Study of Physical Mechanisms of Electrical Reliability for Passivation Layer Applied to Amorphous Indium-Gallium-Zinc-Oxide Thin Film Transistors

• Main Authors: Chang, Geng-Wei, 張耿維
• Other Authors: Tai, Ya-Hsiang
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/08029998550792502353

博士 === 國立交通大學 === 光電工程研究所 === 102 === TFT is a rapid growing field expected to impact all aspects of human life such as energy, health and the environment. TFTs applications include switching and driving devices for active matrix flat panel displays (AMFPDs) based on liquid crystal pixel (AMLCDs) and organic light emitting diodes (AMOLEDs), medical imagers, pressure sensors, low power communication and energy harvesting. When AMLCDs become larger size (>100 inch) and higher resolution (4K2K or 8K4K), the higher mobility of thin film transistor is required. The amorphous indium gallium zinc oxide (a-IGZO) is introduced to the active layer due to the high mobility (>10 cm2/Vs). However, a-IGZO layer is very sensitive to the ambient environment such as oxygen, moisture. Hence, an effectively passivation layer for a-IGZO TFT is necessary to prevent the ambient gas adsorption. In the first part, the sol-gel-processed paraffin wax passivation layer was applied to a-IGZO TFT. The prarffin wax passivated a-IGZO TFT shows a negative threshold voltage shift compared with as-fabricated device because hydrogen may act as a donor in IGZO film. Moreover, the paraffin wax layer has a good passivation ability to prevent gas absorption. Hence, the device exhibits a superior stability against positive gate bias stress under the different ambient environment. Additionally, the light-induced stretch-out phenomenon was suppressed for a-IGZO TFT with paraffin wax passivation layer due to lower density of state. In the second part, this part investigates the effects of ambient atmosphere on electrical characteristics of SiO2 passivated a-IGZO TFT during bias stress. a-IGZO TFT without any passivation layer shows significantly hump phenomenon during positive gate bias stress because H2O adsorption in back channel of a-IGZO film induces the delocalized electron carrier as a parasitic transistor. SiOx passivated IGZO TFT shows the superior stress stability due to effectively suppress the gas adsorption/desorption in the back channel of a-IGZO film. However, the transfer characteristic of a-IGZO TFT exhibits an apparent sub-threshold current stretch-out phenomenon at high temperature which becomes more serious with increasing temperatures. The phenomenon is caused by thermal-induced hole generation and accumulation at the source region that leads to source side barrier lowering. Moreover, the negative gate bias temperature instability results from the thermal-induced hole injected into gate insulator. During positive drain bias stress at high temperature, thermal-induced hole is trapped in gate insulator, especially near the drain region. The non-uniform hole-trapping phenomenon only can be observed at high temperature. Finally, this part investigates N2O plasma treatment applied to a-IGZO TFT. Comparing with the untreated device, the N2O-plasma treated device can significantly suppress the temperature-dependent sub-threshold leakage current stretch-out phenomenon of a-IGZO TFT. The apparent hysteresis phenomenon for the untreated device is attributed to the extra trap states generated during the deposition of SiOx passivation layer by PECVD. However, these trap states did not appear in the N2O-plasma treated device. Because the N2O plasma treatment can generate an oxygen-rich region in the IGZO film surface. The oxygen-rich region can prevent the plasma damage during SiOx passivation layer process. Thus, the N2O-plasma treated device can improve the stability under high temperature environment. Moreover, the hysteresis phenomenon was suppressed because the interface states reduced significantly after N2O plasma treatment.