Abstract
We report on the study and characterization of nanoclusters-related recombination centers within quantum-disks-in-nanowires heterostructure by utilizing microphotoluminescence (μ-PL) and cathodoluminescence scanning transmission electron microscopy (CL-STEM). μ-PL measurement shows that the nanoclusters-related recombination center exhibits different temperature-dependent characteristics compared with the surrounding InGaN quantum-disks-related recombination center. CL-STEM measurements reveal that these recombination centers mainly arise from irregularities within the quantum disks, with a strong, spatially localized emission when measured at low temperature. The spectra obtained from both CL-STEM and μ-PL correlate well with each other. Our work sheds light on the optical and structural properties of simultaneously coexisting recombination centers within nanowires heterostructures.
Highlights
Cathodoluminescence scanning electron microscopy has previously been utilized to spatially characterize the emission characteristics of nanowires structures, the structural characterization is limited to only the surface of the nanowires, and the resolution is not adequate to probe individual nanoscale-sized recombination centers embedded inside the active region of the nanowires.[31,32]
The tapering is caused by the reduction of growth temperature during InGaN Qdisk growth to promote In incorporation, resulting in lower adatom diffusion length and preferential lateral growth
From the μ-PL measurements, we identify the existence of two recombination centers within the active region of the nanowires
Summary
The molecular beam epitaxy (MBE) grown InGaN/GaN-based nanowires heterostructure is attractive as it can be grown spontaneously on a highly mismatched surface, such as silicon,[1,2,3,4,5] oxides,[6,7,8,9] and metal,[10,11,12,13] without threading dislocations.[14,15] In the planar III-nitride material system, random alloy fluctuations and phase segregation of In during the growth of the InGaN layer result in the formation of In-rich clusters.[16,17,18] These clusters result in a separate recombination center with distinct behavior compared with the typical surrounding InGaN matrix. Accurate study of localization centers within nanowires structures is further complicated due to unique nanowires characteristics, such as surface-state-related Fermi-level pinning[25,26] and nonuniform strain distribution.[27,28,29,30] cathodoluminescence scanning electron microscopy has previously been utilized to spatially characterize the emission characteristics of nanowires structures, the structural characterization is limited to only the surface of the nanowires, and the resolution is not adequate to probe individual nanoscale-sized recombination centers embedded inside the active region of the nanowires.[31,32] CL-STEM on the other hand is suitable for probing the nanoscale optical properties of nanowires because of the small interaction volume and high resolution resulting from the high acceleration voltage used.[33,34,35] Through utilizing CL-STEM, it is possible to simultaneously retrieve both the optical and structural features of the nanowires while at the same time utilizing a high-angle annular dark field (HAADF), providing a thorough understanding of how structural features and luminescence properties of nanowires affect each other This capability has been demonstrated to investigate the nanoscale optical properties of nanowire heterostructures,[22,36] with features as small as nanometer-sized clusters.[37]. Our work gives insight on the optical and structural properties of simultaneously coexisting nanoscale luminescence sites within nanowires heterostructures
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