Design and development of batch fabricatable metal ...

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Sensors and Actuators A 142 (2008) 40–47

Design and development of batch fabricatable metal–insulator–metal diode and microstrip slot antenna as rectenna elements S. Krishnan a,b , H. La Rosa a , E. Stefanakos a , S. Bhansali b,∗ , K. Buckle a a

Clean Energy Research Center, College of Engineering, University of South Florida, 4202 E Fowler Ave., Tampa, FL 33620, USA b Bio-MEMS and Microsystems Laboratory, Department of Electrical Engineering, University of South Florida, 4202 E Fowler Ave., Tampa, FL 33620, USA Received 2 October 2006; received in revised form 24 January 2007; accepted 4 April 2007 Available online 19 April 2007

Abstract Thin-film Ni-NiO-Cr metal–insulator–metal (MIM) tunnel diodes with 1 ␮m2 contact area and 1–3 nm insulator layer (NiO) are fabricated for high sensitive far-infrared (IR) detection. Also, a 2.5 GHz microstrip slot antenna is fabricated on a FR-4 substrate and integrated with a commercially available surface mount zero bias Schotky diode. Asymmetric current–voltage (I–V) characteristics of the MIM tunnel diode is observed with a significant degree of non-linearity. Additionally, the sensitivity of the MIM diode is determined to be 5 V−1 at Vbias of 0.1 V. For the antenna coupled detector circuit, the radiation patterns, scattering parameters, output voltage and sensitivity are determined experimentally. Further, when the slot antenna is coupled with the schottky diode detector, a rectified output voltage of 56 mV is obtained by radiating the device with a low frequency tube antenna. © 2007 Elsevier B.V. All rights reserved. Keywords: MIM diode; Ni-NiO-Cr; IR detection; Slot antenna

1. Introduction Terahertz imaging is one of the emerging technologies that has attracted considerable attention towards the next generation military, scientific, and commercial applications. So far, infrared imaging has been achieved by utilizing thermal and photon detectors, however, they require cryogenic cooling for better operation (photon detector) or they exhibit low sensitivity [1] without any cooling mechanism (thermal detectors). In view of these shortcomings, there exists a need for an infrared detector possessing high speed and ambient temperature operation. One such device that satisfies these criteria and offers many advantages over other imaging techniques at high frequencies is the antenna-coupled metal–insulator–metal (MIM) detector known as Rectenna. In these detectors, the incident electromagnetic radiation is amplified by the antenna and coupled to the rectifying element (MIM diode) to generate a useful signal.

∗

Corresponding author. E-mail address: [email protected] (S. Bhansali).

0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.04.021

MIM diodes are utilized since they can operate at high frequencies up to 150 THz with greater switching speed [2]. Also, Antenna coupled MIM diodes have been used for detection and mixing in the IR regime [3]. Although the responsitivity and detectivity of the MIM diodes (106 cm Hz1/2 W−1 ) [4] are lower than the cryo-cooled IR detectors (1010 cm Hz1/2 W−1 ) [5], they are preferred due to their ultra fast response time (10−15 s) [6] as compared to existing microbolometers (10−9 s) [7]. In a MIM diode, the conduction takes place when the electrons flow between the electrodes via an ultra-thin insulator layer ˚ Initially point-contact MIM configurations were used (10–30 A). as mixers and detectors for infrared and visible radiation [8] with tungsten tip acting as an efficient receiving antenna at 30 THz [6]. However, due to poor reproducibility and instability of this configuration, a more reliable device – thin film MIM diode was considered. Various research groups have investigated thinfilm based MIM diodes for IR detection [1,3,4,8–11] and found that MIM diodes with high enough work function difference electrodes exhibit non-linear current–voltage (I–V) behaviour that can produce rectification. They have been fabricated with a wide variety of material combinations like, Al-Al2 O3 -Al [12], Cr-CrO-Au [9,13], Al-Al2 O3 -Ag [14], Nb-NbOx-Au [13], Ni-

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NiO-Au, Ag, Ni, Pt, [4,11,15,16]. In this work Ni-NiO-Cr MIM diodes were fabricated on a Si substrate using standard IC processing technology to exhibit a more pronounced level of non-linearity and asymmetry. In order to study the behavior of a rectenna, a low frequency (2.5 GHz) microstrip slot antenna was fabricated on a FR-4 board and a schottky diode from Agilent (HSMS-2850) was used to determine the rectification voltage of the circuit. This paper primarily focuses on the design, fabrication and characterization aspect of a thin film MIM diode for IR detection. The scattering parameters and radiation patterns of the microstrip slot antenna are analyzed and compared with the simulated response. Also, the dc I–V characteristic curve of the MIM diode is presented along with the sensitivity of the diode. The output voltage of the antenna coupled detector circuit is also measured and discussed. Fig. 1. A schematic representation of the thin film Ni-NiO-Cr MIM diode illustrating the survey from top and cross-section with an ultra thin layer of insulator fabricated on an oxidized Si substrate.

2. Experimental details 2.1. Device design In order to validate the concept of electromagnetic rectification through a rectenna and the feasibility to develop a thin film MIM diode, a bi-level development module was adopted by designing a slot antenna operating at 2.5 GHz with a Schottky diode detector and a MIM diode operating above 94 GHz cut-off frequency. 2.1.1. Design of thin film MIM tunnel junction A planar structure was designed for fabricating the thin film MIM diode on a Si substrate. For the target frequency, the capacitance and contact area of the diode were determined by utilizing the equation formulated by Sanchez et al. [17]. The cut-off frequency of a MIM diode is given by fc =

1 2πRa Cd

(1)

where Ra is the antenna resistance and Cd is the capacitance of the diode given by Cd =

ε0 εr A , d

to form the MIM structure. The top electrode (Cr/Au) cone was designed with a rectangular head and a conical body to minimize the inductance effects of the diode. The area of the head was designed to be 2 ␮m2 . By changing the overlapping positioning of the contact area, the size of the MIM diode and therefore the cut-off frequency could be varied. 2.1.2. Design of low frequency strip antenna coupled diode detector The antenna module operating at 2.5 GHz was designed using Agilent’s advanced design system (ADS) simulator along with a 2.5 GHz detector circuit. A schematic representation of the slot antenna (λ/2 long) along with the detector circuit operating at 2.5 GHz is shown in Fig. 2. The slot antenna was chosen for its similarities to the dipole antenna, its ease of fabrication and bandwidth modulation using tuning stubs. The feed line was purposely placed at an offset in order to match the antenna to a 50  transmission line.

(2)

where A is the contact area of the diode and d is the separation between the two electrodes. From the equations it is clear that a smaller capacitance is required for the diode to operate above the cut-off frequency and can be obtained by decreasing the diode contact area or by increasing the thickness of the dielectric layer. Since increasing the oxide thickness beyond 5 nm will reduce the tunneling current significantly, decreasing the contact area is more appropriate. Thus, the contact area of the diode was determined to be 1.6 ␮m2 for a dielectric thickness of 5 nm. A schematic illustration of the top and cross-sectional view of the MIM diode with a stacked tri-layer structure is shown in Fig. 1. The MIM diode consists of a thick nickel layer deposited over gold contact pads. The bottom electrode (Ni) was oxidized to form an ultra thin layer of nickel oxide. Then, a layer of chromium was deposited and topped off with a layer of gold

Fig. 2. A schematic representation of the 2.5 GHz rectenna device fabricated on FR-4 substrate with a commercially available schottky diode (HSMS-2850). The feed line of the slot antenna is off-center in order to match the 50  transmission line.

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The detector circuit begins with a piece of transmission line that is calibrated by a set of TRL calibration standards followed by a matching network to match the impedance of the antenna (50 ) to the complex conjugate of the detector for maximum power transfer. Then a short circuited stub was designed, λ/4 long at 2.5 GHz to allow only the targeted frequency, while all the other frequencies were short circuited keeping most of the power across the diode. To the right hand side of the diode, an open circuited stub was designed which is equivalent to a 3.9 pf capacitor. At RF most of the power will be shorted by this open circuited stub, and at dc the transmission line will look like an open. In other words, the dc reaches the output while shorting most of the RF signal. The inductor present in the circuit prevents the RF power at the output. The resistor serves as the load resistor where the dc output voltage will be measured. 2.2. Fabrication The fabrication of the antenna coupled MIM detector was carried out in two modules. Initially, the MIM diode with a thin film dielectric was fabricated on a 2 in. oxidized Si substrate. Then, the antenna structure along with the detector circuit was fabricated along with other RF components on a FR-4 board. Fig. 3 schematically illustrates the fabricating sequence for developing the MIM diode. MIM diodes were fabricated using a two level photolithography mask and one level e-beam lithography to achieve fine dimensions. For each photolithography step Futurrex NR-3000PY negative photoresist was used in conjunction with Quintel Q-2000C Mask Aligner. Initially, the Si substrates were chemically cleaned by a standard RCA procedure. Then, SiO2 was thermally grown at 1100 ◦ C for a thickness of ∼450 nm (Fig. 3 step (a)). Following the oxidation process, the wafers were subjected to a photolithography step to define the Au contact pads. Then, the substrate was deposited with a thick layer of Au (100 nm) by thermal evaporation using Denton DV-502 (Fig. 3 step (b)). The excess metal and photoresist were removed by a lift-off process, wherein the wafers were soaked in acetone. Secondly, Ni and NiO patterns were defined by photolithography. Then, 100 nm of Ni was deposited using an in-house dc magnetron sputtering. After the Ni deposition, the surface was oxidized by passing O2 and sputtering Ni at very low power (25 W) to form a thin layer (∼3 nm) of NiO (Fig. 3 step (c)). Later, lift-off was accomplished to achieve the desired features. The thickness of the metal deposition was monitored using a quartz crystal oscillator. Finally, the top electrode was defined using e-beam lithography. A 150 nm thick PMMA (MicroChem 950-K 3% Anisole) was spun on the substrate and baked at 180 ◦ C for 2 h. Later, the devices were patterned by a modified JEOL JSM-840 scanning electron microscope operating at an accelerating voltage of 30 kV with an exposure current of 30 pA. Nabity’s nanometer pattern generation system (NPGS) was used to control the e-beam and pattern the contact area. The patterns were developed in a 3:1 isopropanol:methylisobutylketone (MIBK) solution for 70 s. The samples were then deposited with a 30 nm of Cr followed by 100 nm Au deposition (Fig. 3 step (d)). Fig. 4 shows the micrograph image of the fabricated MIM diode with 1.45 ␮m2 contact area.

Fig. 3. A schematic representation of the MIM diode at the various processing steps of fabrication. A cross-sectional process flow cut across the Ni and Cr/Au layer is shown.

For the fabrication of the antenna and detector circuit, the patterns designed using ADS were exported into a program to etch the design on to the substrate (FR-4 board). Since the feature size of the low frequency device was too large to fit in a 2 in. Si substrate, the device was fabricated in an FR-4 board. The LPKF ProtoMat 91S milling tool was used in conjunction with LPKF CircuitCAM PCB and Board Master to plot the design. Initially a 250 ␮m diameter drill bit was loaded in the milling machine to etch finer features on the board. Then, a 750 ␮m diameter drill bit was replaced to etch away the excess metal, until the desired features remained. For the detector circuit, via holes were drilled on the board to connect the detector circuit to the ground plane. Then, the lumped components that were available off the shelf were soldered along with a commercial surface mount zero bias Schottky diode (Agilent‘s HSMS-2850). Fig. 5 shows the optical

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Fig. 6. A photograph illustrating the measurement set-up used for evaluating the dc characteristics of the MIM diode. The measurement was taken in a noise shielded probe station in conjunction with a Keithley 2400 SMU.

Fig. 4. A micrograph image of the MIM diode showing the top view of 1 ␮m2 contact is fabricated with Ni-NiO-Cr. The conical structure with the blunt head is the active area of the MIM diode.

image of the slot antenna with the 2.5 GHz detector structure milled on the FR-4 substrate. 2.3. Test set-up After the fabrication of the MIM diode and the antenna/detector circuit, the electrical behavior of the diode and the scattering parameters of the antenna were determined to study the performance of the system. The I–V characteristics of the MIM diode were measured to analyze its non-linearity patterns and its suitability for an IR detector. The experimental set-up (Fig. 6) for measuring the I–V characteristics of the diode

Fig. 5. A photograph of an entire low frequency (2.5 GHz) antenna with detector circuit fabricated on a FR-4 substrate. A commercially available schottky diode from Agilent (HSMS-2850) was used as the low frequency diode component.

consisted of a source meter (Keithley model 2400), which can source voltages up to 200 V and measure currents down to 1 pA, connected in series with a noise shielded probe station (Cascade micromanipulator). The measurement unit was connected to a PC via a GPIB-USB adapter (Keithley KUSB-488) and the data points were recorded using LabTracer2TM from Keithley Instruments Inc. The dc characteristics of the diodes were measured under a two-terminal arrangement and the measurements taken under forward and reverse bias conditions. I–V behavior of the MIM diode was obtained by sweeping the voltage range from −0.5 V to +0.5 V at room temperature. The experiments were limited to these voltages since higher bias voltages could result in dielectric breakdown of the insulator layer. In order to measure the radiation patterns of the antenna, an automated rotational stage was used in conjunction with a vector network analyzer (VNA) HP-8753D. This set-up was tested in an anechoic chamber. The slot antenna was excited by a horn antenna and the radiation patterns were measured. Also, the return loss of the antenna was determined. Fig. 7 shows a schematic illustration of the antenna measurement

Fig. 7. A schematic representation of the antenna/detector test set-up showing the source and test antenna placed on a rotational stage positioned automatically at different angle to measure the radiation pattern of the device.

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metal and the insulator. The I–V characteristics were analyzed based on the tunneling theory [18] and the dielectric thickness of the diode was theoretically determined to be ∼2.5 nm. The effect of dielectric thickness on the I–V characteristics of the diode is described in detail elsewhere [19]. The performance of the diode is characterized by its current sensitivity and detectivity. Maximum sensitivity is obtained at a dc bias point where maximum curvature of the I–V characteristics occurs [6]. The sensitivity of the diode is given by the ratio of second derivative and first derivative of the I–V characteristics, S=

I  (V ) I  (V )

(3)

where Fig. 8. Current density–voltage behavior of a Ni-NiO-Cr MIM diode with 2.5 nm insulator layer obtained using Keithley 2400 SMU and LabtracerTM . The J–V of the diode was measured by sweeping the voltage from −0.5 to +0.5 V.

I  (V ) =

set-up. The antenna was then connected to the detector circuit and the rectified voltage and sensitivity were measured.

I  (V ) =

3. Results and discussions 3.1. Diode performance A typical J–V curve of Ni-NiO-Cr MIM diode is shown in Fig. 8. The MIM diodes exhibited non-linear and fairly asymmetric behavior with a current density in the range of 2 A/cm2 at 0.5 V of applied bias. At very low voltages ( 0.2 V) the non-linearity is a function of barrier thickness and barrier height between the

d2 I d2 I (V ) (V ) = (V ) 2 dV dV 2

(4)

and, dI (V ) (5) dV According to Wilke et al. [16], MIM diodes need to be fabricated with strong non-linearity to obtain detectors with large response. Hence, the second derivative, which is a standard measure of non linearity of the diode, is determined. Together with I and I , the sensitivity of the diode is obtained which characterizes the performance of the diode. Typical plots displaying the I–V, first derivative, second derivative and sensitivity are shown in Fig. 9. The maximum sensitivity obtained was 5 V−1 at Vbias = 0.1 V, which is the optimum operating point for low level signal detection. The sensitivity maxima obtained by various research groups using similar and dissimilar MIM diodes with various contact areas are listed in Table 1 for comparison.

Fig. 9. I–V characteristic curve, its first derivative (dI/dV), second derivative (d2 I/dV2 ) and sensitivity (I /I ) vs. bias voltage for a 1.45 ␮m2 Ni-NiO-Cr MIM diode.

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Table 1 A comparative study on experimentally determined sensitivity of thin film diode fabricated Ni and NiO Authors

Type of MIM diode

Sensitivity (V−1 )

Hoofring et al. [6] Abdel-Rahman et al. [4] Pashang Esfandiari et al. [11] This study

Thin film Ni-NiO-Au (0.64 ␮m2 ) Thin film Ni-NiO-Ni (0.075 ␮m2 and 0.0014 ␮m2 ) Thin film Ni-NiO-Pt (0.0025 ␮m2 ) Thin film Ni-NiO-Cr (1.45 ␮m2 )

4.55 2.75 and 1.65 −13 5

It can be seen that the present configuration helps to achieve higher sensitivity with a larger area diode. This might be due to the work function difference between the metal electrodes that cause the conduction. Utilization of metal electrodes with high enough work function difference, gives better non-linearity. Although maximum sensitivity was observed with the fabricated MIM diode, the zero bias resistance was on the order of 500 k. In spite of the high resistance, the diode exhibits a significant degree of non-linearity and asymmetry as required for IR detection. This also indicates that the conduction mechanism is due to electron tunneling through a thin homogeneous insulator layer [18]. 3.2. Antenna and detector properties The low frequency slot antenna was characterized for its radiation patterns and scattering parameters. The antenna was placed in a rotational stage and a tube antenna was used to irradiate the device. The test antenna was placed in line with the source and the stage was rotated automatically from 0◦ to 360◦ and its radiation patterns were recorded. Fig. 10(a) shows the radiation pattern of the slot antenna as measured, in decibels plotted against the angle of incidence, and Fig. 10(b) shows the radiation pattern of the simulated antenna wherein the measured data is in good agreement with the simulated pattern. A detected voltage of −30 dB was recorded while the simulated patterns had a −32 dB with 4.3 dB gain. Further, there exists a back side radiation on the slot antenna since the power was radiating in the back side of the antenna. Hence the slot antenna was modified with a reflector plate and the model was simulated to observe its effects. As shown in Fig. 11, the back side radiations were avoided due to the reflector pad and therefore, the gain of the antenna was increased to 9.3 dB. Being a prototype model, the actual device was not modified with a reflector plate since the fabricated antenna was in good agreement with the simulated results. The back side radiation will be taken into consideration when the device is fabricated on a Si substrate. When fabricated on an oxidized Si substrate the SiO2 layer acts as the reflector enhancing the gain of the antenna. Following the radiation measurements, the reflection co-efficient (S11) of the antenna was measured using HP-8753D VNA. Fig. 12 illustrates the S11 exhibited by the antenna along with the simulated data. The measured value exhibits 21 dB return loss, whereas the simulated S11 exhibits 30 dB return loss. Though there exist a discrepancy, the return loss of the antenna is within the accept-

Fig. 10. Plot showing (a) measured and (b) simulated radiation patterns of the slot antenna without reflector plane. The measured patterns looks slightly tilted due to the placement of the test antenna on the rotational stage.

able limits of comparison. Also, the slot antenna was operating at the designed frequency (2.5 GHz). From the measured data it can be stated that the fabricated and modeled antenna demonstrate good correlation suggesting an acceptable operation of the designed mode. Later, the properties of the detector circuit were measured by connecting it to the VNA. The return loss of the detector circuit was measured and compared with the simulated result. Fig. 13 shows the plot of the detector circuit with −42 dB return loss (measured) and −45 dB (simulated). A good agreement is obtained between the measured and simulated data. A slight shift in frequency is observed from the plot. This shift can be due to the inaccuracy in the dielectric constant of the FR-4 substrate or due to the variation in the values of the lumped elements. Still the detector seems to be operating at 2.47 GHz. Following the scattering properties measurement, the output voltage and the sensitivity of the detector were measured by using an HP8714C VNA. Fig. 14 shows the output voltage obtained from

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Fig. 14. Plot showing the input power vs. the output voltage of the detector circuit measured using HP-8714C VNA.

Fig. 11. Plot showing simulated radiation patterns of the slot antenna with reflector plane.

Fig. 15. Plot showing the input power vs. the sensitivity of the detector circuit measured using HP-8714C VNA.

Fig. 12. Plot of S11 vs. frequency showing the measured and simulated data of the slot antenna operating at 2.5 GHz.

the detector by supplying a power through the VNA in a logarithmic scale. A power level from −37 to 8 dBm was supplied to the detector circuit and the corresponding output voltage was measured and plotted. The detector exhibited a maximum output voltage of 2.5 V at 8 dBm. Then the sensitivity of the detector was measured with respect to the input power. Fig. 15 shows the sensitivity of the detector obtained with power values ranging from −37 to 8 dBm. The detector exhibited a maximum sensitivity of 25,000 mV/mW at a power level of −37 dBm. Thus, the detector circuit was characterized to determine the RF properties. Later, in order to evaluate the rectification response of the slot antenna and the detector, the 2.5 GHz antenna was connected to the detector using a M–M connector. A 2.5 GHz tube antenna was used as the transmitting antenna and it was excited by the HP-8753D VNA with a source power of 0 dBm. An output voltage of 56 mV was observed in the multimeter proving the operation of the prototype rectification system. 4. Conclusions and future work

Fig. 13. Plot of S11 vs. frequency showing the measured and simulated data of the 2.5 GHz detector circuit.

A thin film MIM diode with 1.45 ␮m2 contact area and a slot antenna operating at 2.5 GHz were fabricated together with a detector circuit and a commercially available Schottky diode.

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By measuring the dc electrical characteristics of the diode, a larger degree of asymmetry and non-linearity is demonstrated. The sensitivity of the diode was found to be 5 V−1 at a bias voltage of 0.1 V, through which the detectivity of the system can be determined when integrated with the antenna. The slot antenna exhibited a 4.3 dB gain without a reflector plane which was doubled when the antenna design was simulated with a reflector plane. The measured scattering parameters of the antenna agrees well with the simulated results. Also, the detector circuit demonstrated excellent output voltage and sensitivity at a power of 8 and −37 dBm, respectively. When the antenna and the detector were combined to measure its rectification effects, a significant level of dc output was measured across the system. However, even though the MIM diode and the antenna responded effectively as individual components, certain issues need to be considered before integrating them as a system. The MIM diodes fabricated had a high resistance of 500 k and needs to be optimized to integrate with the antenna to achieve efficient rectification. Also, the detector circuit needs to be modified as per the MIM diode’s dc behavior. References [1] I. Codreanu, F.J. Gonzalez, G.D. Boreman, Detection mechanisms in microstrip antenna-coupled infrared detectors, Infrared Phys. Technol. 44 (2003) 155–163. [2] D.A. Jennings, F.R. Petersen, K.M. Evenson, Extension of absolute frequency measurements to 148 THz: frequencies of 2.0- and 3.5 ␮m Xe laser, Appl. Phys Lett. 26 (1975) 510–511. [3] C. Fumeaux, W. Herrmann, F.K. Kneub¨uhl, H. Rothuizen, Nanometer thin film Ni-NiO-Ni diodes for detection and mixing of 30 THz radiation, Infrared Phys. Technol. 39 (1998) 123–183. [4] M.R. Abdel-Rahman, F.J. Gonzalez, G.D. Boreman, Antenna-coupled metal-oxide-metal diodes for dual-band detection at 92.5 GHz and 28 THz, Electron. Lett. 40 (2004). [5] J. Piotrovski, A. Rogalski, Uncooled long wavelength infrared photon detectors, Infrared Phys. Technol. 46 (2004) 115–131. [6] A.B. Hoofring, V.J. Kapoor, W. Krawczonek, Submicron nickel-oxide-gold tunnel diode detectors for rectennas, J. Appl. Phys. 66 (1989) 430–437. [7] C.C. Fumeaux, D.F. Spenser, G.D. Boreman, Microstrip antenna-coupled infrared detector, Electron. Lett. 35 (1999) 2166–2167. [8] I. Wilke, W. Herrmann, F.K. Kneub¨uhl, Integrated nanostrip dipole antennas for coherent 30 THz infrared radiation, Appl. Phys. B 58 (1994) 87–95. [9] E. Wisendanger, F.K. Kneub¨uhl, Thin-film MOM-diodes for infrared detection, Appl. Phys. Lett. 27 (1977) 343–349. [10] M. Heiblum, S. Wang, J. Whinnery, T.K. Gustafson, Characteristics of integrated MOM junctions at dc and AR optical frequencies, IEEE J. Q. Elect. 159 (1978) 159–169. [11] P. Esfandiari, et al., Tunable antenna-coupled metal-oxide-metal (MOM) uncooled IR detector, Proc. SPIE 5783 (2005) 470–482. [12] G.M. Elchinger, A. Sanchez, C.F. Davis Jr., A. Javan, Mechanism of detection of radiation in a high-speed metal-metal oxide-metal junction in the visible region and at longer wavelengths, J. Appl. Phys. 47 (1976) 591–594. [13] B. Berland, Photovoltaic technologies beyond the horizon: oprical rectenna solar cell, final report, NREL/SR-520-33263, 2003. [14] D. Diesing, A.W. Hassel, M.M. Lohrengel, Aluminium oxide tunnel junctions: influence of preparation technique, sample geometry and oxide thickness, Thin Solid Films 342 (1999) 282–290. [15] C.C. Bradley, G.J. Edwards, Characteristics of metal–insulator–metal point contact diodes used for two-laser mixing and direct frequency measurements, IEEE J. Q. Elect. QE-9 (1973) 548–549.

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[16] I. Wilke, Y. Oppliger, W. Hermann, F.K. Kneub¨uhl, Nanometer thin-film Ni-NiO-Ni diodes for 30 THz radiation, Appl. Phys. A 58 (1994) 329– 341. [17] A. Sanchez, C.F. Davis Jr., K.C. Liu, A. Javan, The MOM tunnelling diode: theoretical estimate of its performance at microwave and infrared frequencies, J. Appl. Phys. 49 (1978) 5270–5277. [18] J.G. Simmons, Electric tunnel effect between dissimilar electrodes separated by a thin insulating film, J. Appl. Phys. 34 (1963) 2581–2590. [19] S. Krishnan, S. Bhansali, E. Stefanakos, Effect of dielectric thickness on I–V characteristics of thin film metal–insulator–metal diodes, Thin Solid Films, under review.

Biographies Subramanian Krishnan received his BE in electronics and communications engineering from the University of Madras, Tamil Nadu, India in 2000, MS degree in microelectronics from the Department of Electrical Engineering at the University of South Florida, Tampa in 2004. He is currently pursuing his doctoral studies in the University of South Florida, Tampa where his research focus is in field of thin films and materials science. His research interests are in the areas of micro/nanofabrication and characterization of antenna coupled detectors for terahertz imaging. Henrry La Rosa received his BS degree in electrical engineering from University of South Florida in 2005, and is currently working towards the MS degree in electrical engineering at the University of South Florida. His current research interests are in the areas of planar millimeter-wave antenna and circuit design for far infrared detection. Shekhar Bhansali received BE in metallurgical engineering (Honors) from the Malaviya Regional Engineering College (MREC), Jaipur, India (1987), MTech Aircraft Production Engineering from Indian Institute of Technology (IIT), Madras, India, (1991), and the PhD in electrical engineering from the Royal Melbourne Institute of Technology (RMIT), Melbourne, Vic., Australia (1997). He is currently an associate professor in the Department of Electrical Engineering and Nanomaterials and Nanomanufacturing Research Center at the University of South Florida. His interests are in the areas of Bio-MEMS, nanostructures, energy storage, sensors, and microsystems. He is the recipient of the NSF CAREER award. He is also the director of NSF-IGERT program, coordinator for Sloan Fellowship Programs and co-director of NSF Bridges to the Doctorate minority fellowship program. He has over 70 international conference and journal publications and seven pending US patents. Kenneth Buckle received BS in electrical engineering from General Motors Institute (1970), MS in electrical engineering from Purdue University (1970) and the PhD in electrical engineering from University of Wisconsin – Madison (1984). He is presently the associate professor in the Department of Electrical Engineering at the University of South Florida, Tampa. His research interests are antennas, electromagnetics, Power Electronics, and Microwave Plasma Processing. Most recently, he has concentrated on the problem of direct conversion of solar radiation to dc power using an antenna/rectifier assembly called the rectenna. Elias (Lee) Stefanakos is presently professor of Electrical Engineering and Director of the Clean Energy Research Center (CERC) at the University of South Florida (USF) located in Tampa, Florida, USA. Upto August 2003 and for 13 years he was Chairman of the Department of Electrical Engineering at USF. He has published and presented over 120 research papers in refereed journals and international conferences in the areas of electronic materials, renewable energy sources and systems, hydrogen and fuel cells, and electric and hybrid vehicles. He has received over US$ 12 million in contracts and grants from agencies such as, National Science Foundation (NSF), US Department of Energy (USDOE), National Aeronautics and Space Administration (NASA), Defense Advanced Research Projects Agency (DARPA), and others. CERC is an interdisciplinary center whose mission is the development of clean energy sources and systems with emphasis on technology development and technology transfer.