Defect induced crystal lattice disorder and its effect on the electron-phonon coupling in Fe doped ZnO thin films

We established a strong correlation between the crystal lattice disorder in Fe doped ZnO (Fe_xZn_1-xO) thin films and its effect on the electron phonon (e-p) coupling strength (I_2LO/I_1LO) which decreased with Fe doping concentration, improved with annealing temperature and showed a mixed increasing/decreasing trend by the variation of the argon (Ar) to oxygen (O₂) gas ratio (Ar + O₂, Ar) in the deposition chamber. Fe doping raised the number of phonon modes, intensity and full width at half maximum (FWHM) of the 1LO phonon modes suggesting the confinement of phonons due to the increased crystal lattice disorder in the Fe_xZn_1-xO films. High temperature annealing enhanced the e-p coupling strength reaching a maximum at 500 °C indicating better crystal quality at high temperature. Furthermore, the e-p coupling strength dropped with the doping concentration for the films prepared in Ar + O₂ atmosphere and that showed a mixed increasing/decreasing trend for films prepared in Ar atmosphere. This unique physical phenomena confirmed that the coupling strength not only depend on the dopant concentration but also the dopant induced micro/nano defects that modulate the crystal lattice disorder and the coupling strength in doped ZnO. The as prepared Fe_xZn_1-xO thin films were highly oriented along the c-axis displaying columnar nanorod (film) like morphology at low (high) Fe doping concentration with the co-existence of both Fe²⁺ and Fe³⁺ ions. Fe doping increased the band gap from 3.28 eV for un-doped ZnO to 3.35 eV (1.40 at.% of Fe) and 3.42 eV (3.5 at.% of Fe). Nearly forty (40) thin films were used for a detailed investigation.