-
Optimizing high-temperature electron mobility in single-crystal Bi$_2$O$_2$Se based on its unconventional dependence on concentration
Authors:
Antonín Sojka,
Petr Knotek,
Jan Zich,
Martin Míšek,
Roman Tesař,
Kyo-Hoon Ahn,
Petr Levinský,
Jiří Navrátil,
Pavlína Ruleová,
Jiří Hejtmánek,
Karel Knížek,
Václav Holý,
Čestmír Drašar
Abstract:
Quasi-2D Bi$_2$O$_2$Se is part of an intensive materials research effort aimed at finding new semiconductors that outperform silicon-based electronics in terms of speed and power consumption. This material exhibits exceptionally high carrier mobility at low temperatures but mediocre mobility at 300 K. Its high mobility is generally associated with its high permittivity ($\varepsilon_r$~500), which…
▽ More
Quasi-2D Bi$_2$O$_2$Se is part of an intensive materials research effort aimed at finding new semiconductors that outperform silicon-based electronics in terms of speed and power consumption. This material exhibits exceptionally high carrier mobility at low temperatures but mediocre mobility at 300 K. Its high mobility is generally associated with its high permittivity ($\varepsilon_r$~500), which is also associated with metallicity persisting down to very low carrier concentrations. This material exhibits a counterintuitive increase in carrier mobility as the concentration increases. The connection between low both carrier concentration and mobility in Se-rich conditions, and both high carrier concentration and mobility in Se-poor conditions suggests that the increase is related to native defects. We demonstrate that these defects can alter the effective mass of charge carriers. Specifically, substitutional Se(Bi) defects, which appear under Se-rich conditions, destroy the Bi$_2$O$_2$ channel and compromise charge transport properties. These defects increase the effective mass of charge carriers transforming the original semiconductor into a semimetal and introducing holes into charge transport. Additionally, we show that single crystals are generally inhomogeneous, particularly those grown under Se-rich conditions. Unlike Se-poor conditions, Se-rich conditions induce a higher concentration of dislocations and extraneous phases. These findings suggest that the perfection of the Bi$_2$O$_2$ channel is crucial for mobility, particularly at room temperature.
△ Less
Submitted 8 July, 2026;
originally announced July 2026.
-
Ytterbium charge state and stabilization in the Ba(Ca)F$_2$ host by electron paramagnetic resonance and infrared photoluminescence
Authors:
David John,
Shelja Sharma,
Marius Stef,
Gabriel Buse,
Zdeněk Remeš,
Anna Artemenko,
Sergii Chertopalov,
Vineet Sikarwar,
Alan Mašláni,
Jafar Fathi,
Jakub Pilař,
Tomáš Hostinský,
Jan Zich,
Tomáš Mates,
Brenda Natalia Lopez Nino,
Michal Hlína,
Karol Bartosiewicz,
Marina Konuhova,
Anatoli Popov,
Ján Lančok,
Maksym Buryi
Abstract:
Lanthanide-doped fluorides are promising materials for advanced photonic and quantum applications due to their wide bandgap, low phonon energy, and chemical stability. In this work, we present a systematic comparative study of ytterbium incorporation at low doping levels (0.05--0.2 mol\%) in BaF$_2$ and CaF$_2$ single crystals, focusing on the interplay between host lattice properties, charge-stat…
▽ More
Lanthanide-doped fluorides are promising materials for advanced photonic and quantum applications due to their wide bandgap, low phonon energy, and chemical stability. In this work, we present a systematic comparative study of ytterbium incorporation at low doping levels (0.05--0.2 mol\%) in BaF$_2$ and CaF$_2$ single crystals, focusing on the interplay between host lattice properties, charge-state stabilization, and defect formation mechanisms. Using a combination of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), transmittance, and infrared photoluminescence (IR PL), we explore how host lattice properties affect the stabilization of Yb$^{3+}$ and Yb$^{2+}$ ions. XRD confirmed cubic phase purity and lattice parameter stability in both hosts, while XPS revealed surface chemical composition variations associated with charge-compensating defects and trace impurities. EPR spectra indicated that BaF$_2$ favored perturbed Yb$^{3+}$ environments with increasing dopant levels, while CaF$_2$ maintained predominantly unperturbed sites, suggesting a more favorable ionic match for Yb$^{2+}$. Photothermal deflection spectroscopy (PDS) and IR PL results showed host-specific optical responses, with CaF$_2$ exhibiting crystal-field splitting and broader local field effects. These results reveal a clear decoupling between long-range structural stability and local lattice perturbations, and demonstrate that host cation identity governs the balance between Yb$^{2+}$ and Yb$^{3+}$ stabilization as well as defect-driven optical behavior. This offers valuable insights for optimizing rare-earth-doped fluoride crystals in laser, scintillator, and quantum device applications.
△ Less
Submitted 11 May, 2026;
originally announced May 2026.
-
The preparation and properties of polycrystalline Bi$_2$O$_2$Se -- pitfalls and difficulties with reproducibility and charge transport limiting parameters
Authors:
Jan Zich,
Tomáš Plecháček,
Antonín Sojka,
Petr Levinský,
Jiří Navrátil,
Pavlína Ruleová,
Stanislav Šlang,
Karel Knížek,
Jiří Hejtmánek,
Vojtěch Nečina,
Čestmír Drašar
Abstract:
Thermoelectric materials allow the direct conversion of waste heat into electricity, and novel materials are being investigated for this purpose. Recently, doped Bi$_2$O$_2$Se has shown high application potential. In this study, we discuss causes for large variation in reported transport properties of pure Bi$_2$O$_2$Se and present a preparation method that improves the reproducibility of undoped…
▽ More
Thermoelectric materials allow the direct conversion of waste heat into electricity, and novel materials are being investigated for this purpose. Recently, doped Bi$_2$O$_2$Se has shown high application potential. In this study, we discuss causes for large variation in reported transport properties of pure Bi$_2$O$_2$Se and present a preparation method that improves the reproducibility of undoped polycrystalline samples and improves their stability under thermal cycling. Key steps of this method include calcination of the Bi$_2$O$_3$ precursor, purification of the synthesized material in a temperature gradient, use of a coarse particle fraction and compaction of the powders in a Si3N4 die instead of a graphite die. The resulting polycrystalline material exhibits improved reproducibility and enhanced resistance to thermal cycling. It has room temperature electrical conductivity σRT ~ 500 S.m-1 and Seebeck coefficient S ~ -300 $μ$V.K$^{-1}$. These properties make it suitable as a reference material for future doping studies. The presented synthesis approach may provide a more reliable platform for investigating the intrinsic behavior and doping response of Bi$_2$O$_2$Se in thermoelectric applications.
△ Less
Submitted 21 April, 2025;
originally announced April 2025.
-
Phonon properties and unconventional heat transfer in quasi-2D $Bi_2O_2Se$ crystal
Authors:
Jan Zich,
Antonín Sojka,
Petr Levinský,
Martin Míšek,
Kyo-Hoon Ahn,
Jiří Navrátil,
Jiří Hejtmánek,
Karel Knížek,
Václav Holý,
Dmitry Nuzhnyy,
Fedir Borodavka,
Stanislav Kamba,
Čestmír Drašar
Abstract:
Bi2O2Se belongs to a group of quasi-2D semiconductors that can replace silicon in future high-speed/low-power electronics. However, the correlation between crystal/band structure and other physical properties still eludes understanding: carrier mobility increases non-intuitively with carrier concentration; the observed $T^2$ temperature dependence of resistivity lacks explanation. Moreover, a very…
▽ More
Bi2O2Se belongs to a group of quasi-2D semiconductors that can replace silicon in future high-speed/low-power electronics. However, the correlation between crystal/band structure and other physical properties still eludes understanding: carrier mobility increases non-intuitively with carrier concentration; the observed $T^2$ temperature dependence of resistivity lacks explanation. Moreover, a very high relative out-of-plane permittivity of about 150 has been reported in the literature. A proper explanation for such a high permittivity is still lacking. We have performed infrared (IR) reflectivity and Raman scattering experiments on a large perfect single crystal with defined mosaicity, carrier concentration and mobility. Five of the eight phonons allowed by factor group theory have been observed and their symmetries determined. The IR spectra show that the permittivity measured in the tetragonal plane is as high as $ε_r{\approx}500$, and this high value is due to a strong polar phonon with a low frequency of ~34 $cm^{-1}$ (~1 THz). Such an unusually high permittivity allows the screening of charge defects, leading to the observation of high electron mobility at low temperatures. It also allows effective modulation doping providing a platform for high performance 2D electronics. DFT calculations suggest the existence of a very low frequency acoustic phonon ~14 $cm^{-1}$ (~0.4 THz). Both the low frequency phonons cause anomalous phonon DOS, which is reflected in the unconventional temperature dependence of the heat capacity, $c_M{\approx}T^{3.5}$. The temperature-dependent, two-component group velocity is proposed to explains the unusual temperature dependence of the thermal conductivity, $κ{\approx}T^{1.5}$
△ Less
Submitted 13 February, 2026; v1 submitted 7 April, 2025;
originally announced April 2025.