Exciton-Phonon Coupling in the Ultraviolet Absorption and Emission Spectra of Bulk Hexagonal Boron Nitride

Abstract

We present an ab initio method to calculate phonon-assisted absorption and emission spectra in the presence of strong excitonic effects. We apply the method to bulk hexagonal BN which has an indirect band gap and is known for its strong luminescence in the UV range. We first analyse the excitons at the wave vector q of the indirect gap. The coupling of these excitons with the various phonon modes at q is expressed in terms of a product of the mean square displacement of the atoms and the second derivative of the optical response function with respect to atomic displacement along the phonon eigenvectors. The derivatives are calculated numerically with a finite difference scheme in a supercell commensurate with q. We use detailed balance arguments to obtain the intensity ratio between emission and absorption processes. Our results explain recent luminescence experiments and reveal the exciton-phonon coupling channels responsible for the emission lines.

Theory of the special displacement method for electronic structure calculations at finite temperature

“…In the case of a real system with many vibrational modes, our strategy is to exploit the same mechanism leading to Eq. (27), by leveraging the cancellation between 'similar' modes, i.e. modes of the same branch at adjacent q-points.…”

“…This problem is becoming increasingly important as we strive to achieve close quantitative agreement between ab initio calculations and experimental data. Furthermore this problem underpins calculations of many important properties, from temperature-dependent optical absorption [24][25][26] and emission spectra [27] to temperature-dependent * fgiustino@oden.utexas.edu transport coefficients [28,29].…”

Boron nitride for excitonics, nano photonics, and quantum technologies

“…We have shown that the band gap of MLBN is direct at K in reciprocal space [40] while the fundamental band gap of bulk hBN is indirect between M and K points of the first Brillouin zone [41,42]. The high internal quantum efficiency is correlated to the strong exciton-phonon coupling with an efficient and fast phonon emission that relaxes the k-dependent selection rule traditionally preventing indirect band gap semiconductors to strongly emit light via intrinsic processes [43][44][45][46][47][48]. This intriguing specificity of hBN is currently under active study, including the physics of the free and bound excitons.…”

“…The c-axis polarized B 1g mode gives rise to the optical branches labelled as ZO 1 and ZO 3 , corresponding to the low-energy and high-energy modes respectively, and the A 2u mode gives rise to the ZO 2 branch. In Figure 10, the different phonon branches are labelled using this systematic scheme [48,74,92], which will allow us to unambiguously identify the phonons that participate in the phonon-assisted emissions. There are three pairs of branches, namely TA and TO 1 , LA and LO 1 , and TO 2 and TO 3 that derive from different zone centre modes and they are non-degenerate at finite q although the branches are very close in frequency within each pair.…”

Thermal evolution of silicon carbide electronic bands

“…We compare our theoretical results with the observed thermal evolution of SiC band edges, and discuss our findings in the light of high temperature SiC electronics and defect qubits operation.Electron-phonon interaction impacts a large variety of fundamental materials properties [1], from the critical temperature of superconductors to the zero-point renormalization and the temperature dependence of the electronic energy bands, from the electronic band gaps [2][3][4][5][6] to the thermal evolution of the optical spectra and excitonic lifetimes [7][8][9]. In addition the electron-phonon coupling contributes to the optical absorption and emission in indirect gap semiconductors [10][11][12], determines the electronic carrier mobility of semiconductors [13], the carrier relaxation rates [14], the distortion of band structures and phonon dispersion giving rise to kinks and Kohn anomalies in photoemission [15].Direct observation of the temperature evolution of individual bands over a wide region of temperatures is not straightforward. Optical techniques are capable of measuring band gaps, and not the absolute values of the valence band maximum (VBM) and the conduction band minimum (CBM) separately.…”