Laterally graded porous silicon optical filter fabricated by diffusion-limited etch process

Abstract

We report a method of producing a lateral gradient in the optical properties of anodically etched porous silicon layers. Lateral gradation details of the porous silicon layer are governed by the etch mask pattern involved. Unlike other methods that rely on uneven hole current distribution, we believe that in our method the diffusion of reactive ions in the etchant plays a key role. As an implementation of the proposed method, we demonstrate a linearly graded optical bandpass filter operating at the lambda=1550 nm range by employing a tapered etch window opening. The resultant optical filter exhibited a approximate 60 nm tuning range with a sharp transmission bandwidth of approximately 3 nm. Computer simulations indicate that an uneven hole current distribution cannot be the reason for the observed gradient along the taper axis, supporting the view that the diffusion-limited etch process plays the key role.

Methods to Evaluate Spatial Uniformity in Porous Silicon

“…It is also possible to deliberately impose spatial variability in selected chemical or physical properties across porous silicon. Lateral pore size gradients can be obtained by appropriate positioning of the counter electrode during etching to give a gradient of current density across the porous silicon (Bohn and Marso 2005;Collins et al 2002;Hunkel et al 1998Hunkel et al , 2000Hwang et al 2008;Ilyas and Gal 2006;Khung and Voelcker 2009;Li et al 2005;Park et al 2010). Radial current density gradients for porous silicon preparation have also been generated in a dual compartment cell separated by the silicon substrate .…”

“…For example, Desplobain et al measured the reflectivity at 17 points in a star pattern across a porous silicon sample using instruments designed for mapping of semiconductor wafers (Desplobain et al 2014). Researchers have also made point measurements in a transect across the sample (Isaji et al 2001;Aoyagi et al 1994), particularly when they have prepared porous silicon with chemical or physical gradients (Bohn and Marso 2005;Hwang et al 2008;Karlsson et al 2004;Park et al 2010). However, this review focuses mainly on approaches that provide a lateral uniformity map by making multiple measurements across the whole surface of an intact porous silicon sample.…”