Subwavelength antireflection structure for optical detector fabricated by fast atom beam etching

We fabricated a two-dimensional subwavelength grating (SWG) on a gallium aluminum arsenide (GaAIAs) double power double hetero (DDH) junction structure. The GaAIAs DDH structure functions as a photodiode (PD). The fabricated grating had 200 nm period and the tapered grating shape with aspect ratio of 1 .38 to prevent reflection in the visible and near-infrared spectral regions. The SWG was patterned by electron beam lithography and etched by a fast atom beam (FAB) with Cl2 and SF6. The novel etching technique using the two kinds of process gas of the FAB for fabricating the tapered grating with high aspect ratio was proposed. The reflectivity was examined at wavelengths from 400 nm to 800 nm. The reflectivity of the SWG was less than 1.0% at wavelengths from 400 nm to 780 nm. For example, at wavelengths of 440 nm and 780 nm, both reflectivities of the SWG decreased to 0.02% from 45.31% and 38. 1% of the flat surface, respectively. The theoretical calculations of the reflectivity were carried out by using rigorous coupled-wave analysis. The calculated reflectivity agreed well with the measured results. The I-V characteristics of the PD were measured by using laser diode light. The open-circuit voltage, short-circuit current and total conversion efficiency were improved by fabricating the SWG.