Invited: Approaches to the Formation and Integration of Large Lattice Mismatched Materials: Metamorphic and Non-Conventional 'buffer' Layers

The growth of large lattice mismatched materials has been studied for many years in order to understand mismatch-derived defects, their mode of introduction, and their mitigation. Traditional approaches focused on the formation of ‘buffer’ layers. ‘Buffer’ layers are intermediate layers within a growing structure which allow a chemical or structural transition between two epitaxial materials. The choice and design of these layers depend critically on the nature of the material transition. Controllably varying the composition across the buffer layer has been the approach most often used in this context. As the composition is graded from the substrate or a lattice matched compound to the substrate to a material which has the desired lattice parameter, the structurally required mismatch dislocations are introduced with a minimum of residual threading dislocations.

The approach of transitional or graded layers has undergone continual development with new effects being realized which can reduce the buffer layer thickness and achieve the dislocation density desired for growth of device layers. Composition grading in compound semiconductors can be accomplished by the change in the cation composition, as in the grading of In_xGa_1-xAs to transition from GaAs to InAs, or on the anion sublattice using InAs_yP_1-y to transition from InP to InAs. Often these graded layers are too rough to use in device applications due to the presence of a surface cross-hatch derived from the introduction of mismatch dislocations. Chemical-mechanical polishing has offered a means to recover the required planar surface. There are alternative approaches to graded buffer layers which are beginning to be studied and utilized. The intent of all these approaches is to generate the required misfit dislocations, have them reside at an appropriate interface and, finally, leave no threading dislocation line segments residing within the device layer. A self-assembled block co-polymer approach to nanoscale patterning, which offers rapid and cost-effective full wafer patterning at the 20-nm length scale, was used to achieve improvements in heteroepitaxial growth of large lattice mismatched materials. The x-ray spectra taken from a 600-nm or less thickness GaSb film grown on a patterned GaAs substrate show a sharp reduction in the full width at half maximum (FWHM). The FWHM of the GaSb peak in this initial study was reduced by at least a factor of two as compared to the film grown to the same thickness on a non-patterned wafer. A reduction of this magnitude is extremely significant for films of this thickness. This improvement in materials properties through nano-patterned growth should be applicable to a wide range of materials and lattice mismatch situations. Views on the strengths, challenges, similarities and applicability of these various approaches will be presented.