By combining nanomasking with thermally resistant materials and sublimation in a molecular beam epitaxy reactor, porous (In)GaN layers can be obtained. The advantages and disadvantages of this technique compared to classical electrochemistry methods will be discussed. The porosity can be adjusted from 0 to 1 and the pore depth can be controlled by the sublimation temperature and time. Preferential sublimation occurs at the dislocation position which strongly enhance the photoluminescence properties. As the porosification process by sublimation does not depend on the doping, fully porous light emitting diodes can be demonstrated.
Hexagonal boron nitride (h-BN), an insulating two-dimensional layered material, has recently attracted a great attention due to its fascinating optical, electrical, and thermal properties, and promising applications across the fields of photonics, quantum optics, and electronics. Here, we exploit the scalable approach to grow h-BN on epitaxial gallium nitride (GaN) substrate by using metal-organic chemical vapor deposition (MOCVD). It was found that at a specific MOCVD growth condition, a very unique h-BN film can be grown on GaN substrates, in which few-layer h-BN film is suspended on GaN nanoneedles. The combination of state-of-the-art microscopic and spectroscopic analyses revealed that the suspended h-BN films exhibit unprecedented DUV photoluminescence spectra. In addition, the h-BN films show unprecedented atomic stacking configuration, the mechanism of which will be discussed with optical and structural characterizations and theoretical calculations.
We study shallow and deep levels in carbon-doped hexagonal boron nitride crystals precipitated from a molten metal solution in a high-temperature furnace. Reflectance and photoluminescence under deep ultraviolet excitation are complemented by spatially resolved experiments by means of a scanning confocal micro-photoluminescence setup operating in the ultraviolet. Isotopically controlled carbon doping does not induce any energy shift of the well-known deep-level emission at 4.1 eV. Our detailed characterization in a series of carbon-doped crystals reveals that the incorporation of carbon during the growth process results in a distinct class of shallow and deep levels in hexagonal boron nitride, calling into question the exact role of carbon in the growth of hexagonal boron nitride and its direct or indirect influence on the formation of the crystal defects.
Deep ultra-violet (DUV) light emitting diodes (LED) are expected to be the next generation of UV sources, offering significant advantages such as compactness, low consumption and long lifetimes. Yet, improvements of their performances are still required and the potential of AlyGa1-yN quantum dots as DUV emitters is investigated in this study. Using a stress induced growth mode transition, quantum dots (QD) are spontaneously formed on Al0.7Ga0.3N/AlN heterostructures grown on sapphire substrates by molecular beam epitaxy. By increasing the QD Al composition, a large shift of the QD photoluminescence in the UV range is observed, going from an emission in the near UV for GaN QD down to the UVC region for Al0.4Ga0.6N QD. A similar behavior is observed for electroluminescence (EL) measurements performed on LED structures and an emission ranging from the UVA (320-340 nm) down to the UVC (265-280 nm) has been obtained. The main performances of Al0.7Ga0.3N based QD LED are presented in terms of electrical and optical characteristics. In particular, the emission dependence on the input current density, including the emitted wavelength, the optical power and the external quantum efficiency are shown and discussed.
We present an investigation of optical properties of yellow light emitting (Ga,In) N-based devices grown by metalorganic vapor phase epitaxy (MOVPE) on c-plane sapphire with different designs: well width, indium composition up to 23 percent in the well layer. Using time-resolved photoluminescence measurements collected in range of 8-300K, temperature – dependent photoluminescence decays are determined, they exhibit similar behaviors for all samples. These quantum devices always display a two-mode exponential decay with a long decay time and a short one in a ratio about four to five. The photoluminescence intensities measured from low temperature to room temperature give large values of activation temperature that indicate the increasing of the non-radiative recombination rate when the temperature increases. The average decay times are found by a procedure using fitting sigmoidal functions. These decay times increase exponentially with the indium concentration and the well width due to influence of quantum confined Stark effect on these devices. Finally, in order to estimate the performances of our samples, we plotted the decay times obtained versus product of the indium content and the well width together with others published decay times.
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