The energy barrier and penetration depth of a hole are studied for ultrathin SiO₂/Si interfaces. Layer thickness dependence and injected carrier concentration dependence are calculated for the superlattice structure, by using a first-principles pseudo-potential approach based on the density-functional theory. A new method to evaluate the energy barrier is proposed, which is derived from calculation of both the energy band structure and behavior of the Bloch function. The energy barrier for a hole is reduced with a decrease in Si layer thickness by confinement effect and is reduced also with the monolayer thickness of SiO₂. The energy barrier increases by electron injection and decreases by hole injection.