Low reflectivity surface relief gratings for photodetectors

Abstract

Photodetectors having low reflectivity triangular groove, surface relief gratings on homogenous material or one layer of a heterostructure. Preferably, the photodetector is a PIN photodiode in which the p-type layer is triangularly grooved. The surface relief gratings have an optical repeat distance greater than the wavelength of light which impinges on the photodetector surface. Thus, zero order backward diffracted waves are not coupled into optical reflections which would thereby decrease the optical return loss (ORL). Furthermore, the surface relief gratings have minimum side-wall angles to limit reflection at the heterostructure interfaces from contributing to the ORL. The side-angles of the gratings are chosen to ensure that the angle of the higher order backward diffracted wave is greater than the capture angle of an optical receiver into which the photodetector is incorporated. The free-space depth of the grooves is a half-wavelength of the light impinging on the photodetector. Additionally, a novel ion milling technique is disclosed. This technique involves providing a rectangular groove grating initially etched by conventional chemical etching and photoresistive techniques and then ion milling at particular ion mill angles to obtain triangular groove surface relief gratings provided in accordance with this invention.