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2008, 112, 20109–20113 Published on Web 12/04/2008
Erbium Emission from Nanoengineered Silicon Surface P. K. Sekhar,† A. R. Wilkinson,‡ R. G. Elliman,‡ T.-H. Kim,‡ and S. Bhansali*,† Nanomaterials and Nanomanufacturing Research Center, Department of Electrical Engineering, UniVersity of South Florida, Tampa, Florida, and Electronic Materials Engineering Department, Research School of Physical Sciences and Engineering, Australian National UniVersity, Canberra, Australia ReceiVed: September 24, 2008; ReVised Manuscript ReceiVed: NoVember 10, 2008
Optically active SiOx nanowires were grown on silicon by ion-implanting it with metallic impurities and annealed at 1100 °C in an Ar ambient. The implanted metals precipitate on the silicon surface and act as catalysts for nanowire growth. Ion implantation of erbium into silicon and subsequent heating in an argon ambient resulted in selective nucleation and growth of optically active silica nanowires. The bottom-up nanowire growth, mediated by vapor liquid solid mechanism, was also demonstrated for a multimetal (Au:Er) implant combinations in Si. The role of Er as a catalyst and dopant resulted in optically active silica nanowires that exhibited photoluminescence emission at 1.53 µm from an Er3+ intra-4f transition. Time resolved photoluminescence (PL) from these nanostructures indicated a luminescence lifetime of 24 ms, larger than that generally observed for Er-doped bulk silica. This increase in luminescence lifetime is attributed to a reduction in the optical density of states of Er in the nanowire samples. Infrared optical emission from erbiumdoped silica nanowires on silicon holds a great potential for implementing wavelength controlled optical nanoswitches, photoresponsive devices and sensitive integrated biosensors. Trivalent rare-earth ions are optically active species with ocular transitions in the visible and near-infrared spectrum. The emission from Er3+ at 1.54 µm (characteristic of the 4I15/2-4I13/2 transition in erbium) is of particular interest as it coincides with standard optical telecommunication wavelength. A critical challenge in the telecom domain is the need to achieve maximum Er emission intensity (high active concentration and long luminescent lifetime). Reports on enhancing the optical activity of Er3+ have established1-3 that the Er emission can be increased by (a) the addition of impurities or clusters that act as sensitizers for Er excitation (b) changing the local atomic environment to increase the fraction of Er in the 3+ oxidation state, and (c) increasing the rate of Er excitation by local field enhancement in the vicinity of metallic particles. The presence of nonradiative defects also has a significant effect on the emission intensity and the passivation of such defects has been shown to lead to a substantial increase in emission intensity.4 Er emission has been investigated in a range of host materials, including Si and SiO2, and in structures with a range of micro-, meso-, and nanoscale architectures.5-7 Among the hosts, silicon has received particular attention due to its potential for the integration of electronic and optical systems.8-15 The fact that the solubility and luminescence efficiency of Er in many materials, especially in bulk Si, is quite low has led to the use of nanometer-scale structures in many applications. For example, Suh et al.16 reported on the optical activation of Si nanowires using sol-gel derived Er-doped silica. The silicon nanowires were grown via the vapor liquid solid (VLS) mechanism using * To whom correspondence should be addressed. Tel.: (813) 974 3593. Fax: (813) 974 5250. E-mail:
[email protected]. † University of South Florida. ‡ Australian National University.
10.1021/jp808462j CCC: $40.75
Au as a catalyst and the resulting wires doped with Er using a spin-coating technique. In this report, significant enhancement intensity was observed and attributed to large surface area of the nanowires. However, the Er3+ luminescence was shown to suffer from thermal quenching, with the PL lifetime decreasing from 8.3 to 6.9 ms when the temperature was varied from 25 to 300 K. In a similar approach, Wang et al.17 studied the size dependence of Si nanowires on Er emission characteristics. They concluded that in relatively small wires (
