Structural, Optical And Electrical Properties Of SnO₂Thin Films

SnO2, which is a transparent conducting oxide, is widely used as transparentelectrode in solar cells, flat panel displays and defrost or anti-moist glasses. On theother hand, due to its high sensitivity in gas, heat and pressure, it is also commonlyapplied in fabricating sensors. As a direct wide band gap semiconductor, SnO2exhibits a high exciton binding of130meV, thus is considered to be a promisingcandidate for ultraviolate photoelectric devices.In this work, undoped and doped (dopant is Mg or Al) SnO2polycrystalline thinfilms were deposited by rf magnetron sputtering. The deposition parameters such asoxygen partial pressure (Opp) and substrate temperature were altered to obtaindifferent series of SnO2thin films. We analyzed the structure, optical and electricalproperties of the films. The the influence of deposition parameters on the properties ofthe films were also carefully studied.The electronic transport property of SnO2was investigated over a widetemperature range. From~380to~2K, the conduction mechanisms of thermallyactivated, nearest neighbour hopping, Mott VRH (variable range hopping) and ESVRH were observed in a sequence. The obtained activation energies (~30and~100meV) in the thermally activated conduction indicates the main contributor ofelectrons in the nominally undoped SnO2is intrinsic oxygen vacancies. Below~80K,the dominate conduction mechanism turns into VRH conduction. With an increase inthe oxygen partial pressure in the deposition process, the disordered degree,crossover temperature from Mott VRH to ES VRH and density of states near Fermilevel (regardless of electron-electron interaction) also increased.The structural analysis shows that the preferred orientation, grain size and surfacemorphology of the films can be effectively tuned by varying oxygen content in thesputtering gas. The grain size and surface roughness decreased with increasing Opp. Inthe undoped SnO2thin films,(001)、(100)、(101) and (211) growth orientationappeared one after another with an increase in the Opp. The shape of (001) and (100)oriented grains is square-based pyramid. The film grew along (101) orientation showsa gravel-like morphology while (211) oriented grains have triangular pyramidal shape.The preferred orientations of Mg (or Al) doped SnO2films, which were deposited under Opphigher than10%, were altered to (301). The crystallization and texture ofthe films was enhanced with a proper increase in the oxygen partial pressure, substratetemperature, or sputtering power.In the optical property aspect, the transmittance of the films in the visible range aswell as the optical band gaps increased with an increase in the Opp. Doping with Mgor Al can also increase the transparency and optical band gap of SnO2thin films.