Gallium Nitride-Based Nanowire Radial Heterostructures for Nanophotonics

We report a new and general strategy for efficient injection of carriers in active nanophotonic devices involving the synthesis of well-defined doped core/shell/shell (CSS) nanowire heterostructures. n-GaN/InGaN/p-GaN CSS nanowire structures were grown by metal-organic chemical vapor deposition. Electron microscopy images reveal that the CSS nanowires are defect-free single crystalline structures, while energydispersive X-ray linescan profile studies confirm that shell thickness and composition can be well controlled during synthesis. Photoluminescence data further show that the optical properties are controlled by the CSS structure with strong emission from the InGaN shell centered at 448 nm. Importantly, electrical devices made by simultaneously contacting the n-type core and outer p-type shell of the CSS nanowires demonstrate that in forward bias these individual nanowires behave as light-emitting diodes (LEDs) with bright blue emission from the InGaN shell. The ability to rationally synthesize gallium nitride-based radial heterostructures should open up new opportunities for nanophotonics, including multicolor LEDs and lasers. Semiconductor nanowires are emerging as versatile building blocks for photonic devices,1 including photodectors,2 lightemitting diodes (LEDs),3-6 and lasers.7-10 The ability to assemble and electrically interconnect these building blocks is especially critical since it represents an approach to active devices that could emit light over a wide range of wavelengths on a single chip, in contrast to conventional technologies. Previous studies have shown that the nand p-type materials required in active devices can be realized in crossed3,5 and axial4,6 structures, yet the nanoscale p-n junctions in these structures limit significantly injection currents. Here we report a new and general strategy involving the metal-organic chemical vapor deposition (MOCVD) synthesis of well-defined doped core/shell/shell (CSS) nanowire heterostructures that overcomes these limitations. We have focused on GaN-based materials since alloys of (Al-Ga-In)N are direct band gap semiconductors with potential for light emission from the ultraviolet through visible regions of the electromagnetic spectrum.11 Previous studies of planar LEDs have shown that n-GaN/InGaN/pGaN and related double heterostructures exhibit enhanced light emission efficiency compared to simple n-GaN/p-GaN diode structures,11-13 and thus suggest that CSS versus core/ shell (CS) structures would be ideal candidates for GaNbased active nanowire devices. The nanowire CSS structures also have potentially significant differences compared to planar heterostructures beyond that of dimensionality. In particular, because nanowire synthesis is essentially substratefree it should prevent formation of dislocations originating from lattice mismatch between GaN and growth substrates, and thereby reduce nonradiative recombination (at these defects) relative to planar structures. Our approach for preparing the GaN-based CSS structures (Figure 1) involves initial metal nanocluster mediated vaporliquid-solid (VLS) growth1 of an n-type GaN core followed by sequential radial growth of intrinsic InGaN and p-type GaN shells. The general concept of controlled, sequential growth of CS structures was described recently for the case of silicon and germanium materials,14 although this earlier work did not realize structures with selective nand p-type doping that are crucial to active photonic devices. To investigate the more complex nanowire structures required for active CSS devices, we exploited MOCVD, which is a technique used extensively for the growth of planar GaNbased heterostructures,11 as a means of delivering Ga and In reactants as well as silicon (n-type) and magnesium (p-type) dopants in a highly controlled and reproducible manner.15 Scanning electron microscopy (SEM) images of Si-doped GaN nanowires obtained following axial elongation (Figure 2a) reveal a high yield of uniform nanowire cores. The lengths of the nanowire cores used in these studies, which depend directly on growth time, were 10-20 μm. A bright * Corresponding author. E-mail: [email protected]. † Department of Chemistry and Chemical Biology. ‡ Division of Engineering and Applied Sciences. § These authors contributed equally to this work. NANO LETTERS 2004 Vol. 4, No. 1