Dodecagonal-faced wurtzite nanowires to fill the green gap in LEDs
In a paper titled “Efficient Green Emission from Wurtzite AlxIn1–xP Nanowires” published in ACS’ Nano Letters, the researchers share their findings as they grew WZ AlxIn1–xP nanowires with high crystal quality on a nanoimprint-patterned (111)A InP substrate. The catalyst-free growth of the nanowires was performed through selective area metalorganic vapor phase epitaxy (SA-MOVPE), using a 50nm silicon nitride mask with a square array of 150nm diameter holes at a 512nm pitch. The researchers found that carefully controlling the V/III material ratio during SA-MOVPE growth was key to achieve high crystal purity and avoid the parasitic growth on the SiNx layer.
What’s more, they studied the morphologies and photoluminescence spectrum of different nanowire growth compositions, yielding differently faceted geometries from hexagonal for InP wurtzite nanowires to dodecagonal for Al0.25In0.75P nanowires. Doing so, they demonstrated a tunable direct band gap room-temperature light emission from the infrared range at 875nm (1.42 eV) to the green range at 555nm (2.23 eV).
a WZ Al0.25In0.75P nanowire (average composition).
Scale bars is 200 nm.
Further study of the nanowire cross section revealed a complex structure spontaneously formed during axial growth, with Al-rich segregations and a 6-fold symmetry which radially connect to an Al-rich AlxIn1–xP inner shell. The authors explained these segregation patterns by an interplay of the relative diffusion rate of the different adatoms on the nanowire surface, leading to the segregation of the most slowly diffusing species (here aluminium) at all edges between two facets.
A joint analysis of the photoluminescence distribution and material structure also revealed that from the two measured peak emissions from the Al0.25In0.75P nanowires, the 615 nm peak was emitted by the core, while the peak around 690nm was emitted by the shell, the Al-rich segregations somehow acting as a barrier for the carriers due to their higher band gap. Next, the researchers aim to improve the radiative efficiency in the green range, possibly by improving the compositional homogeneity of the nanowires and passivating their surfaces.
University of Technology – www.tue.nl
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