Effect of nitrogen species and energy on the crystallinity and luminescent properties of GaN films grown by the reactive neutral beam-assisted evaporation process

Abstract

The low-temperature growth of GaN on (0 0 0 1) sapphire substrates was studied using excited neutral nitrogen molecules as an approach to avoid ion damage in the ion beam assisted evaporation process. The excited neutral nitrogen molecules (N 2 ∗), together with nitrogen ions (N 2 +), were produced by electron impact in the discharge chamber encased in the ion source. To selectively extract N 2 ∗, the single grid configuration was replaced by a grounded triple grid configuration, through which the flux of ionic species to the growing film surface was significantly reduced. The crystallinity and luminescent properties of GaN films were greatly improved due to the increasing ratio flux of excited neutral/ion in the triple-grid configuration. TEM results inferred that the bombardment of ion beam on the growing GaN surface can induce a significant residual stress and more defects, compared to the excited neutral beam. The effect of the non-ionic nitrogen species on the film crystallinity is discussed in the aspect of kinetic energy. As the discharge voltage of the ion source increased, more energetic neutrals were generated by the charge transfer collision between nitrogen ions and thermal neutrals, and degraded the properties of GaN film.