Atypically small temperature-dependence of the direct band gap in the metastable semiconductor copper nitrideCu3N

Abstract

The temperature-dependence of the direct band gap and thermal expansion in the metastable anti-ReO 3 semiconductor Cu 3 N are investigated between 4.2 and 300 K by Fourier-transform infrared spectroscopy and x-ray diffraction. Complementary refractive index spectra are determined by spectroscopic ellipsometry at 300 K. A direct gap of 1.68 eV is associated with the absorption onset at 300 K, which strengthens continuously and reaches a magnitude of 3.5×10 5 cm −1 at 2.7 eV, suggesting potential for photovoltaic applications. Notably, the direct gap redshifts by just 24 meV between 4.2 and 300 K, giving an atypically small band-gap temperature coefficient dE g /dT of −0.082 meV/K. Additionally, the band structure, dielectric function, phonon dispersion, linear expansion, and heat capacity are calculated using density functional theory; remarkable similarities between the experimental and calculated refractive index spectra support the accuracy of these calculations, which indicate beneficially low hole effective masses and potential negative thermal expansion below 50 K. To assess the lattice expansion contribution to the band-gap temperature-dependence, a quasiharmonic model fit to the observed lattice contraction finds a monotonically decreasing linear expansion (descending past 10 −6 K −1 below 80 K), while estimating the Debye temperature, lattice heat capacity, and Grüneisen parameter. Accounting for lattice and electron-phonon contributions to the observed band-gap evolution suggests average phonon energies that are qualitatively consistent with predicted maxima in the phonon density of states. As band-edge temperature-dependence has significant consequences for device performance, copper nitride should be well suited for applications that require a largely temperature-invariant band gap.

Copper interstitial recombination centers in Cu3N

“…From Fig. 3(b), the peak in absorption coefficient at around 2.8 eV is consistent with that observed in the absorption coefficient spectrum measured using spectroscopic ellipsometry in [35]. Although only direct transitions are considered in this calculation, non-zero values of ε 2 (ω) are observed between 0 eV and 1.8 eV, due to the fact that density functional theory calculations are only theoretical approximations to actual materials.…”

“…While the measured absorption coefficient spectrum for Cu 3 N in Fig. 8 is in overall agreement with that reported in [3,17,35], we emphasize that optical absorption below the direct bandgap of 1.82 eV is mediated by Cu i defects and will not be present in the pristine defect-free Cu 3 N crystal.…”

“…3(b)]. The gradual optical absorption tail below the direct bandgap has also been observed in a similar HSE06 dielectric function calculation of Cu 3 N [35], and in other dielectric function calculations based on hybrid functionals [36]. While other first-principles methods like the GW [37] can provide better approximations of the optical properties of Cu 3 N, they require significantly greater computational expense and are beyond the scope of this paper.…”

Perovskite-related ReO3-type structures

“…70 Its crystal structure in space group P m3m is the same as that of 68 tetragonal structure, which is not surprising given the open nature of the structure. 71 There has not been a detailed study of NTE in Cu 3 N, but a recent report indicates that there is little, if any, expansion in the temperature range of 4 K -100 K. 72 More than fifty years from its discovery elapsed before the growth of thin films was reported, 73 and attention was drawn to the interesting semiconducting properties of Cu 3 N. Shortly afterwards, in 1990, it was shown that such films, which are typically green, could be used as an optical data recording medium using infrared light. 74 Since copper does not react directly with nitrogen gas, the films were made by irradiating copper with nitrogen ions during film deposition onto a substrate (though bulk samples can be made from the reaction between CuF 2 and NH 3 ).…”

Photocatalytic Properties of Copper Nitride/Molybdenum Disulfide Composite Films Prepared by Magnetron Sputtering

“…Although the Cu 3 N film alone has better photocatalytic performance, it still has some shortcomings, so researchers often dope or recombine Cu 3 N with other materials. Cu 3 N has a ReO 3 structure, and the eight corners of the cubic crystal are occupied by N atoms; each side of the unit cell has a Cu atom, and many vacancies are present in the center of the unit cell [36][37][38][39]. These vacancies can be filled by other atoms (e.g., Pb [40], Ag [33], and Sc [41]), thereby changing the Cu 3 N band gap width and consequently its electrical and optical properties.…”