Abstract
In this paper, the dependence of threshold voltage (Vth) changes to amorphous-indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) under positive bias light illumination (PBIS) on the height of the conduction band offset was studied. Using SiO2, HfO2, and Al2O3 as gate dielectrics, three different types of a-IGZO TFTs were used in the experiment. Electrical parameters and the density of states of each TFT were measured. Of the devices studied, the interface characteristics between the a-IGZO and the gate dielectric were the best using SiO2 and the worst using Al2O3. HfO2 had the smallest conduction band offsets (CBO) of 2.26 eV. We also performed PBS and PBIS evaluation to confirm the stability of TFTs. The Vth shift in the three samples was insignificant under PBS, but the Vth shifts occurred under PBIS in the order of HfO2, Al2O3, and SiO2. The interfacial characteristics of a-IGZO and the dielectric did not change after PBIS in all three devices; the lower the CBO height, the greater the Vth shift after PBIS. The predominant cause of the Vth shift under PBIS is the accumulation of injected photoelectrons that have sufficient energy to tunnel the CBO barrier into the gate dielectric by positive gate bias.
Highlights
We reported that the mechanism of negative bias light illumination (NBIS) in amorphous-indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) is related to the height of the valence band offset
This study investigates TFTs prepared with gate dielectrics composed of either SiO2, HfO2, or Al2O3 to elucidate the effects of scitation.org/journal/adv
The interfacial traps present in the a-IGZO and the gate dielectric in the a-IGZO TFT remained unchanged after positive bias light illumination (PBIS)
Summary
Oxide thin film transistors (TFTs) have been in the spotlight for application in next-generation flexible displays. Among the various oxide semiconductors, amorphous-indium gallium zinc oxide (a-IGZO) TFTs have good uniformity, transparency, and field effect mobility and are used in active matrix organic light emitting diode (AMOLED) displays. Recently, many studies have reported replacing the dielectric layer of a-IGZO with a high-κ dielectric for implementation in high-resolution displays. instability of Vth has been reported under positive bias light illumination (PBIS) and negative bias light illumination (NBIS)of a-IGZO TFTs used in AMOLEDs. Previously, we reported that the mechanism of NBIS in a-IGZO TFTs is related to the height of the valence band offset. When an OLED emits light in an AMOLED panel with an NMOS oxide TFT, positive bias is applied to drive the TFT. Instability of Vth has been reported under positive bias light illumination (PBIS) and negative bias light illumination (NBIS). When an OLED emits light in an AMOLED panel with an NMOS oxide TFT, positive bias is applied to drive the TFT. Under this condition, light from the OLED or external light can be irradiated to the active oxide TFT by reflection or transmission. One mechanism is that electrons in the valence band or deep state oxygen vacancies (Vo) excited by light are trapped at the interface between the a-IGZO and the gate dielectric or are injected into the gate dielectric after trapping, causing a shift in Vth.. One mechanism is that electrons in the valence band or deep state oxygen vacancies (Vo) excited by light are trapped at the interface between the a-IGZO and the gate dielectric or are injected into the gate dielectric after trapping, causing a shift in Vth. The other mechanism is that Vth instability occurs due to environmental factors such as photodesorption of oxygen molecules (O2) and the existence of a meta-stable subgap state induced by water (H2O) molecules.
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