Photoluminescence studies of carrier dynamics in AlGaN epilayers with different degrees of carrier localization and densities of nonradiative recombination centers show that the prevailing droop mechanism in AlGaN epilayers with strong carrier localization and comparatively high density of nonradiative recombination centers is enhanced nonradiative recombination due to the carrier delocalization at elevated carrier density. The photoluminescence was investigated under quasi-steady-state excitation in the temperature range from 8 to 300 K. The results proved that the onset of this droop effect is below the threshold for the droop due to high-density effects in the epilayers, such as carrier heating, phase space filling, nonradiative Auger recombination, and stimulated emission.

Remote epitaxy via graphene has recently attracted significant attention, since it provides the possibility to lift-off the grown epitaxial layer, reuse the substrate, and produce flexible devices. However, extensive research is still necessary to fully understand the III-nitride formation on the van der Waals surface of a two-dimensional material and utilize remote epitaxy to its full potential. In this work, the growth of a GaN epilayer using a GaN/sapphire template covered with monolayer graphene is presented. Metalorganic vapor phase epitaxy is chosen to fabricate both the template and the nitride epilayer on top as a cost-effective approach toward GaN homoepitaxy. One-step and multi-step growth temperature protocols are demonstrated while paying particular attention to the graphene interface. GaN seed formation on graphene is analyzed to identify remote epitaxy. Crystalline quality improvement of the epilayer by adjusting the growth parameters is further discussed to provide useful insights into GaN growth on a GaN/sapphire template via monolayer graphene.

A set of ${\mathrm{Al}}_{0.35}{\mathrm{Ga}}_{0.65}\mathrm{N}∕{\mathrm{Al}}_{0.49}{\mathrm{Ga}}_{0.51}\mathrm{N}$ multiple quantum wells (MQWs) with fixed barrier width and well widths varying from $1.65\text{to}5.0\mathrm{nm}$ has been grown by metal-organic chemical vapor deposition. Carrier dynamics in the MQWs were studied using time-resolved photoluminescence (PL) spectroscopy and light-induced transient grating (four wave mixing) technique. The authors observed that the lifetime of nonequilibrium carriers (excitons) increases with decreasing well width and interpreted the effect by stronger localization preventing their migration to nonradiative recombination centers. Meanwhile the radiative decay time is also influenced by screening of the built-in electric field, which spatially separates the electrons and holes. It is shown that this effect affects the initial part of PL intensity decay after pulsed excitation. It becomes more pronounced with increase in the initial carrier density but saturates when the carrier density is high enough to completely screen the built-in electric field. The screening effect on PL decay is stronger in wider quantum wells.

Growth of BGaN epitaxial layers by metalorganic chemical vapor deposition (MOCVD) using triethylboron (TEB) as a boron source was studied on 6H-SiC substrate and on GaN and AlN templates on sapphire. X-ray diffraction, atomic force microscopy and photoluminescence spectroscopy were exploited to characterize the structural quality, surface morphology, luminescence efficiency, and boron content. Silicon carbide was shown to be slightly superior to AlN/sapphire and considerably better than GaN/sapphire as the most favorable substrate to incorporate a possibly higher boron content. Increasing TEB flow rate at correspondingly optimized growth temperature and V/III ratio enabled us to achieve the boron content of up to 5.5%, though at the expense of structural quality. We showed that the band gap bowing parameter is similar for the epilayers deposited on all the three templates/substrates under study and is approximately equal to 4 eV, substantially lower than reported before.

The influence of carrier localization on photoluminescence efficiency droop and stimulated emission is studied in AlGaN multiple quantum wells with different strength of carrier localization. We observe that carrier delocalization at low temperatures predominantly enhances the nonradiative recombination and causes the droop, while the main effect of the delocalization at elevated temperatures is enhancement of PL efficiency due to increasing contribution of bimolecular recombination of free carriers. When the carrier thermal energy exceeds the dispersion of the potential fluctuations causing the carrier localization, the droop is caused by stimulated carrier recombination.