70 Search Results

The emergence of hexagonal Ge (2H-Ge) as a candidate direct-gap group-IV semiconductor for Si photonics mandates a rigorous understanding of its optoelectronic properties. Theoretical predictions of a “pseudodirect” band gap, characterized by weak oscillator strength, contrast with a claimed high radiative recombination coefficient 𝐵 comparable to conventional (cubic) InAs. We compute 𝐵 in 2H-Ge from first principles and quantify its dependence on temperature, carrier density, and strain. For unstrained 2H-Ge, our calculated spontaneous emission spectra corroborate that measured photoluminescence corresponds to direct-gap emission, but with 𝐵 being approximately three orders of magnitude lower than in InAs. We confirm a pseudodirect-to-direct-gapmore » transition under ∼2% [0001] uniaxial tension, which can enhance 𝐵 by up to 3 orders of magnitude, making it comparable to that of InAs. Beyond quantifying the strong enhancement of 𝐵 via strain engineering, our analysis suggests the dominance of additional, as-yet unquantified recombination mechanisms in this nascent material.« less

Lead-based halide perovskite solar cells represent a significant advancement in photovoltaic technology, achieving certified power conversion efficiencies of over 27%. However, the toxicity of lead poses a major barrier to widespread commercialization. The demand for environmentally safe alternatives has driven extensive research into Pb-free perovskites. Current efforts include replacing Pb with Sn or Ge; forming double perovskites in which Pb is substituted by a monovalent–trivalent cation pair; and developing chalcogenide perovskites where the B site (ABX₃) adopts tetravalent cations (rather than Pb) to balance the charge. This concept examines the recent progress in developing Pb-free alternatives, revealing fundamental performance bottlenecks,more » inherent material limitations, and persistent development challenges. Through a comparative assessment of material properties and device performance limitations, this work highlights the underlying dilemma between environmental safety and efficiency in perovskite photovoltaics. The analysis identifies fundamental material constraints that create substantial barriers to simultaneously achieving both objectives.« less

Recent angle-resolved photoelectron spectroscopy (ARPES) measurements of the hole effective mass in CsPbBr$$_3$$ revealed an enhancement of $$\sim$$50 % compared to the bare mass computed from first principles for CsPbBr$$_3$$ at $T = 0 K$. This large enhancement was interpreted as evidence of polaron formation. Employing accurate finite-temperature first-principles calculations, we show that the calculated hole effective mass of CsPbBr$$_3$$ at $T = 300 K$ can explain experimental results without invoking polarons. Thermal fluctuations are particularly strong in halide perovskites compared to conventional semiconductors such as Si and GaAs, and cannot be ignored when comparing with experiment. We not onlymore » resolve the debate on polaron formation in halide perovskites, but also demonstrate the general importance of including thermal fluctuations in first-principles calculations for strongly anharmonic materials.« less

Trap-assisted nonradiative recombination has been shown to limit the efficiency of optoelectronic devices. While substitutional carbon (⁠C_N) has been suggested to be a nonradiative recombination center in GaN devices, a complete recombination cycle including the two charge-state transition levels has not been previously described. In this work, we investigate the trap-assisted recombination process due to C_N in GaN, including multiphonon emission, radiative recombination, trap-assisted Auger–Meitner (TAAM) recombination as well as thermal emission of holes. Our study shows the key role of TAAM processes at the high carrier densities relevant for devices. We also reveal the carrier-density regimes where thermal emissionmore » and radiative recombination are expected to play an observable role. Our results highlight that carbon concentrations exceeding ~10¹⁷ cm⁻³ can have a noticeable impact on device efficiency, not just in GaN active layers but also in InGaN and AlGaN. Furthermore, our comprehensive formalism not only offers detailed results for carbon but also provides a general framework for assessing the multiple processes that participate in trap-assisted recombination in semiconductors.« less

Wurtzite LaN (wz-LaN) is a semiconducting nitride with favorable piezoelectric and ferroelectric properties, making it promising for applications in electronics. Here, we use first-principles density functional theory with a hybrid functional to investigate several features that are key for its use in heterostructures. First, for the purposes of growing wz-LaN on a substrate or designing a heterostructure, we show that it can be lattice-matched with a number of cubic materials along their [111] axes. We also evaluate the bound charge at such interfaces, taking into account both the polarization discontinuity and the piezoelectric polarization due to pseudomorphic strain. Second, wemore » investigate band alignments and assess the results for interfaces with zincblende-, rocksalt-, and perovskite-structure compounds, along with chemically similar wurtzite and rocksalt nitrides. Our results provide guidance for the development of electronic devices based on wz-LaN.« less

Chromium is a common transition-metal impurity that is easily incorporated during crystal growth. It is perhaps best known for giving rise to the 694.3 nm (1.786 eV) emission in Cr-doped Al2O3, exploited in ruby lasers. Chromium has also been found in monoclinic gallium oxide, a wide-bandgap semiconductor being pursued for power electronics. In this work, we thoroughly characterize the behavior of Cr in Ga2O3 through theoretical and experimental techniques. β-Ga2O3 samples are grown with the floating zone method and show evidence of a sharp photoluminescence signal, reminiscent of ruby. We calculate the energetics of formation of Cr from first principles, demonstrating thatmore » Cr preferentially incorporates as a neutral impurity on the octahedral site. Cr possesses a quartet ground-state spin and has an internal transition with a zero-phonon line near 1.8 eV. By comparing the calculated and experimentally measured luminescence lineshape function, we elucidate the role of coupling to phonons and uncover features beyond the Franck–Condon approximation. The combination of strong emission with a small Huang–Rhys factor of 0.05 and a technologically relevant host material renders Cr in Ga2O3 attractive as a quantum defect.« less

Toxicity remains one of the major challenges that prevent Pb-based halide perovskites from widespread utilization. Ideally, non-toxic alternatives can be identified while still maintaining the superior power conversion efficiency of the Pb-based perovskite solar cells. Using the currently most promising candidate, the Sn-based halide perovskites, as an example, we show that a trade-off exists between toxicity and efficiency in the Sn- versus Pb-based halide perovskites. Indeed, the dominant nonradiative recombination center in the Sn-based halide perovskites differs from the one in its Pb-based counterparts, resulting in the nonradiative capture coefficient in CsSnI₃ being an order of magnitude higher than thatmore » in CsPbI₃. We attribute this difference to the band alignment. Here, our results indicate that development of halide perovskites beyond the Pb and Sn bases is essential for efficient yet environmentally friendly perovskite solar cells.« less

Wurtzite LaN (wz-LaN) is a semiconducting nitride that has piezoelectric and ferroelectric properties, making it promising for applications in electronics, either as a binary compound or in alloys such as LaAlN. The prospects for wz-LaN in devices are influenced by the properties of point defects and impurities; here, we use first-principles density functional theory with a hybrid functional to calculate their formation energies, as well as their atomic and electronic structures. Among native point defects, we find that nitrogen-related defects, both vacancies ($$V^+_N$$) and interstitials ($$N^-_i$$), are energetically most favorable under most relevant chemical potentials and positions of the Fermimore » level; $$V^0_N$$ may additionally be observed under N-poor conditions, and $$N^0_i$$ may be prominent under N-rich conditions. We also investigate the incorporation of oxygen and hydrogen, which will likely be present as unintentional impurities. We find that the $$O^+_N$$ substitutional species readily forms, but oxygen will not lead to n-type conductivity due to formation of DX centers and compensation by interstitial defects. Similarly, substitutional HN and interstitial H_i can compensate both p- and n-type dopants. Our results provide detailed, microscopic guidance for the development of electronic devices based on wz-LaN.« less

Yttrium oxyhydride (YHO) undergoes a reversible photochromic transition when exposed to ultraviolet light. However, the mechanism for this transformation is not fully understood, and the structure and precise chemical composition of YHO remain under debate. Here, we use first-principles density functional theory calculations with a hybrid functional to study the structure, chemical stability, and point defect properties of YHO. As experiments have shown, we find that YHO prefers a cubic structure, with H and O anions present in equal concentrations and located on tetrahedral sites. Stoichiometric and ordered YHO is chemically stable, but it has a wide band gap ofmore » 5.01 eV, considerably larger than that measured in experiments (2.4–3.8 eV). On the other hand, Y4H10O has a smaller band gap of 2.97 eV and also has a region of chemical stability; thus, the actual material may include some fraction of this H-rich structure. The defect chemistry of YHO is dominated by anionic antisite species (H_O and O_H), with hydrogen interstitials (H_i) and vacancies (V_H) also present in reasonably high concentrations. We show that antisite disorder lowers the band gap relative to the perfectly ordered structure, bringing the magnitude of the gap into closer agreement with experiment. Based on our calculations of defect migration and the positions of defect states relative to the band edges, we link the onset of photochromic behavior to the reaction H_O^– → V_O⁰ + H_i^–, which follows photoexcitation of a H_O⁺ defect. H_i^– can subsequently migrate away and be trapped by additional H_O⁺ defects, contributing to the persistence of the reaction, while the resultant oxygen vacancy, V_O⁰, introduces an occupied defect state that leads to optical absorption at visible wavelengths. Our results can explain reported discrepancies between experimental and computational results for YHO, and they allow us to propose specific atomic-scale processes that can lead to photochromism. In conclusion, understanding these mechanisms is key for unlocking YHO’s application in devices ranging from smart windows and optoelectronics to electrochemical synapses for neural networks.« less

Transition-metal centers exhibit a paramagnetic ground state in wide-bandgap semiconductors and are promising for nanophotonics and quantum information processing. Specifically, there is a growing interest in discovering prominent paramagnetic spin defects that can be manipulated using optical methods. Here, we investigate the electronic structure and magneto-optical properties of Cr and Mn substitutional centers in wurtzite AlN and GaN. We use state-of-the-art hybrid density functional theory calculations to determine level structure, stability, optical signatures, and magnetic properties of these centers. The excitation energies are calculated using the constrained occupation approach and rigorously verified with the complete active space configuration interaction approach.more » Our simulations of the photoluminescence spectra indicate that $Cr$$$$^{1+}_{Al}$$ in AlN and $Cr$$$$^{1+}_{Ga}$$ in GaN are responsible for the observed narrow quantum emission near 1.2 eV. We compute the zero-field splitting (ZFS) parameters and outline an optical spin polarization protocol for $Cr$$$$^{1+}_{Al}$$ and $Cr$$$$^{1+}_{Ga}$$. Our results demonstrate that these centers are promising candidates for spin qubits.« less