Correlation between structural and electrical properties of sprayed tin oxide films with and without fluorine doping

Abstract

The structural and electrical properties of polycrystalline sprayed SnO 2 films formed at 500 °C have been investigated in a wide carrier concentration range (7 × 10 17 cm −3< N a<4 × 10 20 cm −3). Using electron diffraction, transmission electron microscopy (TEM) and high resolution TEM observations, we find that (i) the grain size is in the 20–80 nm range, (ii) there are a number of planar defects (twins) which are thought to reduce the bulk mobility of crystallites and (iii) flourine doping increases the density of twins. Only the films formed from concentrated SnCl 4 solutions present a columnar structure with a 〈100〉 preferred orientation. The dependence of resistivity at room temperature on the carrier concentration N a determined by Hall measurements can be accounted for within the frame of the grain boundary model developed by Seto (J.Y.W. Seto, J. Appl. Phys., 46 (1975) 5247) for polycrystalline silicon films, using the structural data and considering the contribution of the bulk of crystallites and the role of film degeneracy. For N a⩽5×10 18 cm −3, the resistivity is thermally activated and mainly governed by the barrier effect of grain boundaries which act as carrier traps. The mobility drops for N a≈5×10 17 cm −3, i.e. for a total depletion of each grain. The grain boundary trap density Q t is evaluated as about 4.0 × 10 12 cm −2. For N a⩾5 × 10 18 cm −3, the film resistivity is governed indeed by the properties of the bulk material, the mobility of which is limited at 25 cm 2V −1 s −1 because of the high density of structural defects.