Investigation of Electrical Instabilities and Interface Charge in ZnO Thin-Film Transistors with High-k Dielectrics
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Exhibiting high electron mobility compared to amorphous Silicon, transparency in the visible spectrum, and compatibility with large area and flexible substrate applications, the semiconductor Zinc Oxide (ZnO) has become a major focus of research. The performance tradeoffs of inserting high dielectric constant insulators into ZnO thin film transistors were investigated. Hafnium Oxide (HfO2) emerged as the most promising material, however, possible drawbacks include the presence of interface charge and transistor threshold voltage instability. Admittance Spectroscopy was found to be the most comprehensive technique to measure interface charge density and values were found to range from low 1013 to high 1011 cm-2 eV-1. Transistor performance instability was investigated by the Bias – Temperature – Illumination Stress method. Positive bias stress analysis reveal the dominant instability mechanism is carrier injection into the HfO2. A new method was developed to characterize positive bias stress threshold voltage instabilities and extract dielectric charge trap density. Negative bias stress analysis shows the governing instability mechanism is charge state creation, likely, in the ZnO semiconductor or interface. Under illumination, instabilities can be explained via a grain boundary charge (NGB) limited conduction model where illumination causes a reduction in NGB and an increase in carriers due to photo-generation.