  Bottom up nanowires are attractive for realizing semiconductor devices with
extreme heterostructures because strain relaxation through the nanowire
sidewalls allows the combination of highly lattice mismatched materials without
creating dislocations. The resulting nanowires are used to fabricate light
emitting diodes (LEDs), lasers, solar cells and sensors. However, expensive
single crystalline substrates are commonly used as substrates for nanowire
heterostructures as well as for epitaxial devices, which limits the
manufacturability of nanowire devices. Here, we demonstrate nanowire LEDs
directly grown and electrically integrated on metal. Optical and structural
measurements reveal high-quality, vertically-aligned GaN nanowires on
molybdenum and titanium films. Transmission electron microscopy confirms the
composition variation in the polarization-graded AlGaN nanowire LEDs. Blue to
green electroluminescence is observed from InGaN quantum well active regions,
while GaN active regions exhibit ultraviolet emission. These results
demonstrate a pathway for large-scale fabrication of solid state lighting and
optoelectronics on metal foils or sheets.
