Abstract
Epitaxial (0001) AlGaN thin films grown on {111} Si by gas source molecular beam epitaxy were characterized by AFM, XRD, SEM, EBSD, and TEM. The film surfaces exhibit cracks running in <1120>GaN//<110>Si directions. The cracks extend through the coatings (1micron thick) and into the Si substrate to a depth of approx. 1 micron. Crack formation appears due to the large thermal expansion and epitaxial mismatch stresses for this heterostructure material system. The interface between AlGaN and Si is not a uniform epitaxial structure. Instead, nanoscale island nucleation and growth is complicated by the occasional presence of an amorphous interphase or pores, such that island or subgrain misorientations are often relatively large. Thus, large residual stresses exist near the interface. Despite this complex interfacial structure, continued deposition leads to a coarse microstructure, consisting of subgrains 0.2-1.0 microns in size having small relative misorientations from ideal epitaxy and reduced defect concentrations.
