Jinwang Li

High-Performance Solution-Processed ZrInZnO Thin-Film Transistors

We report on the high performance and high stability of thin-film transistors (TFTs) using solution-processed Zr-In-Zn-O (ZIZO) as an active layer. The effects of adding Zr to In-Zn-O, particularly the electrical characteristics of their thin films and TFTs, were systematically investigated. The Zr effectively controlled the oxygen vacancies because of its low standard electrode potential, which was confirmed by modifications in the optical bandgap energy, carrier concentration, and oxygen-vacancy density of the ZIZO thin films. Consequently, we found that the “off” current decreased and the threshold voltage increased with the increasing Zr content. The optimal ZIZO TFT was obtained at a Zr/In/Zn mole ratio of 0.05 : 2 : 1, and its “on/off” ratio, channel mobility, and subthreshold swing voltage were ~ 109 , 6.23 cm2·V-1·s-1 , and 0.19 V/dec, respectively, which are comparable to those of vacuum-processed oxide TFTs. Furthermore, the performance and bias-stress stability of the ZIZO TFTs were improved as a result of the reduced interface charge trapping.

A low-temperature crystallization path for device-quality ferroelectric films

We show a path for low-temperature crystallization of device-quality solution-processed lead zirconate titanate films. The essential aspect of the path is to circumvent pyrochlore formation at around 300 °C during temperature increase up to 400 °C. By maintaining enough carbon via pyrolysis at 210 °C, well below the temperature for pyrochlore formation, Pb2+ can be reduced to Pb0. This leads to the lack of Pb2+ in the film to suppress the development of pyrochlore, which accounts for the usual high-temperature conversion to perovskite. Films on metal, metal/oxide hybrid, and oxide bottom electrodes were successfully crystallized at 400–450 °C.

Rheology printing for metal-oxide patterns and devices

Technologies of device printing have been widely explored, but existing printing techniques still cannot produce well-defined patterns required by fine electronic devices. Here, a new printing method is proposed and the printing of metal-oxide patterns with well-defined shapes was demonstrated. Excellent thin-film transistors with channel lengths around 500 nm were completely printed by this method in an air atmosphere. This printing utilizes a viscoelastic transformation of the precursor gel when imprinted; it softens at a certain temperature during thermal-imprinting so that the gel can be rheologically imprinted. The imprinted pattern shows very small shrinkage during post-annealing, thereby achieving a high shape fidelity to the mould; this results from metal-oxide condensation at imprinting. The viscoelastic transformation and metal-oxide condensation at imprinting constitute the basis for this printing method, which is closely related to the cluster structure in the precursor gel. This method has worked for patterns down to several tens of nanometers.

Optimization of Pt and PZT Films for Ferroelectric-Gate Thin Film Transistors

The polycrystalline Pt film with an excellent (111) orientation and a small grain size of about 30 nm was successfully prepared on a SiO2/Si substrate by the new structured-sputtering system. By optimizing annealing process and using the highly (111)-oriented Pt film as a bottom electrode, an epitaxial-grade (111)-oriented PZT film was successfully prepared by the sol-gel method. Operation of the ferroelectric-gate thin film transistor (FGT) with indium-tin-oxide (ITO) channel, which was based on the optimum Pt and PZT films, has been verified. The FGT device exhibited good properties and performance with high “on/off” current ratio (∼105), adequate memory window (1.2 V) and small swing factor (∼88 mV/decade).

Solution processing of highly conductive ruthenium and ruthenium oxide thin films from ruthenium–amine complexes

Highly conductive ruthenium metal (Ru0) thin films and ruthenium oxide (RuO2) films were prepared by a solution process. Solutions prepared from ruthenium(III) nitrosyl acetate and amines were spin-coated, and Ru0 and RuO2 thin films were formed after annealing in an inert atmosphere (nitrogen or vacuum) and oxygen, respectively. It was observed that amine-coordinated oxo-ruthenium clusters were formed in the precursor solution. The coordinated amines resulted in a change from oxidative decomposition to reductive decomposition of the ruthenium precursor under inert atmospheric conditions. The effects of different amine structures were compared, and alkanolamine and amino acids were found to produce Ru0 films of higher quality (lower resistivity and higher surface flatness) than alkyl amines with primary amines being preferred to secondary and tertiary ones. These results were correlated with the structures of ruthenium complexes revealed by methods including high-energy X-ray diffraction with total correlation function and pair distribution function analyses. The resistivity values of Ru0 and RuO2 thin films prepared from ruthenium–alkanolamine complexes were 2.1 × 10−5 and 4.3 × 10−4 Ω cm, respectively, similar to those of vacuum-processed Ru0 and RuO2 ones. The Ru0 film showed high stability against oxidation during further annealing in oxygen, even at nanometer-thickness (e.g., 25 nm).

Low-Temperature All-Solution-Derived Amorphous Oxide Thin-Film Transistors

We prepared thin-film transistors (TFTs) in which all the layers were fabricated using simple chemical solution-processed, vacuum-free routes, followed by thermal annealing at 400°C. A ruthenium oxide film prepared via low-temperature processing was used for both gate and source/drain electrodes. Amorphous lanthanum-zirconium oxide and zirconium-indium-zinc oxide films were used as the gate insulator and channel layer, respectively, which enabled the fabrication of a TFT with the desired performance at a sufficiently low temperature. The ultraviolet-ozone treatment was adopted to channel layer to facilitate precursor decomposition and condensation processes. As a result, the obtained ON/OFF ratio, subthreshold swing voltage, and channel mobility were ~ 6×105, 250 mV/decade, and 5.80 cm2V1 s-1, respectively. This result contributes to the development of sustainable completely printed inorganic electronics.

Highly conductive p-type amorphous oxides from low-temperature solution processing

We report solution-processed, highly conductive (resistivity 1.3-3.8 m{Omega} cm), p-type amorphous A-B-O (A = Bi, Pb; B = Ru, Ir), processable at temperatures (down to 240 Degree-Sign C) that are compatible with plastic substrates. The film surfaces are smooth on the atomic scale. Bi-Ru-O was analyzed in detail. A small optical bandgap (0.2 eV) with a valence band maximum (VBM) below but very close to the Fermi level (binding energy E{sub VBM} = 0.04 eV) explains the high conductivity and suggests that they are degenerated semiconductors. The conductivity changes from three-dimensional to two-dimensional with decreasing temperature across 25 K.

Hybrid Cluster Precursors of the LaZrO Insulator for Transistors: Properties of High-Temperature-Processed Films and Structures of Solutions, Gels, and Solids

In the solution processing of oxide electronics, the structure of metal–organic precursors in solution and their effect on processability and on the final structure and properties of the oxide have rarely been studied. We have observed that hybrid clusters, having inorganic cores coordinated by organic ligands, are the typical form of metal–organic precursor structures. For insulating ternary LaZrO, improved synthesis of the cluster precursor under solvothermal conditions led to low-temperature deposition of the film at 200 °C, as we will report in another paper. In the current paper, we first briefly show that solvothermal synthesis of the precursor resulted in significantly improved insulating properties (e.g., two orders lower leakage current) of high-temperature-annealed films, and then focus on the structural analysis of the cluster precursors and annealed solids and relate the results to the significant improvement of properties by solvothermal treatment of solutions. A change in the cluster core toward structural unification was brought about by solvothermal treatment, resulting in higher uniformity and higher stability of clusters. The final structure of the material maintained the features of the core structure in solution, even after annealing at high temperatures. These results demonstrate the key role played by designing cluster structure in solution.

P-type conductive amorphous oxides of transition metals from solution processing

We report a series of solution-processed p-type conductive amorphous Ln-M-O (a-Ln-M-O, where M = Ru, Ir, and Ln is a lanthanide element except Ce) having low resistivities (10−3 to 10−2 Ω cm). These oxides are thermally stable to a high degree, being amorphous up to 800 °C, and processable below 400 °C. Their filmsurfaces are smooth on the atomic scale, and the process allows patterning simply by direct imprinting without distortion of the pattern after annealing. These properties have high potential for use in printed electronics. The electron configurations of these oxides are apparently different from existing p-type oxides.

Deposition of platinum patterns by a liquid process

In contrast to the traditional chemical vapor deposition technique under high vacuum, we introduce a deposition method in liquid to prepare Pt patterns on substrate near 100 °C by seed growth.