Design and experimental characterization of antennas and wireless systems for innovative wearable and implantable ultra low–power applications

The present PhD thesis exploits the design skills I have been improving since my master thesis’ research. A brief description of the chapters’ content follows. Chapter 1: the simulation of a complete front–end is a very complex problem and, in particular, is the basis upon which the prediction of the overall performance of the system is possible. By means of a commercial EM simulation tool and a rigorous nonlinear/EM circuit co–simulation based on the Reciprocity Theorem, the above–mentioned prediction can be achieved and exploited for wireless links characterization. This will represent the theoretical basics of the entire present thesis and will be supported by two RF applications. Chapter 2: an extensive dissertation about Magneto–Dielectric (MD) materials will be presented, together with their peculiar characteristics as substrates for antenna miniaturization purposes. A designed and tested device for RF on–body applications will be described in detail. Finally, future research will be discussed. Chapter 3: this chapter will deal with the issue regarding the exploitation of renewable energy sources for low–energy consumption devices. Hence the problem related to the so–called energy harvesting will be tackled and a first attempt to deploy THz solar energy in an innovative way will be presented and discussed. Future research will be proposed as well. Chapter 4: graphene is a very promising material for devices to be exploited in the RF and THz frequency range for a wide range of engineering applications, including those ones marked as the main research goal of the present thesis. This chapter will present the results obtained during my research period at the National Institute for Research and Development in Microtechnologies (IMT) in Bucharest, Romania. It will concern the design and manufacturing of antennas and diodes made in graphene–based technology for detection/rectification purposes

Extraordinary tunability of high-frequency devices using Hf0.3Zr0.7O2 ferroelectric at very low applied voltages

This paper presents the applications of the Hf0.3Zr0.7O2 ferroelectric with a thickness of 10 nm for tuning high-frequency devices such as filters, phase shifters, and phased antenna arrays in the X band when the low bias voltages in the range −3 V–+3 V are applied. In this respect, we show that a bandpass filter shifts its central frequency located at 10 GHz with 3 GHz, a phase shifter produces a phase difference of about 60 degrees in the X band, while the antenna array formed by two patched antennas is steering its lobe with ±32° at 10 GHz. These results open the way for the tunability of high frequency devices for very low power applications, which represent one of the most challenging issues in applied physics

Very large phase shift of microwave signals in a 6 nm Hf x Zr 1− x O 2 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

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

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

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 β-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

Harvesting electromagnetic energy in the V-band using a rectenna formed by a bow tie integrated with a 6-nm-thick Au/HfO2/Pt metal-insulator-metal diode

In this paper, the first demonstration of a bow-tie antenna integrated with a metal-insulator-metal (MIM) diode for electromagnetic energy harvesting in the V-band (i.e., 40-75 GHz) is presented. We have designed, simulated, fabricated, and fully characterized a 60-GHz rectifying antenna (rectenna) based on a vertical Au-HfO2-PtMIM diode with reduced differential resistance. The dielectric used for the MIM structure is a 6-nm-thick amorphous HfO2 grown by atomic layer deposition. For the fabricated MIM device, we report here a current density of 3 x 10(4) A/cm(2) that exceeds the previous values presented in the literature. The vertical MIM-based rectenna is able to efficiently harvest up to 250 mu V from an impinging modulated millimeter-wave signal with -20 dBm of available power, thus offering a voltage responsivity of over 5 V/W. The reported results indicate that the proposed approach is well suited for future low-power solutions much sought after for the energetically autonomous 5G terminal equipment

Oxygen-vacancy induced ferroelectricity in nitrogen-doped nickel oxide

This paper reports the onset of ferroelectricity in NiO by breaking the crystallographic symmetry with oxygen vacancies created by N doping. Nitrogen-doped NiO was grown at room temperature by RF sputtering of Ni target in Ar–O2–N2 plasma on silicon and fused silica substrates. The impact of the nitrogen doping of NiO on microstructural, optical, and electrical properties has been investigated. According to x-ray diffraction investigations, by increasing the N doping level in NiO, a transition from (002) to a (111) preferential orientation for the cubic NiO phase was observed, as well as a lattice strain relaxation, that is usually ascribed to structural defect formation in crystal. The x-ray diffraction pole figures the presence of a distorted cubic structure in NiO and supports the Rietveld refinement findings related to the strain, which pointed out that nitrogen doping fosters lattice imperfections formation. These findings were found to be in agreement with our far-infrared measurements that revealed that upon nitrogen doping a structural distortion of the NiO cubic phase appears. X-ray photo-emission spectroscopy measurements reveal the presence of oxygen vacancies in the NiO film following nitrogen doping. Evidence of ferro-electricity in nitrogen-doped NiO thin films has been provided by using the well-established Sawyer–Tower method. The results reported here provide the first insights on oxygen-vacancy induced ferroelectricity in nitrogen-doped nickel oxide thin films

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