Tuning of Plasmons in Transparent Conductive Oxides by Carrier

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Tuning of Plasmons in Transparent Conductive Oxides By Carrier Accumulation Xiaoge Liu, Juhyung Kang, Hongtao Yuan, Junghyun Park, Yi Cui, Harold Y. Hwang, and Mark L. Brongersma ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01517 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 18, 2018

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Tuning of Plasmons in Transparent Conductive Oxides By Carrier Accumulation Xiaoge Liu1§, Ju-Hyung Kang1§, Hongtao Yuan1,2,3, Junghyun Park1, Yi Cui1,2, Harold Y. Hwang1,2, Mark L. Brongersma1* 1. Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, USA. 2. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA. 3. National Laboratory of Solid-State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China. §

These authors contributed equally to this work. *Corresponding author: [email protected]

Abstract: A metal naturally displays dramatic changes in its optical properties near the plasma frequency where the permittivity changes from a negative to a positive value and the material turns from highly reflective to transparent. For many applications, it is desirable to achieve such large optical changes by electrical gating. However, this is challenging given the high carrier density of most metals that causes them to effectively screen externallyapplied electrical fields. Indium tin oxide (ITO) is a low-electron-density metal that does afford electric tuning of its permittivity in the infrared spectral range. Here, we experimentally show the tuneability of the plasma frequency of an ITO thin film by changing its sheet carrier density via gating with an ionic liquid (IL). By applying moderate gate bias values up to 1.4 V, the electron density increases in a thin (~3 nm) accumulation layer at the surface of the 15-nm-thick ITO film. This results in notable blue shifts in the plasma frequency. These optical and electrical changes are monitored simultaneously, which facilitates construction of a model that provides a consistent picture for the DC electrical and infrared optical properties. It can be used to quantitatively predict the optical changes in the ITO layer with applied bias. This work builds our understanding of electrically-tunable plasmonic materials and aids the design of ultra-compact, active nanophotonic elements. Table of Contents Graphic:

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Keywords: Tunable plasmonics; electrical gating; transparent conductive oxides; surface plasmon polariton; Over the last decade, there is an increasing desire to realize active nanophotonic devices such as ultra-compact optoelectronic modulators1–5 and dynamically reconfigurable nano-antennas6 and metasurfaces7,8. Downscaling of conventional active elements to a subwavelength footprint is challenging as one cannot benefit from long interaction lengths to achieve a desired functionality.1,9 Therefore, new electro-optic materials with very strong electro-absorption or electro-refraction effects need to be developed. The application of plasmon resonances in metal nanostructures has been used to boost electro-optic effects by concentrating light to a deep subwavelength scale.10 This can facilitate an excellent mode overlap between light and an active medium. However, the observation of electro-optic effects in plasmonic metals themselves has been challenging as materials such as Au and Ag feature very high electron densities and this results in effective screening of externally applied electrical fields.11–13 Transparent conductive oxides (TCOs), such as indium tin oxide (ITO), have attracted much attention for their practical application in solar cells, photodetectors and touchscreens, and more recently for their electrically-tunable plasmonic properties in the near-IR spectral range.14,15 ITO behaves as a Drude free-electron metal with a permittivity that can be controlled by changing the carrier density: = −

, =

∗

.

(1)

Here, ε∞ is the background dielectric constant, is the frequency of light, and Γ is the collision frequency. The plasma frequency ωp of ITO depends on the carrier concentration n, electron charge e, vacuum permittivity ε0 and effective mass m* of electrons inside ITO. Since the electron density of ITO (∼1020 cm−3) is much lower than noble metals (∼1023 cm−3), electrical gating can provide very large changes in their permittivity in the IR regime16,17. For this reason, ITO can serve both as a plasmonic material for light concentration and as an active electro-optic material in active nanophotonic devices. In this study, we demonstrate active tuning of the electro-optic properties of thin ITO films through carrier accumulation by electrical gating with an ionic liquid (IL). This study complements a previous study focused on the case of carrier depletion18 and


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elucidates different physics. We then use a series of combined electrical and optical measurements on these films during gating to construct a consistent model for the DC electrical and infrared optical properties.

Figure 1 | Experimental setup for electrical transport measurements and surface plasmon polariton excitation on an ITO film. a, Schematic diagram of the measurement setup used to simultaneously measure the changes in the optical and electrical properties of ITO upon IL gating. A high refractive index (n = 2.4) ZnSe lens is used to evanescently couple a free space beam to a surface plasmon-polariton (SPP) supported by the ITO/Au film. The IL is introduced between the lens and ITO film to electrically gate and accumulate carriers in the surface region of ITO. A Hall bar pattern allows for in-situ transport measurements and ex-situ Hall measurements. Inset shows an optical image of a fabricated sample placed in a chip carrier. b, Color map of the simulated light reflectance versus wavevector β and frequency ω overlaid with light lines of ZnSe for an incident angle of 45o (blue dash-dotted line), IL light line (red dash-dotted line) and the dispersion relation of the SPP supported by the IL/ITO/Au stack (green solid line).

Figure 1a schematically shows the experimental setup that we use to simultaneously measure the changes in the electrical and optical properties of ITO films upon electrical


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gating. We pattern a Hall bar onto the ITO film to facilitate Hall and 4-point probe resistivity measurements. We also monitor the changes in the optical behavior of the ITO film with applied bias using a Fourier transform infrared (FTIR) microscope.19,20 When a gate voltage is applied between the Au gate electrode and the ITO film, the mobile cations and anions in the electrolyte will move towards the oppositely charged electrodes to form electric double layers and produce electrical fields that can either accumulate or deplete electrons in the ITO surface region, depending on the sign (±) of the voltage.21,22 Since the modification in electron density only occurs in a thin (few nm) layer near the surface of the ITO film, a direct observation of the concomitant changes in optical properties can be challenging. We address this challenge in two ways. First, we use a relatively thin film so that the accumulation layer will represent a non-negligible fraction of the total film thickness. Second, we exploit surface plasmon polariton (SPP) excitations supported by the ITO/Au layers to enhance the interaction with the ITO film and to increase the sensitivity to any optical changes. To excite an SPP, we use an attenuated total reflection (ATR) configuration with a ZnSe prism (Fig. 1a).20 The IR light entering through the prism at 45° is totally internally reflected as the incident angle is beyond the critical angle. This produces an evanescent field that overlaps with the SPP mode supported by the ITO surface. At an excitation frequency where the SPP is excited, energy transfers from the excitation beam to the SPP and a reduction in the reflected light intensity is observed. The excitation can be analyzed in more quantitative terms by calculating the dispersion relation (angular frequency versus the parallel wavevector β ) in the IR spectral region for the IL/ ITO/Au/quartz multilayer stack (green line in Fig. 1b). The color map in Fig. 1b shows the reflectance calculated by the transfer matrix method (TMM) at different and β (See Methods). The presence of a low-loss ITO layer induces an oscillation in the SPP dispersion relation in the near-IR range at the surface plasmon frequency ωsp. Here, the value of ITO’s permittivity is about equal in magnitude, but opposite in sign to the permittivity of the IL (ε ITO = -ε IL). As a result, the SPP dispersion relation closely approaches the excitation beam’s light line (blue dash-dotted line in Fig. 1b). Although the two lines do not cross, the optical loss in the ITO broadens the SPP dispersion relation (yellow region in the color map of Fig. 1b)


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and allows its excitation. It leads to a noticeable dip in the reflection spectrum near ωsp (red curve of Fig. 1a). For light beams with a smaller incident angle, the light line tilts up. When the light line is above the IL light line (red dash-dotted line in Fig. 1b), the reflection dip will occur at a higher frequency where εITO features a zero crossing.15,19 In the inset to Fig. 1b, the dispersion relation is shown for a larger frequency range that also includes the surface plasmon resonance frequency for the Au film at much higher frequencies. Given the very high surface plasmon frequency of Au, it serves as a very good mirror in the IR range and prevents light from leaking into the quartz substrate.20

Figure 2 | Tuning of the optical and electrical properties of ITO by IL gating. a, Measured ATR spectra with applied bias of 0 V (red curve) and 1.4 V (blue curve) in the IR region. b, Sheet carrier densities (black curve) and carrier mobility (red curve) of the ITO film against gate voltage Vg, determined from the Hall measurements, as well as the optimum fit for the mobility (blue curve) of ITO film with the approximate two-layer model described in the text.

Figure 2a shows the experimental ATR spectrum for a 15-nm-thick ITO film deposited by magnetron sputtering and a post-anneal in forming gas (See Methods). The film displays a less than 1 nm Root Mean Square roughness as determined by atomic force microscopy (AFM). The ATR spectrum taken at 0 V gate bias shows a reflection dip near 3.9 µm (red line) that corresponds to the surface plasmon frequency of the as-fabricated ITO film. When we apply a gate bias of 1.4 V, a new spectral feature appears near 2.3 µm and the reflectivity at this wavelength is decreased (blue arrow). At the same time, the reflectivity near 3.9 µm increases. We will show that these observations are consistent with


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the formation of a thin accumulation layer that features a higher surface plasmon frequency and a reduced layer thickness in which the original carrier density is maintained.16 To explain the gating-induced changes in the optical response based on changes in the carrier mobility, density and distribution, we also performed measurements of the electrical properties of the same ITO film. Figure 2b shows the measured sheet carrier density σ0 (black line) and carrier mobility µ (red line) for various applied voltages. At VG = 0 V, the sample features a sheet carrier density of σ0 = 2.0 × 1014 cm-2 and its magnitude linearly increases to 2.9 × 1014 cm-2 as we increase the gate voltage VG to 1.4 V. Meanwhile, the mobility decreases from 18 cm2/Vs to 16 cm2/Vs. Such a decrease can be expected as the electrons are pulled towards the ITO interface where they may undergo increased interface scattering. These type of changes have been observed before in studies of fieldinduced carrier accumulation in field-effect transistors.23,24 Inspired by these works, we apply a simple two-layer model to try and understand the changes in the mobility in the ITO film. In this model, we divide the film into an accumulation and bulk layer that can have different carrier densities and mobilities. The total sheet carrier density is given as the sum of the sheet carrier densities of the accumulation and bulk layers: NT = NA + NB and the effective mobility is given by: µT = (µANA +µBNB) /(NA + NB). Here, NT, NA and NB stand for the sheet carrier density of the total film, accumulation layer and bulk layer, while µT, µA and µB are the effective mobility of the total film, accumulation layer and bulk layer. One possible explanation for the lowered mobility with carrier accumulation is an increased interfacial scattering in the accumulation layer due to the presence of roughness or interfacial traps. We fitted the experimental dependence of the effective mobility versus gate bias using the thickness of the accumulation layer tA, the electron mobility of accumulation layer and the electron mobility of the bulk layer as free parameters. The best fit (blue line) was obtained for tA = 3 nm, µA = 13 cm2/Vs and µB = 19.5 cm2/Vs. The mean free path of electrons l in the ITO sample assuming the bulk carrier density of 1020 cm3 can be estimated as the product of the Fermi velocity and collision time as = .

25

Given the measured value of the mobility for the accumulation layer, we find l ≈ 2 nm. This value is comparable to the estimated thickness of the 3-nm-thick accumulation layer in the two-layer model. It thus seems reasonable that the thin accumulation layer can indeed


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display a reduced mobility as compared to the bulk of the film. With the parameters obtained from electrical measurements, the optical properties of the accumulated and bulk ITO layers can be calculated assuming a Drude-like optical response with =

Γ= =

!∗

∗

,

at each bias value, taking the effective mass m* at the considered carrier

densities as 0.4 per reference 20. Using these optical properties, we can then simulate the reflection spectra with a TMM calculation and compare them to the experimental spectra.

Figure 3 | Experimental and simulated attenuated total reflection (ATR) spectra of a 15-nmthick ITO film under electrical gate bias. a, Measured ATR spectra of IL/ITO/Au/quartz stack films taken for 8 different gate bias values and with 0.2V increments (top panel). Simulations with the two-layer model (middle panel) and with the multi-layer model (bottom panel) for these bias values are shown for comparison. Insets illustrate the carrier density distribution assumed inside the ITO film for the two-layer and multi-layer model described in the text. b, Normalized differential reflectance spectra (RVg – R0V)/(1-R0,min) of the sample for experimental results (top panel), two-layer model simulation (middle panel) and multi-layer model simulation (bottom panel). RVg is spectrallydependent reflectance at a given applied gate voltage and R0V is the spectrally-dependent reflectance at a zero gate voltage. R0,min is the minimum spectral reflectance curve at zero gate voltage.


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The top panel in Fig. 3a shows measured ATR spectra taken at different applied bias values as the bias is increased from 0 V with a 0.2 V step size. The middle panel shows the transfer matrix simulations based on the two-layer model approximation. The inset to the middle panel shows the carrier density profile in which we depict the low-mobility accumulation layer in a darker color. The simulations qualitatively agree with the experimental observations as the reflectivity increases near 3.9 µm and decreases near 2.3 µm with similar magnitude changes in the reflectivity. However, the simulations show a pronounced second reflection dip that gradually appears near 2.3 µm, which is not seen in the experiments. This dip can be ascribed to a blue-shifted surface plasmon resonance originating from the accumulation layer that features a higher carrier density and plasma frequency as compared to the bulk. The uniform carrier density assumed in the two-layer model results in the pronounced dip and obviously oversimplifies the real carrier distribution inside the accumulation layer. In an attempt to better reproduce the experimental spectra, a multi-layer model for the carrier density distribution is analyzed next. The bottom panel of Fig. 3a shows the simulation results for the multi-layer model in which the 15-nm-thick ITO film is now divided into 50 layers that can each have a different carrier density. We again link the optical properties of each layer to the local carrier density via the Drude model. The inset illustrates the carrier density profile calculated using the modified Thomas-Fermi approximation (MTFA) method18,26. In an approximate fashion, the MTFA takes account of the quantum-mechanical influence of an infinite potential barrier at the surface and has been shown to be in excellent agreement with full self-consistent Poisson-Schrödinger solutions for semiconductors with surface band bending in nanometer scale26. (See Supplementary Section 2). (See Supplementary Section 2). The calculated carrier density profile of ITO film displays a peak that is slightly inward from the surface. The darker color again denotes the lowered mobility in the approximately 3-nm-thick accumulation region near the ITO film surface, similar to the two-layer model. The integration of the carrier density along depth equals the sheet carrier density obtained by the Hall measurements. The simulated ATR spectra for the multilayer model do not show the


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pronounced dip that was present in the two-layer model and display improved quantitative agreement with the experiments. The experimental reflection equals one minus absorption (Reflection = 1 - Absorption), since there is no transmission going through the underneath Au reflector. The reflection spectrum with no bias is a result of the absorption in the ITO film with surface plasmon resonance around 3.9 µm. By applying a bias, the accumulation layer is formed and the reflection spectrum with bias then results from the absorption in both the accumulation layer and the bulk layer. The bulk layer remains at the original carrier density and induces the absorption at 3.9 µm. This absorption is reduced as the bulk layer thickness is decreases. Thus, the reflection spectrum shows higher reflection around 3.9 µm with increasing bias. The accumulation layer is comprised of multiple sub-layers with different carrier density and thus different surface plasmon resonance wavelengths. For this reason, it impacts the reflection spectrum over the entire short-wavelength size of the spectrum and all the way up to the original reflection dip location at 3.9 µm. As a result, the spectrum ends up broadening and showing a new, shifted reflection minimum at a shorter wavelength of 3.7 µm for a 1.4 V gate bias. However, if the ITO film is gated with even higher voltages to shift the absorption peak to even shorter wavelengths or if the materials loss in the ITO film could be reduced, the reflection dip around 2.3 µm would have been more pronounced and more separated from the 3.9 µm reflection dip. In that scenario, the lowest reflection point would have remained unaffected at 3.9 µm. For practical applications, it is of interest to further analyze the achievable reflectivity changes with applied bias.27–29 For this purpose, we plot the normalized differential reflectance (RVg – R0V)/(1–R0,min), which is proportional to the difference between the reflectance found at a gate bias Vg and the zero-bias reflectance R0V. The quantity (1 - R0,min) is the maximum absorption at zero gate voltage which indicates the energy dissipation by the surface plasmon resonance inside the ITO film. Figure 3b presents the normalized differential reflection change spectrum, which highlight the reflection changes that result from the formation of an accumulation layer. The top panel shows the experimental results for applied bias values up to 1.4 V. A significant reflection reduction of up to 18% near 2.3 µm is achieved and can be linked to the increased carrier density in a small accumulation layer. The formation of the accumulation layer also reduces


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the thickness of the layer in which the original bulk carrier density is maintained. This explains the reduction in the reflectance by 11% at the surface plasmon resonance around 4 µm, where the bulk film displayed its surface plasmon resonance. In the middle panel, the simulations with the two-layer model capture the basic features of the experimental observation but clearly display disagreement at the short wavelength side of the spectrum for larger Vg. In the bottom panel, the simulations with the multi-layer model show a satisfactory reproduction of the experiment results.

In summary, we successfully observed electrical tuning of the plasmonic properties of a thin ITO film by gating. As carriers accumulate at the surface of the film, a blue-shift in the reflection spectrum can be observed. The basic features can conceptually be understood using a simple two-layer model, in which an accumulation layer is formed with a higher carrier density and a reduced mobility. However, a multi-layer model is needed to accurately capture the real carrier distribution in the ITO film and to reproduce the experimentally observed reflectivity changes. The good agreement between the experiment and simulation justifies the extraction of valuable parameters that in a consistent manner can link the DC electrical properties to the infrared optical properties of ITO. This study augments our understanding of the electrical-controllable plasmonic properties of transparent conductive oxides and opens up the possibility of designing ultra-compact optoelectronic modulator by employing the strong electro-optical effects in these materials.


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Methods Sample preparation: A standard photolithography and lift-off procedure was performed on a quartz substrate to fabricate the 50-nm-thick Au structures used in the optical reflection measurements and used as electrical contacts for Hall measurements. A 5-nmthick titanium adhesion layer was used below the Au structures and both metals were deposited using an electron-beam evaporator. The 15-nm-thick ITO film was then deposited by a magnetron sputtering process using an ITO ceramic target of In2O3 90 wt % /SnO2 10 wt% (Heraeus, Inc.) with Ar and O2 flow rates of 80 sccm and 10 sccm, respectively. The sputtering power was 2 kW at 6.5 mTorr and the substrate temperature was kept below 80 °C. A post-annealing process with forming gas (5% H2 / 95% N2) was performed to tune the carrier concentration of the ITO film.20 The sample was annealed at a temperature of 180 °C for 30 min. Electrical experiment setup: The fabricated sample was put into a chip carrier during the experiments (The top left insert of Figure 1a). The bottom metal plate of the chip carrier was used as a counter electrode (The Au pad connected to Vg in Figure 1a). The IL was dropped on the sample covering both the ITO film and the underneath metal plate. We used the

well-etsbalished

ionic

liquid

(IL):

N,

N-diethyl-N-(2-methoxyethyl)-N-

methylammonium bis-(trifluoromethylsulfonyl)-imide (DEME-TFSI), to electrically gate the ITO film. A semiconductor parameter analyzer (Agilent Technologies B1500A) was used to apply the gate bias on the counter electrode and ITO film as well as measure the charge transport characteristics of the devices through a 4-point probe resistivity measurement on the hall bar pattern18,22,30. The switching time of these devices is restricted by the speed of ion motion inside the IL, which was on the order of seconds. However, the switching speed of optical modulators may ultimately limited by the motion of majority carriers inside ITO and GHz should be feasible as for other majority carrier-based devices31. Practically, the speed of majority carrier devices is often limited by the resistances and the capacitances of the


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device/system27,32. Recently we achieved ITO-based switching devices that can operate at RC-limited switching speeds of 125 kHz 27. Avoiding chemical reactions during IL gating: Previous studies with ILs reported on the possibility to induce electrosorption processes or interfacial electrochemical reactions related to the presence of impurities in the IL, such as H2O 13,33. Such processes complicate the data analysis and we have analyzed the possible importance of such undesired processes in our measurements. One common approach is to measure the gate current through the IL. It was shown that when gate bias was more than 1.5 V, a sudden increase in gate current occured18. Below a critical threshold voltage Vthresold ≈ 1.5 V, the resistance of the IL was high, indicative that no undesired processes result in a measurable current. For the measurements in the main text, we kept the magnitude of the applied electrical bias below 1.4 V. Optical experiment setup: Reflectance measurements were made using a Fourier transform infrared microscope (Thermo Scientific). A ZnSe lens with non-polarized light incident within an angular range from 40° to 50° was used to perform the ATR measurement20. The sample with bonded wires was mounted on a piezo-stage to bring the ITO surface close to the ZnSe lens. The reflected light intensity from ITO sample was normalized to the total reflection intensity from Au area to get the reflectance spectra. The high transparency of IL(DEME-TFSI) in the infrared range up to 5 µm wavelengths provided a suitable optical window for ATR measurements19,20.

Optical simulations: Transfer matrix method (TMM) simulations were performed to calculate the ATR reflectance spectra. To match the experimental conditions, the simulations were performed for non-polarized light; the reflection spectra for both transverse electric (TE) and transverse magnetic (TM) polarizations were performed and averaged. The refractive indices of ZnSe and the IL in the simulation were taken as 2.4 and 1.4, respectively. ZnSe and quartz were taken as semi-infinite while the ITO and Au films were 15 nm and 50 nm thickness, respectively. The thickness of the IL was 220 nm as determined from our fitting routine for the reflectivity spectra. In the considered spectral


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range, the Au and ITO layers were modeled with the dielectric function of a free electron gas as Drude model. For ITO, the background dielectric constant ε∞ and the collision frequency Γ were set to be 3.9, and 1.0 × 1013 rad/s for Figure 1b. The carrier concentrations of ITO were obtained from the electrical measurement and effective mass varies from 0.35 to 0.53 depending on the carrier concentration20. For Au, the background dielectric constant ε∞, the plasma frequency ωp and the collision frequency Γ were set to be 12.99, 1.45 × 1016 rad/s and 1.11 × 1014 rad/s, respectively.

Acknowledgement: This work was supported by the Extreme Electron Concentration Devices (EXEDE) MURI program of the Office of Naval Research (ONR) through Grant No. N00014-12-1-0976 and an AFOSR MURI program through Grant No. FA9550-17-1-0002 program. H.T.Y. and Y.C. also acknowledge support by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under contract DE-AC0276SF00515.

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