ELECTRONIC STRUCTURE AND BAND-GAPS OF Eu-DOPED LaSi3N5 TERNARY NITRIDES

摘要

The Eu-doped LaSi3N5 has luminescence in the blue-green light region. With increasing europium content in general formula La_(1-z)Eu_zSi3N_(5-z)O_(3/2z) from z = 0.05 to z = 0.1 the emission intensity increases, indicating the increase of the concentration of the luminescent centres. First-principles Density-Functional Theory (DFT) calculations are performed to enhance the understanding of the electronic structure of the stoichiometric LaSi3N5 and La/Eu and N/O substituted ternary nitrides. To mimic the realistic concentration of Eu and O the cell volume is expanded to the 2x1x2 super-cell with 144 atoms. The calculations show that ternary nitrides are large-gap insulators. The La~(3+)/Eu~(3+) substitution does not lead to a significant change of the gap. The observed concentration of O atoms in the framework can be compensated either by the creation of the M~(3+) hole (M=La, Eu), or by the La~(3+)/Eu~(2+) substitution. Both phenomena lead to a decrease of the gap. Upon the hole creation the DFT-calculated gap decreases from -3.15 eV to -2.55 eV. The La~(3+)/Eu~(2+) substitution leads to the location of the narrow band of Eu 4f-states inside the gap thus narrowing the band-gap to 0.542 eV. This value of the DFT band gap is typically too small. A tuning of the position of the band of the nonbonding 4f-states away from the conduction band towards more negative energies using the LDA+U approach leads to the enlargement of the band gap up to the value of ~3 eV. Finally, a benchmark calculation using a hybrid functional provides the band-gap of 3.1 eV in reasonable agreement with experimental data. Our calculations show that in La/EuSi3N5 ternary nitrides a narrowing of the band gap is caused by the creation of the M~(3+) holes (M=La, Eu) or by the La~(3+)/Eu~(2+) substitution, both phenomena are enabled by N/O substitutions in the framework of ternary nitrides.