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AlGaN-based deep UV (DUV) LEDs generally employ a p-type electron blocking layer (EBL) to suppress electron overflow. However, Al-rich III-nitride EBL can result in challenging p-doping and large valence band barrier for hole injection as well as epitaxial complexity. As a result, wall plug efficiency (WPE) can be compromised. Our systematic studies of band diagram and carrier concentration reveal that carrier concentrations in the quantum well and electron overflow can be significantly impacted because of the slope variation of the quantum barrier (QB) conduction and valence bands, which in turn influence radiative recombination and optical output power. Remarkably, grading the Al composition from 0.60 to 0.70 for the 12-nm-thick AlGaN QB of the DUV LED without the EBL can lead to 13.5% higher output power and similar level of overflown electron concentration (~1 × 1015/cm3) as opposed to the conventional DUV LED with the p-type EBL. This paradigm is significant for the pursuit of higher WPE or shorter emission wavelength for DUV LEDs and lasers, as it provides a new direction for addressing electron overflow and hole injection issues.

AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) suffer from severe quantum confined Stark effect (QCSE) due to the strong polarization field in the quantum wells (QWs) grown on c-plane substrates. In this paper, we propose a novel DUV LED structure embedded with graded QWs in which the Al composition was linearly changed to screen the QCSE. A significant increase of the internal quantum efficiency and thus an enhancement of the light output power by nearly 67% can be achieved, attributing to the improvement of the electron-hole wave function overlap (Γe-hh) to 58.6% in the Increased-Al-composition graded QWs, as compared to the QW without grading (Γe-hh = 40.4%) and reverse grading (Γe-hh = 33.6%). Further investigations show that the grading profile of the Al composition in the QWs, including either linearly increases or decreases along the growth direction and the thickness of graded QWs, determine the polarization electrical field in the QWs and as a result, significantly affecting the performance of the devices. In the end, a careful optimization of the graded QWs is called. The proposed structure with such unique graded QWs provides us an effective solution to suppress the QCSE effect in the pursuit of high-performance DUV emitters.

Deep-ultraviolet emitting structures based on Al0.65Ga0.35N/Al0.8Ga0.2 N multiple quantum wells (MQWs), embedded in compositionally graded AlxGa1-xN films in the form of graded-index-separate-confinement heterostructure, were grown by molecular beam epitaxy. The graded AlGaN layer blocked threading defects (TDs) in the vicinity of the AlN/AlGaN heterointerface, resulting in a reduction of the TDs in the active layer. The intensity of the transverse-magnetic polarized amplified spontaneous emission spectra with peak emission at 270 nm was found to increase linearly with the number of QWs. Furthermore, while the peak intensity for devices with a single QW varies linearly with the pump fluence, it varies superlinearly for larger number of QWs, suggesting light amplification by stimulated emission. These results are consistent with numerical simulations, which indicate that the confinement of the optical mode in this device structure increases with the number of QWs and simultaneously the threshold under optical pumping decreases with increasing of the number of QWs. Finally, the band structure simulations indicate that the device is in the form of a p-n junction due to polarization-induced p- and n-doping in the compositionally graded AlxGa1-xN films on either side of the MQWs, and, thus, could be used for the development of an electrically pumped deep-ultraviolet laser by further optimizing the structures.

The effects of 2-D hexagonal-lattice TiO2 photonic crystal layers on the directional light extraction efficiency of low-scattering (Y1-xCex)3Al5O12 (YAG:Ce 3+) ceramic converters have been investigated. The hexagonal-lattice TiO2 patterns were fabricated by an electron beam lithography process. The optimized yellow directional light extraction enhancement by a factor of 4.4 is achieved with a 2-D photonic crystal layer having a diameter of 430 nm, a lattice constant of 590 nm, and a height of 350 nm. A 3-D finite-difference time-domain model has also been developed to study the geometric parameters of the photonic crystal layer, yielding results that are consistent with our measured data.