Germanium MOS devices integrating high-k dielectric and metal gate
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This dissertation investigates the fabrication and characteristics of the metaloxide-semiconductor (MOS) devices built on germanium substrates integrating HfO2 high-κ dielectric and TaN metal gate electrode. The metal-gate/high-κ/germanium MOS stack, by taking the advantages of the high carrier mobility from the germanium channel and the sub-nm equivalent-oxide-thickness (EOT) scaling capability from the high-κ dielectric and the metal gate electrode, offers a possible solution for the future advanced complementary MOS (CMOS) applications to further boast the transistors’ driving current for faster operation. Due to the unstable and poor-quality natively grown germanium oxide, surface treatment is very critical in germanium device fabrication in order to remove the native oxide and prevent its growth, as well as suppress the interdiffusion across the interface. Several wet cleaning methods and an in situ cleaning technique by Ar anneal have been investigated. Surface passivation techniques, including NH3-based surface nitridation (SN) by forming a GeOxNy layer and silicon interlayer (SiIL) passivation by growing an ultra-thin (several monolayer) silicon layer between the high-κ dielectric and the substrate, have been studied and proved able to improve device performance significantly. Both p- and n-channel germanium transistors have been successfully fabricated. 1.8X enhancement of peak mobility in p-channel and 2.5X in n-channel over the silicon control devices have been achieved. The interface growth mechanism between the germanium substrate and the dielectric layer has been investigated. Two competing processes occurring at the interface determine the formation of the interfacial layer and affect Ge outdiffusion. Substrate dopants are found playing important roles, which causes the variations in the interfacial layer formation on different types of substrates and so on in the electrical properties. The relatively high diffusivity of dopants and germanium atoms in bulk germanium and the induced structural defects near the surface may severely degrade the device performance. This can well explain the very poor performance of the n-channel devices reported recently by several groups. Performance degradation of the germanium devices after thermal anneal, which is resulting from the interdiffusion and germanium oxide desorption, suggests that thermal stability is a concern in high temperature processes and more stable passivation techniques may be required. Long term reliability study indicates that HfO2 dielectric with SN treatment on germanium is robust against TDDB stress and the long term reliability (TDDB) is not a concern for germanium MOS devices.