Suppression of Oxygen Vacancy and Enhancement in Bias Stress Stability of High-Mobility ZnO Thin-Film Transistors with N₂O Plasma Treated MgO Gate Dielectrics

Abstract

ZnO thin-film transistors (TFTs) using MgO dielectrics achieve a high field-effect mobility of around 50 cm 2 /Vs. Plasma treatments with different gases applied on the MgO dielectric surface will amend the TFT's electrical stability and X-ray photoelectron spectroscopy analysis is done nearby the ZnO/MgO interface to study the change of oxygen chemical bonding states. The results show that MgO dielectric without plasma treatment causes the threshold voltage shift of the transistor, which may be attributed to the migration of oxygen ion toward the ZnO/MgO interface to induce the generation of oxygen vacancies in ZnO active layer when devices are under positive gate bias stress. This instability behavior can be eliminated with N 2 O plasma treatment on MgO. N 2 O plasma treatment inhibits the generation of oxygen vacancies in ZnO against gate bias stress thus reduces the effective trap state density in the subgap of ZnO and thereby enhances the TFT's stability. The detailed scenario for the plasma treatment effect is explored to conclude its mechanism on improving the electrical stability.ZnO is a oxide semiconductor which can be applied to many different electronic devices, such as transparent conducting oxide (TCO), 1,2 light emitting diodes (LED) 3 or resistance random access memory (RRAM). 4,5 Especially, the ZnO-based TFTs 6-11 have attracted attention for application in flat panel display owing to their reasonable mobility and transparency in visible light region. For practical applications, the electrical stability of TFTs will be an important issue to overcome. Bias stress test is a general method to determine the electrical stability of the device by tracing the transfer characteristic (I D -V G curves) of the transistor with giving a constant bias in gate electrode for a period of time. The threshold voltage shift ( V TH ) under gate bias stress is a crucial parameter to determine the electrical stability of TFT devices in active matrix liquid crystal display applications since the bias stress-induced threshold voltage shift leads to a change in the individual pixel brightness. At the same time, V TH that is sensitive to light illumination will limit the TFT application to the next generation of transparent electronics because the exposure of switching TFTs to backlight or ambient light would be inevitable during the operation of the electronics such as display panel. The degradation behaviors of ZnO-based TFTs have been investigated by many previous studies. 12-17 They indicated that the V TH under bias stress or light illumination can be attributed to the charge trapping in the gate insulator or in the active layer/insulator interface, 13-15 and oxygen or water absorption/desorption at back channel. 16,17 In a-Si:H TFTs, the charge trapping sites can be eliminated through an annealing of the a-Si. However, high temperature annealing of ZnO 18 is not suitable for glass substrates owing to their low softening/melting temperatures. 19 An alternative solution for reducing the trapping sites is adding an el...

Suppression of Oxygen Vacancy Defects in sALD-ZnO Films Annealed in Different Conditions

“…They may deteriorate or cause instability of the film properties. For instance, they may deteriorate the subthreshold characteristics and induce instability of the oxide TFTs [21]. This kind of oxygen vacancy defects should be suppressed.…”

Improving the electrical properties of transparent ZnO-based thin- film transistors using MgO gate dielectric with various oxygen concentrations

“…Decreasing the number of oxygen vacancies in the MgO films suppresses the interface charge density, thus resulting in a low gate leakage current and a high on/off current ratio in TFTs [18]. The oxygen atoms in the MgO gate insulator compensate for the oxygen vacancies in the ZnO channel layer and enhance the bias stress stability of the TFTs [19]. For MgO layers deposited with various oxygen concentrations ranging from 30% to 70%, the TFTs with the highest oxygen concentration of 70% exhibited the highest dielectric constant (11.35) with a field-effect mobility of 0.0235 cm 2 V −1 s −1 [4].…”

“…For MgO layers deposited with various oxygen concentrations ranging from 30% to 70%, the TFTs with the highest oxygen concentration of 70% exhibited the highest dielectric constant (11.35) with a field-effect mobility of 0.0235 cm 2 V −1 s −1 [4]. However, ZnO-based TFTs are all bottom gates, whereas MgO is a thick film (180-200 nm), which was deposited with a maximum oxygen concentration of 70% [4,15,19]. In this study, ZnO-based top-gate TFTs were fabricated and MgO was deposited as a thin (20 nm) film by magnetron sputtering with oxygen percentages ranging from 0%-100%.…”