Interface study of high-k oxide and Ge for the future Ge based MOSFET device

Abstract

The progress of continuous scaling of metal-oxide-semiconductor field-effect transistors (MOSFET) technology is accompanied by many novel materials and advanced process technologies. High-k dielectrics materials and Germanium (Ge) are two most promising aspects for the further improvement when the international technology roadmap for semiconductors (ITRS) hits 22nm and below. High-k dielectrics gate oxide is critical to replace current SiO2 with thickness limitation of 2nm and alternative high mobility Ge channel can dramatically improve the device performance.

This thesis examines the interface and surface properties of Ge and high-k materials SrTiO3 (STO) from both the experiment and first-principles calculation. In the experiment part, Ge is grown on top of the STO (100) substrate through direct DC sputtering with atomic oxygen source treatment. From high resolution XRD results, it can be concluded that 500 ?C substrate temperature leads to the single crystalline Ge (111) thin film while 650 ?C substrate temperature changes the thin film to Ge (100). This single crystalline structure and clean interface are verified by HRTEM images. STEM EDX line-scan reveals the phenomenon of Ge diffusion at interface, which is much more serious at higher deposition temperature. The stable surface bonding is Ge-TiO2 terminated instead of -SrO in the Si and high-k interface. XPS analysis of the Ge thin films shows the existence of oxidation states at the interface, which is a mix of Ge2O (Ge1+) and GeO (Ge2+) components. The HRTEM and AFM images of samples with 6mins deposition time present the Nanocrystal (NC) islands for the Ge and the density is around 3.68?1012cm-2. This highly integrated NCs (~1012 cm-2) is very useful for the application of floating ? gate (FG) in the nonvolatile memory (NVM) technology.

In the first-principles calculation part, Hybrid-functionals calculations have been employed to study interfacial electronic properties of perovskite SrTiO3 (001)/Ge (100). It is found that the Ge surface states of Ge p-(2?1) can be effectively removed either by one Sr or two O atoms, and the surface passivated by two oxygen atoms is more energetically favorable. Interface structure of SrTiO3 with TiO2 terminated surface is more stable despite the different surface chemical environments of Ge, and the interface structures without dangling bonds show semiconductor character. It is also noted that the relative stability of the insulating interface structures is not affected by the external electric field.