Understanding near infrared absorption in tin doped indium oxide thin films

摘要

Tin doped indium oxide (In2O3:Sn (ITO)) thin films RF sputtered under similar conditions on soda lime glass and single crystal silicon wafers (c-Si) are used to study near infrared optical absorption variations during film growth. The presence of strong free carrier absorption and phonon absorption in the low photon energy spectral range provides an opportunity to disentangle tailing effects extending in the near infrared to visible spectral range. Toward that end, a model describing ITO film optical properties in the form of the complex dielectric function (epsilon = epsilon(1) + i epsilon(2)) from ex situ ellipsometry collected over 0.4 to 4.1 meV, 0.035 to 0.4eV, and 0.75 to 5.89 eV spectral range is developed to describe these features along with other higher energy electronic transitions. In situ real time spectroscopic ellipsometry (RTSE) from 0.75 to 5.89 eV is used to track changes film structure and optical response during thin film growth. Optical emission spectroscopy indicates the plasma is very stable during deposition implying that film property changes with thickness are not correlated with changes in plasma chemistry. Film resistivity (rho), mobility (mu), scattering time (tau), and carrier concentration (n) are determined from the free carrier absorption component of epsilon. Increased near infrared absorption manifested in epsilon obtained from RTSE data analysis is hypothesized to originate from enhancement of OH-group phonon absorption due to the presence of water molecules in the chamber at the beginning of growth.