Myeong Gu Yun

Anomalous tin chemical bonding in indium-zinc-tin oxide films and their thin film transistor performance

We deposited InZnSnO (IZTO) channel layers, where indium (In) composition was controlled from 0 to 28.4 at %. Oxide thin-film-transistors (TFTs) based on these IZTO channels have shown a remarkable field effect mobility of > 30 cm2 V−1 s−1, low sub-threshold swing and good stability, compared to the ZTO TFTs. From x-ray photoelectron spectroscopy results, we found that the IZTO film containing 19.4 at % In showed enhanced intensity from the anomalous Sn2+ related chemical bonding. The incorporation of In elements promotes the formation of Sn2+-O2− binding in the IZTO films, resulting in compensation for the oxygen vacancies in the IZTO films. Thus, moderate incorporation in the ZTO led to improved stability and high field effect mobility simultaneously.

Bi-layer channel structure-based oxide thin-film transistors consisting of ZnO and Al-doped ZnO with different Al compositions and stacking sequences

Zinc oxide (ZnO)-based bi-layers, consisting of ZnO and Al-doped ZnO (AZO) layers grown by atomic layer deposition, were utilized as the channels of oxide thin-film transistors (TFTs). Thin AZO layers (5 nm) with different Al compositions (5 and 14 at. %) were deposited on top of and beneath the ZnO layers in a bi-layer channel structure. All of the bi-layer channel TFTs that included the AZO layers showed enhanced stability (ΔVTh ≤ 3.2 V) under a positive bias stress compared to the ZnO single-layer channel TFT (ΔVTh = 4.0 V). However, the AZO/ZnO bi-layer channel TFTs with an AZO interlayer between the gate dielectric and the ZnO showed a degraded field effect mobility (0.3 cm2/V·s for 5 at. % and 1.8 cm2/V·s for 14 at. %) compared to the ZnO single-layer channel TFT (5.5 cm2/V·s) due to increased scattering caused by Al-related impurities near the gate dielectric/channel interface. In contrast, the ZnO/AZO bi-layer channel TFTs with an AZO layer on top of the ZnO layer exhibited an improved field effect mobility (7.8 cm2/V·s for 14 at. %) and better stability.

Dual Electrical Behavior of Multivalent Metal Cation-Based Oxide and Its Application to Thin-Film Transistors with High Mobility and Excellent Photobias Stability

The effect of multivalent metal cations, including vanadium(V) and tin (Sn), on the electrical properties of vanadium-doped zinc tin oxide (VZTO) was investigated in the context of the fabrication of thin-film transistors (TFTs) using a single VZTO film and VZTO/ZTO bilayer as channel layers. The single VZTO TFT did not show any response to the gate voltage (insulator-like behavior). On the other hand, the VZTO/ZTO bilayer TFT exhibited a typical TFT transfer characteristic (semiconducting behavior). X-ray photoelectron spectroscopy revealed that, in contrast to what is commonly true in many oxides, oxygen vacancies (V(O)) in VZTO did not provide a dominant contribution to the total carrier concentration, because the V(O) peak area in the single VZTO film was 5.4% and reduced to 4.5% in VZTO/ZTO bilayer. Instead, Sn 3d5/2 and V 2p3/2 spectra suggest that the significant reduction in Sn and V ions is strongly related to the insulator-like behavior of the VZTO film. In negative-bias illumination tests and illumination tests with various photon energies, the VZTO/ZTO bilayer TFT had much better stability than the ZTO TFT. This result is attributed to the reduction of donor-like states ([Formula: see text]O) that can be positively ionized by blue and green illumination.

Design of step composition gradient thin film transistor channel layers grown by atomic layer deposition

In this study, we proposed the artificially designed channel structure in oxide thin-film transistors (TFTs) called a “step-composition gradient channel.” We demonstrated Al step-composition gradient Al-Zn-O (AZO) channel structures consisting of three AZO layers with different Al contents. The effects of stacking sequence in the step-composition gradient channel on performance and electrical stability of bottom-gate TFT devices were investigated with two channels of inverse stacking order (ascending/descending step-composition). The TFT with ascending step-composition channel structure (5 → 10 → 14 at. % Al composition) showed relatively negative threshold voltage (−3.7 V) and good instability characteristics with a reduced threshold voltage shift ( Δ 1.4 V), which was related to the alignment of the conduction band off-set within the channel layer depending on the Al contents. Finally, the reduced Al composition in the initial layer of ascending step-composition channel resulted in the best field effect...

Low voltage-driven oxide phototransistors with fast recovery, high signal-to-noise ratio, and high responsivity fabricated via a simple defect-generating process.

We have demonstrated that photo-thin film transistors (photo-TFTs) fabricated via a simple defect-generating process could achieve fast recovery, a high signal to noise (S/N) ratio, and high sensitivity. The photo-TFTs are inverted-staggered bottom-gate type indium-gallium-zinc-oxide (IGZO) TFTs fabricated using atomic layer deposition (ALD)-derived Al2O3 gate insulators. The surfaces of the Al2O3 gate insulators are damaged by ion bombardment during the deposition of the IGZO channel layers by sputtering and the damage results in the hysteresis behavior of the photo-TFTs. The hysteresis loops broaden as the deposition power density increases. This implies that we can easily control the amount of the interface trap sites and/or trap sites in the gate insulator near the interface. The photo-TFTs with large hysteresis-related defects have high S/N ratio and fast recovery in spite of the low operation voltages including a drain voltage of 1 V, positive gate bias pulse voltage of 3 V, and gate voltage pulse width of 3 V (0 to 3 V). In addition, through the hysteresis-related defect-generating process, we have achieved a high responsivity since the bulk defects that can be photo-excited and eject electrons also increase with increasing deposition power density.

Chemically robust solution-processed indium zinc oxide thin film transistors fabricated by back channel wet-etched Mo electrodes

We designed a systematic processing strategy for solution-processed indium zinc oxide thin film transistors (TFTs) with chemically wet-etched Mo electrodes and chemically durable channels prepared by a sol–gel method. First, we explored the effect of H2O2 wet-etchant pH to define efficiently wet-etched Mo source/drain electrodes without Mo residues and with minimal chemical damage to the indium zinc oxide (IZO) channel. Next, sufficient condensation reaction times and a two-step engineering process were performed on the solution-processed IZO thin films to improve their inferior chemical durability (from incomplete metal oxygen bonds and low film density). The solution-processed IZO channels with wet chemical patterning and superior chemical durability preserved the original electrical transfer properties with minimal electrical degradation in the back channel etch (BCE) processes. Finally, additional N2 post-annealing partially recovered the field-effect mobility (2.5 cm2 V−1 s−1), and on-current without oxidation of the Mo electrode, comparable to the lift-off processed TFTs. This approach provides a significant potential for using wet-based BCE processes in sol–gel prepared oxide TFTs.

Expedient floating process for ultra-thin InGaZnO thin-film-transistors and their high bending performance

Recently, much attention has been focused on the development of devices with ultra-thin sample thickness for imperceptible and patchable applications, but the use of inorganic active films in flexible electronics has seen a great obstacle due to brittle mechanical properties during bending, despite better electrical performance and high stability. We report a procedure for fabricating organic electronic devices on ultra-thin polymer substrates using a floating process. This process uses water soluble polyvinyl alcohol (PVA) as a sacrificial layer and the maximum process temperature is increased until around 200 °C, which is a valuable condition for high-quality InGaZnO channels and Al2O3 gate dielectrics. The 400 nm PVA coating produced with low molecular weight characteristics had relatively smooth surface roughness and quick floating process. The ultra-thin InGaZnO TFT showed good transfer and reproducible performance under extreme bending conditions due to the use of ultra-thin compliant substrates.

Periodically pulsed wet annealing approach for low-temperature processable amorphous InGaZnO thin film transistors with high electrical performance and ultrathin thickness

In this paper, a simple and controllable “wet pulse annealing” technique for the fabrication of flexible amorphous InGaZnO thin film transistors (a-IGZO TFTs) processed at low temperature (150 °C) by using scalable vacuum deposition is proposed. This method entailed the quick injection of water vapor for 0.1 s and purge treatment in dry ambient in one cycle; the supply content of water vapor was simply controlled by the number of pulse repetitions. The electrical transport characteristics revealed a remarkable performance of the a-IGZO TFTs prepared at the maximum process temperature of 150 °C (field-effect mobility of 13.3 cm2 V−1 s−1; Ion/Ioff ratio ≈ 108; reduced I-V hysteresis), comparable to that of a-IGZO TFTs annealed at 350 °C in dry ambient. Upon analysis of the angle-resolved x-ray photoelectron spectroscopy, the good performance was attributed to the effective suppression of the formation of hydroxide and oxygen-related defects. Finally, by using the wet pulse annealing process, we fabricated, on a plastic substrate, an ultrathin flexible a-IGZO TFT with good electrical and bending performances.