Chang Young Koo

Compositional influence on sol-gel-derived amorphous oxide semiconductor thin film transistors

We investigated the influence of the chemical compositions of gallium and indium cations on the performance of sol-gel-derived amorphous gallium indium zinc oxide (a-GIZO)-based thin film transistors (TFTs). A systematic compositional study allowed us to understand the solution-processed a-GIZO TFTs. We generated a compositional ternary diagram from which we could predict electrical parameters such as saturation mobility, threshold voltage, and the on/off current ratio as the constituent compositions varied. This diagram can be utilized for tailoring solution-processed amorphous oxide TFTs for specific applications.

Low Temperature Solution-Processed InZnO Thin-Film Transistors

We prepared indium zinc oxide (IZO) semiconductors for low temperature solution-processed thin-film transistors (TFTs). The sol-gel derived IZO films, annealed at 300°C, are uniform and have smooth surface morphology (root-mean-square roughness of 0.27 nm). Both the composition and the film thickness need to be optimized for high performance TFTs. With the composition of In/Zn equal to 50/50 in mol percent, the IZO TFTs with a thickness of 10 nm exhibited the best performance for a clear switching behavior (on/off current ratio of 1.2 × 10 7 and output characteristics (drain current of 3.7 × 10 -4 A), with a relatively high field-effect mobility (0.54 cm 2 V -1 s -1 ) and a low threshold voltage (1.9 V). The nonpassivated IZO-TFT stably operates over a two-month period without any significant change in the on/off current ratio and the mobility.

Bias Stress Stability of Solution-Processed Zinc Tin Oxide Thin-Film Transistors

The effects of bias stress on spin-coated zinc tin oxide (ZTO) transistors are investigated. Applying a positive bias stress results in the displacement of the transfer curves in the positive direction without changing the field-effect mobility or the subthreshold behavior, while a negative stress has no effect on the threshold voltage shift. Device instability appears to be a consequence of the charging and discharging of the temporal trap states at the interface and in the ZTO channel region. All the stressed devices recover their original characteristics after 10 min at room temperature. Furthermore, the inkjet-printed transistor yields similar bias stress effects as those observed in their spin-coated counterparts but has a greater shift in the threshold voltage. Microstructural evidence in conjunction with Rutherford backscattering spectroscopy confirms that severe instability is attributed to the presence of nanopores in the inkjet-printed channel layer.

High performance and high stability low temperature aqueous solution-derived Li–Zr co-doped ZnO thin film transistors

Highly stable and high performance solution-processed amorphous oxide semiconductor thin film transistors (TFTs) were produced using a Li and Zr co-doped ZnO-based aqueous solution. Li and Zr co-doping at the appropriate amounts enhanced the oxide film quality in terms of enhanced oxygen bonding and reduced defect sites. The 0.5 mol% Li and 1.0 mol% Zr co-doped ZnO TFTs annealed at 320 °C exhibited noticeably lower threshold voltage shifts of 3.54 V under positive bias stress and −2.07 V under negative bias temperature stress than the non-doped ZnO TFTs. The transistors revealed a good device mobility performance of 5.39 cm2 V−1 s−1 and an on/off current ratio of 108 when annealed at 320 °C, compared to a mobility performance of 2.86 cm2 V−1 s−1 and an on/off current ratio of ∼107 when annealed at 270 °C. Our results suggest that Li and Zr co-doping can be a useful technique to produce more reliable and low temperature solution-processed oxide semiconductor TFTs.

Dielectric and Electromechanical Properties of Pb(Zr,Ti)O3 Thin Films for Piezo-Microelectromechanical System Devices

The dielectric and electromechanical properties of 1-µm-thick Pb(Zr,Ti)O3 (PZT) thin films with Zr/Ti ratio of 30/70, 52/48 and 70/30 are investigated. The magnitude of the effective longitudinal piezoelectric coefficient (d33) of these films is measured by a double-beam laser interferometric method. With increasing Ti-content, larger remanent polarization and higher coercive voltage are observed. The squareness of the polarization hysteresis loop is optimized in a Ti-rich composition. The remanent d33 values are maximized near a morphotropic phase boundary (MPB) composition, and the trend of d33 values strongly depends on the dielectric constant. It is observed that maximum dielectric constant is achieved near MPB composition, showing the same trend as bulk PZT ceramics. This result indicates that the piezoelectric response in 1-µm-thick films with composition (Zr to Ti ratio) is governed by extrinsic contribution such as that found in bulk PZT ceramics rather than intrinsic contribution.

Enhanced Performance of Solution-Processed Amorphous LiYInZnO Thin-Film Transistors

Solution-processed, amorphous lithium-doped YInZnO (L-YIZO) thin-film transistors (TFTs) are investigated. An appropriate amount of Li doping significantly enhances the field-effect mobility in TFT performance (~15 times greater than that of nondoped YIZO) without controlled annealing under water vapor or O(3)/O(2) environments. The addition of Li into solution-processed YIZO semiconductors leads to improved film quality, which results from enriched metal oxygen bonding and reduced defect sites, such as oxygen vacancies and hydroxyl groups. Li doping of an amorphous ionic oxide semiconductor (AIOS) could serve as an effective strategy for low-temperature and high-performance solution-processed AIOS TFTs.

Thin Film Transistors with Ink-Jet Printed Amorphous Oxide Semiconductors

We investigated the influence of the thickness of printed Ga–In–Zn-O (GIZO) channel on transistor performance. Semiconductor layers were ink-jet printed using sol–gel derived GIZO solution and the thickness of the resulting active layers varied depending on the pre-heated substrate temperature. We found that GIZO film thickness significantly influences device performance. Thin film transistors (TFTs) with thicker active layers showed higher on-current/off-current mobility and a threshold voltage shift in the negative direction. The dependence of the electric characteristics on thickness resulted from the increased intrinsic free charge carriers as the active layer thickness increased. Ink-jet printing conditions need to be carefully controlled to maximize device performance.

Low-Temperature Solution-Deposited Oxide Thin-Film Transistors Based on Solution-Processed Organic–Inorganic Hybrid Dielectrics

We describe low-temperature, solution-deposited, oxide semiconductor thin-film transistors (TFTs) with a solution-processed gate dielectric in this study. The sol–gel-derived indium zinc oxide (IZO) semiconductor matched well with the organic–inorganic hybrid dielectric annealed at 200 °C, forming a coherent interface between the semiconductor and the dielectric without evidence of chemical damage. The IZO-TFTs made with a 420-nm-thick hybrid dielectric layer showed good performance: a low off-current on the order of <10-10 A, a field-effect mobility of 3.3×10-2 cm2 V-1 s-1, and a low threshold gate voltage of ~2.4 V. Spin-coating of the IZO semiconductor on a hybrid dielectric/glass substrate results in TFTs optically transparent in the entire visible region (~90%). Our solution-processable materials of the semiconductor and the gate dielectric can open the possibility of realizing flexible transparent devices using all-solution processing.

Effects of Heterostructure Electrodes on the Reliability of Ferroelectric PZT Thin Films

The effect of the Pt electrode and the Pt-IrO₂ hybrid electrode on the performance of ferroelectric device was investigated. The modified Pt thin films with non-columnar structure significantly reduced the oxidation of TiN diffusion barrier layer, which rendered it possible to incorporate the simple stacked structure of Pt/TiN/poly-Si plug. When a Pt-IrO₂ hybrid electrode is applied, PZT thin film properties are influenced by the thickness and the partial coverage of the electrode layers. The optimized Pt-IrO₂ hybrid electrode significantly enhanced the fatigue properties with minimal leakage current.