Abstract
Electronic band structure in germanium nitride having spinel structure, γ-Ge3N4, was examined using two spectroscopic techniques, cathodoluminescence and synchrotron-based photoluminescence. The sample purity was confirmed by x-ray diffraction and Raman analyses. The spectroscopic measurements provided first experimental evidence of a large free exciton binding energy De≈0.30 eV and direct interband transitions in this material. The band gap energy Eg = 3.65 ± 0.05 eV measured with a higher precision was in agreement with that previously obtained via XES/XANES method. The screened hybrid functional Heyd–Scuseria–Ernzerhof (HSE06) calculations of the electronic structure supported the experimental results. Based on the experimental data and theoretical calculations, the limiting efficiency of the excitation conversion to light was estimated and compared with that of w-GaN, which is the basic material of commercial light emitting diodes. The high conversion efficiency, very high hardness and rigidity combined with a thermal stability in air up to ~ 700 °C reveal the potential of γ-Ge3N4 for robust and efficient photonic emitters.
Highlights
Germanium(IV) nitride having spinel structure is one of the members of the family of spinel nitrides of the group 14 elements (γ-M3N4 where M = Si, Ge, Sn) discovered two decades ago [1,2,3]
The resulting product was annealed under the same conditions for purity purpose and the phase composition was controlled by X-ray diffraction (XRD) to be pure β-Ge3N4
The Raman spectra of γ-Ge3N4 sample shown in Fig. 2 were dominated by two sharp intense bands at 728 cm−1 (T2g) and 329 cm−1 (Tg) and three other less intense bands at 854 cm−1 (Ag), 592 cm−1 (T2g) and 470 cm−1 (Eg), which have been previously experimentally observed and theoretically validated to be the five Raman-active modes permitted for the ideal spinel structure [2, 20,21,22]
Summary
Direct band gap is one of the basic conditions for a material to be considered as an efficient light emitter while a large exciton binding energy (De > > kT) is the second necessary condition. The latter parameter has been evaluated theoretically by Boyko et al [7] for all end-members γ-M3N4 and their hypothetical solid solutions using the hydrogenic-type model. For γ-Ge3N4, the approach led to a large value of De = 174 meV while the highest De = 333 meV has been predicted for γ-Si3N4 and the lowest De = 69 meV for γ-Sn3N4
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