(Invited) High Power Phosphor-Free InGaN/GaN/AlGaN Core-Shell Nanowire White Light Emitting Diodes on Si Substrates
Authors: Mi Zetian , Nguyen Hieu , Djavid Mehrdad , Zhang Shaofei , Connie Ashfiqua , Sadaf Sharif , Wang Qi , Zhao Songrui , Shih Ishiang 
Published: February 2020

Recently, III-nitride nanowire structures have been intensively investigated for applications in solid state lighting and full-color displays. Compared to the conventional GaN based planar LEDs, III-nitride nanowires offer distinct advantages including greatly reduced dislocation densities, polarization fields, and quantum-confined Stark effect, due to the effective lateral stress relaxation, thereby promising superior performance LEDs for lighting and display applications. However, the performance of these devices suffers severely from nonradiative recombination due to the large densities of surface states and defects. As a consequence, currently reported nanowire LEDs still show relatively low output power, which is often in the range of μW, or smaller. It is expected that, by reducing nonradiative surface recombination, the device performance including the light output power and the quantum efficiency can be significantly enhanced. Here, we report that by passivating the InGaN/GaN dot-in-a-wire structures with an in-situgrown AlGaN shell, a record high output power of ~ 1.5mW can be measured, which is more than 100 times stronger than that of InGaN/GaN nanowire white LEDs without using an AlGaN shell.

Vertically aligned InGaN/GaN dot-in-a-wire LED heterostructures were grown on Si(111) substrates by radio frequency plasma-assisted molecular beam epitaxy (MBE) under nitrogen-rich conditions. Shown in figure 1(a), the dot-in-a-wire LED structure consists of ~ 0.4 µm GaN:Si, ten vertically coupled InGaN/GaN dots, and ~ 0.2 µm GaN:Mg segments. The emission wavelengths can be varied across the entire visible spectral range by controlling the indium compositions and sizes of the dots. To reduce/prevent electron overflow, a ~10nm p-doped AlGaN electron blocking layer was incorporated between the InGaN/GaN quantum dot active region and the GaN:Mg segment. To effectively reduce nonradiative surface recombination, an AlGaN layer of ~ 80nm was grown for the formation a shell surrounding the InGaN/GaN core region, due to the diffusion-controlled growth process. The thickness and Al content of the AlGaN shell can be controlled by the growth duration and Al/Ga flux ratio, respectively. Figure 1(b) is a 45 degree-titled scanning electron microscopy (SEM) image for typical InGaN/GaN/AlGaN dot-in-a-wire core-shell LED heterostructures grown on Si (111) substrate, showing the high density and high degree of size uniformity. Illustrated in figure 2, the energy dispersive x-ray (EDX) spectrometry elemental mapping image of InGaN/GaN/AlGaN active region shows clear evidence of AlGaN layer passivating the InGaN/GaN core region, forming a unique core-shell nanowire heterostructure. Moreover, dislocations or stacking faults were not observed in the InGaN/GaN quantum dot active region and in the AlGaN shell.

Illustrated in figure 3(a), the InGaN/GaN/AlGaN core-shell LED structure shows significantly improved photoluminescence (PL) intensity compared to that of non-core-shell LED sample. The significantly improved light intensity is attributed to the greatly reduced nonradiative surface recombination and the effective lateral confinement offered by the large bandgap AlGaN shell. Shown in figure 3(b), under the same current injection conditions, an output power of ~1.5mW was measured for the core-shell LEDs, which is more than two orders of magnitude larger than that of nanowire LEDs without using AlGaN shell, which is attributed to the increased carrier injection efficiency offered by the core-shell structure. Moreover, truly white emission with stable performance, shown in figure 3(c), was achieved, with the derived x and y values in the ranges of ~0.35-0.36 and 0.39-0.40 in the CIE diagram, respectively. Shown in the inset is an optical image of the phosphor-free core-shell nanowire white LED.

In conclusion, the unique core-shell LED structures can lead to significantly reduced surface recombination and enhanced carrier injection efficiency. A record high power of 1.5mW was further measured for InGaN/GaN/AlGaN core-shell phosphor-free white LEDs, showing the tremendous potential for future solid-state lighting applications.