Photoemission of Electrons from Silicon into Silicon Dioxide

Electron transport in thermally grown layers of SiO₂ has been studied. Electrons are introduce into the oxide by photoemission from the adjoining silicon crystal. Specimens consist of a silicon crystal covered with an SiO₂ layer about 2 microns thick and, over this, a semitransparent gold electrode. On illuminating, light passes through the gold electrode and the oxide and is absorbed by the silicon. With appropriate voltage applied between gold and silicon, there is a steady photocurrent for light of wavelengths shorter than 2900. Various possible origins of the photocurrent were investigated, and it is concluded that the current is due to photoemission of electrons from the silicon into the SiO₂ conduction band. By analysis of the spectral response of the photocurrent a photoemission threshold of 4. 25 eV is obtained, independent of whether the silicon is n-type or p-type. This is roughly 0. 9 eV smaller than the threshold for photoemission from silicon into vacuum. Using the vacuum photoemission threshold and the known optical absorption edge for SiO₂, a diagram of the energy relations at the Si-SiO₂ interface is constructed. It is found that deep electron traps are present in concentrations around 310^14/cm^3. Trapped electrons are stable against thermal ionization for hours at room temperature in the dark but may be ionized by visible light. From the spectral response for ionization of traps it is found that the trap levels lie 2 eV below the conduction-band edge. Measurements of the kinetics of trapping give a capture cross section or 1. 310^-12 cm^2, indicating that the trap is a Coulomb attractive center with a positive charge. Comparison of the magnitude of the capture cross section with a current model for capture by a Coulomb attractive center gives an estimate for the microscopic mobility of electron in the SiO₂ conduction band. The estimated value of is either 34 or 17 cm^2/Vsec depending on whether the trapping center is singly or doubly charged.