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![(a) TEM images of the nanostripe MQW structure taken with g ¼ [1–100] (b) enlarged TEM image of the region enclosed by the dashed lines in (a). (c) TEM lattice image of the bottom part of the nanostripe MQW structures (d) Filtered HRTEM image of the stacking fault in the region enclosed by the dashed lines in (c). Blue, green, and red dots represent the A, B, and C lattices, respectively. The blue, green, and red dotted lines represent the locations where A, B, and C lattices align to. The yellow dotted line shows the location of the stacking fault. It is clear that the lattices below the stacking fault align with A and B lattices while the lattices above the stacking fault align with B and C lattices, representing a change of stacking sequence from .ABAB. to .BCBC.](profile/Yoshitake-Nakajima/publication/307866034/figure/fig3/AS:406095819493378@1473832196377/a-TEM-images-of-the-nanostripe-MQW-structure-taken-with-g-14-1-100-b-enlarged-TEM.png)
(a) TEM images of the nanostripe MQW structure taken with g ¼ [1–100] (b) enlarged TEM image of the region enclosed by the dashed lines in (a). (c) TEM lattice image of the bottom part of the nanostripe MQW structures (d) Filtered HRTEM image of the stacking fault in the region enclosed by the dashed lines in (c). Blue, green, and red dots represent the A, B, and C lattices, respectively. The blue, green, and red dotted lines represent the locations where A, B, and C lattices align to. The yellow dotted line shows the location of the stacking fault. It is clear that the lattices below the stacking fault align with A and B lattices while the lattices above the stacking fault align with B and C lattices, representing a change of stacking sequence from .ABAB. to .BCBC.
Source publication
Yellow and green emitting multiple quantum well structures are grown on nanostripe templates with {10-11} facets. SEM and cathodoluminescence measurements show a correlation between rough surface morphology near the bottom of the stripes and non-radiative recombination centers. Transmission electron microscopy (TEM) analysis shows that these surfac...
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... respectively, where R is the displacement vector. The invisibility criterion for the defects is gÁR equals to zero or an integer. The defects were observed with g ¼ [1-100] but disappeared in g ¼ [0002], indicating that the stacking faults are either type I 1 or I 2 . Figure 3(d) shows a filtered HRTEM image near the vicinity of one of the stacking faults shown in the dashed rectangle in Figure 3(c). The stacking sequence is …ABABABCBCBC… which are the characteristics of type I 1 stacking faults. The generation of type I 1 stacking faults is also observed in planar green emitting MQW and LED structures grown on the {10-11}, {20-2-1}, and {10-10} planes. [24][25][26] They are caused by the removal of one half basal plane which releases compressive strain in the QW region. 26 The stacking sequence of the type I 2 stacking fault is …ABABABCACACA… where two consecutive faults occur. The generation of this type of stacking fault requires more energy than the type I 1 and therefore are less commonly observed. 26 Nevertheless, Tischer et al. have observed type I 2 stacking faults in the micron-scale GaN stripe structures with the {10-11} facets. 27,28 The generation of these stacking faults is attributed to the thermal stress during the cool down after the growth. 28,29 By comparison, the type I stacking faults observed in this study are generated to relieve the excessive compressive strain in the InGaN QW. Significantly fewer stacking faults are observed for nanostripe MQW structures with thinner QW thicknesses or reduced In content in the QW (not shown), further confirming that stacking fault generation is a result of strain ...
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... respectively, where R is the displacement vector. The invisibility criterion for the defects is gÁR equals to zero or an integer. The defects were observed with g ¼ [1-100] but disappeared in g ¼ [0002], indicating that the stacking faults are either type I 1 or I 2 . Figure 3(d) shows a filtered HRTEM image near the vicinity of one of the stacking faults shown in the dashed rectangle in Figure 3(c). The stacking sequence is …ABABABCBCBC… which are the characteristics of type I 1 stacking faults. The generation of type I 1 stacking faults is also observed in planar green emitting MQW and LED structures grown on the {10-11}, {20-2-1}, and {10-10} planes. [24][25][26] They are caused by the removal of one half basal plane which releases compressive strain in the QW region. 26 The stacking sequence of the type I 2 stacking fault is …ABABABCACACA… where two consecutive faults occur. The generation of this type of stacking fault requires more energy than the type I 1 and therefore are less commonly observed. 26 Nevertheless, Tischer et al. have observed type I 2 stacking faults in the micron-scale GaN stripe structures with the {10-11} facets. 27,28 The generation of these stacking faults is attributed to the thermal stress during the cool down after the growth. 28,29 By comparison, the type I stacking faults observed in this study are generated to relieve the excessive compressive strain in the InGaN QW. Significantly fewer stacking faults are observed for nanostripe MQW structures with thinner QW thicknesses or reduced In content in the QW (not shown), further confirming that stacking fault generation is a result of strain ...
Citations
... Finally, a number of works have demonstrated monolithic color-tunable LEDs based on nanostructures [16][17][18] that could produce CCT tunable white LEDs. However, the non-planar structures require additional lithography and patterning steps [18] and suffer from growth non-uniformity [19,20]. In general, there is a trade-off between the functionality of the device and its manufacturing cost in these approaches. ...
A color-temperature tunable white light-emitting diode (LED) based on a newly developed monolithic color-tunable LED structure was demonstrated. The color-tunable LED structure consists of three different sets of quantum wells separated by intermediate carrier blocking layers that can independently emit visible lights from 460 to 650 nm under different injection currents. To generate white light, the color-tunable LED is operated under pulsed conditions with each pulse consisting of multiple steps of different current amplitudes and widths emitting different colors. The combined spectrum of different colors is aimed to mimic that of the blackbody radiation light source. The pulse rate is designed to be higher than the human eye response rate, so the human eye will not discern the emission of successive colors but a singular emission of white light. Results of a two-step pulse design show this method is able to generate white light from 2700 K – 6500 K. Moreover, their color coordinates fall within the 4-step MacAdam ellipses about the Planckian locus while achieving the Color Rendering Index (CRI) in the 80-90 range. Finally, simulations show improvement of CRI into the 90-100 range is possible with further optimization to the color-tunable LED spectral emission and use of three-step pulses.
... There are many advantages that core-shell nanostructures may offer in addition to access to polarization-free material in the nonpolar and semipolar planes. Nanostructures, including triangular nanostripes [252][253][254][255] (Figure 18a,b), nanowalls [256,257] (Figure 18c), and nanowires [258][259][260][261] (Figure 18d), provide nonpolar and semipolar planes without the need for costly substrate preparations. On the other hand, nanostructure approaches typically require a crystal re-growth step. ...
The performance of III‐nitride devices is degraded by polarization in a wurtzite crystal structure. Nonpolar and semipolar III‐nitrides have been extensively studied since the early 2000s as a solution to the polarization‐related issues. Removal of polarization is expected to improve the radiative efficiency, optical gain, charge transport, and potentially offer solutions to the challenging problems in III‐nitride light‐emitting diodes (LEDs) known as efficiency droop and the green gap. In addition, use of nonpolar and semipolar orientations is also predicted to offer polarization pinning in some devices due to anisotropic in‐plane strain. Despite the many potential advantages over c‐plane, nonpolar and semipolar optoelectronic devices have not successfully replaced conventional c‐plane devices in the commercial sector. Here, nonpolar and semipolar III‐nitrides are reviewed after more than a decade of development. The successes and challenges of nonpolar and semipolar orientations for applications such as LEDs, laser diodes, superluminescent diodes, and vertical‐cavity surface emitting lasers are discussed. New potential avenues for nonpolar and semipolar III‐nitrides are also highlighted, including visible‐light communication and power electronics. The availability of low‐cost, high‐crystal‐quality substrates with nonpolar or semipolar orientation is discussed and alternative approaches for realizing these orientations are presented, including selective‐area bottom‐up nanostructures.
... It is suggested that at submicrometer scales below the diffusion length of the precursors, variation of In due to mass transport effects is minimized compared to micrometer and larger sized stripes. However, more detailed followon TEM analysis of similar structures shows thickness variations from tip to base as well as rough morphologies and the presence of stacking faults at the base, caused by enhanced surface diffusion of precursors and bending strain from bonding of the overgrown region to the mask [184]. In summary, semipolar nanostructures are promising for green-yellow LEDs, but challenges remain in overcoming nonuniformities in QW thickness and In content due to their 3D morphology, and defects near the bottom of such structures may occur due to interactions with the mask material used in bottom-up SAG approaches. ...
Nonpolar and semipolar III-nitride-based blue and green light-emitting diodes (LEDs) have been extensively investigated as potential replacements for current polar c -plane LEDs. High-power and low-efficiency-droop blue LEDs have been demonstrated on nonpolar and semipolar planes III-nitride due to the advantages of eliminated or reduced polarization-related electric field and homoepitaxial growth. Semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) LEDs have contributed to bridging “green gap” (low efficiency in green spectral region) by incorporating high indium compositions, reducing polarization effects, and suppressing defects. Other properties, such as low thermal droop, narrow spectral linewidth, small wavelength shift, and polarized emission, have also been reported for nonpolar and semipolar LEDs. In this paper we review the theoretical background, device performance, material properties, and physical mechanisms for nonpolar and semipolar III-nitride semiconductors and associated blue and green LEDs. The latest progress on topics including efficiency droop, thermal droop, green-gap, and three-dimensional nanostructures is detailed. Future challenges, potential solutions, and applications will also be covered.
In this work, we demonstrate high-performance electrically injected GaN/InGaN core-shell nanowire-based LEDs grown using selective-area epitaxy and characterize their electro-optical properties. To assess the quality of the quantum wells, we measure the internal quantum efficiency (IQE) using conventional low temperature/room temperature integrated photoluminescence. The quantum wells show a peak IQE of 62%, which is among the highest reported values for nanostructure-based LEDs. Time-resolved photoluminescence (TRPL) is also used to study the carrier dynamics and response times of the LEDs. TRPL measurements yield carrier lifetimes in the range of 1-2 ns at high excitation powers. To examine the electrical performance of the LEDs, current density-voltage (J-V) and light-current density-voltage (L-J-V) characteristics are measured. We also estimate the peak external quantum efficiency (EQE) to be 8.3% from a single side of the chip with no packaging. The LEDs have a turn-on voltage of 2.9 V and low series resistance. Based on FDTD simulations, the LEDs exhibit a relatively directional far-field emission pattern in the range of [Formula: see text]15°. This work demonstrates that it is feasible for electrically injected nanowire-based LEDs to achieve the performance levels needed for a variety of optical device applications.
