Carrier transport in oxygen-rich polycrystalline-silicon films

A symmetrical-Schottky-barrier (SSB) band model is proposed for the carrier transport in oxygen-rich polycrystalline-silicon films. Semiquantitative analyses were made, comparing a one-dimensional depletion approximation, with voltage and temperature-dependent current measurements. The band gap is found to be similar to that of single-crystalline Si, with electrons as the majority carriers. A net donor state density of 2.3×1019 cm−3, 0.12 eV below the conduction band edge, exists in the grains of the polycrystalline silicon, and these donors are completely ionized in the presence of large grain-boundary surface states. This results in a grain-boundary surface-charge density of −2.3×1013q/cm2, setting up overlapped space-charge regions which give rise to the SSB band structure with a maximum band bending of 0.44 eV. The Fermi level is found to be pinned at the midgap at the grain boundaries—suggesting the existence of an amorphous intergrain layer. At and above 373 K, the carrier transport is by thermionic emission, TE. Below 373 K, it is dominated increasingly by a tunneling process, i.e., thermionic field emission, TFE. The current increases with applied voltage according to a sinh function. A calculated effective Richardson constant A** of ∼70 A cm−2 K−2 and a lower quantum-mechanical reflection, but higher electron-optical phonon backscattering than those of a metal-Si Schottky barrier, are found in the TE regime. The grain size deduced from the model agrees with that measured with a TEM: 100±41 Å. The band picture also agrees with the photoelectric threshold measurement.