Semiconducting character of LaN: magnitude of the band gap, and origin of the electrical conductivity

Lanthanum nitride (LaN) has attracted research interest in catalysis due to its ability to activate the triple bonds of N$_2$ molecules, enabling efficient and cost-effective synthesis of ammonia from N$_2$ gas. While exciting progress has been made to use LaN in functional applications, the electronic character of LaN (metallic, semi-metallic, or semiconducting) and magnitude of its band gap have so far not been conclusively determined. Here, we investigate the electronic properties of LaN with hybrid density functional theory calculations. In contrast to previous claims that LaN is semi-metallic, our calculations show that LaN is a direct-band-gap semiconductor with a band-gap value of 0.62 eV at the X point of the Brillouin zone. The dispersive character of the bands near the band edges leads to light electron and hole effective masses, making LaN promising for electronic and optoelectronic applications. Our calculations also reveal that nitrogen vacancies and substitutional oxygen atoms are two unintentional shallow donors with low formation energies that can explain the origin of the previously reported electrical conductivity. Our calculations clarify the semiconducting nature of LaN and reveal candidate unintentional point defects that are likely responsible for its measured electrical conductivity.Comment: 16 pages, 4 figure

Electronic and optical properties of 4H Si from first principles

The cubic polytype of silicon (Si) is the most commercialized semiconductor material and finds applications in numerous electronic and optoelectronic devices such as solar cells. However, recent reports on the synthesis of the hexagonal 4H Si polytype have attracted the attention of the scientific community to understand its functional properties. Here we report the electronic, vibrational, and optical properties of the 4H Si polytype obtained with predictive first-principles calculations. We find that, compared to the cubic polytype, 4H Si shows a slightly narrower indirect gap by $\sim$ 0.05 eV. By calculating its direct and phonon-assisted optical spectra, we show that 4H Si exhibits a stronger absorption coefficient than cubic Si across the visible and IR spectral regions. We further evaluate the short-circuit current density of textured thin-films, and we demonstrate that 4H Si can achieve the same short-circuit current density for a five times thinner film compared to the cubic polytype. Our work demonstrates the advantages of 4H Si for thin-film silicon-based solar-cell applications.Comment: 9 pages, 5 figures in main text, 2 figures in supplemental informatio

Phonon-assisted optical absorption of SiC polytypes from first principles

Silicon carbide (SiC) is an indirect-gap semiconductor material widely used in electronic and optoelectronic applications. While experimental measurements of the phonon-assisted absorption coefficient of SiC across its indirect gap have existed for more than fifty years, theoretical investigations of phonon-assisted absorption have been hampered by their excessive computational cost. In this work, we calculate the phonon-assisted temperature-dependent optical absorption spectra of the commonly occurring SiC polytypes (3C, 2H, 4H and 6H), using first-principles approaches based on density functional theory and related techniques. We show that our results agree with experimentally determined absorption coefficients in the spectral region between the direct and indirect band gaps. The temperature dependence of the spectra can be well-predicted with taking the temperature-dependence of the band gaps into account. Lastly, we compare the spectra obtained with second-order perturbation theory to those determined by the special displacement method, and we show that the full consideration of the electronic energy renormalization due to temperature is important to further improve the prediction of the phonon-assisted absorption in SiC. Our insights can be applied to predict the optical spectra of the less common SiC polytypes and other indirect-gap semiconductors in general.Comment: 13 pages, 9 figure

Strain Effects on Auger-Meitner Recombination in Silicon

We study the effects of compressive and tensile biaxial strain on direct and phonon-assisted Auger-Meitner recombination (AMR) in silicon using first-principles calculations. We find that the application of strain has a non-trivial effect on the AMR rate. For most AMR processes, the application of strain increases the AMR rate. However, the recombination rate for the AMR process involving two holes and one electron is suppressed by 38% under tensile strain. We further analyze the specific phonon contributions that mediate the phonon-assisted AMR mechanism, demonstrating the increased anisotropy under strain. Our results indicate that the application of tensile strain increases the lifetime of minority electron carriers in p-type silicon, and can be leveraged to improve the efficiency of silicon devices.Comment: Supplementary Material included beneath reference