Letter pubs.acs.org/NanoLett
Embedding Plasmonic Nanostructure Diodes Enhances Hot Electron Emission Mark W. Knight,†,§ Yumin Wang,†,§ Alexander S. Urban,†,§ Ali Sobhani,†,§ Bob Y. Zheng,†,§ Peter Nordlander,†,‡,§ and Naomi J. Halas*,†,‡,§ †
Department of Electrical and Computer Engineering, ‡Department of Physics and Astronomy, and §Laboratory for Nanophotonics, Rice Quantum Institute, Rice University, Houston, Texas 77005, United States S Supporting Information *
ABSTRACT: When plasmonic nanostructures serve as the metallic counterpart of a metal−semiconductor Schottky interface, hot electrons due to plasmon decay are emitted across the Schottky barrier, generating measurable photocurrents in the semiconductor. When the plasmonic nanostructure is atop the semiconductor, only a small percentage of hot electrons are excited with a wavevector permitting transport across the Schottky barrier. Here we show that embedding plasmonic structures into the semiconductor substantially increases hot electron emission. Responsivities increase by 25× over planar diodes for embedding depths as small as 5 nm. The vertical Schottky barriers created by this geometry make the plasmon-induced hot electron process the dominant contributor to photocurrent in plasmonic nanostructure-diode-based devices. KEYWORDS: Plasmon, nanoparticle, nanowire, hot electron, embedded, Schottky plasmonically driven hot electron flow have been demonstrated using nanoantennas,2,14,15 strip waveguides,3−6 and nanostructured metallic films.7,16 TiO2-based devices have been used to demonstrate hot electron photoconversion through the visible spectrum.7 Schottky barrier detectors are traditionally based on a planar metal−semiconductor interface, where the metal film or structure is deposited onto the semiconductor as the device is fabricated. While this geometry is most straightforward to fabricate, it allows electrons to emit over the potential barrier only when they fall within a specific cone in momentum space (k-space), corresponding to emission directly into the semiconductor (Figure 1b).17,18 For these electrons, the internal quantum transmission probability (ηi) can be approximated using the modified Fowler equation:17,19
T
he new concept of plasmon-based optoelectronics, exploiting combinations of electrical transport properties and optically excited coherent electron oscillations known as plasmons, is rapidly giving rise to a new generation of electrically active optical elements and devices.1 This new class of devices already includes tunable infrared photodetectors,2−4 complementary metal−oxide−semiconductor (CMOS) compatible on-chip sensors,5,6 and broadband solar cells,7−9 many of which rely on harvesting the energetic (“hot”) electrons generated by plasmon decay. Following optical excitation, a plasmon mode can undergo either radiative (scattering) or nonradiative decay (absorption). When a plasmon decays nonradiatively, the energy of a plasmon quantum is initially transferred to a single, hot electron−hole pair.10,11 For an isolated plasmonic nanostructure, the hot electron will rapidly thermalize with the surrounding electron gas.12,13 However, when a hot electron of sufficient momentum is generated in a metallic nanostructure in direct contact with a semiconductor where an interface potential, known as a Schottky barrier, has been established, the electron may have sufficient momentum to traverse this barrier (Figure 1a). In a plasmonic nanostructure diode, the hot electrons originating within the optically excited metallic nanostructure can contribute to the semiconductor photocurrent, and be replenished, when the nanostructure is included within an electrical circuit. This effect can induce significant photocurrent at photon energies below the bandgap of the semiconductor yet above the Schottky barrier, where the latter can be controlled by selection of the specific metal and semiconductor constituents of the interface. Silicon photodetectors based on © XXXX American Chemical Society
ηi ≈ C F
(hv − qϕB)2 (1)
hv
where CF is a coefficient that depends on device-specific details, qϕB is the Schottky barrier energy, and hν is the energy of the incident photon. For excitation of the surface plasmon by a normal incidence electromagnetic field, injection over this Schottky barrier would require a change in momentum for the coherent electrons of the optically excited plasmon. One approach to overcoming this restriction and increasing Received: January 15, 2013 Revised: February 21, 2013
A
dx.doi.org/10.1021/nl400196z | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
nanobelt width.22 Here we observe a sizable increase in photocurrent relative to nonembedded plasmonic elements by embedding the nanowires just a few nanometers into the semiconductor substrate. Moreover, the size dependence of this effect, strongest for the narrowest nanowires, indicates that the ballistic hot electrons produced by plasmon decay are the major contributor to the observed enhanced photocurrent in embedded plasmonic nanostructure-diode structures. Both the observed embedding depth and size dependence are key parameters that need to be optimized when designing higher quantum efficiency photocurrent harvesting devices based on the plasmonic nanostructure-diode concept. The embedded nanowires were fabricated using a combination of planar lithography and dry etching. Fabrication was performed on a 1−10 Ohm-cm, ⟨100⟩ Si substrate protected by 10 nm of thermal oxide. This oxide thickness was chosen to eliminate shunting between the substrate and the top electrical contacts, while remaining sufficiently thin to permit facile etching of the underlying silicon. Electron beam lithography was used to pattern a 400 nm layer of ZEP-520A (Zeon Chemicals), which was developed for 60 s in n-amyl acetate (ZED-ND50, Zeon Chemicals) and 10 s in methyl isobutyl ketone (MIBK, Zeon Chemicals). The samples were cleaned with an isopropyl alcohol (IPA) rinse and dried under an N2 stream. The high dry etch resistance of the ZEP resist allowed the creation of trenches in the Si substrate using reactive ion etching (RIE with a chamber pressure of 25 mTorr, 150 W forward power, 6 SCCM O2, and 60 sccm CF4). These parameters gave an Si etch rate of ∼0.50 nm/s, measured by atomic force microscopy (AFM). Etch times were selected to yield nominal pit depths in the Si substrate of 5, 15, and 25 nm. Immediately prior to removal from the RIE chamber, we performed a final 5 s RIE etch using only CF4 to expose a fresh Si surface. Post-etch exposure to atmosphere was limited to
