Kyungsoo Jang

Multiferroic properties of epitaxially stabilized hexagonal DyMnO3 thin films

The authors fabricated epitaxial thin films of hexagonal DyMnO3, which otherwise form in a bulk perovskite structure, via deposition on Pt(111)‖Al2O3(0001) and YSZ(111) substrates, each of which has in-plane hexagonal symmetry. The polarization hysteresis loop demonstrated the existence of ferroelectricity in our hexagonal DyMnO3 films at least below 70K. The observed 2.2μC∕cm2 remnant polarization at 25K corresponded to a polarization enhancement by a factor of 10 compared to that of the bulk orthorhombic DyMnO3. Interestingly, this system showed an antiferroelectriclike feature in its hysteresis loop. Our hexagonal DyMnO3 films showed an antiferromagnetic Neel temperature around 60K and a spin reorientation transition around 40K. The authors also found spin-glass-like behavior, which was likely to arise from the geometric frustration of antiferromagnetically coupled Mn spins with an edge-sharing triangular lattice.

Negative gate-bias temperature stability of N-doped InGaZnO active-layer thin-film transistors

Stability of negative bias temperature stress (NBTS) of nitrogen doped amorphous InGaZnO (a-IGZO) thin-film transistor (TFT) is investigated. Undoped a-IGZO TFT stressed at 333 K exhibit a larger negative ΔVTH (−3.21 V) with an unpredictable sub-threshold swing (SS) of hump shaped transfer curve due to the creation of meta-stable traps. Defects related hump formation has disappeared with small ΔVTH (−1.13 V) and ΔSS (0.018 V/dec) in nitrogen doped a-IGZO TFT. It is observed that nitrogen doping enhances device stability by well controlled oxygen vacancy and trap sites in channel and channel/dielectric interface.

Improvement in the performance of an InGaZnO thin-film transistor by controlling interface trap densities between the insulator and active layer

An amorphous InGaZnO film fabricated by radio frequency magnetron sputtering in only an Ar-reactive gas shows high conductivity, and a thin-film transistors (TFTs)-based IGZO active layer expresses a poor on/off current ratio with a high off current and high subthreshold swing (SS). This paper presents the post-annealing effects on IGZO thin films to compensate the oxygen deficiencies in films as well as on TFT devices to reduce the densities of the interface trap between the active layer and insulator. The ratio of oxygen vacancies over total of oxygen (O2/Otot) in IGZO estimated by the XPS measurement shows that they significantly diminish from 24.75 to 17.68% when increasing the temperature treatment to 350 °C, which is related to the enhancement in resistivity of IGZO. The TFT characteristics of IGZO treated in air at 350 °C show a high ION/IOFF ratio of ~1.1 × 107, a high field-effect mobility of 7.48 cm2 V−1 s−1, and a low SS of 0.41 V dec−1. The objective of this paper is to achieve a successful reduction in the interface trap density, ΔDit, which has been reduced about 3.1 × 1012 cm−2 eV−1 and 2.0 × 1012 cm−2 eV−1 for the 350 and 200 °C treatment samples compared with the as-deposited one. The resistivity of the IGZO films can be adjusted to the appropriate value that can be used for TFT applications by controlling the treatment temperature.

Effects of Carrier Concentration, Indium Content, and Crystallinity on the Electrical Properties of Indium-Tin-Zinc-Oxide Thin-Film Transistors

We report the effects of carrier concentration (NCH), indium (In) content, and crystallinity (X_c) on the electrical properties of indium-tin-zinc-oxide (ITZO) thin-film transistors (TFTs). The ITZO TFT with the lowest NCH, In content, and amorphous phase at the optimized oxygen flow rate has high field-effect mobility a (μ_FE) of 37.2 cm²/V·s, high ON/OFF current ratio (I_ON/I_OFF) of ~ 1×10⁷, and low subthreshold swing (S.S) of 0.93. With increasing NCH, In content, and X_c, μ_FE, I_ON/I_OFF, and S.S surprisingly degraded to 14.4 cm²/V·s, ~ 4×10⁴, and 4.01, respectively. Our high ITZO TFTs with μ_FE of 37.2 cm²/V·s, obtained thorough control of the N_CH, In content, and X_c, was suitable for application to next generation ultrahigh resolution displays as well as high frame rate displays.

Suppression of temperature instability in InGaZnO thin-film transistors by in situ nitrogen doping

We have investigated the effect of nitrogen doping on the behavior of hysteresis curve and its suppression of temperature instability in amorphous InGaZnO thin-film transistors (a-IGZO TFTs). The in situ nitrogen doping reduced the temperature induced abnormal sub threshold leakage current and traps generation. Large falling-rate (FR) ~ 0.26 eV V−1, low activation energy (Ea) ~ 0.617 eV and a small hysteresis compared to the pure a-IGZO TFTs, shows the best immunity to thermal instability. This is mainly attributed to the reduction of interface trap density and oxygen vacancies due to the passivation of defects and/dangling bonds.

Drain-Induced Barrier Lowering and Parasitic Resistance Induced Instabilities in Short-Channel InSnZnO TFTs

Effect of short-channel induced instabilities in InSnZnO-based thin-film transistors (TFTs) caused by combination of the drain induced barrier lowering (DIBL) and parasitic resistance is reported. As the active channel length decreased below a critical value of around 8 μm, the draincurrent (2.81 μA) are abruptly increased and N-shaped behavior of the transconductance are observed due to the formation of additional current path in the channel. The magnitude of subgap density of states is also depended on the channel size. The higher value of parasitic resistance RSD (~42 kg) and DIBL coefficient (76.8 mV/V) in short-channel ITZO TFT devices are also discussed.

Bias-stability improvement using Al2O3?interfacial dielectrics in a-InSnZnO thin-film transistors

We report amorphous-indium–tin–zinc-oxide (a-ITZO) thin-film transistors (TFTs) obtained using an aluminum-oxide (Al2O3) interfacial dielectric using atomic layer deposition between a silicon-nitride (SiNX) gate dielectric and an a-ITZO active channel layer. The effect of the Al2O3 interfacial layer on the suppression of charge trapping in a-ITZO TFTs is presented. In transparent oxide TFTs, reducing the shift in threshold voltage by stress-including negative-bias stress (NBS) is one of the key issues in improving the stability performance of TFTs. The NBS stability of an a-ITZO TFT using an Al2O3/SiNX double-layered dielectric is superior to that using an SiNX single-gate dielectric, because of the smooth surface with a root-mean-square roughness of 0.147 nm and a low defect density of less than 3×1011 eV−1 cm−2, which increases hydrophobicity. The a-ITZO TFTs using the Al2O3 interfacial dielectric show little change in the threshold voltage (~0 V), and a long trapping time of ~5000 s when a gate voltage of −25 V and drain voltage of 1 V are applied for 10 000 s. We show that the gate dielectric has a profound effect on the electrical stability, and suggest a way of improving the stability of a-ITZO TFTs.

Operation mechanism of Schottky barrier nonvolatile memory with high conductivity InGaZnO active layer

Influence of Schottky contact between source/drain electrodes and high conductivity a-InGaZnO active layer to the performance of nonvolatile memory devices was first proposed. The Schottky barrier devices faced to the difficulty on electrical discharging process due to the energy barrier forming at the interface, which can be resolved by using Ohmic devices. A memory window of 2.83 V at programming/erasing voltage of ±13 V for Ohmic and 5.58 V at programming voltage of 13 V and light assisted erasing at −7 V for Schottky devices was obtained. Both memory devices using SiO2/SiOx/SiOxNy stacks showed a retention exceeding 70% of trapped charges 10 yr with operation voltages of ±13 V at an only programming duration of 1 ms.

Fabrication of SiO2/SiOx/SiOxNy Non-Volatile Memory with Transparent Amorphous Indium Gallium Zinc Oxide Channels

In this paper, we investigated the electrical and memory properties of nonvolatile memory (NVM) using low temperature multistack gate insulators of SiO2/SiOx/SiOxNy (OOxOn) and an active layer using amorphous InGaZnO (a-IGZO) films. The various amorphous SiOx materials were studied by controlling the gas flow ratio of SiH4:N2O from 2:1 to 1:2 to determine the optimal conditions for the charge-trapping layer. The NVM devices using the OOxOn structure were investigated with SiOxNy tunneling thicknesses changing between 2.2, 2.5, and 2.8 nm. The characteristics of the NVM device with 2.8 nm SiOxNy tunneling thickness showed a retention exceeding 97% of threshold voltage shift after 10 4 s and greater than 93% after 10 years with low þ13 V at an only programming duration of 1 ms. In addition, the optical transmittance of the a-IGZO films was measured the be over 85%. It is a promising candidate for achieving flexible displays and transparency on plastic substrates because of the possibility of the low-temperature deposition and high transparent properties of a-IGZO films. Therefore, the bottom-gate OOxOn NVM using high transparent a-IGZO active layer has become a potential device for flexible memory displays system.

Fabrication and characterization of metal-oxide-nitride-oxynitride-polysilicon nonvolatile semiconductor memory device with silicon oxynitride (SiOxNy) as tunneling layer on glass

A nonvolatile semiconductor memory (NVSM) device with a metal-oxide-nitride-oxynitride-polysilicon (MONOS) structure on a rough polysilicon (poly-Si) surface was fabricated using a low-temperature process and poly-Si thin film transistor (TFT) technology on glass. For the fabrication of the NVSM device on glass, plasma-assisted oxynitridation was carried out using nitrous oxide (N2O) as a reactive gas, due to the very rough surface of the poly-Si on glass annealed using an excimer laser. The ultrathin SiOxNy films obtained using the N2O plasma have a very uniform distribution on poly-Si and similar contents of oxygen and nitrogen in the peaks and valleys of the grains. The NVSM devices having a MONOS structure with a tunneling layer of ultrathin SiOxNy on glass have suitable switching and charge retention characteristics for data storage. The results demonstrate that the NVSM device made using low-temperature poly-Si TFT technology on glass reported in this paper can be used in various types of display de...