Crystalline magnesium nitride ( Mg3N2 ): From epitaxial growth to fundamental physical properties

Abstract

Single-crystalline Mg3N2 thin films are grown on MgO(100) substrates by plasma-assisted molecular beam epitaxy. To prevent the oxidation of the Mg3N2 films and allow further physical characterization, a polycrystalline MgO cap is deposited in-situ. The growth orientation of the Mg3N2 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 Mg3N2 films has been monitored as a function of temperature from 300 K to 900 K, determining thereby the linear thermal expansion coefficient. Transmission measurements indicate an optical bandgap of crystalline Mg3N2 around 2.9 eV at room-temperature, consistent with diffuse reflectance measurements on micrometric Mg3N2 particles. These results demonstrate the possibility of exploiting Mg3N2 as a new crystalline semiconductor in the blue-violet range.

Epitaxial Zn3N2 thin films by molecular beam epitaxy: Structural, electrical, and optical properties

“…The RHEED pattern is streaky and very similar to the one obtained for the growth of Mg 3 N 2 (111) on MgO (100), being isostructural to Zn 3 N 2 . 47,48 However, as the growth proceeds in the (111)-oriented films, the growth direction evolves continuously towards [100], as indicated by the presence of additional spots in the RHEED pattern (not shown here). This instability might be associated to the surface energy of cubic (100) and (111) planes, the (100) planes being thermodynamically favorable.…”

“…This value is roughly four times larger than thermal expansion coefficients corresponding to III-nitrides such as AlN 55 or GaN, 56 but in the same range as the one of the isostructural Mg 3 N 2 . 47 The linear thermal expansion coefficient for both samples has been also extracted point-by-point 57,58 using (4) where is the temperature and the temperature difference between two consecutive points.…”

Microstructure of epitaxial Mg3N2 thin films grown by MBE

“…On the contrary, there are no commercial devices based on group II-nitrides, their growth is not well controlled and some of their basic properties are not yet well determined. Among this family of materials, Mg 3 N 2 seems an interesting candidate for device applications due to its relatively large and direct bandgap of 2.8 to 2.9 eV at room temperature [5][6][7] . Additionally, alloying Mg 3 N 2 with Zn 3 N 2 seems a promising way to address solar energy applications, given the possibility to tune the optical bandgap down to about 1.4 eV or lower 8 .…”

“…Additionally, alloying Mg 3 N 2 with Zn 3 N 2 seems a promising way to address solar energy applications, given the possibility to tune the optical bandgap down to about 1.4 eV or lower 8 . Nevertheless, the epitaxial growth of Mg 3 N 2 crystals has been established only recently 7 , previous reports dealing mainly with Mg 3 N 2 micro-powders, which prevented the development of Mg 3 N 2 -based devices.…”

“…Even though a first attempt of growing epitaxial Mg 3 N 2 (100) on MgO (100) by molecular beam epitaxy (MBE) was reported in 2018 9 , detailed structural analysis of these layers was not possible due to low crystallinity. More recently, we demonstrated the possibility of achieving epitaxial Mg 3 N 2 by plasmaassisted MBE and the tuning of the crystallographic orientation depending on the Mg 3 N 2 growth conditions 7 . Indeed, by changing the Mg/N ratio it is possible to switch the film orientation from (100) to (111).…”