Fabrication And Electrical Characteristics Of High Performance Germanium-channel MOSFETs

Recently it is becoming increasingly difficult to further improve the performance of Si MOSFETs, because the conventional device scaling is approaching its physical limit. Therefore, Germanium (Ge) is considered as one of the most promising channel materials for future CMOS devices due to its high bulk electron and hole mobility. For high performance Ge CMOS devices, advanced high-k/Ge gate stacks with both thin EOT and high mobility are mandatory. In this research, we focus on the fabrication technique for the Ge gate stacks with thin EOT and superior MOS interfaces simultaneously.First, the Ge wafer cleaning method was studied by using Ge (100) substrates. It is found that the improved Ge wafer cleaning process makes it possible to obtain a high quality and low roughness Ge surface without any native oxide and organic molecular on it.In order to overcome the problem of the conventional fabrication process, which inevitably inrtuoduces a GeOx interfacial layer (IL) into the Al2O3/Ge gate stack, we proposed an ozone post oxidation (OPO) technology. With this technology, the interface of Al2O3/Ge gate stack could be effectively passivated. Here, the AI2O3layer could also be served as a barrier layer of oxygen to avoid the rapid oxidation of Ge and provide a protection layer against the harmful species in the atmosphere. The physical structure and chemical components of these ozone oxidation treated AI2O3/Ge structures are analyzed in detail. It has been confirmed that after the careful process optimization, there was no GeOx IL formed after performing the ozone post oxidation process under the room temperature and air pressure.Furthermore, the electrical properties of the Al2O3/Ge gate stacks formed by ozone post oxidation have been investigated systematically. With the OPO treatment, the decreases in the EOT of the Al2O3/Ge MOS capacitors were confirmed. Furthermore, it is confirmed that the ozone post oxidation shows significant advantages in revealing a continuous decrease in the gate leakage current and scaling the EOT. The results could be attributed to the Al2O3film quality improvement due to the OPO treatment.The impact of400℃ozone post oxidation on the electrical properties of the Al2O3/Ge interface and Ge pMOSFETs was also investigated. Due to the decrease in oxygen vacancies in gate oxides, the ozone post annealing could enhance the mobility and reduce the slow traps in IL-free Al2O3/Ge pMOSFETs. These results indicate feasibility for the ozone post oxidation technique to be used to prepare IL-free high-k/Ge gate stacks with high quality MOS interfaces and scaled EOTs simultaneously. The impact of ozone post oxidation on the slow traps density in Al2O3/Ge gate stacks has also been examined using I/V-t sampling of Ge pMOSFETs, which was carried out at Vd=-1V and Vg=Vth-1V. It is found that with increasing the ozone post oxidation time from0min to3.5min, the Id reduction is suppressed from45%to33%after10s’ sampling. This phenomenon is attributable to the reduction in oxygen vacancies in Al2O3/Ge gate stack with increasing the annealing time.With an optimized OPO treatment (high temperature and long time), the high quality Al2O3/Ge gate stack was obtained. Furthermore, with this gate stack, high performance Ge pMOSFETs have also been prepared. A high peak hole mobility of503cm2/Vs is achieved by this optimized ozone post oxidation for25minutes.At last, the reliability behaviors, such as the Negative Bias Temperature Instability (NBTI) characteristic and voltage endurance properties of Ge pMOSFET fabricated by ozone post oxidation technology have also been experimentally investigated. It has been confirmed that the Ge pMOSFET which is fabricated by the ozone post oxidation method under the450℃for15minutes, exhibits improved NBTI and voltage endurance characteristic, indicating the significant suppression of slow traps and significant decrease of oxygen defects by ozone post oxidation method.