Reliability Issues in Germanium and Silicon Carbide MOS Devices
The small band-gap and the non-ideal high-k germanium interface makes Ge p-MOSFETs susceptible to various reliability issues. Similarly, the non-ideal SiO2-SiC interface in SiC MOS capacitors makes them susceptible to radiation damage. In this work radiation and bias temperature stress response of MOS capacitors fabricated on both these materials are studied. Ge p-MOSFETs are shown to be susceptible to enhanced junction leakage in total dose environments. The mechanisms behind this increase in junction leakage are researched in this work. It is shown that the increase in surface generation current component of the Ge p+-n junction is responsible for increase in off-state leakage current in p-MOSFETs. Further modifications in Ge p-MOSFET processing, such as variation in Si monolayer thickness and variation in halo doping, are researched to find an optimum process that provides minimum junction leakage and maximum on/off current ratio. It is shown that a process with 8 Si monolayers provides much better pre-irradiation interface trap properties and maintains a better on/off current ratio than a device with 5 Si mono-layers. An optimum value of halo doping is found which provides the minimum junction leakage. Bias temperature stress (BTS) studies on Ge MOS capacitors showed that the devices without any interlayer (with high-k directly deposited on Ge) are particularly susceptible to temperature stress. Accumulation capacitance and interface trap density was found to decrease temperature stress. This indicates growth of a thin interlayer and diffusion of Ge into the high-k layer with temperature stress. The radiation response of SiC MOS capacitors with SiO2 gate dielectric is also studied in this work. MOS capacitors fabricated on 3C- and 4H-SiC polytypes are studied. These MOS capacitors are nitrided with either NO or N2O as nitridation agent. It is shown that MOS capacitors with N2O nitridation have higher starting interface trap density on both 3C- and 4H-SiC. N2O nitrided MOS capacitors trap more radiation-induced charge than the NO treated MOS capacitors on both 3C- and 4H-SiC. This is due to greater content of nitrogen deposited at the SiO2-SiC interface for NO treated MOS. Secondary ion mass spectroscopy (SIMS) measurements show that NO treated MOS devices indeed deposit a greater content of nitrogen at interface than N2O. 3C-SiC traps more charge than 4H-SiC MOS capacitors. This may be attributed to better quality of 4H-SiC substrates.