Chemical and semiconducting properties of NO₂-activated H-terminated diamond

The H-terminated surface of diamond when activated with NO₂ produces a surface conduction layer that has been used to make field effect transistors (FETs). Previous reports have suggested that during NO₂ exposure (NO₂-activation), NO₂ ⁻ forms on the diamond surface and generates positive carriers (holes) in the diamond, making the diamond surface conductive. We report here on X-ray-photoelectron-spectroscopy (XPS) surface characterization of single crystal diamonds and on infrared absorption of diamond powder. After activation, XPS showed the presence of N atoms on the diamond surface, but infrared absorption found no evidence of NO₂ ⁻, but instead NO₃ ⁻ is present on the diamond surface. Two wet chemistry techniques determined the concentration of NO₃ ⁻ per milligram of diamond powder. With the powder's surface area measured by the BET technique, the surface NO₃ ⁻ concentration was measured to be between 6.2 × 10¹³ and 8.2 × 10¹³ cm⁻². This is in the same range as the carrier densities, 3 × 10¹³ to 9 × 10¹³ cm⁻², determined by Hall mobility and surface conductivity measurements of single crystal diamonds. Using similar techniques, the concentration of NO₂ ⁻ was determined to be <10¹² cm⁻². Both the surface conductance and the surface H atoms are stable in dry nitrogen, with or without NO₂-activation, but the surface conductance, the concentrations of H atoms both with and without activation and NO₃ ⁻ decrease when exposed to laboratory air over a period of hours to days. Infrared absorption measurements showed the reduction of surface NO₃ ⁻ and H atoms during laboratory air exposure, but gave no indication of what reactions are responsible for their loss in laboratory air.