We investigate the temperature effect on the electronic band structure and optical absorption property of wide-band-gap ternary nitride MgSiN$_2$ using first-principles calculations. We find that electron-phonon coupling leads to a sizable reduction in the indirect gap of MgSiN$_2$, which is indispensable in understanding the optoelectronic properties of this material. Taking the band gap renormalization into account, the band gap of MgSiN$_2$ determined by the quasiparticle GW0 calculations shows good agreement with recent experimental result. The predicted phonon-assisted indirect optical absorption spectra show that with increasing temperature the absorption onset undergoes a red-shift. Our work provides helpful insights to the nature of the band gap of MgSiN$_2$ and facilitates its application in ultraviolet optoelectronic devices.
We have performed density functional calculations on the group II-IV nitride MgSiN$_{2}$. At a pressure of about 20~GPa the ground state wurtzite derived MgSiN$_{2}$ structure (LP-MgSiN$_{2}$) transforms into a rock-salt derived structure (HP-MgSiN$_{2}$) in agreement with previous theoretical and experimental studies. Both phases are wide band gap semiconductors with indirect band gaps at equilibrium of 5.58 eV (LP-MgSiN$_{2}$) and 5.87 eV (HP-MgSiN$_{2}$), respectively. As the pressure increases, the band gaps become larger for both phases, however, the band gap in LP-MgSiN$_{2}$ increases f…
Ultra-wide direct band gap semiconductors hold great promise for deep ultraviolet opto-electronic applications. Here we evaluate the potential of MgSiN$_2$-GaN alloys for this purpose. Although MgSiN$_2$ itself has an indirect gap $\sim$0.4 eV below its direct gap of $\sim$6.5 eV, its different sign lattice mismatch from GaN in two different basal plane directions could avoid the tensile strain which limits Al$_x$Ga$_{1-x}$N on GaN for high $x$. Two octet-rule preserving structures (with space groups $Pmn2_1$ and $P1n1$) of a 50% alloy of MgSiN$_2$ and GaN are investigated and are both found t…
Four ternary semiconductors with the chalcopyrite structure (BeCN2, BeSiN2, MgCN2, and MgSiN2) were studied using the first principles methods. The structural, electronic, optical and elastic properties were calculated. All these materials were found to be the indirect band gap semiconductors, with the calculated band gaps in the range from 3.46 eV to 3.88 eV. Comparison of the degree of covalency/ionicity of the chemical bonds in these compounds was performed. Anisotropy of the optical properties of these tetragonal crystals was demonstrated by calculating the real and imaginary parts of the …
Recently, Zacharias et al [Phys. Rev. Lett. 115, 177401 (2015)] developed a new ab initio theory of temperature-dependent optical absorption spectra and band gaps in semiconductors and insulators. In that work the zero-point renormalization and the temperature dependence were obtained by sampling the nuclear wavefunctions using a stochastic approach. In the present work, we show that the stochastic sampling can be replaced by fully deterministic supercell calculations based on a single optimal configuration of the atomic positions. We demonstrate that a single calculation is able to capture th…
The indirect carrier-phonon vertex function is evaluated for cubic zirconia by employing the many-body perturbation theory on top of the frozen phonon model. The phonon-induced carrier velocity and spectral renormalization are represented at each individual lattice vibronic mode. Our results show that for the indirect electron-phonon scattering, optical phonons make up the dominant contribution. It is observed that, despite the phonon-phonon interaction, there is a substantial polaronic character for all the activated lattice optical vibrations which is in accordance with the previous experime…
We present a theory of electronic excitation energies and optical absorption spectra which incorporates energy-level renormalization and phonon-assisted optical absorption within a unified framework. Using time-independent perturbation theory we show how the standard approaches for studying vibronic effects in molecules and those for addressing electron-phonon interactions in solids correspond to slightly different choices for the non-interacting Hamiltonian. Our present approach naturally leads to the Allen-Heine theory of temperature-dependent energy levels, the Franck-Condon principle, the …
We report diffusion quantum Monte Carlo (DMC) and many-body $GW$ calculations of the electronic band gaps of monolayer and bulk hexagonal boron nitride (hBN). We find the monolayer band gap to be indirect. $GW$ predicts much smaller quasiparticle gaps at both the single-shot $G_0W_0$ and the partially self-consistent $GW_0$ levels. In contrast, solving the Bethe-Salpeter equation on top of the $GW_0$ calculation yields an exciton binding energy for the direct exciton at the $K$ point in close agreement with the DMC value. Vibrational renormalization of the electronic band gap is found to be si…
We present femtosecond transient transmission (or absorbance) measurements in silicon nanowires in the energy range 1.1-3.5 eV, from below the indirect band-gap to above the direct band-gap. Our pump-probe measurements allow us to give a complete picture of the carrier dynamics in silicon. In this way we perform an experimental study with a spectral completeness that lacks in the whole literature on carrier dynamics in silicon. A particular emphasis is given to the dynamics of the transient absorbance at the energies relative to the direct band gap at 3.3 eV. Indeed, the use of pump energies b…
Among the transparent conducting oxides, the perovskite barium stannate is most promising for various electronic applications due to its outstanding carrier mobility achieved at room temperature. However, most of its important characteristics, such as band gaps, effective masses, and absorption edge, remain controversial. Here, we provide a fully consistent picture by combining state-of-the-art {\it ab initio} methodology with forefront electron energy-loss spectroscopy and optical absorption measurements. Valence electron energy-loss spectra, featuring signals originating from band gap transi…
This article reviews the theory of electron-phonon interactions in solids from the point of view of ab-initio calculations. While the electron-phonon interaction has been studied for almost a century, predictive non-empirical calculations have become feasible only during the past two decades. Today it is possible to calculate from first principles many materials properties related to the electron-phonon interaction, including the critical temperature of conventional superconductors, the carrier mobility in semiconductors, the temperature dependence of optical spectra in direct and indirect-gap…
EPW is an open-source software for $\textit{ab initio}$ calculations of electron-phonon interactions and related materials properties. The code combines density functional perturbation theory and maximally-localized Wannier functions to efficiently compute electron-phonon coupling matrix elements on ultra-fine Brillouin zone grids. This data is employed for predictive calculations of temperature-dependent properties and phonon-assisted quantum processes in bulk solids and low-dimensional materials. Here, we report on significant new developments in the code that occurred during the period 2016…
The ability to engineer atomically thin nanoscale lateral heterojunctions (HJs) is critical to lay the foundation for future two-dimensional (2D) device technology. However, the traditional approach to creating a heterojunction by direct growth of a heterostructure of two different materials constrains the available band offsets, and it is still unclear if large built-in potentials are attainable for 2D materials. The electronic properties of atomically thin semiconducting transition metal dichalcogenides (TMDs) are not static, and their exciton binding energy and quasiparticle band gap depend…
Deep Neural Network for Phonon-Assisted Optical Spectra in Semiconductors
School of Artificial Intelligence and Data Science, University of Science and Technology of China（中国科学技术大学人工智能与大数据学院）；Suzhou Institute for Advanced Research, University of Science and Technology of China（中国科学技术大学苏州市先进研究院）；AI for Science Institute, Beijing 100080, China（北京人工智能研究院）；Suzhou Big Data & AI Research and Engineering Center, Suzhou 215123, China（苏州大数据与人工智能研究与工程中心）；Birla Institute of Technology & Science, Pilani-Dubai Campus, Dubai 345055, UAE（巴里尔理工学院迪拜校区）；Department of Physics, McGill University（麦吉尔大学物理系）
Within the ionic Hubbard model, electron correlations transmute the single-particle gap of a band insulator into a Mott gap in the strong correlation limit. However understanding the nature of possible phases in between these two extreme insulating phases remains an outstanding challenge. We find two strongly correlated insulating phases in between the above extremes: (i) The insulating phase just before the Mott phase can be viewed as gapping a non-Fermi liquid state of spinons through staggered ionic potential. The quasi-particles of underlying spinons are orthogonal to physical electrons an…
