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Title: Defects and lifetime prediction of germanium MOSFETs
Author: Jigang, M.
ISNI:       0000 0004 5993 0368
Awarding Body: Liverpool John Moores University
Current Institution: Liverpool John Moores University
Date of Award: 2015
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To continue improving device speed, much effort has been made to replace Si by high mobility semiconductors. Ge is considered as a strong candidate for pMOSFETs due to the high hole mobility. Two approaches have been demonstrated: high-k/Si-cap/Ge and high-k/GeO2/Ge. Negative Bias Temperature Instability (NBTI) is still one of the main reliability issues, limiting the device lifetime. In this project, it is found that the conventional lifetime prediction method developed for Si is inapplicable to Ge devicesand defect properties in Ge and Si MOSFETs are different. The threshold voltage degradation in Ge can be nearly 100% recovered under a much lower temperature than that in Si devices. The defect losses observed in Si devices were absent in Ge/GeO2/Al2O3. The generation of interface states is insignificant and the positive charges in GeO2/Al2O3 on Ge dominate the NBTI. These positive charges do not follow the same model as those in SiON/Si and an energy-alternating model has been proposed: there are a spread of energy levels of neutral hole traps below Ev andthey lift up after charging, and return below Ev after neutralization. The energy distribution of positive charges in the Al2O3/GeO2/Ge gate stack was studied by the Discharge-based Multi-pulse (DMP) Technique. The different stress-time dependence of defects below Ev and around Ec indicates that they originate from different defects. Quantization effect, Fermi level pinning, and discharge voltage step were considered. The defect differences in terms of the energy level were investigated by using the DMP technique and the energy alternating model is verified by the defect energy distribution. Based on the understanding of different defect behavior, a new NBTI lifetime prediction method was developed for Ge MOSFETs. Energy alternating defects were separated from as-grown hole traps (AHT), which enables to restore the power law for NBTI kinetics with a constant power exponent. The newly developed Ge method was applicable for NBTI lifetime prediction of the state-of-the-art Si-cap/Ge and GeO2/Ge MOSFETs. When compared with SiON/Si, the optimized Si-cap/Ge shows superior reliability, while GeO2/Ge is inferior and needs further optimization. Preliminary characterization was also carried out to investigate the impacts of energy levels and characteristic times of different defects on the frequency and duty factor dependence of AC NBTI degradation.
Supervisor: Zhang, J. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: TK Electrical engineering. Electronics. Nuclear engineering