Hot-Electron Photodetection with a Plasmonic ... - ACS Publications

Feb 6, 2014 - Instead, absorbed light in one of the metals can produce a current of hot electrons when the incident photon energy exceeds the oxide ba...
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Hot-Electron Photodetection with a Plasmonic Nanostripe Antenna Hamidreza Chalabi, David Schoen, and Mark L. Brongersma* Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: Planar metal−oxide−metal structures can serve as photodetectors that do not rely on the usual electron−hole pair generation in a semiconductor. Instead, absorbed light in one of the metals can produce a current of hot electrons when the incident photon energy exceeds the oxide barrier energy. Despite the desirable traits of convenient fabrication and room-temperature operation at zero bias of this type of device, the low power conversion efficiency has limited its use. Here, we demonstrate the benefits of reshaping one of the metallic contacts into a plasmonic stripe antenna. We use measurements of the voltage-dependence, spectral-dependence, stripewidth dependence, and polarization-dependence of the photocurrent to show that surface plasmon excitations can result in a favorable redistribution in the electric fields in the stripe that enhances the photocurrent. We also provide a theoretical model that quantifies the spectral photocurrent in terms of the electrical and optical properties of the junction. This model provides an accurate estimate of the bias dependence of the external quantum efficiency of different devices and shows that both the spatial and vectorial properties of the electric field distribution are important to its operation. KEYWORDS: Hot electrons, photoemission, photodetection, plasmonics, nanophotonics

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contacts8 and its subsequent transport across the oxide to the other metal.9 This then results in a photocurrent that can be measured and accurately quantified in an external circuit. The efficiency of hot electron production and emission across the barrier tends to be too low to allow for practical applications10 and in this work, we explore the possibility of enhancing hot electron emission in MIM-junctions by reshaping the metallic contact into a stripe that supports plasmonic resonances. Early photoemission studies on metallic films already revealed the potential importance of plasmon resonances to enhance light absorption in the near-surface region of the metal and thereby to enhance the photoemission of electrons.11,12 Very recently, plasmon-enhanced photoemission from metallic nanoantennas placed on top of a Si Schottky-detectors was also demonstrated.13,14 In planar MIM junctions, it was also shown that excitation of propagating surface plasmon polaritons (SPPs) can enhance efficiency.15 In this work, we aim to further enhance this efficiency by reshaping one of the metal contacts of an MIM junction into plasmonic antenna that resonate SPPs. The insights gained from this study can aid the realization of ultracompact and efficient plasmon-enhanced photodetectors.16 Results. Basic Device Geometry and Operation. For our study, we generate MIM-junctions in a crossbar geometry by electron beam lithography (Raith 150 e-beam writer). The

etal−insulator−metal (MIM) junctions are arguably the simplest photodetector/energy harvesting devices one can imagine. They can be reproducibly fabricated using mature thin-film deposition techniques and can operate at room temperature without the need for an applied bias. Thin-film, integrated MIM-junctions have been developed as detector elements operating in the terahertz and mid-infrared since the 1970s.1,2 At these low frequencies, photoejected carriers cannot make it over the high energetic barriers (∼1 eV) typically found in the MIM-junctions employed in photodetectors. For this reason, one typically made the oxide sufficiently thin (∼1 nm) to allow for tunneling through the barrier at a reasonable rate. The diode-like properties of MIM junctions then can rectify the alternating current (ac) driven in the metal by the incident radiation to produce direct current (dc) power. There are many challenges in increasing the operating frequency of these devices. These include shrinking of the device dimensions while maintaining high-quality diode properties. Scaling of the device sizes well below the illumination wavelength also results in an inefficient coupling of electromagnetic energy into the junction. The latter problem was solved by using antennas to effectively collect and then feed electromagnetic energy into the junction. In this way, rectifying antennas or “rectennas” were formed.3−6 Whereas the efficiencies for radiofrequency operation of rectennas can be as high as 90%,7 the realization of such high efficiencies at optical frequencies has not yet been achieved.4 Fortunately, the high photon energies in the visible and UV afford an alternative pathway for photodetection that capitalizes on the generation of energetic, hot electrons in one of the metal © 2014 American Chemical Society

Received: November 29, 2013 Revised: February 4, 2014 Published: February 6, 2014 1374

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piezo-motors and plotting the photocurrent as a function of the illumination-spot position. Figure 1c shows a photocurrent image taken from a device with a 470 nm wide stripe overlaid on top of its SEM image. The bias voltage in this measurement was 0 V. It can clearly be seen that photocurrent is generated only in the device region where the top electrodes overlay the wide bottom electrode to form a series of nanoscale MIMjunctions. A linear dependence of the photocurrent on the illumination power was found for all illumination wavelengths of interest (see Supporting Information Figure S1). This confirms that the photocurrent generation occurs via a linear process. This is consistent with our expectation that photoejected electrons are responsible for the photocurrent generation, which linearly scales with the absorbed light power. Demonstrating That the Photocurrent Is Due to Hot Electrons. As a next goal, we aim to provide further evidence that the photocurrent results from hot electron emission across the oxide barrier and that the hot electron emission can be enhanced by driving a surface plasmon resonance in the top electrode. To this end, we provide experimental data of the voltage-dependence, spectral-dependence, stripe-width dependence, and polarization-dependence of the photocurrent. We will also compare this experimental data to a theoretical model that quantifies the photocurrent in terms of the electrical and optical properties of the junction. First attempts to quantify the current from photoejected electrons were made by R. H. Fowler17 and W. E. Spicer.18,19 Spicer intuitively described the internal photoemission process from a metallic photocathode as proceeding via series of three consecutive steps. Here, we follow a similar approach and describe the photoemission of carriers across a barrier in a series of five steps as depicted in the band diagram shown in Figure 2a and Supporting Information Figure S3. The efficiencies of these steps will be quantified in this article, but we first describe their nature. In step 1, hot electrons are generated in one of the metallic contacts (e.g., top contact) through the absorption of a photon. In this process, electrons are lifted from states below the Fermi level EF,t by the pertinent photon energy hν. It is assumed that the generated hot electrons behave as free electrons and that their initial momentum distribution is isotropic. The latter is a reasonable assumption based on the large mismatch between the wave vectors of the light/surface plasmon and the generated hot electron.20,21 Obviously, more detail about the metal’s electron and phonon dispersion behavior of the metal could improve the accuracy of the predictions made in this work. In step 2, half of the generated hot electrons will move in direction of the metal/ oxide interface. Only some of them will make it to the interface without losing energy in an inelastic collision. In this work, we quantify this fraction with the use of empirical data on the effective mean free path of energetic electrons in solids. We make the simplifying assumption that the electrons that undergo an inelastic collision do not contribute to the photocurrent. The hot electrons arriving at the metal/oxide interface with a kinetic energy exceeding the barrier φB of the oxide have a certain probability to be injected into the oxide (step 3). This injection probability tends to be small as reflections naturally occur as a result of the large wave-vector contrast (i.e., impedance mismatch) between electrons in the metal contact and oxide.22,23 The electrons that get injected into the oxide have a limited probability to propagate across the oxide without inelastic collisions and contribute to the current (step 4). Finally, the electron may be injected into the opposing

devices are shown schematically in Figure 1a and representative scanning electron microcopy images are shown in Figure 1b,c.

Figure 1. (a) Schematic illustration of a typical device featuring a series of MIM crossbar junctions formed between a wide bottom electrode and a set of nanoscale top electrodes. The directions of the electric field for transverse electric (TE) and transverse magnetic (TM) illumination conditions are depicted. (b) An SEM image of a device with 470 nm wide top electrodes. Inset shows a cross-sectional image of the device, composed of oxide layer sandwiched by top and bottom metal layers. (c) Photocurrent image of the device taken at an illumination wavelength of 400 nm with TM polarization overlaid on the SEM image of the device. The red color corresponds to 7 picoamps and blue indicates a close to zero photocurrent.

The bottom metal electrode is a 60 nm thick Au stripe featuring a width of several tens of micrometers. It is deposited by electron-beam evaporation on top of a 4 nm thin Ti adhesion layer on a quartz substrate. This electrode is then overgrown with a layer of Al2O3 by atomic layer deposition (ALD). We used 45 ALD cycles, which translates into a 4.5-nm-thick oxide layer with the growth occurring at approximately 1 Å/cycle under standard deposition conditions (150 °C, 20 sccm N2 carrier flow in a Cambridge Nanotech Savannah deposition chamber). The top electrodes consist of a series of 14 parallel, 90 nm thick Au stripes oriented in an orthogonal direction of the bottom electrodes. Experiments have been carried out on devices with top-electrode widths of 150, 470, and 750 nm. Wire bonds are created to both the top and bottom electrodes to enable photocurrent measurements at different bias conditions, as depicted in Figure 1a. The devices were tested under illumination with light from a super continuum source (Fianium). The illumination wavelength was controlled in the range from 400 nm −700 nm using an acousto-optical tunable filter with a bandwidth of 10 nm. A linear polarizer was used to polarize normally incident light either along the metallic stripes (transverse electric or TE illumination) or orthogonal to them (transverse magnetic or TM illumination). The light was focused on the sample with a 50× microscope objective producing an 8 μm diameter spot size. Photocurrent was measured using the lock-in technique with a SR810 lock-in amplifier together with a SR570 low-noise current preamplifier. The voltage across the junction was also controlled by a Keithley 2612 source meter. Photocurrent images could be generated by moving the sample stage with 1375

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semiconductor interface24 but adds additional physics to model the electron reflections at the metal/oxide boundaries.22,25 Figure 2b shows the spectral dependence of the responsivity of a device with 750 nm- wide stripes upon illumination with TM polarized light. The presented experimental data points were taken from a larger set of spectral photocurrent measurements taken from devices with different stripe widths (Supporting Information Figure S2). Spectra are shown for several bias voltages in the range from −0.4 to +0.4 V. It is of value to plot the square root of the responsivity as a function of the incident photon energy when considering Fowler’s theory of photon emission from a metal surface to vacuum. This theory predicts that the external quantum efficiency ηe for photoejection of electrons scales as ηe ∝ (hν − W)2 when the incident photon energy hν exceeds the metal’s work function W. For internal photoemission across an oxide barrier with a barrier height φB, one intuitively expects a modified Fowler equation in which W is replaced by φB. This type of quadratic dependence is indeed empirically found in many photoemission experiments, and for this reason we plot the square root of the responsivity versus the incident photon energy. In the data, we see that for each applied bias the responsivity shows a clear onset and a more-or-less linear increase with photon energy above the onset. The linear dependence is consistent by the modified Fowler equation. To understand the movement of the photocurrent onset with applied bias, it is important to note that the optical simulations indicate that for the considered devices and wavelengths, the light absorption (and thus the hot electron generation) in the top stripe is at least 1 order of magnitude larger the light absorption in the bottom stripe. As a result, one can interpret the responsivity data to first order by only considering the forward current from the top to bottom stripe. This forward current is expected to turn on when the incident photons have a sufficient energy to lift some of the conduction electrons above the highest point in the oxide barrier. For positive bias voltages, this condition can be expressed mathematically as Eph > φB + eV. When this condition is satisfied, the hottest electrons that are excited from just below the Fermi level in the metal will have sufficient energy to make it over the oxide barrier (without a need to tunnel). For the symmetric Au/Al2O3/Au junction under consideration, the barrier height is expected to be equal to the difference in the work function for Au (WAu = 5.2 eV) and the electron affinity of nanocrystalline Al2O3 grown by ALD (χAl2O3 = 2.6 eV),26,27 that is, φB = 2.6 eV. At zero bias (flat band), the onset photon energy is thus expected to be 2.6 eV, as seen in the plot. For a negative bias, the highest point in the barrier remains the same and as a result the onset energy does not change. For positive biases (as shown in Figure 2b), the highest point in barrier and thus the onset energy moves up linearly with voltage (1 eV per 1 V applied). Figure 2c shows an alternative way of plotting this data. It provides the dependence of the external quantum efficiency (EQE) for a stripe on the applied bias. This quantity is the ratio of the number of hot electrons emitted over the barrier divided by the number of photons incident on the stripe. Plots are shown for several photon energies. Each data set shows a clear onset when the applied voltage satisfies the condition that Vapp < (Eph − φB)/e. The steep increase with decreasing voltage is due to the lowering of the highest point in the barrier that allows significantly more hot electrons to make it across the barrier (see band diagrams in Figure 2c). Recently, it was

Figure 2. (a) Band diagram for the metal-oxide-metal barrier structures. It shows the top metal on the left with a Fermi level EF,t, the oxide barrier with a height φB, and the bottom metal on the right with a Fermi level EF,b. It also illustrates how the barrier can be tilted up (down) upon application of a positive (negative) bias voltage Vapp on the top electrode. Hot electrons can be generated in either contact and contribute to currents Iforward and Ibackward that run the top-tobottom contact or vice versa. The current generation occurs in five consecutive steps that are discussed in the text. (b) Responsivity of the MIM diodes as a function of the incident photon energy for a range of biasing conditions (c) Dependence of the external quantum efficiency of a 750 wide stripe on applied voltage. Experimental data (dots) is shown for several incident photon energies and the solid curves are model calculations.

metal contact (step 5), but reflections can again occur at this interface due to an impedance mismatch. The optical and electronic transport models that quantify the efficiencies of steps 1 through 5 are discussed in detail in section C of the Supporting Information. The optical and electrical models together enable calculation of the forward and backward currents Iforward and Ibackward arising from the hot electron generation in either top or bottom contact. These opposing currents produce a net current Inet = Iforward − Ibackward that will depend on the properties of the metal, the barrier, and the applied voltage Vapp. The model conceptually follows the model described by Scales et al. that was developed for hot electron emission across a Schottky Barrier at a metal/ 1376

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of hot electrons capable of contributing to the forward current. The backward current is negligible due to the much weaker absorption in the bottom contact (1.5%) and the unfavorable biasing conditions that effectively block current flow in the backward direction (see Supporting Information Figure S5). Assuming an isotropic momentum distribution, half of the photogenerated hot electrons will move toward the metal/ oxide interface. Because of the limited mean free path for the hot electrons in Au, a small fraction of 2.4% of the electrons moving to the interface actually reaches the interface without a collision. Here, we have taken the literature value of a 18 nm for the mean free path of electrons with energies of 2−3 eV above the Fermi level29,30 Note that experiments have demonstrated that the electron mean free path is energy-dependent and in this energy range, its magnitude decreases with increasing kinetic energy, going from 70 nm for a 1 eV electron to 20 nm at 2 eV and finally 7 nm at 5 eV.29 This fraction of electrons reaching the oxide interface could be increased substantially by using thinner stripes that support stronger plasmonic resonances and would require shorter travel distances for the electrons to reach the metal/oxide interface. The largest reduction in the EQE lies in the inefficient transmission across the oxide barrier and only a fraction of 2 × 10−4 of the electrons that arrive at the barrier make it across. In traversing the oxide, there is a relatively small loss of 50% of the electrons due to inelastic collisions because of the large mean free path (∼10 nm at 3 eV)31 compared to the oxide thickness. Our calculations indicate that the largest losses (0.36%) occur at the interface with the top electrode due to the presence of a small escape cone that limits the possible angles through which the hot electrons can come out of the metal. This escape cone is analogous to the escape cone for light escaping from a high index medium and in this case results from the fact that the magnitude of the wave vector of the hot electrons in the metal is much larger than that of the electrons at the same energy in the oxide. As the boundary conditions for the electron transmission across the interface require conservation of the momentum parallel to the interface, not all of the electrons in the metal can couple to states in the oxide (see Supporting Information Figure S4). This point has been noted before for Si/metal Schottky junctions.32 It was in fact proposed that roughening the metal/semiconductor surface can help alleviate the issue surrounding the impedance mismatch and experimentally roughened surface have indeed produced higher photocurrents. This hints at the exciting opportunity to engineer optics-inspired, atomic-scale antireflection coatings, resonant tunneling structures, and so forth to manipulate the electrons with characteristic wavelengths of the Fermi-wavelength. The significant reflections that can take place at the metal/oxide and metal/semiconductor boundary may help to understand some of the lower-than-anticipated experimental efficiencies in the earlier literature.15 In the analysis above, it should be kept in mind that the model makes a number of assumptions (see Supporting Information). As such, the efficiencies should not be taken as exact but rather as useful information to guide the design of better hot electron photodetectors. On the basis of the analysis above, it is, for example, very clear addressing the interfacial scattering challenge is of the utmost importance. Demonstrating the Importance of Surface Plasmons in the Generation of Hot Electrons. Next, we analyze the dependence of the photocurrent on the stripe width. Figure 3a,b show full-field simulations of the magnetic field

demonstrated that the clear, voltage-tunable onset in photocurrent generation with applied bias can be used to analyze the spectral content of an incident optical signal.28 At negative voltages, the change in the efficiency with decreasing voltage is less steep as the highest point remains constant. The solid lines in Figure 2c show the predicted dependencies of EQE on bias voltage according to our five-step model detailed in the Supporting Information. The reasonable quantitative agreement makes it valuable to discuss the various efficiencies of the subprocesses that give rise to the overall low EQE. For example, for the most optimal conditions (photon energy of 3.1 eV and a reverse bias of 0.4 V) that give rise to the highest EQE we find the following subefficiencies. First, the optical simulation indicates that about 51% of the light is absorbed in the top metal. In the discussion of Figure 3, we will argue that this strong absorption is due to the excitation of surface plasmon resonance excited on the top surface of the stripe. The plasmon-enhanced absorption causes the generation

Figure 3. Full-field electromagnetic simulation of the magnetic field intensity around (a) a 150 nm wide stripe and (b) a 750 nm wide stripe illuminated at a wavelength λ = 430 nm. (c) Plot of the measured external quantum efficiency per stripe as a function of the incident photon energy for three different devices considered and under 0 V applied voltage. These measurements are carried out for TM polarization. (d) Plot of measured external quantum efficiency per stripe for three different devices considered as well as the net absorption calculated for the three devices. These measurements are carried out for TM polarization and with the wavelength of 400 nm and under a 0 V applied voltage. 1377

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Figure 4. (a) Variation of responsivity of the 470 nm wide device with the polarization angle of the incident light. (b) Variation of the device’s responsivity with photon energy for TE and TM illumination. (c) Magnetic field distribution around the plot of the device under TE and TM polarized light with unity amplitude of the incident magnetic field. (d) Photocurrent ratio between TE and TM polarized incident light and the corresponding values considering different electron mean free paths.

expected as dipolar, 1/2 λSPP, resonance tend to have a higher absorption efficiency than 3/2 λSPP resonance for topillumination conditions. Interestingly, the absorption per stripe appears to increase a bit faster with decreasing stripe width than the EQE. This implies that the internal quantum efficiency (number of hot electrons generated per absorbed photon in the stripe) in fact increases with increasing stripe width. This implies that not only the total amount of light absorption is important but also the spatial distribution. In comparing the spatial distributions of the electric field intensities, the 3/2 λSPP resonance redistributes the electric fields toward the edges of the stripe. Further evidence of the potential importance of the stripe edges in enhancing the hot electron current comes from polarization-dependent measurements of the photocurrent. Figure 4a shows how the responsivity of the device with the 470 nm wide stripes depends on the polarization angle of the incident light. The illumination wavelength for this measurement was 430 nm and the bias was kept at 0 V. Using a halfwave plate the polarization angle was changed from TM to TE and back. Strong polarization dependence is observed with the expected cos2 θ dependence. The photocurrent for TMillumination is significantly larger than for TE illumination. Figure 4b shows that the responsivity for TM illumination is significantly larger for all of the photon energies above the onset. Figure 4c plots the magnetic field distribution around the stripes for both polarizations with the goal of understanding the significant difference in photocurrent production under these distinct illumination conditions. For TE illumination, the field is more-or-less uniformly distributed along the top surface of the stripe. This is quite similar to the field distribution seen in a metal film with the light fields penetrating up to a skin-depth into the metal. For TM polarization, a 3/2 λSPP plasmon resonance is excited and a clear redistribution of the fields toward the edges can be noted. In order to compare the measured results with our model’s prediction, the ratio of the photocurrent between TM and TE

distribution for the narrowest (150 nm) and widest (750 nm) stripes as illuminated with λ = 430 nm light. From these plots, it is clear that a dipolar surface plasmon resonance is excited on the narrowest stripe. Such a resonance can be excited when approximately half a SPP wavelength λSPP fits along the width of the stripe.33 This produces a single maximum in the magnitude of the magnetic field localized on the middle of the stripe. This high magnetic field is linked (via Ampere’s law) to the surface plasmon polariton (SPP) currents running back and forth along the top surface of the stripe. The associated charge distribution that results from the oscillating currents is depicted by the charges (+,−) in the figure. The 750 nm wide stripe supports a 3/2 λSPP resonance (note the excitation of the λSPP resonance is symmetry forbidden for top-illumination). This gives rise to three maxima in the magnetic field intensity along the top surface of the stripe. It is worth noting that gap plasmons are not excited at the considered wavelength due to the much higher mode index of gap modes at these small gap sizes. In this work, we aimed to avoid excitation of gap plasmons as they provide a more symmetric absorption in the two metallic contacts. This is undesired as it leads to more equal forward and backward currents. It is in fact the asymmetric absorption in the two electrodes that allows for photocurrent generation at a 0 V bias in our device. Figure 3c shows the EQE for the three considered stripe widths as a function of the incident photon energy for a bias of 0 V. As expected, all of the stripes show a clear onset at the same photon energy of 2.6 eV, which equals the oxide barrier energy. The EQE increases most rapidly for the device with the narrowest stripes and most slowly for the one with the largest stripe width. Figure 3d provides some insight into this behavior by plotting the dependence of the EQE on the stripe width at the highest photon energy (Eph ≈ 3.1 eV). Because of a local surface plasmon resonance, the effective absorption per stripe and consequently the measured EQE per stripe increases with decreasing the width. It can be seen that the narrower stripes absorb light more effectively than the wider stripes. This can be 1378

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electrodes for which the boundaries play an increasingly important role. As our experiments also showed that not only the spatial but also the vectorial properties of the electric field distribution are important, there is also an exciting possibility to use our growing understanding of the field of plasmonics to create metallic antennas structures that generate high electric fields in desirable directions.

illumination is also calculated based on our model by considering 18 and 4 nm as the mean free path of the hot electrons in the metal. These results are shown in Figure 4d. Using the model, the TM/TE ratio is predicted to be about 1.5 or 3 depending on the mean free path value considered. The higher photocurrent for TM illumination results as the light intensity is redistributed to produce more absorption close to the metal/oxide interface. It is worth noting that this redistribution occurs especially near the stripe edges (see Supporting Information Figure S6). The model predicts only a weak wavelength dependence of the TM/TE current ratio on wavelength over this narrow range. Interestingly, the experimentally found dependence of the current ratio is very strong over this range. Close to the onset (440 nm) the current ratio increases to almost a factor of 12. As our model properly takes into account the differences in the light absorption between TM and TE illumination, the high TM/TE ratio cannot be simply explained by a difference in light absorption. The origin has to be found in the difference in the spatial distribution and/or directionality of the fields in the stripes for the two polarization directions. For TM polarization, the fields redistributed toward the edges. As such, more hot electrons are generated close to a boundary (up to an order of magnitude more). Such abrupt boundaries can provide the missing momentum between the hot electron states in the metal and oxide (i.e., relax the requirement to conserve the parallel momentum upon injection into the oxide). The need to conserve the parallel component of the hot electron momentum together with the limited momentum of the electron states in the oxide results in a limited escape cone for the hot electrons trying to escape the metal (see Supporting Information Section C). Previously, it was shown that roughness can relax the momentum conservation rule and enhance the hot electron current.32 Here, a similar effect may be present. This could possibly explain the higher TM/TE ratio at lower energies as the escape cone is exceedingly small for electrons that barely skim over the barrier. It is also worth noting that the TM illumination produces electric fields in the vertical direction normal to the metal/oxide interface. For TE illumination, fields are purely tangential to the metal/oxide interface. The electric fields normal to the surface are linked to surface plasmon excitation on the side of the stripes. It has been suggested that the hot electrons may be preferentially directed along the SPP momentum direction8,14,34 and the electric fields normal to the oxide could possibly assist in enhancing electron transport across the oxide for TM polarization. Conclusions. In conclusion, we have examined the phenomenon of photocurrent generation by hot electrons in wavelength-scale MIM devices. The voltage-dependence, spectral-dependence, stripe-width dependence and polarization-dependence of the photocurrent were experimentally determined. We also extended previous models developed for hot electron emission in Schottky structures so they can be used to quantify the hot electron current generation in MIM structures. The model provides a quantitative estimate for the efficiency of producing hot electron photocurrent from incident light that is composed of five key steps. From the analysis, it follows that the efficiency is primarily limited by the large wave vector mismatch between hot electron states in the metal contact and the adjacent oxide. This inefficiency may be reduced by roughening surfaces or using smaller plasmonic

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ASSOCIATED CONTENT

S Supporting Information *

Additional spectral- and voltage-dependent measurements of the photocurrent from our presented hot-electron photodetectors. Details of our model used to predict and understand the magnitude and polarization-dependence of the hot electron current from our metal−insulator−metal devices. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (650) 736-2152. Fax: (650) 725-4034. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication was supported by the Department of Energy Grant DE-FG07ER46426 REFERENCES

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