Preparation and Characterization of ZnO based Diluted Magnetic Semiconductor (DMS) Materials

Abstract

ZnO based diluted magnetic semiconductors (DMSs) have been extensively studied during last decade because of their potential applications in spintronic devices. The search for completely new DMS materials with room temperature (RT) ferromagnetism, as well as designing and modification in existing DMS materials are some important and recent issues regarding DMSs. The achievement of RT ferromagnetism in these materials is itself a great challenge. The controversies among researchers suggest that ferromagnetic behavior and its origin in DMSs depends on composition, synthesis conditions, annealing parameters and additional dopants for varying the concentration of carriers as well. A meaningful devotion has been given to magnetic elements (Mn, Ni, Co, and Fe) doped ZnO. Moreover, these compositions have also been treated with some additional thermal treatments and doping of Al for brief and consequential observations. An attention to bulk materials has also been given in this research for more precise studies. We have successfully synthesized the phase pure compositions of Zn0.95Mn0.05O, Zn0.95-xMn0.05AlxO (x = 0, 0.05, 0.10), Zn0.95Ni0.05O, Zn0.95Co0.05O and Zn0.90Co0.05Al0.05O DMSs using sol-gel based auto-combustion and chemically derived co-precipitation techniques. XRD studies revealed the formation of phase pure compositions in most cases without any second or impurity phase. However, some traces of manganese oxide in Mn doped ZnO in Ni doped ZnO were detected in samples thermally treated at higher temperatures and synthesized with co-precipitation technique, respectively. EDX analysis confirmed the incorporation of utilized dopants into ZnO matrix. Morphological studies by SEM and FESEM were carried out to analyze the proportion, uniformity, quality and size of grains. Temperature dependent electrical resistivity measurements depicted the semiconducting behavior of materials. Room temperature ferromagnetism was successfully achieved in Zn0.95Mn0.05O composition. The similar composition of Mn doped ZnO was thermally annealed at various temperatures (400, 600, and 800 °C) for 8 hours. It was observed that crystallinity of the samples improved with increase of temperature of thermal treatment. This improvement in crystallinity also affected the resistivity and magnetization attributed to the dependence of physical properties upon structural arrangement. Temperature dependent electrical resistivity measurements showed a decreasing trend with the doping of Al in ZnMnO, which is attributable to the enhancement of free carriers. The results indicate that Al doping results in significant variation in the concentration of free carriers and correspondingly the carrier-mediated magnetization. Auto-combustion and co-precipitation techniques were employed to prepare the Zn0.95Ni0.05O samples. Room temperature ferromagnetism was detected only in sample synthesized with auto-combustion technique. It was confirmed that synthesis techniques strongly influenced the various properties of diluted magnetic semiconductor materials. Room temperature magnetic measurements reveal the absence of ferromagnetism in Co doped ZnO, while Co and Al co-doped sample display the clear room temperature ferromagnetic behavior. The decrease of resistivity and presence of ferromagnetic behavior in Al doped ZnCoO system corroborates the significant role of free carriers.