LAUSR

Single-crystalline $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ thin films are grown on MgO (100) substrates with plasma-assisted molecular beam epitaxy. To prevent the oxidation of the $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ films and allow further physical characterization, a polycrystalline MgO cap is deposited in situ. The growth orientation of the $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ films can be tuned from (100) to (111) by changing the growth conditions, and the associated epitaxial relationships have been determined by means of x-ray diffraction. The lattice constant of $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ films has been monitored as a function of temperature from 300 to 900 K, determining thereby the linear thermal expansion coefficient. Transmission measurements indicate an optical bandgap of crystalline $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ around 2.9 eV at room temperature, consistent with diffuse reflectance measurements on micrometric $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ particles. These results demonstrate the possibility of exploiting $\mathrm{M}{\mathrm{g}}_{3}{\mathrm{N}}_{2}$ as a new crystalline semiconductor in the blue-violet range.