Electrical, optical and magnetic properties of Er-doped ZnO grown by pulsed laser deposition

Abstract

We have performed a systematic study on the structural, electrical, optical and magnetic properties of Er-doped ZnO grown by the pulsed laser deposition. The effects of oxygen pressure P(O2) during growth, Er doping concentrations and post-growth annealing treatment were investigated. All the samples identified to have a wurtzite crystal structure of ZnO. A decreasing trend in the value of c-parameter was found with the increase of P(O2) during growth. Degradation in the crystalline quality of these samples with the increase in Er concentration was revealed. The surface and cross-sectional morphology of Er-doped films display columnar-grains growth at high P(O2), and an optimal oxygen pressure was found essential to enhance the surface and cross-sectional morphology in these samples. An optical transmittance of ~97% was observed in the visible range of wavelength (400-800 nm) and a low resistivity of 3.8×10-4 Ωcm in the 1% Er-doped films grown in the absence of oxygen. Moreover, all the Er-doped films studied in this work showed an optical transmittance of >85% in the visible range electromagnetic spectrum which makes Er-doped ZnO an attractive transparent conducting oxide. 1% Er doped samples were found to show comparatively strong characteristic Er related emission originated from the intra-4f-shell transition. No intra-4f-shell transitions were observed in the samples with very low concentration (0.5%) of Er. An optimized P(O2) and annealing at 750°C were disclosed crucial for the enhanced intra-4f-shell transitions of Er. The Er doping in ZnO enhances the electron concentration (n) of undoped ZnO from ~1018 to ~1020 cm3. A corresponding widening of the band gap due to Burstein-Moss effect was observed in the degenerate n+ samples. Post-growth annealing treatment would lead to dropping of electron concentration and the corresponding decrease of the band gap. The effect of room temperature hydrogen plasma treatment (HPT) on the optical properties of optically active Er-doped ZnO films was also investigated. It was revealed that the HPT deactivates the Er characteristic emissions and reannealing of the samples after HPT recovers the intra-4f-shell transition 4F9/2→ 4I15/2 and 4S3/2→4I15/2 of Er. As HPT involves surface modification, the loss of Er optical activity indicated the presence of Er optical active centers on the surface of these films. XPS spectra taken on the surface and in the bulk was employed to prove that Er related optical signals that are induced by thermal annealing occur because of the Er atoms residing on the surface of these samples. Field-sweep measurement was performed on Er-doped ZnO as-grown and annealed samples at 300 K. Increasing the Er content in these films leads to a drop in the value of magnetic moment of these samples. Furthermore, an increase in magnetic moment with P(O2) was also observed. Annealing was revealed to degrade magnetic property of Er-doped ZnO. Absence of any bifurcation in the FC/ZFC measurements indicates absence of magnetic nano-particles/phases. Therefore, the observed ferromagnetism was attributed to be an intrinsic property of Er-doped ZnO. The temperature dependent magnetization showed that the curie temperature of as-grown Er-doped ZnO samples is well above room temperature.