pH sensing by surface-doped diamond and effect of the diamond surface termination

For the first time, pH sensing of liquid solutions by a free diamond surface is demonstrated using ungated FET structures. The surface channel is in direct contact with the liquid, forming a liquid gate. The sensors tested were acidic and basic water solutions with pH varying from 1 to 13. Three different types of surface channel were tested: (a) p-type boron-doped channel /hydrogen-terminated surface; (b) p-type hydrogen-induced surface channel; and (c) boron-doped channel/oxygen-terminated surface. The surface termination determined the pH response. For the H-terminated surface (a, b), the surface channel was gradually depleted with increasing pH. No charge transfer across the diamond surface to reach equilibrium between the semiconductor and liquid solution was observed. It is proposed that C-H surface bonds pin the Fermi level at the valence band edge due to a high density of states. Shifting the surface state level into the bandgap results in channel depletion. The effect is believed to be related to interaction between the C-H surface dipoles and negatively charged radicals in solution. The oxygen termination (c) resulted in a high density of surface states at approximately 1.7 eV above the valence band. No pH sensitivity was observed, thus the energy level of the state is not moved. (C) 2001 Elsevier Science B.V. ALL rights reserved.