Temperature induced giant shift of phonon energy in epitaxial boron nitride layers

Abstract

The recent progress in the growth of large-area boron nitride epilayers opens up new possibilities for future applications. However, it remains largely unclear how weakly attached two-dimensional BN layers interact with their substrate and how their properties are influenced by defects. In this work, we investigate hBN layers grown by Metal Organic Vapor Phase Epitaxy (MOVPE) using Fourier-transform Infrared (FTIR) spectroscopy in the temperature range of 160-540 K. Our measurements reveal strong differences in the character of layer-substrate interaction for as-grown and delaminated epitaxial layers. A much weaker interaction of as-grown layers is explained by wrinkles formation that reduces strain at the layer-substrate interface, which for layers transferred to other substrates occurs only in a limited temperature range. The most striking result is the observation of a giant increase in the E1u phonon energy of up to ∼ 6 cm−1 in a narrow temperature range. We show that the amplitude and temperature range of the anomaly is strongly modified by UV light illumination. The observed giant effect is explained in terms of strain generation resulting from charge redistribution between shallow traps and different defects, which can be interpreted as a result of strong electron-phonon coupling in hBN. The observed narrow temperature range of the anomaly indicates that the effect may be further enhanced for example by electrostrictive effects, expected for sp2 boron nitride.

Growth temperature induced changes of luminescence in epitaxial BN: from colour centres to donor–acceptor recombination

“…The second hypothesis suggests that the C300->D330 transformation is due to natural defects or even cubic BN (c-BN), since it was published that it is the most stable phase of BN below 1392 °C. 54 Our XRD measurements (as well as Raman and IR spectroscopy 24,34 ) did not show the c-BN phase; however, the peaks are very broad and we cannot exclude the nanocrystals of c-BN. So it may be that the sharp lines are from c-BN, but since such lines were observed also in samples that are of pure h-BN, 4 this hypothesis has low probability.…”

“…The E 2g low phonon mode energy decreases with temperature, while the E 2g high mode increases its energy. 34,41 This effect is due to the thermal vibrations perpendicular to the BN plane that lead to the shrinkage of the BN lattice. Since the band gap depends on the lattice constant, the anomalous thermal lattice dependence 36 causes anomalous energy dependence.…”

Strain modulation of epitaxial h-BN on sapphire: the role of wrinkle formation for large-area two-dimensional materials

“…For studied layers, the full width at half maximum (FWHM) of the E 2g high phonon mode is about 25 cm −1 which is a low value for epitaxial h-BN indicating the high quality of the grown material [32]. The thickness of thin h-BN layers was assessed by Fourier transform infrared (FTIR) measurements [48]. FTIR spectra were acquired with a Thermo Scientific Nicolet Continuum Infrared Microscope equipped with 32x Schwarzschild infinity-corrected objective (NA 0.65).…”

Effective substrate for the growth of multilayer h-BN on sapphire—substrate off-cut, pre-growth, and post-growth conditions in metal-organic vapor phase epitaxy

“…For the best bulk h-BN material the FWHM is about 5 cm −1 [30,31] and for MOVPE BN about 25 cm −1 [22]. The maximum position is associated with strains in the boron nitride layer, resulting primarily from the interaction with the substrate [32]. Studies on the position of the Raman E 2g peak for a different number of layers [33][34][35] report a shift from about 1370 cm −1 for one monolayer to a value about 1366 cm −1 for bulk material.…”