Work function-driven hot electron extraction in a bimetallic plasmonic

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Work function-driven hot electron extraction in a bimetallic plasmonic MIM device Vårin Renate Andvik Holm, Bob Y. Zheng, Phil M. Denby, Bodil Holst, Naomi J. Halas, and Martin M. Greve ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 11 Mar 2018 Downloaded from http://pubs.acs.org on March 11, 2018

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Work function-driven hot electron extraction in a bimetallic plasmonic MIM device ∗,†,‡

Vårin R. A. Holm,

Bob Y. Zheng,

Halas,

†Department

‡

‡

Phil M. Denby,

and Martin M. Greve

¶

Bodil Holst,

†

Naomi J.

∗,†

of Physics and Technology, University of Bergen, Allégaten 55, 5007 Bergen, Norway

‡Halas

Research Group, Rice University, 6100 Main St, Houston, TX 77005, USA

¶Ensol

AS, Apeltunvegen 2, 5222 Nesttun, Norway

E-mail: [email protected]; [email protected]

Phone: +47 97 54 36 88; +47 90 07 99 74

Abstract The Localized Surface Plasmon Resonance (LSPR) can cause hot electrons to be emitted from metal nanostructures. A key issue is how these hot electrons can be eciently extracted and used for example in energy harvesting or sensing applications. One way is to create a plasmonic metal-insulator-metal device (MIM). In a plasmonic MIM device hot electrons (or holes) are injected from a nanostructured surface electrode across the insulator into the back electrode, and a current ows. Here we present a new, bimetallic plasmonic MIM device where the dierent work functions of the metal electrodes result in a built-in electrical eld which facilitates hot electron extraction. This eliminates the need for an externally applied bias. The nanostructured front electrode, with 52 nm wide and 10 µm long gold nanorods, is separated from the back electrode of aluminum by an insulator layer of Al2 O3 with 1

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thicknesses varying between 3 - 18 nm. A maximum device responsivity of about 2.2 µA/W is measured when the Al2 O3 layer is 3 nm. As the thickness of the insulator

layer increases the LSPR induced photocurrent responsivity decreases, in agreement with theoretical predictions for electron tunneling.

Keywords localized surface plasmon resonance, EBL, zero bias, photocurrent, nanorods.

The demands by society for lighter and more compact electronics is increasing. 1 Most electronics need an external bias to operate. For portable devises this will typically be supplied by a battery, which may be heavy, and have a limited lifetime. 2 Zero-bias devices, which need no external voltage applied, are therefore of particular interest in the context of portable light weight devices. The excitation of hot electrons in metals through the localized surface plasmon resonance (LSPR) eect has attracted increasing interest in recent years with potential applications including photo-voltaic and sensing devices. 3,4 The hot electrons can be generated either directly through interband excitations of d-band electrons or as a result of Landau damping of the LSPR. 5 Depending on the device conguration, relatively large hot electron yields with high charge generation per watt of incident light can be achieved. Photocurrent responsivity in the mA/W range has been reported. 6 Hot electron generation is often investigated by forming a Schottky barrier between a nanostructured metal and a suitable semiconductor. 7,8 Recently metal-insulator-metal (MIM) systems have also been created to study this process. 911 In a classical MIM device, an insulator layer (typically thinner than 20 nm) is sandwiched between two metal electrodes. Current can be generated by light absorption in the metal lms. In a plas2


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monic MIM device, the top electrode is nanostructured allowing the device to absorb light through LSPR and generate hot electrons. A current of hot electrons can be achieved if the electrons: 1) are excited with energies exceeding the metal-insulator barrier height, or 2) can cross the insulator barrier via quantum mechanical tunneling. 1214 The transmission probability through a potential barrier for an electron with energy E above the Fermi level is given as: 13 "

2 Z dq ∗ 2m (ϕ(x) − E)dx T (E) = exp − h ¯ 0

#

(1)

where h ¯ is the reduced Planck constant, m∗ is the eective mass of an electron (here in

Al2 O3 ), ϕ(x) is the potential barrier height, and x = 0 and x = d gives the width of the barrier at the Fermi level. The equation shows that the current decreases exponentially with increasing insulator thickness. In order to increase the eciency of plasmonic MIM devices, the exploitation of tunneling electrons is of key importance as it allows lower energy electrons to be utilized/detected. Previous studies 11,15 used the same metal for both electrodes together with an externally applied bias for extracting the hot electrons. Here we present a new bimetallic plasmonic MIM device, where the work function dierence between the two metals gives rise to a built-in electrical eld, eliminating the need for an externally applied bias. In this paper we investigate both the device current response and the optical properties.

Results and discussion Figure 1 illustrates the working principle of a bimetallic plasmonic MIM device. The nanostructured front electrode consists of gold, the back electrode is of aluminum, and Al2 O3 is used as the insulator layer. Aluminum and gold have been picked because they have a relatively high dierence in work function, and because aluminum naturally forms a native oxide layer. Four devices were fabricated with varying insulator layer thickness; 3, 8, 13, and 18 nm. For each device, 5 identical plasmonic MIM structures were made resulting in a total 3


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of 20 structures investigated. The sample fabrication is described in the methods section. Each individual structure consist of 25 nanorods, 10 µm long, 52 ± 7 nm wide, with 400 nm pitch. The design dimensions were chosen so that the (transverse dipole) gap-plasmon modes occur within the bandwidth of our photocurrent measurement setup (450-690 nm). The contribution from longitudinal modes is known to be negligible and to occur outside this bandwidth. 8,16 A red shift of the LSPR occurs for gap-plasmons upon decreasing gap size. 1719 The nanorods are slightly wider for structures fabricated on thin insulator layers, and slightly thinner for rods fabricated on thick insulator layers, which may also contribute to a redshift of the LSPR for the thinner devices. The nanorods serve as the front electrode, and are contacted via a larger gold pad, see diagram in Fig. 1, and SEM image in Fig. 2a. The back electrode consists of a 50 nm aluminum lm, resting on a high resistance silicon substrate (> 600 Ω/cm).

ϕAu

χAl2O3

φb1

φb2

EF Au

ϕAl

Al2O3

EF

λ

Au Al2O3

Al Si

Al

Figure 1: Left: Energy band diagram of a bimetallic MIM device. The work function (Φ) dierence between the two metals, here gold (Au) and aluminum (Al) leads to a potential dierence ϕAu − ϕAl , without external bias. ϕAu and ϕAl are the barrier heights for gold and aluminum respectively. Right: A 3D drawing of the MIM design used here (not to scale).

4


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a

b -6 -7 -8

log(abs(Current)) [log(A)]

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100 nm

-9 -10 -11 -12 -13 -14 -15 -16

10 m

-17 -1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Voltage [V]

Figure 2: (a) SEM image showing the fabricated gold nanorods, which are approximately 52 nm wide and have a 400 nm pitch, together with the contacting gold pad (large square). The gold nanorods are fabricated directly on top of the Al2 O3 lm. Note also that the underlying aluminum lm appears to be relatively rough (see high magnication inset). (b) Dark I-V curve of the device with 3 nm Al2 O3 . The asymmetry is a consequence of the two metals having dierent work functions. The devices were initially studied by performing current-voltage (I-V) measurements. Figure 2b shows the I-V curve of the thinnest device, which has a 3 nm Al2 O3 layer. The device exhibits a slight nonlinear diode response, where the built-in work function dierence (see Fig. 1) gives rise to an asymmetry around x = 0 V. 20 However, a direct comparison with a theoretically predicted MIM diode response (Simmons 20,21 ) yields unphysical values for the barrier heights. The experimental I-V measurements seem to be a hybrid between diode response (exponential) and Ohmic response (linear). We contribute this to the relatively high surface roughness of the deposited aluminum layer creating a rough Al2 O3 layer, with the possibility of pinholes and/or direct aluminum/gold contact in some places. For this reason experimental values for the barrier heights could not be obtained. In a second experiment we measured the current response as a function of the illumination wavelength, see the description of absorption and photoccurrent measurements in the methods section for details. The results are presented in Fig. 3. The photocurrent responsivity is dened as: 5


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responsivity (λ) =

IAu − IAl P

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(2)

where P is the illuminating power of the unpolarized light source, IAu is the photocurrent measured when illuminating the gold nanorods and IAl is the photocurrent measured when illuminating only the back electrode (Al2 O3 /aluminum lm). The photocurrent is measured once on each structure, resulting in ve IAu measurements per device, and twice for the aluminum back electrode, resulting in two IAl measurements per device. All measured photocurrents IAu and IAl for the devices are presented in Fig. 3a. The averaged responsivity for each device (Eq. 2), where IAl serves as the background signal (see Fig. 3a), are presented in Fig. 3b.

Figure 3: (a) Measured photocurrent for the bimetallic plasmonic MIM devices. The dimension refers to Al2 O3 thickness of 3, 8, 13 and 18 nm. The Au/Al notation refers to which electrode was illuminated when the measurement was taken. (b) Responsivity (R) where the background current has been subtracted (left axis), and normalized Dark-eld (DF) spectroscopy (right axis) of the gold nanorods, revealing the LSPR induced current response. The curves are oset for clarity, and the y axis show zero line and peak values. In a nal experiment Dark-eld (DF) spectroscopy was used to measure the LSPR wavelength for the nanorods (see methods for instrument information). The results are presented 6


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in Fig. 3b, together with the responsivity. The LSPR wavelength shifts with increasing Al2 O3 thickness, from around 680 nm to 520 nm due to the change in separation between the nanorods and aluminum lm. 1719 When the LSPR decays, hot electrons may be generated. The measured hot electron responsivity in Fig. 3b is also showing a shift, and peaks at the same wavelength as the optical LSPR, conrming that the measured current response is due to LSPR in the nanostructured electrode. One thing to note from Fig. 3b is that the responsivities are all negative. This is due to the photocurrent being larger when illuminating the aluminum back electrode than when illuminating the gold nanorods, which is shown in Fig. 3a. For an MIM device a photocurrent can be induced in both electrodes. 22 Hence our device performance is inuenced by the photocurrent generated in the back aluminum electrode, which for clarity we refer to as Al-lm electrons in the remainder of this text. The built-in electric eld will favor electron transport from gold to aluminum (see Fig. 1). However, the light that is not scattered or absorbed by the gold nanorods will reach the aluminum electrode, and be either reected or absorbed there. The absorbed light in the aluminum results in a relatively large background current of Al-lm electrons moving to the gold electrode. The hot electron current generated in the gold nanorods is therefore evident as a decrease in the measured photocurrent at and near the LSPR frequency. Aluminum has an increased optical light absorption centered around 820 nm due to interband transitions. 23,24 In Fig. 3a, the photocurrent of the three thinnest devices can be seen to increase from around 600 nm to the absorption peak at 820 nm, though the spectral bandwidth of our measurement system cuts o at 690 nm. The thickest device shows no response, being too thick to support tunneling at zero-bias. The slight increase in photocurrent for all devices towards the blue in Fig. 3a can be attributed to Al-lm electrons excited in the aluminum electrode reaching energies exceeding the potential barrier, thus allowing electrons to go over it. 11 At wavelengths far away from the LSPR, no responsivity from the nanorods is expected. 7


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However, Fig. 3b shows a non-zero response for the devices with 3 nm and 8 nm Al2 O3 thicknesses, also away from the LSPR peak. This dierence is believed to be due to the small geometrical surface area coverage (approximately 14 %) of the gold nanorods, resulting in non-LSPR-related scattering of a small portion of the incident light across the full spectrum investigated. This leads to a non-zero responsivity. The LSPR induced hot electron responsivity is found to be in the µA/W range. The highest value is approximately 2.2 µA/W, yielded by the device with 3 nm Al2 O3 thickness. The type of structure presented here is limited due to their large time constants. The responsivity can potentially be improved by decreasing the resistance and capacitance. 9 The surface roughness of the back electrode aects the resistance, particularly in the case where the thin insulator layer only consisted of the native oxide. The capacitance can be decreased by decreasing the gold electrode area.

Figure 4: Optical and electrical response of the bimetallic plasmonic MIM devices as a function of insulator (Al2 O3 ) layer thickness. The LSPR peak wavelength is measured using DF spectroscopy (red diamond, measured once on each of the ve structures, then averaged), and simulated using FDTD method (green cross). Responsivity (blue circle) at the LSPR wavelength (from Fig. 3b) is given on the right axis. The wavelength at peak responsivity (black square) coincide well with the LSPR peak measured with DF spectroscopy. Note that for the 18 nm Al2 O3 , no wavelength at peak responsivity is given since it showed no response. Figure 4 show the LSPR peak wavelength measured with Dark-eld spectroscopy, to8


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gether with the hot electron responsivity peak wavelength from Fig. 3b. The optical response of the gold nanorods was simulated using a commercial nite dierence time domain (FDTD) software package (Lumerical inc.) and is included in Fig. 4. Simulation details are given in the methods section. The refractive index for nanometer scale thicknesses of Al2 O3 has been reported in the literature with varying values. 25,26 Here the best t was achieved with the value 1.4. The simulation is done with a perfectly at aluminum lm, where as the actual devices have a comparatively rough surface, as can be seen in gure 2a. This is presumably the reason for the dierence between experiment and simulations for the device with the thinnest Al2 O3 layer (3 nm). Lumdee et al. 27 and Hajisalem et al. 28 report a distinct blue shift of the resonance peak with increasing surface roughness, in agreement with what is observed here. Figure 4 also shows that the responsivity (blue circles, right axis) increases with decreasing Al2 O3 thickness, as predicted by tunneling theory. 29 A similar relation between hot electron extraction and insulator thickness has been shown by Lee et al. for a Au-TiO2 -Ti plasmonic MIM device. 13 The surface roughness of the aluminum back electrode limits the electrical performance of the device due to dielectric imperfections and local variations in insulator thickness. 30

Methods Sample fabrication Four devices were fabricated. The design is shown in Fig. 1 and 2a. On each device, there are ve identical plasmonic structures consisting of gold nanorods with a width of 52 ± 7 nm, and a pitch of 400 nm. The measured variation in rod width is most likely due to proximity eects. A gold pad was patterned in the same lithography step as the gold nanorods serving as one of two electrical contact points for the structure, here our top electrode. Aluminum was chosen as the other electrode material because it naturally forms a thin, uniform native 9


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oxide layer (Al2 O3 ) on the surface, approximately 3 nm thick 31,32 (4 nm if heated). The Al2 O3 serves as the insulator between the two electrodes. The rod size and pitch was chosen so that the absorption peak would be found within the range of the tunable laser used for photo-excitation. The device substrate was a [100] n-type silicon wafers with relatively high resistance (>600 Ω/cm) to prevent recombination of electron hole pairs. The wafers were coated with 50 nm aluminum (bottom electrode) deposited using a SC2000 Dual Evaporator (EBE) from Semicore. Any potential Schottky barrier between the silicon and aluminum would be equally present in all devices, having therefore the same impact on the current for all devices. Additional Al2 O3 was added using atomic layer deposition (ALD) (Cambridge Ultratech Savannah 200 ALD), in order to vary the insulator layer thickness. The devices went through 0, 45, 90 or 135 cycles, resulting in 3, 8, 13, and 18 nm Al2 O3 respectively (± 1 nm for the 3-13 nm layers, ± 2 for the 18 nm layer), where the given thickness include the 3 nm native oxide layer. The Al2 O3 layer was measured using a Filmetrics thin lm analyzer F20-UV (TFA). Standard Electron Beam Lithography (EBL) procedures were used to fabricate the devices. The gold nanorods and pad were patterned into poly(methyl methacrylate) (PMMA) using an FEI Quanta 650 FEG EBL system, at 30 kV acceleration voltage. After developing the pattern, a 35 ± 2 nm layer of gold was deposited using the same EBE as for the aluminum layer. The choice of thickness is the result of a trade-o between higher absorption for a thicker electrode, and allowing more hot electrons to reach the insulator barrier for a thinner electrode. The mean free path for hot electrons in gold is ranges from 25 nm for 2.0 eV to 70 nm for 1.0 eV. 33 A thicker gold electrode is preferred to avoid it being damaged by the needle probes. A lifto step was used to remove the excess PMMA and gold. Finally, a small amount of indium was soldered onto the silicon while scratching its surface. This was done to achieve an optimal electrical contact between the silicon/aluminum electrode and the lock-in-amplier (LIA) or source meter, which records the current. The 10


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scratching removes the thin native oxide layer which has formed on the silicon surface when exposed to air, improving contact. 34 The devices were electrically isolated from the surroundings by placing them on glass slides. The optical response of the structures was simulated using the commercial FDTD software Lumerical inc. A 2D simulation space of 2 x 2 µm2 with perfectly matched layer boarders holds a 52 nm wide, 35 nm tall gold square (2D rod), resting on a layer of Al2 O3 . The thickness of this layer was varied between 3 and 18 nm at 1 nm increments, the width of this layer spanned the full 2 µm of the simulation space. Below the Al2 O3 layer was a 50 nm thick aluminum layer, followed by a silicon layer, which stretches to the limits of the simulation space. A total-eld scattered-eld light source was dened to inject a plane wave from above the rod, with transverse electric (TE) polarization (electric eld perpendicular to the rods). The same plane wave was subtracted behind the structure, leaving only scattered light beyond the dened light source. A rectangular monitor placed within the light source records the incident light on it's external border, and subtracts the transmitted and scattered light it receives on its inside border, leaving the net dierence, i.e. absorption. Only near eld eects are simulated using this approach, due to the placement of the monitor. Both the light source and monitor encompasses the rod structure completely, and was chosen to be 200 nm and 180 nm wide respectively. The nanorods on our experimental devices are far enough apart to avoid coupling eects, so only one rod was simulated. The material refractive indices were taken from the software database, except for Al2 O3 , which was set to be 1.4. The mesh size was set to be 1 nm both in x and y direction. The absorption was recorded between 450 and 690 nm.

Absorption and photocurrent measurements The LSPR absorption peak was determined using a Zeiss Axiovert 200 MAT microscope with Dark-eld objectives and a Princeton Instruments Acton SP2150 Spectrograph for Dark-eld spectroscopy. Five spectra were taken for each device, one on each structure, and the spectra 11


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were background corrected. The opto electronic setup for measuring the device current response consisted of a tunable white light source (Fianium WhiteLase) in combination with a Fianium acousto-optic tunable lter (AOTF) for selecting specic wavelength bands. The tunable laser was unpolarized, and had a scan range from 450 to 690 nm. The light was focused onto the structures using a Mitutoyo NIR objective. Two nickel plated tungsten pico-probes from GGB industries were used to contact the device to a source meter or LIA in order to measure the current. The light source was directed through a chopper and the photocurrent was measured using a LIA (Signal Recovery 7280). A similar setup was used and described by Zheng et al. 8 One probe was touching the gold pad, and the other was touching the indium. This was done to minimize damage to the aluminum lm, and was possible because the LIA is exceptional at detecting low signals even at high noise levels. For the I-V measurements, the LIA could not be used, as the applied voltage interfered with the LIA signal processing. A Keithly 2400 source meter had to be used for those measurements. To decrease the noise of the I-V measurements, and to eliminate any potential eect of the Al/Si barrier, the probe (originally in contact with the indium contact) was moved closer to the structure, and placed directly on the back aluminum electrode. The nanorods on the respective structures were illuminated and the photocurrent was measured. Then a region with only aluminum/Al2 O3 in the vicinity of the nanorods were illuminated and measured, giving the background signal. From these measurements the photocurrent generated only by the gold nanorods (i.e. responsivity) could be calculated. For each device the ve identical structures were illuminated and measured once each, and the aluminum lm was measured at two places in vicinity of one of the gold nanorods. The laser power P was measured for all wavelengths using a Thorlabs PM100 power and energy meter, so the responsivity could be calculated as described in Eq. 2.

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Conclusion We present a new bimetallic plasmonic MIM device, where an electric eld is provided by the work function dierence between the two electrodes, thus eliminating the need for an external bias. We have investigated the hot electron generation for dierent insulator layer thicknesses. A maximum responsivity of 2.2 µA/W was obtained for an insulator thickness of 3 nm. To access the possibilities of this new type of device, a detailed exploration of the parameter space in terms of materials, sizes and shapes of plasmonic structures is required. The responsivity can potentially be improved by making larger/more eective surface coverage designs. The surface roughness of the back electrode also aects the optical and electrical response, particularly where the thin insulator layer only consisted of the native oxide. We propose that further investigations to be carried out should involve using metals with higher work function dierence, such as platinum (instead of gold) which has a reported workfunction of 5.65 eV. 35 This will increase the internal potential dierence. Metal combinations where the LSPR will increase the photocurrent instead of decreasing it should also be explored. Potential applications for our new device includes lightweight electronics, and power generation for remote sensors, where the zero-bias capability presented here will be of great advantage.

Acknowledgement We thank Samuel Gottheim for his assistance. This research was nancially supported by the University of Bergen and the Army Research Oce under Grant W911NF-12-1-0407.

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Thin

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Appl. Phys.

ACS Photonics

ϕAu 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

χAl2O3

φb1

φb2

EF Au

ϕAl

Al2O3

EF

Al ACS Paragon Plus Environment

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λ

Au Al2O3

Al Si

a

b -6

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-7 -8

log(abs(Current)) [log(A)]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

ACS Photonics

100 nm

-9 -10 -11 -12 -13 -14 -15 -16

10 m

-17 -1 -0.8 ACS Paragon Plus Environment

-0.6

-0.4

-0.2

0

0.2

Voltage [V]

0.4

0.6

0.8

1

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Work function-driven hot electron extraction in a bimetallic plasmonic

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