From ZnF2 to ZnO thin films using pulsed laser deposition: Optical and electrical properties

The growth of Zn-based thin films, from pure ZnF2 to pure ZnO, has been investigated as a function of the oxygen partial pressure and substrate temperature using pulsed laser deposition. Starting from (1−x) ZnO−x·ZnF2 target compositions, ZOF films containing a low fluorine content (x<0.10) have been successfully deposited. These films exhibit typical ZnO wurtzite structure while maintaining the (002) preferred orientation. The fluorine doping does not modify the film transparency (T≈90%), whereas for a high fluorine content, a large haze effect is observed. Such effects are associated with increased surface roughness. The Hall effect measurements show a beneficial effect with the addition of F corresponding to a decrease in resistivity for ZOF thin films accounting for oxygen to fluorine substitution in the ZnO structure. In contrast, attempts at annealing in air of successfully deposited pure ZnF2 thin films does not lead to increased conductivity in the films; this is associated with a progressive transformation from ZnF2 to ZnO. Finally, improvements in the electrical properties are proposed for F and Si co-doping, and resistivities as low as 7.2×10−4 Ω cm for SZOF thin films deposited at RT from a ZnO (87 at%)–ZnF2 (10 at%)–SiO2 (3 at%) target are achieved.