Materials Today Energy
Optical rectification through an Al2O3 based MIM passive rectenna at 28.3 THz
Graphical abstract
Introduction
Energy harvesting from a renewable source is a promising alternative for sustainable and clean power generation. The conventional photovoltaic (PV) technology harvests energy only from the visible range of the spectrum (400–750 nm), leaving the infrared (IR) range completely untapped. About 80% of solar radiation is absorbed by the atmosphere and the earth surface; these waves are reemitted as mid-IR radiation (5–15 μm) [1], [2], [3]. The other sources of IR emissions are metal heating, fluid heating, steam generation, heat treatment, and agglomeration. The temperature for these processes fluctuates between 250 K and 1500 K, and the corresponding wavelengths vary in the mid-IR from 2 to 11 μm [4], [5].
An interesting way of thinking about energy harvesting from the IR spectrum is treating the fluctuations as high frequency EM waves, which can be collected and rectified by a combination of a nano-antenna and a rectifier, typically known as a rectenna device, as shown in Fig. 1(a) [6], [7], [8], [9], [10].
Energy harvesting via rectennas in the RF and microwave bands has been demonstrated numerous times [11], [12], [13]. Antennas in microwave frequencies convert free space electromagnetic waves into guided waves, which are then delivered to the load [14], [15]. However, antennas operating at IR frequencies generate an enormous amount of highly localized electric field at the sharp tips of a nanoantenna due to the surface plasmon resonance [5], [16], [17].
In our previous work at 28.3 THz [5], we have shown in simulations that by optimizing the gap and geometry of a bowtie antenna, field enhancements of many orders of magnitude can be achieved at the sharp tips (and in the gap between the sharp tips) of the antenna. These fields were quite localized, and therefore it was necessary to rectify them at the center (gap between the sharp tips) of the bowtie to benefit from the field enhancement. Since no semiconductor based diode can work at such a high frequency, tunneling diodes, typically known as metal-insulator-metal (MIM) diodes [8], [18], [19], were realized by overlapping the arms of the bowtie antenna and sandwiching a sub nano-meter thin oxide between the two arms. This MIM diode was composed of two different metals, namely copper and gold, with a 0.7 nm copper oxide insulator between them to facilitate electron tunneling without any applied bias [5]. Though we were able to demonstrate MIM diode operation experimentally with decent responsivity at DC, we were unable to demonstrate the complete rectenna function at 28.3 THz due to the high permittivity of CuO at that frequency (εr ∼ 7) [20], which severely effects the cutoff frequency of the device, and the fragility of the devices due to a sub-nm oxide.
MIM diode based rectennas have been demonstrated for RF bands [21], [22], [23], [24], however their real utility is for higher frequencies where other semiconductor based diodes cannot work. Eventhough a few papers on high frequency (∼30 THz or so) MIM diode based rectennas have been demonstrated [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], there are numerous unresolved issues related to their optical rectification. Some of them utilize symmetric MIM diodes, which means they require a bias for their operation and thus are not suitable for energy harvesting applications [25], [26]. For others [27], [28], [29], [30], [31], [32], [33], [34], [35], there is some ambiguity as to wether the major contribution to their output a signal from optical rectification or a thermal response due to the Seekbeck effect. Some researchers did attempt to clarify this aspect by comparing the output of their MIM diodes to that of a metal only configuration (without an insulator layer). Results indicated that the main contribution to the overall output voltage of the MIM diode was the Seebeck [33], [34], [35]. The issue of thermal contribution to the MIM diode output is an important but tricky affair and therefore must be regarded with careful study in order to understand the true optical rectification of MIM diodes at these high frequencies.
In view of the above and our previous work [5], we chose aluminum oxide (Al2O3) as the insulator layer for this work. Al2O3 has a lower dielectric constant (ranging from 0.5 to 3.5) at higher THz frequencies [20], [37]. This value is almost half (or even lower) of CuO from our previous work [5], and is expected to help us in enhancing the cutoff frequency of the MIM diode. Au and Ti are used as metal arms, where Au and Ti are expected to have work functions (WF) of 5.1 eV and 4.33 eV, respectively [20], [38]. This higher work function difference between the metals is chosen to facilitate electron tunneling [39], [40]. The other issue we had in our previous work was the fragility of the device due to an extremely thin insulator layer (∼0.7 nm). In this work, we have doubled the oxide thickness to ∼1.5 nm. The above steps enabled us to demonstrate a 28.3 THz MIM diode based rectenna where the antenna can pick up the signal (verified by polarization sensitive output of the rectenna) and the MIM diode can provide optical rectification (verified by the zero bias responsivity and calculations). To the authors' best knowledge, this is the first demonstration of zero bias rectification of a 28.3 THz signal using an MIM diode based rectenna. The paper presents complete electromagnetic (EM) simulations for the antenna part, quantum mechanical simulations for the MIM diode part, the nano-fabrication process of the prototype rectenna and complete DC as well as optical characterization results.
Section snippets
Rectenna design and simulations
Illuminating antennas with IR waves generates plasmon oscillations on metal/dielectric interface [9]. In our previous work, we have compared antennas with different geometries. Bowtie antennas showed higher field enhancement, compared to others, and was thus considered for rectenna design [5]. In this work, bowtie antennas were chosen due to the ease of fabrication and integration simplicity. We performed the simulation studies on the new stack-up to optimize the parameters to have maximum
Device fabrication
After assessing the simulation studies, which highlight the relationship between the device characteristics and the material stack, It can be seen that the choice of material, dielectric constant and chosen thickness play a vital role in altering the MIM diode performance. Considering all these aspects, the fabrication of MIM diode was undertaken. The device studied during this experiment is realized on Si/SiO2 substrate. A Si (100) wafer with high resistivity (p-type; boron doped, resistivity
DC measurements and analysis
For high frequency applications, MIM diodes are characterized by the four parameters: differential resistance, responsivity, nonlinearity and cutoff frequency. The diode resistance (R0) is obtained by differentiating the current with respect to the applied voltage as given by Equation (1). In general, a low value of (R0) is required to achieve good impedance matching to the antenna [5], [50].
Diode responsivity (β0) determines the diode's rectification ability. The (β0), can be
Conclusion
We have demonstrated 28.3 THz (10.6 μm) completely passive rectenna design employing Al2O3 as the insulating layer between two different work function metals, namely Au and Ti. The bowtie antenna has been optimized to enhance the fields in the gap between the two arms. Since these fields are very localized, the two arms have been overlapped to realize the MIM diode exactly at the point with highest concentration of captured fields. Simulation results for the MIM diode have been presented, which
Acknowledgement
We acknowledge financial support from King Abdullah University of Science and Technology (KAUST), Office of Sponsored Research (OSR) for CRG grant OCRF-2014-CRG-62140381. Authors would also like to thank Shuai Yuan and Shuai Yang for their help with optical set-up and SEM analysis.
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