Study of ion beam assisted deposition of low dislocation density gallium nitride thin films using molecular beam epitaxy

In this work, the interactions between ions and GaN thin films are investigated during ion etching and ion beam assisted deposition. The formation of periodic nanoscale patterns during ion-beam assisted deposition of GaN thin films using molecular beam epitaxy (MBE), such as ripple or dimples, are presented. A rate equation based linear continuum equation consisting of a surface roughening term (curvature-dependent sputtering or asymmetric attachment of mobile adatoms/defects), a surface smoothing term (thermal or/and ion induced diffusion), and a noise term, is used to described the nanostructure formation. Rotation can prohibit the formation of these nano scale textured structures. A direct method is developed to measure the curvature-dependent ion etching of GaN. In addition, growth window for GaN is expanded by using low angle ion beam to prevent GaN droplet formation at low growth temperatures. These studies have led to the development of nanostructured GaN which can be used to stop dislocation climbing and terminate dislocations at interfaces, opening the way to grow low dislocation density GaN. This ion-assisted deposition method is applied to prepare dislocation filters for the epitaxial growth of low dislocation density films on both basal plane sapphire substrates and (111) silicon substrates. AFM (atomic force microscopy) and KOH were used to measure the dislocation density in the films. A super lattice structure is deposited using ion-beam assisted MBE. Dislocation density, as low as 2 x 107 cm-2, is achieved by this double-filter technique.