Ziyi Zhang

Deep-UV (DUV) laser diodes with one, two, and three quantum wells were compared. Cavity-length-dependent analysis showed that, under non-uniform carrier injection among the wells, a single quantum well (1QW) is the most advantageous in terms of threshold and slope efficiency. A 1QW device achieved pulsed and CW thresholds of 1.2 and 1.5 kA cm − 2 with a slope efficiency of 1.7 W A − 1 , the best reported to date for a practical 280 nm band DUV-LD, and sustained over 15 h of room-temperature CW operation at 5 mW . The gain contribution is suggested to be non-uniform among multiple quantum wells in the present structures.

We report room-temperature current-injection electroluminescence (EL) from praseodymium (Pr)-related centers in an ion-implanted gallium nitride (GaN) diode structure. Pr ions were introduced into GaN by ion implantation, and the samples were subsequently subjected to ultrahigh-pressure annealing (UHPA) and metal organic vapor phase epitaxy regrowth to form the p-side layers. Under forward bias, a narrow EL peak at 652 nm was observed. This peak coincides with the Pr-related photoluminescence (PL) peak, indicating that Pr-related centers remain optically active after ion implantation, UHPA, regrowth, and device fabrication and can be electrically excited in the processed GaN structure. Secondary ion mass spectrometry and current density–voltage (J–V) measurements reveal pronounced structural and electrical nonidealities in the fabricated structure, including displacement of the effective junction/interface positions from the target design, impurity redistribution near the regrown region, leakage-related transport, and a large apparent turn-on voltage. The optical data therefore indicate that a Pr-related emission process exists under nonideal current injection, while its efficiency is limited by parasitic current paths, nonradiative losses, and the limited availability of local configurations and defect-related states that enable efficient host-to-Pr energy transfer. The saturation-like behavior of the Pr-related EL, together with the simultaneous increase in near-band-edge and Mg-related emission at higher current densities, suggests that additional current does not proportionally enhance the Pr-related emission but increasingly produces radiative recombination from the GaN host and Mg-related states. These results clarify the excitation and limitation mechanisms of Pr-related EL in ion-implanted GaN diode structures and identify requirements for future electrically driven lanthanoid emitters in GaN.

The gain characteristics of electrically injected AlGaN-based deep-ultraviolet laser diodes were studied with respect to their quantum-well (QW) width dependence by the analysis of the threshold currents and gains for various cavity lengths. Among the fabricated QWs with widths of d = 9, 4.5, and 3 nm, the QW with the smallest width of 3 nm was found to have the highest material gain and lowest transparency carrier density. In contrast to recent arguments in favor of wider QWs for deep-ultraviolet laser diodes, the strategy of narrowing QWs is still found to be effective in practical terms of reducing the threshold current density.

The degradation of an AlGaN‐based deep‐ultraviolet laser diode during operation is studied. A rapid increase in threshold current without any facet mirror damage under direct current stress below the threshold current is observed, which is considered to limit the lifetime of continuous wave lasing to the level of seconds. The evolution of current–light and current–voltage characteristics suggests that the dominant mechanism of degradation is the increased loss of carriers before they can reach the active layer, i.e., the decrease in carrier injection efficiency. The decrease in emission intensity, which is more pronounced at lower current densities, and subsequent increase in sub‐threshold current indicate the increase in defect density under current stress, which is similar to the well‐analyzed degradation mechanism found in AlGaN‐based light‐emitting diodes.

Increasing the injection efficiency, a critical factor constraining the reduction in threshold current in AlGaN-based deep-ultraviolet laser diodes, represents one of the paramount remaining technical challenges. In this study, the impact of compositionally graded layers that were unintentionally formed at the interface between the p-cladding and the core layer on carrier injection efficiency was analyzed. Experimental evaluations using laser diodes have shown that the elimination of an unintentionally formed layer increases the injection efficiency above the threshold current, from the conventional 3% to 13%. It has been postulated that the electron overflow toward the p-side exerts a substantial deleterious effect on the injection efficiency. An improvement in this aspect is achieved by increasing the electron-blocking capability due to the improved interface abruptness between the p-cladding layer and the core layer. The lasing threshold was strongly reduced, and characteristic temperature increased from 76 to 107 K for the improved devices.

One approach to improving the output power of ultraviolet (UV-C) light-emitting diodes (LEDs) is to adopt an electron-blocking layer (EBL) with a high barrier. However, the intended effect may not be realized because of the composition pulling effect, which is the unintended occurrence of a gradient layer at an AlGaN/AlGaN hetero-interface with substantial differences in the Al composition. Here, we demonstrate that low-temperature growth (i.e., <1000 °C) can be used to control the unintentional gradient layer at an AlN/AlGaN hetero-interface between a barrier layer and AlN-EBL with a difference in Al compositions of more than 30%. LEDs with an emission wavelength of 265 nm were fabricated, and an AlN-EBL was grown at low temperature to realize an abrupt interface. At an applied current of 100 mA, growing the EBL under low-temperature conditions improved the forward voltage by 0.5 V and remarkably improved the peak luminous intensity by 1.4–1.6 times. Our results can be used to realize UV-C LEDs with a steep EBL and further improve their device characteristics.

Previously reported UV-C laser diode (LD) structures have been subject to design constraints owing to dark line defects at the edge of the mesa stripe after device fabrication. To address this issue, a detailed analysis revealed that the dark line defects were dislocations generated by local residual shear stresses associated with mesa formation on highly strained epitaxial layers. A technique for controlling the local concentration of shear stress using a sloped mesa geometry was proposed based on insight gained by modeling the stress distribution at the edge of the mesa stripe. Experimental results showed that this technique succeeded in completely suppressing the emergence of dark-line defects. This technique will be useful in improving the performance of pseudomorphic AlGaN/AlN-based optoelectronic devices, including UV-C LDs.