AlGaN Microfins as Nonpolar UV Emitters Probed by Time-Resolved Cathodoluminescence

Abstract

Despite the continuous technological progress and recent commercialization of UV light emitting diodes for sterilization and disinfection applications, the performance of solid-state UV emitters still lags far behind that of their InGaNbased counterparts, which emit in the visible blue−green spectrum. Both fundamental physical aspects and material quality restrictions have been discussed as origin of this striking difference. In this study, GaN/AlGaN core−shell microfins with a-plane sidewalls are proposed as a model system to demonstrate the efficiency potential of ultra-low-defect nonpolar UV emitters. Such structures are manufactured by bottom-up selective-area metalorganic vapor phase epitaxy of GaN microfin cores and further overgrowth of the AlGaN active region, employing a compositionally graded GaN/AlGaN short-period superlattice for strain management. Using this approach, mostly crack-free AlGaN quantum wells emitting around 321 nm could be realized with a threading dislocation density of as low as (6 ± 3) × 10 7 cm −2 . The carrier dynamics within the nonpolar AlGaN structures are studied by time-resolved cathodoluminescence spectroscopy, revealing fast radiative recombination lifetimes down to 0.6 ns and distinct signatures of carrier localization at low temperatures. Delayed carrier emission from localized states within the cladding is proposed to be a cause for anomalously high effective carrier lifetimes observed at temperatures below 80 K. By analyzing the characteristic temperature dependence of radiative and nonradiative recombination processes and fitting the lifetime data with an appropriate model across the whole temperature range, a room-temperature internal quantum efficiency of (40 ± 6) % is estimated for the studied sample. The findings corroborate the interest in nonpolar AlGaN structures as highly efficient UV emitters.

A Combination of Ion Implantation and High‐Temperature Annealing: Donor–Acceptor Pairs in Carbon‐Implanted AlN

“…[34][35][36] Consequently, a monotonically decreasing lifetime with emission energy is another fingerprint of a DAP transition. Figure 6a shows measured transients (dots) and fits (lines)-according to the fitting procedure described by Spende et al [39] -with a 20 nm step width and a transient integration window of 5 nm. The decay times extracted from the fit are plotted in Figure 6b, revealing a monotonically decreasing lifetime from 490 to 590 nm.…”

Core–Shell Nanorods as Ultraviolet Light-Emitting Diodes

“…Time-resolved cathodoluminescence (TRCL) measurements find the lifetimes of our m -plane transitions to be remarkably short, even at 80 K (in our case, we generally find that lower temperatures extended the carrier lifetimes) . As seen in Figure d, a convoluted biexponential decay curve can be fitted to determine a carrier lifetime of 19 ps in the QW while accounting for the instrument response function .…”

“…Time-resolved cathodoluminescence (TRCL) measurements find the lifetimes of our m-plane transitions to be remarkably short, even at 80 K (in our case, we generally find that lower temperatures extended the carrier lifetimes). 50 As seen in Figure 3d, a convoluted biexponential decay curve can be fitted to determine a carrier lifetime of 19 ps in the QW while accounting for the instrument response function. 51 Factors such as localization, well widths, and point defect populations will modify these lifetimes, but such short lifetimes can only be explained by the absence of or significant reduction in the internal electric fields.…”

Cathodoluminescence Nanoscopy: State of the Art and Beyond

“…Using a gold photocathode driven by laser pulses and a streak camera for time-resolved detection, they revealed the correlation of local carrier dynamics in GaAs nanostructures with the surface morphology. In parallel with the developments in the laser-driven cathode, advances in ultrafast beam blankers promise a flexible and easily integrated method to generate electron pulses with a broad range of pulse widths and repetition rates, while keeping a good spatial resolution with a typical duration of tens of picoseconds. , In recent years, time-resolved CL with pulsed electron beams in SEM has enabled the dynamic study of exciton diffusion, ,, carrier relaxation, ,− and excited states of DNA . Currently, time-resolved CL studies performed in TEM have also been reported, , making progress toward elucidating structure–function relations at atomic scales.…”

“…Beyond panchromatic and hyperspectral 2D CL imaging, which has been the dominant historical application, more degrees of freedom in CL emission are exploited in nanophotonic research. Polarization-resolved CL has been introduced such that the degree of linear and circular polarization of the emitted CL light can be determined. − In addition, the momentum distribution of CL emission, i.e., the direction in which light is emitted, can be recorded by angle-resolved CL. − By combining these two techniques and performing polarization studies in momentum space, it is viable to fully define the polarization state of CL emission and explore polarization-dependent phenomena with great spatial and angular resolution. − With the advent of ultrafast electron microscopy, time-resolved CL has emerged as a powerful technique for fundamental studies of the dynamics of electron–matter interaction with simultaneous nm–ps–meV space–time–energy resolution, for which it is appealing in the research of exciton/carrier dynamics − and single-photon sources. − On the basis of the synchronized laser and ultrafast electron pulse excitation, the prototypical pump–probe CL spectroscopy has also been reported, enabling analyses of the electron–light–material energy-transfer process. Furthermore, conventional space-resolved CL imaging and spectroscopy have made impressive advances in cathodoluminescence excitation spectroscopy, delocalized imaging, 3D tomography, , and nanothermometry for semiconductors …”