An official website of the United States government
Abstract Oxygen vacancy is the most common type of point defects in functional oxides, and it is known to have profound influence on their properties. This is particularly true for ferroelectric oxides since their interaction with ferroelectric polarization often dictates the ferroelectric responses. Here, we study the influence of the concentration of oxygen vacancies on the stability of ferroelectric domain walls (DWs) in BiFeO3, a material with a relatively narrow bandgap among all perovskite oxides, which enables strong interactions among electronic charge carriers, oxygen vacancies, and ferroelectric domains. It is found that the electronic charge carriers in the absence of oxygen vacancies have essentially no influence on the spatial polarization distribution of the DWs due to their low concentrations. Upon increasing the concentration of oxygen vacancies, charge‐neutral DWs with an originally symmetric polarization distribution symmetric around the center of the wall can develop a strong asymmetry of the polarization field, which is mediated by the electrostatic interaction between polarization and electrons from the ionization of oxygen vacancies. Strongly charged head‐to‐head DWs that are unstable without oxygen vacancies can be energetically stabilized in the off‐stoichiometric BiFeO3−δwithδ∼ 0.02 where ionization of oxygen vacancies provides sufficient free electrons to compensate the bound charge at the wall. Our results delineate the electrostatic coupling of the ionic defects and the associated free electronic charge carriers with the bound charge in the vicinity of neutral and charged DWs in perovskite ferroelectrics.
more »
« less
AlScO 3 perovskite—An ∼8 eV bandgap oxide predicted to exhibit low small hole polaron ionization energies and p -type conductivity at elevated temperatures
We investigate electronic structure and dopability of an ultrawide bandgap (UWBG) AlScO 3 perovskite, a known high-pressure and long-lived metastable oxide. From first-principles electronic structure calculations, HSE06(+G 0 W 0 ), we find this material to exhibit an indirect bandgap of around 8.0 eV. Defect calculations point to cation and oxygen vacancies as the dominant intrinsic point defects limiting extrinsic doping. While acceptor behaving Al and Sc vacancies prevent n-type doping, oxygen vacancies permit the Fermi energy to reach ∼0.3 eV above the valence band maximum, rendering AlScO 3 p-type dopable. Furthermore, we find that both Mg and Zn could serve as extrinsic p-type dopants. Specifically, Mg is predicted to have achievable net acceptor concentrations of ∼10 17 cm −3 with ionization energy of bound small hole polarons of ∼0.49 eV and free ones below 0.1 eV. These values place AlScO 3 among the UWBG oxides with lowest bound small hole polaron ionization energies, which, as we find, is likely due to large ionic dielectric constant that correlates well with low hole polaron ionization energies across various UWBG oxides.
more »
« less
- Award ID(s):
- 1945010
- PAR ID:
- 10400377
- Date Published:
- Journal Name:
- Applied Physics Letters
- Volume:
- 121
- Issue:
- 10
- ISSN:
- 0003-6951
- Page Range / eLocation ID:
- 102103
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
There is increasing interest in the alpha polytype of Ga 2 O 3 because of its even larger bandgap than the more studied beta polytype, but in common with the latter, there is no viable p-type doping technology. One option is to use p-type oxides to realize heterojunctions and NiO is one of the candidate oxides. The band alignment of sputtered NiO on α-Ga 2 O 3 remains type II, staggered gap for annealing temperatures up to 600 °C, showing that this is a viable approach for hole injection in power electronic devices based on the alpha polytype of Ga 2 O 3 . The magnitude of both the conduction and valence band offsets increases with temperature up to 500 °C, but then is stable to 600 °C. For the as-deposited NiO/α-Ga 2 O 3 heterojunction, ΔE V = −2.8 and ΔE C = 1.6 eV, while after 600 °C annealing the corresponding values are ΔE V = −4.4 and ΔE C = 3.02 eV. These values are 1−2 eV larger than for the NiO/β-Ga 2 O 3 heterojunction.more » « less
-
Binary Co 4 Sb 12 skutterudite (also known as CoSb 3 ) has been extensively studied; however, its mixed-anion counterparts remain largely unexplored in terms of their phase stability and thermoelectric properties. In the search for complex anionic analogs of the binary skutterudite, we begin by investigating the Co 4 Sb 12 –Co 4 Sn 6 Te 6 pseudo-binary phase diagram. We observe no quaternary skutterudite phases and as such, focus our investigations on the ternary Co 4 Sn 6 Te 6 via experimental phase boundary mapping, transport measurements, and first-principles calculations. Phase boundary mapping using traditional bulk syntheses reveals that the Co 4 Sn 6 Te 6 exhibits electronic properties ranging from a degenerate p-type behavior to an intrinsic behavior. Under Sn-rich conditions, Hall measurements indicate degenerate p-type carrier concentrations and high hole mobility. The acceptor defect Sn Te , and donor defects Te Sn and Co i are the predominant defects and rationally correspond to regions of high Sn, Te, and Co, respectively. Consideration of the defect energetics indicates that p-type extrinsic doping is plausible; however, Sn Te is likely a killer defect that limits n-type dopability. We find that the hole carrier concentration in Co 4 Sn 6 Te 6 can be further optimized by extrinsic p-type doping under Sn-rich growth conditions.more » « less
-
Teherani, Ferechteh H.; Look, David C.; Rogers, David J. (Ed.)Gallium oxide (Ga2O3), an ultra-wide bandgap semiconductor with potential applications in power devices, may be doped with Mg to control the native n-type conductivity. The charge transitions associated with Mg in Mg-doped β-Ga2O3 crystals are studied using photoinduced electron paramagnetic resonance (photo-EPR) spectroscopy to understand the mechanisms that produce stable semi-insulating substrates. The steady state photo-EPR measurements are performed at 130 K by illuminating the samples with photon energy from 0.7 to 4.7 eV. Our results show that there are two transitions associated with Mg in the bandgap: onset of quenching of neutral Mg at 1.5 eV and excitation at 3.0 eV. The quenching threshold is consistent with several DFT predicted values for Mg-/0 level. Therefore, we suggest the quenching is due to transition of an electron from the valence band to the neutral Mg. For photoexcitation, hole capture is the only viable process due to polaronic nature of neutral Mg in Ga2O3. The measurements demonstrate that electron excitation to impurities, such as Fe and Ir, does not contribute to creation of the holes. Further, gallium vacancies must not participate since their characteristic EPR spectrum is never seen. Thus, we speculate that the defects responsible for the hole formation and consequent excitation of the neutral Mg are oxygen vacancies.more » « less
-
Recent, and somewhat surprising, successful n-type doping of Mg 3 Sb 2 was the key to realizing high thermoelectric performance in this material. Herein, we use first-principles defect calculations to investigate different extrinsic n-type doping strategies for Mg 3 Sb 2 and to reveal general chemical trends in terms of dopant solubilities and maximal achievable electron concentrations. In agreement with experiments, we find that Sb substitution is an effective doping strategy, with Se and Te doping predicted to yield up to ∼8 × 10 19 cm −3 electrons. However, we also find that Mg substitution with trivalent (or higher) cations can be even more effective; in particular, the predicted highest achievable electron concentration (∼5 × 10 20 cm −3 ) with La as an extrinsic dopant exceeds that of Se and Te doping. Interstitial doping (Li, Zn, Cu, Be) is found to be largely ineffective either due to self-compensation (Li) or high formation energy (Zn, Cu, Be). Our results offer La as an alternative dopant to Te and Se and reinforce the need for careful phase boundary mapping in achieving high electron concentrations in Mg 3 Sb 2 .more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1063/5.0097204
