Strain-Induced Valence Band Splitting Enabling Above-Bandgap Exciton Luminescence in Epitaxial Mg₃N₂ Thin Films

Abstract

Excitons are the lowest excited state of the electronic subsystem in semiconductors, composed of a bound electron−hole pair interacting via screened coulomb potential. Compared to the band-to-band transitions, excitons exhibit stronger luminescence with greater oscillator strength at low temperatures. However, achieving room-temperature excitonic luminescence in semiconductors is challenging due to their low binding energies. Magnesium nitride (Mg 3 N 2 ), an emerging IInitride semiconductor, hosts a room-temperature excitonic luminescence in the visible spectral range in the powder phase. Here, we show conclusive experimental evidence of strain-induced valence band splitting in epitaxial and stoichiometric Mg 3 N 2 thin films that leads to excitonic luminescence above and below its direct bandgap at room temperature. Growth-induced biaxial tensile strain splits the light hole and split-off hole bands in Mg 3 N 2 by ∼170 meV, which causes the above-bandgap luminescence. Optical absorption, synchrotron radiation ultraviolet photoemission spectroscopy, and photoluminescence measurements also confirm that Mg 3 N 2 is a direct bandgap semiconductor with an exciton binding energy of ∼40 meV. First-principles calculations with Heyd−Scuseria−Ernzerhof hybrid-functional support the straininduced valence band splitting and above-bandgap excitonic feature. Room-temperature multiple exciton luminescence in Mg 3 N 2 would be useful for visible-light-emitting diodes (LEDs), semiconductor lasers, and other optoelectronic applications.