Electrochemically mediated charge transfer to diamond and other wide band gap semiconductors

High quality diamond is an extremely good insulator with a band gap of 5.5 eV. However, for undoped, hydrogen-terminated diamond exposed to air, a p-type conductive surface layer appears that is mediated by adsorbates from the ambient atmosphere. Although several explanations of this effect have been proposed, none has received wide acceptance and the mechanism remains controversial. Charge transfer to diamond was measured under a wide range of chemical environments and surface terminations. The results unambiguously support the electrochemical transfer doping model.; The surface conductivity of diamond arises from the transfer of electrons from the diamond to an electrochemical redox couple in an adsorbed water film on the surface. When the Fermi level in the hydrogenated diamond is higher than that in water, electrons transfer from the diamond into the film, leaving a positive space charge layer in the diamond and compensating anions in the water. Titration of aqueous solutions of known pH with diamond powder permits the estimation of the position of the diamond Fermi level. Measurements of related properties, e.g., contact angle, work of adhesion, and zeta potential as a function of pH and surface terminations are reported.; This electrochemically mediated charge transfer is seen in other material systems. The work function of semiconducting single walled carbon nanotubes is 4.8--5.1 eV, which places the Fermi level above the electron chemical potential of the oxygen redox couple at pH = 6. Thus electrons transfer out of nanotubes to generate a p-type conductivity similar to that observed with hydrogenated diamond. The results support the model.; It was shown that the intensity of yellow band luminescence in GaN increases with exposure to HCl vapors and decreases with NH3 vapors; the near band edge luminescence shows the opposite behavior. This effect is attributed to electron exchange between the oxygen redox couple in an adsorbed water film and the mid-gap states in GaN responsible for the yellow band luminescence.