61 research outputs found
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 molecules, enabling efficient
and cost-effective synthesis of ammonia from N 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 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
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