A study on Reliability in Amorphous-InGaZnO Thin Film Transistors for Display Application

Abstract

Amorphous-InGaZnO (a-IGZO) thin-film transistors (TFTs) are promising device for display application among many amorphous oxide semiconductor TFTs due to their high electron mobility, large scale threshold voltage Vth uniformity, transparency and low cost for display application because of compatibility with amorphous-Si process. Despite these advantages, a-IGZO TFTs have many problems in reliability such as bias temperature stress (BTS), current stress and ambient atmosphere. Therefore, this thesis covered three topics including BTS, current stress and effect of water molecule for various operation condition. BTS is composed of three topics: positive bias temperature stress (PBTS), negative bias illumination stress (NBIS) and dynamic stress. Under PBTS, hump phenomenon, which is a bidirectional movement in current voltage (I-V) characteristics between on current Ion and off current Ioff is observed. Hump phenomenon is related to parasitic transistor formed by PBTS. For hump phenomenon, Von (Ion movement) is explained by trapped electron in gate insulator and VH (Ioff movement) is caused by migration of positively charged species toward back-channel region or due to positive gate bias. During NBIS, negative Vth is observed under green and blue illumination. It is related to ionization energy of oxygen vacancy VO to VO2+. Energies of blue light (~2.76 eV) and green light (~2.3 eV) are higher than the ionization energy of VO (~2.3 eV). The reason for negative Vth is migration of VO2+ generated under green and blue light in negative bias. Also, Vth is saturated after exceeding a certain intensity of blue illumination. It is related to the saturation of VO2+ due to outward relaxation of VO2+, because it makes ionization of adjacent VO difficult. Under dynamic stress, Vth is affected by duty ratio D and amplitude of pulses. Vth is increased with decreasing 1/D and is decreased with increasing amplitude of pulses. It is related to the number of pulses and transition from low voltage to high voltage in the pulses. Also, Vth is matched well with stretched-exponential equation. Therefore, it is also explained based on a charge trapping mechanism. Additional degradation is occurred by strong electric field in channel rather than on-time. Strong electric field makes electrons easy to directly be trapped in gate insulator and electrons attracted from bulk region become hot electrons. These hot electrons can exceed the conduction band of dielectric and can be trapped. Also, degradation is relaxed with increasing rising time from 0.1 s to 1 ms and illumination. Current stress can be categorized into two parts: on-current stress and off-current stress. Under on current stress, on state region is positively shifted and subthreshold region state region is negatively shifted until 2000 s. However, after 2,000 s, degradation direction of subthreshold region is changed toward positive direction. These hump phenomenon is originated from parasitic current path and the degradation is affected by the electric field and accelerated by joule heating. By decomposing the I-V curve at each time into two curves and measuring the relaxation behavior of the stressed TFTs, abnormal turn-around behavior was found to be related with acceptor-like states. Under off-current stress, I-V characteristics shifted toward negative direction at the early stage, but the hump phenomenon was observed after sufficient stress time. The development of the hump is related to creation of ionized oxygen vacancies which act as shallow donor-like states among defects. To clarify the mechanism about hump, two-dimensional device simulation is used and capacitance-voltage C-V characteristics is also used. Effect of water molecule is investigated under BTS and current stress. Under NBTS, negative Vth was observed at low temperature without illumination. Degradation was caused by molecules that were absorbed and diffused from the outside. These molecules attracted by negative bias make migration of hydrogen ion and caused negative Vth. However, with the addition of illumination, degradation becomes independent with soaking time. Therefore, degradation is related to VO. Hydrogen moved along the VO without illumination and VO2+ generated by photon inhibit the migration of hydrogen under illumination. Current-stress on the soaked TFTs investigated three topics such as off-current, on-current and dynamic current stress. Under-off current stress, negative Vth occurred without degradation of subthreshold swing. In the soaked TFTs Vth was more extreme than in the non-soaked TFTs. This degradation could be explained by hole trapping; the additional degradation was caused by H+ trapping on the soaked TFTs. Under on-current stress or dynamic stress, in non-soaked TFTs, Vth shifted positively and the hump phenomenon occurred; in the soaked TFTs, hump phenomenon was relaxed. The relaxation of the hump phenomenon suggests that after the TFT was soaked in water, H+ produced by that back channel during dynamic stress could combine with VO2+ by getting near electrons; this process reduces the number of VO2+ that cause degradation during dynamic current stress.