A thermalization energy analysis of the threshold voltage shift in amorphous indium gallium zinc oxide thin film transistors under simultaneous negative gate bias and illumination
A thermalization energy analysis of the threshold voltage shift in amorphous indium gallium zinc oxide thin film transistors under simultaneous negative gate bias and illumination
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DOI：
10.1063/1.4870457
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发表时间：
2014-04-07
影响因子：
3.2
通讯作者：
Powell, M. J.
中科院分区：
文献类型：
--
作者：
Flewitt, A. J.;Powell, M. J.
以前已经观察到，使用非晶态铟镓锌氧化物(a-IGZO)半导体沟道的薄膜晶体管(TFT)在同时受到负栅偏压和光照射时，阈值电压发生漂移。在这项工作中，热化能分析已经应用于以前发表的关于a-IGZO TFT在光照应力下的负偏压(NBIS)的数据。得到了0.65-0.75 eV的缺陷转换势垒，这与已报道的氧空位迁移能相一致。尝试逃逸频率为10(6)-10(7)S(-1)，这表明在20-40 nm范围内带尾态的载流子有微弱的局域化。回顾了基于电荷捕获的NBIS机制模型，并提出了一个缺陷池模型，在该模型中，a-IGZO带隙中存在两种不同分布的缺陷态：它们与成膜时形成的中性带电和2+带电的氧空位相关联。在该模型中，阈值电压漂移不是由于缺陷的产生过程，而是由于缺陷迁移时带隙中状态的能量分布的变化，因为这允许形成为中性带电空位的状态被转换为形成2+带电空位的状态，反之亦然。转变过程需要靠近缺陷迁移点的载流子局域化，而这些缺陷迁移点与导电态和价带尾态有关。在负栅偏压下，导带尾部的载流子被耗尽，但偏压不足以在价带尾态积累空穴，因此没有产生阈值电压漂移。只有在光照下，空穴的准费米能级才能充分降低，以允许占据价带尾态。由此产生的电荷局部化然后允许负的阈值电压漂移，但只有在同时负的栅极偏置和照明的条件下，正如实验观察到的NBIS效应。(C)2014 AIP出版有限责任公司。
It has been previously observed that thin film transistors (TFTs) utilizing an amorphous indium gallium zinc oxide (a-IGZO) semiconducting channel suffer from a threshold voltage shift when subjected to a negative gate bias and light illumination simultaneously. In this work, a thermalization energy analysis has been applied to previously published data on negative bias under illumination stress (NBIS) in a-IGZO TFTs. A barrier to defect conversion of 0.65-0.75 eV is extracted, which is consistent with reported energies of oxygen vacancy migration. The attempt-to-escape frequency is extracted to be 10(6)-10(7) s(-1), which suggests a weak localization of carriers in band tail states over a 20-40 nm distance. Models for the NBIS mechanism based on charge trapping are reviewed and a defect pool model is proposed in which two distinct distributions of defect states exist in the a-IGZO band gap: these are associated with states that are formed as neutrally charged and 2+ charged oxygen vacancies at the time of film formation. In this model, threshold voltage shift is not due to a defect creation process, but to a change in the energy distribution of states in the band gap upon defect migration as this allows a state formed as a neutrally charged vacancy to be converted into one formed as a 2+ charged vacancy and vice versa. Carrier localization close to the defect migration site is necessary for the conversion process to take place, and such defect migration sites are associated with conduction and valence band tail states. Under negative gate bias stressing, the conduction band tail is depleted of carriers, but the bias is insufficient to accumulate holes in the valence band tail states, and so no threshold voltage shift results. It is only under illumination that the quasi Fermi level for holes is sufficiently lowered to allow occupation of valence band tail states. The resulting charge localization then allows a negative threshold voltage shift, but only under conditions of simultaneous negative gate bias and illumination, as observed experimentally as the NBIS effect. (C) 2014 AIP Publishing LLC.