, holes were likely to be emitted from the Al2O3/diamond interface states, whereas, in Al-gate oxygen-terminated, Au-gate hydrogen-terminated, and Al-gate hydrogen-terminated diamond capacitors, electrons were emitted from the gate. The barrier heights at the Al/Al2O3 and Au/Al2O3 interfaces were estimated as 1.6–1.7 and 2.8 eV, respectively. However, the analysis of the hole emission from the interface states is left for future studies. Although the substrate of the oxygen-terminated diamond capacitor remained weakly depleted even for highly negative gate voltages, the conduction current was observed as mentioned earlier. To analyze the mechanism for this conduction, the diffusion and drift currents in the depleted substrate were quantitatively estimated, ascribing the current conduction to the diffusion current. As these results could not be obtained by using the conventional graphical methods based on model-specific plots, such as the Fowler–Nordheim and Poole–Frenkel plots, the SCC-FE analysis proved to be a powerful tool for investigating the current conduction in metal-insulator-semiconductor capacitors.