ELECTRICAL AND STRUCTURAL-PROPERTIES OF SEMIINSULATING POLYCRYSTALLINE SILICON THIN-FILMS

Semi-insulating polycrystalline silicon layers with oxygen concentrations ranging from 2 up to 30 at. % 0 have been prepared by low-pressure chemical vapor deposition. After deposition, the samples were annealed at 920-degrees-C for 30 min. Grain-size distributions, high- and low-frequency dielectric constants were measured, respectively, by transmission-electron microscopy, capacitance, and optical measurements as a function of the oxygen content. The average grain radius decreases with the oxygen content from 15 up to 2.5 nm. The current-voltage characteristics have been measured as a function of temperature in the range 80-450 K and under applied transverse electric fields up to almost-equal-to 10(6) V/cm. In weak-transverse-field conditions, the current density as a function of temperature shows two thermally activated regions at low and high temperatures, with activation energies of almost-equal-to 0.14 and almost-equal-to 0.54 eV, respectively. The application of transverse electric fields of the order of almost-equal-to 10(6) V/cm produces a current enhancement depending upon field intensity, temperature, and oxygen content. The results have been modeled by assuming thermionic emission, tunneling, and Frenkel generation in a long series of Schottky barriers formed at the boundary of the adjacent grains. The best-fit values of the model parameters indicate that for 30 at. % 0 a continuous SiO2 shell, two monolayers thick, surrounds each grain. For lower oxygen contents this shell is discontinuous and the carrier transport parameters change considerably.