Qingpu Wang

Graphene ballistic nano-rectifier with very high responsivity

Although graphene has the longest mean free path of carriers of any known electronic material, very few novel devices have been reported to harness this extraordinary property. Here we demonstrate a ballistic nano-rectifier fabricated by creating an asymmetric cross-junction in single-layer graphene sandwiched between boron nitride flakes. A mobility ∼200,000 cm2 V−1 s−1 is achieved at room temperature, well beyond that required for ballistic transport. This enables a voltage responsivity as high as 23,000 mV mW−1 with a low-frequency input signal. Taking advantage of the output channels being orthogonal to the input terminals, the noise is found to be not strongly influenced by the input. Hence, the corresponding noise-equivalent power is as low as 0.64 pW Hz−1/2. Such performance is even comparable to superconducting bolometers, which however need to operate at cryogenic temperatures. Furthermore, output oscillations are observed at low temperatures, the period of which agrees with the lateral size quantization.

Analysis of carrier transport and band tail states in p-type tin monoxide thin-film transistors by temperature dependent characteristics

Tin monoxide (SnO) has drawn much attention in recent years due to its high hole mobility, transparency, and potential for mass production. However, due to its metastable nature, the deposited film often contains multi-phases such as metallic tin and tin dioxide, which may degrade its electrical properties. Here, we presented the temperature dependent characteristics of p-type SnO thin-film transistors. The hole transport mechanism is dominated by band conduction at high temperatures and variable-range hopping at low temperatures. The maximum activation energy was found to be 308 meV, which denotes a bandgap of around 0.6 eV. The density of states was found to be 1.12 × 1021 cm−3 eV−1 at VG = −80 V, and 6.75 × 1020 cm−3 eV−1 at VG = 0 V, respectively.

Performance regeneration of InGaZnO transistors with ultra-thin channels

Thin-film transistors (TFTs) based on ultra-thin amorphous indium gallium zinc oxide (a-IGZO) semiconductors down to 4 nm were studied motivated by the increasing cost of indium. At and below 5 nm, it was found that the field-effect mobility was severely degraded, the threshold voltage increased, and the output characteristics became abnormal showing no saturated current. By encapsulating a layer of polymethyl methacrylate on the IGZO TFTs, the performance of the 5-nm-thick device was effectively recovered. The devices also showed much higher on/off ratios, improved hysteresis, and normal output characteristic curves as compared with devices not encapsulated. The stability of the encapsulated devices was also studied over a four month period.

A Sputtered Silicon Oxide Electrolyte for High-Performance Thin-Film Transistors

Low operating voltages have been long desired for thin-film transistors (TFTs). However, it is still challenging to realise 1-V operation by using conventional dielectrics due to their low gate capacitances and low breakdown voltages. Recently, electric double layers (EDLs) have been regarded as a promising candidate for low-power electronics due to their high capacitance. In this work, we present the first sputtered SiO2 solid-state electrolyte. In order to demonstrate EDL behaviour, a sputtered 200 nm-thick SiO2 electrolyte was incorporated into InGaZnO TFTs as the gate dielectric. The devices exhibited an operating voltage of 1 V, a threshold voltage of 0.06 V, a subthreshold swing of 83 mV dec−1 and an on/off ratio higher than 105. The specific capacitance was 0.45 µF cm−2 at 20 Hz, which is around 26 times higher than the value obtained from thermally oxidised SiO2 films with the same thickness. Analysis of the microstructure and mass density of the sputtered SiO2 films under different deposition conditions indicates that such high capacitance might be attributed to mobile protons donated by atmospheric water. The InGaZnO TFTs with the optimised SiO2 electrolyte also showed good air stability. This work provides a new pathway to the realisation of high-yield low-power electronics.

Extremely Sensitive Dependence of SnOx Film Properties on Sputtering Power

An extremely sensitive dependence of the electronic properties of SnOx film on sputtering deposition power is discovered experimentally. The carrier transport sharply switches from n-type to p-type when the sputtering power increases by less than 2%. The best n-type carrier transport behavior is observed in thin-film transistors (TFTs) produced at a sputtering power just below a critical value (120 W). In contrast, at just above the critical sputtering power, the p-type behavior is found to be the best with the TFTs showing the highest on/off ratio of 1.79 × 104 and the best subthreshold swing among all the sputtering powers that we have tested. A further increase in the sputtering power by only a few percent results in a drastic drop in on/off ratio by more than one order of magnitude. Scanning electron micrographs, x-ray diffraction spectra, x-ray photoelectron spectroscopy, as well as TFT output and transfer characteristics are analyzed. Our studies suggest that the sputtering power critically affects the stoichiometry of the SnOx film.

Effects of substrate and anode metal annealing on InGaZnO Schottky diodes

By studying different annealing effects of substrate and anode metal, high-performance Schottky diodes based on InGaZnO (IGZO) film have been realized. It is observed that a suitable thermal annealing of the SiO2/Si substrate significantly improves the diode performance. On the contrary, annealing of the Pd anode increases surface roughness, leading to degradation in the diode performance. As such, by only annealing the substrate but not the anode, we are able to achieve an extremely high rectification ratio of 7.2 × 107, a large barrier height of 0.88 eV, and a near unity ideality factor of 1.09. The diodes exhibit the highest performance amongst IGZO-based Schottky diodes reported to date where IGZO layer is not annealed. The capacitance vs. voltage measurements indicate that the surface roughness is correlated with the trap state density at the Schottky interface.

Amorphous-InGaZnO Thin-Film Transistors Operating Beyond 1 GHz Achieved by Optimizing the Channel and Gate Dimensions

Amorphous indium–gallium–zinc oxide (a-InGaZnO or a-IGZO) has already started replacing amorphous silicon in backplane driver transistors for large-area displays. However, hardly any progress has been made to commercialize a-IGZO for electronic circuit applications mainly because a-IGZO transistors are not yet capable of operating at gigahertz frequencies. Here, nanoscale a-IGZO thin-film transistors (TFTs) are fabricated on a high-resistivity silicon substrate with a Ta2O5 gate dielectric. Carrier mobilities up to 18.2 cm2V−1s−1 have been achieved. By optimization of the TFT channel length and contact overlap, we are able to demonstrate current gain and power gain cutoff frequencies at 1.24 and 1.14 GHz, respectively, both beyond the 1-GHz benchmark. Such a performance may have implications in developing at least medium performance, a-IGZO-TFTs-based circuits for low-cost or flexible electronics.

Characteristics of sputtered Y-doped IZO thin films and devices*

Yttrium-doped IZO (YIZO) thin films with different thickness have been prepared on soda-lime glass (SLG) and P-Si substrates by radio frequency magnetron sputtering at room temperature. Structural morphology and optical properties of the films have been investigated. YIZO thin film transistors (TFTs) with the bottom-gate-structure are fabricated on P-Si substrates. The output and transfer characteristics of YIZO-TFT have been studied. It has been found that all YIZO thin films prepared at room temperature are amorphous, and the YIZO TFTs exhibit n-channel depletion mode. YIZO-TFT with active layer thickness of 20 nm shows an on/off ratio over 10 5 , a sub-threshold swing of 2.20 V/decade at a low operating voltage of -1.0 V, and saturation mobility values over 0.57 cm 2 /(V· s).

Highly Optimized Complementary Inverters Based on p-SnO and n-InGaZnO With High Uniformity

Oxide semiconductors are desirable for large-area and/or flexible electronics. Here, we report highly optimized complementary inverters based on n-type indium–gallium–zinc oxide and p-type tin monoxide thin-film transistors. Oxide-based inverters with a record voltage gain of 142 have been achieved. The switching point voltage has also been tuned to reach the ideal value, namely half value of the supply voltage. A narrow transition width of 1.04 V (13% of the supply voltage) is achieved which offers a strong anti-jamming ability to avoid logic errors. Rail-to-rail output voltage swing has been achieved. The inverters still maintain high performance at a low supply voltage of 6 V. A very large number of inverters have been fabricated and showed excellent uniformity in a working area of 1 cm

Ambipolar SnOx Thin-Film Transistors Achieved at High Sputtering Power

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