Spatial control of near-infrared (NIR) emission from hexagonal boron nitride (hBN) would facilitate coupling atomic-scale light sources to photonic structures, yet oxygen-related NIR emitters are generally formed at stochastic locations. Here, we combine single-shot femtosecond laser writing with annealing in an oxygen-rich environment to bias NIR activation toward predefined coordinates in exfoliated hBN. Spectra acquired with 532, 635, and 785 nm excitations show narrow and multipeak emission extending to a wavelength of 1 um. Among the spectra collected at written sites, over 83

Although point defects in hexagonal boron nitride (hBN) exhibiting single-photon emission attract considerable interest, a broader understanding of defect physics and chemistry in hBN remains limited, potentially hindering further development. Oxygen is among the most common impurities in hBN, and numerous studies have reported a pronounced photoluminescence (PL) band centered near 3.5 eV following oxygen incorporation, yet its microscopic origin has remained unresolved. Here, we demonstrate that this emission originates from hole capture by neutral oxygen substituting for nitrogen (ON), which acts as a recombination center under optical excitation. The transition mechanism is nontrivial, involving not only a change in charge state but also a substantial structural reconfiguration: while the positively charged defect adopts a high-symmetry in-plane geometry, the neutral state stabilizes in a low-symmetry configuration characterized by out-of-plane displacements of the oxygen atom and its neighboring boron atoms. Strong lattice relaxation accompanying hole capture lowers the emission energy well below the zero-phonon line energy of 5.09 eV. The calculated luminescence spectrum forms a broad emission band with a maximum at 3.66 eV and a full width at half-maximum (FWHM) of 0.69 eV, in excellent agreement with the experimentally observed peak position and line shape in oxygen-doped hBN.

Optically active defects in hexagonal boron nitride (hBN) are promising candidates for active components in emerging quantum technologies, such as single-photon emitters and spin centers. However, further progress in hBN-based quantum technologies requires a deeper understanding of the physics and chemistry of hBN defects. In this work, we employ ab initio calculations to investigate the thermodynamic stability and optical properties of defect complexes involving carbon, boron vacancies, and hydrogen. We demonstrate that the formation of CBVB-nH complexes (0 <= n <= 3) is energetically favorable under nitrogen-rich conditions in the presence of carbon and hydrogen. The low formation energies and high binding energies of these complexes arise from the strong electrostatic attraction between the positively charged carbon substitutional defect (CB) and the negatively charged hydrogen-passivated boron vacancies (VB-nH). These complexes are particularly likely to form in metal-organic vapor-phase epitaxy (MOVPE)-grown samples, where growth occurs in the presence of carbon and hydrogen and is accompanied by a high density of boron vacancies. The optical properties of CBVB-nH complexes are analyzed and compared to recent photoluminescence measurements on MOVPE-grown hBN samples. In particular, we investigate the origin of the emission peaks at 1.90 eV and 2.24 eV and demonstrate that both the energies and lineshapes are consistent with radiative hole capture by negatively charged CBVB and CBVB-H complexes.

The nitrogen vacancy (NV) center has emerged as a powerful quantum sensor in high-pressure research, with the observation of optically detected magnetic resonance at megabar pressures. However, some aspects of NV physics require further investigation to optimize the development of NV-based sensing under pressure. Here, we study both experimentally and theoretically the optical properties of the NV center under hydrostatic pressure. We investigate the evolution of the zero-phonon line (ZPL) position, radiative lifetimes, optical lineshapes, and photoionization thresholds of the NV center under pressures up to ~120 GPa. We also provide spectroscopic guidelines for performing high-pressure optical experiments. Our results confirm that the NV center remains a robust quantum sensor under extreme hydrostatic pressures, especially for magnetic characterization.

We present a comprehensive study of photoluminescence spectra of the CuGaSe2 (CGS) thin films, with a focus on the influence of variation in the copper content on optical transitions involving deep defects. The CGS samples investigated here were grown via the three-stage co-evaporation method, and exhibited varying [Cu]/ ([Cu]+[Ga]) ratios of 1.01, 0.98, and 0.92. We analyzed only the part of the spectrum below 1.3 eV, and carefully considered the interference effects in the analysis. Two prominent peaks were identified in this low-energy region. The first peak (A), located at 0.98 eV, was present in all samples, regardless of their stoichiometry. The second peak (B) at 1.12 eV emerged exclusively in Cu-poor samples, indicating its dependence on copper stoichiometry. Based on an analysis of the PL spectra, ab initio calculations, and literature data, peaks were tentatively assigned to donor-acceptor radiative recombination involving native defects. We proposed two alternative models to explain the nature of peak A at 0.98 eV. The first model assumed a nonintuitive recombination mechanism due to the V VSe charge state change by capturing a hole located on VCu. In the second model, we proposed a transition between a deep donor GaCu(2+/+) and a deep acceptor, which is probably CuGa(0/-). Peak B at 1.12 eV, within both models, is explained as a recombination between a deep donor, i.e. a substitutional defect Ga GaCu GaCu(2 GaCu(2+/ GaCu(2+/+), and a shallow acceptor VCu.

In this work, we present a first-principles density functional theory (DFT) computational investigation of the luminescence and absorption lineshapes associated with the neutral carbon-oxygen interstitial pair (CiOi) defect in silicon. We obtain the lineshapes of the defect in the dilute limit using a computational methodology that constructs dynamical matrices of supercells containing tens of thousands of atoms, utilizing systems directly accessible through DFT. Both perturbed bulk phonons and localized vibrations contribute to the phonon sideband. We achieve excellent agreement with experimental luminescence data. Our findings further reinforce the attribution of the well-known C-line in silicon to the neutral CiOi complex.

Single-photon emitters in hexagonal boron nitride (h-BN) exhibit compelling optical properties, such as high brightness and photostability, making them promising candidates for applications in emerging nanophotonic and quantum technologies. However, their further progress would greatly benefit from the identification of their chemical nature, which in most cases is still under debate. Among the various emitters observed in h-BN, blue quantum emitters (435 nm), also known as B-centers, are unique not only due to their promising optical properties but also because they can be fabricated at specific locations via electron-beam irradiation. Herein, ab initio calculations are used to demonstrate that the properties of a carbon chain tetramer are in excellent agreement with the characteristics of blue quantum emitters. The calculated zero-phonon line (ZPL) energy (3.13 eV) and radiative lifetime (1.62 ns) of the carbon chain tetramer align well with experimental observations. The relatively weak electron-phonon coupling indicates intense emission at the ZPL, and the Debye-Waller factor of 0.25 is consistent with experimental data. We demonstrate that, despite the absence of an inversion center in the carbon chain tetramer, it exhibits a negligible linear Stark effect, consistent with experimental findings. Additionally, our hypothesis explains the experimental observation that the formation of blue emitters is only possible in samples containing numerous ultraviolet emitters, which are probably identical to carbon dimers.

First-principles calculations of defects and electron–phonon interactions play a critical role in the design and optimization of materials for electronic and optoelectronic devices. The late Audrius Alkauskas made seminal contributions to developing rigorous first-principles methodologies for the computation of defects and electron–phonon interactions, especially in the context of understanding the fundamental mechanisms of carrier recombination in semiconductors. Alkauskas was also a pioneer in the field of quantum defects, helping to build a first-principles understanding of the prototype nitrogen-vacancy center in diamond, as well as identifying novel defects. Here, we describe the important contributions made by Alkauskas and his collaborators and outline fruitful research directions that Alkauskas would have been keen to pursue. Audrius Alkauskas’ scientific achievements and insights highlighted in this article will inspire and guide future developments and advances in the field.

Detailed characterization of deep-level color centers requires understanding their electronic and atomic structure, which is most commonly investigated utilizing the Kohn-Sham density functional theory. Standard semilocal functionals based on the generalized gradient approximation (GGA) are inclined toward an imprecise quantitative description of defects' electronic structure. Hybrid functionals provide an improved prediction of electronic properties, albeit at a much higher computational cost. In this work, we test the newly developed Strongly Constrained and Appropriately Normed (SCAN) family of meta-GGA density functionals for selected color centers in diamond. In particular, we study nitrogen-, silicon-, germanium-, and tin-vacancy centers that have been recently investigated for their use in quantum technological applications. We show that SCAN and its derivatives, the rSCAN and r2SCAN functionals, significantly improve the calculated energies of optical transitions within the delta-self-consistent-field approach, almost reaching the accuracy of the hybrid Heyd-Scuseria-Ernzerhof (HSE) functional. In the case of the NV- center, we also show that the SCAN family of functionals improves the description of the adiabatic potential energy surfaces compared to both GGA and hybrid functionals, improving calculated luminescence lineshapes. As a result of these findings, we recommend using the SCAN family of functionals as a promising alternative for studying color centers in solids.

The temperature and light intensity dependences of persistent photoconductivity (PPC) in Cu(In,Ga)Se2 thin films have not been fully understood so far. We show by means of the numerical simulations that experimental characteristics of PPC can be explained by the combination of two factors: a higher than previously assumed energy barrier for the electron capture process (at least 0.26 eV), and stretched-exponential character of transients. Our findings are compatible with the hypothesis linking PPC in Cu(In,Ga)Se2 with a defect exhibiting the large lattice relaxation, like the Se-Cu divacancy complex (VSe-VCu).

We present a first-principles computational study of the thermodynamics of carbon defects in hexagonal boron nitride (hBN). The defects considered are carbon monomers, dimers, trimers, and larger carbon clusters, as well as complexes of carbon with vacancies, antisites, and substitutional oxygen. Our calculations show that monomers (CB, CN), dimers, trimers, and CNON pairs are the most prevalent species under most growth conditions. Compared to these defects, complexes of carbon with vacancies and antisites occur at much smaller concentrations (<1014 cm−3). Our results are discussed in view of the relevance of carbon defects in singlephoton emission in hBN.

We present ab initio calculations of photoionization thresholds and cross sections of the negatively charged nitrogen-vacancy (NV) center in diamond. We consider photoionization from the ground (3)A(2) and the excited E-3 states. After the ionization from the E-3 level, we show that the NV center transitions into the metastable (4)A(2) electronic state of the neutral defect. We reveal how spin polarization of NV- gives rise to spin polarization of the (4)A(2) state, explaining electron spin resonance experiments. We obtain smooth curves of photoionization cross sections as a function of energy by employing dense k-point meshes for the Brillouin-zone integration together with the band unfolding technique to rectify the distortions of the band structure induced by the artificial periodicity of the supercell approach. Our calculations provide a comprehensive picture of photoionization mechanisms of NV-. They will be useful in interpreting and designing experiments on charge-state dynamics at NV centers. In particular, we offer a consistent explanation of recent results of spin-to-charge conversion of NV centers.

Persistent photoconductivity (PPC) in thin Cu(In,Ga)Se-2 films is discussed within a model of relaxing defects acting as donors or acceptors depending on their configurational and charge state. The aim of this work is to identify the factors related to technological processes which affect the magnitude of PPC. We established a method of evaluation of the concentration of metastable defects in thin Cu(In,Ga)Se-2 films relating it to the position of the Fermi level in thermodynamic equilibrium and used it to compare and discuss the impact of preparation details on the PPC value. The main result is that deviation from Cu/(Ga + In) stoichiometry does not change the concentration of metastable defects. Post deposition annealing in selenium affects the PPC depending on the presence of sodium during the treatment, while the impact of sodium itself on the metastable defect concentration apparently depends on whether it is present during the Cu(In,Ga)Se-2 deposition process or whether it is supplied during post-deposition treatment.

Cu(In,Ga)Se2-based solar cells often exhibit fill factor losses at low temperature, in particular after red illumination or reverse biasing. A narrow p+ layer between the absorber and buffer layers is commonly assumed to explain these effects. In this contribution, we analyze by means of analytical and numeric models the influence of the parameters of the p+ layer on current-voltage characteristics. Specifically, we find expressions linking the voltage at which the deterioration of the current-voltage curve begins with the acceptor density and the width of the p+ layer. Moreover, we derive formulas describing the height of the barrier in the conduction band caused by the p+ layer. Examples of the analysis of simulated current-voltage characteristics using our approach are presented. Based on the voltage at which the decrease of the current starts, it is possible to estimate the upper and lower limits of the density of the acceptors in the p+ layer. Furthermore, we analyze the relationship between the fill factor and the height of the barrier in the conduction band, which is determined by the density of acceptors in the p+ layer.

We propose that the carbon dimer defect CBCN in hexagonal boron nitride gives rise to the ubiquitous narrow luminescence band with a zero-phonon line of 4.08 eV (usually labeled the 4.1 eV band). Our first-principles calculations are based on hybrid density functionals that provide a reliable description of wide bandgap materials. The calculated zero-phonon line energy of 4.3 eV is close to the experimental value, and the deduced Huang-Rhys factor of S≈2.0, indicating modest electron-phonon coupling, falls within the experimental range. The optical transition occurs between two localized π-type defects states, with a very short radiative lifetime of 1.2 ns, in very good accord with experiments.

We measured the temperature-dependent internal quantum efficiency (IQE) of Cu(In,Ga)Se-2-based (CIGS) solar cells. The largest differences in IQE spectra measured between 100 and 300 K were observed in the wavelength range, corresponding to the light absorbed exclusively in the CIGS layer. Absorbers in the investigated cells were grown using a one-stage process. Since all elements are supplied at a constant rate, the obtained layers are free of the band gap grading, therefore collection in the bulk of the layer is not affected by a quasi-electrical field. This allows us to discuss temperature changes in collection solely in terms of recombination. We associate the change in IQE with recombination via defects present in the bulk of absorber. The two cases of donor and acceptor defects are discussed. Using SCAPS software and basic handbook formulas that describe the emission and capture rates of carriers, we estimate a range of basic parameters of the possible bulk defects in CIGS that are responsible for the temperature change of IQE spectra. Our results suggest that IQE may be controlled by shallow defects of ionization energy of 45 and 60 meV for the donor and acceptor cases, respectively. We calculate IQE spectra at different temperatures. The temperature change of simulated spectra reproduces the same tendency as experimental characteristics.

The magnitude of the persistent photoconductivity effect (PPC) in two sets of Cu(In,Ga)Se2 samples, differing in the amount of cadmium and sodium, was measured. Using equations describing the magnitude of PPC, metastable defect and shallow acceptor densities were calculated. The method of the analysis of PPC in the presence of a deep acceptor level was presented. Based on obtained results, we drew conclusions about reasons of decreased PPC in Cu(In,Ga)Se2 without sodium as well as the role of (VSe-VCu) complexes in establishing the carrier concentration in Cu(In,Ga)Se2 with and without sodium.

In this contribution, we analyze factors determining the magnitude of the persistent photoconductivity (PPC) effect, defined as a difference in the hole concentration after and before illumination. Based on the Lany-Zunger model, we derive the equation describing the magnitude of the PPC with only two parameters: the density of (VSe-VCu) complexes and the relaxed hole concentration. We apply the derived equation to the experimental data measured on Cu(In,Ga)Se2 solar cells with and without sodium. We demonstrate that significant differences in the PPC can be explained in some cases by differences in the relaxed hole concentration. Furthermore, we show that deep defect levels situated close to the Fermi level can also substantially influence the magnitude of the PPC.

In this contribution, we investigated by means of numerical simulations the influence of relaxation processes related to metastable defects on electrical characteristics of Cu(In,Ga)Se2. In particular, we analyzed the relaxation of a metastable state induced by illumination at a fixed temperature as well as the dependence of the hole concentration on the temperature during cooling. The knowledge of these two relaxation processes is crucial in the evaluation of the hole concentration in the relaxed state and after light soaking. We have shown that the distribution of the metastable defects can be considered frozen below 200 K. The hole capture cross section was estimated as ∼3 × 10−15 cm2. It was shown that the usually used cooling rates may lead to relevant changes of the hole concentration. We calculated the lower limit of the hole concentration after cooling, and we presented how it depends on densities of shallow acceptors and metastable defects. Moreover, we proposed a method which allows for the evaluation of shallow acceptor and metastable defect densities from two capacitance-voltage profiles measured in the relaxed and light soaking states. Finally, we indicated experimental conditions in which the influence of relaxation processes on the accuracy of this method is the smallest.

The persistent increase of conductivity after illumination in CuInGaSe2 thin films was investigated as a function of the temperature and light intensity at its creation. Experimentally observed dependences were compared with the results of calculations based on the Lany–Zunger (L–Z) model relating the phenomenon to configurational changes of VSe–VCu divacancy. The calculations showed that the description of all results within this model would require, in some cases, unrealistic values of cross section for electron capture by the center in donor configuration. We concluded that the model does not fully account for the observed phenomena, and that additional mechanisms, in particular to explain the temperature effect, are required.