Control of ion bombardment energy in the low-temperature deposition of highly transparent and conducting In2O3 and ZnO thin films by activated reactive evaporation

Abstract

Highly transparent and conducting In2O3 and ZnO films have been successfully deposited at temperatures as low as <70 °C by activated reactive evaporation. Ways to control the anode-to-film potential, which can be used as a rough measure of ion bombardment energy, are discussed. It was found that a minimum anode-to-film voltage of ∼20 V is necessary for high mobility (∼18 cm2 V−1 s−1) for In2O3. However, for the case of ZnO, it was found that there is no such a minimum ion bombardment energy. X-ray diffraction data on low-temperature deposited In2O3 and ZnO films are also presented, showing that ZnO films are in general easier to crystallize than In2O3 films. A postulate was also made that microvoids can be filled up more easily in the case of zinc oxide deposition because the low sticking coefficient of zinc allow its vapor to go around corners more easily and thus the process is not pure physical vapor deposition but has a substantial component that behaves like a surface rate-limited chemical vapor deposition process. Hence the explanation is simply that very little energy is necessary to get ZnO films with an electron mobility of ∼40 cm2 V−1 s−1 by activated reactive evaporation. This is, however, not the case for sputtering and thus this may have an important implication that there can be less ion damage on some sensitive substrates for low-temperature deposition of ZnO by activated reactive evaporation.