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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 Zr O 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 HfO could play an important role in the future development of wireless communication systems for very low power applications.

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A barium titanate precursor with a barium:titanium ratio of 1:4 was prepared by controlled coprecipitation of mixed barium and titanium species with an ammonium oxalate aqueous solution at pH 7. The results of thermal analysis and IR measurement show that the obtained precursor is a mixture of BaC 2 O 4 ⅐0.5H 2 O and TiO(OH) 2 ⅐1.5H 2 O in a molar ratio of 1:4. Crystallized BaTi 4 O 9 was obtained by the thermal decomposition of a precipitate precursor at 1300°C for 2 h in air. The dimensions of the powder calcined at 1000°C are between 100 and 300 nm. The grain dimensions of the sintered sample for 2 h at 1300°C are of the order of 10 to 30 m. Dielectric properties of disk-shaped sintered specimens in the microwave frequency region were measured using the TE 011 mode. Excellent microwave characteristics for BaTi 4 O 9 -‫؍‬ 38 ؎ 0.5, Q ‫؍‬ 3800 -4000 at 6 -7 GHz and f ‫؍‬ 11 ؎ 0.7 ppm/°C-were found.

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In memoriam of Prof. Dan Dascalu IntroductionHafnium oxide (HfO 2 ) is the dielectric commonly used in fieldeffect transistors (FETs) fabricated in complementary metal oxide semiconductor (CMOS) technologies and, as such, retrieved in any very large-scale integrated (VLSI) circuits. Examples of such circuits include microprocessors or complex analog integrated circuits found in daily applications, for instance, in computers, smartphones, laptops, or incorporated in cars, airplanes, or medical equipments. Since the discovery of HfO 2 as a dielectric for modern integrated circuits, increasing its permittivity was a leading research issue in itself because higher permittivity values allow decreasing the effective oxide thickness (EOT), and so the leakage current; this is especially important when billions of Si FETs are integrated on a single chip. The solution found to boost the permittivity of HfO 2 is represented by structural phase transformations. [1] HfO 2 has its lowest electrical permittivity (ε r % 18) in its stable phase, which is the monoclinic phase. Higher permittivity values, of ε r % 27 in the cubic phase and ε r % 70 in the tetragonal phase, are achieved, nonetheless, in phases that are stable only at very high temperatures, exceeding 1700 C. The decrease in these temperatures is possible, however, by HfO 2 doping. Thus, the cubic and the tetragonal phases were first obtained at lower temperatures via Y doping [1] and, respectively, Si doping, [2] whereas a stable HfO 2 tetragonal phase at nearly room temperature was observed by growing HfO 2 via atomic layer deposition (ALD) and doping it with ZrO 2 . [3,4] As described in the recent review, [5] the stabilization of these phases is determined by the concentration of dopants, the annealing temperature, and the growing methods.These initial investigations focused on increasing the permittivity of HfO 2 lead eventually to the discovery of the orthorhombic phase of HfO 2 , which was associated with HfO 2 ferroelectricity. [6,7] This phase is obtained under different growth conditions in the presence of dopants, including those mentioned earlier, and requires strict control of dopant concentration, temperature, and mechanical strain. The most used orthorhombic HfO 2 -based ferroelectric, named further as Hf ZrO, is doped with Zr and is usually grown at the wafer level, up to 300 mm, using ALD. It thus benefits from the state-of-the-art CMOS technology. More recently, using Zr-doping and pulsed laser deposition (PLD), another rhombohedral phase of HfO 2 was discovered to be also ferroelectric. [7] The rhombohedral or orthorhombic Hf ZrO are ferroelectrics with similar electric performances; the origin of stable ferroelectricity in both cases being due to the induced strain between the top and/or down substrates sandwiching Hf ZrO. [8] Doping or strain application modifies the atomic structure of a material and can induce a phase transition, in particular can induce ferroelectricity in HfO 2 . Similarly, HfO 2 can transform into a weak magnetic material [9...

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.

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.