RESEARCH REVIEW
Green emission rockets with nanorod array
Switching from a planar sample to a nanorod array increases efficiency by a factor of 88
ENGINEERS at the University of Sheffield, UK, have constructed a novel nanorod array that delivers a tremendous enhancement to emission efficiency via a “coherent” nanocavity effect.
Introducing this nanoscale structure, which increases internal quantum efficiency by a factor of 88, promises to improve the performance of green LEDs.
Today, their performance falls a long way short of that of their blue-emitting cousins, due to the Quantum Confined Stark Effect. At the heart of the green LED is an indium-rich InGaN quantum well, which is surrounded by a GaN barrier with a significant lattice mismatch. This difference in atomic spacing creates strain in the heterostructure – and that introduces a strong internal electric field,
which pulls electrons and holes apart and drives down LED efficiency.
“Our previous work has confirmed that nanorod structures can lead to significant strain relaxation, reducing the QCSE and thus improving optical performance,” says Tao Wang, who explains that the recent, big breakthrough is the hike in the internal quantum efficiency through the introduction of a coherent nanaocavity effect in the green spectral region.
To realise this, the team carefully selected the diameter of the naorods and the geometry of the array. That’s because if these rods, which have a diameter below 300 nm, are isolated or arranged in an array that does not give rise to a coherent nanaocavity effect, the only cavity modes that occur are deep within the ultraviolet.
Arrays of nanorods were formed by taking a multiple quantum well structure with five 2 nm-thick InGaN wells sandwiched between 9 nm-thick InGaN barriers, and depositing silica nanospheres on the surface (see figure). Selective etching of this structure created an array of nanorods with a height of 350 nm.
The team used this approach to fabricate a range of structures. In all cases, separation of the nanodisks were 274 nm, but different nanosphere diameters were employed: 270 nm, 250 nm, 235 nm, 205 nm, 160 nm and 145 nm.
Excitation of these structures with a 375 nm laser produced photoluminescence, which was most intense for the sample with 205 nm-diameter nanorods. Measurements at 12K determined an enhancement factor – defined as the ratio of internal quantum efficiency in the nanodisk sample to that in the as-grown sample – of 88. Structures were also scrutinised by time-resolved photoluminescence. That with 205 nm-diameter rods had the shortest emission lifetime and the highest spontaneous emission rate, confirming the presence of a nanocavity effect in this sample.
To aid the launch of more efficient commercial green LEDs, the team will have to devise a way to put a high- quality p-type contact onto the surface of the nanostructures. However, they will not also need to come up with a new approach for making nanorod arrays, according to Wang, so long as a high degree of uniformity is possible across entire wafers. “Nano-imprinting and electron-beam lithography could be alternative options, but both are too expensive.”
Manufacturers of green LEDs may crave even higher enhancement factors, but that may not be possible − and it would depend on the quality of the epiwafer.
“If the optical performance of the starting wafer would have been very good, the enhancement factor would not be so high,” says Wang. “Therefore, the technique would be particularly useful for the fabrication of green emitters, as the optical performance of green emitters is far from satisfactory.”
Nanorod arrays are formed by depositing a silicon dioxide layer, spin-coating nanospheres, etching the sample, and then removing the spheres with hydrofluoric acid.
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www.compoundsemiconductor.net June 2014
T. Kim et. al. Appl. Phys. Lett. 104 161108 (2014)
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