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We review the recent advance in the understanding of the optoelectronic properties of hexagonal boron nitride (hBN) in the deep ultraviolet. The comparison between bulk hBN and monolayer hBN highlights some of their major differences such the bandgap nature, the excitonic binding energy and the phonon-assisted broadening of the excitonic lines. Perspectives point out the relevance of addressing the regime of hBN samples made of a very few number of monolayers, including twisted hBN monolayers in the context of twistronics.

The sp2-bonded layered compound boron nitride (BN) exists in more than a handful of different polytypes (i.e., different layer stacking sequences) with similar formation energies, which makes obtaining a pure monotype of single crystals extremely tricky. The co-existence of polytypes in a similar crystal leads to the formation of many interfaces and structural defects having a deleterious influence on the internal quantum efficiency of the light emission and on charge carrier mobility. However, despite this, lasing operation was reported at 215 nm, which has shifted interest in sp2-bonded BN from basic science laboratories to optoelectronic and electrical device applications. Here, we describe some of the known physical properties of a variety of BN polytypes and their performances for deep ultraviolet emission in the specific case of second harmonic generation of light. [ABSTRACT FROM AUTHOR]

We report the development of a scanning confocal microscope dedicated to photoluminescence in the 200 nm-wavelength range for samples at cryogenic temperatures (5 K–300 K). We demonstrate the performances of our deep ultraviolet cryomicroscope in high-quality hexagonal boron nitride (hBN) crystals, although it can be utilized for biological studies in its range of operating wavelengths. From the mapping of photoluminescence, we bring evidence for the suppression of extrinsic recombination channels in regions free from defects. The observation of emission spectra dominated by intrinsic recombination processes was never reported before in hBN by means of photoluminescence spectroscopy. We show that photoluminescence tomography now competes with cathodoluminescence and that deep ultraviolet cryomicroscopy by photoluminescence is a novel powerful tool in materials science applications, with the great advantage of an efficient non-invasive photo-excitation of carriers. [ABSTRACT FROM AUTHOR]

The use of novel organosilica materials embedding π-conjugated moieties as semiconductor into field-effect transistors is demonstrated. The chosen π-conjugated core is a [1]benzothieno[3,2-b][1]benzothiophene that is modified with hydrolyzable and crosslinkable triethoxysilyl moieties. After polycondensation, this compound forms a hybrid material composed of charge transport pathways as well as insulating sublayers made of silicon oxide (SiO x). The hybrid material behaves as a semiconductor and is subsequently integrated as active layer into field-effect transistors. These precursors show J-type aggregation that evolves toward H-type aggregates during the sol-gel process, which improve charge transport. Taking advantage of the sol-gel chemistry involved here, hybrid field-effect transistors that are fully crosslinked with covalent bonds are built. Molecules are crosslinked to each other, covalently bonded to the silicon oxide dielectric, and also covalently bonded to the gold electrodes, thanks to the use of an appropriate additional interfacial monolayer. This is the first report of fully covalent transistors. Those devices with modest mobilities show impressive resilience against polar, aliphatic, and aromatics solvents even under sonication. This study opens the route toward a new class of hybrid materials to create highly robust electronic applications. [ABSTRACT FROM AUTHOR]

AlGaN based light emitting diodes (LEDs) will play a key role for the development of applications in the ultra-violet (UV). In the UVB region (280–320 nm), phototherapy and plant lighting are among the targeted uses. However, UVB LED performances still need to be improved to reach commercial markets. In particular, the design and the fabrication process of the active region are central elements that affect the LED internal quantum efficiency (IQE). We propose the use of nanometer-sized epitaxial islands (i.e., so called quantum dots (QDs)) to enhance the carrier localization and improve the IQE of molecular beam epitaxy (MBE) grown UVB LEDs using sapphire substrates with thin sub-µm AlN templates. Taking advantage of the epitaxial stress, AlGaN QDs with nanometer-sized (≤10 nm) lateral and vertical dimensions have been grown by MBE. The IQE of the QDs has been deduced from temperature dependent and time resolved photoluminescence measurements. Room temperature IQE values around 5 to 10% have been found in the 290–320 nm range. QD-based UVB LEDs were then fabricated and characterized by electrical and electroluminescence measurements. On-wafer measurements showed optical powers up to 0.25 mW with external quantum efficiency (EQE) values around 0.1% in the 305–320 nm range. [ABSTRACT FROM AUTHOR]