Microstructure and physical properties of nanostructured tin oxide thin films grown by means of pulsed laser deposition

The pulsed laser deposition (PLD) of tin oxide (SnO2) thin films onto both alumina and quartz substrates have been achieved by ablating poly-SnO2 pellets with a KrF excimer laser. At a laser fluence of ∼ 4.6 J/cm2, the effect of both background pressure (vacuum and 150 mTorr of oxygen) and deposition temperature (Td ranging from 20 to 600 deg. C) were investigated. While all the films deposited under 150 mTorr of oxygen consisted of pure polycrystalline SnO2 phase and were almost stoichiometric ([O]/[Sn] ∼1.9) even for Td as low as 20 deg. C, those deposited under vacuum were found to be composed of both amorphous SnO and poly-SnO2 phases. Scanning and transmission electron microscopy observations have revealed that the micro-/nano-structure of the PLD SnO2 films changes from a relatively porous fine-grained to a completely columnar microstructure as Td is raised from 20 to 600 deg. C. In particular, the average diameter of the SnO2 nanograins was found to increase markedly (from 4 to ∼12 nm) when Td is increased from 20 to 450 deg. C and above. On the other hand, the resistivity (ρ) of the PLD SnO2 films, deposited under 150 mTorr of oxygen, was found to decrease significantly from 200 to ∼2 Ω cm when Td is increased from 150 to 450 deg. C. The optical band gap (Eg) of the PLD SnO2 films increased from ∼2.6 to ∼4.0 eV when the deposition environment is changed from vacuum to 150 mTorr of oxygen while it is slightly affected by the variation of Td. The observed variations of ρ and Eg are correlated to nanograin size variation and oxygen vacancies concentration in the films, respectively