Crossover from small polaron tunneling to correlated barrier hopping and its evolution with thermal instability of SnO surface

In this paper, we have studied the surface and bulk stoichiometry and its effect on DC-AC conductivity mea-surements by x-ray photoelectron spectroscopy (XPS) as well as Raman spectroscopy depth profiling respectively. The SnO surface sheets are readily oxidized to Sn3O4 phases and partially SnO2. The temperature dependence of DC resistance shows three-dimensional variable range hopping (VRH) below 200 K and Nearest Neighbor Hopping (NNH) between 200 and 300 K and undergoes a crossover to the thermally activated band conduction above 300 K till it decomposes at 600 K. This crossover temperature increases beyond 300 K during the sub-sequent heating cycles. In the frequency and temperature dependent conductivity, the bulk SnO throughout 300-600 K shows the frequency exponent of 0.8 which is ascribed to tunneling mechanism. Whereas, the surface progressively changes from correlated barrier hopping, CBH (band conduction) to a crossover of non-overlapping small polaron tunneling to CBH in samples in 300 to 400 K range after a thermal cycling. The oxidized phases bring about the polaronic sites which tunnel below 400 K. The energy of hopping reveals that the pristine sample has band like transport due to shallow Sn vacancies acceptors and the oxidized surface repositions fermi level to midgap due to oxygen vacancy donors.