Miniature Integrated 2.4 GHz Rectennas Using Novel Tunnel Diodes.

This work presents the design, fabrication, and measured results of a fully integrated miniature rectenna using a novel tunnel diode known as the Asymmetrical Spacer Layer Tunnel (ASPAT). The term rectenna is an abbreviation for a rectifying antenna, a device with a rectifier and antenna coexisting as a single design. The ASPAT is the centrepiece of the rectifier used for its strong temperature independence, zero bias, and high dynamic range. The antenna is designed to be impedance matched with the rectifier, eliminating the need for a matching network and saving valuable real estate on the gallium arsenide (GaAs) substrate. The antenna is fully integrated with the rectifier on a single chip, thus enabling antenna miniaturisation due to the high dielectric constant of GaAs and spiral design. This miniaturisation enables the design to be fabricated economically on a GaAs substrate whilst being comparable in size to a 15-gauge needle, thus unlocking applications in medical implants. The design presented here has a total die size of 4 × 1.2 mm2, with a maximum measured output voltage of 0.97 V and a 20 dBm single-tone 2.35 GHz signal transmitted 5 cm away from the rectenna.

Molecular Precursor Route to Bournonite (CuPbSbS3) Thin Films and Powders.

Quaternary metal chalcogenides have attracted attention as candidates for absorber materials for inexpensive and sustainable solar energy generation. One of these materials, bournonite (orthorhombic CuPbSbS3), has attracted much interest of late for its properties commensurate with photovoltaic energy conversion. This paper outlines the synthesis of bournonite for the first time by a discrete molecular precursor strategy. The metal dithiocarbamate complexes bis(diethyldithiocarbamato)copper (II) (Cu(S2CNEt2)2, (1)), bis(diethyldithiocarbamato)lead (II) (Pb(S2CNEt2)2, (2)), and bis(diethyldithiocarbamato)antimony (III) (Sb(S2CNEt2)3, (3)) were prepared, characterized, and employed as molecular precursors for the synthesis of bournonite powders and the thin film by solvent-less pyrolysis and spray-coat-pyrolysis techniques, respectively. The polycrystalline powders and thin films were characterized by powder X-ray diffraction (p-XRD), which could be indexed to orthorhombic CuPbSbS3. The morphology of the powder at the microscale was studied using scanning electron microscopy (SEM). Energy-dispersive X-ray spectroscopy (EDX) was used to elucidate an approximately 1:1:1:3 Cu/Pb/Sb/S elemental ratio. An optical band gap energy of 1.55 eV was estimated from a Tauc plot, which is close to the theoretical value of 1.41 eV.

Dynamic modulation of the Fermi energy in suspended graphene backgated devices.

Freestanding (suspended) graphene films, with high electron mobility (up to ~200,000 cm2V-1s-1), good mechanical and electronic properties, could resolve many of the current issues that are hampering the upscaling of graphene technology. Thus far, attempts at reliably fabricating suspended graphene devices comprising metal contacts, have often been hampered by difficulties in exceeding sizes of 1 µm in diameter, if using UV lithography. In this work, area of suspended graphene large enough to be utilized in microelectronic devices, have been obtained by suspending a CVD graphene film over cavities, with top contacts defined through UV lithography with both wet and dry etching. An area of up to 160 µm2 can be fabricated as backgated devices. The suspended areas exhibit rippling of the surfaces which simultaneously introduces both tensile and compressive strain on the graphene film. Finally, the variations of the Fermi level in the suspended graphene areas can be modulated by applying a potential difference between the top contacts and the backgate. Having achieved large area suspended graphene, in a manner compatible with CMOS fabrication processes, together with enabling the modulation of the Fermi level, are substantial steps forward in demonstrating the potential of suspended graphene-based electronic devices and sensors.

A novel low temperature soft reflow process for the fabrication of deep-submicron (<0.35 µm) T-gate pseudomorphic high electron mobility transistor structures.

We report a new and simple low temperature soft reflow process using solvent vapour. The combination of this soft reflow and conventional i-line lithography enables low cost, highly efficient fabrication at the deep-submicron scale. Compared to the conventional thermal reflow process, the key benefits of the new soft reflow process are its low temperature operation (<50 °C), greater shrinkage of the structure size (up to 75%) and better controllability. Gate openings reflowed from 1 µm to 250 nm have been routinely and reproducibly achieved by utilizing the saturation characteristics of the process. The feasibility of this soft reflow process is demonstrated in the fabrication of a 350 nm T-gate pseudomorphic high electron mobility transistor. By shrinking the gate length by a factor of three (from a 1 µm initial opening), the output current is improved by 60% (500 mA mm(-1) from 300 mA mm(-1)) and f(T) and f(MAX) are increased to 70 GHz (from 20 GHz) and 120 GHz (from 40 GHz) respectively. The proposed soft reflow could potentially be applied on other compatible substrates such as polymer based material for organic or thin film devices, potentially leading to many new possible applications.

Direct synthesis of nanostructured silver antimony sulfide powders from metal xanthate precursors.

Silver(I) ethylxanthate [AgS2COEt] (1) and antimony(III) ethylxanthate [Sb(S2COEt)3] (2) have been synthesised, characterised and used as precursors for the preparation of AgSbS2 powders and thin films using a solvent-free melt method and spin coating technique, respectively. The as-synthesized AgSbS2 powders were characterized by powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. The crystalline AgSbS2 powder was investigated using XRD, which shows that AgSbS2 has cuboargyrite as the dominant phase, which was also confirmed by Raman spectroscopy. SEM was also used to study the morphology of the resulting material which is potentially nanostructured. EDX spectra gives a clear indication of the presence of silver (Ag), antimony (Sb) and sulfur (S) in material, suggesting that decomposition is clean and produces high quality AgSbS2 crystalline powder, which is consistent with the XRD and Raman data. Electronic properties of AgSbS2 thin films deposited by spin coating show a p-type conductivity with measured carrier mobility of 81 cm2 V-1 s-1 and carrier concentration of 1.9 × 1015 cm-3. The findings of this study reveal a new bottom-up route to these compounds, which have potential application as absorber layers in solar cells.

A molecular precursor route to quaternary chalcogenide CFTS (Cu2FeSnS4) powders as potential solar absorber materials.

In the present work we report on the synthesis of tetragonal stannite Cu2FeSnS4 powders using a solvent free melt method using a mixture of Cu, Fe, and Sn(ii)/Sn(iv) O-ethylxanthates heated at different temperatures. The as-synthesized powders were characterized by powder X-ray diffraction (p-XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-Vis absorption spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy, which confirm the successful synthesis of stannite CFTS. Optical measurements show that Cu2FeSnS4 powders have visible light absorption onsets in the far red with direct band gap energies in the range 1.32-1.39 eV which are suitable for acting as efficient absorber layers in solar cells. Electronic characterisation of these materials deposited as thin films by spin coating show that they are p type semiconductors with respectable carrier mobilities of ca. 60 cm2 V-1 s-1 with carrier densities on the order of 1014 cm-1.

The synthesis and characterization of Cu2ZnSnS4 thin films from melt reactions using xanthate precursors.

Kesterite, Cu2ZnSnS4 (CZTS), is a promising absorber layer for use in photovoltaic cells. We report the use of copper, zinc and tin xanthates in melt reactions to produce Cu2ZnSnS4 (CZTS) thin films. The phase of the as-produced CZTS is dependent on decomposition temperature. X-ray diffraction patterns and Raman spectra show that films annealed between 375 and 475 °C are tetragonal, while at temperatures <375 °C hexagonal material was obtained. The electrical parameters of the CZTS films have also been determined. The conduction of all films was p-type, while the other parameters differ for the hexagonal and tetragonal materials: resistivity (27.1 vs 1.23 Ω cm), carrier concentration (2.65 × 10+15 vs 4.55 × 10+17 cm-3) and mobility (87.1 vs 11.1 cm2 V-1 s-1). The Hall coefficients were 2.36 × 103 versus 13.7 cm3 C-1.

Extracting random numbers from quantum tunnelling through a single diode.

Random number generation is crucial in many aspects of everyday life, as online security and privacy depend ultimately on the quality of random numbers. Many current implementations are based on pseudo-random number generators, but information security requires true random numbers for sensitive applications like key generation in banking, defence or even social media. True random number generators are systems whose outputs cannot be determined, even if their internal structure and response history are known. Sources of quantum noise are thus ideal for this application due to their intrinsic uncertainty. In this work, we propose using resonant tunnelling diodes as practical true random number generators based on a quantum mechanical effect. The output of the proposed devices can be directly used as a random stream of bits or can be further distilled using randomness extraction algorithms, depending on the application.