Cation Composition-Dependent Device Performance and Positive Bias Instability of Self-Aligned Oxide Semiconductor Thin-Film Transistors: Including Oxygen and Hydrogen Effect.

Amorphous oxide semiconductor transistors control the illuminance of pixels in an ecosystem of displays from large-screen TVs to wearable devices. To satisfy application-specific requirements, oxide semiconductor transistors of various cation compositions have been explored. However, a comprehensive study has not been carried out where the influence of cation composition, oxygen, and hydrogen on device characteristics and stability is systematically quantified, using commercial-grade process technology. In this study, we fabricate self-aligned top-gate structure thin-film transistors with three oxide semiconductor materials, InGaZnO (In/Ga/Zn = 1:1:1), In-rich InGaZnO, and InZnO, having mobility values of 10, 27, and 40 cm2/V·s, respectively. Combinations of varied amounts of oxygen and hydrogen are incorporated into each transistor by controlling the fabrication process to study the effect of these gaseous elements on the physical nature of the channel material. Electrons can be captured by peroxy linkage (O22-) or undercoordinated In (In* to become In+), which are manifested in the extracted subgap density-of-states profile and first-principles calculations. Energy difference between electron-trapped In+ and O22- σ* is the smallest for IGZO, and In+-O22- annihilation occurs by electron excitation from the subgap In+ state to the O22- σ*. Furthermore, characteristic time constants during positive bias stress and recovery reveal the various microscopic physical phenomena within the transistor structure between different cation compositions.

Significant Performance and Stability Improvements of Low-Temperature IGZO TFTs by the Formation of In-F Nanoparticles on an SiO2 Buffer Layer.

We report the performance improvement of low-temperature coplanar indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFTs) with a maximum process temperature of 230 °C. We treated F plasma on the surface of an SiO2 buffer layer before depositing the IGZO semiconductor by reactive sputtering. The field-effect mobility increases from 3.8 to 9.0 cm2 V-1·s-1, and the threshold voltage shift (ΔVth) under positive-bias temperature stress decreases from 3.2 to 0.2 V by F-plasma exposure. High-resolution transmission electron microscopy and atom probe tomography analysis reveal that indium fluoride (In-F) nanoparticles are formed at the IGZO/buffer layer interface. This increases the density of the IGZO and improves the TFT performance as well as its bias stability. The results can be applied to the manufacturing of low-temperature coplanar oxide TFTs for oxide electronics, including information displays.

Gallium Doping Effects for Improving Switching Performance of p-Type Copper(I) Oxide Thin-Film Transistors.

Copper(I) oxide (Cu2O), which is obtained from copper(II) oxide (CuO) through a reduction process, is a p-type oxide material with a band gap of 2.1-2.4 eV. However, the switching performance of typical Cu2O thin-film transistors (TFTs) is poor because the reduction process increases the concentration of oxygen vacancies (VO), which interfere with the conduction of hole carriers. Ga with high oxygen affinity was doped in Cu2O thin films to decrease VO during the reduction process. As a result, the VO concentration of 1.56 at % for Ga-doped Cu2O (Ga:Cu2O) thin films decreased from 20.2 to 7.5% compared to pristine Cu2O thin films. Accordingly, the subthreshold swing or S-factor, on/off-current ratio (Ion/off), saturation mobility (µsat), and threshold voltage (Vth) of Ga:Cu2O TFTs were improved compared to pristine Cu2O TFTs with values of 7.72 from 12.50 V/dec, 1.22 × 104 from 2.74 × 102, 0.74 from 0.46 cm2/Vs, and -4.56 from -8.06 V, respectively. These results indicate that Ga plays an important role in improving the switching performance of p-type Cu2O TFT.

Negative threshold voltage shift in an a-IGZO thin film transistor under X-ray irradiation.

We investigated the effects of X-ray irradiation on the electrical characteristics of an amorphous In-Ga-Zn-O (a-IGZO) thin film transistor (TFT). The a-IGZO TFT showed a negative threshold voltage (V TH) shift of -6.2 V after 100 Gy X-ray irradiation. Based on spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS) analysis, we found that the Fermi energy (E F) changes from 2.73 eV to 3.01 eV and that the sub-gap state of D1 and D2 changes near the conduction band minimum (CBM) of the a-IGZO film after X-ray irradiation. These results imply that the negative V TH shift after X-ray irradiation is related to the increase in electron concentration of the a-IGZO TFT active layer. We confirmed that the sources for electron generation during X-ray irradiation are hydrogen incorporation from the adjacent layer or from ambient air to the active layer in the TFT, and the oxygen vacancy dependent persistent photocurrent (PPC) effect. Since both causes are reversible processes involving an activation energy, we demonstrate the V TH shift recovery by thermal annealing.

Study on the Lateral Carrier Diffusion and Source-Drain Series Resistance in Self-Aligned Top-Gate Coplanar InGaZnO Thin-Film Transistors.

We investigated the lateral distribution of the equilibrium carrier concentration (n0) along the channel and the effects of channel length (L) on the source-drain series resistance (Rext) in the top-gate self-aligned (TG-SA) coplanar structure amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). The lateral distribution of n0 across the channel was extracted using the paired gate-to-source voltage (VGS)-based transmission line method and the temperature-dependent transfer characteristics obtained from the TFTs with different Ls. n0 abruptly decreased with an increase in the distance from the channel edge near the source/drain junctions; however, much smaller gradient of n0 was observed in the region near the middle of the channel. The effect of L on the Rext in the TG-SA coplanar a-IGZO TFT was investigated by applying the drain current-conductance method to the TFTs with various Ls. The increase of Rext was clearly observed with an increase in L especially at low VGSs, which was possibly attributed to the enhanced carrier diffusion near the source/drain junctions due to the larger gradient of the carrier concentration in the longer channel devices. Because the lateral carrier diffusion and the relatively high Rext are the critical issues in the TG-SA coplanar structure-based oxide TFTs, the results in this work are expected to be useful in further improving the electrical performance and uniformity of the TG-SA coplanar structure oxide TFTs.