Band gap bowings of ternary REN (RE = Sc, Y, La, and Lu) alloys

https://doi.org/10.1016/j.jallcom.2020.153961Get rights and content

Highlights

  • Band gaps for ternary alloys of REN calculated with MBJLDA are presented.

  • Substantial band gap bowings are found in systems studied.

  • Ternary solid solutions of REN may exhibit band gaps below 0.6 eV.

  • The zero band gap was revealed in some Sc1-xLaxN materials.

Abstract

Structural and electronic properties of ternary alloys of ScN, YN, LaN, and LuN semiconductors have been investigated within the density functional theory. Fully relativistic band structures were obtained with the modified Becke-Johnson approach. Although the lattice parameters of solid solutions of rare-earth elements nitrides exhibit a linear Vegard’s law behavior, the dependences of band gaps on alloys compositions are strongly nonlinear, i.e., substantial band gap bowings are predicted for systems studied. Such strong band gap bowings were not found in any group III nitrides. The Sc/Y/Lu-based materials are indirect band gap (ΓX) semiconductors. The ternary REN alloys containing La exhibit mainly direct type of band gaps (XX), which may be completely closed in some Sc1−xLaxN systems. The findings presented in this work suggest that strong band gap bowings in ternary solid solutions of REN semiconductors may enable obtaining band gaps lower than 0.6 eV being characteristic for LaN, which are unavailable in group III nitride materials.

Introduction

Semiconductor alloys of group III nitrides exhibit strong band gap (Eg) bowings [[1], [2], [3]]. This phenomenon is related to characteristic modifications of structural and electronic properties of a host material, caused by a presence of disorder, e.g., relatively bigger indium ions in aluminum nitride. Resulting alloys exhibit Eg which are significantly lower than the predictions based on the simple linear Vegard’s law.

Another family of nitride semiconductors are REN materials, where RE = Sc, Y, La, and Lu, which adopt the cubic rock salt phase. According to the recent experimental and theoretical investigations, indirect EgΓX of 0.9–1.3 eV are expected in (Sc; Y;Lu)N [[4], [5], [6], [7], [8], [9]], whereas the direct XX type of a narrower band gap of 0.6 eV was revealed in theoretical investigations for LaN [[9], [10], [11]].

Several experimental studies reported that solid solutions of group III nitrides with ScN/YN, i.e., Al1−xScxN, Ga1−xScxN, and Al1−xYxN materials [[12], [13], [14], [15], [16], [17]], exhibit rather linear dependences of Eg on Sc/Y contents. However, the band gap of ternary alloys of rare-earth element nitrides was theoretically predicted only for Sc1−xYxN [18,19].

Solid solutions of REN semiconductors may be considered as a new family of promising materials for applications in optoelectronics, in view of their cubic structure and the range of Eg comparable with those available in zinc blende group III arsenides and antimonides [20]. Therefore, some lattice matched heterostructures formed by REN systems and zinc blende group III nitrides may be futher considered.

In this work, the structural and electronic properties of ternary REN alloys, where RE = Sc, Y, La, and Lu, are predicted with the density functional theory (DFT) methods. The local density approximation (LDA) [21] was employed for structural relaxations, whereas the modified Becke–Johnson approach (MBJLDA) [22] was used for the investigations of fully relativistic band structures of materials studied. The discussion of the results is particularly focused on possible Eg bowings in REN alloys and a range of Eg possible to obtain in such novel semiconductor systems.

Section snippets

Computational methods

Structural properties of REN materials have been studied with the use of the Abinit package [23,24], i.e., the equilibrium geometries of the rock salt 2×2×2 supercells of ternary REN alloys were found via stresses/forces relaxation. The PAW atomic datasets taken from the JTH table [27] with the Perdew-Wang [21] (LDA) parameterization of the exchange-correlation energy were employed for this task. Next, the Wien2k package [25] was used for calculations of fully relativistic MBJLDA [22] band

Results and discussion

The cubic lattice parameters of parent REN compounds, a = 4.427, 4.822, 5.226, and 4.714 Å calculated here for ScN, YN, LaN, and LuN, respectively, are in accordance with the results reported in previous LDA-based studies [5,9,11,[28], [29], [30]]. It is worth recalling that the experimental lattice parameters are bigger by 1.5% [[31], [32], [33], [34]], which may be expected considering the well-known correspondence between the LDA predictions and the properties of real materials.

Ternary REN

Conclusions

Theoretical investigations based on DFT predicted approximately linear dependences of lattice parameters on compositions of ternary solid solutions of ScN, YN, LaN, and LuN. Very strong Eg bowings predicted for these systems enables design of materials with Eg significantly lower than those of parent REN compounds. The most complex band structures and rapid changes of Eg are expected in La-doped materials. In particular, Sc1−xLaxN systems may exhibit very narrow or even completely closed Eg.

CRediT authorship contribution statement

Maciej J. Winiarski: Conceptualization, Methodology, Investigation, Writing - review & editing. Dorota A. Kowalska: Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Science Centre (Poland) under research Grant no. 2017/26/D/ST3/00447. Calculations were performed in Wroclaw Center for Networking and Supercomputing (Project nos. 158 and 175).

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