Influence of the postplasma process conditions on the surface conductivity of hydrogenated diamond surfaces

It is a common observation that diamond surface conductivity rises after exposure to hydrogen plasmas. Hydrogenation treatments are known to induce a p-type conductive layer, which is not present on non-hydrogenated samples. However, the particular mechanisms predominant in the plasma treatment process are still controversial, and several antagonist conditions have been reported to be of importance, such as sample temperature (500 °C to 800 °C), duration (a few seconds to 1 h), and microwave (MW) power density, etc. Further, the post-plasma step is also crucial, especially since the surface conductivity has been reported to be affected by the presence of an adsorbate layer on the diamond surface. By setting up the arrangement to enable the in situ measurement of the surface conductivity after treatment, we have been able to control all parameters that could affect the surface conductivity, in order to determine those of importance. Among the parameters studied, we were able to analyze the influence of the surface temperature, the gas phase exposure (dry air, wet air, neutral gas, $CH4,$ $O2,$ and $H2),$ the MW plasma conditions $(O2,H2)$ as well as the exposure to UV (Hg and deuterium) and the importance of the sequence and duration of each of these treatments. We found that hydrogenated surfaces are strongly influenced by the combination of wet air exposure and UV light. We noticed that the effect of UV light is persistent and cannot be related to direct photoconduction and has to be attributed to a modification of the trapped defect population. This can, therefore, be compared with the modification of filled defect density as observed in persistent photoconduction.