Estimation and Verification of the Electrical and Optical Properties of Indium Tin Oxide Based on the Energy Band Diagram

Indium tin oxide (ITO) has been the center of much experimental effort; however, due to its complex crystalline structure, very little effort has gone into the basic understanding of the material properties from a theoretical point of view. Based on ab-initio Hartree-Fock calculations using sc CRYSTAL92, the energy band diagrams for In_2 O_3 and ITO are shown here for the first time. From the energy band diagrams, the electron effective mass is determined. An empirical relationship of the effective mass as a function of the free carrier concentration is established. Estimates of the electron mobility, conductivity and optical transmission are made as function of the ideal free carrier concentration. From these estimations, four separate figures of merit are determined. Each figure of merit predicts an optimum carrier concentration based on the estimated optical and electrical properties. Carrier concentration ranges based on 90% of the maximum predicted figure of merit for each method lie within 4 times 10^{20} cm ^{-3} to 2 times 10^{21} cm ^{-3}. Several ITO films are produced in an argon/oxygen atmosphere using a rf sputtering system. The electrical properties of the films are characterized by four point probe and Hall effect measurement. The optical properties consist of measurements of the film's optical transmission. The optical results show very good agreement with the theoretical predictions. The measured modilities are much lower than expected. Despite the low measured mobilities, the ITO films have conductivities above 500 Omega ^{-1} cm^{ -1} and average optical transmission greater than 90%. The importance of the electron effective mass in predicting the electron mobility, electrical conductivity, and optical properties of ITO is discussed here. The electrical conductivity is directly proportional to the electron concentration and mobility. Improvement in the electron mobility will improve the overall electrical properties. Based on the predicted electron effective mass, electron mobilities of 100 cm^2 V^{ -1} s^{-1} or better should be possible. This will result in the electrical conductivity increasing to above 10 ^4 Omega^{-1} cm^{-1}. The optical properties predicted by Drude's theory are estimated based on the varying effective mass and compare well with the experimental results.