Understanding Oxygen Migration due to Contacts and Gate Bias-Stress in Indium Tin Oxide Transistors

Field-effect transistors with semiconducting oxide channels are promising candidates in several applications due to their low-temperature deposition, ultralow leakage current, as well as processing compatibility with silicon technology [1-3]. However, their stability is under question and their performance can degrade after long-term operation, an effect which is poorly understood. In this work, for the first time, we use <i>in-situ</i> x-ray absorption spectroscopy to probe the instability of indium tin oxide (ITO) transistors by looking into oxygen migration in metal-insulator-semiconductor-metal (MISM) test structures.<br/>To fabricate the MISM devices, we e-beam evaporate ~30 nm Pd as the back (gate-like) contact, followed by ~13 nm Si₃N₄ (as the insulator) with conductively-coupled plasma-enhanced chemical vapor deposition at 350 °C. This nitride is used to avoid interference from any oxygen species other than in the ITO. Next, we deposit ~4 nm thick ITO (as the semiconductor channel), using radio-frequency magnetron sputtering at room temperature, 100 W power and 5 mTorr pressure. Finally, the ~30 nm top metal contact (four types of samples with Ti/Pt, Ni, Pd, or Pt) is deposited using e-beam evaporation. To utilize <i>in-situ</i> scanning transmission x-ray microscopy, these MISM devices are fabricated on SiN_x membranes.<br/>From the spectral absorption measurement of samples with four different workfunction metals, we demonstrate, for the first time, that metal contacts scavenge oxygen from ITO according to a trend consistent with their reactivity. Such oxygen scavenging from the ITO channel in a transistor creates more oxygen vacancies, which in turn generate more carriers in the channel, causing a negative threshold voltage shift, especially at short channel lengths [4] where the contacts are closer together. The observed trend with metal reactivity agrees well with our previous reports of ITO transistors with Ni <i>vs.</i> Pd contacts, where Ni contacts caused a more negative threshold voltage shift at shorter channel lengths <i>vs.</i> Pd contacts [5,6].<br/>To investigate the microscopic physics during device operation and understand the gate bias-stress effect in ITO transistors, we perform <i>in-situ</i> x-ray absorption spectroscopy while applying voltage bias to the MISM structures with Ni contacts, and observe oxygen migration near both the ITO/dielectric and ITO/Ni interfaces. To correlate this effect with transistor behavior, we fabricate transistors using the same composition as the MISM structure and observe a similar trend. Notably, the device behavior upon gate-bias stressing is different with Si₃N₄ dielectric from our previously reported works with Al₂O₃ or HfO₂ dielectrics [7], which reinforces the impact of dielectric material on the stability of ITO transistors.<br/>In short, our results provide new insight of device operation and threshold voltage (in)stability of ITO transistors using <i>in-situ</i> x-ray characterization. Both metal contacts and the gate insulator play major roles in oxygen migration, and are therefore instrumental for optimizing transistor performance and reliability.<br/>This work is partly supported by the Stanford Graduate Fellowship (S.W., M.I.) and the SystemX Alliance.<br/><br/><b>Refs: </b>[1] M. Si <i>et al.</i>, <i>IEEE Trans. Electron Dev.</i> 68, 3195 (2021). [2] S. Li <i>et al.</i>, <i>Nat. Mater.</i> 18, 1091 (2019). [3] W. Chakraborty <i>et al.</i>, <i>IEEE Trans. Electron Dev.</i> 67, 5336 (2020). [4] S. Subhechha <i>et al.</i>, <i>Symp. VLSI Tech</i>. (2021). [5] S. Wahid, E. Pop <i>et al.</i>, <i>IEDM </i>(2022). [6] S. Wahid, E. Pop <i>et al.</i>, <i>DRC</i> (2022). [7] L. Hoang, E. Pop <i>et al.</i>, <i>DRC</i> (2022).