Surface electronic properties of H-terminated diamond in contact with adsorbates and electrolytes

A comprehensive summary of surface electronic properties of undoped hydrogen terminated diamond covered with adsorbates or in electrolyte solutions is given. The formation of a conductive layer at the surface is characterized using Hall effect, conductivity, contact potential difference (CPM), scanning electron microscopy (SEM), and cyclic voltammetry experiments. Data are from measurements on homoepitaxially grown CVD diamond films with atomically smooth hydrogen terminated surfaces. The data show that due to electron transfer from the valence band into empty states in of the electrolyte, a highly conductive surface layer is generated. Holes propagate in the layer with mobilities up to 350 cm(2)/Vs. The sheet hole density in this layer is in the range 10(11) to 5 x 10(12) cm(-2), and dependents on pH of the electrolyte or adsorbate. This has been utilized to manufacture ion sensitive field effect transistors (ISFET) from diamond. The drain source conductivity of single crystalline CVD diamond is pH dependent, with about 66 mV/pH, which is in reasonable agreement with the Nernst law. Due to strong coulomb repulsion between positive ions in the electrolyte and the H+-surface termination of diamond, an enlarged tunneling gap is established which prevents electronic interactions between the electrolyte and diamond. This is a virtual gate insulator of diamond ISFETs. Application of potentials larger than the oxidation threshold of +0.7 V (pH 13) to +1.6 V (pH 1) gives rise to strong leakage currents and to partial surface oxidation. In addition, the electronic interaction of diamond with redox couples is characterized using cyclic voltammetry experiments. The results are well described by the transfer doping model which accounts for the specific properties of undoped diamond immersed in electrolyte solutions are covered simply by adsorbates. In addition, numerical solutions of the Schrodinger and Poisson equations are used to show that the density of state distribution of unoccupied valence band states in case of a perfect interface would be governed by two-dimensional properties (2D). (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.