Investigation of trap states in In-Ga-Sn-O thin-film transistors using photon probing and their correlation with NBIS instability

초록

In this study, the electrical characteristics, stability, and trap-state density of states (DOS) in IGTO thin-film transistors (TFTs) are analyzed based on the O2/(Ar + O2) ratio, and the relationship between carrier traps and device stability is investigated using photo-excited charge collection spectroscopy (PECCS). Thin film samples with varying O2/(Ar + O2) ratios are fabricated, and the electrical characteristics and trap-related binding energies of each sample are examined. In addition, the field-effect mobility, threshold voltage, and subthreshold swing of TFT devices with a 20 nm IGTO channel layer deposited under the same conditions are evaluated. The field-effect mobility gradually decreases as the O2/(Ar + O2) ratio increases, with the highest mobility of 37.3 cm2/Vs observed at an O2/(Ar + O2) ratio of 2 %. To evaluate device stability at different O2/ (Ar + O2) ratios, positive bias stress (PBS) and negative bias illumination stress (NBIS) are applied for 5000 s, and Delta Vth shifts are monitored. In addition, to compare with conventional InGaZnO (IGZO), a control TFT using IGZO deposited at the same thickness and O2/(Ar + O2) ratio (2 %) is also fabricated. Under PBS, Delta Vth values of 0.21 V and 1.57 V are observed at O2/(Ar + O2) = ratios of 2 % and 10 %, respectively. NBIS-induced Delta Vth is evaluated under various light wavelengths. Unlike IGZO, which shows strong UV sensitivity, IGTO exhibits responses across all wavelengths. The DOS is extracted from transfer curves obtained via PECCS to quantify the trap states. The highest trap density appeared in the red-light region with values of 4.83 x 1011 cm-2 eV-1 for both 2 % and 10 % O2/(Ar + O2) ratios. These results suggest that IGO traps more electrons in the red light region than the IGZO under similar conditions while showing fewer traps in the blue and UV regions. Furthermore, the impact of trap states on the NBIS stability is assessed by comparing the Delta Vth shifts with the DOS profile.

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