Seminar Tuesday August 4 – Tristan de Boer

Speaker: Tristan de Boer
Location: Physics 175
Time: 3:30 August 04 2015

Abstract: Broadly speaking, an important factor behind our ever-improving standards of living is the continuous improvement of the basic materials used in the devices around us. Synthesizing new materials and characterizing them is an essential part of developing new and improved devices. To this end I’ll present recent work on two material systems, MSiN2 (M = Mg, Ca) and In2O3. Our approach utilizes spectroscopic X-ray measurements sensitive to the partial electronic density of states together with first principles density functional theory (DFT) calculations to gain insight into the electronic properties of material systems.

Nitridosilicates MSiN2 (M = Mg, Ca) are promising candidates for manifold industrial applications which require excellent thermal, mechanical and electronic properties, as well as for rare-earth doped LED-Phosphors. For MgSiN2 the band gap has been determined to be 5.6 ± 0.2 eV, in agreement with a theoretically predicted band gap of 5.72 eV. For CaSiN2 the band gap has been determined to be 3.8 ± 0.2 eV in contrast with a theoretically predicted value of 4.88 eV. Good agreement between the measured and calculated spectra is obtained, supporting the calculated electronic density of states of these materials.

Recent studies on In2O3  have revealed a rich phase diagram and led to the discovery of new polymorphs, including the synthesis and ambient recovery of Pbcn In2O3. The electronic properties of this new phase are studied together with other better known polymorphs (Ia-3 and R-3c) using soft X-ray absorption and emission spectroscopy. Together with complementary full-potential all electron DFT calculations, this allows important material parameters, such as the electronic band gap and partial density of states, to be elucidated. Excellent agreement between experiment and theory is obtained, with band gaps of  3.2±0.3, 3.1±0.3 and $2.9±0.3 eV determined for the  Ia-3, R-3c and Pbcn In2O3 polymorphs, respectively. The effective mass of carriers in Pbcn In2O3 is predicted to be 12% less than in the widely used Ia-3 polymorph while having a similar effective optical band gap.

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