2D photoluminescence mapping of porous silicon using confocal technique

Abstract

Silicon crystal in porous state can be featured by some extra optical properties. The method widely used in the literature for porous formation is anodic anodization technique due to its simplicity and ease of construction of the measurement cell. However, the porosity distribution attained by this technique strongly depends on anodization cell structure and considered conditions, thus requiring experience and care in fabrication process. Based on some fabrication tolerances and unexpected environmental conditions, surface morphology and optical activity of a created porous layer in two or three dimensions can significantly vary. This challenging circumstance thus requires some specifically designed techniques to measure and evaluate the surface morphology and optical activity of created porous layers. Confocal imaging systems commonly used in the literature for 3D imaging of biological systems due to their exceptional superior resolution, down to nanometer scale. At the same time this technique has a potential of semiconductor characterization. Recent advances in laser technology and highly accurate signal detection systems during the last decade provided significantly better signal/noise ratios in confocal imaging systems. In this study, a two-dimensional optical reflection characteristic from a porous silicon is mapped using a designed confocal system with selected wavelength or photoluminescence. From the mapping, we found the following important points. First, physically unobservable non-uniform porosity formation arising from gas bubbles during anodization process can be detected by the proposed confocal system. Second, comparison of wide- and local-area photoluminescence demonstrated that wide-area luminescence is composition of distinct local-area luminescence values from different areas.