Surface modification of single-crystal boron-doped diamond electrodes for low background current

An all-diamond sub-microelectrode array structure with an insulating cap layer was fabricated on 100-oriented single crystal diamond substrate by epitaxial growth. The cap layer represented a nitrogen-doped layer on top of a highly boron doped film, thus forming a surface p–n junction. The presence of the cap layer reduced the background current down to the pA/mm2 range and shifted the hydrogen—and the oxygen evolution to higher potentials, leading to a potential window of approx. 6 V. Individual electrodes of 600 nm in diameter were introduced using e-beam lithography and plasma etching through the cap layer down to the p+ diamond, leading to a sub-microelectrode array structure. The activity of the sub-microelectrodes could be observed within this 6 V-window at lower potentials. At the same time, the interface capacitance and the background current remained unchanged compared to the initial p–n junction electrode without etched holes.