Towards a Template for Epitaxial Lift-Off Based on Porous n-Si with Reduced Doping Concentration: Increasing Homogeneity and Surface Quality by Photo-Assisted Etching

Porous templates for the epitaxial lift-off approach (ELO) are mainly fabricated based on highly-doped p＋-Si (10-20 mcm). The pore morphology (i.e. porosity and layer homogeneity) realizable on this material is suitable for the application, but among other issues the high dopant concentration of the substrates induces an increase in defect density in epitaxially grown wafers. Hence, it was aimed to find parameters that result in a porous structure on n-Si with a resistivity higher than 20 mcm that is appropriate for this purpose. Porous layers were etched on 6” wafers in the dark and under backside illumination. Both, galvanostatic as well as potentiostatic conditions were used to explore their interplay with photo-generated charge carries. It was found that illumination affects the pore nucleation and thereby, can significantly improve the layer homogeneity, in particular in the galvanostatic regime. With backside illumination surface roughnesses similar to that of an unporosified wafer can be achieved. Being able to etch homogeneous layers with a smooth surface is an important step towards the realization of a high-quality n-Si template for the ELO approach.