Synthesis and characterization of zinc oxide co-doped with gadolinium and aluminum using co-sputtering technique for p-n heterojunction diode application

Diluted magnetic semiconductors (DMS) are being extensively researched as a significant step toward the development of spintronic devices. The discovery of appropriate materials that exhibit ferromagnetic behavior at room temperature and high magnetism is critical for the realization of such devices. However, generating ferromagnetism in ZnO based DMS remains a major obstacle to the fabrication of spintronic devices operating above room temperature and the understanding on the origin of its ferromagnetism is still lacking. In this study, shallow donors of Aluminium (Al) was incorporated into Gadolinium (Gd) doped Zinc Oxide (ZnO) films to explore the possibility of developing a new DMS through co-sputtering technique deposited at room temperature. This study investigated the effect of sputtering parameters and the effect of dopant amount (Gd and Al) on the film characteristics. The findings reveal that 3 at% of (Gd, Al) co-doped ZnO exhibited good physical properties with enhanced magnetic behavior at room temperature as compared to the Gd-doped ZnO and undoped ZnO. X-ray diffraction (XRD) study confirmed the films are well crystalline indexed to the hexagonal wurtzite structure of ZnO with no secondary phases and further supported by energy-dispersive spectroscopy (EDS) analysis study that indicating the existence of Zn, O, Al and Gd elements in the prepared film. Homogeneous nanostructure with well-aligned structure as well as small grains observed field-emission scanning electron microscope (FESEM) and atomic force microscopy (AFM) correlates with the magnetic properties, which contributes to the improvement in saturation magnetization (Ms) and high coercivity (Hc). The optical transmittance obtained above 90% in the visible region with band gap was found red-shifted by Al codoping. The incorporation of Al into Gd-doped ZnO demonstrated a free electron carrier concentration dependence, which increases considerably when the carrier concentration surpasses ~5.3x1026 m-3. The magnetic force microscopy (MFM) measurement proved the existence of room temperature ferromagnetism and spin polarization in 3 at% (Gd, Al) co-doped ZnO film as it exhibited smaller domain size with shorter magnetic correlation length L, larger phase shift Φrms and highest value of δfrms. These findings were further supported by the room temperature M-H curves of the 3 at% (Gd, Al) co-doped ZnO film with improvement of Ms and Hc by 37.9 % and 60.7 %, respectively from 3 at% Gd-doped ZnO film, which the film were induced by carrier-mediated ferromagnetism. Potential n-ZnO based DMS/p-Si heterojunction diode was also demonstrated with the use of (Gd, Al) co-doped ZnO film indicating lowest leakage current of 1.28 x 10-8 A and the ideality factor, n of 1.11 almost to ideal diode behavior of n=1 as compared to the p-Si/n-Gd-doped ZnO and p-Si/n-undoped ZnO heterojunction diodes. The obtained results conclude that (Gd, Al) co-doped ZnO films synthesized by co-sputtering technique have improved electrical and magnetic properties where the films indicate room temperature ferromagnetism with the origin of its magnetism were induced by carrier-mediated ferromagnetism, thus proving that this type of DMS is a promising material for potential spin-based electronic application.