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 100 nm thickness prepared by reactive sputtering in the substrate temperature (T-S) range of 350-450 degrees C. Undoped tin oxide films with high electrical conductivity of 11 Omega(-1) cm(-1) have been achieved at 350 degrees C. The Hall mobility increases from 6.7 to 13.9 cm(2)/V.s and carrier concentration decreases from 11 x 10(18) to 0.9 x 10(18) cm(-3) as T-S is increased. In the 300-100 K temperature range we found two types of conduction mechanisms: thermally activated conduction till 160 K and nearest-neighbour-hopping conduction below 160 K. At temperatures below 90 K, 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 T-S, being 15-50, 0.2-9, and 2-6 meV respectively, consistent with a decrease in oxygen vacancies at higher T-S. 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 70 nm thickness the films behave as three dimensional with regard to the Mott VRH conduction process. (c) 2012 Elsevier B.V. All rights reserved.