Clark I. Bright, Bright Thin Film Solutions LLC, Tucson, AZ
High mobility Transparent Conductive Oxide (TCOs) results in lower absorption than a TCO with higher electron density (carrier concentration, N) for equivalent conductivity (or resistivity) are presented. Lower absorption permits higher potential transmittance for an optimized layer construction. Examples of TCOs with high luminous transmittance and spectral transmittance in the visible and near infrared (NIR) are presented. A design strategy was previously described, which preferably starts with identifying the application type, e.g., electrode, heater, EMI shield, etc., and the specified optical and electrical requirements. This strategy was used to design and deposit an indium oxide (IO), thin film EMI shield exceeding the optical and electrical requirements of the application and achieved superior optical performance to a commercial ITO. Once the IO TCO material was chosen, the deposition process (and any post deposition processing) became the dominant factor determining performance. Controlling the carrier concentration, the physical thickness, and optical thickness during the deposition process also was used to tune the TCO coating properties to match application requirements and achieve high mobility with superior optical performance for other applications. The IO thin films discussed here were deposited by reactive vacuum evaporation of indium metal with oxygen during to control carrier concentration, determined by oxygen vacancies and increased mobility. Using IO also had the advantage of being deposited at considerably lower substrate temperature (~ 230 °C) than typical for TCOs with transition metal doping for high mobility, e.g., Mo, (~ 500 °C) and many commercial ITO. Typically, only DC values of resistivity, or conductivity, for TCO are reported, although both are frequency (wavelength) dependent through mobility. At optical frequencies the corresponding optical mobility values were of more interest for comparing TCO performance. Simulations using ellipsometry for fitting of the measured IO spectral performance were used to determine the plasma wavelength and to derive the optical constants. Knowing the plasma wavelength then allowed calculating carrier concentration. Next the resistivity was determined from measuring the IO coated parts. Using the basic conductivity relationships and the low N values determined from the simulations for the IO coatings developed here, optical mobilities were derived at several visible and NIR wavelengths.