The Preparation And Study Of 3D Transition Metal Doped In₂O₃ Diluted Magnetic Semiconductor Materials

Diluted magnetic semiconductos (DMSs) have attracted great interest for their potential applications in spintronic devices because the charge and spin of carriers can be simultaneously controlled. In2O3 is a wide band gap (3.75 eV) transparent semiconductor with cubic bixbyite crystal structure. The solubility of transition metal (TM) in In2O3 host lattice was found to be very high, which can effectively avoid the formation of magneitc impurities and also makes it possible to obtain homogenous DMSs for application. In this work, Fe, Mn and Cr doped In2O3 samples were prepared by solid state reaction, a vacuum annealing process and pulsed laser deposition technique. Based on the systematical studies of the structure, magnetism and transport properties of the samples, the origin and mechanism of ferromagnetism in In2O3-based DMSs were discussed. The results are summarized as follows:(1) (In1-xFex)2O3 (x=0.02,0.05,0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In1-xFex)2O3 powders as a function of Fe concentration and annealing temperature. The (In1-xFex)2O3 powders appear to be homogeneous and single-phase materials, where Fe elements incorporate into the In sites of the In2O3 lattice rather than forming any secondary phases. The samples were ferromagnetic with the magnetic moment of 0.49-1.73μB/Fe and the Curie temperature around 793 K. The results indicate that the observed high temperature ferromagnetism of vacuum-annealed (In1-xFex)2O3 powders is intrinsic rather than from any magnetic impurities, and is attributed to the ferromagnetic coupling of Fe2+and Fe3+ions via an electron trapped in a bridging oxygen vacancy.(2) Fe-doped In2O3 thin films are deposited on sapphire substrates using pulsed laser ablation. The effects of Fe concentration and oxygen partial pressure on the structure, magnetism and transport properties of (In1-xFex)2O3 films are studied systematically. A detailed analysis of the structural properties suggests the substitution of Fe dopant atoms into In lattice sites and the films are textured with (222) orientation. X-ray photoelectron spectroscopy indicates that the valence of substituted Fe in the (In1-xFex)2O3 films varies with oxygen partial pressure, at lower oxygen partial pressures, Fe behaves as a mixture of+2 and +3 valences, whereas Fe3+dominates in the films grown at higher oxygen pressures. Systematic investigations of transport properties for (In1-xFex)2O3 films with a wide range of carrier densities reveal that they occur in both metallic and insulating regimes. The insulating films exhibit variable range hopping at low temperatures and show temperature dependent ferromagnetism, which can be explained by bound magnetic polarons mechanism. For the metallic films, the carrier densities play a crucial role in their robust ferromagnetism and the resistivity and magnetization are independent of temperature; the carrier-mediated exchange mechanism has been suggested as responsible for magnetic ordering in these metallic films. Optical absorption and magneto-optic studies of (In1-xFex)2O3 films indicate further differences between metallic and semiconducting films and show significant magnetic circular dichroism below the In2O3 band edge at room temperature, which also implies intrinsic ferromagnetism.(3) (In0.92Fe0.05Sn0.03)2O3 and (In0.92Fe0.05Cu0.03)2O3 powders and films were prepared by a vacuum annealing process and a pulsed laser deposition technique, respectively. The effects of codpants Sn and Cu on the structure, magnetism and transport properties of samples have been investigated. The zero-field-cooled and field-cooled magnetic measurement indicated that the room temperature ferromagnetism of (In0.92Fe0.05Sn0.03)2O3 and (In0.92Fe0.05Cu0.03)2O3 powders both originate from the Fe3O4 nanoparticles although the x-ray diffraction results revealed no crystalline phase other than cubic bixbyite structure In2O3 to be present in the powders. However, the origin of ferromagnetism for (In0.92Fe0.05Sn0.03)2O3 and (In0.92Fe0.05Cu0.03)2O3 films is very different. In (In0.92Fe0.05Sn0.03)2O3 film, the Fe-Sn can constitute p-n pairs. The Columbic attraction between the n- and p-type dopants with opposite charge state substantially enhances both the thermodynamic and, in particular, the kinetic solubilities of the dopant pairs in concerted substitutional doping. More profoundly, the noncompensated p-n pairs of Fe-Sn prevented the aggregation of Fe ions. The mictostructure and magnetic analyses also confirmed that there was no segregation of any secondary phases. So the room temperature ferromagnetism of (In0.92Fe0.05Sn0.03)2O3 film is intrinsic rather than from any other magnetic impurity phases. The magnetic behavior is consistent with a carrier-induced ferromagnetism model. While, the (In0.92Fe0.05Cu0.03)2O3 film was superparamagnetic, and the observed ferromagnetism originated from the nanometer-sized Fe clusters, which is not desirable for device application.(4) Mn and Cr-doped In2O3 films with Sn co-doping were deposited on sapphire substrate by pulsed laser deposition. The ferromagnetism of Mn-doped In2O3 films shows reversible behavior, which can be switched between "on" and "off" states by controlling the carrier density via varying Sn concentration. The enhanced ferromagnetism in Cr-doped In2O3 films is observed due to the significant increase in the carrier density with Sn doping, and the saturation magnetization can reach 2.10μB/Cr. Most importantly, both of the experiment results reveal that the carrier density and the net spin are two crucial factors for producing and tuning ferromagnetism.In summary, we have prepared different TMs doped In2O3 DMSs materials and studied the various properties of the samples. The results reveal that the carrier densities play a crucial role in the ferromagnetism and the dependence of the magnetism on the carrier density lend support to carrier-mediated mechanism. This clear demonstration of carrier-mediated ferromagnetism at high carrier concentration implies that TMs-doped In2O3 can be used as n-type DMSs at RT, with consequent potential for exploitation in spintronic applications.