Absence of Quantum-Confined Stark Effect in GaN Quantum Disks Embedded in (Al,Ga)N Nanowires Grown by Molecular Beam Epitaxy

Abstract

Several of the key issues of planar (Al,Ga)N-based deep-ultraviolet light emitting diodes could potentially be overcome by utilizing nanowire heterostructures, exhibiting high structural perfection and improved light extraction. Here, we study the spontaneous emission of GaN/(Al,Ga)N nanowire ensembles grown on Si(111) by plasmaassisted molecular beam epitaxy. The nanowires contain single GaN quantum disks embedded in long (Al,Ga)N nanowire segments essential for efficient light extraction. These quantum disks are found to exhibit intense emission at unexpectedly high energies, namely, significantly above the GaN bandgap, and almost independent of the disk thickness. An in-depth investigation of the ac-tual structure and composition of the nanowires reveals a spontaneously formed Al gradient both along and across the nanowire, resulting in a complex core/shell structure with an Al deficient core and an Al rich shell with continuously varying Al content along the entire length of the (Al,Ga)N segment. This compositional change along the nanowire growth axis induces a polarization doping of the shell that results in a degenerate electron gas in the disk, thus screening the built-in electric fields. The high carrier density not only results in the unexpectedly high transition energies, but also in radiative lifetimes depending only weakly on temperature, leading to a comparatively high internal quantum efficiency of the GaN quantum disks up to room temperature. arXiv:1905.04090v1 [cond-mat.mes-hall]

InGaN micro-light-emitting diodes monolithically grown on Si: achieving ultra-stable operation through polarization and strain engineering

“…As shown in the blue curve in Fig. 3c , the electron densities within the InGaN QWs range from 3 × 10 19 to 6 × 10 19 cm −3 in the presence of induced free electrons, well above the reported Mott density of nitride QWs 50 , 53 . In contrast, the electron density drops below the degenerate doping when a uniform background doping of ~1 × 10 16 cm −3 is adopted throughout the active region, further confirming that the absence of QCSE is due to the relocation of induced free electrons.…”

“…The effect of Al incorporation on the carrier radiative recombination process can be related to the Al distribution in the active region. III-nitrides are polar materials, and the grading of (Al, Ga)N composition along the c -axis induces polarization charges in the bulk nanowire, which further results in accumulated free charge carriers with the opposite sign 50 – 52 . In this study, the Al content features a negative gradient along the direction, which leads to a distribution of positive fixed charge in the shell region.…”

Polariton Lasing of Multiple-Layered InGaN/GaN/AlGaN Axial Heterostructure Nanorods for Tunable Nanolasers

“…The peak position in the heterostructure is similar to that in the homostructure of GaN (inset of Figure d), suggesting that the diffusion of Al and In into the GaN layer is not significant during the fabrication processes of the heterostructure. However, the excitonic emission is slightly red-shifted compared to the band gap energy ( E g ) of 3.4 eV, , which is attributed to the strain-induced polarization along the long axis of nanorods, , although the piezoelectric field of epitaxial films could be much reduced due to the strain release in the nanorod. Besides, during the fabrication processes, shallow defect states would be formed, which might also contribute to the slight shift and broadening of emission as well as the decrease in emission efficiency.…”

“…The shoulder in the short wavelength region (∼355 nm) suggests the excitonic emission of AlGaN, which is more clearly discernable by the decomposition of the spectrum because the emission energy agrees with E g of AlGaN (3.5 eV) at a 5% ratio of Al (Al 0.05 Ga 0.95 N). , Notably, the emission intensity of AlGaN is low (∼15%), considering the volume ratio of AlGaN in the heterostructure (∼50%). In addition, the AlGaN layer is less bright than the GaN and InGaN layers in an optical image (Figure S4), suggesting the carrier transfer from AlGaN to neighboring layers due to the difference in band potentials. − Nevertheless, the emission intensity of AlGaN is not negligible, implying that some of the excited carriers in AlGaN are not transferred but are involved in the radiative recombination due to the large volume of AlGaN compared to the diffusion length of carriers. , In contrast, the emission intensity of InGaN (∼20%) is enhanced by the carrier transfer, , despite the low volume ratio of InGaN (<10%) in the heterostructure.…”

“…The stimulated emission process of AlGaN is barely observed. The quantum yield of AlGaN is known to be low compared to those of GaN and InGaN. − Besides, the carrier transfer to neighboring layers reduces the carrier density in AlGaN, substantially lowering the optical gain due to the nonlinear dependence of the optical gain on the carrier density. Moreover, the guided emission of AlGaN could be absorbed in GaN and InGaN layers during roundtrips (Figure S4) , because the emission energy of AlGaN is larger than E g of GaN and InGaN.…”