Evolution of the surface states during the in situ SiN layer formation on AlN/GaN heterostructures

Abstract

The effect of a monolayer thick SiN film on the surface states of the AlN/GaN heterostructure grown by molecular beam epitaxy was investigated. It was revealed that the submonolayer SiN coating of the AlN surface leads to the formation of an ordered (√3 × √3)R30° structure. Further SiN film growth results in a Si-enriched SiN amorphous film formation. The Fermi level, pinned 1 eV above the valence band maximum of as-grown AlN, jumps 2.3 eV when (√3 × √3)R30° is formed, and then gradually goes to 3.1 eV with an increase in the thickness of the SiN film up to 4 monolayers. The motion of the Fermi level is a consequence of the evolution of surface states, as well as the appearance of donor-like states in a Si-enriched SiN film. The presence of donor-like states was confirmed when studying the effect of current collapse in enhancemet-mode HEMTs made of SiN/AlN/GaN heterostructures.