(a) Schematic of nanopyramid heterostructure design. (b) Strain energy density within an layer on (i) a pyramid-capped GaN seed nanorod of 142 nm height, (ii) a GaN nanowire of 142 nm height, and (iii) a semi-infinite GaN thin film. The width of both the GaN seed nanorod and GaN nanowire is 55 nm. The strain energy density profiles for the seed nanorod and the nanowire were simulated through 3D finite element analysis (Ref. 21 ), while that for the semi-infinite thin film was calculated analytically by assuming pseudomorphic growth (Ref. 22 ). The black lines in the cross-section schematics illustrate the length over which the strain energy density is plotted. The shaded region of the plot corresponds to the layer, and demonstrates that the strain energy density within the on the pyramid-capped GaN seed nanorod is the lowest.
(a) Plan-view FESEM of a GaN seed nanorod array selectively grown through a dielectric template. (b) Bright-field STEM cross-section of two typical GaN seed nanorods (grown at ) with faceted pyramidal caps protruding above the dielectric template.
Bright-field STEM images of GaN seed nanorods with (a) a truncated pyramidal cap grown at and (b) a sharp pyramidal cap grown at .
STEM cross-sections of nanopyramid heterostructures indicating the location of (In,Ga)N grown on both sharp (a) and truncated (b) seed nanorods. The truncated nanorod (b) was additionally thinned by Ar-ion milling, resulting in both the improved clarity over (a), and the resputtered copper particles (the bright dots). (c) X-ray spectral images of four complete nanopyramid heterostructures grown on truncated seed nanorods. The Ga–K signal is plotted in green, the In–L,K in red, and Si–K in blue, such that the yellow in the plot represents the combination of the Ga and In signals. (d) Temperature-dependent of a heterostructure nanopyramid array grown on truncated seed nanorods using Voigt profiles (gray lines) for finding the peak energy of emission at each temperature. Data is normalized and shifted for clarity. Oscillations in the intensity originate from the presence of a Fabry–Perot effect in the sapphire/GaN/air cavity.
Cross-sectional TEM image illustrating planar defects, both stacking faults and zincblende inclusions of thicknesses greater than 5 nm, within a GaN cladding layer grown at from a nanopyramid heterostructure. 62% of nanopyramid heterostructures were free of extended defects, 38% contained clear planar faults, and 9% of the total included distinguishable zincblende inclusions.
Plan-view bright-field STEM images of nanopyramid arrays obtained under similar imaging conditions (a) with (In,Ga)N quantum wells and (b) without (In,Ga)N quantum wells, illustrating the “coffee-bean”-like Ashby–Brown contrast arising from strain related to the (In,Ga)N quantum well. Images were acquired with a slight tilt for . The dashed line in the schematic of (b) illustrates the border between the high temperature GaN and the low temperature GaN.
(a) Schematic of complete LED heterostructure. Black lines in schematic illustrate threading dislocations. (b) Plan-view FESEM after p-GaN growth that results in a partially coalesced film. (c) Bright-field TEM image illustrating three threading dislocations arising from merging nanopyramids. Some of the seed nanorods in this section are almost entirely milled away in the process of making a cross-section thin enough for TEM. Fringes in image result from small changes in the sample thickness. (d) Room temperature spectrum collected from a nanopyramid LED grown on sharp seed nanorods. (e) Bright-field TEM cross-section illustrating the termination of three planar defects (termination points indicated by arrows) during the merging of nanopyramids. The dashed lines in (c) and (e) indicate the locations of (In,Ga)N active regions.
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