Yuzhi Li

Coffee-Ring Defined Short Channels for Inkjet-Printed Metal Oxide Thin-Film Transistors

Short-channel electronic devices several micrometers in length are difficult to implement by direct inkjet printing due to the limitation of position accuracy of the common inkjet printer system and the spread of functional ink on substrates. In this report, metal oxide thin-film transistors (TFTs) with channel lengths of 3.5 ± 0.7 μm were successfully fabricated with a common inkjet printer without any photolithography steps. Hydrophobic CYTOP coffee stripes, made by inkjet-printing and plasma-treating processes, were utilized to define the channel area of TFTs with channel lengths as short as ∼3.5 μm by dewetting the inks of the source/drain (S/D) precursors. Furthermore, by introduction of an ultrathin layer of PVA to modify the S/D surfaces, the spreading of precursor ink of the InOx semiconductor layer was well-controlled. The inkjet-printed short-channel TFTs exhibited a maximum mobility of 4.9 cm(2) V(-1) s(-1) and an on/off ratio of larger than 10(9). This approach of fabricating short-channel TFTs by inkjet printing will promote the large-area fabrication of short-channel TFTs in a cost-effective manner.

High-mobility thin film transistors with neodymium-substituted indium oxide active layer

Thin-film transistors (TFTs) with neodymium-substituted indium oxide (InNdO) channel layer were demonstrated. The structural properties of the InNdO films as a function of annealing temperature have been analyzed using X-ray diffraction and transmission electron microscopy. The InNdO thin films showed polycrystalline nature when annealed at 450 °C with a lattice parameter (cubic cell) of 10.255 A, which is larger than the cubic In2O3 film (10.117 A). The high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy showed that no Nd2O3 clusters were found in the InNdO film, implying that Nd was incorporated into the In2O3 lattice. The InNdO TFTs annealed at 450 °C exhibited more excellent electrical properties with a high mobility of 20.4 cm2 V−1 s−1 and better electric bias stability compared to those annealed at 300 °C, which was attributed to the reduction of the scattering centers and/or charge traps due to the decrease of the |Nd3d5/254f4O2p−1⟩ electron configuration.

All Inkjet-Printed Metal-Oxide Thin-Film Transistor Array with Good Stability and Uniformity Using Surface-Energy Patterns

An array of inkjet-printed metal-oxide thin-film transistors (TFTs) is demonstrated for the first time with the assistance of surface-energy patterns prepared by printing pure solvent to etch the ultrathin hydrophobic layer. The surface-energy patterns not only restrained the spreading of inks but also provided a facile way to regulate the morphology of metal oxide films without optimizing ink formulation. The fully printed InGaO TFT devices in the array exhibited excellent electron transport characteristics with a maximum mobility of 11.7 cm2 V-1 s-1, negligible hysteresis, good uniformity, and good stability under bias stress. The new route lights a general way toward fully inkjet-printed metal-oxide TFT arrays.

Studies on NdxIn1−xO3 semiconducting thin films prepared by rf magnetron sputtering

Neodymium-substituted indium oxide (NdxIn1−xO3, NIO) semiconducting thin films fabricated by rf sputtering were investigated. It was found that the incorporation of Nd atoms would lead to broadening the optical band gap, suppressing the grain growth, and reducing the free carrier concentration. The field-effect transistors with different NIO (5%, 15%, and 25% Nd concentration of the targets) channel layers exhibited similar electrical stability under positive gate-bias-stress, but the ones with 15% and 25% Nd concentration displayed much better stability under negative gate-bias-stress. Detailed studies showed that the content of |Nd3d5/254f4O2p−1> electron configuration decreased as the Nd concentration increased, resulting in the reduction of holes during negative-bias-stress. And the reduction of the |Nd3d5/254f4O2p−1> content as the Nd concentration increased was ascribed to less overlap between the metal and ligand orbitals arose from large lattice expansion.

Solution-processed indium-zinc-oxide thin-film transistors based on anodized aluminum oxide gate insulator modified with zirconium oxide

Solution-processed indium-zinc-oxide (IZO) thin-film transistors (TFTs) based on anodized aluminum oxide gate insulator modified with a zirconium oxide (ZrOx) interlayer were fabricated. By introduction of the ZrOx interlayer, the IZO-TFTs exhibited improved performance with a higher mobility of 7.8 cm2 V−1 s−1, a lower Vth of 4.6 V and a lower SS of 0.21 V dec−1 compared to those without the ZrOx interlayer. Comprehensive studies showed that the Al element easily diffused into the IZO film and formed AlOx clusters which acted as defects to deteriorate TFT performance; and after modification with a ZrOx interlayer, the diffusion of Al was suppressed and the Zr diffusing effect almost could be ignored. These results suggested that the introduction of an interlayer with less diffusing effect as well as an effect of blocking the elements from the gate insulator diffusing into the channel layer could be an effective way to improve the electrical performance for solution-processed oxide TFTs.

High mobility flexible polymer thin-film transistors with an octadecyl-phosphonic acid treated electrochemically oxidized alumina gate insulator

Flexible solution-processed polymer thin-film transistors (PTFTs) with a low band-gap (LBG) donor–acceptor (D–A) conjugated polymer as the active layer and electrochemically oxidized alumina (AlOx:Nd) as the gate insulator are fabricated on polyethylene naphthalate (PEN) substrates. The AlOx:Nd insulator exhibits excellent insulating properties with low leakage current, a high dielectric constant and a high breakdown field. To improve the interface coupling between the polymer active layer and the AlOx:Nd insulator, the AlOx:Nd insulator is treated with octadecyl-phosphonic acid (ODPA), forming self-assembled monolayers (SAMs) on the surface, and great improvement in TFT performance with the highest mobility of 2.88 cm2 V−1 s−1 is attained. The performance improvement is attributed to the smoother surface and lower surface energy of the ODPA-treated AlOx:Nd compared to those of bare AlOx:Nd. In addition, the flexible PTFT exhibits only small shifts in the transfer curves at bending curvatures (R) at 30 mm, but the device shows larger threshold voltage and higher off current (Ioff) after bent at R = 5–20 mm, which may be attributed to the damage in the insulator-semiconductor interface.

Facile patterning of amorphous indium oxide thin films based on a gel-like aqueous precursor for low-temperature, high performance thin-film transistors

A “green precursor” and a “green patterning technique” were used to fabricate low temperature processed indium oxide (InOx) semiconductors. For the InOx precursor, chloride ligand-based indium(III) was dissolved in deionized (DI) water without any additives to form a gel-like precursor. The as-spin-coated precursor films could be facilely patterned using the “green patterning technique”, which requires only ultraviolet (UV) irradiation and DI water. A systematic study was carried out to investigate the chemical reaction of the chloride-based precursor films as well as the semiconductor properties. It was found that UV irradiation and water treatment not only helped to transform In–Cl into In–OH, but also helped to remove the Cl-related impurities. It led to the activation of InOx films at temperatures as low as 180 °C. The mobility of InOx TFTs based on an anodized aluminium oxide (AlOx:Nd) insulator with patterning was improved by more than 1 order compared to that without patterning at an annealing temperature of 280 °C. In addition, flexible InOx TFTs on polyimide (PI) substrates were demonstrated. They showed only a little degradation in the subthreshold region of the transfer curve even at a bending curvature (R) of 5 mm.

Low-temperature, high-stability, flexible thin-film transistors with a novel Sc x In1−x O3 semiconductor

Low-temperature scandium (Sc) incorporating In2O3 (Sc x In1−x O3) thin films as oxide semiconductors were developed and investigated. And flexible thin-film transistors (TFTs) based on Sc x In1−x O3 channel layers were fabricated on polyethylene naphthalate substrate with the highest process temperature of only 150 °C. The Sc x In1−x O3 TFT displayed high on/off ratio of 107 with a turn-on voltage (V on) of only −0.1 V, a subthreshold swing of 0.21 V dec−1, and a mobility of 17.5 cm2 V−1 s−1. Furthermore, the flexible Sc x In1−x O3 TFTs exhibit high stability under both positive bias stability and negative bias stability, which was ascribed to little change of the lattice parameter of In2O3 after incorporation with Sc (since the radius of Sc3+ is similar to that of In3+). More interestingly, the flexible Sc x In1−x O3 TFTs exhibited relatively good stability under negative bias illumination stability compared to those of IGZO TFTs, which was ascribed to fewer oxygen vacancies due to the strong bonding strength of Sc-O. Finally, the transfer curves of the Sc x In1−x O3 TFTs showed only a few changes under a curvature radius of larger than 20 mm.

Thin-Film Transistors With Neodymium-Incorporated Indium–Zinc-Oxide Semiconductors

A thin-film transistor (TFT) with a neodymium-doped indium-zinc-oxide (NIZO) channel layer was fabricated. It was found that the Nd element uniformly distributed in the whole NIZO films, which revealed a nanocrystalline structure, implying that Nd was incorporated into the IZO lattice rather than segregated as clusters. The NIZO TFT annealed at 300 °C showed a saturation mobility of 22.7 cm2V-1s-1, a turn-ON voltage of -1.32 V, a subthreshold swing (SS) of 0.23 V/decade, and small turn-ON voltage shift under negative gate bias stress (-0.42 V) and positive gate bias stress (0.40 V). As the annealing temperature increased, the threshold voltage of the NIZO TFTs became more and more negative. Compared with IZO TFTs without Nd, NIZO TFTs exhibited lower SS and less turn-ON voltage shift as the annealing temperature increased. It was found that the free carriers of NIZO can be lowered even with a small amount of Nd (~1%). When annealed at a temperature of as high as 400 °C, the oxygen out-diffusion effect of NIZO was not as serious as that of IZO. Detailed studies showed that Nd atom could suppress the formation of oxygen vacancies due to the strong bonding strength of Nd-O (703 kJ/mol).

Mobility Enhancement in Amorphous In-Ga-Zn-O Thin-Film Transistor by Induced Metallic in Nanoparticles and Cu Electrodes

In this work, we fabricated a high-mobility amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) based on alumina oxide (Al2O3) passivation layer (PVL) and copper (Cu) source/drain electrodes (S/D). The mechanism of the high mobility for a-IGZO TFT was proposed and experimentally demonstrated. The conductivity of the channel layer was significantly improved due to the formation of metallic In nanoparticles on the back channel during Al2O3 PVL sputtering. In addition, Ar atmosphere annealing induced the Schottky contact formation between the Cu S/D and the channel layer caused by Cu diffusion. In conjunction with high conductivity channel and Schottky contact, the a-IGZO TFT based on Cu S/D and Al2O3 PVL exhibited remarkable mobility of 33.5–220.1 cm2/Vs when channel length varies from 60 to 560 μm. This work presents a feasible way to implement high mobility and Cu electrodes in a-IGZO TFT, simultaneously.