Density of States of the a-IGZO from Temperature Dependent Field-Effect Studies

Charlene Chen and Jerzy Kanicki

Amorphous In-Ga-Zn-O thin film transistor (a-IGZO TFT) has emerged as an important candidate that could be used for future flat panel displays. Besides visible light transparency and the ability to be deposit uniformly over large area at low temperatures, a-IGZO TFTs also demonstrate favorable electrical properties, including high field-effect mobility, high current on-off ratio and sharp subthreshold swing. All these device parameters are highly dependent on the density of localized gap states (DOS) of the a-IGZO. Knowledge of these states is essential for fundamental understanding and improving the material’s electrical properties.

In this work, temperature dependent field-effect measurements were performed on a-IGZO TFTs. We observed that the field effect mobility is weakly thermally activated (activation energy ~ 26meV). The threshold voltage linearly decreases with temperature with a temperature coefficient of -17mV/°C. The current On-Off ratio and subthreshold slope almost remained the same within the investigated temperature range. The density of states of the a-IGZO was calculated using the method based on temperature dependent field effect measurements. The Meyer-Neldel (MN) rule was also taken into account during the calculation. The calculated DOS from the subthreshold regime is low (<10¹⁸ eV^-1cm^-3) with a characteristic energy of ~120meV, and shows good agreement with the density of deep states extracted from SPICE simulations. This work was supported by Canon Corp., Japan.

C. Chen et al., IEEE T-ED, 56 (2009) 1177.

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