Un Ki Kim

Effect of high-pressure oxygen annealing on negative bias illumination stress-induced instability of InGaZnO thin film transistors

Negative-bias illumination stress (NBIS) of amorphous InGaZnO (IGZO) transistors can cause a large negative shift (>7.1 V) in threshold voltage, something frequently attributed to the trapping of photoinduced hole carriers. This work demonstrates that the deterioration of threshold voltage by NBIS can be strongly suppressed by high-pressure annealing under 10 atm O2 ambient. This improvement occurred through a reduction in oxygen vacancy defects in the IGZO film, indicating that a photoinduced transition from VO to VO2+ was responsible for the NBIS-induced instability.

Suppression in the negative bias illumination instability of Zn-Sn-O transistor using oxygen plasma treatment

This study examined the effect of oxygen plasma treatment on light-enhanced bias instability in Zn-Sn-O (ZTO) thin film transistors (TFTs). The treated ZTO TFT exhibited only a threshold voltage (Vth) shift of −2.05 V under negative bias illumination stress (NBIS) conditions, whereas the pristine device suffered from a negative Vth shift of 3.76 V under identical conditions. X-ray photoelectron spectroscopic analysis revealed that the oxygen vacancy defect density was diminished via the oxygen plasma treatment. This suggests the Vth degradation under NBIS is due to photo-transition of oxygen vacancy defects.

Correlation of the change in transfer characteristics with the interfacial trap densities of amorphous In–Ga–Zn–O thin film transistors under light illumination

The change in the transfer characteristics of amorphous In–Ga–Zn–O thin film transistors (TFTs) was investigated under light illumination at various wavelengths. The variations in the interfacial trap density (Dit) were also studied using metal-insulator-semiconductor capacitors. The transfer characteristics of the TFTs were dependent on the wavelength of illuminated light. The increase in subthreshold swing observed under light illumination of wavelengths below 550 nm (∼2.3 eV) was confirmed to be related to the increase in Dit near the conduction band edge. This Dit increase is caused by doubly ionized oxygen vacancies (VO2+) that are temporarily generated under light illumination.

Improvement in Photo-Bias Stability of High-Mobility Indium Zinc Oxide Thin-Film Transistors by Oxygen High-Pressure Annealing

This letter examines the effect of oxygen (O₂) high-pressure annealing (HPA) on indium zinc oxide (IZO) thin-film transistors (TFTs) with a high-quality Al₂O₃ passivation layer. The IZO TFTs anneal under an O₂ atmosphere at 9 atm exhibits a high field-effect mobility, low subthreshold gate swing, moderate threshold voltage (Vth), and high I_ON/OFF ratio of 30.4 cm²/Vs, 0.10 V/decade, 0.79 V, and 10⁸, respectively. In addition, the O₂ HPA-treated IZO TFT has superior reliability (ΔVth= -0.5 V) to that of the 0.2-atm-annealed device (ΔVth=-3.7 V) under negative bias illumination stress conditions. This improvement can be attributed to the reduced concentration of oxygen vacancy defects in the IZO channel layer during the O₂ HPA treatment.

Performance Variation According to Device Structure and the Source/Drain Metal Electrode of a-IGZO TFTs

The transmission-line method (TLM) was adopted to clarify the causes of device performance variation according to the source/drain metal electrode and device structure of a thin-film transistor using an amorphous indium-gallium-zinc-oxide channel. Using the TLM, the channel characteristics independent of contact resistance were extracted for the two different contact metals, i.e., Ti and Mo. Based on these results, the mobility characteristics were compared in terms of device scaling and contact structure in the source/drain overlap region. In addition, the transport characteristics according to the contact structure of the source/drain metal electrode were investigated in detail and reproduced using the simulation model.

Vertically integrated submicron amorphous-In2Ga2ZnO7 thin film transistor using a low temperature process

In this work, vertically integrated amorphous-In2Ga2ZnO7 (a-IGZO) thin film transistors (V-TFTs) with 310 nm channel length were fabricated using a low temperature process (<300  °C), and their device performance was evaluated. The fabricated V-TFTs show well behaved transfer characteristics with an Ion/Ioff current ratio greater than 104 and a threshold voltage of 1.7 V. The influence of the vertical structure on device performance was analyzed in detail. In addition, current polarity characteristics that arise from different metal/a-IGZO contacts were also examined. The non-optimum performance of the V-TFTs was attributed to the fringing-field effect, high defect density, and large source/drain contact resistance.

Theoretical and experimental studies on the electronic structure of crystalline and amorphous ZnSnO3 thin films

The influence of structural disorder on the electronic structure of amorphous ZnSnO3 was examined by ab-initio calculations. The calculation results are compared with the experimental results using as-deposited and annealed ZnSnO3 films grown by atomic layer deposition. The O K-edge X-ray absorption spectroscopy, X-ray diffraction, and thin-film transistors were employed in the experiment. The conduction band minima of amorphous and crystalline ZnSnO3 mainly consisted of Sn 5s state, while a higher non-uniform localization of these states was observed in the amorphous phase compared with the crystalline counterpart. The experimental results coincide well with the theoretical results.

The effects of device geometry on the negative bias temperature instability of Hf-In-Zn-O thin film transistors under light illumination

The negative bias illumination temperature stress instability of amorphous Hf-In-Zn-O thin film transistors with different dimensions was evaluated. The threshold voltage (Vth) shift increased in devices with shorter channel lengths but showed almost no association with the channel width. This behavior was attributed to the diffusion and drift of the photogenerated holes at the channel/dielectric interface from regions near the drain to those near the source, which were due to the simultaneous presence of gate and drain biases. The Vth near the source, which shows the largest shift and hence has the highest local value, governs the overall Vth.

Indium tin oxide/InGaZnO bilayer stacks for enhanced mobility and optical stability in amorphous oxide thin film transistors

Optically more stable, high mobility InGaZnO thin film transistors were fabricated by implementing ultrathin In2O3-SnO2 (ITO) layers at the gate dielectric/semiconductor interface. The optimized device portrayed a high saturation mobility of ∼80 cm2/V s with off current values lower than 10−11A. The ITO layer also acted as a hole filter layer, and hole current and threshold voltage shift values measured under negative bias illumination conditions showed that a significant amount of photo-generated charge carriers were annihilated before reaching the gate insulator. This effect was more evident at larger intensities, showing threshold voltage shift values reduced by more than ∼70% under stress conditions.

The charge trapping characteristics of Si3N4 and Al2O3 layers on amorphous-indium-gallium-zinc oxide thin films for memory application

The charge trapping characteristics of 30-nm-thick Si3N4 and 3-nm-thick Al2O3 layers between amorphous In-Ga-Zn-O thin films and 100-nm-thick blocking oxides made of thermal SiO2 were examined. The Si3N4 layer showed several discrete trap levels with relatively low density, while the Al2O3 layer showed a higher trap density with continuous distribution for electron trapping. When no tunneling oxide was adopted, the trapped carriers were easily detrapped, even at room temperature. Adoption of a 6-nm-thick SiO2 tunneling layer grown by atomic layer deposition largely improved the retention of the trapped charges and retained ∼60% of the trapped charges even after 10 000 s.