Characterization of atomic layer deposited alumina thin films on black silicon textures using helium ion microscopy

Abstract

Black silicon (b-Si) refers to the nanoscale texturing of a silicon surface to reduce the reflectivity across a broad wavelength range. We fabricate b-Si using a metal assisted chemical etch (MACE) process and then electrically passivate the surface using atomic layer deposition (ALD) of alumina. A crucial step towards effective passivation is the necessity of uniform and conformal coverage of the nanostructures by the alumina film, which can be achieved by inserting a diffusion step for the precursors during the ALD process. We demonstrate excellent alumina coverage of b-Si nanostructures through high resolution imaging of texture cross-sections using helium ion microscopy. The images confirm conformal and uniform coverage, even in the narrow trenches between the nanostructures. Moreover, the thickness of the alumina coating is found to be the same for samples with two different nanostructure lengths indicating that precursor supply is sufficient to not limit growth on high surface area structures. Further morphological characterisation is carried out by milling into the nanostructures with a Ne ion beam to reveal the structure in cross-section.