Effect of Exposure to Ultraviolet-Activated Oxygen on the Electrical Characteristics of Amorphous Indium Gallium Zinc Oxide Thin Film Transistors

Abstract

An exposure of the back channel of an indium gallium zinc oxide (IGZO) thin film transistor (TFT) to ultraviolet (UV)-activated oxygen can effectively shift the threshold voltage (V th ) of the TFT. The V th decreases linearly with the exposure time while the on-state current greatly increase with the exposure time. The exposure doesn't have a strong impact on other device parameters. The effect of the exposure on the V th is attributed to the increase in the electron concentration of the channel layer as a result of the creation of oxygen vacancies by exposure. Transparent oxide semiconductors (TOS) have attracted much attention for the application of flat-panel displays due to their high carrier mobility (>1 cm 2 /V s) and high transparency in visible range. 1-3 Amorphous indium gallium zinc oxide (a-IGZO) is one of the most attractive TOS that can be used in the next-generation active-matrix flat panel displays, because it has a few superior properties including relatively high mobility, good uniformity, low temperature process, high optical transparency, high threshold voltage stability, etc. 4,5 It has been recently reported that the threshold voltage (V th ) of a-IGZO thin film transistors (TFTs) is affected by the process parameters such as deposition pressure, target composition, channel thickness, post annealing temperature. 6,7 The oxygen content and the thickness of the a-IGZO channel layer are also reported to play an important role in the electrical characteristics including the V th of the TFTs. 8,9 The V th is determined by the electron concentration in the channel layer of the TFT, thus it is related to the oxygen vacancies which provide the free charge carriers for electrical conduction in the oxide layer. Controlling the V th is required in the TFT application; however, it is not an easy task as the oxygen vacancies can be easily generated during the fabrication processes.In the present work, we have demonstrated that the V th can be easily changed by exposing the back channel of the IGZO TFT to ultraviolet (UV)-activated oxygen. The V th of the TFT is found to be highly sensitive to the exposure to UV-activated oxygen, and a linear relationship between the V th shift and the exposure time is observed. However, the exposure does not have a large impact on other device parameters including the field-effect mobility and subthreshold swing.The IGZO TFTs were fabricated on a heavily-doped n-type Si substrate with resistivity lower than 0.001 · cm, which served as the bottom gate of the TFTs. Firstly, a 30 nm Al 2 O 3 layer was deposited onto the Si substrate by an atomic layer deposition (ALD) process to form the gate insulator. A 50 nm IGZO thin film was subsequently deposited on the Al 2 O 3 thin film layer by radio-frequency sputtering in Ar ambient at room temperature using a IGZO target (In:Ga:Zn mole ratio = 1:1:1). The IGZO thin film was then subject to the exposure to UV-activated oxygen for different durations. Photolithography process followed by wet etching was used to pattern the active ...

Investigation of Low Temperature Process of Solution Processed Oxide Semiconductor as a Thin Film Transistor

“…Further, in both samples, V TH is negatively shifted, which suggests that a decrease in the charge trap took place, increasing the intrinsic carrier in the a-IGZO films. Because oxygen vacancy in the a-IGZO causes an intrinsic carrier [9], the results shows that oxygen vacancy is increased by photo irradiation. Because the increase in mobility and V TH shift simultaneously occurred, we concluded that a decrease in the electron trap took place the V TH shift.…”

Improved bias stress stability of In–Ga–Zn–O thin film transistors by UV–ozone treatments of channel/dielectric interfaces

“…Meanwhile, the behavior of the TFT no longer obeyed the logarithmic time dependence model with longer exposure to the UV irradiation to 60 min (R 2 ¼ À0.259 and r 0 ¼ 0.17), which is ascribed to the removal of the interface traps to sufficiently low level to prevent the device deterioration under prolonged bias stress. The beneficial effect of the UV irradiation on the device performances as observed above can first be attributed to the removal of organics and carbon contaminations on the surfaces of SiO 2 dielectric by activated oxygen or ozone formed by the UV irradiation in oxygen-containing ambient [20,21]. It is known that ozone can remove the surface contaminants by its strong oxidation power and this results in the preparation of atomically clean surface [20].…”

“…The beneficial effect of the UV irradiation on the device performances as observed above can first be attributed to the removal of organics and carbon contaminations on the surfaces of SiO 2 dielectric by activated oxygen or ozone formed by the UV irradiation in oxygen-containing ambient [20,21]. It is known that ozone can remove the surface contaminants by its strong oxidation power and this results in the preparation of atomically clean surface [20]. If the channel layer is deposited on such a clean surface of SiO 2 dielectric, it is expected that interface trap states related to the surface adsorbates can be removed substantially.…”

UV-Cured Reactive Mesogen-YInZnO Hybrid Materials as Semiconducting Channels in Thin-Film Transistors Using a Solution-Process

“…These peaks correspond to stoichiometric oxygen in the hybrid YIZO films (O1), oxygen vacancies in the hybrid YIZO films (O2), and the oxygen loosely bonded at the surfaces of the hybrid YIZO films (O3). 12 The O2/O1 ratio indicates the relative number of oxygen vacancies, and the values of this ratio are 24.65% and 51.86% for the hybrid YIZO films with and without UV irradiation, respectively. These values can be explained by the fact that the oxygen vacancies, which are the origin of the low mobility, were decreased by the application of UV irradiation in the hybrid YIZO system because the UV-cured film affects the channel alignment and densifies the channel.…”

“…These values can be explained by the fact that the oxygen vacancies, which are the origin of the low mobility, were decreased by the application of UV irradiation in the hybrid YIZO system because the UV-cured film affects the channel alignment and densifies the channel. 12,13 We then measured the optical retardation of the hybrid YIZO films with and without UV irradiation to trace the aligned channel material, as shown in Figure 3. Optical-retardation measurements can evaluate and be used to determine the anisotropy of hybrid YIZO films with and without UV irradiation.…”