Influence of rapid thermal annealing time on physical properties of Mg and F co-doped ZnO thin films deposited by radio frequency magnetron sputtering

Abstract

• Mg-F co-doped ZnO (MFZO) thin films are deposited by magnetron sputtering. • Conductivity of MFZO films increases by 37% after the rapid thermal annealing. • The average visible transmittances of the annealed MFZO films exceed 93%. • The annealed MFZO film achieves a high figure of merit (FOM) of 1.18 × 10-2 Ω-1. • The high-FOM MFZO thin films could be used as transparent conducting films. Transparent conducting oxides are commonly used in flat-panel displays, solar cells, light emitting diodes, and other optoelectronic devices. In this study, transparent conducting Mg and F co-doped ZnO (MFZO) thin films were prepared by using radio frequency magnetron sputtering and a post-deposition rapid thermal annealing (RTA) was carried out at 400°C in vacuum. To study the influence of RTA on structural, electrical, and optical properties of the MFZO thin films, the RTA processing time is varied (0–120 s). X-ray diffraction analysis showed that the co-doping of 3.0 at% Mg and 6.0 at% F did not change the (0 0 2) oriented hexagonal wurtzite structure of the MFZO films, while the RTA process effectively improved its crystal quality. The intensity and full width at half maximum of the (0 0 2) peaks revealed that the MFZO films had the optimum crystallinity at the RTA time of 15–60 s and the largest grain size at the RTA time of 30 s. Under this optimal RTA condition, the electrical conductivity increased by 37%, reaching the optimum Hall mobility of 12.5 cm 2 /Vs, carrier concentration of 3.94 × 10 20 cm −3 , and resistivity of 1.27 × 10 −3 Ω cm. The average total transmittance in the visible region (380 – 780 nm) increased to more than 93% at the RTA duration of 15–60 s, and then decreased for a longer RTA time. The optical band gap, determined from the absorption edge of transmittance using Tauc plot, increased from 3.745 to 3.798 eV after RTA for 30 s. The figure of merit (FOM) of the MFZO film increased by 52% to 1.18 × 10 −2 Ω −1 . Thanks to the use of appropriate dopants and RTA process, this high FOM is much superior to previous research results on MFZO thin films with FOMs ranging from 10 −6 to 10 −3 Ω −1 . This work provides a fast and low-temperature approach to realize high-FOM MFZO thin films to fulfill the requirements of a transparent electrode in optoelectronic devices.