First-principles study of B diffusion meachnism at Si/$SiO_2$ interface and the effects of impurities at metal/high-k interface

Abstract

The metal-oxide-semiconductor field effect transistor(MOSFET) is the fundamental semiconductor device. As the size of MOSFET is continuously reduced, it is important to understand dopant segregation at the Si/$SiO_2$ interface and to control the distribution of dopants. While it is reported that B easily segregates to the oxide near the Si/$SiO_2$ interface during oxidation or thermal annealing after ion implantation, the B diffusion mechanisms at the Si/$SiO_2$ interface are not clearly understood. Therefore, in chapter 3, the diffusion pathways and migration barriers for B diffusion at Si/$SiO_2$ interface are investigated. In bulk Si, B forms a bond with self- and becomes B-self-interstitial complex defect. Then B diffuses in Si with preserving B-self-interstitial bond, which called interstitialcy mechanism. When B reaches Si/$SiO_2$ interface, B turns into interstitial B and diffuses toward $SiO_2$. The overall migration barrier for B diffusion at defect-free Si/$\alpha$-quartz $SiO_2$ and Si/amorphous-$SiO_2$ are calculated 2.02 and 2.04±0.44 eV, respectively. They are similar to that in bulk $SiO_2$, so this results indicate that interface does not hinder B diffusion from Si to $SiO_2$. The effect of defects on B diffusion at the Si/α-quartz $SiO_2$ are studied, and found that the roughness defect does not alter the B diffusion pathway, although interface dangling bonds act as traps, and the migration barrier for B diffusion is enhanced. However, the MOSFET with using ultrathin $SiO_2$ gate dielectric has a number of problems, such as gate leakage current, poly-Si depletion, and threshold instability. In order to solve these problems, the replacement of Si/$SiO_2$ gate stack by high-k/metal gate stack is suggested. For high-k metal gate stack, it is important to control the metal work function. The effective work function should be close to the conduction band edge and valence band edge state of Si, 4.05 eV and 5.15 eV for n- p- channel MOSFET, respectively. The one way of control the effective work function is using impurity, such as Al and La. It is reported that Al atoms can reduce effective work function at TiN/$HfO_2$ interface, which is fabricated by using gate-last process. However, there is a lack of theoretical study of the effect of Al atoms at metal/$HfO_2$ interface. Therefore, In chapter 4, the effects of Al atoms on the effective work function and p-type Schottky barrier height are studied. TiN/monoclinic-HfO2(TiN/$m-HfO_2$) interface with single Al defect, $Ti_{1-x} Al_x N/m-HfO_2$ interfaces, and TiAl/TiN/$m-HfO_2$ interfaces are considered, and it will be shown that the Al atoms at high-k/metal gate play an important role in decrease of the effective work function, resulting in n-type shift. In the chapter 5, the another high-k dielectric material, $t-HfO_2$, is studied. The dielectric constant of $m-HfO_2$ is 18~20, which is much higher than that of $SiO_2$(~3.9). However, as the size of MOSFET is continuously reduced, the dielectric constant of m-HfO2 is not sufficient high enough. While the dielectric constant of $t-HfO_2$ is reported as about 70, the $t-HfO_2$ is required very high crystalline temperature, so it is hard to be used as gate dielectric material. However, it is reported that the crystalline temperature of $t-HfO_2$ can be reduced by using Si impurity. Therefore, the $t-HfO_2$ with Si impurities can be replacement of m-HfO2. The bulk $t-HfO_2$ with Si impurities and TiN/$t-HfO_2$ interface structures with and without Si impurities are considered, and the results show that the Si atoms reduce the band gap and dielectric constant of $t-HfO_2$, and increase the effective work function, resulting in p-type shift.