Dependence of Thermal Annealing on Transparent Conducting Properties of HoF_3-Doped ZnO Thin Films

A kind of n-type HoF_3-doped zinc oxide-based transparent conductive film has been developed by electron beam evaporation and studied under thermal annealing in air and vacuum at temperatures 100–500^\circ\!C. Effective substitutional dopings of F to O and Ho to Zn are realized for the films with smooth surface morphology and average grain size of about 50 nm. The hall mobility, electron concentration, resistivity and work function for the as-deposited films are 47.89 cm^2/Vs, 1.39\times 10^20 cm^-3, 9.37\times 10^-4 \Omega\cdotcm and 5.069 eV, respectively. In addition, the average transmittance in the visible region (400–700 nm) approximates to 87%. The HoF_3:ZnO films annealed in air and vacuum can retain good optoelectronic properties under 300^\circ\!C, thereinto, more stable electrical properties can be found in the air-annealed films than in the vacuum-annealed films, which is assumed to be a result of improved nano-crystalline lattice quality. The optimized films for most parameters can be obtained at 200^\circ\!C for the air-annealing case and at room temperature for the vacuum annealing case. The advisable optoelectronic properties imply that HoF_3:ZnO can facilitate carrier injection and has promising applications in energy and light sources as transparent electrodes.