Investigations of the Electronic Structure and Bandgap of the Next‐Generation LED‐Phosphor Sr[LiAl₃N₄]:Eu²⁺—Experiment and Calculations

Abstract

wileyonlinelibrary.com COMMUNICATIONgreatest infl uence on CRI as well as on the luminous effi cacy. [ 12 ] Many red emitters in commercial use produce a signifi cant portion of their light outside of the visible range, limiting their luminous effi cacy. [ 2 ] These infrared emissions result in wasted power input and undesirable heating in the device. Therefore, the discovery of a narrow-band red-emitting phosphor material would be a breakthrough in LED technology. On this front, the recently reported nitridolithoaluminate Sr[LiAl 3 N 4 ]:Eu 2+ (SLA) can be considered a breakthrough material. [ 2 ] With its high emission intensity in the red region of the electromagnetic spectrum and its uniquely narrow bandwidth for an Eu 2+doped phosphor, this material shows great potential for use in illumination-grade pc-LEDs. To fully understand the complex structure-property relationships and to enable further optimization of the material properties of Sr[LiAl 3 N 4 ]:Eu 2+ , an in-depth presentation of the electronic structure determined through experimental soft X-ray spectroscopy measurements and density functional theory calculations is presented here. SLA is found to have an indirect bandgap of (4.56 ± 0.25) eV, highly similar local electronic structure at non-equivalent Sr sites, and the necessary characteristics for a uniform shift in the centroid of the Eu 2+ 5d states. This clarifi es why, with two sites which can be occupied by the activator ion Eu 2+ , such a narrow emission band is still observed and re-enforces the importance of SLA as a phosphor for LED applications.The synthesis and the determination of the crystal structure of SLA are described elsewhere. [ 2 ] The crystal structure of SLA is shown in Figure 1 , where the XN 4 (X = Li, Al) tetrahedra can be seen surrounding the Sr sites. The labeling in this fi gure will be used throughout. The Eu 2+ ions are assumed to occupy the Sr sites due to their similar ionic radii. X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) measurements for the undoped and doped (0.4% Eu 2+ ) samples are shown in Figure 2 , where they are compared to the calculated spectra. The measured spectra for the doped and undoped sample are largely identical. This is expected for the low dopant concentrations dealt with here, because it is the N sites being probed and the RE ions are expected to be largely atomic in nature, having little infl uence on the bonding in the crystal. It can be safely assumed that the calculation results for the undoped sample are largely applicable to the doped sample. In the XES spectra the same peaks ( a , b , c, and d as marked on the non-resonant spectrum) are observed throughout. The high-energy shoulder d becomes suppressed at the lowest excitation energy. This same feature is augmented under 401.30 eV excitation. Feature a increases in intensity under 404.30 eV excitation. These slight changes in the emission spectra are

Electronic Structure, Bandgap, and Thermal Quenching of Sr[Mg3SiN4]:Eu2+ in Comparison to Sr[LiAl3N4]:Eu2+

“…The contributions of magnesium are quite modest and show a degree of sp hybridization (α and β), and mixing with the nitrogen p states (α-γ). The bonding seen here between nitrogen, silicon, and magnesium is very similar to that between nitrogen, aluminum, and lithium in SLA, [ 15 ] as is the high uniformity of pDOS from nonequivalent nitrogen sites. From the agreement between the calculated nitrogen and silicon spectra, it can be expected that the DOS given here is an accurate representation of those in SMS.…”

“…Indeed, this is what is observed when comparing to the reported band gap of SLA. [ 15 ] One also expects that the emptied strontium s states will start affecting the CB minimum, due to their similarity in energy to those of lithium. A few more details can be considered to fi nish the discussion of the differences between the SMS and SLA band structures.…”

“…tetrahedra. [ 15,16 ] The nitrogen p states dominate the VB, while the strontium d states dominate the CB. There is strong mixing of silicon s and nitrogen p states in the lower VB (α), as well as in the mid CB (ε).…”

“…The band structure of SLA using a standard generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA-PBE) calculation has been presented elsewhere. [ 15 ] For comparison with SMS, this band structure was recalculated using the mBJ potential and the resulting character plot is shown with that of SMS in Figure 3 . It should be noted that the VB maximum has the same energetic location in SMS and SLA, making the difference in band gaps due entirely to the positioning of the CB.…”

Critical Review—Narrow-Band Nitride Phosphors for Wide Color-Gamut White LED Backlighting

“…According to the absorption band of the host lattice, the optical bandgap is estimated to be 4.7 eV, which is in good accordance with the calculated value of (4.56 ± 0.25) eV. 54 To further understand the origin of luminescent features, low-temperature measurements at 6 K are performed, as shown in Fig. 16b.…”