Oxide Semiconductor TFT with Oxygen Gradient Gate Insulator
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Solution Overview
Problem
Existing thin-film transistors (TFTs) face challenges with high off-state current and poor on/off ratio due to oxygen defects at the interface between conductive transparent oxide channel layers and gate insulating films, particularly when using ZnO or amorphous oxide semiconductors like a-IGZO, leading to instability and hysteresis in TFT characteristics, especially when formed at low temperatures.
Innovation Solution
Employing an amorphous silicon oxynitride gate insulating film with a controlled oxygen concentration distribution, where the oxygen concentration is higher at the interface with the oxide semiconductor layer and decreases towards the gate electrode, stabilizing the interface and improving film characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a conductive transparent oxide containing ZnO is used as a channel layer, then the TFT can be formed at low temperature and is transparent to visible light, but oxygen defects are generated at the interface with the gate insulating film, causing high off-state current and poor on/off ratio
Solution Approach 1:
The patent applies preliminary action by forming a buffer layer of amorphous silicon oxide between the ZnO channel layer and the gate insulating film before the final device formation. This buffer layer is deposited in advance to prevent oxygen extraction from the ZnO at the interface, thereby preventing oxygen defects and maintaining high on/off ratio even at low formation temperatures
Solution Approach 2:
The patent introduces an intermediary buffer layer (amorphous silicon oxide) between the ZnO channel layer and the gate insulating film. This intermediary layer acts as a mediator that prevents direct interaction between ZnO and the gate insulating film, thereby preventing oxygen extraction and interface defect formation while allowing the device to be formed at low temperatures
2Temperature
If SiNx is used as a gate insulating film, then the TFT can be formed at low temperature, but a large current flows between source and drain terminals even without gate voltage, making it difficult to achieve normally-off operation
Solution Approach 1:
The patent applies preliminary action by forming a buffer layer of amorphous silicon oxide between the ZnO channel layer and the SiNx gate insulating film before final device formation. This buffer layer prevents oxygen extraction from ZnO by SiNx, thereby preventing interface defects that would cause high off-state current, enabling normally-off operation at low temperatures
Solution Approach 2:
The patent introduces an intermediary buffer layer (amorphous silicon oxide) between the ZnO channel layer and the SiNx gate insulating film. This intermediary prevents direct oxygen extraction from ZnO by SiNx, thereby eliminating the harmful off-state current while maintaining low-temperature formation capability
3Stability of the object's composition
If a two-layer insulating structure is used with oxide on semiconductor interface side and SiNx on gate electrode side, then crystallinity can be enhanced, but hysteresis of TFT characteristics is expanded and reproducibility is poor
Solution Approach 1:
The patent applies homogeneity by using a uniform amorphous silicon oxide layer throughout the gate insulating film structure. This homogeneous amorphous structure eliminates the interface between different insulating materials that causes hysteresis, while maintaining good crystallinity of the ZnO channel layer, thereby improving characteristic reproducibility
4Speed
If amorphous oxide semiconductor film (a-IGZO) is used as channel layer, then field effect mobility can be improved, but interface defects are generated between channel layer and gate insulating film, causing high off-state current
Solution Approach 1:
The patent applies preliminary action by forming a buffer layer of amorphous silicon oxide between the a-IGZO channel layer and the gate insulating film before final device formation. This buffer layer prevents oxygen extraction from a-IGZO, thereby preventing interface defects that would cause high off-state current, while preserving the high field effect mobility of a-IGZO
Data Source
AI summary
The thin-film transistor of the present invention has at least a semiconductor layer including: on a substrate, a source electrode, a drain electrode, and a channel region; a gate insulating film; and a gate electrode, wherein the semiconductor layer is an oxide semiconductor layer, and wherein the gate insulating film is amorphous silicon including at least O and N, and the gate insulating film has a distribution of an oxygen concentration in a thickness direction so that the oxygen concentration is high in the side of an interface with an oxide semiconductor layer and the oxygen concentration decreases toward the side of the gate electrode.