Strain-accelerated HF etching of AlAs for epitaxial lift-off

Abstract

Epitaxial lift-off (ELO) is a process which allows for the separation of a single crystallineIII/V thin film or device from the substrate it was deposited on. This process is based onthe selective etching of an intermediate AlAs release layer in an aqueous HF solution.The lateral etch rate of the AlAs release layer through a narrow crevice in theweight-induced epitaxial lift-off (WI-ELO) process is much larger than observed forunobstructed planar AlAs layers. It is possible that this increase in etch rate iscaused by the tensile strain induced upon the AlAs layer in the WI-ELO setup. Inorder to verify this assumption, planar AlAs layers, subjected to a controlledcurvature, were etched in HF solutions and their etch duration was measured. Theapplied curvature reduced the already present compressive strain due to latticemismatch. For large applied bending radii no change in etch rate was observed,because the induced bending is smaller than the already present bending due tothe lattice mismatch. Further bending induces a total compressive strain from−0.126% to−0.11%, resulting in an etch rate variation from 0.054 up to0.066 mm h−1. Measurements on AlAs layers experiencing a tensile strain of+0.286% showed muchhigher etch rates of 0.134 mm h−1.The present results obtained on etching experiments in the lateral plane are extrapolatedto the perpendicular direction so that a combination with the data from previous workbecomes feasible. This results in a better microscopic picture of the etch front inthe WI-ELO process. It is found that the force exerted by the weight can beprojected on an area, limited by the sample width and a depth of approximately6 µm.