Charge transport mechanism in high conductivity undoped tin oxide thin films deposited by reactive sputtering

This paper reports the charge transport mechanism at low temperatures in (110) oriented polycrystalline tin oxide (SnO2) films of less than 100nm thickness prepared by reactive sputtering in the substrate temperature (TS) range of 350–450°C. Undoped tin oxide films with high electrical conductivity of 11Ω–1cm–1 have been achieved at 350°C. The Hall mobility increases from 6.7 to 13.9cm2/V·s and carrier concentration decreases from 11×1018 to 0.9×1018cm–3 as TS is increased. In the 300–100K temperature range we found two types of conduction mechanisms: thermally activated conduction till 160K and nearest-neighbour-hopping conduction below 160K. At temperatures below 90K, the Mott variable-range-hopping (VRH) conduction governs the charge transport. In all the three regimes the activation energies of conduction increase with an increase in TS, being 15–50, 0.2–9, and 2–6meV respectively, consistent with a decrease in oxygen vacancies at higher TS. The analysis of the data in Mott VRH conduction regime suggests a systematic localization of the oxygen vacancy states with an increase in growth temperature. Even at 70nm thickness the films behave as three dimensional with regard to the Mott VRH conduction process. ► High conductivity obtained even at lower thicknesses in undoped SnO2 films ► Oxygen vacancy control by means of growth temperature ► Three conduction mechanisms with activation energies scaling with growth temperature ► Localization of oxygen vacancy states with an increase in growth temperature ► Native donor states lie 15–50meV below conduction band.