DFT study of electron affinity of alkali metal termination on clean and oxygenated β-Si₃N₄

The tuning of electron affinity (EA) by alkali metal (Li, Na, K, Rb and Cs) terminations on clean and oxygenated β-Si₃N₄ surfaces is studied by using density functional theory (DFT) calculations. Large negative EA shifts from - 2.9 to - 6.6 eV relative to the clean surfaces are obtained, resulting in negative electron affinity (NEA) from - 2.1 to - 4.4 eV. The general trend for alkali metal terminations on both clean and oxygenated surfaces is that the EA becomes more negative from Li to Cs, whereas the adsorption strength decreases from Li to Cs. In comparison, alkali terminations on oxygenated surfaces exhibit significantly higher stability than on clean surfaces. The strength of the terminations and their corresponding EA shifts are interpreted using the geometric properties and the charge distribution of the surfaces. Li termination on oxygenated surfaces is predicted to be the most promising candidate to induce NEA due to its high thermal stability and large NEA.