Drain Bias Effect on the Instability of Amorphous InGaZnO Thin-Film Transistors under Negative Gate Bias and Illumination Stress

Recently, amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) have been much investigated for their applications due to their excellent electrical properties such as high mobility (μ) and low subthreshold swing (SS) values. However, device reliability under light exposure has been recognized as an issue remaining to be resolved. In this study, the effect of drain bias (V_DS) on the negative bias and illumination stress (NBIS) stability of a-IGZO TFTs was investigated. The degradation mechanisms of a-IGZO TFTs under NBIS with different V_DSbiases were clarified.

The fabrication process of the bottom-gate IGZO TFT was reported in detail elsewhere (1). NBIS was applied with a gate voltage (V_GS) of −40 V under blue light (460 nm) at an intensity of 0.2 mW/cm². The NBIS was temporarily interrupted when the transfer characteristics were measured by a double-sweeping V_GS mode in darkness at V_DS of 0.1 V, and then NBIS was reapplied up to an accumulated stress time of 10⁴ s. For the double-sweeping V_GS mode, transfer characteristics were measured with V_GSfrom −10 to 20 V (denoted hereafter as forward measurement), and then scanned instantly back to −10 V (denoted hereafter as reverse measurement).

Figure 1 shows the variation in the transfer characteristics under NBIS with grounded V_DS for the forward and reverse measurements, respectively. In the initial stage of NBIS (< 1000 s), the transfer curves shifted in the negative V_GS direction without SS degradation; on-current degradation was then observed in the subsequent stage (> 1000 s) in the forward measurement. In contrast, for the reverse measurements, the transfer curves hardly shifted within the initial NBIS duration of 1000 s, and then these exhibited a parallel shift in the positive V_GS direction without SS degradation with increasing NBIS duration. We considered that the generated donor-like defects, originated from the ionized oxygen vacancies (V_O⁺ and/or V_O²⁺) and contributing to on-current degradation, were stabilized by capturing electrons during the forward measurement, and the trapped holes at the front-channel interface were detrapped by the positive V_GSduring the forward measurement (2); therefore, electron trapping at the back-channel interface was an origin of the positive shift of the transfer curve without SS degradation in the subsequent reverse measurement.

To investigate the effect of the V_DS bias on the NBIS degradation, V_DS biases of 40 and −60 V were applied with V_GS of −40 V. Figure 2 shows the transfer characteristics in the forward and reverse measurements before and after NBIS duration of 10⁴ s with different V_DS biases. NBIS with an additional positive V_DS bias of 40 V accelerated the donor-like state creation near the source side because of the positively charged V_O⁺ and/or V_O²⁺ drifting from the drain to the source side under the influence of the lateral electric field, which contributed to more severe on-current degradation. The NBIS degradation was significantly suppressed by applying a large negative V_DS bias of −60 V, because the positive charges of V_O⁺ and/or V_O²⁺ were easily neutralized by electrons under the accumulation condition in the drain region. The detailed degradation mechanisms of a-IGZO TFTs under NBIS with various V_DSbiases will be presented at the conference.

Reference

1. D. Wang, M. P. Hung, J. Jiang, T. Toda, C. Li, and M. Furuta, Jpn. J. Appl. Phys., 53, 03CC01 (2014).

2. M. P. Hung, D. Wang, J. Jiang, and M. Furuta, ECS Solid State Letters, 3 (3), Q13 (2014).