Anomalous transition of major charge carriers from holes to electrons observed in single-crystal films of tungsten

Tungsten (W) films were grown on SrTiO₃ substrates using pulsed laser deposition. X-ray diffraction and transmission electron microscopy demonstrated that these as-grown films are highly epitaxial and single crystalline with the $[00l]$ orientation. A special lattice stacking for the W/STO interface is observed to significantly reduce the lattice mismatching, which can be explained by the coincidence lattice model. The Hall effect has been investigated over the temperature range of 4--330 K. An anomalous transition of the major charge carriers from holes to electrons was observed in these W films upon cooling. The threshold temperature, in which the sign of the Hall coefficient RH was reversed, was found to increase with the film thinning. With the sample's thickness reduced to several unit cells, its major carriers remained electrons even at room temperature. Calculations using the density functional perturbation theory revealed that such a transition from p type to n type could be attributed to the appearance of an electron pocket along the M-{Γ} direction induced by the lattice mismatching between the W film and SrTiO₃ substrate.