Abstract
We explore the effect of the subwell centers and related carrier dynamics mechanisms in dislocation-free DUV AlGaN/AlGaN multiple quantum wells (MQWs) homoepitaxially grown on an AlN substrate. Cross-sectional imaging and energy-dispersive X-ray compositional analyses using scanning transmission electron microscopy (STEM) reveal epitaxial layers of very high crystalline quality, as well as ultrathin Al-rich subquantum barrier and subwell layers at the interface between the wells and the barriers. Carrier dynamic analyses studied by power- and temperature-dependent time-resolved and time-integrated photoluminescence (PL) and PL excitation measurements, as well as numerical simulations, reveal the carrier repopulation mechanisms between the MQWs and subwell sites. This advanced analysis shows that the subwell/sub-barrier structure results in additional exciton localization centers, enhancing the internal quantum efficiency via staggered carrier repopulation into the MQWs to reach a maximum of ∼83% internal quantum efficiency, which remains high at high injected carrier densities in the droop region. Both experimental and numerical simulation results show that the slight efficiency droop can be due to Auger recombination, counteracted by a simultaneous increase in radiative recombination processes at high power density, demonstrating the role of the subwells/sub-barriers in efficiency enhancement.
Highlights
We explore the effect of the subwell centers and related carrier dynamics mechanisms in dislocation-free Deep ultraviolet (DUV) AlGaN/AlGaN multiple quantum wells (MQWs) homoepitaxially grown on an AlN substrate
Deep ultraviolet (DUV) light-emitting devices have a variety of useful applications, including germicidal irradiation, phototherapy, water, air, and food sterilization, charge management in space-based sensors and photocatalysis, among others.[1−4] Hg vapor-based lamps have traditionally served as UV light source for these applications, their effectiveness is severely limited due to bulkiness, slow response time, fragility, temperature sensitivity, high power consumption, and potential environmental hazards.[5−8] As such, AlGaN-based light-emitting diodes (LEDs) are considered a viable alternative by both researchers and industry practitioners, as their use eliminates the aforementioned drawbacks, while offering the advantage of wide spectral tunability for a multitude of applications.[9]
A 400 nm thick AlN buffer layer was initially regrown on a 1 in. thick (0001) AlN substrate at 1250 °C, followed by a 20 × Al0.4Ga0.6N/AlN Strainedlayer superlattices (SLSs) layer to enhance the quality of the active layer. (The chemical composition of SLS structure was estimated by energy dispersive X-ray spectroscopy (EDX) that are attached to high-resolution transmission electron microscopy (HR-TEM), as shown in Figure S1 in Supporting Information.) Subsequently, a 1 μm thick n-AlGaN layer was grown, followed by a nominal fiveperiod Al0.4Ga0.6N/Al0.5Ga0.5N (3 nm/12 nm) multiple quantum well (MQW) active layer, which was capped by a 30 nm thick p-Al0.65Ga0.35N electron blocking layer (EBL)
Summary
This broadening indicates that, after settling at the main MQW recombination sites, thermal delocalization causes a proportion of the carriers to migrate to other recombination centers in the vicinity of the localized recombination site at 4.55 eV. Auger recombination remains the most plausible loss mechanism in this TD-free MQW structure at high power densities.[62] it is worth noting that, as droop saturation occurred at a relatively high IQE (∼60%), the radiative recombination processes effectively counteracted the droop mechanism This counteraction process may have been aided by the concurrent migration of carriers to the highly confined low energy MQW states (∼4.55 eV) at high power densities.
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