Carrier-Induced Band-Gap Variation and Point Defects in Zn3N2 from First Principles

Abstract

The zinc nitride Zn 3 N 2 is composed of inexpensive and earth-abundant Zn and N elements and shows high electron mobility exceeding 100 cm 2 V −1 s −1 . Although various technological applications of Zn 3 N 2 have been suggested so far, the synthesis of high-quality Zn 3 N 2 samples, especially single crystals, is still challenging, and therefore its basic properties are not yet well understood. Indeed, the reported band gaps of as-grown Zn 3 N 2 widely scatter from 0.85 to 3.2 eV. In this study, we investigate the large gap variation of Zn 3 N 2 in terms of the Burstein-Moss (BM) effect and point-defect energetics using first-principles calculations. First, we discuss the relation between electron carrier concentration and optical gaps based on the electronic structure obtained using the Heyd-Scuseria-Ernzerhof hybrid functional. The calculated fundamental band gap is 0.84 eV in a direct-type band structure. Second, thermodynamic stability of Zn 3 N 2 is assessed using the ideal-gas model in conjunction with the rigid-rotor model for gas phases and firstprinciples phonon calculations for solid phases. Third, carrier generation and compensation by native point defects and unintentionally introduced oxygen and hydrogen impurities are discussed. The results suggest that a significant BM shift occurs mainly due to oxygen substitutions on nitrogen sites and hydrogen interstitials. However, gaps larger than 2.0 eV would not be due to the BM shift because of the Fermi-level pinning caused by acceptorlike zinc vacancies and hydrogen-on-zinc impurities. Furthermore, we discuss details of peculiar defects such as a nitrogen-on-zinc antisite with azidelike atomic and electronic structures.

Point Defects and p -Type Doping in ScN from First Principles

“…The direct gap is also consistent with those evaluated from the optical absorption spectra [25,27]. For the calculation of the Gibbs free energy of formation (ΔG f ) at the standard state, the zero phonon vibrational energy (ZPVE) and the entropic contribution are taken into account only for the N 2 gas phase, as the solid phases largely tend to cancel out these two contributions near room temperature [24,48]. Figure 1 shows the electronic band diagram and the density of states (DOS) projected on each element and each orbital.…”

“…Such an increase of the optical gap is known as the Burstein-Moss (BM) shift [19,20], and it could make ScN suitable for transparent conductors demanding about a 3.0 eV or larger optical absorption onset. The BM shift has also been reported in narrow-gap nitrides such as InN [21][22][23], ZnSnN 2 [23], and Zn 3 N 2 [24]. In these nitrides, oxygen atoms on nitrogen sites and/or hydrogen interstitials have been considered as its origin.…”

“…We use the HSE06 hybrid functional [34][35][36], which can accurately predict the atomic and electronic structures of semiconductors, especially nitrides [5,24]. Indeed, Deng et al reported that the direct gap of ScN calculated by HSE06 is well matched with that determined from the optical absorption spectrum [25].…”

“…E corr ½D q corresponds to an energy for correcting a finite supercell size error. We adopt our extended Freysoldt-Neugebauer-Van de Walle (FNV) correction scheme, which usually estimates the defectformation energies in the dilute limit quite accurately [24,39,41,42]. However, as discussed in the Appendix, the FNV corrections cannot sufficiently remove the finitesize effects in ScN, probably due to large spill out of the defect charges.…”

Temperature dependence of the band gap of zinc nitride observed in photoluminescence measurements

“…However, density of states calculations for Zn 3 N 2 show no evidence for such a band alignment. [16][17][18] Furthermore, the red-shift is too significant to be exclusively caused by the thermal effect. Instead, we suggest that the observed shift with excitation power is related to inhomogeneities in the sample.…”