Search

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 (V Se -V Cu ) complexes and the relaxed hole concentration. We apply the derived equation to the experimental data measured on Cu(In,Ga)Se 2 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. [ABSTRACT FROM AUTHOR]