P-type dipole element screening and diffusion analysis in gate stacks by first-principles calculations

Interface dipole engineering has been regarded as a threshold voltage (Vt) modulation method in advanced logic devices. There are various n-type dipole elements experimentally reported that make NMOS Vt shift negatively. In contrast, p-type dipole elements have been less reported with only aluminum being commonly investigated. In the present work, the valence band offset shift (ΔVBO) of 18 types of elements before and after being doped into the HfO2/SiO2 interface was calculated by first principles to screen the potential p-type dipole candidates. Five promising p-type dipole elements are identified: Ga, B, Nb, Ta, and V have ΔVBO<0, which are −1.28, −0.98, −1.14, −0.86, and −0.6 eV, respectively, and Ga has also been experimentally confirmed. Furthermore, the diffusion barriers of Al and Ga in interstitial form in HfO2 and SiO2 are also compared by the Nudged Elastic Band (NEB) method, which are 3.28, 0.62, 3.27, and 0.78 eV. The results indicate that Al and Ga diffuse more easily in SiO2 than in HfO2, which indicates that the dipole-first process induces severe dipole diffusion into channel than the dipole-last, with Al being worse than Ga.