Axial Diffusion Barriers in Near-Infrared Nanopillar LEDs Adam C. Scofield,* Andrew Lin, Michael Haddad, and Diana L. Huffaker* † Department of Electrical Engineering and California NanoSystems Institute, University of California at Los Angeles, Los Angeles, California 90095, United States * S Supporting Information ABSTRACT: The growth of GaAs/GaAsP axial heterostruc- tures is demonstrated and implemented as diffusion current barriers in nanopillar light-emitting diodes at near-infrared wavelengths. The nanopillar light-emitting diodes utilize an n- GaAs/i-InGaAs/p-GaAs axial heterostructure for current injection. Axial GaAsP segments are inserted into the n- and p-GaAs portions of the nanopillars surrounding the InGaAs emitter region, acting as diffusion barriers to provide enhanced carrier confinement. Detailed characterization of growth of the GaAsP inserts and electronic band-offset measurements are used to effectively implement the GaAsP inserts as diffusion barriers. The implementation of these barriers in nanopillar light-emitting diodes provides a 5-fold increase in output intensity, making this a promising approach to high-efficiency pillar-based emitters in the near-infrared wavelength range. KEYWORDS: Nanopillar, diffusion barrier, light-emitting diode S emiconductor nanowires (NWs) and nanopillars (NPs) have been the subject of intense research because of their promise in new nanoscale electronic and optoelectronic devices. Much of their potential is derived from the ability to form complex axial and radial heterostructures that are either impossible or not easily attained using planar epitaxy. Different combinations of axial and radial heterostructures have been implemented in light-emitting diodes, 1-3 lasers, 4,5 photo- detectors, 6,7 and solar cells. 8 In terms of light-emitting devices, the predominant application of NWs has been in the visible wavelength range for future solid-state lighting, with only a few examples of devices operating in the near-infrared. 9-14 Being less developed, the near-infrared devices are missing one of the key components to making a high internal efficiency emitter: an axial high band gap barrier to limit diffusion of carriers past the active region. In planar GaAs-based emitters, the typical choice for the diffusion barrier layer is Al x Ga 1-x As, as it is lattice-matched for any composition x. For GaAs NW- or NP-based emitters, the use of AlGaAs as a barrier has been demonstrated, but the propensity of Al to adhere to the surrounding substrate and NP sidewalls limits the use of AlGaAs to core-shell-type heterostructures. 15 These core-shell-type emitters require a radial current injection scheme, placing the active region in close proximity to the metal contact, which can be highly detrimental to device performance because of band-bending and recombination at the semiconductor/metal interface. For many applications, an axial current injection scheme is necessary in order to maintain separation between the active region and contacts 16,17 while at the same time having a wide- bandgap shell for surface passivation. 11 If an axial current injection scheme is used, then axial heterostructures must also be used to form the diffusion barriers. In this work, we demonstrate the growth and application of axial GaAsP diffusion barriers in GaAs NPs. In NP devices, the greatly reduced lattice matching requirements of axial heterostructures 18,19 opens up the possibility of using alternative materials to AlGaAs, making GaAsP a strong candidate for the choice of barrier material as it contains no Al. While in planar devices GaAsP is typically used in strain- compensating layers 20 and strained-layer superlattices 21 with critical thicknesses on the order of 40 nm for 20% phosphorus, 22 compositions of GaAsP with phosphorus below 20% can be completely strain relaxed for NPs with diameters below 100 nm following refs 15 and 16. In order to properly implement the axial GaAsP hetero- structures in any device, a detailed study of the material growth and electronic properties is necessary. With planar structures this characterization would normally be accomplished with a combination of photoluminescence and capacitance-voltage measurements. In NPs, however, the impact of the three- dimensional geometry makes it either difficult or impossible to deconvolve the nonideal effects of the surfaces and interfaces on these measurements. Therefore, an alternate set of measurements and characterization is needed to circumvent these effects. In this case, we implement energy-dispersive X-ray (EDS) measurements and temperature-dependent current- voltage characteristics to determine the compositional depend- ence on growth conditions and the electronic band-offsets, respectively. The NP arrays and devices used in this study were grown by selective-area metal-organic chemical vapor deposition with Received: March 18, 2014 Revised: September 19, 2014 Published: October 3, 2014 Letter pubs.acs.org/NanoLett © 2014 American Chemical Society 6037 dx.doi.org/10.1021/nl501022v | Nano Lett. 2014, 14, 6037-6041