On the ion-sensitivity of H-terminated surface channel devices on diamond

Abstract

The transient behavior of ungated ion-sensitive FET-like structures with no reference gate electrode on H-terminated diamond surfaces is studied in various pH solutions. The surface conductive channel of these devices is formed by hydrogen termination of the diamond layers selectively grown on single crystal and polycrystalline diamond substrates. Two types of the ISFET-like structures are used in the experiments: (a) large area devices, where the as-formed surface channel is not subjected to further processing and (b) lithography-patterned (micron-size) ISFET-like structures with the surface channel formed by openings in polyimide passivation via chemical etching. The response of the devices to solutions with different pH was found to be strongly dependent upon the type of substrate and the channel fabrication technology. Reproducible response is observed only for large area devices on single crystal substrates. The response of the large area devices on polycrystalline diamond is less reproducible and is characterized by a large (several hours) relaxation time to restore the initial conductivity. The structures patterned by lithography on polycrystalline diamond undergo fast degradation, once immersed into solution. Then, no ion-sensitivity and no recovery are observed.