Fabrication of graded porous silicon distributed Bragg reflectors via tilted-substrate lateral anodization

Abstract

A laterally graded porous silicon (PSi) distributed Bragg reflector (DBR) was fabricated by electrochemical anodization of p-type Si(100) in a 12% HF–ethanol (1:1) electrolyte, with the substrate tilted at +20° relative to a silver cathode. Fabry–P´erot fringe analysis of single-layer calibration samples etched at 10 mA and 60 mA for 5 min yielded optical path lengths nL = 1436.85 nm and nL = 3857.50 nm, giving quarter-wave etch times of 41.76 s and 15.55 s for a design wavelength of 800 nm. A five-period DBR fabricated with these parameters exhibited a continuously shifting stop band. Multiple repetitions of the color progression were observed along the 2 cm etched length and measurements were performed within the central repetition, where the color bands were visibly wider. Across six measurement points in this region the peak reflectance wavelength shifted from 602 nm to 913 nm, a total gradation of 311 nm, while the maximum reflectance increased from 0.53 to 0.91. The 800 nm design wavelength was reproduced to within 3% at the geometric center of the sample.