Investigating the Rare Earth Dichalcogenides as Functional P-Type Semiconductors

The rare earth polychalcogenides LnCh_2-x(Ln = La-Lu, Ch = S, Se, Te) are a family of compounds that have been investigated for their structural and magnetic tunability: occupying variants of the ZrSSi Zintl-like motif, but with wide variability depending on which chalcogenide, and the proportion of chalcogenide vacancies. The fully stoichiometric, non-magnetic members of the family (Ln = La, Lu; x=0) are semiconductors due to a Peierls distortion within the ‘Ch^-‘ layer: of these, LaSe₂ has seen recent investigation as a promising p-type transport thin-film material with intrinsic carrier concentrations beyond 10¹⁹cm^-3,¹ although with limited exploration for the microscopic origin of its favourable electronic behaviour.<br/>In this study, we thoroughly examine the LnCh₂ family (Ln= La, Lu; Ch = S, Se) using a combination of hybrid Density Functional Theory and GW calculations to investigate their electronic and optical properties. Initially, we demonstrate that despite suitable fundamental band gaps, these systems will be poor photovoltaic absorbers due to forbidden optical transitions, however, we also establish the key role of Se-Se dimerization in the structure of LaSe₂ in engineering valence band dispersion and low hole effective masses.² We then compare LaSe₂ to the lutetium and sulfide analogues in the family using calculations of individual scattering mechanisms within the recently-developed AMSET code to establish the trends and limits in carrier mobilities, and to establish the potential of these compounds as hole conductor layers in functional devices with careful growth control.³ Finally, we calculate their thermal transport properties, with comparison between the hiphive and ALAMODE packages,^{4, 5} to also assess the suitability of LaSe₂ and the other compounds in the family as low-temperature p-type thermoelectrics.<br/><br/>1. Gao, G.; Tong, L.; Yang, L.; Sun, C.; Xu, L.; Xia, F.; Geng, F.; Xue, J.; Gong, H.; Zhu, J., A P-type mid-infrared transparent semiconductor LaSe2 film with small hole effective mass and high carrier concentration. <i>Appl. Phys. Lett. </i><b>2021,</b> <i>118</i> (26), 261602.<br/>2. Huo, L.; Savory, C., Assessing the Electronic and Optical Properties of Lanthanum Diselenide: a computational study. <i>Journal of Materials Chemistry C</i>, Accepted <b>2024 </b>doi: 10.1039/D3TC02833J<br/>3. Ganose, A. M.; Park, J.; Faghaninia, A.; Woods-Robinson, R.; Persson, K. A.; Jain, A., Efficient calculation of carrier scattering rates from first principles. <i>Nat. Commun. </i><b>2021,</b> <i>12</i> (1), 2222.<br/>4. Eriksson, F.; Fransson, E.; Erhart, P., The Hiphive Package for the Extraction of High-Order Force Constants by Machine Learning. <i>Adv. Theory Simul. </i><b>2019,</b> <i>2</i> (5), 1800184.<br/>5. Tadano, T.; Gohda, Y.; Tsuneyuki, S., Anharmonic force constants extracted from first-principles molecular dynamics: applications to heat transfer simulations. <i>J. Phys. Condens. Matter </i><b>2014,</b> <i>26</i> (22), 225402.