O-terminated nano-diamond ISFET for applications in harsh environment

The concept of an ion-sensitive FET (ISFET) on diamond with boron delta-doped channel and oxygen-terminated surface for pH sensing has been successfully transferred to large-area nano-crystalline diamond on silicon substrates. The nano-crystalline diamond layers, including the boron delta-doped channels of the FETs, were grown by hot-filament CVD. The fabricated layers were characterised by their peak concentration of boron of 3 × 1020 1/cm3 and FWHM of about 1 nm. This allowed approx. 50% modulation of the channel current within the gate potential range corresponding to the potential window of water electrolysis on the diamond surface. The O-termination by combination of oxygen-plasma and wet chemical treatments resulted in a pH sensitivity of the ISFETs close to the Nernst's limit in the range between pH1 and pH13. The ISFET characteristics were stable even after anodic treatment in KOH. This allows using nano-crystalline diamond ISFETs with O-termination also as electrodes and even at anodic overpotentials.