Cross-sectional transmission electron microscopy study of femtosecond laser-irradiated selenium-doped 'black' silicon

Abstract

'Black silicon' refers to silicon that has been treated in a laser-ablation process to incorporate large amounts of chalcogen dopants. The material has been found to have greatly increased absorbance of visible and infared wavelength light in comparison to undoped crystalline silicon. Selenium-doped black silicon that had been annealed at different temperatures were studied using transmission electron microscopy (TEM) and electron diffraction. The goal of the investigation was to characterize the structure of the laser-altered regions of the material. In addition, energy dispersive X-ray spectroscopy (EDX) was conducted in a scanning transmission electron microscope (STEM) in order to map spatial distribution of the selenium and the silicon were located within the material. The results of the TEM study showed roughly conical peaks of varying shapes protruding about 1 [mu]m from the surface of the material. The material is altered up to a depth of up to 1-2 [mu]m, where polycrystalline or amorphous layers were observed. Electron diffraction studies revealed increased crystallinity in the annealed sample. A continuous, sharp interface between the affected region and unaltered substrate was found and particles of diameter 5-100 nm embedded within the silicon were observed. The STEM-EDX studies showed that the selenium was dispersed inhomogenously throughout the material. The selenium is concentrated near the interface of the unaltered Si substrate and the laser-altered layer and a high local concentration of selenium in the embedded particles was recorded. The findings in this study provide a first look at the underlying structure of black silicon and will lead to future work characterizing the material.