Subthreshold slope below 60 mV/decade in graphene transistors induced by channel geometry at the wafer-scale.

In this paper, we demonstrate experimentally that field-effect transistors with nanoconstricted graphene monolayer channels have a subthreshold swing (SS) below 60 mV/dec, which is slightly dependent on temperature. Two shapes of nanoconstricted graphene monolayers are considered: (i) a bow-tie shape, representative for a symmetric channel, and (ii) a trapezoidal shape, which illustrates an asymmetric channel. While both types of nonuniform channels are opening a bandgap in graphene, thus showing an on/off ratio of 105, the SS in the graphene bow-tie channel is below 60 mV/dec in the temperature range 25 °C-44 °C.

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.

Reconfigurable horizontal-vertical carrier transport in graphene/HfZrO field-effect transistors.

We have fabricated at wafer level field-effect-transistors (FETs) having as channel graphene monolayers transferred on a HfZrO ferroelectric, grown by atomic layer deposition on a doped Si (100) substrate. These FETs display either horizontal or vertical carrier transport behavior, depending on the applied gate polarity. In one polarity, the FETs behave as a graphene FET where the transport is horizontal between two contacts (drain and grounded source) and is modulated by a back-gate. Changing the polarity, the transport is vertical between the drain and the back-gate and, irrespective of the metallic contact type, Ti/Au or Cr/Au, the source-drain bias modulates the height of the potential barrier between HfZrO and the doped Si substrate, the carrier transport being described by a Schottky mechanism at high gate voltages and by a space-charge limited mechanism at low gate voltages. Vertical transport is required by three-dimensional integration technologies to increase the density of transistors on chip.

Graphene bandgap induced by ferroelectric HfO2 doped with Zr (HfZrO).

Motivated by the need to open a bandgap in graphene, we show experimentally that the CMOS-compatible ferroelectric HfZrO substrate induces a bandgap of 0.18 eV in graphene monolayer, which allows top-gate graphene/HfZrO/SiO2/Si field-effect transistors to have high on/off current ratio, of about 103, at small drain voltages, of 0.5 V, and for gate voltage spans of only 3.5 V. In addition, these transistors have a very high transconductance, of about 1 mS, and carrier mobilities of 7900 cm2 Vs-1. The results show that this relatively simple and wafer-scale compatible bandgap opening method in graphene renders this material useful for digital low-power electronics.

Graphene bandgap induced by ferroelectric Pca21 HfO2 substrates: a first-principles study.

The electronic properties of graphene on top of ferroelectric HfO2 substrates in an orthorhombic phase with space group Pca21 are investigated using density functional theory calculations. The space group Pca21 was recently identified as one of the two potential candidates for ferroelectricity in hafnia, with the polarization direction oriented along the [001] direction. Our results indicate the appearance of sizable energy gaps in graphene induced by the HfO2 substrate, as a consequence of orbital hybridization and the locally deformed graphene structure. The gap sizes depend on the type of HfO2 termination interacting with graphene, showing larger gaps for oxygen terminated slabs compared to hafnium terminated ones. These observations may prove to be highly significant for the development of graphene based field effect transistors using high-k dielectrics.

Ballistic transport in graphene Y-junctions in transverse electric field.

We investigate the prospects for current modulation in single layer graphene Y-junctions in the ballistic regime, under an external electric field. Overcoming the inability of inducing field effect in graphene nanoribbons by a stacked gate, the proposed in-plane electric field setup enables a controlled current transfer between the branches of the Y-junction. This behavior is further confirmed by changing the angular incidence of the electric field. The ballistic transmission functions are calculated for the three terminal system using the non-equilibrium Green's function formalism, in the framework of density functional theory, under finite bias conditions. The edge currents dominating the transport in zigzag nanoribbons are strongly influenced by the induced dipole charge, facilitating the current modulation even for the metallic-like character of the Y-junctions. Spin polarization effects indicate the possibility of achieving spin filtering even in the absence of the external field provided the antiferromagnetic couplings between the edges are asymptotically set. Overall, our results indicate a robust behavior regarding the tunability of the charge current in the two outlet ports, showing the possibility of inducing field effect control in a single layer graphene system.

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.

Wafer-scale very large memory windows in graphene monolayer/HfZrO ferroelectric capacitors.

We have fabricated and electrically characterized at the wafer scale tens of metal-ferroelectric (HfZrO)-semiconductor capacitors and metal-graphene monolayer-ferroelectric (HfZrO)-semiconductor capacitors with the same top electrode dimensions. We have found that the memory windows of the capacitors containing graphene are 3-4 times larger than the ferroelectric capacitors without graphene, and increase even more after annealing. This physical effect can be attributed to the additional electric field exerted by the graphene monolayer on the HfZrO ferroelectric semiconductor capacitor, and to the negative thermal extension coefficient of graphene, respectively.

Room temperature nanostructured graphene transistor with high on/off ratio.

We report the batch fabrication of graphene field-effect-transistors (GFETs) with nanoperforated graphene as channel. The transistors were cut and encapsulated. The encapsulated GFETs display saturation regions that can be tuned by modifying the top gate voltage, and have on/off ratios of at least 2 × 103 at room temperature and at small drain and gate voltages. In addition, the nanoperforated GFETs display orders of magnitude higher photoresponses than any room-temperature graphene detector configurations that do not involve heterostructures with bandgap materials.

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.

Memristive GaN ultrathin suspended membrane array.

We show that ultrathin GaN membranes, with a thickness of 15 nm and planar dimensions of 12 × 184 µm(2), act as memristive devices. The memristive behavior is due to the migration of the negatively-charged deep traps, which form in the volume of the membrane during the fabrication process, towards the unoccupied surface states of the suspended membranes. The time constant of the migration process is of the order of tens of seconds and varies with the current or voltage sweep.

Ultra-lightweight pressure sensor based on graphene aerogel decorated with piezoelectric nanocrystalline films.

In this paper, we report on a pressure sensor based on graphene aerogel functionalized with SnO2 or GaN thin films deposited by magnetron sputtering. Decoration by nanocrystalline SnO2 or GaN was found to enhance the piezoresistive response of the bare aerogel. The responsivity and pressure sensing range (from 1-5 atm) of the sensor are shown to be higher than those inherent in pressure sensors based on graphene membranes.

Graphene-based room-temperature implementation of a modified Deutsch-Jozsa quantum algorithm.

We present an implementation of a one-qubit and two-qubit modified Deutsch-Jozsa quantum algorithm based on graphene ballistic devices working at room temperature. The modified Deutsch-Jozsa algorithm decides whether a function, equivalent to the effect of an energy potential distribution on the wave function of ballistic charge carriers, is constant or not, without measuring the output wave function. The function need not be Boolean. Simulations confirm that the algorithm works properly, opening the way toward quantum computing at room temperature based on the same clean-room technologies as those used for fabrication of very-large-scale integrated circuits.

Negative differential resistance in graphene-based ballistic field-effect transistor with oblique top gate.

Negative differential resistance (NDR) with a room temperature peak-to-valley ratio of 8 was observed in a graphene-based ballistic field-effect transistor (FET) with an oblique top gate. Graphene FETs with a top gate inclination angle of 45° and a drain-source distance of 400 nm were fabricated on a chip cut from a 4-inch graphene wafer grown by chemical vapor deposition (CVD). Of the 60 measured devices, NDR was observed only in the regions where the CVD graphene displayed a Raman signature of defectless monolayers. In other specific positions on the wafer, where graphene quality was not high enough, and the Raman signature indicated the presence of defects, the ballistic character of transport was lost and the graphene FETs displayed nonlinear drain-voltage dependences tuned by the top-gate and back-gate voltage.

Nanomaterials for Energy Harvesting.

Energy harvesting is no longer simply an academic issue; it has grown into a problem with real industrial and even social significance [...].

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.

Tuning the infrared resonance of thermal emission from metasurfaces working in near-infrared.

We simulated numerically and demonstrated experimentally that the thermal emittance of a metasurface consisting of an array of rectangular metallic meta-atoms patterned on a layered periodic dielectric structure grown on top of a metallic layer can be tuned by changing several parameters. The resonance frequency, designed to be in the near-infrared spectral region, can be tuned by modifying the number of dielectric periods, and the polarization and incidence angle of the incoming radiation. In addition, the absorbance/emittance value at the resonant wavelength can be tuned by modifying the orientation of meta-atoms with respect to the illumination direction.