Analysis on thermal properties of tin doped indium oxide films by picosecond thermoreflectance measurement

An attempt of observation of thermal diffusion across tin doped indium oxide (ITO) thin films perpendicular to the film surface has been carried out using a picosecond thermoreflectance measurement. ITO films sandwiched by molybdenum (Mo) films were prepared on fused silica substrate by rf magnetron sputtering using ITO and Mo multitargets. Such Mo/ITO/Mo layered structure was fabricated without exposure to the atmosphere between each deposition. The Mo films with a thickness of 70 nm are necessary because the wavelength of pulse laser used in this study is 780 nm at which wavelength ITO is transparent. The ITO films with a different thickness of 27, 46, and 62 nm were prepared as the intermediate layer in order to estimate thermal resistance at the interface between Mo/ITO. The resistivity, carrier density and Hall mobility of the ITO films ranged from 5.2 to 7.6 × 10 − 4 Ω cm , 3.5 to 3.9 × 10 20 cm − 3 , and 23 to 35 cm 2 ∕ Vs , respectively. The interface between the Mo films and the fused silica substrate was irradiated by picosecond laser pulse (2 ps). Heat generated by the pump laser pulse diffuses toward the top Mo surface across the three-layered films. Then the temperature changes at the Mo film surface, which was probed by reflectivity of another picosecond laser pulse. The heat diffusion time increased with the increase in the thickness of the ITO layers. The thermal diffusivity of the ITO films and thermal resistance at ITO/Mo interface were found to be 1.2 × 10 − 6 m 2 ∕ s and 5.1 × 10 − 9 m 2 K ∕ W , respectively.