This work has been concerned with the measurement and analysis ofthe electro-optical properties of thin films of indium tin oxide deposited by ion-beam sputtering. In the present context the principal use of these film is for window layer solar cells and it is necessary tohave an accurate knowledge of the optical and electricalproperties, and their dependence on deposition conditions, to enable realistic predictions of the internal quantum efficiencyto be made. we have measured the properties of film as a function of deposition rate,partial pressureof oxygen in the sputtering gas, and annealing temperature. The properties studied are the variation with wavelength of the optical constants, the Hall mobility, the film structure and composition.
The compositional data (XPS) suggest that heat treatment causes the Sn to substitute for In atoms, and hence to act as singly ionized donors. FIlms deposited in a lowpartialpressure of oxygen also have doubleionized oxygen vacancies. Estimates of the carrier density have been made using XPS measurments of the Sn content and the atomic deficyency of oxygen. Comparison with the measured values gives reasonable agreement. Films deposited in a relatively high partial pressure of oxygen show avery low carrier concentration before heat treatment. However, in this case also, heating to about 350º-400°C causesan increase in the free carrier concentrationto about thesame levelas for films with a near stoichimetric oxygen content. This demonstratestherole of excess oxygen as an electron trap.
Oxygen deficient filmswhich have not been annealed exhibit a mobility which appears to be tering. After annealing (and the consequent grain growth) the mobility is limited only by ionized impurity scattering. Oxygen rich films which have not been annealed exhibit a mobility which is limited only by grain boundary scattering. After annealing, these also are limited by ionized impurities only. The minimum resistivity achievable therefore appears to depend upon the unavoidable interaction between carrier concentration and mobility.
From the optical data it is shown that the optical gap varies with the free carrier concentration according to the Moss-Burstein effect. In the near IR,films exhibit a plasma absorption edge in accordance with the Drude free carrier model.
