Role of transition metal impurities on the functional properties of dilute magnetic nitride semiconductors and high-performance microwave oxide dielectrics.

摘要

The thesis investigates transition metal doping in two types of materials: wide bandgap semiconductors for spintronic applications and oxide dielectrics for microwave applications.; MBE grown Cr-doped GaN has been found to exhibit ferromagnetism at over 900 K. The measured magnetic moment per Cr atom in Cr-doped GaN varied significantly as a function of Cr concentration, with a maximum magnetic moment occurring at 3% Cr. Transport measurements of Cr-doped GaN revealed properties characteristic of hopping conduction. These measurements also inferred that the carrier concentration is similar in magnitude to the measured concentration of magnetically active Cr. This fits well into the scenario that electrons at the partially filled Cr t2 level contribute to the hopping conduction. These results, along with extensive structural characterization, suggest that ferromagnetism in Cr-doped GaN best fits the double exchange like mechanism as a result of hopping between near-midgap substitutional Cr impurity band.; Exchange biasing effects were observed in sample structures of Cr-doped GaN thin films with an antiferromagnetic MnO overlayer. The center of the magnetic hysteresis loop shifts to negative magnetic field by ∼70 Oe when measured after field cooling. Enhancement of the coercive field of the Cr-doped GaN film is also found. The mechanism responsible for the exchange bias is attributed to the exchange coupling at the ferromagnetic Cr-doped GaN/antiferromagnetic MnO interface. The observed exchange biasing indicates that Cr-doped GaN has properties of a conventional ferromagnet and has potential use in practical magnetoelectronic devices.; The effect of Ni-doping on the structural, dielectric and optical properties of Ba(Cd1/3Ta2/3)O3 (BCT) dielectrics has been explored. Rietveld analysis of the X-ray diffraction (XRD) data indicates that the BCT structure is similar to other Ba(B'1/3B" 2/3)O3 perovskites, although the Ta-O-Cd is distorted to an angle of ∼173°; very close to the earlier theoretical prediction of 172°. The XRD analysis also indicates that Ni doping significantly enhances the extent of Cd-Ta ordering in BCT. The temperature coefficient of resonant frequency decreases with Ni concentration up to 2 wt%. While the loss tangent of BCT is reduced at small levels of Ni doping (up to 0.5 wt%), it increases abruptly at higher concentrations. A correlation exists between the loss tangent of Ni-doped BCT samples and the intensity of a continuous absorption background in the optical spectra. This optical activity results from the presence of optically active point defects and is suggestive that these defects play an important role in the microwave loss in BCT dielectrics.