Effect of Light Intensity and Wavelength on a-Ingazno Thin Film Transistors Under Negative Bias Illumination Stress
Authors: Kim Woo-Sic , Lee Yeol-hyeong , Cho Yong-Jung , Kim Byeong-Koo , Park Kyung Tae , Kim Ohyun 
Published: February 2020

Amorphous InGaZnO (a-IGZO) has excellent properties such as high on/off ratio, good uniformity, and low processing temperature, so they are promising channel material for thin film transistors (TFTs). Instability mechanism of a-IGZO TFTs depending on environmental effects, bias, temperature and illumination stress was widely investigated and reported. Especially in the case of negative bias stress, degradation is accelerated under illumination and many studies have been done to clarify the causes. However, the effects of wavelength and intensity of light during negative bias stress under illumination stress (NBIS) have not been fully explained yet.

 Transfer characteristics of a-IGZO TFTs with back channel etch structure were measured after NBIS is applied. Gate voltage of -40 V was applied at 90 °C for 4000 s while drain and source were grounded. Illumination from white LED was passed through blue, green and red color filters which have peak wavelengths of 450 nm, 530 nm and 700 nm, respectively. To verify the relationship between intensity of light and degradation, measurements were done with various intensities of light.

 Severe negative shifts in transfer curve was shown under NBIS with blue light, while relatively little shifts were shown with green, red light and in darkness (Fig. 1). Apparent degradation in subthreshold slope was observed with blue light, which represents the creation of sub-gap states (Fig. 1d). Shifts in Vth as a function of stress time were extracted with various illumination conditions (Fig.2). The intensities of illumination were 4500 lux for white and 100 lux for blue, green and red light. Despite of much lower intensity compared to that of white light, blue light induced comparable amount of Vth shifts with that of white light; Vthshifts after 4000 s of stress were about -17.7 V for white and -16.3 V for blue light, but were only about -1.0 V for green light, -0.8 V for red light and -0.7 V for in darkness.

 To clarify the effect of intensity in blue light, we adjusted the intensity as 100, 50, 25, 12 and 6 lux with the same stress condition. Amount of Vth shifts were not significantly altered until the illumination was reduced to 25 lux, but started to show drastic decrease below 25 lux (Fig. 3a). It indicates that threshold intensity of illumination and saturation phenomenon exist in NBIS induced degradation of IGZO TFTs. Measurements were repeated at 60 °C and Vthshifts as a function of intensity was plotted with different temperature condition (Fig. 3b). Saturation phenomenon started at higher intensity of light (50 lux) for low temperature, which can be interpreted in terms of activation energy.

 In this work, we investigated the effect of wavelength and intensity of light in degradation of a-IGZO TFTs induced by NBIS. We discovered the existence of threshold light intensity and observed saturated behavior in Vth shifts under NBIS with respect to intensity. We will explain degradation mechanism in NBIS depending on wavelength and introduce new instability mechanism related to intensity of light.

Figure 1