III-nitride materials have different crystal structures and properties than the substrates commonly used for their deposition, including silicon, silicon carbide and sapphire. These differences, such as thermal expansion coefficient and lattice constant, necessitate the use of a transition layer to accommodate the resulting stress between substrate and the epitaxially grown III-N layers. AlxGa1−xN based transition layers are one proven solution used for the growth of device quality GaN layers on Si (111) substrates. The use of such transition layers enables the deposition of state of the art AlGaN/GaN high electron mobility transistor epitaxial structures that, upon fabrication into devices, exhibit high performance and excellent reliability.
Examination of the microstructure of these AlxGa1−xN transition layers, by transmission electron microscopy (TEM) and other methods, reveals some interesting properties that can help explain how high quality III-N epitaxy can be performed in a system with significant thermal and lattice mismatch. Observations that will be reported on and discussed in this presentation are (1) the role that a thin strain absorbing amorphous SiNx layer at the Si substrate/transition layer interface plays in the reduction of the formation of misfit dislocations, (2) the low screw dislocation density (less than ȼ107/cm2) in these III-N films relative to edge and mixed dislocation densities, and (3) the role that the substrate type and quality can play on dislocation type and density.