ABSTRACT
Renewable energy sources, like hydroelectric power, wind power, tidal power, and photovoltaics, have great importance for satisfying the global demand for energy in the future, since the resources of fossil fuels are limited and nuclear power plants can be hazardous radiation sources for thousands of years if the outer shell is damaged. Photovoltaics is one of the most promising energy sources because the solar power density that reaches the earth can be as large as 136 mW/cm2 [1] and will continue to be available for millions of years. The state-of-the-art commercial Si solar cells are autarkic power plants, but they have the drawbacks of a relatively small conversion efficiency, the need for a high-temperature manufacturing process, and the fact that the use of strong chemicals during the fabrication process cannot be avoided. Further, the absorption range of Si solar cells is limited by the large bandgap of Si, so that Si solar cells cannot convert the infrared radiation produced by the sun [3]. These drawbacks can be overcome by utilizing the arrays of nano antennas, either as individual devices for energy harvesting or in combination with organic or inorganic solar cells as hybrid photovoltaic devices to extend the absorption spectrum and thus increase the total efficiency [4–6]. The antenna length is usually set to several micrometers corresponding to the infrared regime, so that the antennas can be easily fabricated by optical lithography. The main challenge in the fabrication process of nano antenna arrays is therefore the implementation of a large number of terahertz rectifying devices, which are required to convert the terahertz alternating current induced in the antennas by the infrared radiation into a direct current. Efficient rectifiers, such as pn-junctions or Schottky diodes, cannot be used in this application, since the cutoff frequency of 134these devices is usually in the megahertz or at the best in the gigahertz range [7]. Recently, Schottky diodes with a cutoff frequency of a few terahertz have been reported [8], but only in the form of individual diodes that cannot be scaled to arrays consisting of millions of such devices. More promising are the metal–oxide–metal (MOM) junctions featuring a thin oxide layer with a thickness in the range of a few nanometers and with metals of different work functions. Such MOM junctions work as tunneling diodes, since the junctions exhibit asymmetric I–V characteristics with respect to the polarities of terahertz (THZ) alternating currents, owing to the fact that electrons tunnel through the thin barrier within femtoseconds [9,10]. However, this is true only if the area of the MOM junction is in the nanometer range, and therefore, simple conventional optical lithography is not applicable for the fabrication of nano antenna arrays, including MOM tunneling diodes. So far, single antenna-coupled MOM diodes (ACMOMDs) fabricated by electron-beam lithography and liftoff techniques have been reported and have shown a promising performances [11,12]. However, these devices have two main drawbacks. The first drawback is that the tunneling dielectric is obtained by the natural oxidation of an aluminum layer, resulting in an insulator with a thickness of 2–3 nm. The advantage of this fabrication method is its simplicity, but this oxide has poor reproducibility and poor electrical stability, resulting in a large fraction of shorted diodes. As an alternative to natural oxidation, a plasma-induced oxide growth has the advantage that it is far more reproducible, that the thickness of the oxide is limited to 3.6 nm, and that the physical properties of this oxide, such as its compactness, result in a dielectric that is electrically stable at high electrical fields. The second drawback of fabricating MOM diodes by electron-beam lithography is that this process is very time-consuming, which is problematic when fabricating arrays of millions of nano antennas. A more promising fabrication technique is the direct transfer-printing of complete arrays of nano antennas and MOM-rectifying diodes. Stacks of several metals are deposited onto a pretreated stamp having nanometer-size structures and are then transfer-printed onto a target substrate, which may consist of Si, SiO2, glass, or other materials [2]. Ultrathin dielectrics with a thickness of a few nanometers can also be fabricated and transferred in this way. The stamp can be either a soft stamp (e.g., polydimethylsiloxane, PDMS) or a hard stamp (e.g., silicon). Hard stamps have the advantages of providing higher resolution of the printed structures and the possibility of reusing the stamps several tens of times, provided the stamps are properly cleaned after each transfer step.
