Detection of chemically induced hot charge carriers with ultrathin metal film Schottky contacts

Energy dissipation during chemical reactions at metal surfaces may excite electron–hole pairs in the metal. Direct detection of such reaction-induced hot electrons and holes is feasible using solid state barrier devices like Schottky diodes with ultrathin metal films. While exposing the diodes to reactive gases, a chemicurrent is observed in the diodes. The concept of hot charge carrier detection by chemicurrent measurements and the dependence of the current strength on device properties are discussed in detail. Data recorded from thin film Cu/n-Si(1 1 1) and Ag/n-Si(1 1 1) diodes exposed to atomic hydrogen and atomic oxygen are presented. The current detection sensitivity is improved by a factor of 10 if the metal films are annealed to room temperature after low-temperature deposition. This annealing effect is related to a reduced scattering of hot electrons in the metal. Chemicurrents are attenuated exponentially with increasing metal film thickness. Attenuation lengths between 6 and 11 nm are observed. They are much smaller than attenuation lengths for photo- and internal photoemission currents. The results demonstrate that chemicurrents are due to hot charge carrier excitation and transport and are not attributed to surface chemiluminescence and photon reabsorption in the device.