Interface Barrier Energy Determination from Voltage Dependence of Photoinjected Currents

Interface barrier energies and their dependence on applied field may be obtained from the voltage dependence of currents produced by electron photoinjection from an electrode. Unlike the conventional Fowler plot method, direct measurement is possible and knowledge of the absorbed light intensity is not required. A theoretical analysis gives the dependence of photocurrent on voltage and photon energy, and it is shown that barrier heights are obtainable by examination of the V-I characteristics to determine the sign of the second derivative. Theoretical forms of the V-I characteristics are derived for specific electron energy distributions which should be generally applicable. Experimental results from MIS structures using thermally grown SiO2 on degenerate n-type silicon are found to be in good agreement with theoretical predictions. Thresholds for emission from the conduction band (3.2 eV) and from the valence band (4.16 eV) are clearly defined in the V-I characteristics. The fact that the difference between these values is significantly less than the Si bandgap is explained by phonon scattering effects in Si and it is concluded that the actual separation between the Si and SiO2 conduction bands is about 3.0 eV. The barrier energy for Au on SiO2 was found to be 4.1 eV, and a value of 2.2 was determined for the SiO2 image force dielectric constant.