A Study on Stability in Amorphous-InGaZnO Thin Film Transistors for Display Application

Abstract

DoctorIn display industry, amorphous Indium-Galium-Zinc-Oxide (a-IGZO) thin film transistors (TFTs) are very promising device because of their good uniformity, transparency, low cost and high electron mobility compared to a-Si:H TFTs, which is main device in display industry. However, in spite of these advantages, a-IGZO TFTs have many reliability issues related to bias stress, illumination stress, current stress, ambient stress and temperature stress. In this thesis, among these problems, bias stress, illumination stress, current stress and moisture stress would be treated. Furthermore, I use capacitance voltage (C V) measurement and technology computer aided design tool (TCAD) simulation tool from Silvaco Cop. This doctoral dissertation is divided to two parts. Firstly, bias stress and illumination stress will be dealt with by experiment of drain bias illumination stress (DBIS). Second, analyzing the results of negative bias stress after H2O soaking stress, I will treat the topic related to reliability of moisture stress. In the first DBIS part, when drain bias stress (+40 V) is applied on a-IGZO TFTs with white LED 5000 lux, two step degradation occurred: ‘on’ current (ION) drop and re elevation. Firstly, ION dropped to 35% of its original value and mobility decreased from 11.8 cm2/(V∙s) to 3.0 cm2/(V∙s) because of acceptor-like tail states (ATAIL), which are generated by breakage of weak oxygen bonds near drain region; hot carriers are generated by the strong electric field near drain region, then these hot carriers break weak oxygen bonds near drain region. The ATAIL trap electrons, and therefore ATAIL are negatively charged when energy level of ATAIL < Fermi level. The negatively charged ATAIL cause scattering of charged carriers and decrease in FE. As DBIS time elapsed, generation of ATAIL are saturated and ionized oxygen vacancies (VO2+) are consistently generated near the drain region, so ION is re elevated to 1.57 times as high as its dropped value (i.e., to 55% of its initial value); FE increased from 3.0 cm2/(V∙s) to 6.9 cm2/(V∙s) (i.e., to 59% of its origin value);. These compensation effects between ATAIL and VO2+ happened when ATAIL and VO2+ exist in the same place, so generation of VO2+ near drain region are the main cause of the re-elevation. The mechanism of ION drop and re elevation is identified by C V measurements and Silvaco TCAD simulation. Second, I observed abnormal threshold voltage (VT) shift in a-IGZO TFTs under negative gate bias stress (NBS) after soaking them in H2O (pH 8). During application of NBS, VT decreased by -0.43 V, then increased to nearly the initial value. I hypothesize that the electrical field that is applied during NBS causes some dissociation of H2O to hydrogen ions (H+) and hydroxide ions (OH ); the effects between H+ and OH- are responsible for the abnormal changes of VT. The initial decrease is a result of trapping of H+ at the front channel; the subsequent increase is caused by neutralization of the H+ and the OH-, if the a IGZO is very thin, so the front channel and the back channel could affect each other. Therefore, neutralization of energy band bending is possible. Recovery also occurs in two phases: VT first increases then decreases to its initial value. During the recovery process, accumulation of an OH- layer generates a negatively charged field that attracts H+ so that the two species recombined. Increase in VT occurs due to desorption of H+ from the front channel interface, and decrease in VT occurs by recombination