Nickel Foam as a Substrate for III-nitride Nanowire Growth

Abstract

This article presents the use of flexible metal foam substrates for the growth of III-nitride nanowire light emitters to tackle the inherent limitations of thin-film light emitting diodes as well as fabrication and application issues of traditional substrates. A dense packing of gallium nitride nanowires were grown on a nickel foam substrate. The nanowires grew predominantly along the a-plane direction, normal to the local surface of the nickel foam. Strong luminescence was observed from undoped GaN and InGaN quantum well light emitting diode nanowires. INTRODUCTIONThe GaN-based light emitting diode (LED) market has grown into a multi-billion dollar market in just the last two decades. Despite this rapid progress, certain restrictions are inherent to the thin-film on a planar substrate design. These constraints can be generalized into light extraction, defectivity, substrate cost, and processing cost limitations as well as a lack of mechanical flexibility. 1 Sapphire is the predominant substrate for epitaxy of III-nitride light emitting diode thin films. Sapphire is non-conductive and presents a large lattice mismatch with the III-nitride material system. Silicon carbide substrates are expensive but have a closer lattice mismatch to GaN and can be supplied in a conductive state. 2-4 Silicon is available in larger diameters but GaN epitaxy on silicon suffers from thermal stress constraints. 5 The cost of the wafer is significant but often overstated when compared to the processing and balance of system costs. 6 A move to larger wafers allows a significant increase in back-end processing throughput and commensurate decrease in the cost per die. 7 An ongoing idea is to produce III-nitride LEDs on low-cost, large-area substrates such as glass akin to the thin film photovoltaic technologies. 8,9 Despite some progress for producing III-nitride LEDs on glass, 10 a poly-crystal type growth is inherently produced when the underlying substrate, such as glass, does not present crystalline order to which reactant atoms can align to form an ordered thin film. Poly-crystal or fine-grain III-nitride material presents an exceedingly large number of dislocations and other defects that effectively destroy operation of the pn junction. 11 Even for high-quality thin film growth on sapphire, lattice mismatch leads to the formation of dislocations with densities greater than 10 8 cm -2 , which limit the internal quantum efficiency of LEDs. 12 Furthermore, green LEDs require high indium content in