Method for inferring the mechanical strain of GaN-on-Si epitaxial layers using optical profilometry and finite element analysis

3D integration of optoelectronic devices is an important future technology for applications in areas such as quantum dot in cavity, multi-layer photonic integrated circuits, flexible and advanced displays, and biosensors [1-3]. Transfer printing [1,4] is a particularly attractive method to achieve 3D integration, its key benefits including: the transfer of fully fabricated devices, the integration of multiple materials on a single target substrate, the opportunity to stack devices, and the lack of need for post-transfer material processing (when substrate removal is best avoided, especially in the case of multi-layer and multi-material assemblies). GaN-on-Si has become a useful fabrication route for many GaN devices and applications, but the mechanical stress incorporated throughout the material stack can impact the viability of this approach. The transfer printing of GaN membrane devices, a promising emerging technology, is most effective with flat membranes, but in practice many GaN structures released from their Si substrate are highly bowed due to the strain in the epitaxial nitride stack. Our approach uses the optical profiles of epitaxial wafers and membranes as inputs for inferring the mechanical strain state of the material by multi-variable numerical model fitting using COMSOL Multiphysics. This versatile, adaptable and scalable method was tested on samples from two GaN-on-Si wafers, revealing the relationship between built-in strain and material bow in principal-component fashion, returning 3-4 & times;10 & minus;4 strain estimates for the AlGaN (compressive) and GaN (tensile) layers, and suggesting the occurrence of plastic deformation during transfer printing.