Work Function Tuning Of Magnetic Field, Electric Field, Stress And Interfacial Microstructure For Metal Gate And Metal/High K Gate

In the rapid development of semiconductor integrated circuits, The size downscaling of Metel-Oxide-Semiconductor field effect transistors (MOSFETs) follows the famous law-Moore’s Law. A new generation of metal/high-K dielectric substituting for the traditional polysilicon/SiO2 has become a reality for 45nm and 32nm technology nodes. For metal gate, work function (WF) is one of the most important factors, which determines the threshold voltage and the performance of MOSFET. Therefore it is one of the most important parameters for MOSFET devices. There are many factors, including not only gate material but also interfacial properties between metal and high-K dielectric can affect the work function of the metal gate. These factors include interfacial chemical composition, interfacial bond formation, interfacial atomic vacancy, crystal orientation of polycrystalline granular structure of metal gate and so on. Currently, the influence mechanism how these factors affect the WF has not in-depth understood. Therefore, in the current and future development of MOSFET devices, how to effectively modulate the WF of metal/high-K gate stack is still an important issue and remains a challenge. In this thesis, using first-principles calculations, we investigate the impacts of magnetic configuration of metal surfaces, the applied electric field, strain and interfacial intrinsic defects which include intrinsic atom substitution doping and atom vacancy. The main findings are as follows:(1) Based on the studies of Cr/Fe(001) and C adsorbed Cr/Fe(001) surfaces, we found that the magnetic configuration of the system plays a significant role in determining the WF. For all systems with various C coverages, the WFs of the systems with parallel states are evidently larger than those with antiparallel states. C adsorption leads to the relaxation of surfacial atoms and results in an important role on the WF. The studies on Cr/Ni(111), Cr/Ni(100) and Cr/Ni(110) magnetic systems prove again that the magnetic configuration plays a significant role on the WF. The calculated results also reveal that WFs vary with crystal orientations. Our work strongly suggests that controlling magnetic configurations is a promising way for modulating the WF of magnetic metal gate.(2) The studies on the effects of external electric field on the WFs for Ni(001), Ni(111), HfO2(001), HfO2(111) films and Ni(001)/HfO2(001), Ni(111)/HfO2(111) interfaces reveal that the WFs for all the systems change linearly with the strength of external electric field. Comparing the slopes of the WF change versus external electric field for Ni, HfO2 films and Ni/HfO2 interfaces, we have found that the response of the effective WF to external electric field for Ni/HfO2 interfaces was determined by the response of the HfO2 side to external electric field.(3) The formation energies and WFs for Ni/HfO2 interfaces with various defects including interfacial intrinsic atom substitution and atom vacancy in interfacial layer have studied. The calculated results indicate that ① the O-Ni combining bonds may energetically be superior to Hf-Ni combining bonds during Ni/HfO2 interface preparation, and a small amount of O vacancy is comparatively easy to form in O-Ni interface, especially under O-rich condition; Hf vacancy is prone to exist in Hf-Ni interface while Ni vacancy is hard to form in Ni interfacial region. ② The EWF strongly depends on the microstructure in interfaces. For Hf-Ni interfaces, the EWF increases with Ni substituting for Hf or Hf vacancies, but it is not sensitive to Ni vacancy. For O-Ni interfaces, oxygen vacancies can result in a decrease of WF. ③ The variations of the WFs are in proportion to that of interface dipole density. Finally, ionic valence state and localized state are used to qualitatively analyze and explain the effects of interfacial defects on the WF in metal-oxide interfaces. Our work suggests that controlling interfacial intrinsic atom substitution doping and atom vacancies (interface roughness) are attractive and promising ways for modulating the effective WF of Ni/HfO2 interfaces.(4) The effects of strain on WF for Ni/HfO2 are studied. The calculated results indicate that the WFs are strongly affected by the type of interface and the strain states. The changed value of the WFs linearly increases with increasing strain. Our work suggests that controlling the strain and interface structure is a promising way for modulating the WF of Ni/HfO2 interfaces.