Thanh Thuy Trinh

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.

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.

The mechanisms of negative oxygen ion formation from Al-doped ZnO target and the improvements in electrical and optical properties of thin films using off-axis dc magnetron sputtering at low temperature

Transparent conducting aluminum-doped zinc oxide (AZO) films have been prepared on glass substrates by dc magnetron sputtering using ceramic ZnO with 2 wt% Al2O3 target. The mechanism of negative oxygen ion generation on an AZO target surface and its influence on the conductivity of films were discussed. The negative ion generation on an AZO target was contributed by the surface ionization leading to the spot emission from Al atoms adsorbed on the AZO target surface. The contribution of negative ions’ current was mainly from the erosion area of the target due to its higher temperature. To reduce the damage caused by negative ion bombardment to film growth, an off-axis sputtering system was proposed, where the substrates were placed perpendicular to the target. The effects of distance (d) on the electrical properties of films were experimentally verified in detail. A low resistivity of 3.7 × 10 −4 � cm, an average transmittance above 85% in the visible range (300‐800 nm) and reflectance higher than 85% in the infrared range (2500‐4000 nm) were obtained for the films deposited at d = 2.5 cm. The overall analysis revealed that the generation of negative ions on the AZO target has a great influence on film growth, especially in the ultra-low pressure deposition process. Our work demonstrates the feasibility of reducing the negative effects of ion bombardment on the quality of films, which would be of great merit for industrial applications. (Some figures in this article are in colour only in the electronic version)

Effect of high conductivity amorphous InGaZnO active layer on the field effect mobility improvement of thin film transistors

High mobility thin film transistors (TFTs) with a high conductivity amorphous InGaZnO (a-IGZO) active layer were successfully fabricated. The operation of the high-carrier-IGZO thin film transistor with a Schottky barrier (SB) was proposed and clearly experimentally explained. The switching characteristic of SB-TFT does not rely on the accumulation process but due to the Schottky barrier height control. Leakage current can be reduced by Schottky contact at the source/drain (S/D), while it was as high as the on current so that the switch properties could not achieve in ohmic ones. The a-IGZO SB-TFTs with Ag S/D contact express the high performance with μFE of 20.4 cm2 V−1 s−1, Vth of 5.8 V, and ION/IOFF of 2 × 107 @ VD = 1V. The introduction of operating mechanism for TFTs using high conductivity a-IGZO promises an expansion study for other active layer materials.

High performance non-volatile memory with the control of charge trapping states in an amorphous InSnZnO active channel

In this study, the influence of interface states between an indium tin zinc oxide (ITZO) active layer and a gate insulator on memory characteristics was examined as a function of annealing temperature. The annealing nonvolatile memory (NVM) devices have shown the best electrical characteristics such as high field effect mobility (27.22 cm2 V−1 s−1), low threshold voltage (0.15 V), low subthreshold slope (0.17 V dec−1), and high on/off current ratio (7.57 × 107) in comparison with as-deposited devices. By annealing at 250 °C, the number of ITZO/insulator interface trap densities was reduced. The effect of the remaining trap states on the retention characteristic of memory devices is negligible. The performance of NVM devices using different annealing temperatures of ITZO and a multi-stack gate insulator SiO2/SiOx/SiOxNy with Si-rich SiOx for the charge storage layer was also reported. The 250 °C annealed ITZO-based NVM device showed a retention exceeding ~94% of the threshold voltage shift after 104 s and greater than ~90% after 10 years with a low operating voltage of +11 V at only 1 μs programming duration time. Therefore, the NVM devices, which were fabricated by the low ITZO/insulator interface trap densities, were highly suitable for potential application in memory systems.