Fabrication and Characterization of ZnO LangmuirâBlodgett Film and

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Fabrication & Characterization of ZnO Langmuir-Blodgett films and its use in Metal-Insulator-Metal Tunnel Diode Ibrahim Azad, Manoj Kumar Ram, Dharendra Yogi Goswami, and Elias K Stefanakos Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02182 • Publication Date (Web): 27 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016

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Fabrication &Characterization of ZnO Langmuir-Blodgett films and its use in Metal-Insulator-Metal Tunnel Diode

Ibrahim Azad1,2, Manoj K. Ram2, D. Yogi Goswami2, and Elias Stefanakos1,2* 1

Electrical Engineering Department, and 2 Clean Energy Research Center

University of South Florida, 4202 E Fowler Avenue, Tampa, FL 33620 *Corresponding author. E-mail: [email protected]

ABSTRACT: Metal–insulator–metal tunnel diodes have great potential for use in infrared detection and energy harvesting applications. The quantum based tunneling mechanism of electrons in MIM (Metal-Insulator-Metal) or MIIM (Metal-Insulator-Insulator-Metal) diodes can facilitate rectification at THz frequencies. In this study, the required nanometer thin insulating layer (I) in the MIM diode structure was fabricated using the Langmuir-Blodgett technique. The zinc stearate LB film was deposited on Au/Cr coated quartz, FTO and silicon substrates, and then heat treated by varying the temperature from 100 to 550 oC to obtain nm thin ZnO layers. The thin films were characterized by XRD, AFM, FTIR and cyclic voltammetry methods. The final MIM structure was fabricated by depositing chromium/nickel over the ZnO on Au/Cr film. The current voltage (I-V) characteristics of the diode showed that the conduction mechanism is electron tunneling through the thin insulating layer. The sensitivity of the diodes was as high as 32 V-1. The diode resistance was ~80 ohms (at a bias voltage of 0.78 V), and the rectification ratio at that bias point was about 12 (for a voltage swing of ±200mV). The diode response exhibited significant non-linearity and high asymmetry at the bias point, very desirable diode performance parameters for IR detection applications.

ACS Paragon Plus Environment

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INTRODUCTION In recent years, Metal–insulator–metal (MIM) structures have received considerable attention for applications in various electronic and optoelectronic devices such as energy harvesting high-frequency infrared detectors memory based devices

10, 11

4, 5, 6

, high frequency mixers

7, 8, 9

1, 2, 3

,

, and switching and static

. The MIM diode must ideally display current–voltage (I–V)

characteristics with high nonlinearity, high sensitivity, high asymmetry and low turn-on voltage 3

. The quantum based tunneling mechanism for electrons in MIM diodes facilitate rectification at

near infrared, visible light or ultra-visible light frequency

12, 13

. There are other diodes with fast

response time, such as Schottky diodes, but they are frequency limited to far infrared

14

. The

MIM diode consists of two metallic electrodes separated by an ultrathin (nm) insulating layer, a potential barrier being formed between the Fermi level of the metal electrode and the conduction band of the insulator. Electrons could tunnel through this potential barrier by applying a voltage across the electrodes 9. For optimal response and minimization of the parasitic diode capacitance, the insulating layer thickness must be ultra-thin, few nanometers15. Moreover, the insulating layer should be designed pinhole free for avoiding a short circuit between the conductors, and, furthermore, it should be uniform with very low surface roughness. These issues could be minimized by the careful fabrication of the insulating layer and design of the active diode area. The Langmuir-Blodgett (LB) deposition technique could be used to fabricate a closely packed pinhole free insulating layer. This technique can facilitate the fabrication of molecular films with a high degree of structural order using materials that produce y – configuration films 16, 17, 18. In this method molecules of amphiphilic nature first spread on the air/water sub-phase and then transferred onto the desired substrate through contact surface pressure and controlled barrier speed. Most of the reports related to the LB technique are studies of organic films. Sharma et al.


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characterized and explored the use of a polymerized 10, 12-pentacosadiynoic acid LB monolayer 19

as the insulator layer in the MIM diode

. Ram et al. used the LB deposition technique to

fabricate a polyaniline thin film as the insulator layer between silver and an ITO glass plate that exhibited

nonlinear

rectification

20

.

The

use

of

conductive

LB

films

of

bis(ethylenedioxy)tetrathiafulvalene as the top electrode of the MIM structure was studied by Mochizuki et al.

21

. Iwamoto fabricated MIM tunnel diodes using polyimide LB films and

studied their electrical properties over a range of temperature

22

, however, the thin LB organic

films could be easily damaged during the top metal deposition resulting in short circuited devices 19

. In this work, ZnO has been used as an insulating layer in the MIM structure. Gupta et al.

demonstrated ZnO as an insulating material in a MIM structure and presented capacitancevoltage (C-V) and conductance-voltage (G-V) measurements of that structure 23. Dutta et al. also showed the use of insulating ZnO films on silicon for the M-I-S application

24

. Ábrahám et al.

studied the effect of gold nanoparticles on photoluminescence property of ZnO film deposited by LB technique

25

. In a different work, interaction between cysteine or glutathione coated Au

nanoparticles and lipid membranes were demonstrated where Au particles were deposited by LB method

26

. Several methods have been reported previously about the fabrication of inorganic

ZnO film by LB technique

27, 28, 29, 30

. A procedure similar to that reported by Feng et al. was

followed to fabricate the ZnO multilayers in this work 31. ZnO layer in MIM diode structure has been used for high –frequency rectification at power transfer efficiency at 1 THz32. ZnO rectification abilities for resonant tunneling diode at high frequencies have been studied by Grover et al33 and Guziewicz et al34. In an effort to produce a MIM tunnel junction device that exhibits very high sensitivity, low resistance and high non-linearity, the synthesis of a thin film of inorganic ZnO has been carried


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out from an organic precursor, zinc acetate, using the LB method. Zinc acetate was used as the sub-phase, while the spreading liquid was a mixture of stearic acid and chloroform. The surface tension vs area isotherms of the zinc stearate monolayer at the air-sub-phase interface was studied to find the target surface tension for the LB deposited film. The monolayer formation parameters, such as concentration of the sub-phase and spreading volume, and deposition parameters, such as barrier speed, lifting speed, and wait time, were optimized to obtain a thin compact pinhole free monolayer. The zinc stearate monolayers were then deposited on different substrates such as glass, silicon and Au/Cr. The transfer ratio of the deposition process was monitored to observe the quality of the deposited multilayers. The multilayers of zinc stearate were then annealed at 300 °C for 0.25 h and 550 °C for 2 h to obtain the ZnO thin film. The structural and surface characteristics of the thin films were studied by grazing incidence X-Ray diffraction and atomic force microscopy. The electrical measurements of the Au/Cr-ZnO (LB)Ni structure were analyzed to determine the tunneling activity of the MIM structure. EXPERIMENTAL DETAILS: Materials: For the experiments, zinc acetate dehydrate (Zn(CH3COO)2, ACS, 98.0-101.0%) from Alfa Aesar and ultra-pure water (18.2 MΩ.cm) were used as sub-phase. Stearic Acid (C18H36O2, analytical standard, ≥ 98.5%) and chloroform (CHCl3, ACS regent, ≥ 99.8%) from SigmaAldrich were used for spreading the liquid. Ethanol (C2H5OH, ≥ 99.5%, A.R.) was used to clean the barrier and the trough, and acetone (CH3COCH3, ≥ 99.5% A>R.), methanol (CH3OH, ≥ 99.8% A.R) and 2-propanol ((CH3)2CHOH, ≥ 99.5%, A.R.) were used to clean and prepare the substrates. Synthesis of ZnO multi layers: Multilayers of ZnO on different substrates were formed by using the method described in Feng et al

31

. The substrates were cleaned thoroughly with acetone,


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methanol and 2-propanol sequentially to make them hydrophobic. They were finally rinsed thoroughly with ultra-pure water and dried at room temperature. Zinc stearate monolayers and multilayers were deposited on different substrates using a process similar to that reported previously 35. A KSV NIMA Langmuir-Blodgett trough system equipped with a paper Wilhelmy plate was used to deposit the multilayer on the substrates. The Wilhelmy plate was used to measure the surface tension of the sub-phase with different concentrations by suspending it in the sub-phase in the trough. Zinc acetate and ultra-pure water were mixed to form the sub-phase with different concentrations. Varying volumes of 1 mg/ml stearic acid/chloroform solution were spread carefully using a micro-liter syringe to form a monolayer on the sub-phase. From the surface tension-surface area (П-A) isotherms of different samples, the monolayer formation parameters can be obtained. The deposition parameters were determined from the transfer ratio, which is the ratio of the decreased area at the sub-phase surface during deposition to the total substrate area 19. The feedback control system of the barrier compression of the trough assures a closely packed monolayer at the air-sub-phase surface. Subsequently, the substrate was dipped into the sub-phase, maintaining all deposition parameters, to transfer the monolayer from the airsub-phase interface to the surface of the sub-phase. After deposition on the substrate, the fabricated Langmuir –Blodgett monolayers were cured at 100 °C for 15 min. Then annealed at 300 °C for 15 min and at 550 °C for 120 min to form the ZnO multilayer schematic of the ZnO deposition process.


31

. Figure 1 shows a

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Figure 1: Schematic of the ZnO deposition process. The monolayer formation parameters such as sub-phase concentration, spreading liquid volume, barrier speed and evaporation time were determined from the surface tension-surface area (П-A) isotherms. Multilayer deposition parameters like target pressure and lifting speed were determined from the transfer ratio. A XRD (Philips X’Pert Pro X-ray diffractometer) was used to investigate the crystal structure of the ZnO thin films. It was operated at a grazing incidence (GI) XRD setup with the 2θ range being 25°-75° at a voltage of 45 kV and a current of 40 mA. For this experiment, ZnO was deposited on a glass surface. An AFM (Digital Instrument Co., USA) was used to observe the film surface morphology. Bruker AFM probe tips (0.01-0.025 Ohm-cm Antimony (n) doped Si) were used for the AFM measurements in the tapping mode. DC I-V measurements were used to characterize the diode performance. The diode consisted of a 100 nm Au/Cr layer (bottom electrode) deposited on a silicon wafer with a silicon dioxide passivation layer (Cr being used as an adhesion layer). 10 and 20 layers of Zinc Acetate


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monolayers were deposited on the Au/Cr layer using the LB technique. The multilayers were then dried and annealed at different temperatures to produce the ZnO insulating thin film. Finally, the top 50 nm thick Ni electrode was deposited by sputtering, at a pressure of 2-3 mtorr and 80 watts of power, using a shadow mask to control the shape and size of the top contact. Figure 2 shows a schematic of the Au/Cr-ZnO-Ni MIM tunnel diode structure. A micromanipulator setup with Dumet (Cu-Fe) probe tips was used to measure the current-voltage characteristics of the Au/Cr-ZnO-Ni MIM diodes. A 4145B Semiconductor Parameter Analyzer was used to analyze the measurements.

Shadow Mask

Ni ZnO

Au/Cr

SiO2

Si

Figure 2: A schematic of the Au/Cr-ZnO-Ni MIM tunnel diode structure. RESULTS AND DISCUSSION LB film Figure 3 shows the surface tension-surface area (П-A) isotherms of the monolayer formed at the air-water interface at different sub-phase (zinc acetate) concentrations. Here the sub-phase concentrations were 1.0, 0.1, 0.05 and 0.025 mmol/L. The spreading volume of stearic


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acid was 15 µL, the wait time was 25 min and the barrier speed 3 mm/min. As shown in figure 3, the isotherms have different collapse points for different concentrations of the sub-phase (zinc acetate). At different concentrations of the sub-phase the molecules at the surface have different inter-molecular distances. For higher concentrations the inter-molecular distance is low and for lower concentrations high. When the surface tension increases and reaches a certain point where the inter-molecular force is high and the inter-molecular distance is very small, the highest point suggests the collapse point. At that point, a compact monolayer has been formed at the airsubphase interface. Further increase in the surface tension results in the collapse and overlapping of monolayers and the intermolecular force has been found to drop significantly. At higher concentrations of zinc acetate (1.0 mmol/L), more Zn2+ reacts with the stearic acid and forms a slightly overlapped monolayer. The surface tension keeps increasing after the collapse point. At lower concentrations (0.1, 0.05 and 0.025 mmol/L), collapse points are prominent implying that no overlapping monolayers would exist at those concentrations of zinc acetate. At concentrations of 0.05 and 0.025 mmol/L, isotherms prominently exhibit gas, liquid and solid phases during the formation of a monolayer. Thus, at a phase concentration of 0.05 mmol/L a stearic acid/ZnO monolayer could be formed.


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ZnA 1.0 mmol/L

ZnA 0.1mmol/L

ZnA 0.05 mmol/L

ZnA 0.025 mmol/L

30

25

Surface Tension (mN/m)

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 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

15

10

5

0 28

38

48

58

68

78

Surface Area (cm2) Figure 3: Surface tension vs Surface area isotherms at different subphase concentrations. Figure 4 depicts the surface tension-surface area (П-A) isotherms of the stearic acid/ZnO monolayer at different spreading volumes (14, 15, 16 and 17 µL) of stearic acid. For a sub-phase concentration of 0.05 mmol/L, a wait time of 500 s and a barrier speed of 3 mm/min was maintained during the experiments. From figure 4, one sees that there is no difference in the shape and the trend of the graphs, with the collapse point shifting to the right at higher spreading volumes. Also, at higher spreading volumes, the number of molecules at the air-water interface increases and this increases the inter-molecular force and decreases the inter-molecular distance. As the inter-molecular distance decreases, the collapse pressure point appears earlier for a higher spreading volume. It would not provide enough space to the barrier of the trough to maintain a target pressure. On the other hand, a lower number of molecules at the air-water interface (lower spreading volume) would give enough time for the self-orientation and attainment of a well-


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ordered monolayer. Taking these restrictions into consideration, a 16 µl of spreading volume was chosen to fabricate the monolayer. The compactness of the monolayer deposited on a solid surface was determined by using the transfer ratio which was computed during each deposition (the transfer ratio being defined as the ratio of the decreased area in the monolayer to the area of the substrate). For an ideal case, the transfer ratio is 1. Values between 0.8-1.2 are considered as good transfer ratios with no accumulation of monolayer film 36. Several experiments were carried out to optimize the deposition parameters, such as dipping speed (3.5 mm/min), barrier speed (3 mm/min), wait time (500 s) and target pressure (17 mN/m) for obtaining the best layer configuration.

14 microl

15 microl

16 microl

17 microl

25

20

Surface Tension (mN/m)

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 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15

10

5

0 50

55

60

65

Surface Area (cm2)

70

75

Figure 4: Surface tension vs Surface area isotherms with different spreading volumes. X-ray diffraction: The XRD patterns of the ZnO thin film and stearic acid/Zn2+ multilayers are shown in figure 5. The scan was perform in the 2θ range of 25°-75° and the instrument setup was


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in the GI-XRD mode. In the zinc stearate multilayer diffraction pattern (a) there are no observable peaks, which implies possible failure of the ZnO crystal structure without annealing. After annealing at 300 °C for 15 min and at 550 °C for 120 min, the diffraction pattern (b) shows sharp and strong peaks related to (100), (002), (101) and (102) planes. This suggests that after annealing the multilayers form the hexagonal ZnO wurtzite structure 37. The decrease in intensity

ZnO(102)

ZnO(101)

ZnO(002)

ZnO(100)

of the diffracted peaks could be due to the film thickness.

b)

Intensity (a.u)

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 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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25

a)

35

45

2θ (degree)

55

65

75

Figure 5: XRD patters of (a) the ZnO film before annealing, and (b) the ZnO film after annealing. AFM study Figure 6 shows the AFM image of a representative area of the fabricated ZnO LB film on a Si substrate. The average roughness, Ra, of the deposited films was measured to be 1.076 nm for 10 LB layers whereas the root-mean-square (rms) value was found to be 1.38 nm over an area of 100 µm2. The ZnO Bruker AFM tips (0.01-0.025 ohm-cm antimony (n) doped Si) were used for this purpose 38. This value has to be compared with the typical value of Ra = 0.3


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nm for a Si wafer. The diameter of the particles ranges from 3.1-7.3 nm where the average size is 5 nm, which is consistent with previously reported results 25. The section analysis in figure 6(b) shows the surface topography along the white line which suggests the flatness of the ZnO film surface. During the annealing process of the LB monolayers deposited on Au/Cr films, the thin metal film of Au/Cr layer shows a de-wetting behavior

39

. The underlying thin metal film of

Au/Cr becomes agglomerated at a temperature of 550 °C, well below its melting temperature. As a result the Au/Cr thin layer forms an array of islands. The de-wetting temperature depends on the film thickness and the metal type. It was found that a 100 nm thick Au/Cr layer could be sustained at an annealing temperature of 550 °C without any de-wetting of the thin metal film 40.

a)

b)

Figure 6: AFM images of a) the ZnO thin film and b) a section analysis along the white line. FTIR studies: Figure 7 shows the FTIR spectra of a 10 monolayer stearic acid LB on a silicon substrate and the ZnO layers fabricated after sequential annealing of the SA LB precursor film. It shows peaks at 2848 and 2920 cm-1 due to the symmetric and asymmetric methyl stretching bands in the Zn


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stearate LB film41. The peaks observed at 1538 and 1570 cm-1 are due to the carbohydrate group, indicative of the antisymmetric νaCOO- and also the νaCH3 bands. However, the blue annealed ZnO does not show the characteristic peaks of 1538 and 1570 cm-1

and 1538 and 1570 cm-1

suggesting the formation of a ZnO film from a precursor of LB. The inlet shows the characteristic ZnO peak at 390 cm-1 which was absent in the Zn stearate LB film. The inlet figure on the right shows no characteristic peaks at 2848 and 2920 cm-1 due to symmetric and asymmetric methyl stretching after annealing to make the ZnO film from the zinc stearate LB film.

Figure 7. FTIR spectra of LB 10 monolayers stearic acid and ZnO layers after annealing of the SA LB film precursor. The left inlet shows the stearic acid peak and the right inlet shows the ZnO FTIR peak. Cyclic voltammetry study: Figure 8 shows the cyclic voltammogram of ZnO layers fabricated on FTO glass from a zinc stearate precursor as the anode, platinum as the cathode and Ag/AgCl


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as the reference electrode in 0.01 M HCl at a scan rate of 2 mV/sec. The scan direction is also shown in figure 8. It shows the cathodic peak at -630 mV and a small anodic peak at -760 mV. However, it also shows the quasi reversible cyclic voltammetric characteristics. The cathodic peak obtained as -630 nm indicates that the ZnO film has been formed using the zinc stearate precursor LB film.

0

-50 Current Density (µA/cm2)

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 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-100 2mV/s -150

-200

-250 -860

-760

-660 Potential (mV)

-560

-460

Figure 8 Cyclic voltammogram of ZnO monolayers at FTO glass plate in 0.01 M HCl at 2 mV/sec I-V characteristics: The I-V characteristics, 1st and 2nd derivatives including the sensitivity of fabricated MIM diodes for 20 monolayers LB ZnO monolayers are shown in figure 9 (a-d). Similarly, I-V characteristics, 1st and 2nd derivatives including the sensitivity of fabricated MIM diodes for 10 monolayers LB ZnO monolayers are shown in figure 10 (a-d). The typical I-V characteristics of the fabricated MIM diodes are shown in figures 9(a) and 10(a). A 4145B Semiconductor Parameter Analyzer was used for this purpose. The diode displays a linear response at low voltages (< 0.7V), making it unsuitable for recognition. However, for the voltage


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> 0.7V it shows non-linearity which can be a function of both barrier height and thickness. The tunneling theory was applied to characterize the diode, with the thickness of the diode theoretically calculated to be ∼4.0 nm for 10 monolayers. For each device, along with the I-V curve, the 1st derivative curves (figures 9(b) and 10(b)) and the 2nd derivative curves were plotted. The I-V characteristics of the fabricated diode reveal a tunneling behavior. The MIM diode of 10 LB monolayers demonstrates higher conductivity than the 20 monolayer diode which endorses the tunnel diode mechanism

42

. The turn-on voltage for the 10 monolayer appears

earlier than for the 20 monolayer insulating films.


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Figure 9: A 20 layer Au/Cr-ZnO-Ni diode: a) I-V characteristics; b) 1st derivative; c) 2nd derivative and d) sensitivity.

The current sensitivity and detectivity are measures of the diode’s performance. The 1st derivative, I'=dI/dV, shows the conductivity of the diode, and the 2nd derivative, I"= d2I/dV2, is a measure of the diode nonlinearity. The sensitivity refers to the diode’s rectification ability. The sensitivity of the diode is defined as the ratio of the second derivative to the first derivative at each point of the I-V curve

43, 44

. The second derivative also suggests the rate change of the

conductivity of the diode. The maximum sensitivity for the ZnO film fabricated using 20 monolayer and 10 monolayer diodes were found to be 20 V-1(fig 9(d)) and 32V-1(fig 10(d)), respectively, which are high enough for efficient rectification 45, 46. The rectification ratio of the diode is calculated as the ratio of the forward current, If, to the reverse current, Ir, for an equal deviation of the voltage around the bias point. The rectification ratio for the 10 monolayer diode was thus calculated to be as high as ~12 for 200 mV of voltage deviation at a 0.78 V bias voltage. The LB deposited Au/Cr-ZnO-Ni structured MIM diode exhibits a competitive sensitivity, as shown in Table 1.


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Figure 10: A 10 layer Au/Cr-ZnO-Ni diode: a) I-V characteristics; b) 1st derivative; c) 2nd derivative; and d) sensitivity

Table 1: Published data for the sensitivity of state-of-the-art MIM diodes. Assembly

Maximum Sensitivity (V-1)

Ref.

Ni-NiO-Au

5.5

36

Ni-NiO-Ni

1.6

7

Ni-NiO-Ni

2.75

47


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Ni-NiO-Pt

-13

48

Ni-NiO-Cr/Au

5

5

Polysilicon-SiO2-polisilicon

-31

49

Polysilicon-SiO2-Au

-14.5

50

Al-AlOx-Pt

-2.3

51

Ni-NiO-Cu

7.3

52

Al-Al2O3-Pt

0.03

53

Graphene-Air-Graphene

0.24

54

Nb-Nb2O5-Pt

20

45

Cu-CuO-Au

4.0

44

Ni-NiO-ZnO-Cr

16

55

Ni-NiO-Ag

8.5

13

Au/Cr-ZnO-Ni

32

This work

CONCLUSIONS In summary, due to the tunneling based conduction mechanism, MIM/MIIM diodes together with an appropriate antenna could be used as IR detectors or IR energy harvesters. The sensitivity and the turn-on voltage determine whether the diode can be used as a detector or an energy harvester. For a diode optimal performance, the insulating layer should be pinhole free and conformal to the underlying layer, with a high structural order being maintained during deposition. Using zinc acetate as the sub-phase and stearic acid/chloroform mixture as the spreading liquid, a zinc stearate monolayer is formed at the air-water interface. After optimizing the monolayer formation and deposition parameters, the zinc acetate monolayer is deposited on a


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thin film of Au/Cr covering different substrates like glass or silicon with a passivation layer, gold on a Si wafer, etc. A shadow musk was used to deposit the top Ni electrode. Thus, the LB method has been effectively used to fabricate Au/Cr-ZnO-Ni MIM tunnel diodes. The I-V characteristics of the fabricated diodes with different insulator thickness were successfully measured and, from the I-V measurements, the conductivity, nonlinearity and sensitivity of the diodes were calculated. A rectification ratio of ~12 was calculated at a bias voltage of 0.78 V for ±200mV. A sensitivity value as high as 32 V-1 was also obtained which compares well with published results and suggests a promising device for high frequency applications.

AUTHOR INFORMATION

Corresponding author. *E-mail: [email protected] Notes The authors declare no competing financial interest ASSOCIATED CONTENT Supporting information: none ACKNOWLEDGMENTS The authors are thankful to National Science Foundation (NSF) for financial support under the NSF ECCS award number 1343228.

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