A Study of Electrical and Optical Properties of Transparent Conducting Sno2 and In203 Thin Films Prepared by Pyrosol Process

Abstract

The design and construction of a simple and low-cost device for the deposition of thin films by the pyrosol process has been described. Doped and undoped films of SnO2 and In203 have been pre- pared on glass substrates. These films were highly conducting and at the same time highly transparent. They were non-stoichiometric but homogeneous and polycrystalline in structure. It is found that an activation energy of about 0.2 eV is associated with the growth process of these films. The sheet resistance of these films shows a size effect below a thickness of about 0.3 um. Doping has been performed by adding autimony (Sb) and fluorine (F) in SnO2 film and higher figure of merit in comparison to other earlier reports has been obtained. In In203 film Sn has been used as doping element to obtain films of higher conductivity and all mobility. The electrical transport properties and the optical transmission coefficients in the UV-VIS range of these films in the carrier concon-tration range~ 6 x 1018 to ~ 1.3 x 1021 cm3 for Sno2 and from ~3x1018 to~ X1020 са-3 for In203 films with thickness about 0.2 μm have been studied in detail to establish a tontative energy band picture of these degenerate samples. The thermoelectric power of the films were measured in the temperature range 27° to 160°C for Sno2 and upto 175°C for In20 films. Using the thermopower and optical transmission data the effective masses of electrons and of holes have res- pectively been determined as a function of the carrier coneen- tration. The optical band gap and the position of the Fermi levels were determined. It has been observed that the Formi levels are pinned near the conduction band edge at decreasing carrier concentrations, and the optical band gap shows an appre- ciable loss-Burstein shift in the investigatod range of carrier concentration. The effect of post-deposition heat treatment on some of the SnO2 and In203 films has been studied. It has been obser- ved that heat treatment in different ambients have remarkable effects on the film resistivity. No appreciable change of carrier concentration was noticed in SnO2 film but it was noticed in the In203 films. Hall mobilities in these films are found thermally activated at temperatures ranging from room temperature upto about 250°c. Oxygen diffusion mechanism at the grain boundaries of these films has been discussed critically to explain the observed mobility data. It is inferred from the heat treatment experiments that the role of oxygen diffusion process in the film is only to modulate the inter-grain boundary potential barrier heights. For SnO thin films two distinct scattering processes viz the grain boundary scattering and the ionized impurity scatte- ring are found to play a predominant role in limiting the Hall mobility in the two different ranges of carrier concentrations at room temperature. For the In203 films however the lattice scattering is found to be the dominant process limiting the carrier mobility. Finally, an attempt has been made to determine the electron affinity and work function of these samples on an empiri- cal basis using the electro negativity concept. The crystal bindings of these samples were also determined and were found to be of fairly ionic in nature.