Change in bulk defect density of hydrogenated amorphous silicon by bias stress in thin film transistor structures

Abstract

The changes in defect density of hydrogenated amorphous silicon (a-Si:H) in thin film transistor (TFT) structures by prolonged bias stress or light soaking have been studied through constant photocurrent method (CPM) measurements. The CPM absorptions due to defect absorptions do not change after positive or negative bias-stress. On the other hand, the CPM absorptions due to dangling bond defects increase remarkably after light soaking. These experimental results demonstrate that the bulk defect density of a-Si:H in TFT structure is not changed after long time bias stress even though the characteristics of the TFT degrade significantly.

New Findings and Interpretation on Atmospheric Adsorption Induced Instability in Microcrystalline Silicon Films

“…The positive Fermi level shift and the dark conductivity decrease in atmospheric adsorption of µc-Si:H may correspond to the positive threshold voltage shift and the drain current decrease in positive bias-stress induced instabilities in a-Si:H TFTs, respectively. Furthermore, the same resultswere observed for the CPM spectra in µc-Si:H and a-Si:H TFTs with a built-in upward band-bending [8,12,13].Furthermore, dark conductivity increase and negative Fermi level shift in atmospheric adsorption of µc-Si:H may correspond to the drain current increase and the negative threshold voltage shift in negative bias-stress induced instabilities in a-Si:H TFTs. In TFTs, Negative bias-stress makes downward built-in band-bending due to the trapped holes within gate insulator.…”

“…Positive (Negative) bias-stress makes upward (downward)built-in band-bending due to the trapped electrons (holes) within gate insulator. Previously, we applied CPM to measure optical absorption spectra of a-Si:H films in TFT structure and the behavior of CPM spectra upon band-bending between gate and source-drain channel were investigated [12,13]. The observed spectra showed a reduced defect absorption, compared with that under flat-band condition, when band-bending (either upward or downward) is formed at a-Si:H/a-SiN:H interface.…”

P‐16: Turnaround Phenomenon of Threshold Voltage Shifts in Bias‐Stressed a‐Si:H Thin‐Film Transistor under Extremely High Intensity Illumination

“…In contrast, when the a-Si:H TFT was under lower intensity illumination (6000 cd/cm 2 ) stress, no abnormal behavior of Vth was observed (Figure 1e). Generally, with respect to the Vth turnaround phenomenon occurring under high-intensity illumination during negative gate bias, there are two possible sources of trapped charge in a-Si:H TFTs, including increase in electron trapping or decrease in hole trapping [6][7] . According to previous reports, electrons can be injected into the gate insulator under high intensity negative gate bias-stress, the number of injected electrons is expected to be higher under high-intensity illumination.…”

Stable amorphous-silicon thin-film transistors

“…This is confirmed by recent constant-photocurrent measurements ͑CPM͒ of PECVD a-Si:H TFTs under illumination and gate bias stress. Park et al 16 found that the CPM signal due to bulk defect absorptions does not change after positive or negative bias stress, whereas a significant change of the transfer characteristics is observed. This showed that the bulk electronic defect density of the a-Si:H is not changed upon bias stress.…”