Optical and electrical properties of radio frequency sputtered tin oxide films doped with oxygen vacancies, F, Sb, or Mo

Abstract

Tin oxide films doped with oxygen vacancies, F, Sb, or Mo were made by reactive rf magnetron sputtering of Sn, Sn-Sb, or Sn-Mo in Ar+O2(+CF4) onto glass heated to a temperature up to 530 °C. Electrical dc resistivity, mobility, free-electron density, spectral optical properties, and microstructure were investigated as a function of sputtering parameters. Optimized deposition parameters gave SnOx:(Sb,F) films with high luminous transmittance, low luminous absorptance, high infrared reflectance, and dc resistivity down to 9.1×10−4 Ω cm. Refractive index n and extinction coefficient k were evaluated from spectrophotometric transmittance. In the luminous range, the films had 1.90<n<2.0 and k of the order of 10−2. Hall-effect measurements showed n-type conduction with electron densities in the 1020–1021 cm−3 range. Band-gap broadening from 4.06 to 4.45 eV was observed with increasing electron density. X-ray diffractometry and transmission electron microscopy showed that the structure factor of the films depended on the oxygen content as well as on the specific doping species. A preferred direction of film growth was probably also present. Transmission electron microscopy indicated different grain sizes, between 6 and 30 nm, depending on oxygen content, substrate temperature, and doping species. Optical and electrical properties were compared with results from a quantitative model for wide band-gap semiconductors. The theory is based on heavy n doping by oxygen vacancies or by Sb or/and F and encompasses ionized impurity scattering of the free electrons. It was found that ionized impurity scattering, as well as an additional scattering mechanism tentatively ascribed to grain boundaries, prevailed in the films.