A Study on the Electrical Properties of Atomic Layer Deposition Grown InO_<i>x</i> on Flexible Substrates with Respect to N₂O Plasma Treatment and the Associated Thin-Film Transistor Behavior under Repetitive Mechanical Stress

Indium oxide (InO_<i>x</i>) films were deposited at low processing temperature (150 °C) by atomic layer deposition (ALD) using [1,1,1-trimethyl-<i>N</i>-(trimethylsilyl)­silanaminato]­indium (InCA-1) as the metal precursor and hydrogen peroxide (H₂O₂) as the oxidant. As-deposited InO_<i>x</i> exhibits a metallic conductor-like behavior owing to a relatively high free-carrier concentration. In order to control the electron density in InO_<i>x</i> layers, N₂O plasma treatment was carried out on the film surface. The exposure time to N₂O plasma was varied (600–2400 s) to evaluate its effect on the electrical properties of InO_<i>x</i>. In this regard, thin-film transistors (TFTs) utilizing this material as the active layer were fabricated on polyimide substrates, and transfer curves were measured. As the plasma treatment time increases, the TFTs exhibit a transition from metal-like conductor to a high-performance switching device. This clearly demonstrates that the N₂O plasma has an effect of diminishing the carrier concentration in InO_<i>x</i>. The combination of low-temperature ALD and N₂O plasma process offers the possibility to achieve high-performance devices on polymer substrates. The electrical properties of InO_<i>x</i> TFTs were further examined with respect to various radii of curvature and repetitive bending of the substrate. Not only does prolonged cyclic mechanical stress affect the device properties, but the bending direction is also found to be influential. Understanding such behavior of flexible InO_<i>x</i> TFTs is anticipated to provide effective ways to design and achieve reliable electronic applications with various form factors

Facile Route to the Controlled Synthesis of Tetragonal and Orthorhombic SnO₂ Films by Mist Chemical Vapor Deposition

Two types of tin dioxide (SnO₂) films were grown by mist chemical vapor deposition (Mist-CVD), and their electrical properties were studied. A tetragonal phase is obtained when methanol is used as the solvent, while an orthorhombic structure is formed with acetone. The two phases of SnO₂ exhibit different electrical properties. Tetragonal SnO₂ behaves as a semiconductor, and thin-film transistors (TFTs) incorporating this material as the active layer exhibit n-type characteristics with typical field-effect mobility (μ_FE) values of approximately 3–4 cm²/(V s). On the other hand, orthorhombic SnO₂ is found to behave as a metal-like transparent conductive oxide. Density functional theory calculations reveal that orthorhombic SnO₂ is more stable under oxygen-rich conditions, which correlates well with the experimentally observed solvent effects. The present study paves the way for the controlled synthesis of functional materials by atmospheric pressure growth techniques

High-Performance Zinc Tin Oxide Semiconductor Grown by Atmospheric-Pressure Mist-CVD and the Associated Thin-Film Transistor Properties

Zinc tin oxide (Zn–Sn–O, or ZTO) semiconductor layers were synthesized based on solution processes, of which one type involves the conventional spin coating method and the other is grown by mist chemical vapor deposition (mist-CVD). Liquid precursor solutions are used in each case, with tin chloride and zinc chloride (1:1) as solutes in solvent mixtures of acetone and deionized water. Mist-CVD ZTO films are mostly polycrystalline, while those synthesized by spin-coating are amorphous. Thin-film transistors based on mist-CVD ZTO active layers exhibit excellent electron transport properties with a saturation mobility of 14.6 cm²/(V s), which is superior to that of their spin-coated counterparts (6.88 cm²/(V s)). X-ray photoelectron spectroscopy (XPS) analyses suggest that the mist-CVD ZTO films contain relatively small amounts of oxygen vacancies and, hence, lower free-carrier concentrations. The enhanced electron mobility of mist-CVD ZTO is therefore anticipated to be associated with the electronic band structure, which is examined by X-ray absorption near-edge structure (XANES) analyses, rather than the density of electron carriers

Influence of Dielectric Layers on Charge Transport through Diketopyrrolopyrrole-Containing Polymer Films: Dielectric Polarizability vs Capacitance

Field-effect mobility of a polymer semiconductor film is known to be enhanced when the gate dielectric interfacing with the film is weakly polarizable. Accordingly, gate dielectrics with lower dielectric constant (<i>k</i>) are preferred for attaining polymer field-effect transistors (PFETs) with larger mobilities. At the same time, it is also known that inducing more charge carriers into the polymer semiconductor films helps in enhancing their field-effect mobility, because the large number of traps presented in such a disorder system can be compensated substantially. In this sense, it may seem that employing higher <i>k</i> dielectrics is rather beneficial because capacitance is proportional to the dielectric constant. This, however, contradicts with the statement above. In this study, we compare the impact of the two, i.e., the polarizability and the capacitance of the gate dielectric, on the transport properties of poly­[(diketopyrrolopyrrole)-<i>alt</i>-(2,2′-(1,4-phenylene)­bisthiophene)] (PDPPTPT) semiconductor layers in an FET architecture. For the study, three different dielectric layers were employed: fluorinated organic CYTOP (<i>k</i> = ∼2), poly­(methyl methacrylate) (<i>k</i> = ∼4), and relaxor ferroelectric poly­(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) (<i>k</i> = ∼60). The beneficial influence of attaining more carriers in the PDPPTPT films on their charge transport properties was consistently observed from all three systems. However, the more dominant factor determining the large carrier mobility was the low polarizability of the gate dielectric rather than its large capacitance; field-effect mobilities of PDPPTPT films were always larger when lower <i>k</i> dielectric was employed than when higher <i>k</i> dielectric was used. The higher mobilities obtained when using lower <i>k</i> dielectrics could be attributed to the suppressed distribution of the density of localized states (DOS) near the transport level and to the resulting enhanced electronic coupling between the macromolecules

Improving the Stability of High-Performance Multilayer MoS₂ Field-Effect Transistors

In this study, we propose a method for improving the stability of multilayer MoS₂ field-effect transistors (FETs) by O₂ plasma treatment and Al₂O₃ passivation while sustaining the high performance of bulk MoS₂ FET. The MoS₂ FETs were exposed to O₂ plasma for 30 s before Al₂O₃ encapsulation to achieve a relatively small hysteresis and high electrical performance. A MoO<i>_x</i> layer formed during the plasma treatment was found between MoS₂ and the top passivation layer. The MoO<i>_x</i> interlayer prevents the generation of excess electron carriers in the channel, owing to Al₂O₃ passivation, thereby minimizing the shift in the threshold voltage (<i>V</i>_th) and increase of the off-current leakage. However, prolonged exposure of the MoS₂ surface to O₂ plasma (90 and 120 s) was found to introduce excess oxygen into the MoO<i>_x</i> interlayer, leading to more pronounced hysteresis and a high off-current. The stable MoS₂ FETs were also subjected to gate-bias stress tests under different conditions. The MoS₂ transistors exhibited negligible decline in performance under positive bias stress, positive bias illumination stress, and negative bias stress, but large negative shifts in <i>V</i>_th were observed under negative bias illumination stress, which is attributed to the presence of sulfur vacancies. This simple approach can be applied to other transition metal dichalcogenide materials to understand their FET properties and reliability, and the resulting high-performance hysteresis-free MoS₂ transistors are expected to open up new opportunities for the development of sophisticated electronic applications