First-principles investigation of interface phenomena in hafnium-based metal-insulator-metal diodes.

Metal-insulator-metal (MIM) diodes are very interesting in many different applications exploiting environment-friendly renewable energy solutions. Moreover, since the dimensions of such devices are at the nanoscale, the size and the characteristics of their constitutive elements can drastically influence their macroscale performance. As it could be difficult to describe in detail the physical phenomena occurring among materials in nanoscale systems, in this work first-principles calculations have been used to study the structural and electrical properties of three different hafnium oxide (HfO2)-MIM diodes. These devices have been simulated at the atomistic level by interposing 3 nm of HfO2 between drain and source electrodes made of gold and platinum, respectively. The monoclinic and orthorhombic polymorphs of HfO2 have been considered to model different types of MIM diodes, and the interface geometries have been optimized to compute the current-voltage characteristics, reflecting the tunneling mechanisms occurring in such devices. The calculation of the transmission pathways has also been carried out to investigate the effects of atomistic coordinates despite the use of the same material. The results demonstrate the role of the Miller indices of metals and the influence of the HfO2 polymorphs on the MIM properties. In this study, the importance of interface phenomena on the measurable properties of the proposed devices has been investigated in detail.

Multilayer Smart Holographic Label with Integrated RFID for Product Security and Monitoring.

Counterfeiting presents a major economic problem and an important risk for the public health and safety of individuals and countries. To make the counterfeiting process more difficult, and to ensure efficient authentication, a solution would be to attach anti-counterfeit labels that include a radio frequency identification (RFID) element to the products. This can allow real-time quality check along the entire supply chain. In this paper we present the technology optimized to obtain a multilayer holographic label with a high degree of security, patterned on a thin zinc sulfide film of a semi-transparent holographic foil rather than on the standard substrate for diffractive optical elements (metallized foil). The label is applied onto the product surface or packaging for anti-counterfeit protection. The developed multilayer structure contains various elements such as: a holographic background, nanotext-type elements, holographic elements, and an RFID antenna. The employed semi-transparent holographic foil offers the RFID antenna the possibility to transmit the electromagnetic signal through the label and thus to maximize the antenna footprint, achieving up to 10 m reading distance, with a 6 cm × 6 cm label, much smaller than the commercial standard (minimum 10 cm × 10 cm).

Nanomaterials and Devices for Harvesting Ambient Electromagnetic Waves.

This manuscript presents an overview of the implications of nanomaterials in harvesting ambient electromagnetic waves. We show that the most advanced electromagnetic harvesting devices are based on oxides with a thickness of few nanometers, carbon nanotubes, graphene, and molybdenum disulfide thanks to their unique physical properties. These tiny objects can produce in the years to come a revolution in the harvesting of energy originating from the Sun, heat, or the Earth itself.

Harvesting microwave energy using pyroelectricity of nanostructured graphene/zirconium-doped hafnium oxide ferroelectric heterostructures.

In this work, we present the design, atomistic/circuit/electromagnetic simulations, and the experimental results for graphene monolayer/zirconium-doped hafnium oxide (HfZrO) ultra-thin ferroelectric-based field effect transistors fabricated at the wafer scale, regarding the pyroelectricity generation directly from microwave signals, at room temperature and below it, namely at 218 K and at 100 K. The transistors work like energy harvesters, i.e. they collect low-power microwave energy and transform it into DC voltages with a maximum amplitude between 20 and 30 mV. The same devices function as microwave detectors in the band 1-10.4 GHz and at very low input power levels not exceeding 80µW when they are biased by using a drain voltage, with average responsivity values in the range 200-400 mV mW-1.

Ultralow voltage (1µV) electrical switching of SnS thin films driven by a vertical electric field.

In this paper, we show in a series of experiments on 10 nm thick SnS thin film-based back-gate transistors that in the absence of the gate voltage, the drain current versus drain voltage (ID-VD) dependence is characterized by a weak drain current and by an ambipolar transport mechanism. When we apply a gate voltage as low as 1µV, the current increases by several orders of magnitude and theID-VDdependence changes drastically, with the SnS behaving as ap-type semiconductor. This happens because the current flows from the source (S) to the drain (D) electrode through a discontinuous superficial region of the SnS film when no gate voltage is applied. On the contrary, when minute gate voltages are applied, the vertical electric field applied to the multilayer SnS induces a change in the flow path of the charge carriers, involving the inner and continuous SnS layer in the electrical conduction. Moreover, we show that high gate voltages can tune significantly the SnS bandgap.

Ultrathin tin sulfide field-effect transistors with subthreshold slope below 60 mV/decade.

In this paper, we present for the first time a field-effect-transistor (FET) having a 10 nm thick tin sulfide (SnS) channel fabricated at the wafer scale with high reproducibility. SnS-based FETs are in on-state for increasing positive back-gate voltages up to 6 V, whereas the off-state is attained for negative back-gate voltages not exceeding -6 V, the on/off ratio being in the range 102-103depending on FET dimensions. The SnS FETs show a subthreshold slope (SS) below 60 mV/decade thanks to the in-plane ferroelectricity of SnS and attaining a minimum value SS = 21 mV/decade. Moreover, the low SS values can be explained by the existence of a negative value of the capacitance of the SnS thin film up to 10 GHz (for any DC bias voltage between 1 and 5 V), with the minimum value being -12.87 pF at 0.1 GHz.

The microwave properties of tin sulfide thin films prepared by RF magnetron sputtering techniques.

In this paper we present the microwave properties of tin sulfide (SnS) thin films with the thickness of just 10 nm, grown by RF magnetron sputtering techniques on a 4 inch silicon dioxide/high-resistivity silicon wafer. In this respect, interdigitated capacitors in coplanar waveguide technology were fabricated directly on the SnS film to be used as both phase shifters and detectors, depending on the ferroelectric or semiconductor behaviour of the SnS material. The ferroelectricity of the semiconducting thin layer manifests itself in a strong dependence of the electrical permittivity on the applied DC bias voltage, which induces a phase shift of 30 degrees mm-1at 1 GHz and of 8 degrees mm-1at 10 GHz, whereas the transmission losses are less than 2 dB in the frequency range 2-20 GHz. We have also investigated the microwave detection properties of SnS, obtaining at 1 GHz a voltage responsivity of about 30 mV mW-1in the unbiased case and with an input power level of only 16µW.

Perspectives on Atomic-Scale Switches for High-Frequency Applications Based on Nanomaterials.

Nanomaterials science is becoming the foundation stone of high-frequency applications. The downscaling of electronic devices and components allows shrinking chip's dimensions at a more-than-Moore rate. Many theoretical limits and manufacturing constraints are yet to be taken into account. A promising path towards nanoelectronics is represented by atomic-scale materials. In this manuscript, we offer a perspective on a specific class of devices, namely switches designed and fabricated using two-dimensional or nanoscale materials, like graphene, molybdenum disulphide, hexagonal boron nitride and ultra-thin oxides for high-frequency applications. An overview is provided about three main types of microwave and millimeter-wave switch: filament memristors, nano-ionic memristors and ferroelectric junctions. The physical principles that govern each switch are presented, together with advantages and disadvantages. In the last part we focus on zirconium-doped hafnium oxide ferroelectrics (HfZrO) tunneling junctions (FTJ), which are likely to boost the research in the domain of atomic-scale materials applied in engineering sciences. Thanks to their Complementary Metal-Oxide Semiconductor (CMOS) compatibility and low-voltage tunability (among other unique physical properties), HfZrO compounds have the potential for large-scale applicability. As a practical case of study, we present a 10 GHz transceiver in which the switches are FTJs, which guarantee excellent isolation and ultra-fast switching time.

Tunable Microwave Filters Using HfO2-Based Ferroelectrics.

In this paper, we present microwave filters that are based on 6-nm-thick ferroelectric thin films of hafnium oxide doped with zirconium (HfZrO), which are tunable continuously in targeted bands of interest within the frequency range 0.1-16 GHz, when the applied direct current (DC) voltage is swept between 0 V and 4 V. Here, we exploit the orthorhombic polar phase in HfO2 through a careful doping using zirconium in an Atomic Layer Deposition (ALD) process, in order to guarantee phase stabilization at room temperature. Polarization versus voltage characterization has been carried out, showing a remanent polarization (Pr) of ~0.8 µC/cm2 and the coercive voltage at ~2.6 V. The average roughness has been found to be 0.2 nm for HfZrO films with a thickness of 6 nm. The uniform topography, without holes, and the low surface roughness demonstrate that the composition and the structure of the film are relatively constant in volume. Three filter configurations (low-pass, high-pass, and band-pass) have been designed, modelled, fabricated, and fully characterized in microwaves, showing a frequency shift of the minimum of the reflection coefficient between 90 MHz and 4.4 GHz, with a minimum insertion loss of approximately 6.9 dB in high-pass configuration.

Aero-Ga2O3 Nanomaterial Electromagnetically Transparent from Microwaves to Terahertz for Internet of Things Applications.

In this paper, fabrication of a new material is reported, the so-called Aero-Ga2O3 or Aerogallox, which represents an ultra-porous and ultra-lightweight three-dimensional architecture made from interconnected microtubes of gallium oxide with nanometer thin walls. The material is fabricated using epitaxial growth of an ultrathin layer of gallium nitride on zinc oxide microtetrapods followed by decomposition of sacrificial ZnO and oxidation of GaN which according to the results of X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) characterizations, is transformed gradually in ß-Ga2O3 with almost stoichiometric composition. The investigations show that the developed ultra-porous Aerogallox exhibits extremely low reflectivity and high transmissivity in an ultrabroadband electromagnetic spectrum ranging from X-band (8-12 GHz) to several terahertz which opens possibilities for quite new applications of gallium oxide, previously not anticipated.

MoS2 radio: detecting radio waves with a two-dimensional transition metal dichalcogenide semiconductor.

In this paper, we designed, fabricated and tested a microwave circuit based on a MoS2 self-switching diode. The MoS2 thin film (10-monolayers nominal thickness) was grown on a 4 inch Al2O3/high-resistivity silicon wafer by chemical vapor deposition process. The Raman measurements confirm the high quality of the MoS2 over the whole area of the 4 inch wafer. We show experimentally that a microwave circuit based on a few-layers MoS2 self-switching diode fabricated at the wafer level is able to detect the audio spectrum from amplitude-modulated microwave signals in the band 0.9-10 GHz, i.e. in the frequency range mostly used by current wireless communications. In particular, the 900 MHz band is widely exploited for GSM applications, whereas the 3.6 GHz band has been identified as the primary pioneer band for 5G in the European Union.

Electromagnetic interference shielding in X-band with aero-GaN.

We investigate the electromagnetic shielding properties of an ultra-porous lightweight nanomaterial named aerogalnite (aero-GaN). Aero-GaN is made up of randomly arranged hollow GaN microtetrapods, which are obtained by direct growth using hydride vapor phase epitaxy of GaN on the sacrificial network of ZnO microtetrapods. A 2 mm thick aero-GaN sample exhibits electromagnetic shielding properties in the X-band similar to solid structures based on metal foams or carbon nanomaterials. Aero-GaN has a weight four to five orders of magnitude lower than the weight of metals.

Electromagnetic energy harvesting based on HfZrO tunneling junctions.

HfZrO ferroelectrics with a thickness of 6 nm were grown directly on Si using atomic layer deposition, top and bottom metallic electrodes being subsequently deposited by electron-beam metallization techniques. Depending on the polarity of the ±10 V poling voltages, the current-voltage dependence of these tunneling diodes shows a rectifying behavior for different polarizations, the ON-OFF ratio being about 104. Because the currents are at mA level, the HfZrO tunneling diodes coupled to an antenna array can harvest electromagnetic energy at 26 GHz (a bandwidth designated for internet of things), with a responsivity of 63 V W-1 and a NEP of 4 nW/Hz0.5.

Very large phase shift of microwave signals in a 6 nm Hf x Zr1-x O2 ferroelectric at ±3 V.

In this letter, we report for the first time very large phase shifts of microwaves in the 1-10 GHz range, in a 1 mm long gold coplanar interdigitated structure deposited over a 6 nm Hf x Zr1-x O2 ferroelectric grown directly on a high resistivity silicon substrate. The phase shift is larger than 60° at 1 GHz and 13° at 10 GHz at maximum applied DC voltages of ±3 V, which can be supplied by a simple commercial battery. In this way, we demonstrate experimentally that the new ferroelectrics based on HfO2 could play an important role in the future development of wireless communication systems for very low power applications.