Condensed-Matter Physics & Materials Science Seminar

Compared to Group IV, III-V, and II-VI semiconductors, transition metal oxide semiconductors are relatively unexplored. Fundamental insight into doping and the associated effects on properties are lacking. To this end, we use state-of-the-art epitaxial film growth methods (MBE, PLD and MOCVD), in conjunction with definitive materials characterization plus magnetic, electronic transport, optical and photochemical measurements, to prepare very well-defined materials and elucidate structure-function relationships based on atomic-scale properties. In this talk, I will illustrate this research theme with two examples: (i) Ti-doped a-Fe2O3 and, (ii) N-doped TiO2. Doping a-Fe2O3 with Ti constitutes a novel way to approach the synthesis of diluted magnetic semiconductors for spintronics. a-Fe2O3 is a canted antiferromagnet. Ti(IV), a d0 cation, substitutes for Fe(III), a d5 cation, leading to a localized reduction in moment. The spin orientations in adjacent cation layers of a-Fe2O3 are antiparallel. The spin orientation and layer structure along the c axis can be represented schematically as (Ma-Ma-O3-Mb-Mb-O3…), where M-M represents a buckled cation layer. If Ti(IV) locates preferentially in one particular spin sublattice (either Ma-Ma or Mb-Mb), the total spin will be nonzero, and should increase with dopant concentration. Alternatively, if Ti(IV) is randomly distributed in both spin sublattices, a dilute system with essentially zero net spin is expected to form. MBE-grown a-TixFe2-xO3 is ferromagnetic, but not to the extent expected if Ti replaces Fe exclusively in one magnetic sublattice (the “non-random” phase). Our combined magnetic and structural results suggest that a minority (~1/8) of Ti forms the non-random phase. N-doped TiO2 is of potential interest for bandgap reduction and enhanced visible solar light absorption for water splitting to produce H2 as an energy source. TiO2-xNx rutile was grown on TiO2(110) and a-Al2O3(0001) by MBE, and TiO2-xNx anatase was gro